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Response of tree regeneration and understory plant species diversity to stand density in mature Pinus tabulaeformis plantations in the hilly area of the Loess Plateau, China
Ecological Engineering 73 (2014) 238–245
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Response of tree regeneration and understory plant species diversity to stand density in mature Pinus tabulaeformis plantations in the hilly area of the Loess Plateau, China Yunming Chen a,b , Yang Cao a,b, * a b
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F University, Yangling 712100, China Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling 712100, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 May 2014 Received in revised form 29 August 2014 Accepted 17 September 2014 Available online xxx
Due to historic land degradation and severe erosion on the Loess Plateau of China, vegetation restoration projects have been undertaken with various levels of success. This study had the objective of assessing the restoration capability of large-scale forest plantations in achieving natural regeneration and biodiversity in the hilly area of the Loess Plateau. We investigated three mature Pinus tabulaeformis plantations with 1875, 2215, and 3925 trees ha1, that represented low (LD), medium (MD), and high (HD) stand densities, respectively, and compared them with clear-cut stands (CC) after almost 30 years following density reduction. Tree regeneration, understory plant species diversity, and soil nutrient and water contents were assessed. The results showed that the number, base diameter and height of seedlings were not significantly affected by stand density, and that the base diameters and heights of seedlings in CC stands were significantly greater than in the forested stands. The age of seedlings significantly decreased with increasing stand density, and the ages in the LD and CC stands were similar. With increasing stand density, the shrub layer’s species richness and diversity indices tended to decrease, and the evenness index tended to increase, but differences among the stands were not significant; the values for the CC stands were similar to those for the forested stands. The CC stands had the highest values of the herb layer’s species richness, diversity indices and evenness index, but none of these indices in the forested stands were significantly affected by stand density. The soil organic matter, total N and soil water contents all followed the trend of HD < MD < LD < CC. These results indicated that a lower stand density is best suited to integrate species diversity, natural regeneration, and the replenishment of soil fertility and water with wood production on the Loess Plateau. Therefore, timely density management can promote sustainable development of P. tabulaeformis plantations. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Afforestation Density management Naturalisation Seedling Regeneration
1. Introduction Plantations are primarily established in order to achieve economic objectives, such as the profitable income derived from the timber production and from other wood products. However, plantations are also established for the purposes of soil and water conservation as well as of carbon sequestration during reforestation and afforestation. Plantations currently cover approximately 187 million ha worldwide and are being established
* Corresponding author at: No. 26, Xinong Road, Institute of Soil and Water Conservation, Yangling, Shaanxi 712100, PR China. Tel.: +86 029 8701 4869/153 8924 5368. E-mail address: [email protected] (Y. Cao). http://dx.doi.org/10.1016/j.ecoleng.2014.09.055 0925-8574/ ã 2014 Elsevier B.V. All rights reserved.
at an annual rate of 4.5 million ha (Stephens and Wagner, 2007; Bremer and Farley, 2010). As plantations become an increasingly ubiquitous land use, an improved understanding of the potential ability of a plantation to perform other ecosystem services is critical when carrying out socially and ecologically sustainable land-use policies (Goldman et al., 2008). These services include biodiversity maintenance and natural regeneration (Aubin et al., 2008; Bremer and Farley, 2010; Duan et al., 2009). Natural forests are commonly considered the reference state when assessing the sustainability of forest management practices (Wesolowski, 2005; Paillet et al., 2009). There is an ongoing global debate over whether the effects of plantations on biodiversity and community succession are more beneficial as compared with those of natural forests (Brockerhoff et al., 2008; Paillet et al., 2009; Lima and Vieira, 2013). In order to clarify the impacts of planted forests on biodiversity and natural
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regeneration, studies have considered the effect of land-use transition as well as the effects of plantation age, tree species, location, land-use history and management (Carnus et al., 2006; Duan et al., 2009; Gonzalez-Alday et al., 2009; Bremer and Farley, 2010). For example, Carnus et al. (2006) concluded that biodiversity within plantations tended to increase over time. However, a metaanalysis carried out in Europe showed that species richness in plantations increased during the first 20 years when compared to natural forests, but declined after that (Paillet et al., 2009). The Loess Plateau of China is susceptible to severe soil erosion and water losses. Consequently, there are many widely distributed large-scale plantations across the plateau that is part of governmental ecosystem restoration and erosion control programs, especially the “Grain for Green Program” launched in 1999 (Sun et al., 2006; Li, 2004). Approximately 5.2 million ha of plantations were established on the Loess Plateau between 1999 and 2008 (Xu et al., 2012). Despite some of the achievements of these ecological restoration programs, a number of scientists have noted that assessments of the ecological success of restoration by afforestation should focus on ecosystem functionality and stability (Sun et al., 2006; Yang et al., 2010; Cao et al., 2011; Deng et al., 2012; Yin and Yin, 2010). Restoration success can be viewed as a continuous process from initial plant establishment to successfully developing those attributes that ensure a selfsustaining and naturally functioning ecosystem (Reay and Norton, 1999). Preliminary field investigations have found that the composition and diversity of understory vegetation in plantations on the Loess Plateau were relatively poor and that the tree seedlings could not readily survive or establish themselves. Furthermore, dried soil layers have been observed under plantations where the water consumption of the tree species, especially non-native species, exceeds soil water recharge (Wang et al., 2012). This represents soil degradation that leads to poor development of forest plantations mainly due to exacerbating water deficits. This suggested that natural restoration could possibly offer a more adaptive and appropriate method of ecological restoration (Chirino et al., 2006; Wang et al., 2002, 2009; Wang, 2010; Wen et al., 2005; Jiao et al., 2012). An increasing number of studies on the Loess Plateau have recently considered the influencing factors that would enable large-scale monoculture plantations to achieve natural succession and functional ecosystems during the ecological restoration process; these factors included forest configurations, habitats, slope gradients and growth age phases (Zhang et al., 2007; Duan et al., 2009; Wang et al., 2009; Zheng et al., 2010; Jiao et al., 2012; Han et al., 2012). For example, species richness and diversity benefits from the main characteristic of shady slopes, i.e., having more available water than sunny slopes that results in less water stress, while the evenness index appears to be increased when water stress and increased light and heat are present, as is the case on sunny slopes (Wang and Zhang, 2009; Wei et al., 2013). Stand density, as one of the influencing factors, also has an effect on stand structural components, on shrub and herb cover, and on species richness and composition. Intensive density management could increase canopy openness to ensure adequate resources for understory development (Wilson and Puettmann, 2007). Understory development is an important factor in soil erosion control since the plants protect the soil surface from both raindrop and throughfall impacts, thereby reducing soil particle detachment, increasing soil surface roughness, which reduces the transport capacity of overland flow, and increase soil stability with their roots. However, it would be necessary to investigate whether density management would be sufficient to promote species diversity and community stability, especially for large old trees (Alaback and Herman, 1988; Wilson and Puettmann, 2007; Wang et al., 2002; Bu et al., 2004). In Oregon, Latham and
