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Stemflow volume per unit rainfall as a good variable to determine the relationship between stemflow amount and morphological metrics of shrubs
Journal of Arid Environments 141 (2017) 1e6
Contents lists available at ScienceDirect
Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv
Stemflow volume per unit rainfall as a good variable to determine the relationship between stemflow amount and morphological metrics of shrubs Ya-feng Zhang*, Xin-ping Wang, Rui Hu, Yan-xia Pan Shapotou Desert Research and Experiment Station, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, 320 Donggang West Road, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 November 2014 Received in revised form 6 February 2017 Accepted 8 February 2017
In this study, we determined the relationship between stemflow amount and morphological metrics of plants. Previous studies on this issue generally neglected the influence of differential rainfall amounts on stemflow, which somehow bias the relationship between stemflow amount and morphological metrics. Here, field observation of stemflow in two xerophytic shrubs (Caragana korshinskii and Artemisia ordosica) of varying sizes were conducted during three growing seasons in 2011e2013 to evaluate this relationship by using a simple variable, i.e., stemflow volume per unit rainfall (SfVPR, mL mm1). This stemflow index directly defines the influence of rainfall depth on the morphological metrics of plants, and it has the following advantages over the conventional stemflow coefficient that has been used to determine the influence of tree/shrub morphological metrics on the stemflow amount: (1) it avoids the bias caused by the influences of differential rainfall amount on stemflow yield and (2) it is suitable for use in hydrological models. Our results showed that SfVPR can well be used to determine the relationship between stemflow amount and shrub morphological metrics. Moreover, by using multiple regression model, we found that projected canopy area (or canopy volume, or basal area), plant area index, and stem diameter are the most influential factors for the stemflow amount of C. korshinskii, whereas no significant explanatory variables were found for that of A. ordosica. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Stemflow Morphological metrics C. korshinskii A. ordosica Simple regression Multiple linear regression
1. Introduction Stemflow refers to a part of rainfall that is intercepted by leaves, twigs, and branches and eventually channeled into soil through trunk or stem, which further could be transported and redistributed into deeper soil layers through preferential pathways such as roots. Although volumetrically minor, stemflow is an important source of soil moisture and nutrients for plant growth, this being particular important in arid and semiarid ecosystems (Aboal et al., 1999; Navar and Bryan, 1990; Navar et al., 2009; Navar, 2011; Whitford et al., 1997; Zhang et al., 2016). The hydrological and biogeochemical importance of stemflow was systematically reviewed by Levia and Frost (2003). Stemflow amount is a function of canopy structure, rainfall characteristics, and meteorological variables (Crockford and
* Corresponding author. E-mail address: [email protected] (Y.-f. Zhang). http://dx.doi.org/10.1016/j.jaridenv.2017.02.002 0140-1963/© 2017 Elsevier Ltd. All rights reserved.
Richardson, 1990; Navar, 1993; Levia and Frost, 2003). The influence of individual canopy structure metrics on stemflow amount is difficult to determine (Germer et al., 2010; Levia et al., 2013). Simple regressions were first performed between a conventionally used stemflow coefficient (the total volume of stemflow during the observation periods) and the morphological metrics of plants, and then a multiple linear regression model was developed after eliminating colinearity between these metrics (e.g., Aboal et al., 1999; Martinez-Meza and Whitford, 1996; Wang et al., 2013; Yang et al., 2008). However, by using the accumulated stemflow volumes during the observational periods, the previous authors neglected the influence of differential rainfall amount on the stemflow amount that obscures the morphology effects because it is well known that stemflow significantly increases with the rainfall amount after a threshold value for stemflow generation (e.g., Li et al., 2008; Manfroi et al., 2004; Navar, 2011; Zhang et al., 2013, 2015). As such, the contribution of rainfalls with high depths was probably overestimated, whereas that of rainfall with low depths was underestimated or under-represented. In terms of accurate
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predictions, the results are not so desirable without fully considering the influence of differential rainfall depths. This relationship, based on volumetrically expressed unstandardized stemflow, is also unsuitable for use in ecohydrological or hydropedological models (Levia and Germer, 2015). In this study, we used a simple variable, stemflow volume per unit rainfall (SfVPR, mL mm1), to evaluate the relationship between stemflow amount and shrub morphological metrics. This standardized variable demonstrates the amount of rainfall that a tree channels to the ground as SfVPR, which has been used by Honda et al. (2015) and Manfroi et al. (2004), among others. However, no attempts have been made to use SfVPR to determine the relationship between stemflow amount and shrub morphological metrics, although SfVPR allows the comparison of stemflow among trees in localities with variable rainfall, as Honda et al. (2015) suggested. Therefore, it is expected that SfVPR can reasonably and accurately determine the relationships between stemflow amount and inter- and intraindividual morphological metrics of shrubs. 2. Materials and methods 2.1. Site information Field measurements were conducted during three growing seasons in 2011e2013 at the Shapotou Desert Research and Experiment Station (SDRES) of Chinese Academy of Sciences (37 320 N, 105 020 E, at an elevation of 1300 m a.s.l.), southeastern fringe of the Tengger Desert in northwestern China. The mean annual rainfall in this area is only 191 mm (1955e2005, SDRES). Detailed information on other rainfall and meteorological characteristics and soil properties are given by Zhang et al. (2015). Extensive revegetation efforts had been undertaken during the 1950s to the 1980s to protect the Baotou-Lanzhou railway from encroaching sand dunes in the Shapotou area. A 16,000-m-long artificial protection system had been established along the railway that is 500 m wide to the north and 200 m wide to the south. Straw checkerboards were set up on the mobile sand dunes on both sides of the railway, and subsequently, xerophytic shrubs (mainly Caragana korshinskii, Hedysarum scoparium, and Artemisia ordosica) were planted within these checkerboards. A detailed description of the revegetation procedure is given by Li et al. (2006). The 1-ha Water Balance Experimental Field (WBEF) is one of the revegetated enclosures studied here. The WBEF was established in 1989 by planting two morphologically different shrubs, i.e.,
C. korshinskii and A. ordosica (Fig. 1). C. korshinskii is a multistemmed deciduous perennial leguminous shrub and resembles an inverted cone. Stems have smooth bark. Leaves are 6e10 cm long, pinnately compound, and opposite or subopposite in arrangement. Each pinna has 5e8 pairs of ovate leaflets (7e8 mm in length and 2e5 mm in width). A. ordosica is a highly branched dwarf shrub with plumose, full split needled leaves (10e30 mm in length and 0.3e1 mm in width). It has only one rough stem with thick, loose, and inclinedly fractured bark.
