Precipitation and nitrogen addition enhance biomass allocation to aboveground in an alpine steppe

There are two important allocation hypotheses in plant biomass allocation: allometric and isometric. We tested these two hypotheses in an alpine steppe using plant biomass allocation under nitrogen (N) addition and precipitation (Precip) changes at a community level. An in situ field manipulation experiment was conducted to examine the two hypotheses and the responses of the biomass to N addition (10 g N m^?2 y^?1) and altered Precip (±50% precipitation) in an alpine steppe on the Qinghai–Tibetan Plateau from 2013 to 2016. We found that the plant community biomass differed in its response to N addition and reduced Precip such that N addition significantly increased aboveground biomass (AGB), while reduced Precip significantly decreased AGB from 2014 to 2016. Moreover, reduced Precip enhanced deep soil belowground biomass (BGB). In the natural alpine steppe, the allocation between AGB and BGB was consistent with the isometric hypotheses. In contrast, N addition or altered Precip enhanced biomass allocation to aboveground, thus leading to allometric growth. More importantly, reduced Precip enhanced biomass allocation into deep soil. Our study provides insight into the responses of alpine steppes to global climate change by linking AGB and BGB allocation.     

Biomass allocation and productivity–richness relationship across four grassland types at the Qinghai Plateau

Aboveground biomass (AGB) and belowground biomass (BGB) allocation and productivity–richness relationship are controversial. Here, we assessed AGB and BGB allocation and the productivity–richness relationship at community level across four grassland types based on the biomass data collected from 80 sites across the Qinghai Plateau during 2011–2012. The reduced major axis regression and general linear models were used and showed that (a) the median values of AGB were significantly higher in alpine meadow than in other three grassland types; the ratio of root to shoot (R/S) was significantly higher in desert grassland (36.06) than intemperate grassland (16.60), alpine meadow (13.35), and meadow steppe (19.46). The temperate grassland had deeper root distribution than the other three grasslands, with about 91.45% roots distributed in the top 30 cm soil layer. (b) The slopes between log AGB and log BGB in the temperate grassland and meadow steppe were 1.09 and 1, respectively, whereas that in the desert grassland was 1.12, which was significantly different from the isometric allocation relationship. A competitive relationship between AGB and BGB was observed in the alpine meadow with a slope of ?1.83, indicating a trade‐off between AGB and BGB in the alpine meadow. (c) A positive productivity–richness relationship existed across the four grassland types, suggesting that the positive productivity–richness relationship might not be affected by the environmental factors at the plant location. Our results provide a new insight for biomass allocation and biodiversity–ecosystem functioning research.     

Effects of soil C:N:P stoichiometry on biomass allocation in the alpine and arid steppe systems

Soil nutrients strongly influence biomass allocation. However, few studies have examined patterns induced by soil C:N:P stoichiometry in alpine and arid ecosystems. Samples were collected from 44 sites with similar elevation along the 220‐km transect at spatial intervals of 5 km along the northern Tibetan Plateau. Aboveground biomass (AGB) levels were measured by cutting a sward in each plot. Belowground biomass (BGB) levels were collected from soil pits in a block of 1 m × 1 m in actual root depth. We observed significant decreases in AGB and BGB levels but increases in the BGB:AGB ratio with increases in latitude. Although soil is characterized by structural complexity and spatial heterogeneity, we observed remarkably consistent C:N:P ratios within the cryic aridisols. We observed significant nonlinear relationships between the soil N:P and BGB:AGB ratios. The critical N:P ratio in soils was measured at approximately 2.0, above which the probability of BGB:AGB response to nutrient availability is small. These findings serve as interesting contributions to the global data pool on arid plant stoichiometry, given the previously limited knowledge regarding high‐altitude regions.     

