Environmental controls over carbon exchange of three forest ecosystems in eastern China

Net ecosystem productivity (NEP) was continuously measured using the eddy covariance (EC) technique from 2003 to 2005 at three forest sites of ChinaFLUX. The forests include Changbaishan temperate mixed forest (CBS), Qianyanzhou subtropical coniferous plantation (QYZ), and Dinghushan subtropical evergreen broad‐leaved forest (DHS). They span wide ranges of temperature and precipitation and are influenced by the eastern Asian monsoon climate to varying extent. In this study, we estimated ecosystem respiration (RE) and gross ecosystem productivity (GEP). Comparison of ecosystem carbon exchange among the three forests shows that RE was mainly determined by temperature, with the forest at CBS exhibiting the highest temperature sensitivity among the three ecosystems. The RE was highly dependent on GEP across the three forests, and the ratio of RE to GEP decreased along the North–South Transect of Eastern China (NSTEC) (i.e. from the CBS to the DHS), with an average of 0.77 ± 0.06. Daily GEP was mainly influenced by temperature at CBS, whereas photosynthetic photon flux density was the dominant factor affecting the daily GEP at both QYZ and DHS. Temperature mainly determined the pattern of the interannual variations of ecosystem carbon exchange at CBS. However, water availability primarily controlled the interannual variations of ecosystem carbon exchange at QYZ. At DHS, NEP attained the highest values at the beginning of the dry seasons (autumn) rather than the rainy seasons (summer), probably because insufficient radiation and frequent fog during the rainy seasons hindered canopy photosynthesis. All the three forest ecosystems acted as a carbon sink from 2003 to 2005. The annual average values of NEP at CBS, QYZ, and DHS were 259 ± 19, 354 ± 34, and 434 ± 66 g C m⁻² yr⁻¹, respectively. The slope of NEP that decreased with increasing latitude along the NSTEC was markedly different from that observed on the forest transect in the European continent. Long‐term flux measurements over more forest ecosystems along the NSTEC will further help verify such a difference between the European forest transect and the NSTEC and provide insights into the responses of ecosystem carbon exchange to climate change in China.     

基于模型数据融合的中国温带和亚热带典型森林生态系统碳通量模拟

生态系统碳循环过程对水分响应的研究已成为全球变化关注的焦点问题之一。基于长白山温带针阔混交林与千烟洲亚热带人工针叶林观测站2003—2009年生长季的碳通量(NEE)和气象观测数据,综合考虑水分对光合、呼吸作用的影响,构建不同的NEE模型,并应用模型数据融合方法优化模型参数、遴选最适模型,系统分析了水分因子对不同森林生态系统碳循环的影响。结果表明:(1)优化后的模型参数均能被NEE实测数据较好约束。长白山生长季的光合、呼吸参数值均高于千烟洲,未考虑空气饱和水汽压差(VPD)的模型高估了千烟洲温度敏感性参数(Q10)值、低估了千烟洲基础呼吸速率参数(BR)值;(2)仅考虑VPD对光合作用影响的模型是长白山生长季碳通量模拟的最优模型,但模拟精度提高不显著。不同模型间碳通量组分模拟结果差异较小;(3)考虑VPD和土壤含水量对光合、呼吸作用共同影响的模型是千烟洲生长季碳通量模拟的最优模型,并且显著提高了模拟精度。未考虑水分的模型在生长季高估了总生态系统生产力(GEP)总量2.0%(21.85 g C/m~2),同时更大幅度地高估了生态系统呼吸(RE)总量4.4%(38.02 g C/m~2),从而导致NEE总量低估于实测值7.8%(18.55 g C/m~2)。     

Urban Tree Species Show the Same Hydraulic Response to Vapor Pressure Deficit across Varying Tree Size and Environmental Conditions

Background

The functional convergence of tree transpiration has rarely been tested for tree species growing under urban conditions even though it is of significance to elucidate the relationship between functional convergence and species differences of urban trees for establishing sustainable urban forests in the context of forest water relations.

Methodology/Principal Findings

We measured sap flux of four urban tree species including Cedrus deodara, Zelkova schneideriana, Euonymus bungeanus and Metasequoia glyptostroboides in an urban park by using thermal dissipation probes (TDP). The concurrent microclimate conditions and soil moisture content were also measured. Our objectives were to examine 1) the influence of tree species and size on transpiration, and 2) the hydraulic control of urban trees under different environmental conditions over the transpiration in response to VPD as represented by canopy conductance. The results showed that the functional convergence between tree diameter at breast height (DBH) and tree canopy transpiration amount (E _c) was not reliable to predict stand transpiration and there were species differences within same DBH class. Species differed in transpiration patterns to seasonal weather progression and soil water stress as a result of varied sensitivity to water availability. Species differences were also found in their potential maximum transpiration rate and reaction to light. However, a same theoretical hydraulic relationship between G _c at VPD = 1 kPa (G _cref) and the G _c sensitivity to VPD (−dG _c/dlnVPD) across studied species as well as under contrasting soil water and R _s conditions in the urban area.

Conclusions/Significance

We concluded that urban trees show the same hydraulic regulation over response to VPD across varying tree size and environmental conditions and thus tree transpiration could be predicted with appropriate assessment of G _cref.     

Water‐use efficiency in response to climate change: from leaf to ecosystem in a temperate steppe

Water‐use efficiency (WUE) has been recognized as an important characteristic of ecosystem productivity, which links carbon (C) and water cycling. However, little is known about how WUE responds to climate change at different scales. Here, we investigated WUE at leaf, canopy, and ecosystem levels under increased precipitation and warming from 2005 to 2008 in a temperate steppe in Northern China. We measured gross ecosystem productivity (GEP), net ecosystem CO₂ exchange (NEE), evapotranspiration (ET), evaporation (E), canopy transpiration (T_c), as well as leaf photosynthesis (P_max) and transpiration (T_l) of a dominant species to calculate canopy WUE (WUE_c=GEP/T), ecosystem WUE (WUE_gep=GEP/ET or WUE_nee=NEE/ET) and leaf WUE (WUE_l=P_max/T_l). The results showed that increased precipitation stimulated WUE_c, WUE_gep and WUE_nee by 17.1%, 10.2% and 12.6%, respectively, but decreased WUE_l by 27.4%. Climate warming reduced canopy and ecosystem WUE over the 4 years but did not affect leaf level WUE. Across the 4 years and the measured plots, canopy and ecosystem WUE linearly increased, but leaf level WUE of the dominant species linearly decreased with increasing precipitation. The differential responses of canopy/ecosystem WUE and leaf WUE to climate change suggest that caution should be taken when upscaling WUE from leaf to larger scales. Our findings will also facilitate mechanistic understanding of the C–water relationships across different organism levels and in projecting the effects of climate warming and shifting precipitation regimes on productivity in arid and semiarid ecosystems.     

Response of transpiration to rain pulses for two tree species in a semiarid plantation

Responses of transpiration (E _c) to rain pulses are presented for two semiarid tree species in a stand of Pinus tabulaeformis and Robinia pseudoacacia. Our objectives are to investigate (1) the environmental control over the stand transpiration after rainfall by analyzing the effect of vapor pressure deficit (VPD), soil water condition, and rainfall on the post-rainfall E _c development and recovery rate, and (2) the species responses to rain pulses and implications on vegetation coverage under a changing rainfall regime. Results showed that the sensitivity of canopy conductance (G _c) to VPD varied under different incident radiation and soil water conditions, and the two species exhibited the same hydraulic control (?dG _c/dlnVPD to G _cref ratio) over transpiration. Strengthened physiological control and low sapwood area of the stand contributed to low E _c. VPD after rainfall significantly influenced the magnitude and time series of post-rainfall stand E _c. The fluctuation of post-rainfall VPD in comparison with the pre-rainfall influenced the E _c recovery. Further, the stand E _c was significantly related to monthly rainfall, but the recovery was independent of the rainfall event size. E _c enhanced with cumulative soil moisture change (ΔVWC) within each dry–wet cycle, yet still was limited in large rainfall months. The two species had different response patterns of post-rainfall E _c recovery. E _c recovery of P. tabulaeformis was influenced by the pre- and post-rainfall VPD differences and the duration of rainless interval. R. pseudoacacia showed a larger immediate post-rainfall E _c increase than P. tabulaeformis did. We, therefore, concluded that concentrated rainfall events do not trigger significant increase of transpiration unless large events penetrate the deep soil and the species differences of E _c in response to pulses of rain may shape the composition of semiarid woodlands under future rainfall regimes.     

塔里木河下游河岸柽柳林冠层导度变化特征及模拟

冠层导度(G_c)对植被的蒸腾和光合作用具有重要影响。利用涡度相关仪器实测了塔里木河下游河岸柽柳林地的蒸散发,以及气象因子(温度、湿度、总辐射、光和有效辐射),并利用Penman-Monteith公式计算了柽柳林在2013年生长季的冠层导度。结果显示:柽柳林冠层导度日变化过程在8:00左右迅速增大,于10:30左右达到最大值,之后缓慢下降,18:00左右快速降低;柽柳林冠层导度季节变化过程总体显示,展叶期缓慢上升,落叶期迅速下降,生长盛期缓慢波动下降;研究区,叶面积指数(LAI)是影响柽柳冠层导度季节变化的主要因素,其次为温度(T)、光合有效辐射(PAR)、总辐射(S)、空气饱和差(VPD);四元线性回归方程可以较好地拟合冠层导度与各因子的关系,利用2013年奇数天数据建立回归方程,对偶数天冠层导度值进行模拟和验证,RMSE值为0.169 mm/s,NSE值为0.814,达到了较高的模拟精度。     

Partitioning Climatic and Biotic Effects on Interannual Variability of Ecosystem Carbon Exchange in Three Ecosystems

Understanding the climatic and biotic controls of interannual variability (IAV) in net ecosystem exchange (NEE) is important for projecting future uptake of CO₂ in terrestrial ecosystems. In this study, a statistical modeling approach was used to partition climatic and biotic effects on the IAV in NEE, gross primary productivity (GPP) and ecosystem respiration (RE) at a subtropical evergreen plantation in China (QYZ), a deciduous forest (MOZ), and a grassland (DK1) in the USA. The climatic effects in the study are defined as the interannual anomalies in carbon (C) fluxes directly caused by climatic variations, whereas the biotic effects are those caused by the IAV in photosynthetic and respiratory traits. The results showed that the contribution of biotic effects to the IAV in NEE increased significantly as the temporal scale got longer from daily to annual scales. At the annual scale, the contribution of biotic effects to the IAV in NEE was 47, 69, and 77% at QYZ, MOZ, and DK1, respectively. However, the IAV in NEE was mainly controlled by GPP at QYZ, and by RE at DK1, whereas the contributions of GPP and RE to the IAV in NEE were similar at MOZ, indicating different mechanisms regulating the IAV in NEE among ecosystems. Interestingly, there was a strong negative correlation between the climatic and biotic effects at the annual scale from 2003 to 2009 at QYZ (r ² = 0.80, P < 0.01), suggesting these two effects counteracted each other and resulted in a relatively stable C sink, whereas no correlations were found at the other two sites. Overall, our study revealed the relative importance of climatic and biotic effects on the IAV in NEE and contributed to our understanding of their underlying mechanisms.     

Root functioning in tropical high-elevation forests: Environmental vs. biological control of root water absorption

Lowered temperatures may reduce the root water uptake of tropical trees at high elevations through several mechanisms; however, field studies to test their relevance are lacking. We measured sap flux density (J) in small-diameter tree roots across a 2000-m elevation transect in a tropical mountain forest for quantifying the effects of temperature (T_a), VPD and soil moisture (θ) on root water flow and uptake at different elevations. Recently developed miniature heat balance-sap flow gauges were applied to roots of about 10 mm in diameter in mountain forest stands at 1050, 1890 and 3060 m a.s.l. in the Ecuadorian Andes and the measured flow was related to anatomical properties of the root xylem. Between 1050 and 3060 m, mean J decreased to about a third. VPD was the most influential environmental factor controlling J at 1050 and 1890 m, while T_a was the key determinant at 3060 m. Large vessels were absent in the root xylem of high-elevation trees which resulted in a 10-fold decrease of theoretical hydraulic conductivity (k_h^theor) between 1050 and 3060 m. We conclude that both physical limitations (reduced VPD, increased viscosity of water) and biological constraints (large decrease of k_h^theor) result in a significantly reduced J and root water uptake of the trees in high-elevation tropical forests.     

Asymmetric response of Amazon forest water and energy fluxes to wet and dry hydrological extremes reveals onset of a local drought-induced tipping point

Understanding the effects of intensification of Amazon basin hydrological cycling—manifest as increasingly frequent floods and droughts—on water and energy cycles of tropical forests is essential to meeting the challenge of predicting ecosystem responses to climate change, including forest “tipping points”. Here, we investigated the impacts of hydrological extremes on forest function using 12+ years of observations (between 2001–2020) of water and energy fluxes from eddy covariance, along with associated ecological dynamics from biometry, at the Tapajós National Forest. Measurements encompass the strong 2015–2016 El Niño drought and La Niña 2008–2009 wet events. We found that the forest responded strongly to El Niño-Southern Oscillation (ENSO): Drought reduced water availability for evapotranspiration (ET) leading to large increases in sensible heat fluxes (H). Partitioning ET by an approach that assumes transpiration (T) is proportional to photosynthesis, we found that water stress-induced reductions in canopy conductance (G_s) drove T declines partly compensated by higher evaporation (E). By contrast, the abnormally wet La Niña period gave higher T and lower E, with little change in seasonal ET. Both El Niño-Southern Oscillation (ENSO) events resulted in changes in forest structure, manifested as lower wet-season leaf area index. However, only during El Niño 2015–2016, we observed a breakdown in the strong meteorological control of transpiration fluxes (via energy availability and atmospheric demand) because of slowing vegetation functions (via shutdown of G_s and significant leaf shedding). Drought-reduced T and G_s, higher H and E, amplified by feedbacks with higher temperatures and vapor pressure deficits, signaled that forest function had crossed a threshold, from which it recovered slowly, with delay, post-drought. Identifying such tipping point onsets (beyond which future irreversible processes may occur) at local scale is crucial for predicting basin-scale threshold-crossing changes in forest energy and water cycling, leading to slow-down in forest function, potentially resulting in Amazon forests shifting into alternate degraded states.     

Microclimatic buffering in forests of the future: the role of local water balance

Forest canopies buffer climate extremes and promote microclimates that may function as refugia for understory species under changing climate. However, the biophysical conditions that promote and maintain microclimatic buffering and its stability through time are largely unresolved. We posited that forest microclimatic buffering is sensitive to local water balance and canopy cover, and we measured this effect during the growing season across a climate gradient in forests of the northwestern United States (US). We found that forest canopies buffer extremes of maximum temperature and vapor pressure deficit (VPD), with biologically meaningful effect sizes. For example, during the growing season, maximum temperature and VPD under at least 50% forest canopy were 5.3°C and 1.1 kPa lower on average, respectively, compared to areas without canopy cover. Canopy buffering of temperature and vapor pressure deficit was greater at higher levels of canopy cover, and varied with water balance, implying that buffering effects are subject to changes in local hydrology. We project changes in the water balance for the mid‐21st century and predict how such changes may impact the ability of western US forests to buffer climate extremes. Our results suggest that some forests will lose their capacity to buffer climate extremes as sites become increasingly water limited. Changes in water balance combined with accelerating canopy losses due to increases in the frequency and severity of disturbance will create potentially non‐linear changes in the microclimate conditions of western US forests.     

Associations between carbon isotope ratios of ecosystem respiration, water availability and canopy conductance

We tested the hypothesis that the stable carbon isotope signature of ecosystem respiration (δ¹³C_R) was regulated by canopy conductance (G_c) using weekly Keeling plots (n=51) from a semiarid old‐growth ponderosa pine (Pinus ponderosa) forest in Oregon, USA. For a comparison of forests in two contrasting climates we also evaluated trends in δ¹³C_R from a wet 20‐year‐old Douglas‐fir (Pseudotsuga menziesii) plantation located near the Pacific Ocean. Intraannual variability in δ¹³C_R was greater than 8.0‰ at both sites, was highest during autumn, winter, and spring when rainfall was abundant, and lowest during summer drought. The δ¹³C_R of the dry pine forest was consistently more positive than the wetter Douglas‐fir forest (mean annual δ¹³C_R: ?25.41‰ vs. ?26.23‰, respectively, P=0.07). At the Douglas‐fir forest, δ¹³C_R–climate relationships were consistent with predictions based on stomatal regulation of carbon isotope discrimination (Δ). Soil water content (SWC) and vapor pressure deficit (vpd) were the most important factors governing δ¹³C_R in this forest throughout the year. In contrast, δ¹³C_R at the pine forest was relatively insensitive to SWC or vpd, and exhibited a smaller drought‐related enrichment (～2‰) than the enrichment observed during drought at the Douglas‐fir forest (～5‰). Groundwater access at the pine forest may buffer canopy–gas exchange from drought. Despite this potential buffering, δ¹³C_R at the pine forest was significantly but weakly related to canopy conductance (G_c), suggesting that δ¹³C_R remains coupled to canopy–gas exchange despite groundwater access. During drought, δ¹³C_R was strongly correlated with soil temperature at both forests. The hypothesis that canopy‐level physiology is a critical regulator of δ¹³C_R was supported; however, belowground respiration may become more important during rain‐free periods.     

Stomatal acclimation to vapour pressure deficit doubles transpiration of small tree seedlings with warming

Future climate change is expected to increase temperature (T) and atmospheric vapour pressure deficit (VPD) in many regions, but the effect of persistent warming on plant stomatal behaviour is highly uncertain. We investigated the effect of experimental warming of 1.9–5.1 °C and increased VPD of 0.5–1.3 kPa on transpiration and stomatal conductance (g_s) of tree seedlings in the temperate forest understory (Duke Forest, North Carolina, USA). We observed peaked responses of transpiration to VPD in all seedlings, and the optimum VPD for transpiration (D_opt) shifted proportionally with increasing chamber VPD. Warming increased mean water use of Carya by 140% and Quercus by 150%, but had no significant effect on water use of Acer. Increased water use of ring‐porous species was attributed to (1) higher air T and (2) stomatal acclimation to VPD resulting in higher g_s and more sensitive stomata, and thereby less efficient water use. Stomatal acclimation maintained homeostasis of leaf T and carbon gain despite increased VPD, revealing that short‐term stomatal responses to VPD may not be representative of long‐term exposure. Acclimation responses differ from expectations of decreasing g_s with increasing VPD and may necessitate revision of current models based on this assumption.     

南亚热带不同演替阶段森林优势树种叶片构建成本与机械抗性的协同关系

为探究不同演替阶段森林优势种叶片资源获取策略的差异以及叶片构建成本与机械抗性的关系,对我国南亚热带不同演替阶段森林14优势种的叶片构建成本、机械抗性、角质层厚度和比叶重等结构性状进行测定。结果表明,与演替早期相比,演替晚期优势种具有更高的单位面积叶片构建成本、叶片撕裂力以及穿透力,但其叶片最大光合速率较低;同时,单位面积叶片构建成本与机械抗性呈显著正相关关系,而叶片角质层厚度、比叶重等结构性状也与叶片构建成本、机械抗性均呈显著正相关。因此,从叶片能量投资策略上反映了南亚热带森林演替进程中叶片构建成本与机械抗性的协同关系。     

Assessment of annual carbon exchange in a water-stressed Pinus radiata plantation: an analysis based on eddy covariance measurements and an integrated biophysical model

We used a combination of eddy flux, chamber and environmental measurements with an integrated suite of models to analyse the seasonality of net ecosystem carbon uptake (F_CO2) in an 8-year-old, closed canopy Pinus radiata D.Don plantation in New Zealand (42°52′ S, 172°45′ E). The analyses utilized a biochemically based, big-leaf model of tree canopy photosynthesis (A_c), coupled to multiplicative environmental-constraint functions of canopy stomatal conductance (G_c) via environmental measurements, a temperature-dependent model of ecosystem respiration (R_eco), and a soil water balance model. Available root zone water storage capacity at the measurement site is limited to about 50 mm for the very stony soil, and annual precipitation is only 660 mm, distributed evenly throughout the year. Accordingly the site is prone to soil moisture deficit throughout the summer. G _c and A_c obtained maximum rates early in the growing season when plentiful soil water supply was associated with sufficient quantum irradiance (Q_abs), and moderate air saturation deficit (D) and temperature (T). From late spring onwards, soil water deficit and D confined G_c and A_c congruously, which together with the solely temperature dependency of R_eco resulted in the pronounced seasonality in F_CO2. Reflecting a light-limitation of A_c in the closed canopy, modelled annual carbon (C) uptake was most sensitive to changes in Q_abs. However, Q_abs did not vary significantly between years, and changes in annual F_CO2 were mostly due to variability in summer rainfall and D. Annual C-uptake of the forest was 717 g C m^–2 in a near-average rainfall year, exceeding by one third the net uptake in a year with 20% less than average rainfall (515 g C m^–2).     

西北干旱荒漠绿洲区葡萄园水热通量特征及其主要影响因素

为提高干旱绿洲区农田地表过程的科学认识及改善农业水管理，该研究基于涡度相关技术，运用葡萄园生长季能量平衡观测资料，分析了不同时间尺度的水热通量变化特征，以及干旱平流和冠层导度(G_c)在不同生长阶段上对水热通量的影响，并应用通径分析研究了环境因子对潜热通量(LE)的影响路径和程度。结果表明：(1)日尺度上，LE呈现出不同程度多峰状，其余水热通量总体上呈单峰状。整体上，各生长阶段日间净辐射(R_n)>LE>感热通量(H)>土壤热通量(G)。G相较于R_n呈现出明显的滞后现象。(2)整个生长季上，LE和H分别占白天可利用能量(R_n-G)的86%和14%,表明LE始终是白天葡萄园可利用能量的主要消耗项。干旱平流对白天LE的贡献范围为5%～59%,全生长季平均贡献为28%;G_c对LE的影响在生长季上呈动态变化，且在新梢生长期和落叶期对LE的影响强于中间生长阶段。(3)LE主要受R_n的影响，饱和水汽压差(VPD)和气温(T_a)对LE...     

Variations in bulk canopy conductance of an irrigated olive (Olea europaea L.) orchard

Measurements of transpiration (E_p) and bulk canopy conductance (G_c) of an irrigated olive orchard during three years are presented here. Transpiration was obtained subtracting the hourly soil evaporation (E_s, calculated with a specific model) from the total evapotranspiration of the orchard measured with eddy covariance. G_c was obtained inverting the Penman–Monteith model for the transpiration flux. The orchard G_c showed a seasonal pattern of variation, very similar in all the three years of experiment. G_c is very low in the beginning of the growing cycle (spring), then it rises slowly until a maximum until the beginning of July. From that point on, it shows a sharp fall in August, leading it to approximately half the maximum value of July. During Autumn, high conductances can be reached again, when favourable conditions are met. To help discerning the direct effect of the meteorological environment on the stomata, an analysis with a stomatal conductance model was performed. The Jarvis model was optimised on small periods of G_c data in different times of the year, and used to obtain a simulated G_c for a standard environment. This modelled-normalized G_c showed essentially the same seasonal pattern than the measured G_c, indicating that the source of seasonal variation is not due to the direct environmental effect on the stomatal system.     

Long-term drought effects on the thermal sensitivity of Amazon forest trees

The continued functioning of tropical forests under climate change depends on their resilience to drought and heat. However, there is little understanding of how tropical forests will respond to combinations of these stresses, and no field studies to date have explicitly evaluated whether sustained drought alters sensitivity to temperature. We measured the temperature response of net photosynthesis, foliar respiration and the maximum quantum efficiency of photosystem II (F_v/F_m) of eight hyper-dominant Amazonian tree species at the world's longest-running tropical forest drought experiment, to investigate the effect of drought on forest thermal sensitivity. Despite a 0.6°C–2°C increase in canopy air temperatures following long-term drought, no change in overall thermal sensitivity of net photosynthesis or respiration was observed. However, photosystem II tolerance to extreme-heat damage (T₅₀) was reduced from 50.0 ± 0.3°C to 48.5 ± 0.3°C under drought. Our results suggest that long-term reductions in precipitation, as projected across much of Amazonia by climate models, are unlikely to greatly alter the response of tropical forests to rising mean temperatures but may increase the risk of leaf thermal damage during heatwaves.     

Do the energy fluxes and surface conductance of boreal coniferous forests in Europe scale with leaf area?

Earth observing systems are now routinely used to infer leaf area index (LAI) given its significance in spatial aggregation of land surface fluxes. Whether LAI is an appropriate scaling parameter for daytime growing season energy budget, surface conductance (G_s), water‐ and light‐use efficiency and surface–atmosphere coupling of European boreal coniferous forests was explored using eddy‐covariance (EC) energy and CO₂ fluxes. The observed scaling relations were then explained using a biophysical multilayer soil–vegetation–atmosphere transfer model as well as by a bulk G_s representation. The LAI variations significantly alter radiation regime, within‐canopy microclimate, sink/source distributions of CO₂, H₂O and heat, and forest floor fluxes. The contribution of forest floor to ecosystem‐scale energy exchange is shown to decrease asymptotically with increased LAI, as expected. Compared with other energy budget components, dry‐canopy evapotranspiration (ET) was reasonably ‘conservative’ over the studied LAI range 0.5–7.0 m² m^?2. Both ET and G_s experienced a minimum in the LAI range 1–2 m² m^?2 caused by opposing nonproportional response of stomatally controlled transpiration and ‘free’ forest floor evaporation to changes in canopy density. The young forests had strongest coupling with the atmosphere while stomatal control of energy partitioning was strongest in relatively sparse (LAI ~2 m² m^?2) pine stands growing on mineral soils. The data analysis and model results suggest that LAI may be an effective scaling parameter for net radiation and its partitioning but only in sparse stands (LAI <3 m² m^?2). This finding emphasizes the significance of stand‐replacing disturbances on the controls of surface energy exchange. In denser forests, any LAI dependency varies with physiological traits such as light‐saturated water‐use efficiency. The results suggest that incorporating species traits and site conditions are necessary when LAI is used in upscaling energy exchanges of boreal coniferous forests.     

Phylogenetic Structure of Tree Species across Different Life Stages from Seedlings to Canopy Trees in a Subtropical Evergreen Broad-Leaved Forest

Investigating patterns of phylogenetic structure across different life stages of tree species in forests is crucial to understanding forest community assembly, and investigating forest gap influence on the phylogenetic structure of forest regeneration is necessary for understanding forest community assembly. Here, we examine the phylogenetic structure of tree species across life stages from seedlings to canopy trees, as well as forest gap influence on the phylogenetic structure of forest regeneration in a forest of the subtropical region in China. We investigate changes in phylogenetic relatedness (measured as NRI) of tree species from seedlings, saplings, treelets to canopy trees; we compare the phylogenetic turnover (measured as β_NRI) between canopy trees and seedlings in forest understory with that between canopy trees and seedlings in forest gaps. We found that phylogenetic relatedness generally increases from seedlings through saplings and treelets up to canopy trees, and that phylogenetic relatedness does not differ between seedlings in forest understory and those in forest gaps, but phylogenetic turnover between canopy trees and seedlings in forest understory is lower than that between canopy trees and seedlings in forest gaps. We conclude that tree species tend to be more closely related from seedling to canopy layers, and that forest gaps alter the seedling phylogenetic turnover of the studied forest. It is likely that the increasing trend of phylogenetic clustering as tree stem size increases observed in this subtropical forest is primarily driven by abiotic filtering processes, which select a set of closely related evergreen broad-leaved tree species whose regeneration has adapted to the closed canopy environments of the subtropical forest developed under the regional monsoon climate.     

Whole-Plant Water use and Canopy Conductance of Cassava Under Iimited Available Soil Water and Varying Evaporative Demand

Cassava (Manihot esculenta Crantz), a perennial woody shrub, is known to be highly productive under favourable conditions and produce reasonably well under adverse conditions where other crops fail. Using constant heat sap flow sensors, sap flow density (F _d ) of cassava was monitored for 10 days in December 2002. Sap flow was highly correlated (R ² =0.72, P<0.05) to incoming solar radiation (R _s) than to other climatic factors. Using cross-correlation analysis, no time shift was detected between F _d and solar radiation, whereas vapour pressure deficit (VPD) lags F _d by 110 min. Solar radiation and VPD together explained 83% of diurnal variation in sap flow. Whole-plant transpiration ranged from 0.8 to 1.2 mm day⁻¹ and daily canopy conductance (g _c), computed based on the inverted Penman–Monteith model, varied between 0.7 and 2.1 mm s⁻¹ (mean = 1.4 ± 0.5 mm s⁻¹). For the measurement period, characterized by high evaporative demand coupled with low available soil water, transpiration accounted for 21% of the available energy and was only able to meet 24% of the atmospheric water demand. Average decoupling factor (Ω) of 0.05±0.02 estimated suggested that a 10% change in g _c may lead to more than 9% change in transpiration which further supports the notion that stomata play significant role in regulating cassava water use compared to other known mechanisms. Beyond light saturation (R _s >300 W m⁻²) and at higher VPD (>1.0 kPa), wind effects on the canopy transpiration under water stress condition were low, while VPD explains 94% of the observed variance in daily canopy conductance.