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Identifying suitable tree species for evapotranspiration covers of landfills in humid regions using seedlings
Accepted Manuscript Title: Identifying suitable tree species for evapotranspiration covers of landfills in humid regions using seedlings Authors: LC Hui, LM Chu PII: DOI: Reference:
S1618-8667(17)30395-3 https://doi.org/10.1016/j.ufug.2018.12.004 UFUG 26255
To appear in: Received date: Revised date: Accepted date:
28 June 2017 11 December 2018 12 December 2018
Please cite this article as: Hui L, Chu L, Identifying suitable tree species for evapotranspiration covers of landfills in humid regions using seedlings, Urban Forestry and amp; Urban Greening (2018), https://doi.org/10.1016/j.ufug.2018.12.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Identifying suitable tree species for evapotranspiration covers of landfills in humid regions using seedlings LC Hui1 and LM Chu2 1
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School of Life Sciences, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR, China; [email protected] 2 School of Life Sciences, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR, China; [email protected]
Corresponding author: LM Chu (603A, Mong Man Wai Building, The Chinese University of
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Hong Kong, Shatin, NT, Hong Kong SAR; [email protected]) Highlights
ET covers are potentially effective on closed landfills even in humid areas by planting suitable species.
Species that have a high growth rate and small leaves are likely to have a high leaflevel transpiration rate.
A high growth rate and a high maximum quantum yield are good indicators for the selection of species to be planted on ET covers in humid regions.
Exotic species should be included for ET covers, with consideration of their transpiration potential and ecological implications.
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Abstract Post-closure landfills are usually managed for restoration and revegetation. To minimize percolation and the negative environmental effects of landfill-associated factors, evapotranspiration (ET) covers are becoming more widely used on landfills to allow the establishment of a functional plant-soil system. The applicability of ET covers in humid regions can be enhanced by using plant species with high transpiration rates. This is determined using plant traits that are useful indicators for screening species with high transpiration rates. In this study, the transpiration rates of seedlings of six common tree species in Hong Kong were measured using a gravimetric method. Acacia confusa, the only exotic species, showed the best performance in terms of both growth and transpiration potential at tree-level transpiration (60.4 to 93.7 g/day) and leaf-level transpiration (0.080 to 0.172 g/day/cm2) among the six species studied. Whole-seedling transpiration was found to be highly dependent on the size of tree seedlings, whereas a high growth rate and small leaves were possible indicators of a high transpiration rate per unit leaf area. We suggest that the growth rate and maximum quantum yield are good criteria for the selection of species to 1
be planted on ET covers. The possibility of using suitable exotic species as pioneer species on ET covers is also discussed. Keywords Landfill restoration; Maximum quantum yield; Species selection; Transpiration; Urban trees
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Introduction Many landfills are located in or near urban areas, especially in areas of rapid urbanization (Zurbrugg et al., 2003; Owusu et al., 2012). For example, there are more than 500 operating urban landfills in China (Zhou et al., 2015). The restoration of urban landfills with vegetation has several benefits, including the creation of habitats for local wildlife (Simmons, 1999;
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Rahman et al., 2011; Do et al., 2014) and improvement of the aesthetic value of the landscape (Wang et al., 2016). Meanwhile, alleviation of the potential negative environmental effects of landfills on the surrounding environment is important. Water pollution from landfill leachate
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is a serious problem, especially for urban landfills without good pollution control (Chofqi et al., 2004).
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Evapotranspiration (ET) cover, also known as phytocap, has become increasingly popular as an alternative for landfill cover in recent years because of its potentially lower cost and longer-term sustainability (Hauser et al., 2001). It makes use of the transpiration of vegetation to minimize percolation. In this approach, water is allowed to enter the soil during a rainfall event and is stored in the soil for later removal by plant transpiration and soil evaporation.
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The vegetation also reduces methane emissions by enhancing methane oxidation through improving soil aeration (Venkatraman and Ashwath, 2008; Abichou et al., 2015). The reliance on vegetation and the use of local soils for this type of cover promotes the habitat restoration of landfills in and near urban areas. However, the application of ET cover in humid regions is still limited (Albright et al., 2004) because of the relatively high amount of precipitation relative to ET under this type of climate. To enhance the effectiveness of an ET cover in humid regions, one useful approach is to use plants with good transpiration ability (Barnswell and Dwyer, 2012). The influence of plants on the soil water balance, which is not normally taken into great consideration for typical rehabilitation programs for conventional landfill covers, is more critical for an ET cover. In addition, although the water removal ability of a species is important, its transpiration provides additional advantages such as cooling (Ballinas and Barradas, 2016) and soil stabilization (Simon and Collison, 2002). Although some studies have suggested the importance of tree species on an ET cover, which show great variation in transpiration rate (Venkatraman and Ashwath, 2009) and their effectiveness in reducing percolation (Abichou et al., 2012), there is still a paucity of information on the suitable tree species to be planted on ET covers and on the criteria for tree species selection with 2
consideration of transpiration potential.
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A plant’s transpiration rate is determined mainly by two factors: the surrounding environment, which is largely controlled by the local climate, and plant traits, which are species specific. Because of the interaction between these two factors, the degree of control of different plant traits on transpiration is dependent on meteorological characteristics (Meinzer et al., 1995). Thus, by identifying plant traits that are correlated with a high transpiration potential in local meteorological conditions, the species that are most suitable for ET covers can be identified. Hong Kong is located on the southern coast of China in a humid subtropical region
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characterized by wet, hot summers and dry, cool winters. Therefore, our study on ET covers in the context of local conditions in Hong Kong can provide useful insights into the design of ET covers in regions with similar climates. The objectives of this experiment were i) to
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determine and compare the transpiration potential of selected tree seedlings; ii) to identify the plant characteristics associated with transpiration potential; iii) to evaluate the plant traits that are good indicators of the transpiration potential of tree species; and iv) to provide implications for species selection in terms of the transpiration potential for ET covers in humid regions. We hypothesized that different tree species will have different transpiration potential because of their morphological and physiological differences, and that the measured plant traits in this study can predict transpiration rate.
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Materials and Methods Study area and experimental setup The experiment was carried out outdoors in the open space of the Physical Geography Experimental Station at The Chinese University of Hong Kong (22°24'55.8"N 114°12'44.1"E). A metal frame with a transparent plastic cover was erected to allow the tree seedlings to experience natural environmental conditions but prevent precipitation. Seedlings (1 to 2 years old) with similar initial total leaf areas were purchased from the Tai Tong Forest Nursery of the Agriculture, Fisheries and Conservation Department of the Hong Kong Government. They were transplanted into 19-cm-diameter polyvinylchloride (PVC) pots filled with soil to 28 cm for acclimation for two weeks, during which the seedlings were irrigated adequately. The bottom of each pot had five 6-mm perforations to allow free water drainage. There were six species and six replicates of each species, giving a total of 36 seedlings. They were arranged in randomized blocks without overlapping canopies. The soil substrate used was completely decomposed granite (CDG), a locally available soil from areas commonly used for land restoration in Hong Kong that typically has a high sand 3
content and poor fertility (Jim, 1996; Cheng and Chu, 2011). Gravel was removed with a 2cm sieve, and soils were compacted to a dry density of 1.44 Mg/m3, equivalent to relative compaction of 80% of the CDG used, a level that allows normal root development for sandy soils and is within the field-compaction range for ET cover (Reynolds et al., 2002; McGuire et al., 2009).
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Plant species studied Five native species that are commonly used in revegetation in Hong Kong were investigated: Myrica rubra (Myricaceae), Reevesia thyrsoidea (Sterculiaceae), Schefflera heptaphylla (Araliaceae), Schima superba (Theaceae), and Syzygium hancei (Myrtaceae). A common exotic species was also included: Acacia confusa (Fabaceae), a nitrogen-fixing species widely
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used as a pioneer species for revegetation in Hong Kong. Totally, six tree species were used in this study.
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Measurement of transpiration rate The transpiration potential of the different tree species was determined by a gravimetric method. Seven days were taken as one complete cycle. During the night before the start of each cycle (Day 0), water was added to each polyvinylchloride pot to bring the soil to saturation and excess water was allowed to drain overnight. Bringing the soil to saturation avoided the occurrence of depletion of the transpiration potential caused by low soil water availability (Sinclair et al., 2005). The weight of each PVC pot was determined the next morning (Day 1). During the night on Day 7, the weight was determined again. The
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difference in weight between the start and the end of the cycle was assumed to be the total water loss from the soil-plant system. Since in plants most of the absorbed water is lost through transpiration and in our experiment the change in weight of the soil-plant system was most probably a result of water loss from the plants, the change in weight of the soil-plant system was taken as the transpiration rate of the plant over seven days.
