On the monoterpene emission under heat stress and on the increased thermotolerance of leaves of Quercus ilex L. fumigated with selected monoterpenes

Leaves of the monoterpene emitter Quercus ilex were exposed to a temperature ramp with 5 °C steps from 30 to 55 °C while maintained under conditions in which endogenous emission of monoterpenes was allowed or suppressed, or under fumigation with selected exogenous monoterpenes. Fumigation with monoterpenes reduced the decline of photosynthesis, photorespiration and monoterpene emission found in non-fumigated leaves exposed to high temperatures. It also substantially increased respiration when photosynthesis and photorespiration were inhibited by low O₂ and CO₂-free air. These results indicate that, as previously reported for isoprene, monoterpenes may help plants cope with heat stress. Monoterpenes may enhance membrane stability, thus providing a rather non-specific protection of photosynthetic and respiratory processes. Monoterpene emission was maximal at a temperature of 35 °C and was inhibited at higher temperatures. This is likely to be the result of the temperature dependency of the enzymes involved in monoterpene synthesis. In contrast to other monoterpenes, cis- and trans- β -ocimene did not respond to exposure to high temperatures. Cis- β -ocimene also did not respond to low O₂ or to fumigation. These results indicate that cis and trans- β -ocimene may have a different pathway of formation that probably does not involve enzymatic synthesis.     

Drought controls over conductance and assimilation of a Mediterranean evergreen ecosystem: scaling from leaf to canopy

Drought control over conductance and assimilation was assessed using eddy flux and meteorological data monitored during four summer periods from 1998 to 2001 above a closed canopy of the Mediterranean evergreen oak tree Quercus ilex. Additional discrete measurements of soil water content and predawn leaf water potential were used to characterize the severity of the drought. Canopy conductance was estimated through the big‐leaf approach of Penman–Monteith by inverting latent heat fluxes. The gross primary production ( GPP ) was estimated by adding ecosystem respiration to net ecosystem exchange. Ecosystem respiration was deduced from night flux when friction velocity ( u _*) was greater than 0.35 m s^?1. Empirical equations were identified that related maximal canopy conductance and daily ecosystem GPP to relative soil water content ( RWC) , the ratio of current soil water content to the field capacity, and to the predawn leaf water potential. Both variables showed a strong decline with soil RWC for values lower than 0.7. The sharpest decline was observed for GPP . The curves reached zero for RWC =0.41 and 0.45 for conductance and GPP , respectively. When the predawn leaf water potential was used as a surrogate for soil water potential, both variables showed a hyperbolic decline with decreasing water potential. These results were compared with already published literature values obtained at leaf level from the same tree species. Scaling up from the leaf to ecosystem highlighted the limitation of two big‐leaf representations: Penman–Monteith and Sellers' Π factor. Neither held completely for comparing leaf and canopy fluxes. Tower measurements integrate fluxes from foliage elements clumped at several levels of organization: branch, tree, and ecosystem. The Q. ilex canopy exhibited non‐random distribution of foliage, emphasizing the need to take into account a clumping index, the factor necessary to apply the Lambert–Beer law to natural forests. Our results showed that drought is an important determinant in water losses and CO₂ fluxes in water‐limited ecosystems. In spite of the limitations inherent to the big‐leaf representation of the canopy, the equations are useful for predicting the influence of environmental factors in Mediterranean woodlands and for interpreting ecosystem exchange measurements.     

Estimation of plant and leaf area index using three techniques in a mature native eucalypt canopy

Abstract Leaf area index (L) is a critical variable in monitoring and modelling forest condition and growth and is therefore important for foresters and environmental scientists to measure routinely and accurately. We compared three different methods for estimating L: a plant canopy analyser (PCA), a point‐quadrat camera method and digital hemispherical photography at a native eucalypt forest canopy at Tumbarumba in southern New South Wales, Australia. All of these methods produced indirect estimates of L based on the close coupling between radiation penetration and canopy structure. The individual L estimates were compared, and the potential advantages and disadvantages of each method were discussed in relation to use in forest inventory and in field data collection programmes for remote sensing calibration and verification. The comparison indicated that all three methods, PCA, digital hemispherical photography and the modified point‐quadrat camera method, produced similar estimates with a standard error between techniques of less than 0.2 L units. All methods, however, provided biased estimates of L and calibration is required to derive true stand L. A key benefit, however, of all of these estimation methods is that observations can be collected in a short period of time (1–2 h of field‐work per plot).     

Long-term transpiration change with rainfall decline in a Mediterranean Quercus ilex forest

In the Mediterranean basin, precipitation is expected to decline as a consequence of climate change. The response of a Quercus ilex forest in southern France to such a decline in water availability was studied using a 4-year throughfall exclusion experiment. Seasonal courses of sap flow and leaf water potential were obtained from 2004 to 2007 and used to characterize tree water relations in a control and a dry treatment. The experiment reduced the average precipitation input to the soil by 29%, and resulted in a 23% reduction in annual transpiration. Soil water potential was significantly lower in the dry treatment only during summer drought, but transpiration was reduced all year round even during well-watered periods. Despite a tight stomatal control over transpiration, whole-tree hydraulic conductance was found to be lower in the trees growing in the driest conditions. This reduction in water transport capacity was observed jointly with a reduction in leaf transpiring area. Canopy leaf area decreased by 18% in the dry treatment as a consequence of the throughfall exclusion, which was found to validate the ecohydrological equilibrium theory.     

不同沙地生境下黄柳(Salix gordejevii)的根系分布和冠层结构特征

对分布于3种不同沙地生境（半固定沙丘、固定沙丘、丘间低地）黄柳灌丛的根系分布（根的数目与深度分布）和冠层结构特征（叶面积指数、平均叶倾角以及当年枝长度分布）进行了研究。半固定沙丘上的黄柳根系最发达（根系分布深,数量大）,固定沙丘的黄柳明显不及前者,而丘间低地的黄柳根系最不发达。粗根（Φ≥5mm）的分布与土壤水分和土壤容重呈显著相关性,细根（Φ<5mm）的分布与土壤容重和紧实度呈极显著相关。不同程度的根系发育导致地上部分的繁茂程度不同分布于半固定沙丘上的黄柳灌丛叶面积指数较大,当年枝平均长度较大,枝长频度分布趋于正态分布;丘间低地的黄柳灌丛叶面积指数较小,当年枝平均长度较小,枝长频度分布趋于对数正态分布;固定沙丘上灌丛的上述特征大致介于二者之间,而其当年枝长度频度符合Weibull分布。     

Transpiration rates of the understory annual Impatiens capensis (Balsaminaceae) in response to the autumnal changes in canopy leaf area

Leaf transpiration rates of Impatiens capensis were measured beneath a broadleaved deciduous forest canopy over successive growing seasons using a steady-state porometer. The transpiration measurements, which continued into early autumn, provided a framework for assessing whether I. capensis exhibits stomatal opening in response to the autumnal increase in available direct-beam radiation reaching the forest floor. The deciduous canopy LAI (leaf area index) decreased from a growing season maximum of 3.94 m² m⁻², while the understory I. capensis population located along a stream channel maintained LAI values ranging from 0.58 to 1.05 m² m⁻² late into the growing season. Late morning and early afternoon leaf transpiration rates during the months of June and July averaged about 8 μg cm⁻² s⁻¹, with a mean stomatal conductance of 0.5 cm s⁻¹. In August, leaf transpiration averaged almost 12 μg cm⁻² s⁻¹, with stomatal conductance exceeding 1.5 cm s⁻¹. However, beginning in early to mid-September, before canopy leaf-fall, the persistent green leaves of I. capensis exhibited a sharp decline in transpiration, possibly a result of decreasing vapor pressure deficits or non-lethal physiological damage induced by cold stress. This physiological decline offsets any advantage that could have been gained by the increased exposure to direct-beam radiation after canopy leaf-fall in mid-October. Although green leaf area and seed-bearing capsules may persist until the first frost in October or early November, there is no evidence of stomatal opening suggestive of carbon assimilation for enhanced seed development during this early autumn period. We conclude that the persistent green leaf area of I. capensis fails to exploit the increase in available direct-beam radiation in the final stage of its life cycle.     

Strong correlation between isoprene emission and gross photosynthetic capacity during leaf phenology of the tropical tree species Hymenaea courbaril with fundamental changes in volatile organic compounds emission composition during early leaf development