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Tappeiner (2002) concluded that there was a positive response of old-growth conifer trees to a wide range of density reduction treatments. As they noted, the vigor of trees can be improved without intensive density reduction. For the response of understory vegetation to stand density, Alaback and Herman (1988) found that over seventeen years after density reduction treatments did not appear to fundamentally change the pattern of understory succession and did not significantly change shrub and herb productivity in Picea–Tsuga forests in Oregon. However, the question remains: what would be the response of mature Pinus tabulaeformis plantations to stand density changes on the Loess Plateau? In order to address this issue, we conducted the present study that investigated the effects of three different stand densities, as the primary influencing factor, on the differences in the species diversity and natural regeneration in mature P. tabulaeformis plantations (more than 40 years after afforestation) in the hilly area of the Loess Plateau. P. tabulaeformis is a preferred tree species that is widely planted on the Loess Plateau for the purpose of soil and water conservation. Furthermore, we investigated clear-cut stands in which P. tabulaeformis seedlings were naturally regenerating for comparison purposes. 2. Material and methods 2.1. Study site The study was conducted at the Yichuan water and soil conservation experimental station (35 590 N, 110 060 E, 900–1200 m a.s.l.), which is located in Yichuan county in northern Shaanxi Province, China (Fig. 1). The station covers approximately 2.94 ha of the hilly area of the Loess Plateau. The study area is in the semi-arid temperate climate zone with an annual mean temperature of 9.7 C. The annual mean rainfall is 523 mm, and approximately 60% of the rain falls during the rainy season from June to September. The annual variation in rainfall ranges from 308 to 843 mm, and brief and intense (1.0 mm min1) rainfall events occur one to three times each year. The annual frost-free season is 180 days. The annual potential evapotranspiration is approximately 1560 mm, which is almost three times greater than the total precipitation. The mean atmospheric humidity is approximately 61.4%. The soil at the study site had a silty clay loam texture: 34.4% clay (<0.01 mm), 61.3% silt (0.01–0.05 mm) and 4.30% sand (>0.05 mm). The mean soil bulk density was 2.64 g cm3, but at the surface (0–10 cm) the value was only 1.14 g cm3. An extensive P. tabulaeformis plantation was implemented over the entire study area in 1966, with an initial planting density of 6000 plants ha1. Thinning was conducted in 1983 and 1995, after which the plantation was left unmanaged. Major shrub species present at the site were Rosa xanthina,Lonicera japonica, and microphylla, and the main herb species were Carex stenophylloides and Artemisia sacrorum. 2.2. Data collection Three experimental blocks, which had been established since 1983 and had similar site conditions (slope: 20–25 ; altitude: 1071–1085 m; aspect: NW; location: middle–upper slope), were used in this current study. The three experimental blocks were close to each other separated by about 200 m. Each block included three 20 m 5 m forested plots with different stand densities and one 20 m 5 m clear-cut plot with no trees (CC). In the current study, all of the plots were investigated during July–August 2011. The three forested plots had stand densities that were low (LD; mean = 1875 trees ha1), medium (MD; mean = 2215 trees ha1),
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Fig. 1. Location of the study site on the Loess Plateau. 1
and high (HD; mean = 3925 trees ha ). The plots were used to count individual trees, which were considered to have a diameter at breast height (DBH) that was greater than 5 cm. Understory plants (DBH <5 cm, including shrubs and P. tabulaeformis seedlings) and herbs were surveyed in three 5 m 5 m and five 1 m 1 m sub-plots, respectively, within each plot. The height and DBH of all of the trees in the plots were measured. All individual species and species coverage in the shrub and herb sub-plots were identified by two observers working together. Information about the P. tabulaeformis seedlings, i.e., the numbers, base diameters, heights and ages, was recorded. In each plot, soil samples were taken from the upper 20 cm soil layer at eight points in an S-shape pattern. The samples were air-dried and mixed together to form a composite sample that was representative of the soil in the plot. Soil organic matter was determined using the Walkley–Black K2Cr2O7–H2SO4 wet oxidation method (Nelson and Sommers, 1996). Total N was measured using the Kjeldahl method (Bremner and Mulvaney, 1982), and total P was determined using the HClO4–H2SO4 colorimetric method (Parkinson and Allen, 1975). Soil samples were also collected at 20 cm depth intervals to a depth of 300 cm in order to determine soil water contents within the 0–300 cm profile. Gravimetric soil water content was determined from the loss in mass when the samples were dried at 105 C in an oven until the mass was constant.
where S is the number of species, Pi is the proportion of individuals or the abundance of the ith species expressed as a proportion of the total in the community, and ln is log base-e. The data were statistically analysed using SPSS (version 20, USA). One-way ANOVA tests were used to compare results for different categories and LSD post hoc tests were conducted to identify significant differences between means. The level of statistical significance was taken as P < 0.05. 3. Results 3.1. Trees growth conditions Generally, tests for normality of tree DBH and height data showed that these data were normally distributed (P < 0.001), with the exception of tree height in the MD stands (Fig. 2). The stand density had a significant effect on the tree DBH (P < 0.01). The mean DBH of P. tabulaeformis tended to decrease with increases in stand density. However, the only significant differences were between the mean DBH of the HD stands (11.4 0.3 mm) and that of both the MD (14.4 0.5 mm) and LD stands (15.1 0.4 mm). Furthermore, stand density had no significant effect on the mean height of P. tabulaeformis although the highest trees were observed in the HD (12.4 0.1 m) stands and slightly shorter trees were found in the MD (11.9 0.2 m) and the LD (11.8 0.2 m) stands.
2.3. Data analysis 3.2. Tree regeneration Indices used to assess plant diversity were species richness, Simpson, Shannon–Wiener and Pielou, as determined from the following equations: Species number as the richness index (S) Simpson index: D ¼ 1 SðPi Þ2 Shannon–Wiener index: H ¼ SðPi ÞlnðPi Þ 0
Pielou index: Jsw ¼
SPi lnPi lnS
The number, age, base diameter and height of P. tabulaeformis seedlings were employed to describe the community regeneration capability. A normality test of the seedling data showed that seedling numbers, ages, base diameters and heights were not normally distributed in the forested P. tabulaeformis stands. However, these parameters were normally distributed for the seedlings in the CC plots (P < 0.05). The number of seedlings varied between 2533 352 (LD) and 4080 783 (MD) plants ha1, but seedling number was not significantly different among the four stands (Fig. 3(A)). Stand density significantly affected the age of seedlings (P < 0.001) such that seedlings were older as stand density decreased (Fig. 3(B)). Significant differences were found among the three planted P. tabulaeformis stands but there was no
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Fig. 2. Frequency distributions of diameter at breast height (DBH) ((A), (B) and (C)) and of tree height ((D), (E), (F)) of Pinus tabulaeformis in plantations with high (HD), medium (MD) and low (LD) stand density; curves fitted by a Gaussian function.