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To minimize soil evaporation, the pots were enclosed with a plastic bag to cover the soil (Singh and Thompson, 1995). Thus, the weight differences were mainly due to transpiration. The experiment started on 9 September 2015 and ended on 11 November 2015; eight cycles of measurement were made during the experimental period for analysis. The data for one cycle (30 September to 6 October) were excluded due to a typhoon. Transpiration at tree-level and leaf-level were analyzed, with the former for determining the size effect of tree on transpiration, while the latter helps decide size-independent characteristics of tree that favor transpiration. Leaf-level transpiration was estimated by treelevel transpiration over total leaf area. 4
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Determination of plant traits potentially associated with tree-level transpiration The total leaf area of each seedling was determined every seven days by counting the number of leaves and multiplying by the average size of ten leaves of each individual for every species. The leaf size was determined with ImageJ by analyzing digital photos of the leaves. The seedlings were harvested at the end of the experiment. The crown spread was determined by taking the average length of the greatest spread of the crown and the length of crown perpendicular to the greatest spread. The dry mass of the different parts of the seedlings was determined after drying at 80°C for 48 hours. Determination of plant traits potentially associated with leaf-level transpiration
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Growth rate and biomass allocation, that is, leaf weight ratio (LWR) and root weight ratio (RWR), specific leaf area (SLA) and leaf area index (LAI), were derived from the primary data. Growth rate was determined by the percentage change in total leaf area at each cycle.
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LAI was calculated by one-sided leaf area over crown projection. Stomatal density was determined by observing the imprints of leaf surfaces under a light microscope at 400×. Each imprint was prepared by applying clear nail polish on the leaf surfaces, avoiding the midveins and leaf margins (Camargo and Marenco, 2011). After drying, the imprint was transferred from the leaf surface to a glass slide with transparent adhesive tape. For each individual, five imprints from five different leaves were examined, and four counts per imprint were made. With the exception of Acacia confusa, stomata were only found in abaxial surfaces for all species. In addition, the photosynthetic capacity of each seedling was
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measured by determining the fluorescence-based maximum photochemical yield of photosystem II (Fv/Fm), a commonly used indicator of photosynthesis (Baker, 2008), using a photosynthesis yield analyzer MINI-PAM (Walz, Germany). For each individual, the Fv/Fm of four fully expanded leaves was determined after dark adaptation (at least 30 minutes after sunset).
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Weather information Weather data were collected from the nearest weather station of the Hong Kong Observatory (www.hko.gov.hk). The temperature and relative humidity were obtained from the Shatin automatic weather station (22°24'09"N 114°12'36"E), which is 1.45 km away, and the solar radiation was obtained from the King’s Park automatic weather station (22°18'43"N 114°10'22"E), which is 12.2 km from our experimental site. Statistics analysis SPSS (v24) was used to carry out all statistical analyses. One-way analysis of variance was carried out to compare the differences in the transpiration rate and plant traits if the data were 5
normally distributed. One-way analysis of covariance was performed to consider the effects
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of covariance on transpiration. Tukey’s honestly significant difference (HSD) test was used as the post-hoc test if variances were assumed to be equal. If equal variances could not be assumed, Dunnett’s T3 was used as the post-hoc test. Mann-Whitney U-tests (p<0.05) were carried out to compare species differences in the transpiration rate and plant traits if the data were not normally distributed. Correlations of different plant traits with the transpiration rate were determined by Pearson correlation analysis. During our experiment, one replicate of S. hancei and one replicate of S. superba died. The measurements of these two individuals were therefore discarded.
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Results Morphological characteristics and ecophysiological performance The values of the plant traits in relation to the sizes of the studied species are shown in Table 1. Great variation was found in the size of the different species. A. confusa was the largest
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species in terms of the crown spread, leaf area, and dry mass of leaves, stems, and roots. Among the native species, M. rubra, S. superba, and S. hancei were the largest in terms of total leaf area and mass, with M. rubra having the greatest size. The LAI of M. myrica and S. hancei were the highest, whereas A. confusa, R. thyrsoidea, and S. heptaphylla had sparser crowns. Table 2 shows the ecophysiological and morphological plant traits of species that Table 1, may affect leaf-level transpiration. Again, great variation in these traits was found among Table 2 species. Only A. confusa, M. rubra, and R. thyrsoidea produced new leaves throughout the experiment, and A. confusa had the highest growth rate in terms of leaf area. For
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morphological traits, A. confusa had the smallest leaf size, and M. myrica, S. superba, and S. heptaphylla had the largest leaves. S. hancei had the lowest SLA, followed by A. confusa, and R. thyrsoidea had a relatively high SLA. A. confusa tended to allocate less biomass to leaves, whereas R. thyrsoidea and S. heptaphylla showed particularly low biomass allocation to roots. M. rubra had the highest stomatal density, followed by S. superba. In general, the values of different traits varied two- to three-fold among species. A. confusa showed the highest Fv/Fm.
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Transpiration performance The tree-level transpiration rate, that is, the transpiration rate of a whole tree seedling, varied remarkably among species (Fig. 1). Exotic A. confusa showed the highest transpiration rate, which was slightly more than double those of the other native species except M. rubra, which was the only species that showed no significant difference with A. confusa in transpiration rate. Of the native species, M. rubra had the highest transpiration rate, approximately double those of the other natives. However, its transpiration rate was only significantly higher than S. heptaphylla and R. thyrsoidea, whereas no significant difference could be found among the 6
Fig. 1, Fig. 2
remaining four native species. Fig. 2 shows the transpiration rates of the various species at the leaf level (i.e., transpiration rate per unit leaf area). Exotic A. confusa and native R. thyrsoidea and S. hancei had higher leaf-level transpiration rates than the other three native species studied. M. rubra had a high tree-level transpiration rate but a comparatively low leaf-level transpiration rate.
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Although the superiority of A. confusa in terms of tree-level transpiration was significant throughout the eight cycles, the difference declined after the first three cycles (Fig. 3). Similarly, the variation in leaf-level transpiration decreased after three cycles (Fig. 4). These differences diminished probably as a result of the lower temperatures seen during the last five cycles (Table 3). Association of tree characteristics with transpiration potential The plant traits that had significant correlations with transpiration at the tree level are
Fig. 3, Fig. 4, Table 3
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presented in Table 4. The tree-level transpiration rate was significantly correlated with the total leaf area, the final dry mass of leaves, stems, and root, and crown spread, but no correlation was found with LAI, which shows that larger plants with a higher total leaf area and dry biomass tend to have high rates of tree-level transpiration. However, even if leaf area was taken into account, analysis of covariance showed that species still had a significant effect on the transpiration rate (F(5.27)=9.947, p<0.01). This finding is in accord with the fact that the leaf-level transpiration rates of the various species differed significantly. The linear relationship between the tree-level transpiration rate and different correlated plant traits
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within the observed range are shown in Fig 5.
Table 4, Fig. 5
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The plant traits that had significant correlations with transpiration at the leaf level are presented in Table 5. The leaf-level transpiration rate had a moderate positive correlation with the growth rate and a moderate negative correlation with leaf size. The linear relationship between the leaf-level transpiration rate and these two plant traits within the range studied are shown in Fig 6. Table 5, Figure 6
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Discussion Key findings Six studied species varied in their transpiration potential because of the differences in plant traits. Crown spread, leaf area, and dry mass of leaves, stems and roots were all positively related to tree-level transpiration rate, while growth rate and leaf size are positively and negatively related to leaf-level transpiration rate respectively; hence, their use in predicting transpiration rate. 7
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Plant traits favoring high transpiration rate At the tree level, all size-related traits measured in our experiment can be used to predict the transpiration rate. It was suggested that the tree-level transpiration rate was size-dependent rather than species-dependent for mature trees of five species in a tropical forest (Goldstein et al., 1998). However, even if leaf area was taken into account, analysis of covariance showed that species still had a significant effect on the transpiration rate (F(5.27)=9.947, p<0.01) in our experiment. This finding is in accord with the fact that the various species in our experiment showed significant differences in the leaf-level transpiration rate. Therefore, plant size is not the only important determining factor of transpiration, and the effect of species should not be neglected, at least for tree seedlings. Our results show that leaf size and growth
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rate were the three traits most closely linked to a high transpiration rate at the leaf level.
Leaf size can predict the leaf-level transpiration rate in tropical secondary forests (Juhrbandt
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et al., 2004). Smaller leaves are favorable to transpiration because they lead to lower boundary layer resistance for water loss through transpiration (Yates et al., 2010) because the smaller the leaf size, the smaller the surface area relative to the perimeter length, which thus allows greater heat and moisture transfer. However, the greater degree of cooling in smaller leaves caused by greater heat transfer may contrarily reduce transpiration rate, because of the reduced leaf temperature. Therefore, further experiments may be required to confirm if the relationship between leaf size and transpiration holds in the prevailing environmental conditions.
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In contrast, some traits that were suggested to be possible indicators of a high transpiration rate, including RWR or LWR (Poorter and Farguhar, 1994), SLA (Poorter and Bongers, 2006), and stomatal density (Juhrbandt et al., 2004), did not show any significant relationship with the transpiration rate in our experiment. Two qualitative traits that were proposed to promote the transpiration rate, the presence of a toothed margin on leaves (Wilf, 1997) and shade intolerance (Siegert and Levia, 2011), also failed to predict the transpiration rate in our experiment. Species that have leaves with a toothed margin (M. rubra, S. heptaphylla, and S. superba) and species that are shade tolerant (S. heptaphylla and S. hancei) did not show lower leaf-level transpiration rates. One possible reason is that the effects of other traits masked the effects of these traits. For example, the species in our experiment that have leaves with a toothed margin also had a comparatively large leaf size. Suitable indicators for the selection of species for ET cover Great variation in the transpiration rates of seedlings reflects the importance of selecting suitable species to enhance the effectiveness of ET covers. However, it can be seen that 8
simple generalization is difficult because one favorable trait for a species’ transpiration rate
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may be counteracted by another trait that is unfavorable to transpiration. Although leaf size and growth rate were found to determine leaf-level transpiration, the use of leaf size as a selection criterion is less appropriate than growth rate. If the growth rate within a tree species varies greatly, the contribution of leaf-level transpiration to tree-level transpiration is very likely masked by variations in tree size during development. Therefore, although a species may produce an effect on transpiration at the leaf level because of its morphological traits, it is not a reliable criterion for selection without considering its physiology. Growth rate is better selection criterion because it hints at both the photosynthesis and transpiration rates of the species. Moreover, it implies that seedlings can achieve a larger size
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in a shorter time and hence a higher overall transpiration rate. Although there is no directional relationship of Fv/Fm with transpiration rate, Fv/Fm was highly correlated with growth rate (Jeannine and Fakhri, 2004). Indeed, a moderately positive correlation between growth rate
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and Fv/Fm was found in our experiment (r=0.0359, p<0.05). Particularly, Fv/Fm reflects the adaptability of plants to a given environment, including drought tolerance (Percival et al., 2006) and compaction tolerance (Philip and Azlin, 2005), which are important for successful growth on restored landfills. Therefore, Fv/Fm is a possible reference for selection from this point of view. Local field survey of the growth conditions of various species on restored land, by measuring the Fv/Fm of species or growth measurement in terms of size, is therefore important to screen for tree species for their growth rate and transpiration potential. The use of morphological traits alone to estimate the transpiration potential of trees is obviously less
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desirable. In other words, we should not only consider the transpiration rates of two species when they are of similar size, but also their long-term growth performance, to get a better picture of their transpiration potential.
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Inclusion of exotic species Our results suggest that the inclusion of suitable exotic species in landfill restoration was desirable. A successful revegetation plan requires the selection of suitable species according to the local climate and site conditions and to the specific aims of reforestation (Wishnie et al., 2007). One of the objectives of planting of an ET cover is to maximize transpiration to remove soil water between rainfall events, which is normally not taken into consideration for typical restoration programs. A. confusa, a heavily planted exotic, showed superior performance in terms of transpiration potential in our experiment. In Hong Kong, Acacia spp. have been observed to grow well on many landfills and quarries compared with other native species because of their nitrogen fixing ability (Jim, 2001; Wong et al., 2016). Although fertilizers are routinely applied to the topsoil of landfill cover for vegetation establishment in Hong Kong, the effect of fertilizers is limited over the long term. Therefore, nutrient 9
deficiency is a common limitation to the growth of native tree species on landfill covers
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(Wong et al., 2016). From a functional point of view, they are suitable species because of their high transpiration potential, as evidenced in this study and predicted by their fast growth rate. From an ecological point of view, they have the potential to promote colonization of native species on local degraded land by providing shelter and increasing the soil nitrogen content (Zhang et al., 2013; Wong et al., 2016). It has been proposed that the use of pioneer species that enhance plant diversity will lead to lower soil water content and higher slope stability over the long term because of the resultant high biomass and extensive root systems (Osman and Barakbah, 2011). The use of exotic species may therefore not necessarily be an evil because of their possible ecological advantages (D’Antonio and Meyerson, 2002). Yet, exotic species that are invasive to local region should be avoided for planting. Use of larger seedlings In our experiment, Acacia confusa showed the highest transpiration rate in first cycle with
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93.7g/day, which is equivalent to 0.826 mm/day. This value was much lower than the average daily rainfall in Hong Kong in wet season, which is around 12 mm/day. This implies that saplings of larger size should be employed to remove soil water and achieve soil drying.
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Limitations We studied only the effects of species on soil water removal in terms of transpiration. However, soil water content is also influenced by many other interspecific differences. For example, crown traits may affect interception (Park and Cameron, 2008), which reduces the
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amount of rainfall that reaches the soil. In addition, the effects of soil evaporation were not considered. The magnitude of water loss from evaporation of vegetated soil is affected by the light interception by the tree seedlings, which depends partially on the crown characteristics. More species should be included in further studies to confirm plant traits as good predictors of transpiration. Moreover, because of the limited experimental period, the influence of environmental conditions on the transpiration potential of species could not be fully evaluated. Another issue is that we used tree seedlings in our experiment, so the results are not directly applicable to adult trees. Besides, interspecific traits between laboratory-grown seedlings and field-grown adult trees can be disparate (Cornelissen et al., 2003). Despite these defects of measurement in the use of seedlings, from the plantation point of view, planting tree seedlings is a more economical and practical choice. Therefore, the performance of tree seedlings is worthwhile to study to ensure the effectiveness of cover during the early stages of plantation. Conclusions The study assessed the transpiration rates of six species of tree seedlings and determined 10
suitable indicators for the selection of species for ET cover. Exotic A. confusa had the highest
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transpiration rate of all the species at the tree and leaf levels. Growth rate provides good implications of a plant’s size development and leaf transpiration rate; it is good criterion for species selection, from the perspective of maximizing transpiration, to achieve the target of reducing percolation. Meanwhile, Fv/Fm gives information on the adaptability of the species to the often-harsh environment of restored landfills. Leaf size is an additional parameter to be considered, but is comparatively less significant. Acacia spp. that give both functional and ecological advantages can be considered as pioneer species on ET covers, the primary goal of which is to reduce percolation, in our study region. This provides implications for the recruitment of suitable exotic species that can maximize the performance of ET cover to reduce percolation and restore habitats in urban areas. Acknowledgements The work was supported by the Research Grants Council (RGC) of the Hong Kong Special
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Administrative Region [grant number HKUST6/CRF/12R]. We would like to thank Mr Ben Yeung for technical assistance and the Department of Geography and Resource Management, The Chinese University of Hong Kong, for lending us the venue for our experiment.