Changes of the volatile organic compounds (VOC) emission capacity and composition of different developmental stages of the tropical tree species Hymenaea courbaril were investigated under field conditions at a remote Amazonian rainforest site. The basal emission capacity of isoprene changed considerably over the course of leaf development, from young to mature and to senescent leaves, ultimately spanning a wide range of observed isoprene basal emission capacities from 0.7 to 111.5 µg C g^?1 h^?1 during the course of the year. By adjusting the standard emission factors for individual days, the diel courses of instantaneous isoprene emission rates could nevertheless adequately be modelled by a current isoprene algorithm. The results demonstrate the inadequacy of using one single standard emission factor to represent the VOC emission capacity of tropical vegetation for an entire seasonal cycle. A strong linear correlation between the isoprene emission capacity and the gross photosynthetic capacity (GP_max) covering all developmental stages and seasons was observed. The present results provide evidence that leaf photosynthetic properties may confer a valuable basis to model the seasonal variation of isoprenoid emission capacity; especially in tropical regions where the environmental conditions vary less than in temperate regions. In addition to induction and variability of isoprene emission during early leaf development, considerable amounts of monoterpenes were emitted in a light‐dependent manner exclusively in the period between bud break and leaf maturity. The fundamental change in emission composition during this stage as a consequence of resource availability (supply side control) or as a plant's response to the higher defence demand of young emerging leaves (demand‐side control) is discussed. The finding of a temporary emergence of monoterpene emission may be of general interest in understanding both the ecological functions of isoprenoid production and the regulatory processes involved.     

Canopy structure,light microclimate and leaf gas exchange of Quercus coccifera L. in a Portuguese macchia: measurements in different canopy layers and simulations with a canopy model

Summary The structural characteristics of a diverse array of Quercus coccifera canopies were assessed and related to measured and computed light attenuation, proportion of sunlit foliage, foliage temperatures, and photosynthesis and diffusive conductance behavior in different canopy layers. A canopy model incorporating all components of shortwave and longwave radiation, and the energy balance, conductance, and CO₂ and H₂O exchanges of all leaf layers was developed and compared with measurements of microclimate and gas exchange in canopies in four seasons of the year. In the denser canopies with a leaf area index (LAI) greater than 5, there is little sunlit foliage and the diffuse radiation (400–700 nm) is attenuated to 5% or less of the global radiation (400–700 nm) incident on the top of the canopy. Foliage of this species is nonrandomly distributed with respect to azimuth angle, and within each canopy layer, foliage azimuth and inclination angles are correlated. A detailed version of the model which computed radiation interception and photosynthetic light harvesting according to these nonrandom distributions indicated little difference in whole-canopy gas exchange from calculations of the normal model, which assumes random azimuth orientation. The contributions of different leaf layers to canopy gas exchange are not only a function of the canopy microclimate, but also the degree to which leaves in the lower layers of the canopy exhibit more shade-leaf characteristics, such as low photosynthetic and respiratory capacity and maximal conductance. On cloudless days, the majority of the foliage in a canopy of 5.4 LAI is shaded —70%–90% depending on the time of year. Yet, the shaded foliage under these conditions is calculated to contribute only about one-third of the canopy carbon gain. This contribution is about the same as that of the upper 13% of the canopy foliage. Computed annual whole-canopy carbon gain and water use are, respectively, 60% and 100% greater for a canopy of 5 LAI than for one of 2 LAI. Canopy water-use efficiency is correspondingly less for the canopy of 5 LAI than for that of 2 LAI, but most of this difference is apparent during the cool months of the year, when moisture is more abundant.     

Linking physiological processes with mangrove forest structure: phosphorus deficiency limits canopy development, hydraulic conductivity and photosynthetic carbon gain in dwarf Rhizophora mangle

Spatial gradients in mangrove tree height in barrier islands of Belize are associated with nutrient deficiency and sustained flooding in the absence of a salinity gradient. While nutrient deficiency is likely to affect many parameters, here we show that addition of phosphorus (P) to dwarf mangroves stimulated increases in diameters of xylem vessels, area of conductive xylem tissue and leaf area index (LAI) of the canopy. These changes in structure were consistent with related changes in function, as addition of P also increased hydraulic conductivity ( K _s), stomatal conductance and photosynthetic assimilation rates to the same levels measured in taller trees fringing the seaward margin of the mangrove. Increased xylem vessel size and corresponding enhancements in stem hydraulic conductivity in P fertilized dwarf trees came at the cost of enhanced midday loss of hydraulic conductivity and was associated with decreased assimilation rates in the afternoon. Analysis of trait plasticity identifies hydraulic properties of trees as more plastic than those of leaf structural and physiological characteristics, implying that hydraulic properties are key in controlling growth in mangroves. Alleviation of P deficiency, which released trees from hydraulic limitations, reduced the structural and functional distinctions between dwarf and taller fringing tree forms of Rhizophora mangle .     