significant difference between the ages of the seedlings in the LD stands and those of the seedlings in the CC stands. However, the seedlings in the CC stands had much greater base diameters (1.2 0.1 cm) that were significantly different (P < 0.001) from those of the seedlings in the three P. tabulaeformis stands with differing stand densities (Fig. 3(C)). Furthermore, the base diameters of seedlings did not differ significantly among the three forested stands with different stand densities. The heights of seedlings in the CC stands were the highest (76.7 9.0 cm), and differed significantly from those in the forested stands (P < 0.001) (Fig. 3(D)). Although the height of seedlings decreased as stand density increased from LD (21.0 2.5 cm) to MD (11.9 1.3 cm), and to HD (6.6 0.7 cm), no significant differences were observed among the three stand densities (Fig. 3(D)). 3.3. Richness and diversity Among the shrub layer’s species, the dominant species with the highest mean coverages were Pyrus betulifolia Bge., Viburnum schensianum, Cotoneaster multiflora and Rosa xanthina in the LD, MD, HD and CC stands, respectively. There were no significant differences among the species richness, diversity indices and evenness index of the four stands (Table 1). However, the species richness and diversity indices in the LD stands were the highest and descended in the order LD > MD > CC > HD. In contrast, the values of the evenness index were the highest in the HD stands, and descended in the order HD > MD > CC > LD. Among the herbaceous plants, the dominant species, having the highest mean coverage, in all of the stands was Carex lancifolia. The species richness, diversity indices and evenness index had the highest values in the CC stands. Species richness tended to decrease with increases in the forested stand density but there were no significant differences among either the forested stands or between each of them and the CC stands (Table 1). Diversity indices of the forested stands did not different significantly among
the different stand densities. However, the diversity indices were both significantly higher in the CC stands than in the forested stands (P < 0.05). The evenness index did not differ significantly among the different forested stands and only the lowest value, which occurred in the MD stands, was significantly lower than that in the CC stands (P < 0.05). 3.4. Soil properties The concentrations of soil organic matter, total N and total P were not significantly different among the three forested stands (Table 2). However, organic matter, total N and total P in the CC stands were all higher than in the forested stands, but the only significant differences observed were for soil organic matter and total N between the CC and HD stands. In contrast, stand density had a significant effect on total K concentration in the forested stands, in that it was significantly lower in the LD than in either the MD or the HD stands (P < 0.05). Furthermore, there was no significant difference in total K concentration between the CC and any of the forested stands. The mean soil water content (0–300 cm) followed the trend: HD (13.17%) < MD (13.34%) < LD (13.61%) < CC (16.92%) (Fig. 4). The mean soil water content in the CC stands was significantly higher than in the forested stands (P < 0.001). Soil water content significantly decreased with soil depth (P < 0.001), and the mean soil water content was significantly different between the CC and the forested stands for soil depths from 140 to 300 cm (P < 0.001). The mean soil water content in the 140–300 cm soil layer was 14.94%, 10.34%, 9.65%, and 9.82% in the CC, LD, MD and HD stands, respectively. There were no significant differences in soil water content among the different forested stand densities (Fig. 4). In the forested stands, the water content was lower and the distribution was relatively constant when the soil was deeper than 120 cm as compared with that of the soil shallower than 120 cm (Fig. 4).
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Fig. 3. Comparison of the number (A), age (B), base diameter (C) and height (D) of Pinus tabulaeformis seedlings in three stand densities (high, HD; medium, MD; low LD) and in clear-cuts (CC). Error bars indicate the standard error of the mean value. Bars having the same lowercase letter above them indicate no significant difference between them (P < 0.05).
4. Discussion There is an intense debate over whether large-scale plantations on the Loess Plateau can achieve self-sustaining forest ecosystems after long-term succession. Forest management practices, such as the choice of tree species, site preparation, plant density, thinning and rotation length, greatly affects species diversity and forest composition, stand structure and ecosystem function (Hunter, 1999). 4.1. Effect of density management on trees growth In this study, we report the effect of density management on tree seedlings, understory plant species diversity, and soil properties in mature P. tabulaeformis plantations that are over 40 years old in the hilly area of the Loess Plateau, China. The growing space available for individual trees in stands is an important factor governing tree and stand vigor. Therefore, stand density has a significant effect on tree DBH (Fig. 2). The mean tree DBH of LD and MD stands were significantly larger than that of the HD stand, although there was no significant difference between the DBH in the LD and MD stands. A similar result was found by
Latham and Tappeiner (2002), who did not find a significant difference in old-growth conifer trees in low and medium stand density categories in Oregon. In addition, the influence of density or thinning management on the increment of tree growth is expected to decline with time as the stand canopy closes (Soucy et al., 2012). The normal distributions of tree DBH and height indicated that the local weather conditions in the study area were suitable for the growth requirements of P. tabulaeformis (Fig. 2). However, dryer sites located in the northern Loess Plateau have been found to be unsuitable for P. tabulaeformis growth resulting in distributions of tree DBH and height that were clearly not normal (Wang, 2010). 4.2. Effect of density management on P. tabulaeformis seedlings The seedling stage is a crucial part of the plant life cycle and it is important in natural vegetation restoration. The regeneration performance of plantations as a restoration strategy may differ depending on initial species composition, planting density, and site conditions (Sansevero et al., 2011). In this study, the stand density only had a significant effect on the age of P. tabulaeformis seedlings (Fig. 3(B)). This could be attributed to the limited time in which
Table 1 Species diversity indices of the shrub and herb layers under three Pinus tabulaeformis plantation stand densities and in clear-cut (CC) stands. a
Stands
HD MD LD CC a b
b
Shrub layer
Herb layer
Species richness
Simpson
Shannon–Wiener
Pielou index
Species richness
Simpson
Shannon–Wiener
Pielou index
6.00 1.2a 8.67 1.3a 10.00 1.0a 9.33 1.9a
0.36 0.1a 0.45 0.1a 0.60 0.02a 0.52 0.1a
0.56 0.2a 0.80 0.2a 1.00 0.04a 0.83 0.1a
0.95 0.03a 0.92 0.02a 0.87 0.08a 0.87 0.04a
9.33 2.8a 11.5 2.5a 13.00 1.2a 18.00 2.0a
0.28 0.07a 0.24 0.03a 0.35 0.02a 0.63 0.06b
0.54 0.16a 0.50 0.08a 0.62 0.16a 1.30 0.09b
0.56 0.09ab 0.34 0.05a 0.51 0.01ab 0.74 0.09b
HD, high density (3925 trees ha1); MD, medium density (2215 trees ha1); LD, low density (1875 trees ha1) and CC, clear-cut stands. Data is reported as mean standard error (n = 3); within a column, values followed by the same lowercase letter indicate that they did not differ significantly (P < 0.05).