2.2. Shrubs selection and measurements Seventeen robust and healthy shrubs (10 C. korshinskii and 7 A. ordosica) were selected for field observation as a representation of the range of sizes of the two xerophytic shrub species in WBEF (Tables 1 and 2). Stem diameter was measured at each stem's base using a vernier caliper. Stem angle was determined using a protractor. Plant area index (PAI) of individual shrubs was estimated using a LAI-2000 plant canopy analyzer (Li-Cor., Inc., USA) with a 30 view cap on the ground by the stem, assuming shrub canopy shapes such as a hemisphere. PAI measurements were obtained under uniform sky conditions around sunset to avoid the interference of direct sunlight (Leblanc and Chen, 2001). Shrub canopy height was measured at the center of the canopy. Canopy area (approximated as an ellipse) was calculated by taking the eastewest and northesouth diameters through the center of the fullest part of the canopy (Martinez-Meza and Whitford, 1996). Shrub canopy was approximated as an inverted elliptic cone, and canopy volume was thus determined from canopy height and canopy area.
2.3. Stemflow and rainfall measurements Stemflow was collected using collars constructed from aluminum foil plates that were fitted around the circumference of the shrub stem (Fig. 1). The volume of stemflow was measured by a graduated cylinder for each individual stem (A. ordosica has only one stem) after the cessation of each rainfall event. SfVPR was calculated as the stemflow volume collected from each individual stem divided by the rainfall amount in that period. A standard tipping bucket rain gauge (Adolf Thies GMVH & Co. KG, Germany) with a resolution of 0.1 mm and a mini logger recording 10-min rainfall intensity values were installed in an open area approximately 50 m from the study plot.
Fig. 1. Photographs showing the morphological features of C. korshinskii (a) and A. ordosica (b) and the corresponding method of collecting stemflow.
Y.-f. Zhang et al. / Journal of Arid Environments 141 (2017) 1e6
3
Table 1 Descriptive statistics (mean ± SE) of canopy morphology and plant area index (PAI) of C. korshinskii selected in the experiments. No.
Stem
1 2 3 4 5 6 7 8 9 10 Mean SE
Canopy
Number
Diameter (cm)
7 6 9 12 11 2 5 1 1 3 6 1
1.9 2.5 2.5 1.8 1.7 2.2 1.4 1.8 1.8 1.5 1.9 0.1
± ± ± ± ± ± ± ± ± ±
0.5 0.2 0.4 0.1 0.2 0.0 0.2 NA NA 0.2
PAI
Length (cm)
Angle ( )
Height (cm)
Area (m2)
Volume (m3)
119 ± 14.3 159 ± 18.3 151 ± 12.3 103 ± 9.1 83 ± 2.6 94 ± 5.5 50 ± 5.9 57 ± NA 98 ± NA 77 ± 1.7 107 5.7
72 ± 73 ± 60 ± 60 ± 56 ± 77 ± 59 ± 72 ± 50 ± 60 ± 62 2.1
216 245 203 182 158 115 92 66 114 126 152 18
2.23 3.69 5.19 2.07 2.69 0.59 0.66 0.11 0.39 0.60 1.82 0.53
1.61 3.02 3.51 1.26 1.42 0.23 0.20 0.02 0.15 0.25 1.26 0.40
5 3 8 4 3 5 3 NA NA 19
0.78 0.71 1.02 1.01 0.68 0.77 0.95 0.46 0.69 0.54 0.76 0.06
NA: not available.
Table 2 Descriptive statistics (mean ± SE) of canopy morphology and plant area index (PAI) of A. ordosica selected in the experiments. No.
Stem
1 2 3 4 5 6 7 Mean SE
Canopy
PAI
Diameter (cm)
Length (cm)
Angle ( )
Height (cm)
Area (m2)
Volume (m3)
2.81 3.16 1.58 3.08 3.65 5.45 3.74 3.35 0.44
7 10 9 14 8 15 16 11 1.4
90 78 88 68 78 89 82 82 3.0
74 85 50 69 95 75 80 75 5.5
0.52 0.75 0.17 0.51 0.78 1.02 1.59 0.76 0.17
0.13 0.21 0.03 0.12 0.25 0.26 0.42 0.20 0.05
1.40 1.79 1.65 1.72 1.69 2.00 1.64 1.70 0.07
2.4. Statistical analyses Descriptive statistics were compiled for shrub morphological metrics. Simple regression equations were developed between stemflow volume and rainfall depth and between SfVPR and shrub morphological metrics. In addition, we used multiple linear regression to determine the relationship between SfVPR and shrub morphological metrics. All the descriptive statistics and regressions were performed using the SPSS 16.0 statistical software (SPSS Inc., Chicago, USA). 3. Results and discussion 3.1. Rainfall characteristics and stemflow volume
Stemflow volume (L)
7 (a) C. korshinskii
6 5 4 3 2 1
y=-0.09+0.165x, R2=0.80, P<0.01
0
Stemflow volume (L)
0.7 (b) A. ordosica
0.6 0.5 0.4 0.3 0.2 0.1 0.0
y=0.02+0.014x, R2=0.50, P<0.01
0
5
10
15 20 Rainfall (mm)
25
30
35
Fig. 2. Stemflow volume as a function of rainfall depth. Bars represent 95% confidence level. n ¼ 10 for C. korshinskii and n ¼ 7 for A. ordosica.