青藏高原高寒草甸生物量动态变化及与环境因子的关系——基于模拟增温实验

以青藏高原高寒草甸为研究区,设置模拟增温实验样地,于2010年开始持续增温,2012和2013年调查植被地上-地下生物量,探讨气候变暖背景下高寒草甸生物量的动态变化及其与环境因子的关系。结果表明:(1)增温处理下地上-地下生物量与根冠比的中值和平均值大于对照,其中地下生物量(变异系数为0.30)的增加幅度大于地上生物量(变异系数为0.27),根冠比的变异系数(0.33)大于地上-地下生物量,这表明增温可导致高寒草甸植被生物量分配出现差异。(2)地上-地下生物量呈极显著的幂指数函数关系(R~2=0.147,P0.001),表现为异速生长,但在增温处理下异速生长出现减缓(R~2=0.102,P0.05)。(3)地上生物量受深层土壤水分和浅层土壤温度影响较大,地下生物量受深层土壤水分和深层土壤温度影响较大;土壤温度对地上-地下生物量的影响强于土壤水分,表现为20 cm深度土壤温度对地上生物量(R=0.582,P0.01)和根冠比(R=-0.238,P0.05)影响较大,60 cm深度土壤温度对地下生物量影响较大(R=0.388,P0.01),100 cm深度土壤水分对地上生物量(R=0.423,P0.01)和地下生物量(R=0.245,P0.05)影响较大,这说明增温导致浅层土壤温度对生物量分配产生影响,使生物量更多分配到地上部分,而冻土融化致使深层土壤水分对生物量产生影响。     

Divergent biomass partitioning to aboveground and belowground across forests in China

Aims Belowground to aboveground biomass (BGB/AGB) ratio is a highly valued parameter of the terrestrial carbon cycle and productivity. However, it remains far from clear whether plant biomass partitioning to aboveground and belowground is isometric (equal partitioning) or allometric (unequal partitioning) at community levels and what factors are necessary in order to regulate the partitioning. This study aimed to comprehensively find out the patterns of biomass partitioning and their regulatory factors across forests in China.Methods The data of AGB and BGB were compiled from 1542 samples for communities across forests in China. Standardized major axis regression was conducted to examine whether AGB and BGB were allocated isometrically or allometrically at a community level. Redundancy analysis was used to analyze the relationships of BGB/AGB ratio with climatic factors and soil properties.Important findings We found that the slopes of the relationship between logAGB and logBGB were not always comparable to 1.0 (isometric allocation) at community levels, including primary forest, secondary forest, and planted forest. Meanwhile, samples in clay, loam, and sand soil types also presented the same phenomenon. Furthermore, the radically different allocations of AGB and BGB were found in northern and southern China. Environmental factors totally explained 3.86% of the variations in the BGB/AGB ratio at the community level, which include the mean annual precipitation, mean annual temperature, potential water deficit index, soil carbon content, soil nitrogen content, soil clay, soil loam, soil sand, soil pH, and soil bulk density. In addition, the environmental factors also have effects on the BGB/AGB ratio in other categories. The patterns revealed in this study are helpful for better understanding biomass partitioning and spreading the carbon circle models.     

Soil respiration in Tibetan alpine grasslands: belowground biomass and soil moisture, but not soil temperature, best explain the large-scale patterns

The Tibetan Plateau is an essential area to study the potential feedback effects of soils to climate change due to the rapid rise in its air temperature in the past several decades and the large amounts of soil organic carbon (SOC) stocks, particularly in the permafrost. Yet it is one of the most under-investigated regions in soil respiration (Rs) studies. Here, Rs rates were measured at 42 sites in alpine grasslands (including alpine steppes and meadows) along a transect across the Tibetan Plateau during the peak growing season of 2006 and 2007 in order to test whether: (1) belowground biomass (BGB) is most closely related to spatial variation in Rs due to high root biomass density, and (2) soil temperature significantly influences spatial pattern of Rs owing to metabolic limitation from the low temperature in cold, high-altitude ecosystems. The average daily mean Rs of the alpine grasslands at peak growing season was 3.92 μmol CO(2) m(-2) s(-1), ranging from 0.39 to 12.88 μmol CO(2) m(-2) s(-1), with average daily mean Rs of 2.01 and 5.49 μmol CO(2) m(-2) s(-1) for steppes and meadows, respectively. By regression tree analysis, BGB, aboveground biomass (AGB), SOC, soil moisture (SM), and vegetation type were selected out of 15 variables examined, as the factors influencing large-scale variation in Rs. With a structural equation modelling approach, we found only BGB and SM had direct effects on Rs, while other factors indirectly affecting Rs through BGB or SM. Most (80%) of the variation in Rs could be attributed to the difference in BGB among sites. BGB and SM together accounted for the majority (82%) of spatial patterns of Rs. Our results only support the first hypothesis, suggesting that models incorporating BGB and SM can improve Rs estimation at regional scale.     

