Elevational Distribution of Adult Trees and Seedlings in a Tropical Montane Transect, Southwest China

Montane habitats are characterized by high variation of environmental factors within small geographic ranges, which offers opportunities to explore how forest assemblages respond to changes in environmental conditions. Understanding the distributional transition of adult trees and seedlings will provide insight into the fate of forest biodiversity in response to future climate change. We investigated the elevational distribution of 156 species of adult trees and 152 species of seedlings in a tropical montane forest in Xishuangbanna, southwest China. Adult trees and seedlings were surveyed within 5 replicate plots established at each of 4 elevational bands (800, 1000, 1200, and 1400 m above sea level). We found that species richness of both adult trees and seedlings changed with elevation, showing a notable decline in diversity values from 1000 to 1200 m. Tree species composition also demonstrated distinct differences between 1000 and 1200 m, marking the division between tropical seasonal rain forest (800 and 1000 m) and tropical montane evergreen broad-leaved forest (1200 and 1400 m). The results suggested that soil moisture and temperature regimes were associated with elevational distribution of tree species in this region. We also observed that seedlings from certain species found at high elevations were also distributed in low-elevation zones, but no seedlings of species from low elevations were distributed in high-elevation zones. The increase in temperature and droughts predicted for this region may result in the contraction of tropical seasonal rain forest at lower elevations and a downhill shift of higher tropical montane tree species.


Introduction
Understanding species distribution along spatial and environmental gradients is one of the core aims of ecological and biogeographical research (Whittaker 1960;Itow 1991;Wolf 1993;Vazquez and Givnish 1998;Buckley and Jetz 2008). The elevational gradient encompasses multiple environmental gradients within very short geographic distances (K€ orner 2007), offering opportunities for investigating the distribution of species in response to changing environments (Lomolino 2001;Grytnes and McCain 2007;K€ orner 2007).
Elevational distribution of plants has been widely reported (eg Van Steenis 1984;Kessler 2000;Hemp 2006;Jones et al 2011;Martin et al 2011). However, the patterns of species elevational distribution vary depending on the study locations, because they are likely to be influenced by multiple factors, including human and natural disturbances (eg Pereira et al 2007;Angelo and Daehler 2013), biotic interactions (reviewed by Anthelme and Dangles 2012), edaphic variables (eg Sanchez et al 2013), topography (eg Bohlman et al 2008), moisture (eg Patten and Ellis 1995;Leuschner 2000;Oliveira-Filho and Fontes 2000;You et al 2013), and temperature (eg Vazquez and Givnish 1998;Salas-Morales and Meave 2012). It is therefore important to account for multiple factors that may be related to plants' distribution patterns along elevational gradients.
In response to recent climate change, many species have been observed to shift their distributional ranges (Parolo and Rossi 2008;Hillyer and Silman 2010;Chen et al 2011;Meier et al 2012). In general, tree species on montane gradients are predicted to shift their distributions upslope due to increased temperature (eg Feeley et al 2011). However, Crimmins et al (2011) suggested that tree species in California forests are likely to move downhill as increased precipitation may render drier, lower elevations more suitable for high-elevation  Ledo et al (2009) also found that the dryer conditions at low elevations led to the migration of tropical cloud forest to gullies and higher elevations with humid conditions. By measuring environmental factors and elevational distributions of species, future impacts of climate change on the diversity of local forests may be predicted.
Some studies have also examined elevational distributions of seedlings under forest canopy, which can indicate potential future changes of forest structure (Lenoir et al 2009;Vitasse et al 2012). Tree seedlings were found to migrate upslope at faster rates than their adult counterparts (Lenoir et al 2009). The distribution of seedlings is also considered to reflect changes in climatic factors (Vera 1997;Ross et al 2012;Marcora et al 2013).
The seedling assemblage plays a key role in forest regeneration (Teketay 1997;Li et al 2010;Bace et al 2012), and range extension of tree species may be indicated by seedling migration under climate change scenarios. However, the elevational distribution of tree seedlings may not be the same as those of adult trees. Seeds of some species may be distributed far from parent trees to other less suitable elevations and sustain their populations as seedling banks (Lenoir et al 2009). Those seedlings may mature and change the forest structure significantly when the habitat becomes more favorable due to climate change.
Most studies of tree and seedling migration have been carried out in temperate forests (eg Bansal and Germino 2010;Urbieta el al 2011), with very few reported from tropical forests (Rehm and Feeley 2013). This study aimed to help fill that gap with an investigation in Xishuangbanna Dai Autonomous Prefecture in southwestern China, an area rich in biodiversity across various types of seasonal tropical forests (Cao and Zhang 1997;Zhu et al 2006). A number of studies of forest species composition and assemblage structures have been conducted in this region (Wu 1980;Wu et al 1987;Zhang and Cao 1995;Cao and Zhang 1997;Zhu et al 2005;. In Xishuangbanna, vegetation below 800 m in elevation has been classified as tropical seasonal rain forest, vegetation between 1000 m and 1200 m as tropical montane rain forest, and vegetation between 1200 m and 1400 m as tropical montane evergreen broad-leaved forest (Wu et al 1987). However, this classification is based on the elevational distribution of adult trees, and the classification of seedlings has not yet been quantitatively described within this region.
We investigated changes in the composition of adult tree and seedling species along an elevational transect in this region to address the following questions: 1. What are the elevational species richness and composition patterns of adult trees and seedlings? Do adult tree and seedling species have the same range of elevational distributions? 2. How does the composition of tree species respond to variation in environmental factors (eg temperature and soil moisture) as a consequence of elevational change? 3. What are the potential effects of local climate change scenarios on the distribution of tree species in mountains?