During our experimental periods in the three years of 2011 (from 26 June to 7 November), 2012 (from 11 April to 25 September), and 2013 (from 15 May to 31 November), we observed that a rainfall of <2.4 mm was negligible for generating stemflow. Stemflow was measurable for 37 rainfall events, with a total of 412.5 mm, accounting for 89.4% of the total incident rainfall amount and 36.3% of the total number of storms. The 37 rainfall events ranged from 2.4 to 28.8 mm, with a mean of 11.2 mm. The rainfall intensity ranged from 0.37 to 21.7 mm h1, with a mean of 2.9 mm h1. Thirty-five out of the 37 rainfall events had rainfall intensities of <6 mm h1. Two storms had intensities of 21.7 and 21.2 mm h1, which can be considered as extreme rainfall events in our study area. Stemflow volume significantly increased with rainfall amount for both shrubs species (Fig. 2). The total stemflow volume collected from the 10 shrubs of C. korshinskii amounted to 638.7 L during the experimental periods, with a mean of 63.87 L for each shrub and a coefficient of variation of 86.3%. The total stemflow volume amounted to 43.07 L for the 7 shrubs of A. ordosica, with a mean of 6.15 L for each shrub and a coefficient of variation of 45.5%. These volumes were very small compared to those measured from tree species in forested ecosystems with high rainfall (Aboal et al., 1999; Germer et al., 2010; Herwitz, 1986). The volumes were, however, similar to the stemflow volumes measured in several arid and semiarid shrubs (Navar and Bryan, 1990; Navar et al., 1999a,b; Navar, 2011). An average of 4.5 and 3.1 L of rainwater would have been collected in the open in the same area as projected canopy area of C. korshinskii and A. ordosica, respectively. The values would be 0.78 and 0.28 L in the open in the same area as stem basal area of C. korshinskii and A. ordosica, respectively. Moreover, it should be noted that a weak linear relationship can be found between SfVPR
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and rainfall, with R2 (the coefficient of determination) < 0.3 for C. korshinskii and <0.1 for A. ordosica. 3.2. Simple regression C. korshinskii is a multi-stemmed shrub; it thus has inter- and intraindividual differences in morphological metrics. The interindividual ones include the projected canopy area, basal area, shrub height, PAI, and shrub volume. The intraindividual ones include stem angle, stem diameter, branch length, and stem height. A. ordosica has only a single stem; therefore, no intraindividual morphological variables were considered in the current study. As can be seen in Fig. 3, the SfVPR of C. korshinskii was significantly positively correlated to the projected canopy area
(P < 0.001), basal area (P < 0.001), shrub height (P ¼ 0.013), shrub volume (P < 0.001), number of stems (P ¼ 0.02), stem diameter (P < 0.001), branch length (P < 0.001), but was negatively correlated to stem height (P ¼ 0.012), respectively; it showed increasing tendency with increase in PAI, although not statistically significant (P ¼ 0.057); it showed increasing tendency and attained the highest value with stem angle until 65 and then showed decreasing tendency. As shown in Fig. 4, the SfVPR of A. ordosica showed increasing tendency with increase in the projected canopy area, basal area, shrub height, PAI, shrub volume, stem diameter, and stem height, although not statistically significant (P > 0.05). Moreover, the relationship between the SfVPR of A. ordosica and stem angle was a downward-facing parabolic curve, with the highest value being
Fig. 3. Regression equations between stemflow volume per unit rainfall (SfVPR) and morphological metrics of C. korshinskii. Bars represent 95% confidence level.
Y.-f. Zhang et al. / Journal of Arid Environments 141 (2017) 1e6
5
Fig. 4. Regression equations between stemflow volume per unit rainfall (SfVPR) and morphological metrics of A. ordosica. Bars represent 95% confidence level.
attained at 80 . In our study, we used SfVPR (instead of the conventionally used variable, the total stemflow volume) to evaluate the relationships between stemflow amount and shrub morphological metrics. This variable has been reported by Honda et al. (2015) and Manfroi et al. (2004); no attempts, however, have been made to use it to determine the relationship between stemflow amount and shrub morphological metrics. In fact, our results showed that SfVPR can be used to determine the relationship between stemflow amount and the inter- and intraindividual morphological metrics of two shrubs (Figs. 3 and 4). Although stemflow is an important metric for calculating hydrologic inputs and overall budgets (e.g., Siegert and Levia, 2014), the conventionally used method that is based on the total stemflow volume (e.g., Aboal et al., 1999; Martinez-Meza and Whitford, 1996) has two main shortcomings: (1) it overestimates the contribution of rainfalls with high depths but underestimates that of rainfalls with low depths and (2) the volumetrically expressed unstandardized stemflow is unsuitable for use in ecohydrological or hydropedological models (Levia and Germer, 2015). In contrast, SfVPR takes into account the influences of differential rainfall depth on stemflow amount, is a suitable index for determining the influence of the tree size on stemflow volume (Honda et al., 2015; Levia and Germer, 2015), and can easily be used in hydrologic models. Therefore, SfVPR is expected to be widely used to determine the relationships between the stemflow amount and tree/shrub morphological metrics in other ecosystems.
