Carbon sequestration in a high-elevation, subalpine forest

We studied net ecosystem CO₂ exchange (NEE) dynamics in a high‐elevation, subalpine forest in Colorado, USA, over a two‐year period. Annual carbon sequestration for the forest was 6.71 mol C m^?2 (80.5 g C m^?2) for the year between November 1, 1998 and October 31, 1999, and 4.80 mol C m^?2 (57.6 g C m^?2) for the year between November 1, 1999 and October 31, 2000. Despite its evergreen nature, the forest did not exhibit net CO₂ uptake during the winter, even during periods of favourable weather. The largest fraction of annual carbon sequestration occurred in the early growing‐season; during the first 30 days of both years. Reductions in the rate of carbon sequestration after the first 30 days were due to higher ecosystem respiration rates when mid‐summer moisture was adequate (as in the first year of the study) or lower mid‐day photosynthesis rates when mid‐summer moisture was not adequate (as in the second year of the study). The lower annual rate of carbon sequestration during the second year of the study was due to lower rates of CO₂ uptake during both the first 30 days of the growing season and the mid‐summer months. The reduction in CO₂ uptake during the first 30 days of the second year was due to an earlier‐than‐normal spring warm‐up, which caused snow melt during a period when air temperatures were lower and atmospheric vapour pressure deficits were higher, compared to the first 30 days of the first year. The reduction in CO₂ uptake during the mid‐summer of the second year was due to an extended drought, which was accompanied by reduced latent heat exchange and increased sensible heat exchange. Day‐to‐day variation in the daily integrated NEE during the summers of both years was high, and was correlated with frequent convective storm clouds and concomitant variation in the photosynthetic photon flux density (PPFD). Carbon sequestration rates were highest when some cloud cover was present, which tended to diffuse the photosynthetic photon flux, compared to periods with completely clear weather. The results of this study are in contrast to those of other studies that have reported increased annual NEE during years with earlier‐than‐normal spring warming. In the current study, the lower annual NEE during 2000, the year with the earlier spring warm‐up, was due to (1) coupling of the highest seasonal rates of carbon sequestration to the spring climate, rather than the summer climate as in other forest ecosystems that have been studied, and (2) delivery of snow melt water to the soil when the spring climate was cooler and the atmosphere drier than in years with a later spring warm‐up. Furthermore, the strong influence of mid‐summer precipitation on CO₂ uptake rates make it clear that water supplied by the spring snow melt is a seasonally limited resource, and summer rains are critical for sustaining high rates of annual carbon sequestration.     

Comparisons of photosynthesis‐related traits of 27 abundant or subordinate bryophyte species in a subalpine old‐growth fir forest

Bryophyte communities can exhibit similar structural and taxonomic diversity as vascular plant communities, just at a smaller scale. Whether the physiological diversity can be similarly diverse, and whether it can explain local abundance patterns is unknown, due to a lack of community‐wide studies of physiological traits. This study re‐analyzed data on photosynthesis‐related traits (including the nitrogen, phosphorus and chlorophyll concentrations, photosynthetic capacities, and photosynthetic nutrient use efficiencies) of 27 bryophyte species in a subalpine old‐growth fir forest on the eastern Tibetan Plateau. We explored differences between taxonomic groups and hypothesized that the most abundant bryophyte species had physiological advantages relative to other subdominant species. Principal component analysis (PCA) was used to summarize the differences among species and trait values of the most abundant and other co‐occurring subdominant species. Species from the Polytrichaceae were separated out on both PCA axes, indicating their high chlorophyll concentrations and photosynthetic capacities (axis 1) and relatively high‐light requirements (axis 2). Mniaceae species also had relatively high photosynthetic capacities, but their light saturation points were low. In contrast, Racomitrium joseph‐hookeri and Lepidozia reptans, two species with a high shoot mass per area, had high‐light requirements and low nutrient and chlorophyll concentrations and photosynthetic capacities. The nutrient concentrations, photosynthetic capacities, and photosynthetic nutrient use efficiencies of the most abundant bryophyte species did not differ from co‐occurring subdominant species. Our research confirms the links between the photosynthesis‐related traits and adaptation strategies of bryophytes. However, species relative abundance was not related to these traits.     