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Table 2 Soil nutrient contents in the 0–20 cm soil layer under three Pinus tabulaeformis plantation stand densities and in clear-cut (CC) stands. a
Stands
HD MD LD CC a b
b
Organic matter (g/kg)
14.68 0.3a 18.35 1.0ab 19.89 1.8ab 23.15 1.0b
Total N (g/kg)
Total P (g/kg)
Total K (g/kg)
1.24 0.05a 1.27 0.1ab 1.61 0.09ab 1.86 0.2b
0.16 0.02a 0.17 0.03a 0.18 0.01a 0.20 0.01a
19.85 0.2a 19.86 0.2a 18.62 0.4b 18.86 0.2ab
HD, high density (3925 trees ha1); MD, medium density (2215 trees ha1); LD, low density (1875 trees ha1) and CC, clear-cut stands. Data is reported as mean standard error (n = 3); within a column, values followed by the same lowercase letter indicate that they did not differ significantly (P < 0.05)
stand density would have a great effect on the number of seedlings, i.e., within just 1–3 years after thinning operations (GonzalezAlday et al., 2009). In the CC stands, the higher soil water and nutrient contents enhanced seedling growth so that the base diameter and height of the seedlings were significantly greater than those of seedlings in the forested stands (Figs. 3 and 4; Table 2). It is noteworthy that the number of P. tabulaeformis seedlings in this study’s plantations was greater than that of seedlings in natural P. tabulaeformis forests close to the study area. Furthermore, the number of P. tabulaeformis seedlings in plantations increased with age for at least 40 years (Zhang et al., 2008, 2009). This demonstrates that the plantations were exhibiting a greater amount of regeneration over the forty years than the natural forest was. Environmental conditions on the Loess Plateau, especially rainfall, are the key factors that determine seedling survival and tree regeneration during ecosystem restoration (Wang, 2010). As annual precipitation decreases from south to north across the Loess Plateau, the number of P. tabulaeformis seedlings decreases and their growth is adversely affected (Wang, 2010). There are examples where there are no seedlings at all in P. tabulaeformis plantations in the north of the Loess Plateau (Wang, 2010). Therefore, Wang (2010) suggested that the region where annual precipitation was greater than 550 mm was suitable for the cultivation and regeneration of P. tabulaeformis plantations on the Loess Plateau. 4.3. Effect of density management on plant species diversity This study demonstrated that the effect of stand density did not significantly affect the species richness, diversity indices and the evenness index of either the shrub or herb layers (Table 1). This can be explained by the decline over time of the influence of stand density on the understory species diversity that occurs as the stand
canopy closes (Soucy et al., 2012). Furthermore, the herb layer was generally less responsive to stand density than the shrub layer (Wilson and Puettmann, 2007). Over time, plantation habitats become increasingly complex, which benefits forest species (Paillet et al., 2010). Compared with natural forests, the mature P. tabulaeformis plantations displayed similar species diversity (Wang et al., 2013). Shrub cover plays a very important role in erosion processes (Selkimäki et al., 2012), and it has been found to be inversely correlated with mean elevation and related to the basal area of dominant species in Spain (Coll et al., 2011; Selkimäki et al., 2012). Mean shrub cover in the present study was only 15%, ranging from 8–20%. There were no significant a difference among the four stands and explained no relation with stand basal area probably. In addition, plot size and number of samples collected in the present study was limited and only large differences among species richness, diversity indices and the evenness index could be detected. Therefore, it is interesting to expand the research in other studies under different site conditions in order to develop a more comprehensive understanding of ecological restoration in plantations on the Loess Plateau. Clear-cutting is a controversial forest management practice in terms of its effect on the forest ecology. Therefore, it is of interest to investigate the impacts of clear-cutting on the diversity of the forest structure and on the forest ecological processes (Keenan and Kimmins, 1993). Clear-cutting results in six patterns of species diversity over time: an increase with succession; a decrease during succession; an increase followed by a decrease; a decline followed by an increase; peaks in the early and later succession; and diversity being independent of succession (Schoonmaker and McKee, 1988; Keenan and Kimmins, 1993; Gonzalez-Alday et al., 2009). A high species richness, diversity indices and evenness index in both the shrub and the herb layers were found in this study’s CC stands (Table 1). The number of herb and shrub species has been reported to increase following clear-cutting in southern pine stands in Texas and Florida, but the number of understory plants decreased as the tree canopy closed (Keenan and Kimmins, 1993). However, increasing the proportions of non-crop vegetation could reduce timber yields. 4.4. Effect of density management on soil nutrient and water content
Fig. 4. Soil water content within the 0–300 cm soil profiles under Pinus tabulaeformi plantations with three stand densities (high, HD; medium, MD; low LD) and in clear-cuts (CC).
Nutrient circulation and water availability are important factors in forest ecosystem restoration and its processes, in addition to the photosynthetic rate and net primary production. Stands planted at higher densities need to use and uptake greater amounts of the soil resources, i.e., nutrients and water (Will et al., 2005). Therefore, the values of total N and soil water content followed the trend: HD < MD < LD < CC (Table 2 and Fig. 4). Soil organic matter also followed the same trend even though stands with higher densities might also produce greater amounts of litter that would become incorporated into the soil; however, in this case, the use of soil organic matter exceeded its production. Higher soil water contents in the CC stands, especially in the deeper soil layers, were primarily due to less root mass, reduced evapotranspiration and were only
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slightly impacted by increases in temperature. Soil water on the Loess Plateau is one key factor that limits vegetation development and recovery. Current evidence indicates that plantations have significant negative effects on soil desiccation and negative impacts on the long-term sustainability of plantation ecosystems (Sun et al., 2006; Cao et al., 2011). In this study, it is probable that a dried soil layer has been developing below 200 cm under all of the forested stands (Fig. 4), which could be detrimental to the future development of the plantation (Wang et al., 2012). Given that the annual evapotranspiration (1560 mm) exceeds the usual rainfall amount (308–843 mm), the planted trees have been relying on water stored in deep soil layers, which cannot be fully recharged except under clear-cuts as noted above, and the trees may begin to experience water stress. Therefore, experts have suggested that natural revegetation offers a more adaptive and appropriate method of ecological restoration (Jiao et al., 2012). However, some studies have shown that there were no significant differences among the soil nutrients between plantations and natural sites (Jiao et al., 2012; Zheng et al., 2010). A previous study showed that obvious increases in soil nutrients occurred for a period of time after clear-cutting because plant uptake was eliminated and decomposition continued or even increased (Keenan and Kimmins, 1993). However, soil nutrient levels returned to, or were lower than, pre-felling levels after 4 years. 5. Conclusion The large plantation areas on the hilly Loess Plateau effectively conserve both water and soil by reducing runoff and erosion, and enhance carbon sequestration within the forest ecosystem. We can conclude that within the range of stand densities considered in this study, which despite thinning were still high, there was little difference in the light and soil resources available to understory species and, thus, the effects of stand density did not significantly affect the diversity of understory species and tree regeneration. Our comparison with CC stands suggests that a lower stand density would better integrate the conservation of species diversity and ecosystem functions and the replenishment of soil fertility and water with wood production. To address the compromise between biodiversity conservation and timber production in plantations, further study is needed to determine plantation potential and mechanisms in order to provide basic forest ecological attributes and to promote the sustainable management of plantations on the Loess Plateau. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 41201088), the Doctoral Fund of the Ministry of Education of China (No. 20120204120014), and the West Light Foundation of the Chinese Academy of Sciences. We thank the staff at the Yichuan Water and Soil Conservation Experimental Station for valuable assistance. References Alaback, P.B., Herman, F.R., 1988. Long-term response of understory vegetation to stand density in Picea–Tsuga forests. Can. J. Forest Res. 18 (12), 1522–1530. Aubin, I., Messier, C., Bouchard, A., 2008. Can plantations develop understory biological and physical attributes of naturally regenerated forests? Biol. Conserv. 8, 2461–2476. Bremer, L.L., Farley, K.A., 2010. Does plantation forestry restore biodiversity or create green deserts? A synthesis of the effects of land-use transitions on plant species richness. Biodivers. Conserv. 19, 3893–3915. Bremner, J.M., Mulvaney, C.S., 1982. Nitrogen-total. In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties. Am. Soc. Agron., Madison, WI, pp. 595–624.