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Engineering 37, 139-147. Owusu, G., Oteng-Ababio, M., Afutu-Kotey, R.L., 2012. Conflicts and governance of landfills in a developing country city, Accra. Landscape and Urban Planning 104, 105-113. Park, A. and Cameron, J.L., 2008. The influence of canopy traits on throughfall and stemflow in five tropical trees growing in a Panamanian plantation. Forest Ecology and Management 255, 1915-1925. Percival, G.C., Keary, I.P., Sulaiman, A.H., 2006. An assessment of the drought tolerance of Fraxinus genotypes for urban landscape plantings. Urban Forestry and Urban Greening 5, 17-
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27. Philip, E., Azlin, Y.N., 2005. Measurement of soil compaction tolerance of Lagestromia speciosa (L.) Pers. using chlorophyll fluorescence. Urban Forestry and Urban Greening 3, 203-208. Poorter, H., Farquhar, G.D., 1994. Transpiration, intracellular carbon dioxide concentration and carbon-isotope discrimination of 24 wild species differing in relative growth rate. Functional Plant Biology 21, 507-516. Poorter, L., Bongers, F. 2006. Leaf traits are good predictors of plant performance across 53 rain forest species. Ecology 87, 1733-1743. Rahman, M.L., Tarrant, S., McCollin, D., Ollerton, J., 2011. The conservation value of restored landfill sites in the East Midlands, UK for supporting bird communities. Biodiversity and Conservation 20, 1879-1893. Reynolds, W.D., Bowman, B.T., Drury, C.F., Tan, C.S. and Lu, X., 2002. Indicators of good soil physical quality: density and storage parameters. Geoderma 110, 131-146. Siegert, C.M., Levia, D.F., 2011. Stomatal conductance and transpiration of co-occurring seedlings with varying shade tolerance. Trees 25, 1091-1102. 13
Simmons, E., 1999. Restoration of landfill sites for ecological diversity. Waste Management
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and Research 17, 511-519. Simon, A., Collison, A.J., 2002. Quantifying the mechanical and hydrologic effects of riparian vegetation on streambank stability. Earth Surface Processes and Landforms 27, 527546. Sinclair, T.R., Holbrook, N.M., Zwieniecki, M.A., 2005. Daily transpiration rates of woody species on drying soil. Tree Physiology 25, 1469-1472. Singh, K.A., Thompson, F.B., 1995. Effect of lopping on water potential, transpiration, regrowth, 14C-photosynthate distribution and biomass production in Alnus glutinosa. Tree Physiology 15, 197-202. Venkatraman, K., Ashwath, N., 2008. Can phytocapping technique reduce methane emission
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from municipal landfills? International Journal of Environmental Technology and Management 10, 4-15. Venkatraman, K., Ashwath, N., 2009. Phytocapping: importance of tree selection and soil
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thickness. Water, Air, and Soil Pollution: Focus 9, 421-430. Wang, R., Zhao, J. Liu, Z., 2016. Consensus in visual preferences: the effects of aesthetic quality and landscape types. Urban Forestry and Urban Greening 20, 210-217. Wilf, P., 1997. When are leaves good thermometers? A new case for leaf margin analysis. Paleobiology 23, 373-390. Wishnie, M.H., Dent, D.H., Mariscal, E., Deago, J., Cedeno, N., Ibarra, D. Condit, R. Ashton, P.M.S., 2007. Initial performance and reforestation potential of 24 tropical tree species planted across a precipitation gradient in the Republic of Panama. Forest Ecology and
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Management 243, 39-49. Wong, J.T.F., Chen, X.W., Mo, W.Y., Man, Y.B., Ng, C.W.W., Wong, M.H., 2016. Restoration of plant and animal communities in a sanitary landfill: a 10‐year case study in Hong Kong.
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Land Degradation and Development 27, 490-499. Yates, M.J., Anthony Verboom, G., Rebelo, A.G., Cramer, M.D., 2010. Ecophysiological significance of leaf size variation in Proteaceae from the Cape Floristic Region. Functional Ecology 24, 485-492. Zhang, H., Zhuang, X., Chu, L.M., 2013. Plant recruitment in early development stages on rehabilitated quarries in Hong Kong. Restoration Ecology 21, 166-173. Zhou, C., Gong, Z., Hu, J., Cao, A., Liang, H., 2015. A cost-benefit analysis of landfill mining and material recycling in China. Waste Management 35, 191-198. Zurbrugg, C., 2003. Urban solid waste management in low-income countries of Asia: how to cope with the garbage crisis. In Scientific Committee on Problems of the Environment (SCOPE) Urban Solid Waste Management Review Session, pp. 1-13. Durban, South Africa: Sandec Publications.
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Fig. 1.Mean tree-level transpiration rate (g/day) of the six studied species over eight cycles. Different letters denote significant differences among species (p<0.05) by Dunnett’s T3 test.
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Fig. 2.Mean leaf-level transpiration rate (g/day/cm2) of six studied species over eight cycles. Different letters denote significant differences among species (p<0.05) by Tukey’s HSD test.
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Fig. 3.Daily tree-level transpiration rate (g/day) of the six studied species over eight cycles. Different letters denote significant differences among species within the same cycle (p<0.05) by Dunnett’s T3 test.
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Fig. 4.Daily leaf-level transpiration rate (g/day/cm2) of the six studied species over eight cycles. Different letters denote significant differences among species within the same cycle (p<0.05) by Dunnett’s T3 test (Cycle 1) or Tukey’s HSD test (Cycles 2 to 8).
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Fig. 5 Linear regression between tree-level transpiration rate and a) crown spread, b) leaf dry mass, c) root dry mass, d) stem dry mass and, e) total leaf area. **p<0.01.
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Fig. 6 Linear regression between leaf-level transpiration rate and a) growth rate and, b) leaf size. *p<0.05, **p<0.01.
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Table 1. Size-related plant traits of the six tree species studied: Acacia confusa (Acc), Myrica rubra (Myr), Reevesia thyrsoidea (Ret), Schefflera heptaphylla (Sch), Schima superba (Scs), Syzygium hancei (Syh). Species
Crown spread* Total leaf area# Leaf dry mass^ Stem dry mass# Root dry mass# LAI*
(cm2)
(g)
(g)
(g)
Acc
41.3±3.6a
757±167a
9.92±2.58a
10.5±2.5a
7.89±0.99a
0.578±0.185bc
Myr
23.0±6.3bc
568±215a
5.03±2.21ab
6.32±1.73bc
7.51±1.76bc
1.66±1.38a
Ret
21.0±4.7bc
162±50b
1.22±0.43b
2.82±0.66c
4.22±0.75c
0.505±0.219bc
Sch
32.2±7.1bc
266±94b
2.40±0.89b
3.48±0.79c
5.97±0.59c
0.361±0.206c
Scs
23.3±0.4b
319±66b
2.84±0.80ab
5.07±1.57bc
6.82±1.00bc
0.746±0.140b
Syh
16.1±2.4bc
239±58b
3.49±0.83ab
6.28±1.36bc
1.10±0.164a
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(cm)
5.38±1.33bc
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Different letters denote significant differences among species (p<0.05) by #Tukey’s HSD test, ^Dunnett’s T3 test or *Mann-Whitney U test.
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Table 2.Plant ecophysiological and morphological traits of the six species studied. Species Leaf size#
(cm2)
Growth rate* Fv/Fm*
SLA*
(%)
(cm2/g)
LWR*
RWR^
Stomatal
density#
(no./mm2)
6.12±0.75d
39.9±23.7a
0.812±0.009a
77.5±9.5b 0.267±0.063a
0.317±0.031a 331±38bc
Myr
17.2±3.6a
9.04±35.29ab 0.777±0.025b
116±17cd 0.260±0.105a
0.418±0.066bc 637±71a
Ret
11.0±1.6bc
34.8±25.3ab 0.737±0.024bc
13512d
0.0887±0.0338b 0.553±0.044d 252±55c
Sch
15.2±3.0ab
-7.2±17.8b
0.662±0.080d
111±12c
0.137±0.035b
Scs
15.5±2.7a
-3.52±4.96b
0.771±0.016bc
114±9c
Syh
8.31±1.53cd 0.00±0.00b
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0.174±0.012ab
0.723±0.034bcd 68.4±2.2a 0.221±0.005ab
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Acc
0.562±0.057d 260±54c
0.489±0.036cd 406±28b
0.428±0.025b 335±9bc
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Different letters denote significant differences among species (p<0.05) by #Tukey’s HSD test, ^Dunnett’s T3 test or *Mann-Whitney U test.
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Table 3. Mean temperature, relative humidity, and global solar radiation of eight measurement cycles. Temperature (oC)
Relative humidity (%)
Global solar radiation (MJ/m2)
1
27.6±0.4
70.9±3.3
21.7±1.8
2
27.7±0.5
78.7±3.1
16.1±5.1
3
29.0±0.6
72.3±12.0
18.0±5.9
4
24.4±2.5
76.1±8.2
11.5±7.4
5
24.2±0.4
72.9±4.2
18.1±2.1
6
25.8±0.5
74.3±3.7
13.7±4.0
7
24.3±2.1
74.4±6.1
8
25.5±1.0
79.1±2.3
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Cycle
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14.9±2.6
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11.8±4.2
Table 4. Plant traits that had a significant correlation with tree-level transpiration rate across
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Crown spread
0.624**
Leaf dry mass
0.953**
Root dry mass
0.693**
Stem dry mass
0.874**
Total leaf area
0.923**
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Trait
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species and their correlation coefficient (r).