Simulated N and S deposition affected soil chemistry and understory plant communities in a boreal forest in western Canada

Aims We conducted a simulated nitrogen (N) and sulfur (S) deposition experiment from 2006 to 2012 to answer the following questions: (i) does chronic N and S deposition decrease cation concentrations in the soil and foliage of understory plant species, and (ii) does chronic N and S deposition decrease plant diversity and alter species composition of the understory plant community in a boreal forest in western Canada where intensifying industrial activities are increasing N and S deposition. Methods Our field site was a mixedwood boreal forest stand located ~100 km southeast of Fort McMurray, Alberta, Canada. The experiment involved a 2 × 2 factorial design, with two levels each of N (0 and 30 kg N ha^-1 yr^-1; applied as NH₄NO₃) and S addition (0 and 30 kg S ha^-1 yr^-1; applied as Na₂SO₄). Four blocks were established in July 2006, each with four plots of 20 × 20 m randomly assigned to the treatments. Soil and understory vegetation were sampled and cover (%) of individual species of herb (height ≤ 0.5 m) and shrub (height 0.5–1 m) layers was determined in August 2012. Important findings Seven years after the treatments began, N addition increased dissolved organic carbon and N in the mineral soil (P < 0.05), whereas S addition decreased exchangeable cations (P < 0.05) in the forest floor. In the shrub layer, species evenness, and overall diversity were decreased by N addition (P < 0.05) due to increases in abundance of nitrophilous species and S addition (P < 0.01) due to decreased cation concentrations in soils. Total shrub cover decreased with S addition (P < 0.10). Nitrogen and S addition affected neither species richness nor evenness in the herb layer. However, permutational multivariate analysis of variance and non-metric multidimensional scaling analyses (based on plant cover) indicated that the effect of N and S addition on understory plant species composition in the both shrub and herb layers was species-specific. Addition of N decreased foliar phosphorus and potassium concentrations in some species, suggesting potential risk of N-meditated nutrient imbalance in those species. Our results indicate that long-term elevated levels of N and S deposition can negatively impact plant nutrition and decrease the diversity of the understory plant community in boreal forests in northern Alberta, Canada. However, considering that the current N and S deposition rates in northern Alberta are much lower than the rates used in this study, N and S deposition should not negatively affect plant diversity in the near future.     

Functional characteristics of corticolous lichens in the understory of a tropical lowland rain forest

In tropical lowland forests, corticolous crustose green algal lichens are abundant and highly diverse. This may be related to adaptation to prevailing microenvironmental conditions including, for example, high precipitation and low light intensities. In the understory of a tropical lowland rain forest in French Guiana, we studied the morphology of crustose green algal lichens and measured gas exchange and chlorophyll a fluorescence. We found that (i) periods of thallus suprasaturation with water were reduced by the presence of water-repelling surface structures of mycobiont hyphae at the thallus surface and the medulla; (ii) photosynthesis was adapted to the low light intensities present in the understory; and (iii) photosynthesis was rapidly activated in fluctuating light. The combination of these three mechanisms enables corticolous lichens to implement specific morphological and physiological strategies, which may favour growth in the limiting understory habitat of tropical lowland rain forests.     

黄土丘陵区植物功能性状对微地形变化的响应

植物功能性状是连接植物与环境的桥梁,可以反映植物对环境的适应能力.在地形复杂地区,研究不同水平上的植物功能性状随微地形的变化规律,对认识不同物种对复杂地形条件下多变环境的适应策略具有重要意义.以黄土丘陵区森林草原区自然植被为对象,分析土壤养分以及植物各功能性状值在科属和群落水平上微地形的响应关系.结果表明: 不同植物的功能性状差异较大,植物科属组成差异是影响植物功能性状的主要因素,其次是坡向,而坡位对其影响较小;群落水平的性状值在微地形上无显著差异;豆科和菊科植物的叶氮含量和根氮含量在不同坡位上均存在显著差异,禾本科只有根组织密度在不同坡位上存在显著差异;群落水平性状值的根组织密度与土壤氮含量和土壤有机质含量呈显著正相关;豆科植物的叶氮含量和根氮含量与土壤磷含量呈显著正相关,菊科和禾本科叶氮含量和根氮含量与土壤氮含量呈显著正相关.这表明不同科属植物在对环境变化响应上存在较大差异,不同科属物种对环境适应策略不同.
     