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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Response of tree regeneration and understory plant species diversity to stand density in mature Pinus tabulaeformis plantations in the hilly area of the Loess Plateau, China Yunming Chen a,b , Yang Cao a,b, * a b
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F University, Yangling 712100, China Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling 712100, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 May 2014 Received in revised form 29 August 2014 Accepted 17 September 2014 Available online xxx
Due to historic land degradation and severe erosion on the Loess Plateau of China, vegetation restoration projects have been undertaken with various levels of success. This study had the objective of assessing the restoration capability of large-scale forest plantations in achieving natural regeneration and biodiversity in the hilly area of the Loess Plateau. We investigated three mature Pinus tabulaeformis plantations with 1875, 2215, and 3925 trees ha1, that represented low (LD), medium (MD), and high (HD) stand densities, respectively, and compared them with clear-cut stands (CC) after almost 30 years following density reduction. Tree regeneration, understory plant species diversity, and soil nutrient and water contents were assessed. The results showed that the number, base diameter and height of seedlings were not significantly affected by stand density, and that the base diameters and heights of seedlings in CC stands were significantly greater than in the forested stands. The age of seedlings significantly decreased with increasing stand density, and the ages in the LD and CC stands were similar. With increasing stand density, the shrub layer’s species richness and diversity indices tended to decrease, and the evenness index tended to increase, but differences among the stands were not significant; the values for the CC stands were similar to those for the forested stands. The CC stands had the highest values of the herb layer’s species richness, diversity indices and evenness index, but none of these indices in the forested stands were significantly affected by stand density. The soil organic matter, total N and soil water contents all followed the trend of HD < MD < LD < CC. These results indicated that a lower stand density is best suited to integrate species diversity, natural regeneration, and the replenishment of soil fertility and water with wood production on the Loess Plateau. Therefore, timely density management can promote sustainable development of P. tabulaeformis plantations. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Afforestation Density management Naturalisation Seedling Regeneration
1. Introduction Plantations are primarily established in order to achieve economic objectives, such as the profitable income derived from the timber production and from other wood products. However, plantations are also established for the purposes of soil and water conservation as well as of carbon sequestration during reforestation and afforestation. Plantations currently cover approximately 187 million ha worldwide and are being established
* Corresponding author at: No. 26, Xinong Road, Institute of Soil and Water Conservation, Yangling, Shaanxi 712100, PR China. Tel.: +86 029 8701 4869/153 8924 5368. E-mail address: [email protected] (Y. Cao). http://dx.doi.org/10.1016/j.ecoleng.2014.09.055 0925-8574/ ã 2014 Elsevier B.V. All rights reserved.
at an annual rate of 4.5 million ha (Stephens and Wagner, 2007; Bremer and Farley, 2010). As plantations become an increasingly ubiquitous land use, an improved understanding of the potential ability of a plantation to perform other ecosystem services is critical when carrying out socially and ecologically sustainable land-use policies (Goldman et al., 2008). These services include biodiversity maintenance and natural regeneration (Aubin et al., 2008; Bremer and Farley, 2010; Duan et al., 2009). Natural forests are commonly considered the reference state when assessing the sustainability of forest management practices (Wesolowski, 2005; Paillet et al., 2009). There is an ongoing global debate over whether the effects of plantations on biodiversity and community succession are more beneficial as compared with those of natural forests (Brockerhoff et al., 2008; Paillet et al., 2009; Lima and Vieira, 2013). In order to clarify the impacts of planted forests on biodiversity and natural
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regeneration, studies have considered the effect of land-use transition as well as the effects of plantation age, tree species, location, land-use history and management (Carnus et al., 2006; Duan et al., 2009; Gonzalez-Alday et al., 2009; Bremer and Farley, 2010). For example, Carnus et al. (2006) concluded that biodiversity within plantations tended to increase over time. However, a metaanalysis carried out in Europe showed that species richness in plantations increased during the first 20 years when compared to natural forests, but declined after that (Paillet et al., 2009). The Loess Plateau of China is susceptible to severe soil erosion and water losses. Consequently, there are many widely distributed large-scale plantations across the plateau that is part of governmental ecosystem restoration and erosion control programs, especially the “Grain for Green Program” launched in 1999 (Sun et al., 2006; Li, 2004). Approximately 5.2 million ha of plantations were established on the Loess Plateau between 1999 and 2008 (Xu et al., 2012). Despite some of the achievements of these ecological restoration programs, a number of scientists have noted that assessments of the ecological success of restoration by afforestation should focus on ecosystem functionality and stability (Sun et al., 2006; Yang et al., 2010; Cao et al., 2011; Deng et al., 2012; Yin and Yin, 2010). Restoration success can be viewed as a continuous process from initial plant establishment to successfully developing those attributes that ensure a selfsustaining and naturally functioning ecosystem (Reay and Norton, 1999). Preliminary field investigations have found that the composition and diversity of understory vegetation in plantations on the Loess Plateau were relatively poor and that the tree seedlings could not readily survive or establish themselves. Furthermore, dried soil layers have been observed under plantations where the water consumption of the tree species, especially non-native species, exceeds soil water recharge (Wang et al., 2012). This represents soil degradation that leads to poor development of forest plantations mainly due to exacerbating water deficits. This suggested that natural restoration could possibly offer a more adaptive and appropriate method of ecological restoration (Chirino et al., 2006; Wang et al., 2002, 2009; Wang, 2010; Wen et al., 2005; Jiao et al., 2012). An increasing number of studies on the Loess Plateau have recently considered the influencing factors that would enable large-scale monoculture plantations to achieve natural succession and functional ecosystems during the ecological restoration process; these factors included forest configurations, habitats, slope gradients and growth age phases (Zhang et al., 2007; Duan et al., 2009; Wang et al., 2009; Zheng et al., 2010; Jiao et al., 2012; Han et al., 2012). For example, species richness and diversity benefits from the main characteristic of shady slopes, i.e., having more available water than sunny slopes that results in less water stress, while the evenness index appears to be increased when water stress and increased light and heat are present, as is the case on sunny slopes (Wang and Zhang, 2009; Wei et al., 2013). Stand density, as one of the influencing factors, also has an effect on stand structural components, on shrub and herb cover, and on species richness and composition. Intensive density management could increase canopy openness to ensure adequate resources for understory development (Wilson and Puettmann, 2007). Understory development is an important factor in soil erosion control since the plants protect the soil surface from both raindrop and throughfall impacts, thereby reducing soil particle detachment, increasing soil surface roughness, which reduces the transport capacity of overland flow, and increase soil stability with their roots. However, it would be necessary to investigate whether density management would be sufficient to promote species diversity and community stability, especially for large old trees (Alaback and Herman, 1988; Wilson and Puettmann, 2007; Wang et al., 2002; Bu et al., 2004). In Oregon, Latham and
