大兴安岭东部主要林分类型乔木层生物量估算模型

大尺度森林生物量的估算方法是人们目前关注的焦点,建立林分生物量模型成为一种趋势.本研究以大兴安岭东部6个主要林分类型为研究对象,构建了其总量及各分项一元、二元可加性林分生物量模型.采用似然分析法判断总量及各分项生物量异速生长模型的误差结构(可加型或相乘型),采用非线性似乎不相关回归模型方法估计模型参数.结果表明: 经似然分析法判断,大兴安岭东部6个主要林分类型总量及各分项生物量异速生长模型的误差结构都是相乘型的,对数转换的可加性生物量可以被选用.各林分类型可加性生物量模型的调整后确定系数为0.78～0.99,平均相对误差为-2.3%～6.9%,平均相对误差绝对值6.3%～43.3%.增加林分平均高可以提高绝大多数生物量模型的拟合效果和预测能力,而且总量、地上和树干生物量模型效果较好,树根、树枝、树叶和树冠生物量模型效果较差.为了使模型参数估计更有效,所建立的生物量模型应当考虑林分总生物量及各分项生物量的可加性.本研究建立的林分总量与各分项生物量模型都能对大兴安岭东部6个主要林分类型生物量进行较好的估计.     

Carbon allocation in forest ecosystems

Carbon allocation plays a critical role in forest ecosystem carbon cycling. We reviewed existing literature and compiled annual carbon budgets for forest ecosystems to test a series of hypotheses addressing the patterns, plasticity, and limits of three components of allocation: biomass, the amount of material present; flux, the flow of carbon to a component per unit time; and partitioning, the fraction of gross primary productivity (GPP) used by a component. Can annual carbon flux and partitioning be inferred from biomass? Our survey revealed that biomass was poorly related to carbon flux and to partitioning of photosynthetically derived carbon, and should not be used to infer either. Are component fluxes correlated? Carbon fluxes to foliage, wood, and belowground production and respiration all increased linearly with increasing GPP (a rising tide lifts all boats). Autotrophic respiration was strongly linked to production for foliage, wood and roots, and aboveground net primary productivity and total belowground carbon flux (TBCF) were positively correlated across a broad productivity gradient. How does carbon partitioning respond to variability in resources and environment? Within sites, partitioning to aboveground wood production and TBCF responded to changes in stand age and resource availability, but not to competition (tree density). Increasing resource supply and stand age, with one exception, resulted in increased partitioning to aboveground wood production and decreased partitioning to TBCF. Partitioning to foliage production was much less sensitive to changes in resources and environment. Overall, changes in partitioning within a site in response to resource supply and age were small (<15% of GPP), but much greater than those inferred from global relationships. Across all sites, foliage production plus respiration, and total autotrophic respiration appear to use relatively constant fractions of GPP – partitioning to both was conservative across a broad range of GPP – but values did vary across sites. Partitioning to aboveground wood production and to TBCF were the most variable – conditions that favored high GPP increased partitioning to aboveground wood production and decreased partitioning to TBCF. Do priorities exist for the products of photosynthesis? The available data do not support the concept of priorities for the products of photosynthesis, because increasing GPP increased all fluxes. All facets of carbon allocation are important to understanding carbon cycling in forest ecosystems. Terrestrial ecosystem models require information on partitioning, yet we found few studies that measured all components of the carbon budget to allow estimation of partitioning coefficients. Future studies that measure complete annual carbon budgets contribute the most to understanding carbon allocation.     

Optimal co-allocation of carbon and nitrogen in a forest stand at steady state

Nitrogen (N) is essential for plant production, but N uptake imposes carbon (C) costs through maintenance respiration and fine-root construction, suggesting that an optimal C:N balance can be found. Previous studies have elaborated this optimum under exponential growth; work on closed canopies has focused on foliage only. Here, the optimal co-allocation of C and N to foliage, fine roots and live wood is examined in a closed forest stand. Optimal co-allocation maximizes net primary productivity (NPP) as constrained by stand-level C and N balances and the pipe model. Photosynthesis and maintenance respiration increase with foliar nitrogen concentration ([N]), and stand-level photosynthesis and N uptake saturate at high foliage and fine-root density. Optimal NPP increases almost linearly from low to moderate N availability, saturating at high N. Where N availability is very low or very high, the system resembles a functional balance with a steady foliage [N]; in between, [N] increases with N availability. Carbon allocation to fine roots decreases, allocation to wood increases, and allocation to foliage remains stable with increasing N availability. The predicted relationships between biomass density and foliage [N] are in reasonable agreement with data from coniferous stands across Finland. All predictions agree with our qualitative understanding of N effects on growth.     