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**p < 0.01.
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Table 5. Plant traits that had a significant correlation with leaf-level transpiration rate across species and their correlation coefficient (r). r
Leaf size
-0.610**
Growth rate
0.351*
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*p< 0.05, **p < 0.01.
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S1618-8667(17)30395-3 https://doi.org/10.1016/j.ufug.2018.12.004 UFUG 26255
To appear in: Received date: Revised date: Accepted date:
28 June 2017 11 December 2018 12 December 2018
Please cite this article as: Hui L, Chu L, Identifying suitable tree species for evapotranspiration covers of landfills in humid regions using seedlings, Urban Forestry and amp; Urban Greening (2018), https://doi.org/10.1016/j.ufug.2018.12.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Identifying suitable tree species for evapotranspiration covers of landfills in humid regions using seedlings LC Hui1 and LM Chu2 1
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School of Life Sciences, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR, China; [email protected] 2 School of Life Sciences, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR, China; [email protected]
Corresponding author: LM Chu (603A, Mong Man Wai Building, The Chinese University of
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Hong Kong, Shatin, NT, Hong Kong SAR; [email protected]) Highlights
ET covers are potentially effective on closed landfills even in humid areas by planting suitable species.
Species that have a high growth rate and small leaves are likely to have a high leaflevel transpiration rate.
A high growth rate and a high maximum quantum yield are good indicators for the selection of species to be planted on ET covers in humid regions.
Exotic species should be included for ET covers, with consideration of their transpiration potential and ecological implications.
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Abstract Post-closure landfills are usually managed for restoration and revegetation. To minimize percolation and the negative environmental effects of landfill-associated factors, evapotranspiration (ET) covers are becoming more widely used on landfills to allow the establishment of a functional plant-soil system. The applicability of ET covers in humid regions can be enhanced by using plant species with high transpiration rates. This is determined using plant traits that are useful indicators for screening species with high transpiration rates. In this study, the transpiration rates of seedlings of six common tree species in Hong Kong were measured using a gravimetric method. Acacia confusa, the only exotic species, showed the best performance in terms of both growth and transpiration potential at tree-level transpiration (60.4 to 93.7 g/day) and leaf-level transpiration (0.080 to 0.172 g/day/cm2) among the six species studied. Whole-seedling transpiration was found to be highly dependent on the size of tree seedlings, whereas a high growth rate and small leaves were possible indicators of a high transpiration rate per unit leaf area. We suggest that the growth rate and maximum quantum yield are good criteria for the selection of species to 1
be planted on ET covers. The possibility of using suitable exotic species as pioneer species on ET covers is also discussed. Keywords Landfill restoration; Maximum quantum yield; Species selection; Transpiration; Urban trees
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Introduction Many landfills are located in or near urban areas, especially in areas of rapid urbanization (Zurbrugg et al., 2003; Owusu et al., 2012). For example, there are more than 500 operating urban landfills in China (Zhou et al., 2015). The restoration of urban landfills with vegetation has several benefits, including the creation of habitats for local wildlife (Simmons, 1999;
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Rahman et al., 2011; Do et al., 2014) and improvement of the aesthetic value of the landscape (Wang et al., 2016). Meanwhile, alleviation of the potential negative environmental effects of landfills on the surrounding environment is important. Water pollution from landfill leachate
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is a serious problem, especially for urban landfills without good pollution control (Chofqi et al., 2004).
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Evapotranspiration (ET) cover, also known as phytocap, has become increasingly popular as an alternative for landfill cover in recent years because of its potentially lower cost and longer-term sustainability (Hauser et al., 2001). It makes use of the transpiration of vegetation to minimize percolation. In this approach, water is allowed to enter the soil during a rainfall event and is stored in the soil for later removal by plant transpiration and soil evaporation.
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The vegetation also reduces methane emissions by enhancing methane oxidation through improving soil aeration (Venkatraman and Ashwath, 2008; Abichou et al., 2015). The reliance on vegetation and the use of local soils for this type of cover promotes the habitat restoration of landfills in and near urban areas. However, the application of ET cover in humid regions is still limited (Albright et al., 2004) because of the relatively high amount of precipitation relative to ET under this type of climate. To enhance the effectiveness of an ET cover in humid regions, one useful approach is to use plants with good transpiration ability (Barnswell and Dwyer, 2012). The influence of plants on the soil water balance, which is not normally taken into great consideration for typical rehabilitation programs for conventional landfill covers, is more critical for an ET cover. In addition, although the water removal ability of a species is important, its transpiration provides additional advantages such as cooling (Ballinas and Barradas, 2016) and soil stabilization (Simon and Collison, 2002). Although some studies have suggested the importance of tree species on an ET cover, which show great variation in transpiration rate (Venkatraman and Ashwath, 2009) and their effectiveness in reducing percolation (Abichou et al., 2012), there is still a paucity of information on the suitable tree species to be planted on ET covers and on the criteria for tree species selection with 2
consideration of transpiration potential.
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A plant’s transpiration rate is determined mainly by two factors: the surrounding environment, which is largely controlled by the local climate, and plant traits, which are species specific. Because of the interaction between these two factors, the degree of control of different plant traits on transpiration is dependent on meteorological characteristics (Meinzer et al., 1995). Thus, by identifying plant traits that are correlated with a high transpiration potential in local meteorological conditions, the species that are most suitable for ET covers can be identified. Hong Kong is located on the southern coast of China in a humid subtropical region
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characterized by wet, hot summers and dry, cool winters. Therefore, our study on ET covers in the context of local conditions in Hong Kong can provide useful insights into the design of ET covers in regions with similar climates. The objectives of this experiment were i) to
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determine and compare the transpiration potential of selected tree seedlings; ii) to identify the plant characteristics associated with transpiration potential; iii) to evaluate the plant traits that are good indicators of the transpiration potential of tree species; and iv) to provide implications for species selection in terms of the transpiration potential for ET covers in humid regions. We hypothesized that different tree species will have different transpiration potential because of their morphological and physiological differences, and that the measured plant traits in this study can predict transpiration rate.
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Materials and Methods Study area and experimental setup The experiment was carried out outdoors in the open space of the Physical Geography Experimental Station at The Chinese University of Hong Kong (22°24'55.8"N 114°12'44.1"E). A metal frame with a transparent plastic cover was erected to allow the tree seedlings to experience natural environmental conditions but prevent precipitation. Seedlings (1 to 2 years old) with similar initial total leaf areas were purchased from the Tai Tong Forest Nursery of the Agriculture, Fisheries and Conservation Department of the Hong Kong Government. They were transplanted into 19-cm-diameter polyvinylchloride (PVC) pots filled with soil to 28 cm for acclimation for two weeks, during which the seedlings were irrigated adequately. The bottom of each pot had five 6-mm perforations to allow free water drainage. There were six species and six replicates of each species, giving a total of 36 seedlings. They were arranged in randomized blocks without overlapping canopies. The soil substrate used was completely decomposed granite (CDG), a locally available soil from areas commonly used for land restoration in Hong Kong that typically has a high sand 3
content and poor fertility (Jim, 1996; Cheng and Chu, 2011). Gravel was removed with a 2cm sieve, and soils were compacted to a dry density of 1.44 Mg/m3, equivalent to relative compaction of 80% of the CDG used, a level that allows normal root development for sandy soils and is within the field-compaction range for ET cover (Reynolds et al., 2002; McGuire et al., 2009).
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Plant species studied Five native species that are commonly used in revegetation in Hong Kong were investigated: Myrica rubra (Myricaceae), Reevesia thyrsoidea (Sterculiaceae), Schefflera heptaphylla (Araliaceae), Schima superba (Theaceae), and Syzygium hancei (Myrtaceae). A common exotic species was also included: Acacia confusa (Fabaceae), a nitrogen-fixing species widely
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used as a pioneer species for revegetation in Hong Kong. Totally, six tree species were used in this study.