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Tappeiner (2002) concluded that there was a positive response of old-growth conifer trees to a wide range of density reduction treatments. As they noted, the vigor of trees can be improved without intensive density reduction. For the response of understory vegetation to stand density, Alaback and Herman (1988) found that over seventeen years after density reduction treatments did not appear to fundamentally change the pattern of understory succession and did not significantly change shrub and herb productivity in Picea–Tsuga forests in Oregon. However, the question remains: what would be the response of mature Pinus tabulaeformis plantations to stand density changes on the Loess Plateau? In order to address this issue, we conducted the present study that investigated the effects of three different stand densities, as the primary influencing factor, on the differences in the species diversity and natural regeneration in mature P. tabulaeformis plantations (more than 40 years after afforestation) in the hilly area of the Loess Plateau. P. tabulaeformis is a preferred tree species that is widely planted on the Loess Plateau for the purpose of soil and water conservation. Furthermore, we investigated clear-cut stands in which P. tabulaeformis seedlings were naturally regenerating for comparison purposes. 2. Material and methods 2.1. Study site The study was conducted at the Yichuan water and soil conservation experimental station (35 590 N, 110 060 E, 900–1200 m a.s.l.), which is located in Yichuan county in northern Shaanxi Province, China (Fig. 1). The station covers approximately 2.94 ha of the hilly area of the Loess Plateau. The study area is in the semi-arid temperate climate zone with an annual mean temperature of 9.7 C. The annual mean rainfall is 523 mm, and approximately 60% of the rain falls during the rainy season from June to September. The annual variation in rainfall ranges from 308 to 843 mm, and brief and intense (1.0 mm min1) rainfall events occur one to three times each year. The annual frost-free season is 180 days. The annual potential evapotranspiration is approximately 1560 mm, which is almost three times greater than the total precipitation. The mean atmospheric humidity is approximately 61.4%. The soil at the study site had a silty clay loam texture: 34.4% clay (<0.01 mm), 61.3% silt (0.01–0.05 mm) and 4.30% sand (>0.05 mm). The mean soil bulk density was 2.64 g cm3, but at the surface (0–10 cm) the value was only 1.14 g cm3. An extensive P. tabulaeformis plantation was implemented over the entire study area in 1966, with an initial planting density of 6000 plants ha1. Thinning was conducted in 1983 and 1995, after which the plantation was left unmanaged. Major shrub species present at the site were Rosa xanthina,Lonicera japonica, and microphylla, and the main herb species were Carex stenophylloides and Artemisia sacrorum. 2.2. Data collection Three experimental blocks, which had been established since 1983 and had similar site conditions (slope: 20–25 ; altitude: 1071–1085 m; aspect: NW; location: middle–upper slope), were used in this current study. The three experimental blocks were close to each other separated by about 200 m. Each block included three 20 m 5 m forested plots with different stand densities and one 20 m 5 m clear-cut plot with no trees (CC). In the current study, all of the plots were investigated during July–August 2011. The three forested plots had stand densities that were low (LD; mean = 1875 trees ha1), medium (MD; mean = 2215 trees ha1),
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Fig. 1. Location of the study site on the Loess Plateau. 1
and high (HD; mean = 3925 trees ha ). The plots were used to count individual trees, which were considered to have a diameter at breast height (DBH) that was greater than 5 cm. Understory plants (DBH <5 cm, including shrubs and P. tabulaeformis seedlings) and herbs were surveyed in three 5 m 5 m and five 1 m 1 m sub-plots, respectively, within each plot. The height and DBH of all of the trees in the plots were measured. All individual species and species coverage in the shrub and herb sub-plots were identified by two observers working together. Information about the P. tabulaeformis seedlings, i.e., the numbers, base diameters, heights and ages, was recorded. In each plot, soil samples were taken from the upper 20 cm soil layer at eight points in an S-shape pattern. The samples were air-dried and mixed together to form a composite sample that was representative of the soil in the plot. Soil organic matter was determined using the Walkley–Black K2Cr2O7–H2SO4 wet oxidation method (Nelson and Sommers, 1996). Total N was measured using the Kjeldahl method (Bremner and Mulvaney, 1982), and total P was determined using the HClO4–H2SO4 colorimetric method (Parkinson and Allen, 1975). Soil samples were also collected at 20 cm depth intervals to a depth of 300 cm in order to determine soil water contents within the 0–300 cm profile. Gravimetric soil water content was determined from the loss in mass when the samples were dried at 105 C in an oven until the mass was constant.
where S is the number of species, Pi is the proportion of individuals or the abundance of the ith species expressed as a proportion of the total in the community, and ln is log base-e. The data were statistically analysed using SPSS (version 20, USA). One-way ANOVA tests were used to compare results for different categories and LSD post hoc tests were conducted to identify significant differences between means. The level of statistical significance was taken as P < 0.05. 3. Results 3.1. Trees growth conditions Generally, tests for normality of tree DBH and height data showed that these data were normally distributed (P < 0.001), with the exception of tree height in the MD stands (Fig. 2). The stand density had a significant effect on the tree DBH (P < 0.01). The mean DBH of P. tabulaeformis tended to decrease with increases in stand density. However, the only significant differences were between the mean DBH of the HD stands (11.4 0.3 mm) and that of both the MD (14.4 0.5 mm) and LD stands (15.1 0.4 mm). Furthermore, stand density had no significant effect on the mean height of P. tabulaeformis although the highest trees were observed in the HD (12.4 0.1 m) stands and slightly shorter trees were found in the MD (11.9 0.2 m) and the LD (11.8 0.2 m) stands.
2.3. Data analysis 3.2. Tree regeneration Indices used to assess plant diversity were species richness, Simpson, Shannon–Wiener and Pielou, as determined from the following equations: Species number as the richness index (S) Simpson index: D ¼ 1 SðPi Þ2 Shannon–Wiener index: H ¼ SðPi ÞlnðPi Þ 0
Pielou index: Jsw ¼
SPi lnPi lnS
The number, age, base diameter and height of P. tabulaeformis seedlings were employed to describe the community regeneration capability. A normality test of the seedling data showed that seedling numbers, ages, base diameters and heights were not normally distributed in the forested P. tabulaeformis stands. However, these parameters were normally distributed for the seedlings in the CC plots (P < 0.05). The number of seedlings varied between 2533 352 (LD) and 4080 783 (MD) plants ha1, but seedling number was not significantly different among the four stands (Fig. 3(A)). Stand density significantly affected the age of seedlings (P < 0.001) such that seedlings were older as stand density decreased (Fig. 3(B)). Significant differences were found among the three planted P. tabulaeformis stands but there was no
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Fig. 2. Frequency distributions of diameter at breast height (DBH) ((A), (B) and (C)) and of tree height ((D), (E), (F)) of Pinus tabulaeformis in plantations with high (HD), medium (MD) and low (LD) stand density; curves fitted by a Gaussian function.
