QuestionKnowledge of how nutrient use strategies differ between forest trees and alpine shrubs/grasses is important to understand the mechanisms of vegetation changes from montane forests to alpine shrubs/grasslands along altitudinal gradients. We tested the hypothesis that, to maximize the nitrogen use efficiency (NUE) of canopy production, forest trees tend to have a higher mean residence time (MRT) of nitrogen in the plants through increased leaf life span, whereas alpine shrublands and grasslands tend to have higher nitrogen productivity through increased below-ground biomass fraction. We further tested whether similar patterns are found in phosphorus use efficiency (PUE).LocationTwenty-one sampling sites along Tibetan Alpine Vegetation Transects (TAVT) at altitudes from 1900 m to 4900 m.MethodsWe measured the maximum biomass of new canopy leaves and twigs and the concentrations of their nutrients N and P and associated ecosystem variables along the TAVT. NUE (PUE) was calculated as the product of nutrient productivity (dry matter production per unit N or P in new canopy leaves and twigs) and MRT (the ratio of foliage nutrient pool to annual nutrient uptake).ResultsWith increasing altitude, leaf life span increased in forest trees but decreased in shrublands and grasslands, while below-ground fraction increased when vegetation changed from montane forests to alpine shrubs/grasslands. In forest trees, higher N-MRT and P-MRT and lower P productivity were associated with longer leaf life span and lower below-ground fraction, while N productivity varied little. In alpine shrublands and grasslands, N-MRT, P-MRT and P productivity varied little with leaf life span and below-ground fraction, while N productivity was positively correlated with below-ground fraction.ConclusionsOur data supported the hypothesis, suggesting that NUE of canopy production would be a measure of changes in ecosystem functioning from montane forests to alpine shrublands and grasslands along altitudinal gradients. The findings provide an insight into the linkage between biogeochemistry and phytogeographic processes across ecosystems. The data along Tibetan Alpine Vegetation Transects indicate that, to maximize nitrogen use efficiency of canopy production, forest trees tend to have a higher mean residence time of nitrogen in the plants through increased leaf life span, whereas alpine shrubs and grasslands tend to have higher nitrogen productivity through increased below-ground fraction of biomass. Similar patterns are found in phosphorus use efficiency.image

IntroductionShrub 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.MethodsWe 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.ResultsWe 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.DiscussionOur 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.

green twigs is a poor predictor for the variation in leaf longevity. In summary, for the J. saltuaria species in timberline or nearby subalpine forests, the effects of elevation and canopy depths on leaf longevity are minor, and the leaf trait analysis is in accordance with the prediction of LES.

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.

Few data have demonstrated why early-season freezing events may increase under a warmer climate at alpine treelines, which is critical to understand the key limiting factors determining treeline dynamics under future climate warming. Here we test the hypothesis that the increase of early-season freezing events under a warmer climate is mainly associated with advanced onset of growing season and enhanced radiative cooling effect in the pre-monsoon season. We conducted 11-year observations of microclimate factors across north-facing and southfacing treelines in the Sergyemla Mountains, southeast Tibet. We found that the frequency, intensity, and duration of early-season freezing events were generally higher under a warmer climate on the south-facing slope or in the warmer years within a slope, in which the frequency of early-season freezing events significantly increased with increasing annual mean air-temperature. During 2006-2016, the frequency, intensity and duration of early-season freezing events typically showed a negative correlation with the onset date of growing season, while their frequency was positively correlated with the early-season global radiation. In each of both slopes, global radiation was significantly higher and long-wave radiation balance was much more negative on days with daily minimum air-temperature (Tmin) < 0 C than with Tmin > 0 degrees C, indicating the generality of radiative cooling effect on early-season freezing events at high elevations. Our data support the hypothesis, revealing physical mechanisms for the increase of early-season freezing events with climate warming. The physical mechanisms should provide a general explanation for the topography-dependent pattern of tree species distribution and alpine treeline stability under climate warming in high mountainous regions like the Himalayas.