Study site
Xishuangbanna Dai Autonomous Prefecture (21808 0 -22835 0 N, 99856 0 -101850 0 E) borders Myanmar in the southwest and Laos in the southeast (Figure 1). The region has a mountainous topography, with mountain ridges running in a north-south direction and a southward decrease in elevation. It has a typical monsoon climate with distinct wet and dry seasons. Mean annual air temperature and rainfall are 21.88C and 1493 mm, respectively. Approximately 80% of the annual rainfall occurs in the wet season, which lasts from May to October. The frequent occurrence of heavy fog in the dry season in the lower hills and valleys may assist the expansion of the northern limit of tropical rain forest to this region .

Data collection
In August 2012, we established a transect consisting of 4 elevational bands (800, 1000, 1200, and 1400 m above sea level- Figure 1 and Table S1, Supplemental material, http:// dx.doi.org/10.1659/MRDJOURNAL-D-15-00109.S1). in the Xishuangbanna National Nature Reserve. Elevations below 800 m above sea level were not included in this study, as the forests were cleared or heavily disturbed. Within each elevational band, we set up 5 replicate plots of 20 m 3 20 m that were spaced approximately 200 m from each other. Locations impacted by anthropogenic and natural disturbances, such as large canopy gaps, were avoided. Within each plot, all trees with a diameter at breast height !5 cm were measured and tagged. Most trees were identified on-site by local botanists; those that could not be identified were collected and brought back to Xishuangbanna Tropical Botanical Garden herbarium for further determination. In December 2013, five 1 m 3 1 m subplots were established at the 4 corners and the center of each 20 m 3 20 m plot to survey seedlings. Within each subplot, all stems of tree species ,1 cm diameter at breast height (hereafter referred to as seedlings) were tagged and identified by species. We recorded hourly temperatures from May 2013 to April, 2014 using a thermo-logger (DS1923 Hygrochron iButton, Maxim, CA, USA) set at the height of 1.3 m in the vicinity of each plot. Due to technical problems, no temperature measurements at 1200 m were made in July and August, and thermo-loggers at three 1000 m plots were lost during the dry season. We measured soil moisture at 5 cm below the ground at 5 randomly selected points within each subplot (a total of 25 measurements per plot) using a conductivity probe (Theta probe MPM-160B, ICT International Pty Ltd, Armidale, New South Wales, Australia) at the end of the wet (December 2013) and dry (April 2014) seasons. We used a geologic compass (HRB DQY-1, Harbin Optical Instrument Factory, Heilongjiang Province, China) to measure the slope and aspect of each plot.