Diversification and phylogenetic correlation of functional traits for co-occurring understory species in the Chinese boreal forest

Functional traits impact species interactions, community composition, and ecosystem functioning. However, few studies have focused on the diversification and phylogenetic correlation of multiple functional traits over geological time. We conducted phylogenetic comparative analysis for boreal forest understory species in northeast China to examine the diversification and phylogenetic correlation in several functional traits: leaf area (LA), leaf carbon content (LCC), leaf dry matter content (LDMC), leaf nitrogen content (LNC), plant height (PH), and specific leaf area (SLA). Phylogenetic signals showed that there were very low levels of phylogenetic niche conservatism (PNC) in understory leaf-related traits and plant height, suggesting divergence of functional traits for the co-occurring understory species. The disparity through time analyses (DTT) indicated that trait disparities mainly originated during recent divergence events and there were no differences in the observed trait disparities compared with that expected under Brownian motion. Furthermore, we found both positive and negative phylogenetic correlations among the measured functional traits. The very low levels of PNC suggest that these functional traits diverged among co-occurring understory species, and that those species are distantly phylogenetically related. The phylogenetic correlations among traits may be caused by both positively and negatively correlated adaptions that correspond to resource acquisition strategies. This study provides evidence that divergence in functional traits may reflect understory adaptions to boreal conditions.     

Comparative cryptogam ecology: a review of bryophyte and lichen traits that drive biogeochemistry

BACKGROUND: Recent decades have seen a major surge in the study of interspecific variation in functional traits in comparative plant ecology, as a tool to understanding and predicting ecosystem functions and their responses to environmental change. However, this research has been biased almost exclusively towards vascular plants. Very little is known about the role and applicability of functional traits of non-vascular cryptogams, particularly bryophytes and lichens, with respect to biogeochemical cycling. Yet these organisms are paramount determinants of biogeochemistry in several biomes, particularly cold biomes and tropical rainforests, where they: (1) contribute substantially to above-ground biomass (lichens, bryophytes); (2) host nitrogen-fixing bacteria, providing major soil N input (lichens, bryophytes); (3) control soil chemistry and nutrition through the accumulation of recalcitrant polyphenols (bryophytes) and through their control over soil and vegetation hydrology and temperatures; (4) both promote erosion (rock weathering by lichens) and prevent it (biological crusts in deserts); (5) provide a staple food to mammals such as reindeer (lichens) and arthropodes, with important feedbacks to soils and biota; and (6) both facilitate and compete with vascular plants. APPROACH: Here we review current knowledge about interspecific variation in cryptogam traits with respect to biogeochemical cycling and discuss to what extent traits and measuring protocols needed for bryophytes and lichens correspond with those applied to vascular plants. We also propose and discuss several new or recently introduced traits that may help us understand and predict the control of cryptogams over several aspects of the biogeochemistry of ecosystems. CONCLUSIONS: Whilst many methodological challenges lie ahead, comparative cryptogam ecology has the potential to meet some of the important challenges of understanding and predicting the biogeochemical and climate consequences of large-scale environmental changes driving shifts in the cryptogam components of vegetation composition.     

亚热带天然常绿阔叶林林下9种灌木细根形态和C、N化学计量特征

林下灌木是亚热带常绿阔叶林重要的构成部分,但林下灌木细根功能性状变异规律及地下生态策略仍不清楚。以福建建瓯万木林自然保护区内9种灌木为研究对象,对细根直径、根长、比根长、组织密度、碳浓度和氮浓度6个细根性状进行研究,采用序级划分法,分析不同树种细根性状序级间的变化特征、常绿和落叶灌木细根性状之间的差异,不同序级细根性状之间的关系以及细根性状变异维度。结果表明:树种和序级对9种灌木细根形态和化学性质有显著影响。直径、根长、根组织密度随着序级的增加而逐渐增加,比根长和氮浓度逐渐减小,碳浓度在序级间的变化趋势不一,未表现出明显的规律。落叶灌木细根直径、根长和氮浓度均显著高于常绿灌木,碳浓度和组织密度显著低于常绿灌木,表明与常绿灌木相比落叶灌木更偏向于资源获取型生态策略,常绿灌木则更偏向于保守型策略。灌木细根在不同序级间的直径与比根长、组织密度,氮浓度与组织密度有较强的相关性,细根其他性状间的关系并不密切或因序级而异。主成分分析结果表明灌木细根性状变异沿一个主成分轴发生变异,该轴表示灌木细根的资源获取和保守的权衡策略。     

高寒草甸对刈割、施肥和浇水发生响应的最优植物性状集和功能型

基于植物性状和功能型的特征变化对于研究植被动态和生态系统功能变化具有重要意义。通过在高寒矮嵩草(Kobresia humilis)草甸为期5年(2007-2011年)的刈割(不刈割、留茬3 cm、留茬1 cm)、施肥(施肥、不施肥)和浇水(浇水、不浇水)控制实验, 采用递归算法(recursive algorithm)和多元回归分析筛选对模拟放牧发生响应的最优植物性状集和响应功能型, 以及影响群落生产力变化的作用功能型。研究结果显示: (1)在不施肥不浇水、仅施肥、仅浇水和既施肥又浇水4种条件下的最优植物性状集不同, 它们分别是叶缘形状-株高-叶干质量-比叶面积、生活周期-株高-叶干质量-比叶面积、生活周期-叶片叶绿素含量-叶表面结构-株高-叶干质量-比叶面积和繁殖结构-叶缘-株高。其中, 株高、叶干质量和比叶面积是对刈割和土壤资源变化更为敏感的植物性状。(2)在这4种处理条件下, 共获得14个最优响应功能型和4个作用功能型。作用功能型对群落生产力变异的解释能力在50.3%-86.4%之间。(3)最优响应功能型和作用功能型分别占功能型总数的70%和20%。作用功能型占最优响应功能型的28.5%, 两者间仅存在部分重叠。上述结果说明, 植物功能性状和功能型变化能够准确地反映植被的放牧响应和生态系统功能变化, 但是不同资源条件下群落的最优响应性状集和功能型不同。作用功能型是同时反映植被放牧响应和生态系统功能变化的最优功能型。     

Competition, traits and resource depletion in plant communities

Although of primary importance to explain plant community structure, general relationships between plant traits, resource depletion and competitive outcomes remain to be quantified across species. Here, we used a comparative approach to test whether instantaneous measurements of plant traits can capture both the amount of resources depleted under plant cover over time (competitive effect) and the way competitors perceived this resource depletion (competitive response). We performed a large competition experiment in which phytometers from a single grass species were transplanted within 18 different monocultures grown in a common-garden experiment, with a time-integrative quantification of light and water depletion over the phytometers’ growing season. Resource-capturing traits were measured on both phytometers (competitive response traits) and monocultures (competitive effect traits). The total amounts of depleted light and water availabilities over the season strongly differed among monocultures; they were best estimated by instantaneous measurements of height and rooting depth, respectively, performed when either light or water became limiting. Specific leaf area and leaf water potential, two competitive response traits measured at the leaf level, were good predictors of changes in phytometer performance under competition, and reflected the amount of light and water, respectively, perceived by plants throughout their lifespan. Our results demonstrated the relevance of instantaneous measures of plant traits as indicators of resource depletion over time, validating the trait-based approach for competition ecology. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Morphological and physiological traits in relation to carbon balance in a diverse clade of dryland mosses

Plant functional trait analyses have focused almost exclusively on vascular plants, but bryophytes comprise ancient and diverse plant lineages that have widespread global distributions and important ecological functions in terrestrial ecosystems. We examined a diverse clade of dryland mosses, Syntrichia, and studied carbon balance during a precipitation event (C‐balance), a functional trait related to physiological functioning, desiccation tolerance, survival, and ecosystem carbon and nitrogen cycling. We examined variability in C‐balance among 14 genotypes of Syntrichia and measured an additional 10 physiological and 13 morphological traits at the cell, leaf, shoot, and clump level. C‐balance varied 20‐fold among genotypes, and highest C‐balances were associated with long, narrow leaves with awns, and small cells with thick cell walls, traits that may influence water uptake and retention during a precipitation event. Ordination analyses revealed that the axis most strongly correlated with C‐balance included the maximum chlorophyll fluorescence, F_m, indicating the importance of photosystem II health for C exchange. C‐balance represents a key functional trait in bryophytes, but its measurement is time intensive and not feasible to measure on large scales. We propose two models (using physiological and morphological traits) to predict C‐balance, whereby identifying simpler to measure traits for trait databases.     

Water relations and photosynthetic characteristics in different functional groups of epiphytic lichens in montane forest of Ailaoshan

Aims There are abundant epiphytic lichens in the tropical and subtropical montane forest ecosystems, which are important components of forest canopy and play a vital role in biodiversity conservation, environmental monitoring and nutrient cycling. In accordance with photobiont type, growth form and reproductive strategy, the epiphytic lichens can be divided into different functional groups, with different distribution patterns. In this study we aim to explain this phenomenon from the perspective of physiological ecology. Methods The maximum water content, water loss curves, photosynthetic water and light response curves were determined in four epiphytic lichen functional groups, including cyanolichens, fruticose lichens, broadly lobed foliose lichens and narrowly lobed foliose lichens. Important findings The functional characteristics of epiphytic lichens influence their maximum water-holding capacity and rate of water loss. The cyanolichens have higher maximum water content, while the fruticose lichens have a faster water loss. The cyanolichens that are widely distributed in the moist habitats require particularly high moisture for their photosynthetic activities; their optimal water content for photosynthesis is higher in comparison with other groups. They also have a low light compensation point and a high light saturation point, which explain the wide range of light intensity of the habitat. The fruticose lichens, widely distributed in the relatively arid habitats with high irradiance, have high light compensation point and light saturation point, and low optimum water content for photosynthesis. The broadly lobed foliose lichens and the narrowly lobed foliose lichens have a high light compensation point and light saturation point; they preferably occur in habitats with strong light.     

Coupling between carbon cycling and climate in a high-elevation,subalpine forest: a model-data fusion analysis

Fundamental questions exist about the effects of climate on terrestrial net ecosystem CO₂ exchange (NEE), despite a rapidly growing body of flux observations. One strategy to clarify ecosystem climate–carbon interactions is to partition NEE into its component fluxes, gross ecosystem CO₂ exchange (GEE) and ecosystem respiration (R _E), and evaluate the responses to climate of each component flux. We separated observed NEE into optimized estimates of GEE and R _E using an ecosystem process model combined with 6 years of continuous flux data from the Niwot Ridge AmeriFlux site. In order to gain further insight into the processes underlying NEE, we partitioned R _E into its components: heterotrophic (R _H) and autotrophic (R _A) respiration. We were successful in separating GEE and R _E, but less successful in accurately partitioning R _E into R _A and R _H. Our failure in the latter was due to a lack of adequate contrasts in the assimilated data set to distinguish between R _A and R _H. We performed most model runs at a twice-daily time step. Optimizing on daily-aggregated data severely degraded the model’s ability to separate GEE and R _E. However, we gained little benefit from using a half-hourly time step. The model-data fusion showed that most of the interannual variability in NEE was due to variability in GEE, and not R _E. In contrast to several previous studies in other ecosystems, we found that longer growing seasons at Niwot Ridge were correlated with less net CO₂ uptake, due to a decrease of available snow-melt water during the late springtime photosynthetic period. Warmer springtime temperatures resulted in increased net CO₂ uptake only if adequate moisture was available; when warmer springtime conditions led into mid-summer drought, the annual net uptake declined.     

南极地区苔藓地衣植物的地球化学元素营养富集特征

研究南极苔藓地衣中地球化学元素的营养富集特征,发现K,Ca为苔藓地衣中最活跃元素,主要以主动吸收的方式累积于苔藓地衣植物中,P极易富集在地衣的藻层,参与藻类的有机合成过程,苔藓容易富集环境中的S,Al,Si以被动吸收的方式累积于地衣中,同时Fe,Mg以被动吸收的方式累积于苔藓体内,根据元素的含量和营养作用,研究认为K,Ca苔地衣的大量无机营养元素,S,P为苔藓地衣的中等营养元素,Al,Si为苔藓地衣的环境累积元素。     

哀牢山山地森林不同附生地衣功能群的水分关系和光合生理特征

附生地衣是热带和亚热带山地森林生态系统中重要的结构性组分, 在生物多样性保护、环境监测、养分循环中发挥着重要作用。附生地衣按共生藻、生活型和繁殖策略的不同可划分为不同的功能群, 不同附生地衣功能群的分布格局存在较大的差异, 然而其生理生态机制仍不清楚。该研究以我国西南地区哀牢山亚热带山地森林中的附生地衣优势类群为研究对象, 对该地区蓝藻地衣、阔叶地衣、狭叶地衣及枝状地衣4种功能群的8种附生地衣的水分关系、光合生理特征等进行了测定分析, 结果显示: 不同功能群附生地衣的持水力和失水速率均存在差异, 其中蓝藻地衣具有较高的最大水分含量, 而枝状地衣的失水速率较快; 过高和过低的水分含量都会抑制附生地衣的光合作用, 但抑制程度有所差异; 蓝藻地衣的光合作用最适水分含量比较高, 表明它们的光合生理活动对水分条件要求较高, 所以它们偏好潮湿的生境, 同时蓝藻地衣的光补偿点比较低但光饱和点却不低, 反映出它们具有较宽的光强适应范围, 所以蓝藻地衣能够同时分布于强光和弱光生境中; 枝状地衣的光合最适水分含量较低, 表明它们的光合生理活动对水分条件要求不是很高, 能够适应较为干旱的环境, 同时枝状地衣的光补偿点和光饱和点都很高, 说明它们的光合生理活动对光照条件要求比较高, 所以它们广泛分布于强光生境中; 阔叶和狭叶地衣的光补偿点比较高, 说明它们更适应有充足光照条件的生境。     

Stoichiometric traits (N:P) of understory plants contribute to reductions in plant diversity following long‐term nitrogen addition in subtropical forest

Nitrogen enrichment is pervasive in forest ecosystems, but its influence on understory plant communities and their stoichiometric characteristics is poorly understood. We hypothesize that when forest is enriched with nitrogen (N), the stoichiometric characteristics of plant species explain changes in understory plant diversity. A 13‐year field experiment was conducted to explore the effects of N addition on foliar carbon (C): N: phosphorus (P) stoichiometry, understory plant species richness, and intrinsic water use efficiency (iWUE) in a subtropical Chinese fir forest. Four levels of N addition were applied: 0, 6, 12, and 24 g m⁻² year⁻¹. Individual plant species were categorized into resistant plants, intermediate resistant plants, and sensitive plants based on their response to nitrogen addition. Results showed that N addition significantly decreased the number of species, genera, and families of herbaceous plants. Foliar N:P ratios were greater in sensitive plants than resistant or intermediate resistant plants, while iWUE showed an opposite trend. However, no relationship was detected between soil available N and foliar N, and soil N:P and foliar N:P ratios. Our results indicated that long‐term N addition decreased the diversity of understory plants in a subtropical forest. Through regulating water use efficiency with N addition, sensitive plants change their N:P stoichiometry and have a higher risk of mortality, while resistant plants maintain a stable N:P stoichiometry, which contributes to their survival. These findings suggest that plant N:P stoichiometry plays an important role in understory plant performance in response to environmental change of N.