significant difference between the ages of the seedlings in the LD stands and those of the seedlings in the CC stands. However, the seedlings in the CC stands had much greater base diameters (1.2 0.1 cm) that were significantly different (P < 0.001) from those of the seedlings in the three P. tabulaeformis stands with differing stand densities (Fig. 3(C)). Furthermore, the base diameters of seedlings did not differ significantly among the three forested stands with different stand densities. The heights of seedlings in the CC stands were the highest (76.7 9.0 cm), and differed significantly from those in the forested stands (P < 0.001) (Fig. 3(D)). Although the height of seedlings decreased as stand density increased from LD (21.0 2.5 cm) to MD (11.9 1.3 cm), and to HD (6.6 0.7 cm), no significant differences were observed among the three stand densities (Fig. 3(D)). 3.3. Richness and diversity Among the shrub layer’s species, the dominant species with the highest mean coverages were Pyrus betulifolia Bge., Viburnum schensianum, Cotoneaster multiflora and Rosa xanthina in the LD, MD, HD and CC stands, respectively. There were no significant differences among the species richness, diversity indices and evenness index of the four stands (Table 1). However, the species richness and diversity indices in the LD stands were the highest and descended in the order LD > MD > CC > HD. In contrast, the values of the evenness index were the highest in the HD stands, and descended in the order HD > MD > CC > LD. Among the herbaceous plants, the dominant species, having the highest mean coverage, in all of the stands was Carex lancifolia. The species richness, diversity indices and evenness index had the highest values in the CC stands. Species richness tended to decrease with increases in the forested stand density but there were no significant differences among either the forested stands or between each of them and the CC stands (Table 1). Diversity indices of the forested stands did not different significantly among
the different stand densities. However, the diversity indices were both significantly higher in the CC stands than in the forested stands (P < 0.05). The evenness index did not differ significantly among the different forested stands and only the lowest value, which occurred in the MD stands, was significantly lower than that in the CC stands (P < 0.05). 3.4. Soil properties The concentrations of soil organic matter, total N and total P were not significantly different among the three forested stands (Table 2). However, organic matter, total N and total P in the CC stands were all higher than in the forested stands, but the only significant differences observed were for soil organic matter and total N between the CC and HD stands. In contrast, stand density had a significant effect on total K concentration in the forested stands, in that it was significantly lower in the LD than in either the MD or the HD stands (P < 0.05). Furthermore, there was no significant difference in total K concentration between the CC and any of the forested stands. The mean soil water content (0–300 cm) followed the trend: HD (13.17%) < MD (13.34%) < LD (13.61%) < CC (16.92%) (Fig. 4). The mean soil water content in the CC stands was significantly higher than in the forested stands (P < 0.001). Soil water content significantly decreased with soil depth (P < 0.001), and the mean soil water content was significantly different between the CC and the forested stands for soil depths from 140 to 300 cm (P < 0.001). The mean soil water content in the 140–300 cm soil layer was 14.94%, 10.34%, 9.65%, and 9.82% in the CC, LD, MD and HD stands, respectively. There were no significant differences in soil water content among the different forested stand densities (Fig. 4). In the forested stands, the water content was lower and the distribution was relatively constant when the soil was deeper than 120 cm as compared with that of the soil shallower than 120 cm (Fig. 4).
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Fig. 3. Comparison of the number (A), age (B), base diameter (C) and height (D) of Pinus tabulaeformis seedlings in three stand densities (high, HD; medium, MD; low LD) and in clear-cuts (CC). Error bars indicate the standard error of the mean value. Bars having the same lowercase letter above them indicate no significant difference between them (P < 0.05).
4. Discussion There is an intense debate over whether large-scale plantations on the Loess Plateau can achieve self-sustaining forest ecosystems after long-term succession. Forest management practices, such as the choice of tree species, site preparation, plant density, thinning and rotation length, greatly affects species diversity and forest composition, stand structure and ecosystem function (Hunter, 1999). 4.1. Effect of density management on trees growth In this study, we report the effect of density management on tree seedlings, understory plant species diversity, and soil properties in mature P. tabulaeformis plantations that are over 40 years old in the hilly area of the Loess Plateau, China. The growing space available for individual trees in stands is an important factor governing tree and stand vigor. Therefore, stand density has a significant effect on tree DBH (Fig. 2). The mean tree DBH of LD and MD stands were significantly larger than that of the HD stand, although there was no significant difference between the DBH in the LD and MD stands. A similar result was found by
Latham and Tappeiner (2002), who did not find a significant difference in old-growth conifer trees in low and medium stand density categories in Oregon. In addition, the influence of density or thinning management on the increment of tree growth is expected to decline with time as the stand canopy closes (Soucy et al., 2012). The normal distributions of tree DBH and height indicated that the local weather conditions in the study area were suitable for the growth requirements of P. tabulaeformis (Fig. 2). However, dryer sites located in the northern Loess Plateau have been found to be unsuitable for P. tabulaeformis growth resulting in distributions of tree DBH and height that were clearly not normal (Wang, 2010). 4.2. Effect of density management on P. tabulaeformis seedlings The seedling stage is a crucial part of the plant life cycle and it is important in natural vegetation restoration. The regeneration performance of plantations as a restoration strategy may differ depending on initial species composition, planting density, and site conditions (Sansevero et al., 2011). In this study, the stand density only had a significant effect on the age of P. tabulaeformis seedlings (Fig. 3(B)). This could be attributed to the limited time in which
Table 1 Species diversity indices of the shrub and herb layers under three Pinus tabulaeformis plantation stand densities and in clear-cut (CC) stands. a
Stands
HD MD LD CC a b
b
Shrub layer
Herb layer
Species richness
Simpson
Shannon–Wiener
Pielou index
Species richness
Simpson
Shannon–Wiener
Pielou index
6.00 1.2a 8.67 1.3a 10.00 1.0a 9.33 1.9a
0.36 0.1a 0.45 0.1a 0.60 0.02a 0.52 0.1a
0.56 0.2a 0.80 0.2a 1.00 0.04a 0.83 0.1a
0.95 0.03a 0.92 0.02a 0.87 0.08a 0.87 0.04a
9.33 2.8a 11.5 2.5a 13.00 1.2a 18.00 2.0a
0.28 0.07a 0.24 0.03a 0.35 0.02a 0.63 0.06b
0.54 0.16a 0.50 0.08a 0.62 0.16a 1.30 0.09b
0.56 0.09ab 0.34 0.05a 0.51 0.01ab 0.74 0.09b
HD, high density (3925 trees ha1); MD, medium density (2215 trees ha1); LD, low density (1875 trees ha1) and CC, clear-cut stands. Data is reported as mean standard error (n = 3); within a column, values followed by the same lowercase letter indicate that they did not differ significantly (P < 0.05).