Above- and Belowground Biomass Allocation in Shrub Biomes across the Northeast Tibetan Plateau

Biomass partitioning has been explored across various biomes. However, the strategies of allocation in plants still remain contentious. This study investigated allocation patterns of above- and belowground biomass at the community level, using biomass survey from the Tibetan Plateau. We explored above- and belowground biomass by conducting three consecutive sampling campaigns across shrub biomes on the northeast Tibetan Plateau during 2011–2013. We then documented the above-ground biomass (AGB), below-ground biomass (BGB) and root: shoot ratio (R/S) and the relationships between R/S and environment factors using data from 201 plots surveyed from 67 sites. We further examined relationships between above-ground and below-ground biomass across various shrub types. Our results indicated that the median values of AGB, BGB, and R/S in Tibetan shrub were 1102.55, 874.91 g m^-2, and 0.85, respectively. R/S showed significant trend with mean annual precipitation (MAP), while decreased with mean annual temperature (MAT). Reduced major axis analysis indicated that the slope of the log-log relationship between above- and belowground biomass revealed a significant difference from 1.0 over space, supporting the optimal hypothesis. Interestingly, the slopes of the allometric relationship between log AGB and log BGB differed significantly between alpine and desert shrub. Our findings supported the optimal theory of above- and belowground biomass partitioning in Tibetan shrub, while the isometric hypothesis for alpine shrub at the community level.     

Above- and belowground biomass in relation to environmental factors in temperate grasslands, Inner Mongolia

Above- and belowground biomasses of grasslands are important parameters for characterizing re- gional and global carbon cycles in grassland ecosystems. Compared with the relatively detailed in- formation for aboveground biomass (AGB), belowground biomass (BGB) is poorly reported at the re- gional scales. The present study, based on a total of 113 sampling sites in temperate grassland of the Inner Mongolia, investigated regional distribution patterns of AGB, BGB, vertical distribution of roots, and their relationships with environmental factors. AGB and BGB increased from the southwest to the northeast of the study region. The largest biomass occurred in meadow steppe, with mean AGB and BGB of 196.7 and 1385.2 g/m2, respectively; while the lowest biomass occurred in desert steppe, with an AGB of 56.6 g/m2 and a BGB of 301.0 g/m2. In addition, about 47% of root biomass was distributed in the top 10 cm soil. Further statistical analysis indicated that precipitation was the primary determinant factor in shaping these distribution patterns. Vertical distribution of roots was significantly affected by precipitation, while the effects of soil texture and grassland types were weak.     

Aggravated phosphorus limitation on biomass production under increasing nitrogen loading: a meta‐analysis

Nitrogen (N) and phosphorus (P), either individually or in combination, have been demonstrated to limit biomass production in terrestrial ecosystems. Field studies have been extensively synthesized to assess global patterns of N impacts on terrestrial ecosystem processes. However, to our knowledge, no synthesis has been done so far to reveal global patterns of P impacts on terrestrial ecosystems, especially under different nitrogen (N) levels. Here, we conducted a meta‐analysis of impacts of P addition, either alone or with N addition, on aboveground (AGB) and belowground biomass production (BGB), plant and soil P concentrations, and N : P ratio in terrestrial ecosystems. Overall, our meta‐analysis quantitatively confirmed existing notions: (i) colimitation of N and P on biomass production and (ii) more P limitation in tropical forest than other ecosystems. More importantly, our analysis revealed new findings: (i) P limitation on biomass production was aggravated by N enrichment and (ii) plant P concentration was a better indicator of P limitation than soil P availability. Specifically, P addition increased AGB and BGB by 34% and 13%, respectively. The effect size of P addition on biomass production was larger in tropical forest than grassland, wetland, and tundra and varied with P fertilizer forms, P addition rates, or experimental durations. The P‐induced increase in biomass production and plant P concentration was larger under elevated than ambient N. Our findings suggest that the global limitation of P on biomass production will become severer under increasing N fertilizer and deposition in the future.     