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Measurement of transpiration rate The transpiration potential of the different tree species was determined by a gravimetric method. Seven days were taken as one complete cycle. During the night before the start of each cycle (Day 0), water was added to each polyvinylchloride pot to bring the soil to saturation and excess water was allowed to drain overnight. Bringing the soil to saturation avoided the occurrence of depletion of the transpiration potential caused by low soil water availability (Sinclair et al., 2005). The weight of each PVC pot was determined the next morning (Day 1). During the night on Day 7, the weight was determined again. The
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difference in weight between the start and the end of the cycle was assumed to be the total water loss from the soil-plant system. Since in plants most of the absorbed water is lost through transpiration and in our experiment the change in weight of the soil-plant system was most probably a result of water loss from the plants, the change in weight of the soil-plant system was taken as the transpiration rate of the plant over seven days.
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To minimize soil evaporation, the pots were enclosed with a plastic bag to cover the soil (Singh and Thompson, 1995). Thus, the weight differences were mainly due to transpiration. The experiment started on 9 September 2015 and ended on 11 November 2015; eight cycles of measurement were made during the experimental period for analysis. The data for one cycle (30 September to 6 October) were excluded due to a typhoon. Transpiration at tree-level and leaf-level were analyzed, with the former for determining the size effect of tree on transpiration, while the latter helps decide size-independent characteristics of tree that favor transpiration. Leaf-level transpiration was estimated by treelevel transpiration over total leaf area. 4
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Determination of plant traits potentially associated with tree-level transpiration The total leaf area of each seedling was determined every seven days by counting the number of leaves and multiplying by the average size of ten leaves of each individual for every species. The leaf size was determined with ImageJ by analyzing digital photos of the leaves. The seedlings were harvested at the end of the experiment. The crown spread was determined by taking the average length of the greatest spread of the crown and the length of crown perpendicular to the greatest spread. The dry mass of the different parts of the seedlings was determined after drying at 80°C for 48 hours. Determination of plant traits potentially associated with leaf-level transpiration
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Growth rate and biomass allocation, that is, leaf weight ratio (LWR) and root weight ratio (RWR), specific leaf area (SLA) and leaf area index (LAI), were derived from the primary data. Growth rate was determined by the percentage change in total leaf area at each cycle.
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LAI was calculated by one-sided leaf area over crown projection. Stomatal density was determined by observing the imprints of leaf surfaces under a light microscope at 400×. Each imprint was prepared by applying clear nail polish on the leaf surfaces, avoiding the midveins and leaf margins (Camargo and Marenco, 2011). After drying, the imprint was transferred from the leaf surface to a glass slide with transparent adhesive tape. For each individual, five imprints from five different leaves were examined, and four counts per imprint were made. With the exception of Acacia confusa, stomata were only found in abaxial surfaces for all species. In addition, the photosynthetic capacity of each seedling was
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measured by determining the fluorescence-based maximum photochemical yield of photosystem II (Fv/Fm), a commonly used indicator of photosynthesis (Baker, 2008), using a photosynthesis yield analyzer MINI-PAM (Walz, Germany). For each individual, the Fv/Fm of four fully expanded leaves was determined after dark adaptation (at least 30 minutes after sunset).
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Weather information Weather data were collected from the nearest weather station of the Hong Kong Observatory (www.hko.gov.hk). The temperature and relative humidity were obtained from the Shatin automatic weather station (22°24'09"N 114°12'36"E), which is 1.45 km away, and the solar radiation was obtained from the King’s Park automatic weather station (22°18'43"N 114°10'22"E), which is 12.2 km from our experimental site. Statistics analysis SPSS (v24) was used to carry out all statistical analyses. One-way analysis of variance was carried out to compare the differences in the transpiration rate and plant traits if the data were 5
normally distributed. One-way analysis of covariance was performed to consider the effects
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of covariance on transpiration. Tukey’s honestly significant difference (HSD) test was used as the post-hoc test if variances were assumed to be equal. If equal variances could not be assumed, Dunnett’s T3 was used as the post-hoc test. Mann-Whitney U-tests (p<0.05) were carried out to compare species differences in the transpiration rate and plant traits if the data were not normally distributed. Correlations of different plant traits with the transpiration rate were determined by Pearson correlation analysis. During our experiment, one replicate of S. hancei and one replicate of S. superba died. The measurements of these two individuals were therefore discarded.
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Results Morphological characteristics and ecophysiological performance The values of the plant traits in relation to the sizes of the studied species are shown in Table 1. Great variation was found in the size of the different species. A. confusa was the largest
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species in terms of the crown spread, leaf area, and dry mass of leaves, stems, and roots. Among the native species, M. rubra, S. superba, and S. hancei were the largest in terms of total leaf area and mass, with M. rubra having the greatest size. The LAI of M. myrica and S. hancei were the highest, whereas A. confusa, R. thyrsoidea, and S. heptaphylla had sparser crowns. Table 2 shows the ecophysiological and morphological plant traits of species that Table 1, may affect leaf-level transpiration. Again, great variation in these traits was found among Table 2 species. Only A. confusa, M. rubra, and R. thyrsoidea produced new leaves throughout the experiment, and A. confusa had the highest growth rate in terms of leaf area. For
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morphological traits, A. confusa had the smallest leaf size, and M. myrica, S. superba, and S. heptaphylla had the largest leaves. S. hancei had the lowest SLA, followed by A. confusa, and R. thyrsoidea had a relatively high SLA. A. confusa tended to allocate less biomass to leaves, whereas R. thyrsoidea and S. heptaphylla showed particularly low biomass allocation to roots. M. rubra had the highest stomatal density, followed by S. superba. In general, the values of different traits varied two- to three-fold among species. A. confusa showed the highest Fv/Fm.
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Transpiration performance The tree-level transpiration rate, that is, the transpiration rate of a whole tree seedling, varied remarkably among species (Fig. 1). Exotic A. confusa showed the highest transpiration rate, which was slightly more than double those of the other native species except M. rubra, which was the only species that showed no significant difference with A. confusa in transpiration rate. Of the native species, M. rubra had the highest transpiration rate, approximately double those of the other natives. However, its transpiration rate was only significantly higher than S. heptaphylla and R. thyrsoidea, whereas no significant difference could be found among the 6
Fig. 1, Fig. 2
remaining four native species. Fig. 2 shows the transpiration rates of the various species at the leaf level (i.e., transpiration rate per unit leaf area). Exotic A. confusa and native R. thyrsoidea and S. hancei had higher leaf-level transpiration rates than the other three native species studied. M. rubra had a high tree-level transpiration rate but a comparatively low leaf-level transpiration rate.
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Although the superiority of A. confusa in terms of tree-level transpiration was significant throughout the eight cycles, the difference declined after the first three cycles (Fig. 3). Similarly, the variation in leaf-level transpiration decreased after three cycles (Fig. 4). These differences diminished probably as a result of the lower temperatures seen during the last five cycles (Table 3). Association of tree characteristics with transpiration potential The plant traits that had significant correlations with transpiration at the tree level are
Fig. 3, Fig. 4, Table 3
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presented in Table 4. The tree-level transpiration rate was significantly correlated with the total leaf area, the final dry mass of leaves, stems, and root, and crown spread, but no correlation was found with LAI, which shows that larger plants with a higher total leaf area and dry biomass tend to have high rates of tree-level transpiration. However, even if leaf area was taken into account, analysis of covariance showed that species still had a significant effect on the transpiration rate (F(5.27)=9.947, p<0.01). This finding is in accord with the fact that the leaf-level transpiration rates of the various species differed significantly. The linear relationship between the tree-level transpiration rate and different correlated plant traits
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within the observed range are shown in Fig 5.
Table 4, Fig. 5
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The plant traits that had significant correlations with transpiration at the leaf level are presented in Table 5. The leaf-level transpiration rate had a moderate positive correlation with the growth rate and a moderate negative correlation with leaf size. The linear relationship between the leaf-level transpiration rate and these two plant traits within the range studied are shown in Fig 6. Table 5, Figure 6
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Discussion Key findings Six studied species varied in their transpiration potential because of the differences in plant traits. Crown spread, leaf area, and dry mass of leaves, stems and roots were all positively related to tree-level transpiration rate, while growth rate and leaf size are positively and negatively related to leaf-level transpiration rate respectively; hence, their use in predicting transpiration rate. 7
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Plant traits favoring high transpiration rate At the tree level, all size-related traits measured in our experiment can be used to predict the transpiration rate. It was suggested that the tree-level transpiration rate was size-dependent rather than species-dependent for mature trees of five species in a tropical forest (Goldstein et al., 1998). However, even if leaf area was taken into account, analysis of covariance showed that species still had a significant effect on the transpiration rate (F(5.27)=9.947, p<0.01) in our experiment. This finding is in accord with the fact that the various species in our experiment showed significant differences in the leaf-level transpiration rate. Therefore, plant size is not the only important determining factor of transpiration, and the effect of species should not be neglected, at least for tree seedlings. Our results show that leaf size and growth
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rate were the three traits most closely linked to a high transpiration rate at the leaf level.