Uncertainty still exists on the directions and intensity of changes in leaf herbivory under scenarios of global warming. We, therefore, conducted an investigation on insect herbivory along an elevational gradient to explore how leaf herbivory may respond to future climate warming using a space-for-time substitution approach. We hypothesize that the leaf herbivory for alpine woody species should decline with elevation. We also guess the leaf herbivory may not differ between different leaf-age groups, for the old leaves are less attractive to insect due to their lower nutrients. To approve these assertions, we measured different aspects of leaf herbivory, i.e., the intensity (leaf area consumed per damaged leaf), frequency (percentage of leaves damaged), and rate (percentage of consumed leaf area over the total number of leaves), across different leaf-age groups for Rhododendron aganniphum var. schizopeplum along an elevational gradient (4280–4640 m) in the Sergymla Mountains, southeast Tibet. Related leaf traits of leaf mass per area (LMA) and nitrogen (Nmass), as well as total phenolics for 1-year-old leaves, were also investigated. In contrast with our expectation, the rate of herbivory did not vary with elevation, while the frequency and intensity reflected increasing and declining patterns, respectively. LMA and total phenolics tended to increase with elevation, while Nmass significantly declined. Further analysis indicated that Nmass and total phenolics mainly explained the variation of frequency of herbivory, while Nmass reflected a significant effect on the variation of intensity. No differences in herbivory were found between the leaf-age groups. Our results suggest that the lower intensity of leaf herbivory at higher elevations is mainly associated with the declined nutritional levels, while the higher frequency might be related to the higher costly anti-herbivore defenses like phenolics and the lower nutritional levels. Although the older leaves are exposed to herbivore attacks for a longer time, they possessed the same herbivory levels as current-year leaves partly due to their lower nitrogen concentrations. Both supporting the nutrient limitation hypothesis, i.e., plants with lower leaf nutrient levels possess less leaf herbivory. In all, the herbivory on the alpine Rhododendron is small in magnitude, but given the higher herbivory (for intensity at least) under persistent warming conditions and its potential impacts on mediating plant–insect interactions, insect herbivory should be included in predictions of climate change impacts on alpine ecosystems.

Knowledge of how leaf mass per area (LMA) and its components, leaf thickness (LT) and leaf density (LD), and related leaf traits may vary with altitude concerning different plant life forms at extremely high altitudes is scarce. We measured LMA, leaf nitrogen (N-mass), LT, LD, and the related anatomical characteristics of an evergreen shrub (Rhododendron aganniphum var. schizopeplum, RAS) and a wintergreen herb (Bergenia purpurascen, BP) along an altitudinal gradient above 4200 m a.s.l. to disclose how LMA for two different growth forms may vary with altitude and which leaf attribute determines the within-species variation of LMA. LMA significantly increased with rising altitude, at a pace of 40 g m(-2)/100 m for RAS and 12 g m(-2)/100 m for BP, respectively. LT was mainly responsible for the variability in LMA for RAS, and the variation of LT and LD contributed almost equally for BP. N-mass tended to decrease with rising altitude, and was negatively related to LMA and LT for both species. Foliar delta C-13 showed significantly rising trends with altitude, and was positively correlated with LMA and LT. The results indicated that plants at the extremely high alludes are adapted to possessing especially higher LMA and steeper slopes of the relationships between LMA and allude, as well as lower leaf nitrogen concentrations and higher delta C-13 to cope with the low-temperature environment. Compared with many other studies, LMA for both species are assumed to be more sensitive to the changing environment (especially warming). Also, Rhododendron shrub displayed steeper slope compared with dwarf herbs beneath the shrub layer indicating the former species should be more sensitive to warming climate. In all, exploring the plant strategies via leaf functional traits is of importance to understand the geography of evergreen species in alpine regions and its responses to future climate change.

Few data have examined the warming-related controls on seedling recruitment above the treeline, which is critical to understand causes for treeline stability under climate warming. We tested the hypothesis that compared to controls on seed dispersal and germination, the warming-induced decrease of seedling survival above the treeline is more likely to cause treeline stability. Long-term microclimate observations and 4-year seed and seedling transplant experiments were conducted in two contrasting treeline ecotones on the opposite slopes of a U-shaped valley at the peak of the Sergyemla Mountains. Seeds and young seedlings of Smith-fir (Abies georgei var. smithii) were transplanted to the treeline (4320 m) and Rhododendron shrub (4390 m) on the north-facing slope and to the juniper treeline (4425 m) on the south-facing slope. Transplanted seeds were collected from the Smith-fir treeline and its distribution center (3800 m). We also investigated the distribution of naturally-established seedlings and its correlation with shrub cover along elevation transects above the treeline. Smith-fir seedlings growing under shrubs were observed up to 40 m in elevation higher above the treeline, and there was either no correlation or a low correlation between seedling density and shrub cover. Harsh environments above the treeline did not limit the germination of seeds from different seed sources. However, transplanted seedlings cannot survival well above the treeline or at the juniper treeline due to high frequency of early-season freezing events and strong light intensity, while the absence of early-season freezing events in the Smith-fir treeline forest was more beneficial to seedling survival. During 2006-2015, the frequency of early-season freezing events increased with increasing annual mean air temperature above the treeline and at the juniper treeline but not in the Smith-fir treeline forest. The warming-induced increase of early-season freezing events, combined with high light intensity, mainly limits the survival of young seedlings above the treeline, which is more likely to cause treeline stability. Early-season freezing events should be an important index for evaluating the vulnerability of treeline forests to global warming.