Data analysis
The seedling data collected from the five 1 m 3 1 m subplots in each plot were pooled before analysis. The abundances of the adult trees and the seedlings were logtransformed before calculating 3 species diversity indices: the Shannon-Wiener index, Simpson index, and Pielou evenness index (Magurran 1988).
To test whether diversity and species richness differed in the different elevational bands, we used one-way analysis of variance (ANOVA) with post hoc Tukey's tests. Differences in assemblage composition were investigated using nonmetric multidimensional scaling (NMDS) ordinations with 10 random restarts (Minchin 1987), based on 4 types of similarity matrices: the Jaccard, Bray-Curtis (using log-transformed abundances), Chao-Jaccard (Chao et al 2005), and Raup-Crick (Chase et al 2011) indices. All species that occurred in 1 site only were removed before calculating similarity values to reduce overestimation of beta-diversity. Permutational multivariate analysis of variance (PERMANOVA) was used to test if the species compositions of adult trees and seedlings were significantly different among the 4 elevational bands (Anderson 2001;McArdle and Anderson 2001).
We adopted an information-theoretic approach (Burnham and Anderson 2002) to test whether assemblage composition of adult and seeding species was related to temperature, soil moisture, or other environmental factors, including elevation and geographic factors. We generated 12 environmental variables, which were grouped into 5 classes with similar attributes (temperature, soil moisture, elevation, topography, and spatial arrangement). For temperature data, we first calculated the average air temperature during the day (7:00 to 18:00 h) and at night (19:00 to 6:00 h) of each day in January-April (the dry season). These values were then used to calculate mean, maximum, and minimum air temperatures and their coefficient of variation. Wet season temperature data were not used due to the lack of data at 1200 m (explained earlier) and negligible differences between elevational bands in this season ( Figure 2). Soil moisture measurements within each plot were averaged to generate wet-season (December) and dry-season (April) soil moisture and its seasonal variation. Topography consisted of slope and aspect measured within each plot. Spatial arrangement of the plots was quantified using the primary and secondary vectors of the principal coordinates of neighbor matrices (PCNM1 and PCNM2), which showed positive spatial correlation based on Moran's I (Dray et al 2006;Legendre and Legendre 2012).
We employed a model averaging method (Burnham and Anderson 2002) to investigate the relative importance of individual predictor variables in explaining variation in the composition of adult tree and seedling species. Using  (2012), we selected plausible predictor variables by testing whether the sum of the Akaike weights of each predictor variable was significantly greater than the summed Akaike weights obtained from a null distribution of the plots. The null model was generated by permuting plots (not individuals), as this preserved the tree species composition, and thus species interactions, of each plot. We compared the observed Akaike weights with those derived from 999 permutations. Due to the loss of temperature thermo-loggers from 3 plots at 1000 m during the dry season, we included only 17 plots (instead of 20) for the analysis using all 12 predictors. We then reran the same analysis using all 20 plots but excluding temperature predictors. Abundances were square-root transformed before analysis (Nakamura et al 2015).
To investigate differences (and similarities) in the elevational distributions of adult tree and seedling species, we used the indicator value protocol developed by Dufrêne and Legendre (1997), using the labdsv R package version 1.6-1. We calculated the indicator value of a given species for single elevational bands (eg 800 m, 1200 m) and all possible sequential elevational bands (eg 800-1000 m, 1000-1400 m) omitting the entire elevational range (ie 800-1400 m), giving a total of 9 indicator values per species. A random reallocation procedure with 4999 permutations was performed to test for the significance of indicator values. We selected the species with significant indicator values greater than 60% (a subjective benchmark value). We also calculated the importance value, which additively summarizes the relative abundance and the relative frequency of the selected adult and seedling tree species in each of the elevational bands (Curtis and McIntosh 1951).