3.3. Multiple linear regressions To explain the causes of stemflow variation among shrubs, stepwise regression analysis was used to determine the effects of morphological metrics on SfVPR. Multicollinearity existed between the projected canopy area, canopy volume, and basal area, which first should be eliminated. When canopy volume and basal area were eliminated in the multiple linear regression model, we found projected canopy area (PCA) and PAI to be the two statistically significant explanatory variables (interindividual morphological variables) for stemflow amount (SfVPR ¼ 72.2 þ 69.9PCAþ124.2PAI, R2 ¼ 0.96, P < 0.01). Regarding the intraindividual morphological variables (stem angle, stem diameter, branch length, and stem length) of C. korshinskii, we found the stem diameter (SD) to be a statistically significant explanatory variable of stemflow amount (SfVPR ¼ 12.3 þ 20.1SD, R2 ¼ 0.57, P ¼ 0.016). Therefore, SfVPR is most influenced by the projected canopy area (or canopy volume, or basal area), PAI, and stem diameter for C. korshinskii, the first two being interindividual morphological variables and the latter being an intraindividual morphological variable. Likewise, Martinez-Meza and Whitford (1996), Aboal et al. (1999), and Yang et al. (2008) found relationships between stemflow volumes and canopy area. For A. ordosica, no individual morphological metrics, however, were found to be statistically significant explanatory variables for
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stemflow amount from stepwise regression analysis. We assume other vegetative traits such as leaf shape and bark microrelief may exert more influences on its stemflow amount. Acknowledgements This study was supported by the National Natural Science Foundation of China (41530750, 41501108, and 41371101) and the CAS “Light of West China” Program. We appreciate the editors and two anonymous reviewers for their constructive comments. References Aboal, J.R., Morales, D., Hernandez, M., Jimenez, M.S., 1999. The measurement and modelling of the variation of stemflow in a laurel forest in Tenerife, Canary Islands. J. Hydrol. 221, 161e175. Crockford, R.H., Richardson, D.P., 1990. Partitioning of rainfall in a eucalypt forest and pine plantation in Southeastern Australia .2. Stemflow and factors affecting stemflow in a dry sclerophyll eucalypt forest and a pinus-radiata plantation. Hydrol. Process 4, 145e155. Germer, S., Werther, L., Elsenbeer, H., 2010. Have we underestimated stemflow? Lessons from an open tropical rainforest. J. Hydrol. 395, 169e179. Herwitz, S.R., 1986. Infiltration-excess caused by stemflow in a cyclone-prone tropical rain-forest. Earth Surf. Process. 11, 401e412. Honda, E.A., Mendonca, A.H., Durigan, G., 2015. Factors affecting the stemflow of trees in the Brazilian Cerrado. Ecohydrology 8 (7), 1351e1362. Leblanc, S.G., Chen, J.M., 2001. A practical scheme for correcting multiple scattering effects on optical LAI measurements. Agr. For. Meteorol. 110 (2), 125e139. Levia, D.F., Frost, E.E., 2003. A review and evaluation of stemflow literature in the hydrologic and biogeochemical cycles of forested and agricultural ecosystems. J. Hydrol. 274, 1e29. Levia, D.F., Germer, S., 2015. A review of stemflow generation dynamics and stemflow-environment interactions in forests and shrublands. Rev. Geophys 53. http://dx.doi.org/10.1002/2015RG000479. Levia, D.F., Michalzik, B., N€ athe, K., Bischoff, S., Richter, S., Legates, D., 2013. Differential stemflow yield from European beech saplings: the role of individual canopy structure metrics. Hydrol. Process. http://dx.doi.org/10.1002/hyp.10124. Li, X., Xiao, H., He, M., Zhang, J., 2006. Sand barriers of straw checkerboards for habitat restoration in extremely arid desert regions. Ecol. Eng. 28, 149e157. Li, X.Y., Liu, L.Y., Gao, S.Y., Ma, Y.J., Yang, Z.P., 2008. Stemflow in three shrubs and its effect on soil water enhancement in semiarid loess region of China. Agr. For. Meteorol. 148, 1501e1507.