Leaf size can predict the leaf-level transpiration rate in tropical secondary forests (Juhrbandt
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et al., 2004). Smaller leaves are favorable to transpiration because they lead to lower boundary layer resistance for water loss through transpiration (Yates et al., 2010) because the smaller the leaf size, the smaller the surface area relative to the perimeter length, which thus allows greater heat and moisture transfer. However, the greater degree of cooling in smaller leaves caused by greater heat transfer may contrarily reduce transpiration rate, because of the reduced leaf temperature. Therefore, further experiments may be required to confirm if the relationship between leaf size and transpiration holds in the prevailing environmental conditions.
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In contrast, some traits that were suggested to be possible indicators of a high transpiration rate, including RWR or LWR (Poorter and Farguhar, 1994), SLA (Poorter and Bongers, 2006), and stomatal density (Juhrbandt et al., 2004), did not show any significant relationship with the transpiration rate in our experiment. Two qualitative traits that were proposed to promote the transpiration rate, the presence of a toothed margin on leaves (Wilf, 1997) and shade intolerance (Siegert and Levia, 2011), also failed to predict the transpiration rate in our experiment. Species that have leaves with a toothed margin (M. rubra, S. heptaphylla, and S. superba) and species that are shade tolerant (S. heptaphylla and S. hancei) did not show lower leaf-level transpiration rates. One possible reason is that the effects of other traits masked the effects of these traits. For example, the species in our experiment that have leaves with a toothed margin also had a comparatively large leaf size. Suitable indicators for the selection of species for ET cover Great variation in the transpiration rates of seedlings reflects the importance of selecting suitable species to enhance the effectiveness of ET covers. However, it can be seen that 8
simple generalization is difficult because one favorable trait for a species’ transpiration rate
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may be counteracted by another trait that is unfavorable to transpiration. Although leaf size and growth rate were found to determine leaf-level transpiration, the use of leaf size as a selection criterion is less appropriate than growth rate. If the growth rate within a tree species varies greatly, the contribution of leaf-level transpiration to tree-level transpiration is very likely masked by variations in tree size during development. Therefore, although a species may produce an effect on transpiration at the leaf level because of its morphological traits, it is not a reliable criterion for selection without considering its physiology. Growth rate is better selection criterion because it hints at both the photosynthesis and transpiration rates of the species. Moreover, it implies that seedlings can achieve a larger size
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in a shorter time and hence a higher overall transpiration rate. Although there is no directional relationship of Fv/Fm with transpiration rate, Fv/Fm was highly correlated with growth rate (Jeannine and Fakhri, 2004). Indeed, a moderately positive correlation between growth rate
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and Fv/Fm was found in our experiment (r=0.0359, p<0.05). Particularly, Fv/Fm reflects the adaptability of plants to a given environment, including drought tolerance (Percival et al., 2006) and compaction tolerance (Philip and Azlin, 2005), which are important for successful growth on restored landfills. Therefore, Fv/Fm is a possible reference for selection from this point of view. Local field survey of the growth conditions of various species on restored land, by measuring the Fv/Fm of species or growth measurement in terms of size, is therefore important to screen for tree species for their growth rate and transpiration potential. The use of morphological traits alone to estimate the transpiration potential of trees is obviously less
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desirable. In other words, we should not only consider the transpiration rates of two species when they are of similar size, but also their long-term growth performance, to get a better picture of their transpiration potential.
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Inclusion of exotic species Our results suggest that the inclusion of suitable exotic species in landfill restoration was desirable. A successful revegetation plan requires the selection of suitable species according to the local climate and site conditions and to the specific aims of reforestation (Wishnie et al., 2007). One of the objectives of planting of an ET cover is to maximize transpiration to remove soil water between rainfall events, which is normally not taken into consideration for typical restoration programs. A. confusa, a heavily planted exotic, showed superior performance in terms of transpiration potential in our experiment. In Hong Kong, Acacia spp. have been observed to grow well on many landfills and quarries compared with other native species because of their nitrogen fixing ability (Jim, 2001; Wong et al., 2016). Although fertilizers are routinely applied to the topsoil of landfill cover for vegetation establishment in Hong Kong, the effect of fertilizers is limited over the long term. Therefore, nutrient 9
deficiency is a common limitation to the growth of native tree species on landfill covers
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(Wong et al., 2016). From a functional point of view, they are suitable species because of their high transpiration potential, as evidenced in this study and predicted by their fast growth rate. From an ecological point of view, they have the potential to promote colonization of native species on local degraded land by providing shelter and increasing the soil nitrogen content (Zhang et al., 2013; Wong et al., 2016). It has been proposed that the use of pioneer species that enhance plant diversity will lead to lower soil water content and higher slope stability over the long term because of the resultant high biomass and extensive root systems (Osman and Barakbah, 2011). The use of exotic species may therefore not necessarily be an evil because of their possible ecological advantages (D’Antonio and Meyerson, 2002). Yet, exotic species that are invasive to local region should be avoided for planting. Use of larger seedlings In our experiment, Acacia confusa showed the highest transpiration rate in first cycle with
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93.7g/day, which is equivalent to 0.826 mm/day. This value was much lower than the average daily rainfall in Hong Kong in wet season, which is around 12 mm/day. This implies that saplings of larger size should be employed to remove soil water and achieve soil drying.
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Limitations We studied only the effects of species on soil water removal in terms of transpiration. However, soil water content is also influenced by many other interspecific differences. For example, crown traits may affect interception (Park and Cameron, 2008), which reduces the
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amount of rainfall that reaches the soil. In addition, the effects of soil evaporation were not considered. The magnitude of water loss from evaporation of vegetated soil is affected by the light interception by the tree seedlings, which depends partially on the crown characteristics. More species should be included in further studies to confirm plant traits as good predictors of transpiration. Moreover, because of the limited experimental period, the influence of environmental conditions on the transpiration potential of species could not be fully evaluated. Another issue is that we used tree seedlings in our experiment, so the results are not directly applicable to adult trees. Besides, interspecific traits between laboratory-grown seedlings and field-grown adult trees can be disparate (Cornelissen et al., 2003). Despite these defects of measurement in the use of seedlings, from the plantation point of view, planting tree seedlings is a more economical and practical choice. Therefore, the performance of tree seedlings is worthwhile to study to ensure the effectiveness of cover during the early stages of plantation. Conclusions The study assessed the transpiration rates of six species of tree seedlings and determined 10
suitable indicators for the selection of species for ET cover. Exotic A. confusa had the highest
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transpiration rate of all the species at the tree and leaf levels. Growth rate provides good implications of a plant’s size development and leaf transpiration rate; it is good criterion for species selection, from the perspective of maximizing transpiration, to achieve the target of reducing percolation. Meanwhile, Fv/Fm gives information on the adaptability of the species to the often-harsh environment of restored landfills. Leaf size is an additional parameter to be considered, but is comparatively less significant. Acacia spp. that give both functional and ecological advantages can be considered as pioneer species on ET covers, the primary goal of which is to reduce percolation, in our study region. This provides implications for the recruitment of suitable exotic species that can maximize the performance of ET cover to reduce percolation and restore habitats in urban areas. Acknowledgements The work was supported by the Research Grants Council (RGC) of the Hong Kong Special
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Administrative Region [grant number HKUST6/CRF/12R]. We would like to thank Mr Ben Yeung for technical assistance and the Department of Geography and Resource Management, The Chinese University of Hong Kong, for lending us the venue for our experiment.