Severe freezing events could be one of the crucial causes preventing the advance of alpine timberline.However,studies on the characteristics of freezing events across altitudes and vegetation types at high elevations are scarce.Based on the measurements of air and soil temperature for forests and shrubs across an Abies georgei var.Smithii timberline ecotone along a north-facing slope in the Sergyemla Mountains,southeast Tibet,we analyzed altitudinal variations of freezing events for different vegetation types during growing season based on two different deftnitions-canopy temperature and soil temperature.Results are as follows:1) compared with canopy temperature of the growing season,soil temperature lagged behind nearly one month,and the length of growing season based on canopy temperature was significantly longer than that calculated by soil temperature for subalpine and timberline forests.2) Although the growing season length varied between different definitions,there were more freezing events in Rhododendron shrubs in the growing season than in the subalpine and timberline forests,and the frequency for the former was 1 to 3 times more than the latter.Besides,the intensity was stronger and the duration was longer in Rhododendron shrubs than in the subalpine and/or timberline forests.Our results indicated that there were more growing-season freezing events in Rhododendron shrubs at high elevations,with their frequency,intensity,and duration all tending to strengthen with rising altitudes.This might result in the difficulty of seedling establishment above the timberline.

Advances of alpine timberline forests during last century are not ubiquitous worldwide,suggesting additional factors and mechanisms likely affect the response of alpine timberline forests to climate warming.Upward shifts of treelines begin with seed dispersal and germination,and seedling establishment above the treeline and any limiting factors during these processes may affect treeline migration.Therefore,investigation of mechanisms controlling seedling recruitment at alpine treeline will be helpful to elucidate treeline formation and its response to future climate change.We reviewed recent advances in tree seedling recruitment at alpine treelines from the key seed and seedling stages.For the seed stage,the seed quantity and quality generally decreased with the sum temperature during summer;the seed dispersal to elevations above treeline was impeded by low wind speed,dense dwarf shrub and grass cover;the ability of seed germination above the treeline was impaired by frost and water stresses near the ground.Also,the allelochemical properties of shrubs had negative effects on seed germination.For the seedling stage,large temperature amplitudes and freezing events during the growing season,as well as the extremely low temperature during winter,were important factors affect seedling mortality.Also the low-temperature photoinhibition resulted from the combination of low temperature and high sunlight significantly decreased seedling photosynthesis during the growing season.Besides,frost-heave activity induced by large soil temperature amplitude and soil water deficits during summer impeded seedling establishment at and above the treeline.Snowpack could keep the seedlings away from the extremely low air temperature during the winter and supply snowmelt water in the early growing season.However,too long duration of the snowpack might increase the possibility of fungal infection that promote seedling mortality.Dense shrub and grass cover above the treeline and the presence of herbivores might decrease seedling survival.In all,the influence of climate warming on seedling establishment across the timberline ecotone is complex and uncertain.Further research is needed to explore the exact effects of warmth-induced environmental changes to seedling recruitment at the alpine treeline.Since the beginning of the growing season might advance under scenarios of climate warming,which in turn led to more early-season freezing events at and above the treeline,it is important to define the temperature threshold of freezing events to analyze the relationship between growing-season freezing events and increasing temperature in the future.Based on this threshold,we can further disclose the effects of growing-season freezing events on seedling establishment at alpine treeline,which will be helpful to elucidate treeline formation and predict treeline dynamics under future climate change.

Aims The expansion of shrublands is considered as one of the key reasons leading to the increase of carbon den-sity in terrestrial ecosystems in China. In the present study, our aims were to explore the biomass allocation and carbon density of Sophora moorcroftiana shrublands in Xizang.Methods We sampled the biomass of S. moorcroftiana shrubs from 18 sites in the middle reaches of Yarlung Zangbo River, Xizang. Using concentrations of different organs, we estimated the carbon density of different lay-ers in S. moorcroftiana shrublands.Important findings The plant cover rather than biomass volume (the product of cover and height) provided the best fit for aboveground biomass. The average of the total biomass was 5.71 Mg·hm–2, ranging from 2.32 to 8.96 Mg·hm–2. The average biomass of shrub layer, the main component of shrub ecosystem, was 4.08 Mg·hm–2, ac-counting for 71% of the total biomass. The belowground biomass of shrub and herb layers was 2.08 and 0.86 Mg·hm–2, respectively, which was higher than the corresponding aboveground biomass. The average biomass carbon density was 2.48 Mg·hm–2. Shrub vegetation in the eastern part of the middle reaches has lower carbon density than that in the western part. The relatively high biomass allocation to roots to increase water and nutrient undertake as well as physical support for plants is an important strategy of S. moorcroftiana to cope with the arid environment on the Qinghai-Xizang Plateau. Moreover, the lower carbon density in the eastern part of the middle reaches might be due to the dry environment resulted from high temperature and evapotranspiration and enhanced human activities at low altitudes. The continuous decrease of evapotranspiration under scenarios of future climate change may lead to increase in carbon density in S. moorcroftiana shrublands.