Fire Severity and Long-term Ecosystem Biomass Dynamics in Coniferous Boreal Forests of Eastern Canada

The objective of this study was to characterize the effects of soil burn severity and initial tree composition on long-term forest floor dynamics and ecosystem biomass partitioning within the Picea mariana [Mill.] BSP-feathermoss bioclimatic domain of northwestern Quebec. Changes in forest floor organic matter and ecosystem biomass partitioning were evaluated along a 2,355-year chronosequence of extant stands. Dendroecological and paleoecological methods were used to determine the time since the last fire, the soil burn severity of the last fire (high vs. low severity), and the post-fire tree composition of each stand (P. mariana vs. Pinus banksiana Lamb). In this paper, soil burn severity refers to the thickness of the organic matter layer accumulated above the mineral soil that was not burned by the last fire. In stands originating from high severity fires, the post-fire dominance by Pinus banksiana or P. mariana had little effect on the change in forest floor thickness and tree biomass. In contrast, stands established after low severity fires accumulated during the first century after fire 73% thicker forest floors and produced 50% less tree biomass than stands established after high severity fires. Standing tree biomass increased until approximately 100 years after high severity fires, and then decreased at a logarithmic rate in the millennial absence of fire. Forest floor thickness also showed a rapid initial accumulation rate, and continued to increase in the millennial absence of fire at a much slower rate. However, because forest floor density increased through time, the overall rate of increase in forest floor biomass (58 g m⁻² y⁻¹) remained constant for numerous centuries after fire (700 years). Although young stands (< 200 years) have more than 60% of ecosystem biomass locked-up in living biomass, older stands (> 200 years) sequester the majority (> 80%) of it in their forest floor. The results from this study illustrate that, under similar edaphic conditions, a single gradient related to time since disturbance is insufficient to account for the full spectrum of ecosystem biomass dynamics occurring in eastern boreal forests and highlights the importance of considering soil burn severity. Although fire severity induces diverging ecosystem biomass dynamics in the short term, the extended absence of fire brings about a convergence in terms of ecosystem biomass accumulation and partitioning.     

三种林分生物量估算方法的比较

区域森林生物量的估算方法是人们目前关注的焦点,建立林分生物量模型成为一种趋势.本文以吉林省落叶松人工林固定样地为例,采用非线性似乎不相关回归法构建2种林分生物量模型,即基于林分变量的林分生物量模型(模型系统Ⅰ)和基于生物量换算系数的林分生物量模型(模型系统Ⅱ),给出落叶松人工林固定生物量换算系数值,并比较了3种林分生物量估算方法的预估精度.结果表明: 所建立的2种林分生物量模型中,总生物量和树干生物量模型拟合和预测效果较好,其R_a²>0.95,且均方根误差(RMSE)、平均预测误差(MPE)和平均绝对误差(MAE)都较小.树叶和树枝生物量模型拟合和预测效果相对较差,其模型的R_a²<0.95.模型系统Ⅰ和模型系统Ⅱ的预测精度均优于固定生物量换算系数法.基于生物量换算系数的林分生物量模型属于材积源生物量法,其本质与基于林分变量的林分生物量模型不同,但二者的预测效果相当.固定生物量换算系数的预测能力较差,将生物量与蓄积量之比假定为恒定常数是不恰当的.此外,为了使模型参数估计更有效,所建立的生物量模型应当考虑林分总生物量及各分项生物量的可加性.     