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Table 2 Soil nutrient contents in the 0–20 cm soil layer under three Pinus tabulaeformis plantation stand densities and in clear-cut (CC) stands. a
Stands
HD MD LD CC a b
b
Organic matter (g/kg)
14.68 0.3a 18.35 1.0ab 19.89 1.8ab 23.15 1.0b
Total N (g/kg)
Total P (g/kg)
Total K (g/kg)
1.24 0.05a 1.27 0.1ab 1.61 0.09ab 1.86 0.2b
0.16 0.02a 0.17 0.03a 0.18 0.01a 0.20 0.01a
19.85 0.2a 19.86 0.2a 18.62 0.4b 18.86 0.2ab
HD, high density (3925 trees ha1); MD, medium density (2215 trees ha1); LD, low density (1875 trees ha1) and CC, clear-cut stands. Data is reported as mean standard error (n = 3); within a column, values followed by the same lowercase letter indicate that they did not differ significantly (P < 0.05)
stand density would have a great effect on the number of seedlings, i.e., within just 1–3 years after thinning operations (GonzalezAlday et al., 2009). In the CC stands, the higher soil water and nutrient contents enhanced seedling growth so that the base diameter and height of the seedlings were significantly greater than those of seedlings in the forested stands (Figs. 3 and 4; Table 2). It is noteworthy that the number of P. tabulaeformis seedlings in this study’s plantations was greater than that of seedlings in natural P. tabulaeformis forests close to the study area. Furthermore, the number of P. tabulaeformis seedlings in plantations increased with age for at least 40 years (Zhang et al., 2008, 2009). This demonstrates that the plantations were exhibiting a greater amount of regeneration over the forty years than the natural forest was. Environmental conditions on the Loess Plateau, especially rainfall, are the key factors that determine seedling survival and tree regeneration during ecosystem restoration (Wang, 2010). As annual precipitation decreases from south to north across the Loess Plateau, the number of P. tabulaeformis seedlings decreases and their growth is adversely affected (Wang, 2010). There are examples where there are no seedlings at all in P. tabulaeformis plantations in the north of the Loess Plateau (Wang, 2010). Therefore, Wang (2010) suggested that the region where annual precipitation was greater than 550 mm was suitable for the cultivation and regeneration of P. tabulaeformis plantations on the Loess Plateau. 4.3. Effect of density management on plant species diversity This study demonstrated that the effect of stand density did not significantly affect the species richness, diversity indices and the evenness index of either the shrub or herb layers (Table 1). This can be explained by the decline over time of the influence of stand density on the understory species diversity that occurs as the stand
canopy closes (Soucy et al., 2012). Furthermore, the herb layer was generally less responsive to stand density than the shrub layer (Wilson and Puettmann, 2007). Over time, plantation habitats become increasingly complex, which benefits forest species (Paillet et al., 2010). Compared with natural forests, the mature P. tabulaeformis plantations displayed similar species diversity (Wang et al., 2013). Shrub cover plays a very important role in erosion processes (Selkimäki et al., 2012), and it has been found to be inversely correlated with mean elevation and related to the basal area of dominant species in Spain (Coll et al., 2011; Selkimäki et al., 2012). Mean shrub cover in the present study was only 15%, ranging from 8–20%. There were no significant a difference among the four stands and explained no relation with stand basal area probably. In addition, plot size and number of samples collected in the present study was limited and only large differences among species richness, diversity indices and the evenness index could be detected. Therefore, it is interesting to expand the research in other studies under different site conditions in order to develop a more comprehensive understanding of ecological restoration in plantations on the Loess Plateau. Clear-cutting is a controversial forest management practice in terms of its effect on the forest ecology. Therefore, it is of interest to investigate the impacts of clear-cutting on the diversity of the forest structure and on the forest ecological processes (Keenan and Kimmins, 1993). Clear-cutting results in six patterns of species diversity over time: an increase with succession; a decrease during succession; an increase followed by a decrease; a decline followed by an increase; peaks in the early and later succession; and diversity being independent of succession (Schoonmaker and McKee, 1988; Keenan and Kimmins, 1993; Gonzalez-Alday et al., 2009). A high species richness, diversity indices and evenness index in both the shrub and the herb layers were found in this study’s CC stands (Table 1). The number of herb and shrub species has been reported to increase following clear-cutting in southern pine stands in Texas and Florida, but the number of understory plants decreased as the tree canopy closed (Keenan and Kimmins, 1993). However, increasing the proportions of non-crop vegetation could reduce timber yields. 4.4. Effect of density management on soil nutrient and water content
Fig. 4. Soil water content within the 0–300 cm soil profiles under Pinus tabulaeformi plantations with three stand densities (high, HD; medium, MD; low LD) and in clear-cuts (CC).
Nutrient circulation and water availability are important factors in forest ecosystem restoration and its processes, in addition to the photosynthetic rate and net primary production. Stands planted at higher densities need to use and uptake greater amounts of the soil resources, i.e., nutrients and water (Will et al., 2005). Therefore, the values of total N and soil water content followed the trend: HD < MD < LD < CC (Table 2 and Fig. 4). Soil organic matter also followed the same trend even though stands with higher densities might also produce greater amounts of litter that would become incorporated into the soil; however, in this case, the use of soil organic matter exceeded its production. Higher soil water contents in the CC stands, especially in the deeper soil layers, were primarily due to less root mass, reduced evapotranspiration and were only
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slightly impacted by increases in temperature. Soil water on the Loess Plateau is one key factor that limits vegetation development and recovery. Current evidence indicates that plantations have significant negative effects on soil desiccation and negative impacts on the long-term sustainability of plantation ecosystems (Sun et al., 2006; Cao et al., 2011). In this study, it is probable that a dried soil layer has been developing below 200 cm under all of the forested stands (Fig. 4), which could be detrimental to the future development of the plantation (Wang et al., 2012). Given that the annual evapotranspiration (1560 mm) exceeds the usual rainfall amount (308–843 mm), the planted trees have been relying on water stored in deep soil layers, which cannot be fully recharged except under clear-cuts as noted above, and the trees may begin to experience water stress. Therefore, experts have suggested that natural revegetation offers a more adaptive and appropriate method of ecological restoration (Jiao et al., 2012). However, some studies have shown that there were no significant differences among the soil nutrients between plantations and natural sites (Jiao et al., 2012; Zheng et al., 2010). A previous study showed that obvious increases in soil nutrients occurred for a period of time after clear-cutting because plant uptake was eliminated and decomposition continued or even increased (Keenan and Kimmins, 1993). However, soil nutrient levels returned to, or were lower than, pre-felling levels after 4 years. 5. Conclusion The large plantation areas on the hilly Loess Plateau effectively conserve both water and soil by reducing runoff and erosion, and enhance carbon sequestration within the forest ecosystem. We can conclude that within the range of stand densities considered in this study, which despite thinning were still high, there was little difference in the light and soil resources available to understory species and, thus, the effects of stand density did not significantly affect the diversity of understory species and tree regeneration. Our comparison with CC stands suggests that a lower stand density would better integrate the conservation of species diversity and ecosystem functions and the replenishment of soil fertility and water with wood production. To address the compromise between biodiversity conservation and timber production in plantations, further study is needed to determine plantation potential and mechanisms in order to provide basic forest ecological attributes and to promote the sustainable management of plantations on the Loess Plateau. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 41201088), the Doctoral Fund of the Ministry of Education of China (No. 20120204120014), and the West Light Foundation of the Chinese Academy of Sciences. We thank the staff at the Yichuan Water and Soil Conservation Experimental Station for valuable assistance. References Alaback, P.B., Herman, F.R., 1988. Long-term response of understory vegetation to stand density in Picea–Tsuga forests. Can. J. Forest Res. 18 (12), 1522–1530. Aubin, I., Messier, C., Bouchard, A., 2008. Can plantations develop understory biological and physical attributes of naturally regenerated forests? Biol. Conserv. 8, 2461–2476. Bremer, L.L., Farley, K.A., 2010. Does plantation forestry restore biodiversity or create green deserts? A synthesis of the effects of land-use transitions on plant species richness. Biodivers. Conserv. 19, 3893–3915. Bremner, J.M., Mulvaney, C.S., 1982. Nitrogen-total. In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties. Am. Soc. Agron., Madison, WI, pp. 595–624.
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