Relationships of Biomass with Environmental Factors in the Grassland Area of Hulunbuir,China

Many studies have focused on the relationship between vegetation biomass and environmental factors in grassland. However, several questions remain to be answered, especially with regards to the spatial pattern of vegetation biomass. Thus, the distributed mechanism will be explored in the present study. Here, plant biomass was measured at 23 sites along a transect survey during the peak growing season in 2006. The data were analyzed with a classification and regression tree (CART) model. The structural equation modeling (SEM) was conducted to explicitly evaluate the both direct and indirect effects of these critical environmental elements on vegetation biomass. The results demonstrated that mean annual temperature (MAT) affected aboveground biomass (AGB) scored at −0.811 (P<0.05). The direct effect of MAT on belowground biomass (BGB) was −0.490 (P<0.05). The results were determined by SEM. Our results indicate that AGB and BGB in semi-arid ecosystems is strongly affected by precipitation and temperature. Future work shall attempt to take into account the integrated effects of precipitation and temperature. Meanwhile, partitioning the influences of environmental variations and vegetation types are helpful in illuminating the internal mechanism of biomass distribution.     

Patterns of Below- and Aboveground Biomass in Eucalyptus populnea Woodland Communities of Northeast Australia along a Rainfall Gradient

In vegetated terrestrial ecosystems, carbon in below- and aboveground biomass (BGB, AGB) often constitutes a significant component of total-ecosystem carbon stock. Because carbon in the BGB is difficult to measure, it is often estimated using BGB to AGB ratios. However, this ratio can change markedly along resource gradients, such as water availability, which can lead to substantial errors in BGB estimates. In this study, BGB and AGB sampling was carried out in Eucalyptus populnea-dominated woodland communities of northeast Australia to examine patterns of BGB to AGB ratio and vertical root distribution at three sites along a rainfall gradient (367, 602, and 1,101 mm). At each site, a vegetation inventory was undertaken on five transects (100 × 4 m), and trees representing the E. populnea vegetation structure were harvested and excavated to measure aboveground and coarse-root (diameter of at least 15 mm) biomass. Biomass of fine and small roots (diameter less than 15 mm) at each site was estimated from 40 cores sampled to 1 m depth. The BGB to AGB ratio of E. populnea-dominated woodland plant communities declined from 0.58 at the xeric end to 0.36 at the mesic end of the rainfall gradient. This was due to a marked decline in AGB with increased aridity whereas the BGB was relatively stable. The vertical distribution of fine roots in the top 1 m of soil varied along the rainfall gradient. The mesic sites had more fine-root biomass (FRB) in the upper soil profile and less at depth than the xeric site. Accordingly, at the xeric site, a much larger proportion of FRB was found at depth compared to the mesic sites. The vertical distribution patterns of small roots of the E. populnea woodland plant communities were consistently )-shaped, with the highest biomass occurring at 15–30-cm depth. The potential significance of such a rooting pattern for grass–tree and shrub–tree co-existence in these ecosystems is discussed. Overall, our results revealed marked changes in BGB to AGB ratio of E. populnea woodland communities along a rainfall gradient. Because E. populnea woodlands cover a large area (96 M ha), their contribution to continental-scale carbon sequestration and greenhouse gas emission can be substantial. Use of the rainfall-zone-specific ratios found in this study, in lieu of a single generic ratio for the entire region, will significantly improve estimates of BGB carbon stocks in these woodlands. In the absence of more specific data, our results will also be relevant in other regions with similar vegetation and rainfall gradients (that is, arid and semiarid woodland ecosystems).     

Environmental driving factors affecting plant biomass in natural grassland in the Loess Plateau,China

Plant biomass is a key parameter for estimating terrestrial ecosystem carbon (C) stocks, which varies greatly as a result of specific environmental conditions. Here, we tested environmental driving factors affecting plant biomass in natural grassland in the Loess Plateau, China. We found that above-ground biomass (AGB) and below-ground biomass (BGB) had a similar change trend in the order of Stipa bungeana > Leymus secalinus > Artemisia sacrorum > Artemisia scoparia, whereas shoot ratio (R/S) displayed an opposite change trend. There was a significantly positive linear relationship between the AGB and BGB, regardless of plant species (p < 0.05). Furthermore, more than 50% of the AGB were found in 20–50 cm of plant height in Compositae plants (A. sacrorum, A. scoparia), whereas over 60% of the AGB were found in 20–80 cm of plant height in Gramineae plants (S. bungeana, L. secalinus). For each plant species, more than 75% of the BGB was distributed in 0–10 cm soil depth, and 20% was distributed in 10–20 cm soil depth, while less than 5% was distributed in 20–40 cm soil depth. Further, AGB and BGB were highly affected by environmental driving factors (soil properties, plant traits, topographic properties), which were identified by the structural equation model (SEM) and the generalized additive models (GAMs). In addition, AGB was directly affected by plant traits, and BGB was directly affected by soil properties, and soil properties associated with plant traits that affected AGB and BGB through interactive effects were 9.12% and 3.59%, respectively. However, topographic properties had a weak influence on ABG and BGB (as revealed by the lowest total pathway effect). Besides, soil organic carbon (SOC), soil microbial biomass carbon (MBC), and plant height had a higher relative contribution to AGB and BGB. Our results indicate that environmental driving factors affect plant biomass in natural grassland in the Loess Plateau.     

Forest biomass stocks and dynamics across the subtropical Andes

Forest biomass plays an important role in the global carbon cycle. Therefore, understanding the factors that control forest biomass stocks and dynamics is a key challenge in the context of global change. We analyzed data from 60 forest plots in the subtropical Andes (22–27.5° S and 300–2300 m asl) to describe patterns and identify drivers of aboveground biomass (AGB) stocks and dynamics. We found that AGB stocks remained roughly constant with elevation due to compensating changes in basal area (which increased with elevation) and plot‐mean wood specific gravity (which decreased with elevation). AGB gain and loss rates both decreased with elevation and were explained mainly by temperature and rainfall (positive effects on both AGB gains and losses). AGB gain was also correlated with forest‐use history and weakly correlated with forest structure. Mean annual temperature and rainfall showed minor effects on AGB stocks and AGB change (gains minus losses) over recent decades. Although AGB change was only weakly correlated with climate variables, increases in AGB gains and losses with increasing rainfall—together with observed increases in rainfall in the subtropical Andes—suggest that these forests may become increasingly dynamic in the future. Abstract in Spanish is available with online material     

Moderate grazing promotes the root biomass in Kobresia meadow on the northern Qinghai–Tibet Plateau

Grazing is an important modulator of both plant productivity and biodiversity in grassland community, yet how to determine a suitable grazing intensity in alpine grassland is still controversy. Here, we explore the effects of different grazing intensities on plant biomass and species composition, both at community level and functional group level, and examines the productivity–species richness relationship under four grazing patterns: no grazing (CK), light grazing (LG), moderate grazing, (MG) and heavy grazing (HG), attempt to determine a suitable grazing intensity in alpine grassland. The results were as follows. The total aboveground biomass (AGB) reduced with increasing grazing intensity, and the response of plant functional groups was different. AGB of both sedges and legumes increased from MG to HG, while the AGB of forbs reduced sharply and the grass AGB remained steady. There was a significant positive relationship between productivity and species richness both at community level and functional group level. In contrast, the belowground biomass (BGB) showed a unimodal relationship from CK to HG, peaking in MG (8,297.72 ± 621.29 g/m²). Interestingly, the grassland community tends to allocate more root biomass to the upper soil layer under increasing grazing intensities. Our results suggesting that moderate levels of disturbance may be the optimal grassland management strategy for alpine meadow in terms of root production.     