Manfroi, O.J., Koichiro, K., Nobuaki, T., Masakazu, S., Nakagawa, M., Nakashizuka, T., Chong, L., 2004. The stemflow of trees in a Bornean lowland tropical forest. Hydrol. Process 18, 2455e2474. Martinez-Meza, E., Whitford, W.G., 1996. Stemflow, throughfall and channelization of stemflow by roots in three Chihuahuan desert shrubs. J. Arid. Environ. 32, 271e287. Navar, J., 1993. The causes of stemflow variation in 3 semiarid growing species of northeastern Mexico. J. Hydrol. 145, 175e190. Navar, J., 2011. Stemflow variation in Mexico's northeastern forest communities: its contribution to soil moisture content and aquifer recharge. J. Hydrol. 408, 35e42. Navar, J., Bryan, R., 1990. Interception loss and rainfall redistribution by 3 semiarid growing shrubs in northeastern Mexico. J. Hydrol. 115, 51e63. Navar, J., Carlyle-Moses, D.E., Martinez, A., 1999a. Interception loss from the Tamaulipan matorral thornscrub of north-eastern Mexico: an application of the Gash analytical interception loss model. J. Arid. Environ. 41, 1e10. Navar, J., Charles, F., Jurado, E., 1999b. Spatial variations of interception loss components by Tamaulipan thornscrub in northeastern Mexico. For. Ecol. Manag. 124, 231e239. Navar, J., Gonzalez, J.M., Gonzalez, H., 2009. Gross precipitation and throughfall chemistry in legume species planted in Northeastern Mexico. Plant Soil 318, 15e26. Siegert, C., Levia, D., 2014. Seasonal and meteorological effects on differential stemflow funneling ratios for two deciduous tree species. J. Hydrol. 519, 446e454. Wang, X.P., Zhang, Y.F., Wang, Z.N., Pan, Y.X., Hu, R., Li, X.J., Zhang, H., 2013. Influence of shrub canopy morphology and rainfall characteristics on stemflow within a revegetated sand dune in the Tengger Desert, NW China. Hydrol. Process 27, 1501e1509. Whitford, W.G., Anderson, J., Rice, P.M., 1997. Stemflow contribution to the 'fertile island' effect in creosotebush, Larrea tridentata. J. Arid. Environ. 35, 451e457. Yang, Z.P., Li, X.Y., Liu, L.Y., Wu, J.J., Hasi, E.D., Sun, Y.L., 2008. Characteristics of stemflow for sand-fixed shrubs in Mu Us sandy land, Northwest China. Chin. Sci. Bull. 53, 2207e2214. Zhang, Y.F., Wang, X.P., Hu, R., Pan, Y.X., Zhang, H., 2013. Stemflow in two xerophytic shrubs and its significance to soil water and nutrient enrichment. Ecol. Res. 28, 567e579. Zhang, Y.F., Wang, X.P., Hu, R., Pan, Y.X., Paradeloc, M., 2015. Rainfall partitioning into throughfall, stemflow and interception loss by two xerophytic shrubs within a rain-fed re-vegetated desert ecosystem, northwestern China. J. Hydrol. 527, 1084e1095. Zhang, Y.F., Wang, X.P., Pan, Y.X., Hu, R., 2016. Variations of nutrients in gross rainfall, stemflow, and throughfall within revegetated desert ecosystems. Water Air Soil Poll. 227 (6). DOI: Artn 18310.1007/S11270-016-2878-Z.
Contents lists available at ScienceDirect
Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv
Stemflow volume per unit rainfall as a good variable to determine the relationship between stemflow amount and morphological metrics of shrubs Ya-feng Zhang*, Xin-ping Wang, Rui Hu, Yan-xia Pan Shapotou Desert Research and Experiment Station, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, 320 Donggang West Road, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 November 2014 Received in revised form 6 February 2017 Accepted 8 February 2017
In this study, we determined the relationship between stemflow amount and morphological metrics of plants. Previous studies on this issue generally neglected the influence of differential rainfall amounts on stemflow, which somehow bias the relationship between stemflow amount and morphological metrics. Here, field observation of stemflow in two xerophytic shrubs (Caragana korshinskii and Artemisia ordosica) of varying sizes were conducted during three growing seasons in 2011e2013 to evaluate this relationship by using a simple variable, i.e., stemflow volume per unit rainfall (SfVPR, mL mm1). This stemflow index directly defines the influence of rainfall depth on the morphological metrics of plants, and it has the following advantages over the conventional stemflow coefficient that has been used to determine the influence of tree/shrub morphological metrics on the stemflow amount: (1) it avoids the bias caused by the influences of differential rainfall amount on stemflow yield and (2) it is suitable for use in hydrological models. Our results showed that SfVPR can well be used to determine the relationship between stemflow amount and shrub morphological metrics. Moreover, by using multiple regression model, we found that projected canopy area (or canopy volume, or basal area), plant area index, and stem diameter are the most influential factors for the stemflow amount of C. korshinskii, whereas no significant explanatory variables were found for that of A. ordosica. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Stemflow Morphological metrics C. korshinskii A. ordosica Simple regression Multiple linear regression
1. Introduction Stemflow refers to a part of rainfall that is intercepted by leaves, twigs, and branches and eventually channeled into soil through trunk or stem, which further could be transported and redistributed into deeper soil layers through preferential pathways such as roots. Although volumetrically minor, stemflow is an important source of soil moisture and nutrients for plant growth, this being particular important in arid and semiarid ecosystems (Aboal et al., 1999; Navar and Bryan, 1990; Navar et al., 2009; Navar, 2011; Whitford et al., 1997; Zhang et al., 2016). The hydrological and biogeochemical importance of stemflow was systematically reviewed by Levia and Frost (2003). Stemflow amount is a function of canopy structure, rainfall characteristics, and meteorological variables (Crockford and
* Corresponding author. E-mail address: [email protected] (Y.-f. Zhang). http://dx.doi.org/10.1016/j.jaridenv.2017.02.002 0140-1963/© 2017 Elsevier Ltd. All rights reserved.