Knowledge of the quantitative relationship between plant cover and its corresponding biomass for shrubs is not well known, especially for those on the Tibetan Plateau. Based on investigations of 35 sites, 90 plots and 95 standard individuals for two typical shrub species (Rhododendron nivale Hook. f. and Sophora moorcroftiana (Benth.) Baker) across Tibet, we developed allometric models for biomass estimation from measurements of crown diameter and/or height. We found that the parameters of crown projection area (CPA), height and their product (volume) were all significantly (p < 0.01) correlated with dry mass of different organs for both species at individual level. The CPA rather than volume best predicted aboveground dry mass. This is because that the bulk density declined significantly with increasing plant height, leading to the inappropriateness for plant height itself being employed as a parameter in biomass estimation, especially for shrubs in smaller size groups. At community level, cover was tightly correlated with the aboveground, belowground and total biomass (R-2 = 0.97-0.99). Therefore, biomass for the two shrubs can be simply estimated by measuring plant cover, which enables rapid estimation of shrubland carbon stock at large scales by using satellite data and repeated experiments over time. This non-destructive method using cover to estimate shrub biomass can be applied not only in arid ecosystems but also in alpine or subalpine environment. (C) 2015 Elsevier Ltd. All rights reserved.

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.

Aims Our objectives were to disclose why evergreen shrubs, but not deciduous shrubs, dominate above timber- line in humid southeastern Qinghai-Xizang Plateau, and to test if different functional types converge in response to the warming climate from aspect of nitrogen limitation. Methods Based on investigations of nitrogen concentration in senesced leaves of seven shrubs across timberline ecotones in the Sergymla Mountains, Southeast Xizang, we analyzed differences in leaf mass- and area-based ni- trogen resorption proficiency among different functional types (evergreen vs. deciduous), altitudes and aspects at 4 200-4 400 m a.s.l. Important findings Leaf mass-based nitrogen resorption proficiency was higher in the evergreen shrub Rhodo- dendron aganniphum var. schizopeplum than in deciduous shrubs. However, the leaf area-based N resorption pro- ficiency was relatively higher in deciduous shrubs due to their lower leaf mass per unit area. Although no signifi- cant difference in the resorption proficiency was found between altitudes or aspects for the deciduous shrubs of Salix oritrepha and Berberis hemsleyana, leaf mass-based N resorption proficiency was higher at higher altitude for the evergreen shrub Rhododendron aganniphum var. schizopeplum. Decreasing N concentration in senesced leaves, i.e., increasing resorption proficiency, which can improve N use efficiency, is an important strategy for the evergreen shrub to cope with the stressful alpine environment across timberline ecotones. Compared with the de- ciduous shrubs, N resorption proficiency in the evergreen shrub Rhododendron aganniphum var. schizopeplum is assumed to be more sensitive to future climate warming.

Seedling mortality is important to the formation and dynamics of alpine treeline. There is a need to understand the mechanisms governing seedling mortality at and above treelines under a warmer climate. We tested the hypothesis that under a warmer climate, seed-based treeline seedlings are especially vulnerable to freezing events in the early growing season. Using space-for-time substitution, we conducted a 5-yr reciprocal transplant experiment for >10 yr-old seedlings of seed-based fir and root-sprouting juniper between north-facing and south-facing slopes of a valley with elevations of 4200-4600 m in the Sergyemla Mountains, and additional experiments of seed germination and younger seedling transplants (with ages of 3-5 yr) at and above the fir treeline. Between both slopes, annual precipitation was similar but annual mean air-temperature above the treeline differed by 2.0 degrees C, being comparable to the temporal difference of 2.3 degrees C between the warmest and coldest years and the unchanged trend of precipitation during 1960-2008 at Nyingchi station nearby the study sites. The frequency, intensity and duration of growing-season freezing events were much higher under the warmer climate on the south-facing slope. Across years and non-forested sites above both treelines, annual mean air-temperature was well correlated with the early-season (April-June) freezing events. In pooled data across years and sites, annual mortality increased in fir seedlings but varied little in juniper seedlings with increasing freezing events in the early growing season. Similar patterns were also found in their annual growth rates. Partial correlation analysis indicated that the early-season freezing event was the major limiting factor determining annual mortality of fir seedlings, while that of juniper seedlings varied little with all the microclimate factors. Harsh environments above the treeline did not limit fir seed germination. The finding that the early-season freezing events under a warmer climate increased fir seedling mortality can explain the cause for the unique distribution pattern of fir and juniper treelines on opposite slopes of a valley in southeast Tibet, and suggests an explanation for the phenomenon that the world's highest fir-treeline position did not advance with climatic warming in past 200 years. (C) 2013 Elsevier B.V. All rights reserved.