Results
We recorded a total of 1034 adult trees (diameter at breast height ! 5 cm) representing 156 species from 103 genera and 51 families. A comparable number of seedlings (1105 individuals) were recorded, representing 152 species from 121 genera and 57 families. The most dominant adult tree species within each elevational belt were Parashorea chinensis at 800 m, Alseodaphne petiolaris at 1000 m, Castanopsis echinocarpa at 1200 m, and Castanopsis mekongensis at 1400 m. The most dominant seedling species were Pittosporopsis kerrii at 800 m, C. echinocarpa at 1000 m and 1200 m, and Castanopsis calathiformis at 1400 m. A large number of adult tree and seedling species were relatively rare, with distributions in only 1 elevational band (Tables  S2 and S3, Supplemental material, http://dx.doi.org/10.1659/ MRDJOURNAL-D-15-00109.S1

Diversity and species composition across elevations
Individual-based rarefaction curves showed greater observed and estimated (by extrapolation) species richness at lower (800 and 1000 m) than higher (1200 and 1400 m) elevational bands for both adult trees and seedlings ( Figure  3). Within each of the lower and higher elevational bands, the differences in species richness were indistinguishable, with overlapping 95% confidence intervals. This was also statistically confirmed, with the results of ANOVA showing both adult trees and seedlings having higher species richness within lower elevational bands (F ¼ 67.56 for adult trees, F ¼ 39.77 for seedlings; see also Figure 4).
Regardless of the similarity indices used, NMDS ordinations showed clear elevational stratification in both adult tree and seedling species composition (Figure 3). PERMANOVA confirmed significant elevational differences (P , 0.001) for both adult tree and seedling species composition. All of the ordinations showed a notable gap between the low elevations (800 and 1000 m) and the high elevations (1200 and 1400 m), and the sample dispersion was generally greater at the low elevations.

Temperature and soil moisture
Mean monthly temperature showed little difference in May-August among the elevational bands (Figure 2).
Although the temperature was highly variable in September-December, it was generally warmer in the lower elevational bands. In contrast, temperatures in January-April were cooler at 800 m than at other elevational bands.
We found significant differences in soil moisture among the elevational bands in both wet and dry seasons (F ¼ 7.109 and F ¼ 13.88 in wet and dry seasons, respectively). Post hoc tests showed that the wet season soil moisture was significantly greater at lower (800 and 1000 m) elevational bands ( Figure 5). This pattern is consistent with those of adult and seedling species richness ( Figure 4) and assemblage composition (Figure 3). In the dry season, only the soil moisture at 800 m was significantly higher, and soil moisture at 1000 m was not different from that at 1200 and 1400 m.

Relationships with environmental factors
For both partial (17 plots with 12 predictors) and full (20 plots with 8 predictors) datasets, the results of the information-theoretic approach suggested a number of plausible predictors that may explain the assemblage composition of adult and seedling species. Due to the spatially aggregated nature of the elevational transect, the spatial predictor (PCNM1) was plausible for both adult trees and seedlings (Tables 1 and 2). Elevation was also a plausible predictor for both adult trees and seedlings, with the exception of adult trees when using the full dataset (20 plots). No environmental predictors were selected for seedling assemblage composition, whereas mean temperature and dry season soil moisture were significantly associated with adult trees when using partial and full datasets, respectively.

Elevational distribution of adult trees and seedlings
We found 20 and 16 significant indicator species for adult trees and seedlings, respectively (Table 3). Of these, 19 were indicative of only 1 elevation (10 and 12 for adult trees and seedlings, respectively), 15 were indicative of 2 elevations (10 and 5 for adult trees and seedlings, respectively), and none was indicative of 3 or 4 elevations. All indicator species were restricted to either low (800 and 1000 m) or high (1200 and 1400 m) elevations. The only exception was Aporosa yunnanensis, whose adult trees and seedlings indicated elevations of 1000 and 1200 m. When both the adult trees and the seedlings of the same species were found to be indicators, their indicated elevations generally matched, with the exception of C. echinocarpa, whose seedlings showed a narrower elevational range than the adult trees.