Richardson, 1990; Navar, 1993; Levia and Frost, 2003). The influence of individual canopy structure metrics on stemflow amount is difficult to determine (Germer et al., 2010; Levia et al., 2013). Simple regressions were first performed between a conventionally used stemflow coefficient (the total volume of stemflow during the observation periods) and the morphological metrics of plants, and then a multiple linear regression model was developed after eliminating colinearity between these metrics (e.g., Aboal et al., 1999; Martinez-Meza and Whitford, 1996; Wang et al., 2013; Yang et al., 2008). However, by using the accumulated stemflow volumes during the observational periods, the previous authors neglected the influence of differential rainfall amount on the stemflow amount that obscures the morphology effects because it is well known that stemflow significantly increases with the rainfall amount after a threshold value for stemflow generation (e.g., Li et al., 2008; Manfroi et al., 2004; Navar, 2011; Zhang et al., 2013, 2015). As such, the contribution of rainfalls with high depths was probably overestimated, whereas that of rainfall with low depths was underestimated or under-represented. In terms of accurate
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Y.-f. Zhang et al. / Journal of Arid Environments 141 (2017) 1e6
predictions, the results are not so desirable without fully considering the influence of differential rainfall depths. This relationship, based on volumetrically expressed unstandardized stemflow, is also unsuitable for use in ecohydrological or hydropedological models (Levia and Germer, 2015). In this study, we used a simple variable, stemflow volume per unit rainfall (SfVPR, mL mm1), to evaluate the relationship between stemflow amount and shrub morphological metrics. This standardized variable demonstrates the amount of rainfall that a tree channels to the ground as SfVPR, which has been used by Honda et al. (2015) and Manfroi et al. (2004), among others. However, no attempts have been made to use SfVPR to determine the relationship between stemflow amount and shrub morphological metrics, although SfVPR allows the comparison of stemflow among trees in localities with variable rainfall, as Honda et al. (2015) suggested. Therefore, it is expected that SfVPR can reasonably and accurately determine the relationships between stemflow amount and inter- and intraindividual morphological metrics of shrubs. 2. Materials and methods 2.1. Site information Field measurements were conducted during three growing seasons in 2011e2013 at the Shapotou Desert Research and Experiment Station (SDRES) of Chinese Academy of Sciences (37 320 N, 105 020 E, at an elevation of 1300 m a.s.l.), southeastern fringe of the Tengger Desert in northwestern China. The mean annual rainfall in this area is only 191 mm (1955e2005, SDRES). Detailed information on other rainfall and meteorological characteristics and soil properties are given by Zhang et al. (2015). Extensive revegetation efforts had been undertaken during the 1950s to the 1980s to protect the Baotou-Lanzhou railway from encroaching sand dunes in the Shapotou area. A 16,000-m-long artificial protection system had been established along the railway that is 500 m wide to the north and 200 m wide to the south. Straw checkerboards were set up on the mobile sand dunes on both sides of the railway, and subsequently, xerophytic shrubs (mainly Caragana korshinskii, Hedysarum scoparium, and Artemisia ordosica) were planted within these checkerboards. A detailed description of the revegetation procedure is given by Li et al. (2006). The 1-ha Water Balance Experimental Field (WBEF) is one of the revegetated enclosures studied here. The WBEF was established in 1989 by planting two morphologically different shrubs, i.e.,
C. korshinskii and A. ordosica (Fig. 1). C. korshinskii is a multistemmed deciduous perennial leguminous shrub and resembles an inverted cone. Stems have smooth bark. Leaves are 6e10 cm long, pinnately compound, and opposite or subopposite in arrangement. Each pinna has 5e8 pairs of ovate leaflets (7e8 mm in length and 2e5 mm in width). A. ordosica is a highly branched dwarf shrub with plumose, full split needled leaves (10e30 mm in length and 0.3e1 mm in width). It has only one rough stem with thick, loose, and inclinedly fractured bark.
2.2. Shrubs selection and measurements Seventeen robust and healthy shrubs (10 C. korshinskii and 7 A. ordosica) were selected for field observation as a representation of the range of sizes of the two xerophytic shrub species in WBEF (Tables 1 and 2). Stem diameter was measured at each stem's base using a vernier caliper. Stem angle was determined using a protractor. Plant area index (PAI) of individual shrubs was estimated using a LAI-2000 plant canopy analyzer (Li-Cor., Inc., USA) with a 30 view cap on the ground by the stem, assuming shrub canopy shapes such as a hemisphere. PAI measurements were obtained under uniform sky conditions around sunset to avoid the interference of direct sunlight (Leblanc and Chen, 2001). Shrub canopy height was measured at the center of the canopy. Canopy area (approximated as an ellipse) was calculated by taking the eastewest and northesouth diameters through the center of the fullest part of the canopy (Martinez-Meza and Whitford, 1996). Shrub canopy was approximated as an inverted elliptic cone, and canopy volume was thus determined from canopy height and canopy area.
2.3. Stemflow and rainfall measurements Stemflow was collected using collars constructed from aluminum foil plates that were fitted around the circumference of the shrub stem (Fig. 1). The volume of stemflow was measured by a graduated cylinder for each individual stem (A. ordosica has only one stem) after the cessation of each rainfall event. SfVPR was calculated as the stemflow volume collected from each individual stem divided by the rainfall amount in that period. A standard tipping bucket rain gauge (Adolf Thies GMVH & Co. KG, Germany) with a resolution of 0.1 mm and a mini logger recording 10-min rainfall intensity values were installed in an open area approximately 50 m from the study plot.
Fig. 1. Photographs showing the morphological features of C. korshinskii (a) and A. ordosica (b) and the corresponding method of collecting stemflow.
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Table 1 Descriptive statistics (mean ± SE) of canopy morphology and plant area index (PAI) of C. korshinskii selected in the experiments. No.
Stem
1 2 3 4 5 6 7 8 9 10 Mean SE
Canopy
Number
Diameter (cm)
7 6 9 12 11 2 5 1 1 3 6 1
1.9 2.5 2.5 1.8 1.7 2.2 1.4 1.8 1.8 1.5 1.9 0.1
± ± ± ± ± ± ± ± ± ±
0.5 0.2 0.4 0.1 0.2 0.0 0.2 NA NA 0.2
PAI
Length (cm)
Angle ( )
Height (cm)
Area (m2)
Volume (m3)
119 ± 14.3 159 ± 18.3 151 ± 12.3 103 ± 9.1 83 ± 2.6 94 ± 5.5 50 ± 5.9 57 ± NA 98 ± NA 77 ± 1.7 107 5.7
72 ± 73 ± 60 ± 60 ± 56 ± 77 ± 59 ± 72 ± 50 ± 60 ± 62 2.1
216 245 203 182 158 115 92 66 114 126 152 18
2.23 3.69 5.19 2.07 2.69 0.59 0.66 0.11 0.39 0.60 1.82 0.53
1.61 3.02 3.51 1.26 1.42 0.23 0.20 0.02 0.15 0.25 1.26 0.40
5 3 8 4 3 5 3 NA NA 19
0.78 0.71 1.02 1.01 0.68 0.77 0.95 0.46 0.69 0.54 0.76 0.06
NA: not available.