环境和冠层结构对华北落叶松林树干液流的影响

准确量化环境和冠层结构变化对树干液流的影响,对于深入理解变化环境下的树木水分利用机制具有重要意义.在六盘山香水河小流域于2019年6-9月利用热扩散式探针监测了华北落叶松林的树干液流,同步观测林外气象、林内根系层土壤含水量和冠层结构动态,分析树干液流速率与潜在蒸散(PET)、土壤相对可利用水分(REW)和林冠层叶面积指...     

Using temperature‐dependent changes in leaf scaling relationships to quantitatively account for thermal acclimation of respiration in a coupled global climate–vegetation model

The response of plant respiration (R) to temperature is an important component of the biosphere's response to climate change. At present, most global models assume that R increases exponentially with temperature and does not thermally acclimate. Although we now know that acclimation does occur, quantitative incorporation of acclimation into models has been lacking. Using a dataset for 19 species grown at four temperatures (7, 14, 21, and 28 °C), we have assessed whether sustained differences in growth temperature systematically alter the slope and/or intercepts of the generalized log–log plots of leaf R vs. leaf mass per unit leaf area (LMA) and vs. leaf nitrogen (N) concentration. The extent to which variations in growth temperature account for the scatter observed in log–log R–LMA–N scaling relationships was also assessed. We show that thermal history accounts for up to 20% of the scatter in scaling relationships used to predict R, with the impact of thermal history on R–LMA–N generalized scaling relationships being highly predictable. This finding enabled us to quantitatively incorporate acclimation of R into a coupled global climate–vegetation model. We show that accounting for acclimation of R has negligible impact on predicted annual rates of global R, net primary productivity (NPP) or future atmospheric CO₂ concentrations. However, our analysis suggests that accounting for acclimation is important when considering carbon fluxes among thermally contrasting biomes (e.g. accounting for acclimation decreases predicted rates of R by up to 20% in high‐temperature biomes). We conclude that acclimation of R needs to be accounted for when predicting potential responses of terrestrial carbon exchange to climatic change at a regional level.     

Sensitivity of African biomes to changes in the precipitation regime

Aim Africa is identified by the Inter‐governmental Panel on Climate Change (IPCC) as the least studied continent in terms of ecosystem dynamics and climate variability. The aim of this study was (1) to adapt the Lund‐Postdam‐Jena‐GUESS (LPJ‐GUESS) ecological modelling framework to Africa by providing new parameter values for tropical plant functional types (PFT), and (2) to assess the sensitivity of some African biomes to changes in precipitation regime. Location The study area was a representative transect (0–22° N and 7–18° E) through the transition from equatorial evergreen forests to savannas, steppes and desert northwards. The transect showed large latitudinal variation in precipitation (mean rainfall ranged from 50 to 2300 mm year^?1). Methods New PFT parameters used to calibrate LPJ‐GUESS were based on modern pollen PFTs and remote sensed leaf area index (LAI). The model was validated using independent modern pollen assemblages, LAI and through comparison with White's modern potential vegetation map. Several scenarios were developed by combining changes in total rainfall amount with variation in the length of the dry season in order to test the sensitivity of African biomes. Results Simulated vegetation compared well to observed data at local and regional scales, in terms of ecosystem functioning (LAI), and composition (pollen and White's vegetation map). The assessment of the sensitivity of biomes to changes in precipitation showed that none of the ecosystems would shift towards a new type under the range of precipitation increases suggested by the IPCC (increases from 5 to 20%). However, deciduous and semi‐deciduous forests may be very sensitive to small reductions in both the amount and seasonality of precipitation. Main conclusions This version of LPJ‐GUESS parameterized for Africa simulated correctly the vegetation present over a wide precipitation gradient. The biome sensitivity assessment showed that, compared with savannas and grasslands, closed canopy forests may be more sensitive to change in precipitation regime due to the synergetic effects of changed rainfall amounts and seasonality on vegetation functioning.     