Diversity and species composition across elevations
We found a marked decrease in species richness from 1000 to 1200 m for both the adult tree and the seedling assemblages ( Figure 4 and Table 4). Such a step-wise change in diversity patterns was also reported from other tropical mountains in Malesia (Van Steenis 1984), Borneo (Aiba and Kitayama 1999), Bolivia (Kessler 2000), Tanzania (Hemp 2006), and the Caribbean (Martin et al 2011).
Furthermore, we found a clear division in assemblage composition between low (800 and 1000 m) and high (1200 and 1400 m) elevations (Figure 3). This division was also evident in the indicator species, where only 1 species (A. yunnanensis) was indicative of both low and high elevational bands. Our results were consistent with previous research showing that large members of tropical tree species were restricted to a single forest type (Pitman et al 1999). The dominant tree species at low and high elevations belong to the genera Parashorea and Castanopsis, respectively (Table 3). These findings were consistent with previous studies based on adult trees (Wu et al 1987;Ohsawa 1993;Zhu et al 2006). The relative narrowness of the elevational distributions of dominant tree species is caused by opposing environmental constraints (Williams et al 2010;Arellano and Mac ıa 2014). The vegetation changes observed in our study were likely influenced by strong seasonal dryness (and perhaps temperaturediscussed later), with tropical rain forest occurring in wet valleys and tropical montane evergreen broad-leaved forest occurring in dry montane habitats (Zhu et al 2005).
Species composition was more similar for both adult trees and seedlings at high elevations ( Figure 3). This may have resulted from drier environmental conditions at high elevations acting as an environmental filter and reducing species richness (Zhu et al 2005;Cao et al 2006). Aggregation of high-elevation plots was observed in NMDS ordinations of all 4 similarity indices, including the  Raup Crick index (a probabilistic measure unaffected by the number of species-Chase et al 2011), suggesting that the high similarity of these plots is not a simple artifact caused by depauperate flora at these elevations.

Associations with environmental factors
Elevation is a suitable proxy for varying environmental factors, as the assemblage composition of organisms and environmental characteristics generally change with   For adult trees, we found that mean temperature and dry-season soil moisture were plausible predictors, significantly associated with assemblage composition. Temperature, generally speaking, decreases linearly with elevation (Barry 1992). Our results, however, showed much lower mean temperature at 800 m (17.5 6 0.34SE) than at higher elevations (20.0 6 0.02, 19.4 6 0.18, and 18.4 6 0.07 at 1000, 1200, and 1400 m, respectively) in the dry season ( Figure 2). Several of the previous surveys in this area reported similar patterns in temperature (Guo and Zhong 1986;Zhong and Guo 1997). As the 800 m plots were all located within a mountain valley, our results may be attributable to cool air draining down into the valley and lifting the warm air to higher elevations (Miller et al 1983;Goulden et al 2006). However, our results have caveats because the analysis included partial data (n ¼ 17), which excluded 3 of the 1000 m plots due to loss of the thermo-loggers. Using the full 20 plots, we found that soil moisture in the dry season was a plausible environmental factor (Table 2). Soil moisture was significantly higher at 800 m than at other elevational bands at the end of the dry season, perhaps due to the presence of heavy fog in the lower valley . Although similar results were found during the wet season, where soil moistures at both 800 and 1000 m were significantly higher than at 1200 and 1400 m, wet season soil moisture was not selected as plausible. This implies that limited soil moisture availability during the dry season may shape the elevational distribution of adult trees in this area. Water shortage has been associated with the distribution of tree species in many parts of the world. For example, in tropical montane forest, gullies and high elevations with light water stress are dominated by humidcondition species, while low elevations are dominated by dry-condition species (Ledo et al 2009). In subtropical forest, soil moisture drives change in elevational vegetation types (You et al 2013). This could even be aggravated in mountains dominated by monsoon climate. In the temperate forest, drought resistance ability significantly influenced the elevational positions of 5 pine species (Barton and Teeri 1993). In Panama, soil water availability was considered to be the main factor controlling the distribution of tropical tree species (Bunker and Carson 2005;Engelbrecht et al 2007), perhaps because the survival and growth of seedlings is constrained by the severity of drought in the dry season (Engelbrecht et al 2005;Slot and Poorter 2007;Comita and Engelbrecht 2009). Our study suggested that moisture availability and temperature regime, which are caused by the change of elevation, may determine the elevational distribution pattern of adult tree species.
We found that seedling composition, in contrast to that of adult trees, was only associated with elevation and spatial arrangement. Soil moisture and temperature were not selected as plausible predictors of seedling composition (Tables 1 and 2). The lack of plausible environmental predictors for seedling assemblage composition may suggest that environmental filtering only operates while trees are maturing (Grubb 1977;Poorter et al 2008). Our result suggests that the seedlings of high-elevation tree species established at low elevations may play an important role in shifting species ranges to stay in a suitable environmental envelope.