Ambient precipitation determines the sensitivity of soil respiration to precipitation treatments in a marsh

The effects in field manipulation experiments are strongly influenced by amplified interannual variation in ambient climate as the experimental duration increases. Soil respiration (SR), as an important part of the carbon cycle in terrestrial ecosystems, is sensitive to climate changes such as temperature and precipitation changes. A growing body of evidence has indicated that ambient climate affects the temperature sensitivity of SR, which benchmarks the strength of terrestrial soil carbon–climate feedbacks. However, whether SR sensitivity to precipitation changes is influenced by ambient climate is still not clear. In addition, the mechanism driving the above phenomenon is still poorly understood. Here, a long-term field manipulation experiment with five precipitation treatments (−60%, −40%, +0%, +40%, and +60% of annual precipitation) was conducted in a marsh in the Yellow River Delta, China, which is sensitive to soil drying–wetting cycle caused by precipitation changes. Results showed that SR increased exponentially along the experimental precipitation gradient each year and the sensitivity of SR (standardized by per 100 mm change in precipitation under precipitation treatments) exhibited significant interannual variation from 2016 to 2021. In addition, temperature, net radiation, and ambient precipitation all exhibited dramatic interannual variability; however, only ambient precipitation had a significant negative correlation with SR sensitivity. Moreover, the sensitivity of SR was significantly positively related to the sensitivity of belowground biomass (BGB) across 6 years. Structural equation modeling and regression analysis also showed that precipitation treatments significantly affected SR and its autotrophic and heterotrophic components by altering BGB. Our study demonstrated that ambient precipitation determines the sensitivity of SR to precipitation treatments in marshes. The findings underscore the importance of ambient climate in regulating ecosystem responses in long-term field manipulation experiments.     

Biomass and Its Allocation in Relation to Temperature,Precipitation, and Soil Nutrients in Inner Mongolia Grasslands,China

Aim

Understanding and predicting ecosystem functioning such as biomass accumulation requires an accurate assessment of large-scale patterns of biomass distribution and partitioning in relation to climatic and soil environments.

Methods

We sampled above- and belowground biomass from 26 sites spanning 1500 km in Inner Mongolian grasslands, compared the difference in aboveground, belowground biomass and below-aboveground biomass ratio (AGB, BGB, and B/A, respectively) among meadow steppe, typical steppe, and desert steppe types. The relationships between AGB, BGB, B/A and climatic and soil environments were then examined.

Results

We found that AGB and BGB differed significantly among three types of grasslands while B/A did not differ. Structural equation model analyses indicated that mean annual precipitation was the strongest positive driver for AGB and BGB. AGB was also positively associated with soil organic carbon, whereas B/A was positively associated with total soil nitrogen.

Conclusions

These results indicated that precipitation positively influence plant production in Inner Mongolian grasslands. Contrary to the prediction from the optimal partitioning hypothesis, biomass allocation to belowground increased with soil total nitrogen, suggesting that more productive sites may increase belowground allocation as an adaptive strategy to potentially high fire frequencies.     

Belowground biomass of alpine shrublands across the northeast Tibetan Plateau

Although belowground biomass (BGB) plays an important role in global cycling, the storage of BGB and climatic effects on it are remaining unclear. With data from 49 sites, we aimed to investigate BGB and its climatic controls in alpine shrublands in the Tibetan Plateau. Our study showed that the BGB (both grass‐layer and shrub‐layer biomass) storage in the alpine shrublands was 67.24 Tg, and the mean BGB density and shrublands area were 1,567.38 g/m² and 4.29 × 10⁴ km², respectively. Shrub layer had a larger BGB stock and accounted for 66% of total BGB this area, while only 34% was accumulated in the grass layer. BGB of the grass layer in the Tibetan Plateau shrublands was larger than that of Tibetan alpine grasslands, indicating that shrubland ecosystem played a critical importance role in carbon cycle on the Tibetan Plateau. The BGB in the grass layer and shrub layer demonstrated different correlations with climatic factors. Specifically, the effects from mean annual temperature on shrub‐layer BGB were not significant, similarly to the relationship between mean annual precipitation and grass‐layer BGB. But shrub‐layer BGB had a significantly positive relationship with mean annual precipitation (p < .05), while grass‐layer BGB showed a trend of decrease with increasing mean annual temperature (p < .05). Consequently, the actual and potential increases of BGB varied due to different increases of mean annual precipitation and temperature among different areas of the Tibetan Plateau. Therefore, in the warmer and wetter scenario, due to contrary relationships from mean annual precipitation and temperature on shrub‐layer BGB and grass‐layer BGB, it is necessary to conduct a long‐term monitoring about dynamic changes to increase the precision of assessment of BGB carbon sequestration in the Tibetan Plateau alpine shrublands.     

Community species diversity mediates the trade‐off between aboveground and belowground biomass for grasses and forbs in degraded alpine meadow,Tibetan Plateau

Although many empirical experiments have shown that increasing degradation results in lower aboveground biomass (AGB), our knowledge of the magnitude of belowground biomass (BGB) for individual plants is a prerequisite for accurately revealing the biomass trade‐off in degraded grasslands. Here, by linking the AGB and BGB of individual plants, species in the community, and soil properties, we explored the biomass partitioning patterns in different plant functional groups (grasses of Stipa capillacea and forbs of Anaphalis xylorhiza). Our results indicated that 81% and 60% of the biomass trade‐off variations could be explained by environmental factors affecting grasses and forbs, respectively. The change in community species diversity dominated the biomass trade‐off via either direct or indirect effects on soil properties and biomass. However, the community species diversity imparted divergent effects on the biomass trade‐off for grasses (scored at −0.72) and forbs (scored at 0.59). Our findings suggest that plant communities have evolved two contrasting strategies of biomass allocation patterns in degraded grasslands. These are the “conservative” strategy in grasses, in which plants with larger BGB trade‐off depends on gigantic roots for soil resources, and the “opportunistic” strategy in forbs, in which plants can adapt to degraded lands using high variation and optimal biomass allocation.     

高寒草甸植物群落生长发育特征与气候因子的关系

为合理利用高寒草甸资源,探讨近年来气候变化对高寒草甸的影响,以青海省甘德县高寒草甸为例,基于牧业气象站1976-2006年的气象资料和1994-2006年的牧草观测资料,分析了草地植被地上生物量、高度、盖度和物候期等群落特征以及当地气温、降雨等气象因素的年际变化趋势,采用典型相关分析法和逐步回归分析法对草地植物群落特征变化与气象因子的关系进行了研究,综合分析了影响植被生长状况的关键因子,结果表明：（1）青藏高原高寒草甸总体呈年均气温和平均地温上升、年降水量下降的"暖干化"趋势,牧草盖度高度增大,产量减少,整体观测水平下的牧草物候期推迟。（2）牧草的高度、盖度及产量对不同气候因子的响应程度不同。牧草高度与盖度对温度因子的变化更敏感,牧草产量对水分因子的变化更敏感。平均地温和相对湿度越高,牧草高度越高,产量越多。（3）不同牧草的物候期受不同气象因子的影响,变化趋势也不相同。从整体水平上看,牧草物候期对温度因子更敏感,温度越高,物候期越提前。     

The Effects of Interannual Rainfall Variability on Tree–Grass Composition Along Kalahari Rainfall Gradient

Precipitation variability has been predicted to increase in a global warmer climate, and is expected to greatly affect plant growth, interspecies interactions, plant community composition, and other ecosystem processes. Although previous studies have investigated the effect of intra-annual rainfall variability on plant growth and ecosystem dynamics, the impacts of interannual rainfall variability remain understudied. This paper uses satellite data and develops a new mechanistic model to investigate the response of tree–grass composition to increasing interannual rainfall variability in arid to sub-humid ecosystems along the Kalahari Transect in Southern Africa. Both satellite data and model results show that increasing interannual rainfall fluctuations favor deep-rooted trees over shallow-rooted grasses in drier environments (that is, mean annual rainfall, MAP < 900–1000 mm) but favor grasses over trees in wetter environments (that is, MAP > 900–1000 mm). Trees have a competitive advantage over grasses in dry environments because their generally deeper root systems allow them to have exclusive access to the increased deep soil water resources expected to occur in wet years as a result of the stronger interannual rainfall fluctuations. In relatively wet environments, grasses are favored because of their high growth rate that allows them to take advantage of the window of opportunity existing in years with above average precipitation and thus increase fire-induced tree mortality. Thus, under increasing interannual rainfall fluctuations both direct effects on soil water availability and indirect effects mediated by tree–grass interactions and fire dynamics are expected to play an important role in determining changes in plant community composition.