Table 2 Descriptive statistics (mean ± SE) of canopy morphology and plant area index (PAI) of A. ordosica selected in the experiments. No.
Stem
1 2 3 4 5 6 7 Mean SE
Canopy
PAI
Diameter (cm)
Length (cm)
Angle ( )
Height (cm)
Area (m2)
Volume (m3)
2.81 3.16 1.58 3.08 3.65 5.45 3.74 3.35 0.44
7 10 9 14 8 15 16 11 1.4
90 78 88 68 78 89 82 82 3.0
74 85 50 69 95 75 80 75 5.5
0.52 0.75 0.17 0.51 0.78 1.02 1.59 0.76 0.17
0.13 0.21 0.03 0.12 0.25 0.26 0.42 0.20 0.05
1.40 1.79 1.65 1.72 1.69 2.00 1.64 1.70 0.07
2.4. Statistical analyses Descriptive statistics were compiled for shrub morphological metrics. Simple regression equations were developed between stemflow volume and rainfall depth and between SfVPR and shrub morphological metrics. In addition, we used multiple linear regression to determine the relationship between SfVPR and shrub morphological metrics. All the descriptive statistics and regressions were performed using the SPSS 16.0 statistical software (SPSS Inc., Chicago, USA). 3. Results and discussion 3.1. Rainfall characteristics and stemflow volume
Stemflow volume (L)
7 (a) C. korshinskii
6 5 4 3 2 1
y=-0.09+0.165x, R2=0.80, P<0.01
0
Stemflow volume (L)
0.7 (b) A. ordosica
0.6 0.5 0.4 0.3 0.2 0.1 0.0
y=0.02+0.014x, R2=0.50, P<0.01
0
5
10
15 20 Rainfall (mm)
25
30
35
Fig. 2. Stemflow volume as a function of rainfall depth. Bars represent 95% confidence level. n ¼ 10 for C. korshinskii and n ¼ 7 for A. ordosica.
During our experimental periods in the three years of 2011 (from 26 June to 7 November), 2012 (from 11 April to 25 September), and 2013 (from 15 May to 31 November), we observed that a rainfall of <2.4 mm was negligible for generating stemflow. Stemflow was measurable for 37 rainfall events, with a total of 412.5 mm, accounting for 89.4% of the total incident rainfall amount and 36.3% of the total number of storms. The 37 rainfall events ranged from 2.4 to 28.8 mm, with a mean of 11.2 mm. The rainfall intensity ranged from 0.37 to 21.7 mm h1, with a mean of 2.9 mm h1. Thirty-five out of the 37 rainfall events had rainfall intensities of <6 mm h1. Two storms had intensities of 21.7 and 21.2 mm h1, which can be considered as extreme rainfall events in our study area. Stemflow volume significantly increased with rainfall amount for both shrubs species (Fig. 2). The total stemflow volume collected from the 10 shrubs of C. korshinskii amounted to 638.7 L during the experimental periods, with a mean of 63.87 L for each shrub and a coefficient of variation of 86.3%. The total stemflow volume amounted to 43.07 L for the 7 shrubs of A. ordosica, with a mean of 6.15 L for each shrub and a coefficient of variation of 45.5%. These volumes were very small compared to those measured from tree species in forested ecosystems with high rainfall (Aboal et al., 1999; Germer et al., 2010; Herwitz, 1986). The volumes were, however, similar to the stemflow volumes measured in several arid and semiarid shrubs (Navar and Bryan, 1990; Navar et al., 1999a,b; Navar, 2011). An average of 4.5 and 3.1 L of rainwater would have been collected in the open in the same area as projected canopy area of C. korshinskii and A. ordosica, respectively. The values would be 0.78 and 0.28 L in the open in the same area as stem basal area of C. korshinskii and A. ordosica, respectively. Moreover, it should be noted that a weak linear relationship can be found between SfVPR
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and rainfall, with R2 (the coefficient of determination) < 0.3 for C. korshinskii and <0.1 for A. ordosica. 3.2. Simple regression C. korshinskii is a multi-stemmed shrub; it thus has inter- and intraindividual differences in morphological metrics. The interindividual ones include the projected canopy area, basal area, shrub height, PAI, and shrub volume. The intraindividual ones include stem angle, stem diameter, branch length, and stem height. A. ordosica has only a single stem; therefore, no intraindividual morphological variables were considered in the current study. As can be seen in Fig. 3, the SfVPR of C. korshinskii was significantly positively correlated to the projected canopy area
(P < 0.001), basal area (P < 0.001), shrub height (P ¼ 0.013), shrub volume (P < 0.001), number of stems (P ¼ 0.02), stem diameter (P < 0.001), branch length (P < 0.001), but was negatively correlated to stem height (P ¼ 0.012), respectively; it showed increasing tendency with increase in PAI, although not statistically significant (P ¼ 0.057); it showed increasing tendency and attained the highest value with stem angle until 65 and then showed decreasing tendency. As shown in Fig. 4, the SfVPR of A. ordosica showed increasing tendency with increase in the projected canopy area, basal area, shrub height, PAI, shrub volume, stem diameter, and stem height, although not statistically significant (P > 0.05). Moreover, the relationship between the SfVPR of A. ordosica and stem angle was a downward-facing parabolic curve, with the highest value being
Fig. 3. Regression equations between stemflow volume per unit rainfall (SfVPR) and morphological metrics of C. korshinskii. Bars represent 95% confidence level.
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Fig. 4. Regression equations between stemflow volume per unit rainfall (SfVPR) and morphological metrics of A. ordosica. Bars represent 95% confidence level.
attained at 80 . In our study, we used SfVPR (instead of the conventionally used variable, the total stemflow volume) to evaluate the relationships between stemflow amount and shrub morphological metrics. This variable has been reported by Honda et al. (2015) and Manfroi et al. (2004); no attempts, however, have been made to use it to determine the relationship between stemflow amount and shrub morphological metrics. In fact, our results showed that SfVPR can be used to determine the relationship between stemflow amount and the inter- and intraindividual morphological metrics of two shrubs (Figs. 3 and 4). Although stemflow is an important metric for calculating hydrologic inputs and overall budgets (e.g., Siegert and Levia, 2014), the conventionally used method that is based on the total stemflow volume (e.g., Aboal et al., 1999; Martinez-Meza and Whitford, 1996) has two main shortcomings: (1) it overestimates the contribution of rainfalls with high depths but underestimates that of rainfalls with low depths and (2) the volumetrically expressed unstandardized stemflow is unsuitable for use in ecohydrological or hydropedological models (Levia and Germer, 2015). In contrast, SfVPR takes into account the influences of differential rainfall depth on stemflow amount, is a suitable index for determining the influence of the tree size on stemflow volume (Honda et al., 2015; Levia and Germer, 2015), and can easily be used in hydrologic models. Therefore, SfVPR is expected to be widely used to determine the relationships between the stemflow amount and tree/shrub morphological metrics in other ecosystems.
3.3. Multiple linear regressions To explain the causes of stemflow variation among shrubs, stepwise regression analysis was used to determine the effects of morphological metrics on SfVPR. Multicollinearity existed between the projected canopy area, canopy volume, and basal area, which first should be eliminated. When canopy volume and basal area were eliminated in the multiple linear regression model, we found projected canopy area (PCA) and PAI to be the two statistically significant explanatory variables (interindividual morphological variables) for stemflow amount (SfVPR ¼ 72.2 þ 69.9PCAþ124.2PAI, R2 ¼ 0.96, P < 0.01). Regarding the intraindividual morphological variables (stem angle, stem diameter, branch length, and stem length) of C. korshinskii, we found the stem diameter (SD) to be a statistically significant explanatory variable of stemflow amount (SfVPR ¼ 12.3 þ 20.1SD, R2 ¼ 0.57, P ¼ 0.016). Therefore, SfVPR is most influenced by the projected canopy area (or canopy volume, or basal area), PAI, and stem diameter for C. korshinskii, the first two being interindividual morphological variables and the latter being an intraindividual morphological variable. Likewise, Martinez-Meza and Whitford (1996), Aboal et al. (1999), and Yang et al. (2008) found relationships between stemflow volumes and canopy area. For A. ordosica, no individual morphological metrics, however, were found to be statistically significant explanatory variables for
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stemflow amount from stepwise regression analysis. We assume other vegetative traits such as leaf shape and bark microrelief may exert more influences on its stemflow amount. Acknowledgements This study was supported by the National Natural Science Foundation of China (41530750, 41501108, and 41371101) and the CAS “Light of West China” Program. We appreciate the editors and two anonymous reviewers for their constructive comments. References Aboal, J.R., Morales, D., Hernandez, M., Jimenez, M.S., 1999. The measurement and modelling of the variation of stemflow in a laurel forest in Tenerife, Canary Islands. J. Hydrol. 221, 161e175. Crockford, R.H., Richardson, D.P., 1990. Partitioning of rainfall in a eucalypt forest and pine plantation in Southeastern Australia .2. Stemflow and factors affecting stemflow in a dry sclerophyll eucalypt forest and a pinus-radiata plantation. Hydrol. Process 4, 145e155. Germer, S., Werther, L., Elsenbeer, H., 2010. Have we underestimated stemflow? Lessons from an open tropical rainforest. J. Hydrol. 395, 169e179. Herwitz, S.R., 1986. Infiltration-excess caused by stemflow in a cyclone-prone tropical rain-forest. Earth Surf. Process. 11, 401e412. Honda, E.A., Mendonca, A.H., Durigan, G., 2015. Factors affecting the stemflow of trees in the Brazilian Cerrado. Ecohydrology 8 (7), 1351e1362. Leblanc, S.G., Chen, J.M., 2001. A practical scheme for correcting multiple scattering effects on optical LAI measurements. Agr. For. Meteorol. 110 (2), 125e139. Levia, D.F., Frost, E.E., 2003. A review and evaluation of stemflow literature in the hydrologic and biogeochemical cycles of forested and agricultural ecosystems. J. Hydrol. 274, 1e29. Levia, D.F., Germer, S., 2015. A review of stemflow generation dynamics and stemflow-environment interactions in forests and shrublands. Rev. Geophys 53. http://dx.doi.org/10.1002/2015RG000479. Levia, D.F., Michalzik, B., N€ athe, K., Bischoff, S., Richter, S., Legates, D., 2013. Differential stemflow yield from European beech saplings: the role of individual canopy structure metrics. Hydrol. Process. http://dx.doi.org/10.1002/hyp.10124. Li, X., Xiao, H., He, M., Zhang, J., 2006. Sand barriers of straw checkerboards for habitat restoration in extremely arid desert regions. Ecol. Eng. 28, 149e157. Li, X.Y., Liu, L.Y., Gao, S.Y., Ma, Y.J., Yang, Z.P., 2008. Stemflow in three shrubs and its effect on soil water enhancement in semiarid loess region of China. Agr. For. Meteorol. 148, 1501e1507.
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