Elevational distributions of adult trees versus seedlings
We observed very similar elevational distributions for adult trees and seedlings of most indicator species (Table  3). Zhu et al (2014) also reported that most tree species in North America are not yet migrating in response to climate change. In our study, we found several tree species that dominate the evergreen broad-leaved forest at 1200 and 1400 m, such as C. echinocarpa and C. calathiformis, occurring at lower elevations where the tropical rain forest is distributed. However, we did not observe seedlings of tree species dominant at 800 m and 1000 m distributed in higher elevations. Under potential climate change regimes, these changes in the elevational distribution of seedlings may expand the range of tree species in the future and ultimately alter forest structure and processes. This suggests that the species currently dominant at high elevations might expand their range downward if future climate change causes habitats at lower elevations to become more suitable for their establishment (eg dryer). However, under the same climate change scenarios, species currently dominant at low elevations are unlikely to expand upslope because their seedlings are less adapted for the dryer habitats found at high elevations.
In a climate warming scenario, shifts in distribution range of seedlings may lead to shifts in the distribution range of tree species, which ultimately influences the structure and dynamics of the forest (Lenoir et al 2009; Rehm and Feeley 2013). Previous studies indicated that some species in temperate areas shifted their distribution to higher elevations because of climate warming (Wilson et al 2005;Lenoir et al 2008). In some tropical forests, climate warming is considered to be the main factor driving tree species to migrate upslope Savage and Vellend 2015). However, such a situation may not take place in our study region, as the relatively dryer conditions at higher elevations may limit the uphill shift of tree species in tropical rain forests. Our results suggest that, as this region is predicted to become drier and hotter with climate change (He and Zhang 2005), the tropical rain forests at lower elevations may contract their distribution ranges as the habitats at lower elevations become drier, and species at higher elevations may shift their distributions downward. Habitat modification can also be an alternative mechanism causing downslope range shifts (Lenoir et al 2010). A study in California suggested that species shift downhill in response to regional changes in climatic water balance (Crimmins et al 2011). Furthermore, our research region may face a significant loss of species diversity if tree species cannot shift their elevational distributions as quickly as climate change occurs (Corlett and Westcott 2013). Some tree species of the Fagaceae family may expand their local distribution. High-elevation species such as C. echinocarpa and C. calathiformis could likely shift to low-elevation areas, but the representative tree species of low-elevation tropical rain forests, such as P. chinensis and Baccaurea ramiflora, may narrow their distribution range. Under climate change regimes, this downhill migration of high-elevation species might also occur in other tropical and subtropical montane forests dominated by monsoon climate.

Conclusion
A step-wise decline in diversity was observed in this tropical montane forest. Our study also observed that a large number of tropical tree species are restricted to a narrow elevational range, and environmental filtering regulates species distribution while trees are maturing. Furthermore, we found a trend in which the seedlings of high-elevation species migrate downhill, suggesting that local climate change might result in the contraction of tropical rain forest at lower elevations and the expansion of higher tropical montane tree species downhill. Longterm monitoring of seedling dynamics and a transplant experiment would help us to predict potential changes in tree species distribution under climate change regimes, and this is strongly recommended for future research. All found at DOI: 10.1659/MRD-JOURNAL-D-15-00109.S1 (145 KB PDF).