Pattern of carbon allocation across three different stages of stand development of a Chinese pine (Pinus tabulaeformis) forest

The pattern of carbon (C) allocation across different stages of stand development of Chinese pine (Pinus tabulaeformis) forests is poorly documented. In order to understand the effects of stand age on the C pool of the Chinese pine forest ecosystem, we have examined the above- and belowground C pools in three differently aged stands of Chinese pine in the northern mountains of Beijing, China, by plot-level inventories and destructive sampling. Our results suggest that tree branch and foliage biomass should be estimated by age-specific equations. Reasonably accurate estimates of tree stem, tree root, aboveground, and total tree biomass in a Chinese pine forest at different development stages were obtained using age-independent allometric equations from tree diameter only. The ratio of belowground to aboveground tree biomass was relatively constant with stand aging, remaining around 21?%. The contribution of aboveground tree biomass C increased from 21?% of the total ecosystem C in a 25-year-old stand to 44?% in a 65-year-old stand, subsequently falling to 41?% in a 105-year-old stand, while the contribution of mineral soil C decreased from 64?% of the total ecosystem C in 25-year-old stand to 38?% in a 65-year-old stand, subsequently increasing to 41?% in a 105-year-old stand. The C stock of the total ecosystem and its aboveground tree, tree root, forest floor, and mineral soil components continuously increased with stand ageing, whereas the C stock of the understory showed a declining trend and contributed little to the total site C pool.     

Effect of intraspecific competition on biomass partitioning of Larix principis-rupprechtii

It is acknowledged that trees biomass allocation in response to environmental conditions. However, it remains poorly understood what strategies of plant biomass allocation with inter- and intraspecific interactions of tree species in forest stands. Such information is important for revealing strategies of plant biomass allocation with plant competition. To address this problem, a study was conducted in Larix principis-rupprechtii plantations to evaluate the impact of plant competition on plant biomass allocation in Shanxi Province, China. We measured a competition index (CI), stem, branch, foliage, and root biomass as well as element content (Carbon (C), Nitrogen (N), Phosphorus (P), Potassium (K)). Stem-foliage ratio (S/F), aboveground–belowground biomass ratio (T/R), average annual increment of biomass (AAB), height (AAH), and DBH (AAD) were calculated. The study found that the competition intensity of neighboring trees was closely related to the partitioning of biomass. Our results demonstrated that competition pressure of neighboring trees was a crucial factor to drive and regulate the distribution of biomass. Predicting biomass allocation–competition relationships could represent a supportive method for improving management of Larix principis-rupprechtii plantations in Mountain Taiyue areas.     

Patterns of Biomass and Carbon Distribution across a Chronosequence of Chinese Pine (Pinus tabulaeformis) Forests

Patterns of biomass and carbon (C) storage distribution across Chinese pine (Pinus tabulaeformis) natural secondary forests are poorly documented. The objectives of this study were to examine the biomass and C pools of the major ecosystem components in a replicated age sequence of P. tabulaeformis secondary forest stands in Northern China. Within each stand, biomass of above- and belowground tree, understory (shrub and herb), and forest floor were determined from plot-level investigation and destructive sampling. Allometric equations using the diameter at breast height (DBH) were developed to quantify plant biomass. C stocks in the tree and understory biomass, forest floor, and mineral soil (0–100 cm) were estimated by analyzing the C concentration of each component. The results showed that the tree biomass of P. tabulaeformis stands was ranged from 123.8 Mg·ha^–1 for the young stand to 344.8 Mg·ha^–1 for the mature stand. The understory biomass ranged from 1.8 Mg·ha^–1 in the middle-aged stand to 3.5 Mg·ha^–1 in the young stand. Forest floor biomass increased steady with stand age, ranging from 14.9 to 23.0 Mg·ha^–1. The highest mean C concentration across the chronosequence was found in tree branch while the lowest mean C concentration was found in forest floor. The observed C stock of the aboveground tree, shrub, forest floor, and mineral soil increased with increasing stand age, whereas the herb C stock showed a decreasing trend with a sigmoid pattern. The C stock of forest ecosystem in young, middle-aged, immature, and mature stands were 178.1, 236.3, 297.7, and 359.8 Mg C ha^–1, respectively, greater than those under similar aged P. tabulaeformis forests in China. These results are likely to be integrated into further forest management plans and generalized in other contexts to evaluate C stocks at the regional scale.     

Looking for age-related growth decline in natural forests: unexpected biomass patterns from tree rings and simulated mortality

Forest biomass growth is almost universally assumed to peak early in stand development, near canopy closure, after which it will plateau or decline. The chronosequence and plot remeasurement approaches used to establish the decline pattern suffer from limitations and coarse temporal detail. We combined annual tree ring measurements and mortality models to address two questions: first, how do assumptions about tree growth and mortality influence reconstructions of biomass growth? Second, under what circumstances does biomass production follow the model that peaks early, then declines? We integrated three stochastic mortality models with a census tree-ring data set from eight temperate forest types to reconstruct stand-level biomass increments (in Minnesota, USA). We compared growth patterns among mortality models, forest types and stands. Timing of peak biomass growth varied significantly among mortality models, peaking 20–30 years earlier when mortality was random with respect to tree growth and size, than when mortality favored slow-growing individuals. Random or u-shaped mortality (highest in small or large trees) produced peak growth 25–30 % higher than the surviving tree sample alone. Growth trends for even-aged, monospecific Pinus banksiana or Acer saccharum forests were similar to the early peak and decline expectation. However, we observed continually increasing biomass growth in older, low-productivity forests of Quercus rubra, Fraxinus nigra, and Thuja occidentalis. Tree-ring reconstructions estimated annual changes in live biomass growth and identified more diverse development patterns than previous methods. These detailed, long-term patterns of biomass development are crucial for detecting recent growth responses to global change and modeling future forest dynamics.     

林龄和竞争对日本落叶松各组分生物量异速关系的影响

基于7-、17-、30-和40年生日本落叶松生物量测定数据,应用方差分析和多重比较分析了林龄和林分内树木竞争类型(优势木、平均木和被压木)对各组分生物量分配比例和异速关系的影响,构建了含林龄和树木竞争类型作为哑变量的生物量异速方程,为准确估算日本落叶松人工林生物量和碳储量提供依据。结果表明:(1)林龄显著影响生物量分配比例的异速关系。随林龄增加干生物量比例增大,枝叶生物量比例减小,根生物量逐渐稳定。加入林龄的干、枝和叶生物量方程显著改善。年龄效应在幼龄林阶段作用最显著,需单独构建生物量模型。(2)树木竞争类型对生物量分配的影响小于林龄。立地条件一致下,虽然相同胸径的优势木比劣势木积累更多的枝叶生物量和少的干生物量,但它们分配生物量到不同器官的比例和方式是基本相同的,林内竞争不会导致生物量分配规律由"异速关系理论"向"环境优先理论"转化。因此,常规采用平均木法估算各组分生物量是可行的。(3)在近成熟林分中不同竞争类型树木的根生物量分配比例均较为稳定,采用根茎比比值来估算根生物量是可行的。     

基于不同预测变量的天然椴树可加性地上生物量模型构建

森林生物量是森林生态系统的最基本数量特征,生物量数据是研究许多林业问题和生态问题的基础,因此,准确测定生物量对于计算碳储量以及研究气候变化、森林健康、森林生产力、养分循环等十分重要.目前,测算森林生物量常用的方法为生物量模型估算法.本研究基于小兴安岭地区和张广才岭地区97株实测生物量数据,建立了3个天然椴树立木可加性生物量模型系统(基于胸径的一元可加性生物量模型系统、基于胸径和树高的二元可加性生物量模型系统、基于最优变量的最优可加性生物量模型系统),采用非线性似乎不相关回归法进行参数估计,用加权方法解决模型的异方差问题,并采用“刀切法”进行模型检验.结果表明: 3种可加性生物量模型系统均能较好地对椴树各部分生物量进行拟合和预测(调整后确定系数R_a²>0.84,平均预测误差百分比MPE<8.5%,平均绝对误差MAE<16.3 kg,平均百分标准误差MPSE<28.5%),其中,树干和地上生物量的拟合效果优于树叶、树枝和树冠;在引入树高和树冠因子后,提高了模型的拟合效果和预测能力(R_a²提高0.01～0.04,MAE降低0.01～4.55 kg),缩小了预测值置信区间的范围,树干、树叶和地上生物量提高较多,树枝和树冠提高较少.总体来看,最优生物量模型系统效果最好,其次为二元生物量模型系统,再次是一元生物量模型系统,添加树高和树冠因子进行生物量模型的构建十分必要.     

森林生态系统碳循环的基本概念和野外测定方法评述

全球气候变化与森林生态系统碳循环息息相关,定量评估森林碳收支是生态系统与全球变化研究的重要任务。30年来森林生态系统碳循环研究已经取得了长足的进展,但全球和区域森林碳收支仍然存在很大的不确定性。这一方面与森林生态系统本身的复杂性有关,另一方面也与具体研究方法有关。评述了森林生态系统碳循环的基本概念和主要野外测定方法,为我国森林生态系统碳循环研究提供可参考的方法论。从生态系统碳浓度、密度、通量、分配和周转5个方面回顾了碳循环相关概念,指出碳浓度和碳储量是对碳库的静态描述,而碳通量和碳周转是对碳库的动态描述。净初级生产力是测量最普遍的碳通量组分,但大多数情况下因忽略了一些细节而被系统低估。普遍使用的净生态系统生产力,由于没有包含非CO2形式的水文、气象和干扰过程产生的碳通量,通常情况下高于生态系统净碳累积速率。在详细介绍碳通量组分的基础上,改进了森林生态系统碳循环的概念模型。重点讨论了碳通量的3种地面实测方法:测树学方法、箱法和涡度协方差法,并指出了其注意事项和不确定性来源。针对当前碳循环研究的突出问题,建议从4个方面减小碳循环测定的不确定性:(1)恰当运用生物量方程估算乔木生物量;(2)尽可能全面测定生态系统碳组分;(3)给出碳通量估算值的不确定性;(4)多种途径交互验证。     

Biomass and Carbon Storage in an Age-Sequence of Korean Pine (<Emphasis Type="Italic">Pinus koraiensis</Emphasis>) Plantation Forests in Central Korea

This study examined the biomass and carbon pools of the main ecosystem components in an age sequence of five Korean pine plantation forest stands in central Korea. The C contents in the tree and ground vegetation biomass, coarse woody debris, forest floor, and mineral soil were estimated by analyzing the C concentration of each component. The aboveground and total tree biomass increased with increasing stand age. The highest C concentration across this chronosequence was found in the tree branch while the lowest C concentration was found in the ground vegetation. The observed C contents for tree components, ground vegetation, and coarse woody debris were generally lower than the predicted C contents estimated from a biomass C factor of 0.5. Forest floor C content was age-independent. Total mineral soil C content appeared to decline initially after establishing Korean pine plantations and recover by the stand age of 35 years. Although aboveground tree biomass C content showed considerable accumulation with increasing age, the relative contribution of below ground C to total ecosystem C content varied substantially. These results suggest that successional development as temporal factor has a key role in estimating the C storage in Korean pine plantation forests.     

Do Boreal Forests Need Fire Disturbance to Maintain Productivity?

Fire is considered as a major driver of ecosystem processes of the boreal forest with important effects on soil and forest productivity. When the interval between successive forest fires is long, a thick organic layer can develop and eventually interfere with processes involved in tree nutrient uptake. We thus hypothesized that the organic layer of well-drained boreal stands increases with time since last fire and that thick organic layers are associated with low values of soil temperature, nutrient availability, and site productivity. This was tested on a chronosequence composed of 90 boreal stands ranging from 1 to more than 2000 years after fire within which we measured organic layer thickness (OLT), mineral soil and foliage nutrient concentrations, soil temperature, ground cover of Sphagnum sp. and Ericaceae sp., leaf area index, aboveground biomass production, and growth efficiency index (GEI). The OLT increased during the first 64 years after fire but stayed statistically constant thereafter. This initial increase in OLT was accompanied by an increase in the C/N ratio and decreases in soil temperature, foliar N, and GEI. The absence of a significant decrease in productivity from 80 to 2000 years post-fire suggests that these characteristics reach a steady state early in the chronosequence that persists in the absence of major disturbances or changes in site conditions. These results imply that management practices may not be necessary to maintain boreal forest productivity in the absence of fire on well-drained sites.     

Old trees contribute bio-available nitrogen through canopy bryophytes

Symbiotic cyanobacteria??bryophyte associations on the forest floor are shown to contribute significantly to stand-level nitrogen budgets through the process of biological nitrogen fixation (BNF), but few studies have considered the role of canopy bryophytes. Given the high biomass of epiphytic bryophytes in many tree species of the North American temperate rain forest, we suggest that canopy bryophytes may contribute substantially to stand-level N dynamics. We confirm the presence of cyanobacteria and measure rates of BNF at three heights (0, 15 and 30 m) in Sitka spruce trees across three watershed estuaries of Clayoquot Sound, British Columbia, Canada. This study is the first to report BNF by cyanobacteria associated with epiphytic and forest floor bryophytes in the coastal temperate rain forest of North America. Cyanobacteria density was significantly greater in epiphytic bryophytes compared to mosses on the forest floor, and rates of BNF were highest at 30 m in the canopy. The majority of total stand-level BNF (0.76 kg N · ha^-1 · yr^-1) occurs in the canopy, rather than on the forest floor (0.26 kg N · ha^-1 · yr^-1). We suggest that BNF by cyanobacterial-bryophyte associations in the canopy of coastal temperate rain forests is a unique source of ecosystem N, which is dependent on large, old trees with high epiphytic bryophyte biomass.     

A size-distribution-based model of forest dynamics along a latitudinal environmental gradient

A geographically extended model of the dynamics of tree size structure of forests is proposed to simulate the change of forest zonation along latitude in response to global environmental change. To predict the response of forests to global change, it is necessary to construct functional models of forest tree populations. The size-structure-based model requires far less memory and steps of calculation compared with individual-based models, and it is easy to incorporate the dimension of geographic locations into the model to describe large-scale dynamics of forest-type distributions. The effect of increasing size growth rate, expected from increasing atmospheric carbon dioxide, was diminished at the stand-level basal area density, because of regulation by one-sided competition. Model simulations of a century-long global warming at around 3 °C predicted that (1) biomass changed in resident forests rather simultaneously in response to warming, and that (2) there was a considerable time lag in movement at the boundaries of different forest types, particularly under the existence of resident forest types that would be finally replaced. It required several thousand years after a century-long warming spell for forest types to attain new steady-state distributions after shifting. As a consequence, global warming created a zigzag pattern of biomass distribution along a latitudinal gradient, i.e., an increase in the cooler-side boundary of forest types and a decrease in the warmer-side boundary.     

Importance of short-lived components of a dry tropical forest for biomass production and nutrient cycling

Abstract. Short-lived components in a dry tropical forest ecosystem in India - tree foliage, fine roots and herbaceous plants - are shown to be important for biomass production and nutrient cycling. With 62 % they contribute much more to the dry matter production than the long-lived components- tree boles, branches and coarse roots - which make up only 38 %. The contribution of short-lived components to the total uptake of different nutrients was also high: 18 - 30 % for tree foliage, 26 - 34 % for fine roots and 6–19 % for herbs; their share in the total nutrient storage is less: 6–19 % for tree foliage, 4–8 % for fine roots and 0.6–1.3 % for herbs. The transfer of nutrients by the short-lived components was also substantial: 31 - 46 % for foliage, 7–24% for herbs and 33–45% for fine roots. The results indicate that the short-lived components play a significant role in the functioning of a dry tropical forest.     

Increased forest carbon storage with increased atmospheric CO2 despite nitrogen limitation: a game‐theoretic allocation model for trees in competition for nitrogen and light

Changes in resource availability often cause competitively driven changes in tree allocation to foliage, wood, and fine roots, either via plastic changes within individuals or through turnover of individuals with differing strategies. Here, we investigate how optimally competitive tree allocation should change in response to elevated atmospheric CO₂ along a gradient of nitrogen and light availability, together with how those changes should affect carbon storage in living biomass. We present a physiologically‐based forest model that includes the primary functions of wood and nitrogen. From a tree's perspective, wood is an offensive and defensive weapon used against neighbors in competition for light. From a biogeochemical perspective, wood is the primary living reservoir of stored carbon. Nitrogen constitutes a tree's photosynthetic machinery and the support systems for that machinery, and its limited availability thus reduces a tree's ability to fix carbon. This model has been previously successful in predicting allocation to foliage, wood, and fine roots along natural productivity gradients. Using game theory, we solve the model for competitively optimal foliage, wood, and fine root allocation strategies for trees in competition for nitrogen and light as a function of CO₂ and nitrogen mineralization rate. Instead of down‐regulating under nitrogen limitation, carbon storage under elevated CO₂ relative to carbon storage at ambient CO₂ is approximately independent of the nitrogen mineralization rate. This surprising prediction is a consequence of both increased competition for nitrogen driving increased fine root biomass and increased competition for light driving increased allocation to wood under elevated CO₂.     

Alteration of forest succession and carbon cycling under elevated CO2

Regenerating forests influence the global carbon (C) cycle, and understanding how climate change will affect patterns of regeneration and C storage is necessary to predict the rate of atmospheric carbon dioxide (CO₂) increase in future decades. While experimental elevation of CO₂ has revealed that young forests respond with increased productivity, there remains considerable uncertainty as to how the long‐term dynamics of forest regrowth are shaped by elevated CO₂ (eCO₂). Here, we use the mechanistic size‐ and age‐ structured Ecosystem Demography model to investigate the effects of CO₂ enrichment on forest regeneration, using data from the Duke Forest Free‐Air Carbon dioxide Enrichment (FACE) experiment, a forest chronosequence, and an eddy‐covariance tower for model parameterization and evaluation. We find that the dynamics of forest regeneration are accelerated, and stands consistently hit a variety of developmental benchmarks earlier under eCO₂. Because responses to eCO₂ varied by plant functional type, successional pathways, and mature forest composition differed under eCO₂, with mid‐ and late‐successional hardwood functional types experiencing greater increases in biomass compared to early‐successional functional types and the pine canopy. Over the simulation period, eCO₂ led to an increase in total ecosystem C storage of 9.7 Mg C ha^‐1. Model predictions of mature forest biomass and ecosystem–atmosphere exchange of CO₂ and H₂O were sensitive to assumptions about nitrogen limitation; both the magnitude and persistence of the ecosystem response to eCO₂ were reduced under N limitation. In summary, our simulations demonstrate that eCO₂ can result in a general acceleration of forest regeneration while altering the course of successional change and having a lasting impact on forest ecosystems.     

Aboveground biomass allocation and water content relationships in Mediterranean trees and shrubs in two climatological regions in Israel

Thisstudy investigated the variation along basipetal gradients of the relationshipsbetween the foliage/wood allocation ratios of biomass and of water content, inMediterranean trees and shrubs, at two different locations along a climaticgradient. Understanding of the biomass allocation and water relations inMediterranean trees and shrubs provides useful information on growth patternsofthese species, and on resource dynamics of these plant communities. Twoexperimental sites were selected along a climatological transect that runs fromthe foothills of the Judean Hills to the northern Negev desert in Israel. Ateach site, 16 quadrats of 10 × 10 m (eight on south-facingslopes and eight on north-facing slopes) were marked. The aboveground biomassofdominant tree and shrub species were estimated. Main branches of trees andshrubs were cut, their foliage and wood biomass were separately weighed, andtheir respective water contents were determined. The species studied includedthe evergreen sclerophylls, Quercus calliprinos, Phillyrealatifolia and Pistacia lentiscus, and thesemi-deciduous species, Cistus creticus, Coridothymuscapitatus and Sarcopoterium spinosum. Theresults indicated that the foliage/wood ratio decreased from the periphery ofthe crown to the interior of the trees and shrubs: foliage biomass and waterwere mainly limited to the top 30 cm of the crown in all studiedspecies. Leaves had higher relative water contents than woody tissues in theupper part of the crown. However; when the whole tree or shrub was considered,the relative water content was found to be mostly allocated to the woodystructures. The results are discussed in terms of biomass allocation in variouslife forms of the eastern-Mediterranean plant communities and how they areaffected by slope aspect and climatic conditions.