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Engineering 37, 139-147. Owusu, G., Oteng-Ababio, M., Afutu-Kotey, R.L., 2012. Conflicts and governance of landfills in a developing country city, Accra. Landscape and Urban Planning 104, 105-113. Park, A. and Cameron, J.L., 2008. The influence of canopy traits on throughfall and stemflow in five tropical trees growing in a Panamanian plantation. Forest Ecology and Management 255, 1915-1925. Percival, G.C., Keary, I.P., Sulaiman, A.H., 2006. An assessment of the drought tolerance of Fraxinus genotypes for urban landscape plantings. Urban Forestry and Urban Greening 5, 17-
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27. Philip, E., Azlin, Y.N., 2005. Measurement of soil compaction tolerance of Lagestromia speciosa (L.) Pers. using chlorophyll fluorescence. Urban Forestry and Urban Greening 3, 203-208. Poorter, H., Farquhar, G.D., 1994. Transpiration, intracellular carbon dioxide concentration and carbon-isotope discrimination of 24 wild species differing in relative growth rate. Functional Plant Biology 21, 507-516. Poorter, L., Bongers, F. 2006. Leaf traits are good predictors of plant performance across 53 rain forest species. Ecology 87, 1733-1743. Rahman, M.L., Tarrant, S., McCollin, D., Ollerton, J., 2011. The conservation value of restored landfill sites in the East Midlands, UK for supporting bird communities. Biodiversity and Conservation 20, 1879-1893. Reynolds, W.D., Bowman, B.T., Drury, C.F., Tan, C.S. and Lu, X., 2002. Indicators of good soil physical quality: density and storage parameters. Geoderma 110, 131-146. Siegert, C.M., Levia, D.F., 2011. Stomatal conductance and transpiration of co-occurring seedlings with varying shade tolerance. Trees 25, 1091-1102. 13
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from municipal landfills? International Journal of Environmental Technology and Management 10, 4-15. Venkatraman, K., Ashwath, N., 2009. Phytocapping: importance of tree selection and soil
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thickness. Water, Air, and Soil Pollution: Focus 9, 421-430. Wang, R., Zhao, J. Liu, Z., 2016. Consensus in visual preferences: the effects of aesthetic quality and landscape types. Urban Forestry and Urban Greening 20, 210-217. Wilf, P., 1997. When are leaves good thermometers? A new case for leaf margin analysis. Paleobiology 23, 373-390. Wishnie, M.H., Dent, D.H., Mariscal, E., Deago, J., Cedeno, N., Ibarra, D. Condit, R. Ashton, P.M.S., 2007. Initial performance and reforestation potential of 24 tropical tree species planted across a precipitation gradient in the Republic of Panama. Forest Ecology and
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Management 243, 39-49. Wong, J.T.F., Chen, X.W., Mo, W.Y., Man, Y.B., Ng, C.W.W., Wong, M.H., 2016. Restoration of plant and animal communities in a sanitary landfill: a 10‐year case study in Hong Kong.
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Land Degradation and Development 27, 490-499. Yates, M.J., Anthony Verboom, G., Rebelo, A.G., Cramer, M.D., 2010. Ecophysiological significance of leaf size variation in Proteaceae from the Cape Floristic Region. Functional Ecology 24, 485-492. Zhang, H., Zhuang, X., Chu, L.M., 2013. Plant recruitment in early development stages on rehabilitated quarries in Hong Kong. Restoration Ecology 21, 166-173. Zhou, C., Gong, Z., Hu, J., Cao, A., Liang, H., 2015. A cost-benefit analysis of landfill mining and material recycling in China. Waste Management 35, 191-198. Zurbrugg, C., 2003. Urban solid waste management in low-income countries of Asia: how to cope with the garbage crisis. In Scientific Committee on Problems of the Environment (SCOPE) Urban Solid Waste Management Review Session, pp. 1-13. Durban, South Africa: Sandec Publications.
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Fig. 1.Mean tree-level transpiration rate (g/day) of the six studied species over eight cycles. Different letters denote significant differences among species (p<0.05) by Dunnett’s T3 test.
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Fig. 2.Mean leaf-level transpiration rate (g/day/cm2) of six studied species over eight cycles. Different letters denote significant differences among species (p<0.05) by Tukey’s HSD test.
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Fig. 3.Daily tree-level transpiration rate (g/day) of the six studied species over eight cycles. Different letters denote significant differences among species within the same cycle (p<0.05) by Dunnett’s T3 test.
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Fig. 4.Daily leaf-level transpiration rate (g/day/cm2) of the six studied species over eight cycles. Different letters denote significant differences among species within the same cycle (p<0.05) by Dunnett’s T3 test (Cycle 1) or Tukey’s HSD test (Cycles 2 to 8).
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Fig. 5 Linear regression between tree-level transpiration rate and a) crown spread, b) leaf dry mass, c) root dry mass, d) stem dry mass and, e) total leaf area. **p<0.01.
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Fig. 6 Linear regression between leaf-level transpiration rate and a) growth rate and, b) leaf size. *p<0.05, **p<0.01.
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Table 1. Size-related plant traits of the six tree species studied: Acacia confusa (Acc), Myrica rubra (Myr), Reevesia thyrsoidea (Ret), Schefflera heptaphylla (Sch), Schima superba (Scs), Syzygium hancei (Syh). Species
Crown spread* Total leaf area# Leaf dry mass^ Stem dry mass# Root dry mass# LAI*
(cm2)
(g)
(g)
(g)
Acc
41.3±3.6a
757±167a
9.92±2.58a
10.5±2.5a
7.89±0.99a
0.578±0.185bc
Myr
23.0±6.3bc
568±215a
5.03±2.21ab
6.32±1.73bc
7.51±1.76bc
1.66±1.38a
Ret
21.0±4.7bc
162±50b
1.22±0.43b
2.82±0.66c
4.22±0.75c
0.505±0.219bc
Sch
32.2±7.1bc
266±94b
2.40±0.89b
3.48±0.79c
5.97±0.59c
0.361±0.206c
Scs
23.3±0.4b
319±66b
2.84±0.80ab
5.07±1.57bc
6.82±1.00bc
0.746±0.140b
Syh
16.1±2.4bc
239±58b
3.49±0.83ab
6.28±1.36bc
1.10±0.164a
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(cm)
5.38±1.33bc
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Different letters denote significant differences among species (p<0.05) by #Tukey’s HSD test, ^Dunnett’s T3 test or *Mann-Whitney U test.
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Table 2.Plant ecophysiological and morphological traits of the six species studied. Species Leaf size#
(cm2)
Growth rate* Fv/Fm*
SLA*
(%)
(cm2/g)
LWR*
RWR^
Stomatal
density#
(no./mm2)
6.12±0.75d
39.9±23.7a
0.812±0.009a
77.5±9.5b 0.267±0.063a
0.317±0.031a 331±38bc
Myr
17.2±3.6a
9.04±35.29ab 0.777±0.025b
116±17cd 0.260±0.105a
0.418±0.066bc 637±71a
Ret
11.0±1.6bc
34.8±25.3ab 0.737±0.024bc
13512d
0.0887±0.0338b 0.553±0.044d 252±55c
Sch
15.2±3.0ab
-7.2±17.8b
0.662±0.080d
111±12c
0.137±0.035b
Scs
15.5±2.7a
-3.52±4.96b
0.771±0.016bc
114±9c
Syh
8.31±1.53cd 0.00±0.00b
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0.174±0.012ab
0.723±0.034bcd 68.4±2.2a 0.221±0.005ab
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Acc
0.562±0.057d 260±54c
0.489±0.036cd 406±28b
0.428±0.025b 335±9bc
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Different letters denote significant differences among species (p<0.05) by #Tukey’s HSD test, ^Dunnett’s T3 test or *Mann-Whitney U test.
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Table 3. Mean temperature, relative humidity, and global solar radiation of eight measurement cycles. Temperature (oC)
Relative humidity (%)
Global solar radiation (MJ/m2)
1
27.6±0.4
70.9±3.3
21.7±1.8
2
27.7±0.5
78.7±3.1
16.1±5.1
3
29.0±0.6
72.3±12.0
18.0±5.9
4
24.4±2.5
76.1±8.2
11.5±7.4
5
24.2±0.4
72.9±4.2
18.1±2.1
6
25.8±0.5
74.3±3.7
13.7±4.0
7
24.3±2.1
74.4±6.1
8
25.5±1.0
79.1±2.3
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14.9±2.6
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11.8±4.2
Table 4. Plant traits that had a significant correlation with tree-level transpiration rate across
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Crown spread
0.624**
Leaf dry mass
0.953**
Root dry mass
0.693**
Stem dry mass
0.874**
Total leaf area
0.923**
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species and their correlation coefficient (r).
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**p < 0.01.
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Table 5. Plant traits that had a significant correlation with leaf-level transpiration rate across species and their correlation coefficient (r). r
Leaf size
-0.610**
Growth rate
0.351*
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*p< 0.05, **p < 0.01.
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