2008特大冰冻灾害后大明山常绿阔叶林林冠结构动态

林冠结构是研究森林生态系统众多关键生态功能和过程的重要参数,常绿阔叶林是亚热带林区具有代表性的森林类型,对其林冠结构及动态特征的研究还很不深入。在广西大明山中山区选择了一个斜坡水平长200 m、宽160 m的典型坡面,在整个坡面建立了80个20 m×20 m的样地,将样地均匀分为5个坡段,每个坡段包含16个连续的样地,在2009—2012年的生长季测定了林冠高度(CH)、林冠体积(CV)、林冠覆盖度(CC)、林冠上/下冠盖比(HLr)和林冠叶面积指数(LAI),分析了各林冠结构指标的坡位及年际动态,揭示了亚热带常绿阔叶林的林冠结构特征及短期动态规律。研究结果表明,大明山常绿阔叶林林冠结构的一般特征是:平均CH(12.09±0.05)m,平均CV(2642.51±278.33)m~3(每400 m~2样地),平均CC(59.90±3.29)%,平均HLr2.48±0.23,平均LAI 2.00±0.06。大明山常绿阔叶林的林冠结构存在多层性,上层林冠覆盖度平均为42.20%,中层为30.35%,下层为18.05%。大明山常绿阔叶林的林冠结构存在坡面和年际差异,坡面变异系数为CV(29.84%—55.89%)HLr(32.90%—53.52%)LAI(22.48%—43.89%)CC(16.61%—25.74%)CH(8.26%—12.77%);年际变异系数为HLr(47.33%—57.00%)CV(39.70%—49.06%)LAI(21.58%—48.13%)CC(20.35%—24.15%)CH(9.19%—12.59%),表明CH有较强的稳定性。林冠LAI存在明显的坡面尺度效应,即向下顺坡每滑动100 m冠层LAI升高0.34。坡位对CH、HLr有显著(P=0.022)和极显著(P0.001)影响;年份对HLr有显著影响(P=0.013),对CV和CC有极显著影响(P0.001);坡位×年份对CV和LAI的交互作用显著(P=0.016,P=0.017)。回归分析发现树冠面积与林木胸径呈极显著的线性关系。此研究结果表明大明山常绿阔叶林冠层高度较低、林冠体积较小、林冠覆盖度不高、上/下冠盖比和叶面积指数偏小,这与研究区域的海拔较高(934—1223 m),土层浅薄(30—45 cm)以及经常受到冰冻灾害(特别是2008年的特大冰冻灾害)的影响有关,是山地常绿阔叶林树冠结构与山地环境条件长期适应的结果。     

Effects of photoinhibition on whole-plant carbon gain assessed with a photosynthesis model

A canopy photosynthesis model was modified to assess the effect of photoinhibition on whole‐plant carbon gain. Photoinhibitory changes in maximum quantum yield of photosystem II (F_v/F_m) could be explained solely from a parameter (Lflux) calculated from the light micro‐environment of the leaves. This relationship between F_v/F_m and the intercepted cumulative light dose, integrated and equally weighted over several hours was incorporated into the model. The effect of photoinhibition on net photosynthesis was described through relationships between photoinhibition and the shaping parameters of the photosynthetic light‐response curve (quantum use efficiency, convexity, and maximum capacity). This new aspect of the model was then validated by comparing measured field data (diurnal courses of F_v/F_m) with simulation results. Sensitivity analyses revealed that the extent of photoinhibitory reduction of whole‐plant photosynthesis was strongly dependent on the structural parameters (LAI and leaf angle). Simulations for a Mediterranean evergreen oak, Quercus coccifera, under climatic conditions which cause mild photoinhibition revealed a daily loss of 7·5–8·5% of potential carbon gain in the upper sunlit canopy layers, a 3% loss in the bottom canopy, and an overall loss of 6·1%. Thus, this canopy photoinhibition model (CANO‐PI) allows the quantitative evaluation of photoinhibition effects on primary production.     

The differential role of mechanisms for drought resistance in a Mediterranean evergreen shrub: a simulation approach

Plants have the ability to dampen the effects of variability in water resources. Various mechanisms contribute to these properties: reduction of leaf area, increased rooting depth and stomatal conductance. To evaluate the differential roles and interactions of these mechanisms, we have built a model and simulated flows of water in Mediterranean evergreen scrub. The essential concept of this model is that the water status of the canopy is governed by the water lost by transpiration, the availability of soil water and the hydraulic resistances to water flow in soil and plant. The amount of water supplied by the roots is related to changes in water potential between the soil and the leaf. The amount of water lost to the atmosphere is regulated by an interaction between atmospheric demand and canopy water potential. Water uptake by plant is assumed equal to plant water loss. Leaf area appears to affect largely the annual water balance. The critical leaf water potential required to reduce the maximum stomatal conductance by half has a dominant effect on annual leaf water potential. Reducing rooting depth induces a new functional equilibrium for the plant. This new equilibrium is reached by decreasing leaf area and the critical leaf water potential. Our results show the complexity of interactions of these mechanisms and highlight the importance of the coordination between them. Finally, we suggest a reconsideration of these mechanisms in a context of the survival and long-term persistence of the plant.     

2001-2019年太行山南麓栓皮栎人工林叶面积指数与气候因子的关系

栓皮栎(Quercus variabilis)是我国天然分布最广的树种之一,太行山南麓属我国林业工程重点区域,对气候变化较敏感,研究该地区栓皮栎人工林叶面积指数(LAI)与气候因子的关系对我国林业生态工程建设、温带森林碳汇功能评估有重要意义。依托河南黄河小浪底地球关键带国家野外科学观测研究站,利用LAI实测数据对2001—2019年基于MODIS地表反射率反演的LAI产品进行校正,分析了近19年来太行山南麓的气候变化趋势,以及40年生栓皮栎-刺槐-侧柏混交人工林LAI的变化特征及其与气候因子的关系。结果表明：2001—2019年实验区气候为变暖、变湿润趋势,气温升高温变化率为0.70℃/10a,生长季增温高于非生长季,生长季和非生长季降水量分别为增加和减少趋势。近10年来气候暖湿化趋势加快,高温天气发生的频率和强度增加。实验区栓皮栎混交人工林LAI为增加趋势,近19年平均值为2.09,增长率为0.21/10a,近熟林时期LAI增长率低于中龄林时期。气温是影响LAI季节和年际变异的主导气候因子,降水量主要影响LAI的季节变异。近熟林时期高温天气超过35 d的年份LAI比上一年度减小。     

华北落叶松人工林蒸散及产流对叶面积指数变化的响应

定量评价林地蒸散和产流等水文过程对冠层叶面积指数(LAI)的响应,对于深入认识森林植被的生态水文过程及其发生机制,实现半干旱区林水综合管理和区域可持续发展是非常必要的。应用集总式生态水文模型BROOK90,模拟分析了不同降水年型(丰水年、平水年、枯水年)下,位于半干旱区的宁夏六盘山叠叠沟小流域内华北落叶松(Larix principis-rupprechtii)人工林的水文过程对冠层LAI变化的响应关系。结果发现:林分总蒸散量、冠层截留量、蒸腾量与LAI都呈显著的正相关关系(R~20.99,P0.01),而土壤蒸发量、产流量则与LAI均呈显著的负相关关系(R~20.99,P0.01);在不同的降水年型下,各水文过程变量与LAI的关系都可以很好地用指数函数来表达,且都存在着一个LAI阈值。当LAI低于阈值时,各水文过程变量随LAI的变化幅度较大;但高于阈值时,各变量的变化十分缓慢并趋于稳定。在不同降水年型下,各变量LAI阈值之间存在着一定的差异。一般地,丰水年各变量的LAI阈值要大于枯水年,尤其是冠层截留和土壤蒸发。在丰水年,各水文过程变量随LAI增加而变化的速率要比在平水年、枯水年更快,说明在水分充足年份中各变量的波动更多取决于LAI变化,而在水分亏缺的年份中则可能更多地受到水分条件的限制。模拟结果表明,通过减少冠层LAI(如间伐)导致的林分的降低蒸散耗水和增加产流的作用是有限的,这是由于林分蒸散降低的幅度要比LAI降低的幅度小。例如,在平水年,当LAI从4.2变为2.0(减少幅度52.4%)时,林分年蒸散仅从357.2 mm减少至333.9 mm(减少幅度6.5%)。     

Satellite‐derived estimates of forest leaf area index in southwest Western Australia are not tightly coupled to interannual variations in rainfall: implications for groundwater decline in a drying climate

There is increasing concern that widespread forest decline could occur in regions of the world where droughts are predicted to increase in frequency and severity as a result of climate change. The average annual leaf area index (LAI) is an indicator of canopy cover and the difference between the annual maximum and minimum LAI is an indicator of annual leaf turnover. In this study, we analyzed satellite‐derived estimates of monthly LAI across forested coastal catchments of southwest Western Australia over a 12 year period (2000–2011) that included the driest year on record for the last 60 years. We observed that over the 12 year study period, the spatial pattern of average annual satellite‐derived LAI values was linearly related to mean annual rainfall. However, interannual changes to LAI in response to changes in annual rainfall were far less than expected from the long‐term LAI‐rainfall trend. This buffered response was investigated using a physiological growth model and attributed to availability of deep soil moisture and/or groundwater storage. The maintenance of high LAIs may be linked to a long‐term decline in areal average underground water storage and diminished summer flows, with an emerging trend toward more ephemeral flow regimes.