Soil water balance and ecosystem response to climate change

Some essential features of the terrestrial hydrologic cycle and ecosystem response are singled out by confronting empirical observations of the soil water balance of different ecosystems with the results of a stochastic model of soil moisture dynamics. The simplified framework analytically describes how hydroclimatic variability (especially the frequency and amount of rainfall events) concurs with soil and plant characteristics in producing the soil moisture dynamics that in turn impact vegetation conditions. The results of the model extend and help interpret the classical curve of Budyko, which relates evapotranspiration losses to a dryness index, describing the partitioning of precipitation into evapotranspiration, runoff, and deep infiltration. They also provide a general classification of soil water balance of the world ecosystems based on two governing dimensionless groups summarizing the climate, soil, and vegetation conditions. The subsequent analysis of the links among soil moisture dynamics, plant water stress, and carbon assimilation offers an interpretation of recent manipulative field experiments on ecosystem response to shifts in the rainfall regime, showing that plant carbon assimilation crucially depends not only on the total rainfall during the growing season but also on the intermittency and magnitude of the rainfall events.     

差巴嘎蒿灌丛土壤和根系含水量对降雨的响应

以科尔沁沙地半固定沙丘灌木差巴嘎蒿(Artemisia halodendron)为对象,用土钻取样法和壕沟法研究了2006年生长季降雨后差巴嘎蒿周围土壤和根系含水量的时空分布特征及其相互关系,并计算了该灌丛的水量平衡.结果表明:该灌丛在降雨后对水分有暂时存储作用;降雨结束后,灌丛主干的茎流作用使得灌丛中心位置的土壤含水量高于灌丛丛幅垂直投影1/4和3/4处的土壤含水量;雨后6 h灌丛根系含水量与灌丛中心位置的土壤含水量呈极显著的负相关(r=-0.89,P＜0.01);灌丛根系含水量在雨后126 h内出现胁迫脉冲间歇反应.水量平衡计算表明,灌丛边缘外1 m处土壤蒸散量高于灌丛覆盖区的蒸散量,灌丛覆盖可降低土壤水分蒸散速率.     

Increased Drought Impacts on Temperate Rainforests from Southern South America: Results of a Process-Based,Dynamic Forest Model

Increased droughts due to regional shifts in temperature and rainfall regimes are likely to affect forests in temperate regions in the coming decades. To assess their consequences for forest dynamics, we need predictive tools that couple hydrologic processes, soil moisture dynamics and plant productivity. Here, we developed and tested a dynamic forest model that predicts the hydrologic balance of North Patagonian rainforests on Chiloé Island, in temperate South America (42°S). The model incorporates the dynamic linkages between changing rainfall regimes, soil moisture and individual tree growth. Declining rainfall, as predicted for the study area, should mean up to 50% less summer rain by year 2100. We analysed forest responses to increased drought using the model proposed focusing on changes in evapotranspiration, soil moisture and forest structure (above-ground biomass and basal area). We compared the responses of a young stand (YS, ca. 60 years-old) and an old-growth forest (OG, >500 years-old) in the same area. Based on detailed field measurements of water fluxes, the model provides a reliable account of the hydrologic balance of these evergreen, broad-leaved rainforests. We found higher evapotranspiration in OG than YS under current climate. Increasing drought predicted for this century can reduce evapotranspiration by 15% in the OG compared to current values. Drier climate will alter forest structure, leading to decreases in above ground biomass by 27% of the current value in OG. The model presented here can be used to assess the potential impacts of climate change on forest hydrology and other threats of global change on future forests such as fragmentation, introduction of exotic tree species, and changes in fire regimes. Our study expands the applicability of forest dynamics models in remote and hitherto overlooked regions of the world, such as southern temperate rainforests.     

Land-use change and water losses: the case of grassland afforestation across a soil textural gradient in central Argentina

Vegetation changes, particularly those involving transitions between tree‐ and grass‐dominated covers, often modify evaporative water losses as a result of plant‐mediated shifts in moisture access and demand. Massive afforestation of native grasslands, particularly important in the Southern Hemisphere, may have strong yet poorly quantified effects on the hydrological cycle. We explored water use patterns in Eucalyptus grandis plantations and the native humid grasslands that they replace in Central Argentina. In order to uncover the interactive effects that land cover type, soil texture and climate variability may have on evaporative water losses and water use efficiency, we estimated daily evapotranspiration (ET) in 117 tree plantations and grasslands plots across a soil textural gradient (clay‐textured Vertisols to sandy‐textured Entisols) using radiometric information from seven Landsat scenes, existing timber productions records, and ¹³C measurements in tree stems. Tree plantations had cooler surface temperatures (?5°C on average) and evaporated more water (+80% on average) than grasslands at all times and across all sites. Absolute ET differences between grasslands and plantations ranged from ～0.6 to 2 mm day^?1 and annual up‐scaling suggested values of ～630 and ～1150 mm yr^?1 for each vegetation type, respectively. The temporal variability of ET was significantly lower in plantations compared with grasslands (coefficient of variation 36% vs. 49%). Daily ET increased as the water balance became more positive (accumulated balance for previous 18 days) with a saturation response in grassland vs. a continuous linear increase in plantations, suggesting lower ecophysiological limits to water loss in tree canopies compared with the native vegetation. Plantation ET was more strongly affected by soil texture than grassland ET and peaked in coarse textured sites followed by medium and fine textured sites. Timber productivity as well as ¹³C concentration in stems peaked in medium textured sites, indicating lower water use efficiency on extreme textures and suggesting that water limitation was not responsible for productivity declines towards finer and coarser soils. Our study highlighted the key role that vegetation type plays on evapotranspiration and, therefore, in the hydrological cycle. Considering that tree plantations may continue their expansion over grasslands, problematic changes in water management and, perhaps, in local climate can develop from the higher evaporative water losses of tree plantations.     

黄土高原苹果园土壤水分及水分生产力模拟

以长武地区为例,采用WinEPIC模型模拟1980—2018年间黄土高原旱作苹果园地深剖面土壤水分和水分生产力变化动态,以期为该区苹果产业的可持续发展提供科学依据。结果表明: 长武地区苹果园年均产量为26.37 t·hm^-2,年均蒸散量为673.66 mm,年均水分生产力为4.07 kg·m^-3,成龄果树水分胁迫天数主要受降雨量影响,果树生长后期年均胁迫天数为46.46 d,深层土壤含水量最早于9龄果树开始接近凋萎湿度。长武地区苹果整个生长周期内供水量是对果园产量影响最大的因素,深层土壤有效水含量降低是制约果树生长中后期产量提高的最主要因素,在降水不足的年份果树会利用更深层土壤水分。当深层土壤可利用水分较少时,过多的降水并未被果树利用,而是转化为浅层土壤水分蒸发。对于成龄果树在年供水量低于500 mm或高于700 mm时都会造成产量的下降。针对不同生长时期的果园,在不同的降雨年份应该调整果园水分管理策略,可以通过补充灌溉、拦蓄集聚雨水、覆盖、修剪枝条等管理措施,降低果树非生产性耗水及自身奢侈性耗水,延缓深层土壤干层的出现时间,在保证果树生长的同时避免水资源的浪费。     

Association between Plant Canopies and the Spatial Patterns of Infiltration in Shrubland and Grassland of the Chihuahuan Desert,New Mexico

Shrubs have invaded extensive areas of grassland in the southwestern United States. The zones of nutrient-rich soil found beneath plant canopies, referred to as “islands of fertility,” are more intense and spaced farther apart in shrubland than in grassland. This difference in the spatial pattern of soil nutrients may reinforce shrub invasion. Changes in water availability in the soil could also influence shrub invasion. Here we compare the spatial patterns of infiltration, defined as the total equivalent water depth entering the soil following individual rainfall events or summed over many events, at adjacent grass- and shrub-dominated sites in the Sevilleta National Wildlife Refuge. We use two infiltration data sets. First, following four rainfall events, we measured soil moisture and wetting front depth at 10-cm intervals along 24-m transects. We estimate infiltration from these data. Second, we use vertical arrays of soil moisture probes to compare infiltration between adjacent canopies and interspaces following 31 storms. In both the grassland and shrubland, infiltration is typically greater beneath plant canopies than beneath interspaces. Canopies are oases where soil moisture is higher than in the surrounding areas. However, infiltration is not greater beneath canopies when surface runoff is limited. In the shrubland, the canopy–interspace infiltration ratio increases as storm size, and therefore runoff, increases. This relationship also exists in the grassland, but it is not as strong or clear. The magnitude of spatial variability of infiltration is similar in shrubland and grassland. In addition, the distance over which infiltration is correlated is approximately 50 cm in both environments. Most of the spatial variability exists between the stem and canopy margin in the shrubland and straddling the canopy margin in the grassland. The most notable difference is that subcanopy oases are spread farther apart in the shrubland because canopies are separated by larger interspaces in this environment. Received 30 October 2001; accepted 1 August 2002.     

The Influence of Spatial Patterns of Soil Moisture on the Grass and Shrub Responses to a Summer Rainstorm in a Chihuahuan Desert Ecotone

The cycling of surface water, energy, nutrients, and carbon is different between semiarid grassland and shrubland ecosystems. Although differences are evident when grasslands are compared to shrublands, the processes that contribute to this transition are more challenging to document. We evaluate how surface redistribution of precipitation and plant responses to the resulting infiltration patterns could contribute to the changes that occur during the transition from grassland to shrubland. We measured soil water potential under grasses (Bouteloua eriopoda), shrubs (Larrea tridentata) and bare soil and changes in plant water relations and gas exchange following a 15 mm summer storm in the grassland–shrubland ecotone at the Sevilleta National Wildlife Refuge in central New Mexico USA. Following the storm, soil water potential (Ψ_s) increased to 30 cm depth beneath both grass and shrub canopies, with the greatest change observed in the top 15 cm of the soil. The increase in Ψ_s was greater beneath grass canopies than beneath shrub canopies. Ψ_s under bare soil increased only to 5 cm depth. The substantial redistribution of rainfall and different rooting depths of the vegetation resulted in high Ψ_s throughout most of the rooting volume of the grasses whereas soil moisture was unchanged throughout a large portion of the shrub rooting volume. Consistent with this pattern, predawn water potential (Ψ_PD) of grasses increased more than 5 MPa to greater than −1 MPa whereas Ψ_PD of shrubs increased to −2.5 MPa, a change of less than 2 MPa. Transpiration increased roughly linearly with Ψ_PD in both grasses and shrubs. In grasses, assimilation was strongly correlated with Ψ_PD whereas there was no relationship in shrubs where assimilation showed no significant response to the pulse of soil moisture following the storm. These data show that preferential redistribution of water to grass canopies enhances transpiration and assimilation by grasses following large summer storms. This process may inhibit shrubland expansion at the ecotone during periods without extreme drought.     

Effects of tree clumps on soil characteristics in a humid savanna of West Africa (Lamto,Côte d'Ivoire)

The effect of tree clumps on soil characteristics was investigated in a humid savanna (Lamto, Côte d'Ivoire). Soil texture and field capacity were not significantly different under tree clumps compared to open grassland. On the other hand, bulk density was lower under tree clumps, likely due to a greater soil fauna activity under the trees. The pH, available phosphorus, cation exchange capacity, total carbon and total nitrogen contents were higher under tree clumps due to greater organic matter input beneath canopies. Potential soil respiration and mineral nitrogen accumulation were also enhanced, indicating a higher potential microbial activity under tree clumps. Soil water content was slightly lower beneath canopies (from July to November only between 0 and 10 cm depth) when soil moisture was above field capacity. During the other months, no significant difference was measured.     

Soil and plant water stress in an Appalachian oak forest in relation to topography and stand age

Summary A Forest Site Quality Index (FSQI) formulated to predict site quality in Ridge and Valley terrain based on the topographic parameters of aspect, slope inclination and slope position was used to verify moisture gradients along the southeast face of Potts Mountain in Craig County, Virginia. A gradient of site quality index values representing xeric to mesic sites was established in both recently clearcut and adjacent uncut second-growth forest stands. Soil moisture content was determined gravimetrically at ten day intervals from May to October, 1981. Plant moisture stress measurements were taken in conjunction with soil moisture sampling using the pressure chamber technique on three dominant hardwood tree species.For both clearcut and uncut forest stands, a general gradient of increasing soil moisture availability with increasing FSQI was evident, although differences were not large between index values of 8 and 11 in either stand type. Soil water potential and predawn plant water potential exhibited a strong seasonal trend, their direct relationship suggesting that available soil water is probably the critical factor controlling base P levels. Growth limiting stress levels began in late July and continued for the remainder of the growing season.Funding for this research was granted through Cooperative Research Agreement # 18-882, USDA SE Forest Experiment Station and the Forestry Department of Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.     

内蒙古西鄂尔多斯荒漠区降水入渗氘同位素特征

探讨我国干旱半干旱地区大气降水在土壤剖面中的时空分布特征将为西鄂尔多斯荒漠退化生态系统恢复和维持提供科学依据.本研究利用氘同位素技术研究了内蒙古西鄂尔多斯荒漠的大气降水、土壤水、地下水中的氘同位素值(δD),运用二元线性混合模型计算降水对各层土壤水的贡献率,并结合土壤含水量分析了不同降水条件下土壤剖面各层土壤水δD的时空分布特征.结果表明: 雨后9 d内,小雨(0～10 mm)影响0～10 cm土壤含水量和土壤水δD值,对表层土壤(0～10 cm)的贡献率在30.3%～87.9%;中雨(10～20 mm)影响0～40 cm土壤含水量和土壤水δD值,对0～40 cm土壤水的贡献率为28.2%～80.8%;大雨(20～30 mm)和特大暴雨(＞30 mm)影响0～100 cm土壤含水量和土壤水δD值.降水对100～150 cm深层土壤水δD值影响不显著.西鄂尔多斯荒漠土壤水δD介于大气降水δD与地下水δD之间,表明西鄂尔多斯荒漠土壤水主要来源于大气降水与地下水.在同一降水强度下,表层土壤水(0～10 cm)受降水的直接影响显著,随着土壤深度的增加,土壤水δD变化幅度降低,100～150 cm深层土壤水δD基本趋于稳定.降水强度越大,对土壤水δD影响的时间越长,影响的土壤深度也越深.     

Water source niche overlap increases with site moisture availability in woody perennials

Classical niche partitioning theory posits increased competition for and partitioning of the most limiting resource among coexisting species. Coexisting plant species may vary in rooting depth, reflecting niche partitioning in water source use. Our goal was to assess the soil water partitioning of woody plant communities across northern Arizona along an elevational moisture gradient using stem and soil water isotopes from two sampling periods to estimate the use of different water sources. We hypothesized that niche overlap of water sources would be higher and monsoon precipitation uptake would be lower at sites with higher moisture availability. Pairwise niche overlap of coexisting species was calculated using mixing model estimates of proportional water use for three sources. Across the moisture gradient, niche overlap increased with site moisture index (precipitation/potential evapotranspiration) across seasons, and site moisture index explained 37% of the variation in niche overlap of intermediate and deeper sources of water. Desert trees utilized more winter source water than desert shrubs, suggesting the partitioning of water sources between functional groups. However, seasonal differences in surface water use were primarily found at intermediate levels of site moisture availability. Our findings support classical niche partitioning theory in that plants exhibit higher overlap of water sources when water is not a limiting resource.     

荒漠地区柽柳人工固沙林土壤水分动态研究

柽柳林土壤剖面的含水量与裸沙地土壤含水量的变化趋势有相似性,表层（0－30cm)含水量均受蒸发量和降雨量的影响变化较大,由于树冠的遮荫作用林地该层平均含水量高于裸沙地的含水量,30－200cm范围内林地土壤的各测量层平均含水量较裸沙地的低,林地和裸沙地土壤含水量均值随深度的变化可用二次函数描述;降雨对每月的0－200cm的土壤水量没有明显的影响,说明降雨多数在当月就被蒸散和深层渗漏所消耗;发现柽柳人工固沙林能在沙土壤的0－200cm范围内有较低的含水量条件下生存。     

Variation in soil respiration under the tree canopy in a temperate mixed forest,central China,under different soil water conditions

The forest canopy cover can directly and indirectly affect soil conditions and hence soil carbon emission through soil respiration. Little is known, however, on the effects of canopy cover on soil respiration under the canopy of different tree species and soil water conditions. We have examined the variation in soil respiration at different soil water conditions (dry <10 %, wet >20 %, v/v) under different tree canopy covers in comparison with the canopy interspace in a temperate coniferous (Pinus armandii Franch) and broadleaved (Quercus aliena var. acuteserrata) mixed forest in central China. The results show that soil respiration measured under tree canopy cover varied with canopy size and soil water content. Soil respiration under small-sized canopies of P. armandii (PS) was higher than that under large-sized (PL) canopies, but the difference was only significant under the dry soil condition. However, soil respiration under large-sized canopies of Q. aliena (QL) was significantly greater than that under small-sized (QS) canopies under both dry and wet soil conditions. The difference in soil respiration between differently sized canopies of Q. aliena (33.5–35.8 %) was significantly greater than that between differently sized canopies of P. armandii (2.4–8.1 %). Differences in soil respiration between inter-plant gaps and under QS canopies in both the dry and wet soil conditions were significant. Significant increases in soil respiration (9.7–32.2 %) during the transition from dry to wet conditions were found regardless of canopy size, but the increase of soil respiration was significantly lower under P. armandii canopies (9.7–17.7 %) than under Q. aliena canopies (25.9–31.5 %). Our findings that the canopy cover of different tree species influences soil respiration under different soil moisture conditions could provide useful information for parameterizing and/or calibrating carbon flux models, especially for spatially explicit carbon models.     

内蒙古典型草原小叶锦鸡儿灌丛化对水分再分配和利用的影响

灌丛化是全球草原地区存在的主要环境问题。通过对内蒙古典型草原区小叶锦鸡儿灌丛和草地斑块冠层降雨再分配、地表径流、土壤含水量的对比观测,研究了小叶锦鸡儿灌丛化对该区水分再分配和利用的影响。结果表明,灌丛和草地斑块的冠层截留量分别占降雨量的20.86%和7.88%,灌丛和草地斑块的平均地表径流系数分别为5.95%和17.19%。土壤含水量观测结果显示,0—60 cm土层中,降雨事件过程中,灌丛斑块较草地斑块能捕获更多水分,灌丛斑块植被冠层下方土壤含水量高于草地斑块;而在雨后无有效降水补充土壤水分的前提下,0—60 cm土层中,灌丛斑块土壤水分蒸散发量高于草地斑块,其中0—10cm土层中灌丛斑块土壤水分蒸散发速率低于草地斑块,10—60 cm土层中灌丛斑块土壤水分蒸散发速率高于草地斑块。研究认为,在水分为关键性限制因子的干旱半干旱区,小叶锦鸡儿灌丛化过程增加草原生态系统中水分分布的空间异质性,灌丛斑块能捕获、利用更多水分以维持更多的生物量。     

黄土区荒草地和裸地土壤水分的循环特征

在人工、天然降雨条件下,研究了黄土高原地区荒草地和裸地土壤水分循环特征.结果表明,干旱年(天然降雨条件),荒草地和裸地剖面内土壤水分变异系数随土层深度的增加而降低,基于标准差和变异系数两个指标,采用聚类分析可以将土壤剖面水分垂直变化划分为4层.丰水年(人工降雨条件),由于持续降雨入渗和强烈的蒸发蒸腾作用,荒草地和裸地剖面内土壤水分变异系数随土层深度的增加呈现“降-升-降”的变化趋势,且表层土壤水分变异程度明显降低.与裸地相比,荒草地土壤水分循环深度和强度加剧,表现为活跃层、次活跃层深度和蒸散量增大.土壤水分的动态变化主要受降雨和蒸散过程的影响,尤其是浅层,而深层具有相对滞后性.土壤水分的动态变化具有明显的季节性,一般可划分为3个主要时期,如春季失墒期、夏秋增失交替期和冬季相对稳定期.干旱年,土壤水分收支负平衡,入渗雨量全为蒸发蒸腾所消耗;丰水年,土壤水分收支正平衡,但入渗雨量的大部分(＞80%)为强烈蒸发蒸腾所消耗.     

Use of differential water sources by Prosopis flexuosa DC: a dendroecological study

In central-western Argentina, there is a pronounced water deficit gradient, from semiarid climate conditions with 500-mm rainfall/year to arid climate conditions with 80-mm rainfall/year. This climatic transition, governed by the rainfall gradient, occurs between the Arid Chaco and Monte phytogeographic regions and is evidenced by differences in vegetation type, structure, dynamics and tree growth. In turn, the availability of soil moisture, particularly access to the water table, modifies water use strategies by trees along this gradient. We analyzed how water availability, expressed as differences in accessibility to the water table, influences Prosopis flexuosa tree rings along a precipitation gradient. In this manner, we try to interpret the growth of species according to the use of differential water sources. P. flexuosa showed highly varying growth reactions (tree-ring width and hydraulic anatomic parameters) with climate, depending on the ecology of the site. Along the Arid Chaco-Monte gradient, the growth of P. flexuosa is more dependent on variations in rainfall in those areas where water depth is greater than root spread. The climate signal was hidden in those regions where the water table is accessible to the root system.     

Forest-scale sap flux responses to rainfall in a dryland eucalyptus plantation

Dryland salinity is caused by rising saline water tables, the result of relatively recent landscape-scale clearance of deep-rooted vegetation. One obvious solution to this problem is the reintroduction of deep-rooted vegetation into these landscapes, most likely non-deciduous trees. Ideally, continually-transpiring deep-rooted trees would remove moisture from throughout the soil profile, increasing the capacity of the soil to store water, thus lowering water tables by effectively reducing the number of rainfall events that contribute to groundwater recharge. In this study, we examined how water use by a Eucalyptus sideroxylon A. Cunn. ex Woolls plantation, growing in a salinity-prone landscape, varied in response to rainfall events across four years of sap flux monitoring. Responses of the plantation were observed across multiple seasons, from above average to well below average rainfall. We observed that the plantation forest, while capable of continuous water use during drought, was also quite responsive to rainfall events. During the driest periods, during which shallow soil moisture was reduced to a stable minimum, the forest continued using water at around 1 mm/day. Generally we observed increases in forest water use following only 5 mm of rainfall, in contrast to 20 mm for neighbouring native vegetation. We compared a range of plausible empirical models for describing forest water use responses to rainfall. The best model demonstrated that rainfall size, post-rainfall PET and the interaction between rainfall size and antecedent soil moisture made significant contributions to variation in forest water use across rainfall events. Interestingly, the model showed that all else equal, higher antecedent soil moisture tended to reduce potential increases in forest water use in response to rainfall.     

北京山区干旱胁迫下侧柏叶片水分吸收策略

干旱与半干旱地区,水分是限制树木生长的重要影响因子。由于降水稀缺且分配不均,叶片吸收水分是此地区树木吸收和利用小量级降水和凝结水的主要方式。北京山区处于易旱少雨的生态脆弱地带,森林植被经常遭受干旱胁迫,所以对该地区的森林系统而言,叶片直接吸收利用截留的降雨是干旱时期树木获得水分的重要途径。基于野外对比控制试验和室内盆栽模拟试验,选取北京山区的主要造林树种侧柏为研究对象,进行利用天然降雨与模拟降雨试验,研究降雨前后侧柏叶片吸水特征,探究侧柏在干旱环境下如何通过叶片吸水缓解干旱胁迫。结果表明：当侧柏长期处于干旱胁迫状态时,叶片可以利用降雨,从中获益用来缓解树木的干旱胁迫状态;叶片的吸水能力与降雨强度呈正相关关系,与土壤含水率呈负相关关系;重度干旱下侧柏植株在降雨强度为15 mm/h时叶片吸水现象最明显,叶水势变化最大为(1.18±0.17) MPa,叶片含水率变化最大为(8.47±1.00)mg/cm~2;当土壤水率高于20.8%时,基本不发生叶片吸水现象。试验结果说明在干旱地区叶片吸水是树木除根系吸水外的重要水分来源方式,并且对干旱地区有效利用短缺水资源,减轻植物水分亏缺具有重要意义。     

羊草群落的水分利用

羊草(Leymus chinensis (Trin.)Tzvel.)群落的土壤水分具有明显的成层性:0～40 cm是根系集中分布层,受降水和蒸散的直接影响,称为蒸散与降水相互作用层;40～120 cm贮水变化滞后于根系层贮水和群落蒸散的季节变化,称为主要贮水层;120 cm以下称为水分相对稳定/平衡层.1996年属平水年,生长季末土壤水盈余18 mm;1998年属丰水年,在连续强降雨时发生渗漏,生长季末土壤水亏缺15 mm.蒸腾-蒸散比(T/ET)不仅反映群落的繁茂和活力,而且反映植物对环境水资源的利用状况.1998年8月T/E7值较小(0.5),6月达0.7,7月受降水少影响而有所降低(0.6),8月水分利用效率达到最大(0.9),9月降到0.6.水分利用效率(WUE)在良好的水分条件下(1998年),主要受植物自身生长速度的限制,其季节变化与生长大周期吻合.深入分析WUE和T/E的内涵,提出蒸散效率(ETE)的概念,能更好地反映植物对环境水资源利用的状况或程度,具有实际意义.     

Modeled ecohydrological responses to climate change at seven small watersheds in the northeastern United States

A cross‐site analysis was conducted on seven diverse, forested watersheds in the northeastern United States to evaluate hydrological responses (evapotranspiration, soil moisture, seasonal and annual streamflow, and water stress) to projections of future climate. We used output from four atmosphere–ocean general circulation models (AOGCMs; CCSM4, HadGEM2‐CC, MIROC5, and MRI‐CGCM3) included in Phase 5 of the Coupled Model Intercomparison Project, coupled with two Representative Concentration Pathways (RCP 8.5 and 4.5). The coarse resolution AOGCMs outputs were statistically downscaled using an asynchronous regional regression model to provide finer resolution future climate projections as inputs to the deterministic dynamic ecosystem model PnET‐BGC. Simulation results indicated that projected warmer temperatures and longer growing seasons in the northeastern United States are anticipated to increase evapotranspiration across all sites, although invoking CO₂ effects on vegetation (growth enhancement and increases in water use efficiency (WUE)) diminish this response. The model showed enhanced evapotranspiration resulted in drier growing season conditions across all sites and all scenarios in the future. