Leaf carbon isotope tracks the facilitation pattern of legume shrubs shaped by water availability and species replacement along a large elevation gradient in Trans-Himalayas.

BACKGROUND AND AIMS: Understanding patterns and mechanisms of nurse plant facilitation is important to predict the resilience of arid/semi-arid ecosystems to climate change. We investigate whether water availability and nurse species turnover interact to shape the facilitation pattern of widespread legume shrubs along a large elevation gradient. We also investigate whether leaf δ13C of nurse plants can track the facilitation pattern. METHODS: We measured the relative interaction index (RII) of the number of species within and outside the canopy of two widespread legume shrub species (Caragana gerardiana and Caragana versicolor) alternatively distributed along a large elevation gradient in the Trans-Himalayas. We also assessed the proportional increase of species richness (ISR) at the community level using the paired plot data. To determine site-specific water availability, we measured the leaf δ13C of nurse shrubs and calculated the Thornthwaite moisture index (MI) for each elevation site. KEY RESULTS: Elevational variations in RII, ISR and δ13C were mainly explained by the MI when the effects of soil nitrogen and plant traits (leaf nitrogen and shrub size) were controlled. Variations in RII and ISR across the two nurse species were explained better by δ13C than by smoothly changing climatic factors along elevation. At the transition zone between the upper limit of C. gerardiana (4100 m) and the lower limit of C. versicolor (4200 m), RII and ISR were much higher in C. versicolor than in C. gerardiana under a similar MI. Such an abrupt increase in facilitation induced by nurse species replacement was well tracked by the variation of δ13C. CONCLUSIONS: Water availability and nurse species replacement are crucial to shaping facilitation patterns by legume shrubs along a large elevation gradient in dry mountainous regions, such as the Trans-Himalayas. Turnover in nurse species under global change might significantly alter the pattern of nurse plant facilitation associated with water availability, which can be well tracked by leaf δ13C.

Precipitation and local adaptation drive spatiotemporal variations of aboveground biomass and species richness in Tibetan alpine grasslands.

The Tibetan Plateau contains the highest and largest alpine pasture in the world, which is adapted to the cold and arid climate. It is challenging to understand how the vast alpine grasslands respond to climate change. We aim to test the hypothesis that there is local adaptation in elevational populations of major plant species in Tibetan alpine grasslands, and that the spatiotemporal variations of aboveground biomass (AGB) and species richness (S) can be mainly explained by climate change only when the effect of local adaptation is removed. A 7-year reciprocal transplant experiment was conducted among the distribution center (4950 m), upper (5200 m) and lower (4650 m) limits of alpine Kobresia meadow in central Tibetan Plateau. We observed interannual variations in S and AGB of 5 functional groups and 4 major species, and meteorological factors in each of the three elevations during 2012-2018. Relationships between interannual changes of AGB and climatic factors varied greatly with elevational populations within a species. Elevation of population origin generally had a greater or an equal contribution to interannual variation in AGB of the 4 major species, compared to temperature and precipitation effects. While the effect of local adaptation was removed by calculating differences in AGB and S between elevations of migration and origin, relative changes in AGB and S were mainly explained by precipitation change rather than by temperature change. Our data support the hypothesis, and further provide evidence that the monsoon-adapted alpine grasslands are more sensitive to precipitation change than to warming.

Shrub-mediated effects on soil nitrogen determines shrub-herbaceous interactions in drylands of the Tibetan Plateau.

Introduction: Shrub promotes the survival, growth and reproduction of understory species by buffering the environmental extremes and improving limited resources (i.e., facilitation effect) in arid and semiarid regions. However, the importance of soil water and nutrient availability on shrub facilitation, and its trend along a drought gradient have been relatively less addressed in water-limited systems. Methods: We investigated species richness, plant size, soil total nitrogen and dominant grass leaf δ13C within and outside the dominant leguminous cushion-like shrub Caragana versicolor along a water deficit gradient in drylands of Tibetan Plateau. Results: We found that C. versicolor increased grass species richness but had a negative effect on annual and perennial forbs. Along the water deficit gradient, plant interaction assessed by species richness (RIIspecies) showed a unimodal pattern with shift from increase to decrease, while plant interaction assessed by plant size (RIIsize) did not vary significantly. The effect of C. versicolor on soil nitrogen, rather than water availability, determined its overall effect on understory species richness. Neither the effect of C. versicolor on soil nitrogen nor water availability affected plant size. Discussion: Our study suggests that the drying tendency in association with the recent warming trends observed in drylands of Tibetan Plateau, will likely hinder the facilitation effect of nurse leguminous shrub on understories if moisture availability crosses a critical minimum threshold.

Increased precipitation leads to earlier green-up and later senescence in Tibetan alpine grassland regardless of warming.

It is debatable whether warming or increased precipitation primarily drives the changes of spring and autumn phenology in alpine grasslands at high elevations like the Tibetan Plateau. We aim to test the hypothesis that increased precipitation and soil moisture rather than warming significantly advance spring green-up dates (GUD) of dominant species in a semiarid alpine grassland, while both increases of temperature and precipitation delay their autumn senescence dates (SD). We conducted a 2-year manipulative experiment with infrared warming (ambient, +2 °C) and precipitation increase for each of rainfall events (ambient, +15 %, +30 %) during the growing season in a Tibetan alpine grassland. GUD and SD of three dominant species and the relevant soil temperature (ST) and moisture (SM) were observed. Rainy season onset as well as Pre-GUD or Pre-SD (30 days before GUD or SD) mean air-temperature (T-30d) and precipitation (P-30d) and relevant soil temperature (ST-30d) and moisture (SM-30d) were calculated for each experimental treatment. GUD dates of the three dominant species were advanced by increased precipitation rather than by warming, which showed a robust positive correlation with rainy season onset. SD dates were independently delayed by both increases of temperature and precipitation. There was no interactive effect of warming and increased precipitation on GUD and SD across species and years. In general, GUD had a significant negative correlation with Pre-GUD P-30d (SM-30d) but not with Pre-GUD T-30d (ST-30d), while SD showed a significant positive correlation with Pre-SD T-30d and P-30d or Pre-SD ST-30d and SM-30d. Our data support the hypothesis, indicating that spring and autumn phenology of monsoon-adapted alpine vegetation are more sensitive to precipitation change than to warming. The prolonged growing season length under increased temperature and precipitation is more depended on the delay of autumn senescence than the advance of spring green-up.

Enhanced leaf turnover and nitrogen recycling sustain CO2 fertilization effect on tree-ring growth.

Whether increased photosynthates under elevated atmospheric CO2 could translate into sustained biomass accumulation in forest trees remains uncertain. Here we demonstrate how tree radial growth is closely linked to litterfall dynamics, which enhances nitrogen recycling to support a sustained effect of CO2 fertilization on tree-ring growth. Our ten-year observations in two alpine treeline forests indicated that annual (or seasonal) stem radial increments generally had a positive relationship with the previous year's (or season's) litterfall and its associated nitrogen return and resorption. Annual tree-ring width, annual litterfall and annual nitrogen return and resorption all showed an increasing trend during 2007-2017, and most of the variations were explained by elevated atmospheric CO2 rather than climate change. Similar patterns were found in the longer time series of tree-ring width index from 1986-2017. The regional representativeness of our observed patterns was confirmed by the literature data of six other tree species at 11 treeline sites over the Tibetan Plateau. Enhanced nitrogen recycling through increased litterfall under elevated atmospheric CO2 supports a general increasing trend of tree-ring growth in recent decades, especially in cold and nitrogen-poor environments.

Rainy season onset mainly drives the spatiotemporal variability of spring vegetation green-up across alpine dry ecosystems on the Tibetan Plateau.

It is still debatable whether temperature or precipitation mainly triggers spring vegetation green-up (SVG) in alpine dry ecosystems on the Tibetan Plateau. As phenological sensitivity to the arrival of monsoon-season rainfall would allow plants to simultaneously avoid drought and frost damages in the early growing season, we hypothesize that rainy season onset (RSO) rather than temperature mainly drives the spatiotemporal variability of SVG across alpine dry ecosystems over the Tibetan Plateau. Dates of RSO and SVG across 67 target areas nearby 67 weather stations over the Tibetan Plateau were calculated from time-series data of daily mean temperature and precipitation (1974-2013) and of the Normalized Difference Vegetation Index from the Moderate Resolution Imaging Spectroradiometer (2001-2013), respectively. Satellite-derived SVG was validated by 7-year observations (2007-2013) for leaf emergence of dominant species in alpine meadows along elevations (4400-5200 m) in Damxung of Tibet. We found that SVG generally synchronized with or was somewhat later than RSO although seasonal air temperatures were already continuously above 0 °C in 1 month before SVG dates. In pooled data across sites and years, the analysis of linear mixed model indicated that RSO (F = 42.109) and its interactions with pre-SVG precipitation (F = 6.767) and temperature (F = 4.449) mainly explained the spatio-temporal variability of SVG, while pre-SVG temperature and its interaction with precipitation did not have significant effects on SVG. Our data supported the hypothesis, suggesting that synchronization of SVG and RSO is a general spring phenological strategy across alpine dry ecosystems under influence of monsoon climate.

Effect of increasing precipitation and warming on microbial community in Tibetan alpine steppe.

Soil microorganisms play an important role in regulating the feedback of Alpine steppe ecosystems to future climate change. However, the interaction effect of warming and increasing precipitation on soil microorganisms remains unclear, in the face of an ongoing warmer and wetter climate on the Tibetan Plateau. In this study, we investigate the multi-factorial effects on soil microbial diversity, community structure, and microbial interactions in a three-year climate change experiment established in an Alpine steppe on the Tibetan Plateau, involving warming (+2 °C), +15% increasing precipitation and +30% increasing precipitation. Compared to warming, warming plus increasing precipitation alleviated the decrease in microbial diversity, and increased the dissimilarities in microbial community structures, largely influenced by water and substrate availability. We further observed differences in moisture increased the differences in microbial diversity and dissimilarities in microbial community structures across different precipitation levels under ambient temperature. Interestingly, warming plus increasing precipitation could create more ecological niches for microbial species to coexist but may lessen the strength of microbial interactions in contrast to increasing precipitation alone. Collectively, our findings indicate that microbial responses to future climate change in Alpine steppe soils will be more complex than those under single-climate-factor conditions.

Warming and increased precipitation indirectly affect the composition and turnover of labile-fraction soil organic matter by directly affecting vegetation and microorganisms.

Global warming accompanied by precipitation changes impacts soil carbon sequestration. A three-year field manipulation experiment with warming (+2 °C above ambient temperature) and increased precipitation (+15% and +30% above ambient precipitation) was conducted in an alpine grassland to investigate the response of soil organic matter (SOM) to future climate change on the Qinghai-Tibet Plateau (QTP). Labile-fraction SOM (LF-SOM) fingerprints were characterized by pyrolysis-gas chromatography/tandem-mass spectrometry (Py-GC-MS/MS), and organic compounds in LF-SOM were used as indicators to quantify the contributions of vegetation input and microbial degradation to LF-SOM transformation. Increased precipitation promoted LF-SOM accumulation, which were mainly due to the positive effect of increased precipitation on vegetation productivity. Plant-derived compounds in LF-SOM (including lignin, long-chain alkyl compounds, polysaccharides and phenols) were more sensitive to increasing soil moisture than microbial-derived (including short-chain alkyl compounds, N compounds and chitin) and aromatic-derived compounds (including aromatics and polyaromatics). In contrast, warming alone intensified the effect of drought on the alpine grassland, which had negative effects on both vegetation and microorganisms and reduced LF-SOM. Warming plus increased precipitation not only alleviated the water loss caused by warming but also increased soil temperature, which was more favorable for the growth of microorganisms. This was reflected in the increase in microbial-derived compounds in LF-SOM with increasing soil temperature, which contributed to LF-SOM degradation. Aromatic-derived compounds, as refractory compounds in soil, showed no significant response to either warming or increased precipitation treatments. Acidobacteria (approximately 25%) and Actinobacteria (approximately 20%), as the dominant soil bacterial communities in the alpine grassland, were significantly correlated with plant-derived compounds. At the same time, there were significant correlations between Proteobacteria and microbial-derived compounds, as well as between Firmicutes and aromatic-derived compounds (relative abundance). Under future climate change, microbial activity will increase as temperature increases, which will promote LF-SOM degradation only if precipitation also increases.

Optimal balance of water use efficiency and leaf construction cost with a link to the drought threshold of the desert steppe ecotone in northern China.

BACKGROUND AND AIMS: In arid environments, a high nitrogen content per leaf area (Narea) induced by drought can enhance water use efficiency (WUE) of photosynthesis, but may also lead to high leaf construction cost (CC). Our aim was to investigate how maximizing Narea could balance WUE and CC in an arid-adapted, widespread species along a rainfall gradient, and how such a process may be related to the drought threshold of the desert-steppe ecotone in northern China. METHODS: Along rainfall gradients with a moisture index (MI) of 0·17-0·41 in northern China and the northern Tibetan Plateau, we measured leaf traits and stand variables including specific leaf area (SLA), nitrogen content relative to leaf mass and area (Nmass, Narea) and construction cost (CCmass, CCarea), δ(13)C (indicator of WUE), leaf area index (LAI) and foliage N-pool across populations of Artemisia ordosica KEY RESULTS: In samples from northern China, a continuous increase of Narea with decreasing MI was achieved by a higher Nmass and constant SLA (reduced LAI and constant N-pool) in high-rainfall areas (MI > 0·29), but by a lower SLA and Nmass (reduced LAI and N-pool) in low-rainfall areas (MI ≤ 0·29). While δ(13)C, CCmass and CCarea continuously increased with decreasing MI, the low-rainfall group had higher Narea and δ(13)C at a given CCarea, compared with the high-rainfall group. Similar patterns were also found in additional data for the same species in the northern Tibetan Plateau. The observed drought threshold where MI = 0·29 corresponded well to the zonal boundary between typical and desert steppes in northern China. CONCLUSIONS: Our data indicated that below a climatic drought threshold, drought-resistant plants tend to maximize their intrinsic WUE through increased Narea at a given CCarea, which suggests a linkage between leaf functional traits and arid vegetation zonation.

Seasonal Shift in Climatic Limiting Factors on Tree Transpiration: Evidence from Sap Flow Observations at Alpine Treelines in Southeast Tibet.

Alpine and northern treelines are primarily controlled by low temperatures. However, little is known about the impact of low soil temperature on tree transpiration at treelines. We aim to test the hypothesis that in cold-limited forests, the main limiting factors for tree transpiration switch from low soil temperature before summer solstice to atmospheric evaporative demand after summer solstice, which generally results in low transpiration in the early growing season. Sap flow, meteorological factors and predawn needle water potential were continuously monitored throughout one growing season across Smith fir (Abies georgei var. smithii) and juniper (Juniperus saltuaria) treelines in southeast Tibet. Sap flow started in early May and corresponded to a threshold mean air-temperature of 0°C. Across tree species, transpiration was mainly limited by low soil temperature prior to the summer solstice but by vapor pressure deficit and solar radiation post-summer solstice, which was further confirmed on a daily scale. As a result, tree transpiration for both tree species was significantly reduced in the pre-summer solstice period as compared to post-summer solstice, resulting in a lower predawn needle water potential for Smith fir trees in the early growing season. Our data supported the hypothesis, suggesting that tree transpiration mainly responds to soil temperature variations in the early growing season. The results are important for understanding the hydrological response of cold-limited forest ecosystems to climate change.

Leaf unfolding of Tibetan alpine meadows captures the arrival of monsoon rainfall.

The alpine meadow on the Tibetan Plateau is the highest and largest pasture in the world, and its formation and distribution are mainly controlled by Indian summer monsoon effects. However, little is known about how monsoon-related cues may trigger spring phenology of the vast alpine vegetation. Based on the 7-year observations with fenced and transplanted experiments across lower to upper limits of Kobresia meadows in the central plateau (4400-5200 m), we found that leaf unfolding dates of dominant sedge and grass species synchronized with monsoon onset, regardless of air temperature. We also found similar patterns in a 22-year data set from the northeast plateau. In the monsoon-related cues for leaf unfolding, the arrival of monsoon rainfall is crucial, while seasonal air temperatures are already continuously above 0 °C. In contrast, the early-emerging cushion species generally leafed out earlier in warmer years regardless of precipitation. Our data provide evidence that leaf unfolding of dominant species in the alpine meadows senses the arrival of monsoon-season rainfall. These findings also provide a basis for interpreting the spatially variable greening responses to warming detected in the world's highest pasture, and suggest a phenological strategy for avoiding damages of pre-monsoon drought and frost to alpine plants.

Bacterial community in alpine grasslands along an altitudinal gradient on the Tibetan Plateau.

The Tibetan Plateau, 'the third pole', is a region that is very sensitive to climate change. A better understanding of response of soil microorganisms to climate warming is important to predict soil organic matter preservation in future scenario. We selected a typically altitudinal gradient (4400 m-5200 m a.s.l) along south-facing slope of Nyainqentanglha Mountains on central Tibetan Plateau. Bacterial communities were investigated using terminal restriction fragment length polymorphism analysis (T-RFLP) combined with sequencing methods. Acidobacteria and Proteobacteria were dominant bacteria in this alpine soil. Redundancy analysis revealed that soil bacterial communities were significantly different along the large altitudinal gradient, although the dominant environmental driving factors varied at different soil depth. Specifically, our results showed that precipitation and soil NH4 + were dominant environmental factors that influence bacterial communities at 0-5 cm depth along the altitudinal gradients, whereas pH was a major influential factor at 5-20 cm soil. In this semi-arid region, precipitation rather than temperature was a main driving force on soil bacterial communities as well as on plant communities. We speculate that an increase in temperature might not significantly change soil bacterial community structures along the large altitudinal gradient, whereas precipitation change would play a more important role in affecting soil bacterial communities.

Little change in the fir tree-line position on the southeastern Tibetan Plateau after 200 years of warming.

• As one of the world's highest natural tree lines, the Smith fir (Abies georgei var. smithii) tree line on the southeastern Tibetan Plateau is expected to vary as a function of climate warming. However, the spatial patterns and dynamics of the Smith fir tree line are not yet well understood. • Three rectangular plots (30 m × 150 m) were established in the natural alpine tree-line ecotone on two north-facing (Plot N1, 4390 m asl; Plot N2, 4380 m asl) and one east-facing (Plot E1, 4360 m asl) slope. Dendroecological methods were used to monitor the tree-line patterns and dynamics over a 50-yr interval. • The three study plots showed a similar pattern of regeneration dynamics, characterized by increased recruitment after the 1950s and an abrupt increase in the 1970s. Smith fir recruitment was significantly positively correlated with both summer and winter temperatures. However, Smith fir tree lines do not show a significant upward movement, despite warming on the Tibetan Plateau. • The warming in the past 200 yr is already having a significant impact on the population density of the trees, but not on the position of the Smith fir tree line.

Linking carbon supply to root cell-wall chemistry and mechanics at high altitudes in Abies georgei.

BACKGROUND AND AIMS: The mobile carbon supply to different compartments of a tree is affected by climate, but its impact on cell-wall chemistry and mechanics remains unknown. To understand better the variability in root growth and biomechanics in mountain forests subjected to substrate mass movement, we investigated root chemical and mechanical properties of mature Abies georgei var. smithii (Smith fir) growing at different elevations on the Tibet-Qinghai Plateau. METHODS: Thin and fine roots (0·1-4·0 mm in diameter) were sampled at three different elevations (3480, 3900 and 4330 m, the last corresponding to the treeline). Tensile resistance of roots of different diameter classes was measured along with holocellulose and non-structural carbon (NSC) content. KEY RESULTS: The mean force necessary to break roots in tension decreased significantly with increasing altitude and was attributed to a decrease in holocellulose content. Holocellulose was significantly lower in roots at the treeline (29·5 ± 1·3 %) compared with those at 3480 m (39·1 ± 1·0 %). Roots also differed significantly in NSC, with 35·6 ± 4·1 mg g(-1) dry mass of mean total soluble sugars in roots at 3480 m and 18·8 ± 2·1 mg g(-1) dry mass in roots at the treeline. CONCLUSIONS: Root mechanical resistance, holocellulose and NSC content all decreased with increasing altitude. Holocellulose is made up principally of cellulose, the biosynthesis of which depends largely on NSC supply. Plants synthesize cellulose when conditions are optimal and NSC is not limiting. Thus, cellulose synthesis in the thin and fine roots measured in our study is probably not a priority in mature trees growing at very high altitudes, where climatic factors will be limiting for growth. Root NSC stocks at the treeline may be depleted through over-demand for carbon supply due to increased fine root production or winter root growth.

[Distribution characteristics of delta13C values in different organs of Abies georgei growing at alpine timberline].

Based on the analysis of the distribution characteristics of delta13C values in the needle, twig, branch, trunk, and root of different age Abies georgei growing at the timberline of Sergyemla Mountain on the southeast edge of Tibetan Plateau, this paper studied the process or extent of post-photosynthetic delta13C fractionation and its affecting factors. The results showed that the organ-specific difference in delta13C values was highly significant (P < 0.001), with the sequence of trunk (-24.19 per thousand +/- 0.34 per thousand) > branch (-24.56 per thousand +/- 0.62 per thousand) > root (-25.05 per thousand +/- 1.08 per thousand) > twig (-25.12 per thousand +/- 0.54 per thousand) > needle (-7.25 per thousand +/- 0.63 per thousand), which suggested that an obvious post-photosynthetic delta13C fractionation was existed between photosynthetic and non-photosynthetic organs. With increasing age of needle and twig, the delta13C value in needle decreased significantly, while that in twig showed a reverse pattern (P < 0.01). The delta13C value in needle increased significantly (P < 0.01) from lower canopy to higher canopy, whereas no significant difference (P > 0.05) of delta13C value in twig was observed among different canopy heights. A significant vertical gradient of delta13C value existed in the branches at a distance of 2.5 m from trunk (P < 0.01), but disappeared at a distance of 1.5 m or 0.5 m. At the same canopy height, the delta13C value in twig decreased with increasing distance from trunk, which was most obvious in the twigs of middle and lower canopies. All of these suggested that post-photosynthetic delta13C fractionation occurred in alpine A. georgei, and the carbon need for the growth of height-specific tree stems or branches was not wholly supplied by the corresponding segment of the tree crown at the same height.

[Root biomass and underground C and N storage of primitive Korean pine and broad-leaved climax forest in Changbai Mountains at its different succession stages].

With more than 200 years old primitive Korean pine and broad-leaved climax forest and its two 20 and 80 years old secondary Populus davidiana and Betula platyphylla forests in Changbai Mountains as test objects, this paper studied their root biomass and underground C and N storage. The results showed that with forest succession, the root biomass of 20 years old, 80 years old, and climax forests was 2.437, 2.742 and 4.114 kg x m(-2), respectively. The root C storage was 1.113, 1.323 and 2.023 kg x m(-2), soil C storage was 11.911, 11.943 and 12.587 kg x m(-2), and underground C storage was 13.024, 13.266 and 14.610 kg x m(-2), respectively, while the root N storage was 0.035, 0.032 and 0.038 kg x m(-2), soil N storage was 1.207, 1.222 and 0.915 kg x m(-2), and underground N storage was 1.243, 1.254 and 0.955 kg x m(-2), respectively, which indicated that along with forest succession, forest underground became a potential "carbon sink", whereas underground N storage did not change obviously.

[Root biomass of different stand-age Pinus yunnanensis forests and its distribution pattern in different soil depths].

With average-tree-specific sampling method, the authors measured the root biomass of Pinus yunnanensis forests with different stand ages in Yongren County, Yunnan Province. The results indicated that total root biomass increased from 8.50 t x hm(-2) in young stand (15-17 yr) to 11.70 t x hm(-2) in middle-aged stand (30-32 yr), and to 18.91 t x hm(-2) in mature stand (60-130 yr). Among different stand age classes, the biomass of coarse roots (>10 mm) varied greatly (1.5-12.3 t x hm(-2)), whereas those of medium (5-10 mm) and fine (<5 mm) roots showed less variation (medium roots 1.4-1.6 t x hm(-2), fine roots 5.3-6.2 t x hm(-2)). The root biomass of mature stand decreased with soil depth, approximately 93% of total underground biomass being distributed in surface layer (0-30 cm), and only 7% in deeper layer (30-115 cm).

Leaf traits and associated ecosystem characteristics across subtropical and timberline forests in the Gongga Mountains, Eastern Tibetan Plateau.

Knowledge of how leaf characteristics might be used to deduce information on ecosystem functioning and how this scaling task could be done is limited. In this study, we present field data for leaf lifespan, specific leaf area (SLA) and mass and area-based leaf nitrogen concentrations (N(mass), N(area)) of dominant tree species and the associated stand foliage N-pool, leaf area index (LAI), root biomass, aboveground biomass, net primary productivity (NPP) and soil available-N content in six undisturbed forest plots along subtropical to timberline gradients on the eastern slope of the Gongga Mountains. We developed a methodology to calculate the whole-canopy mean leaf traits to include all tree species (groups) in each of the six plots through a series of weighted averages scaled up from leaf-level measurements. These defined whole-canopy mean leaf traits were equivalent to the traits of a leaf in regard to their interrelationships and altitudinal trends, but were more useful for large-scale pattern analysis of ecosystem structure and function. The whole-canopy mean leaf lifespan and leaf N(mass) mainly showed significant relationships with stand foliage N-pool, NPP, LAI and root biomass. In general, as elevation increased, the whole-canopy mean leaf lifespan and leaf N(area) and stand LAI and foliage N-pool increased to their maximum, whereas the whole-canopy mean SLA and leaf N(mass) and stand NPP and root biomass decreased from their maximum. The whole-canopy mean leaf lifespan and stand foliage N-pool both converged towards threshold-like logistic relationships with annual mean temperature and soil available-N variables. Our results are further supported by additional literature data in the Americas and eastern China.

[Biomass and net primary productivity of secondary evergreen broadleaved forest in Huangmian Forest Farm, Guangxi].

By the methods of sampling plot harvesting method and allometric dimension analysis, we measured the belowground and aboveground standing biomass and net primary productivity (NPP) of the secondary evergreen broadleaved forest in Huangmian Forest farm of Guangxi, southern China, with the location of 24 degrees 51'N and 109 degrees 51'E and an altitude of about 315 m. The total biomass was 99.96 t x hm(-2), aboveground and belowground biomasses accounted for 69.41% and 30.59%, respectively. The leaf area index of trees and undergrowth shrubs was 6.50, and the total annual NPP was 24.65 t x hm(-2) x yr(-1) by estimate, aboveground and belowground NPP accounted for 44.54% and 55.46%, respectively. The NPP of fine roots was 11.79 t x hm(-2) x yr(-1), being 86.24% of the belowground NPP.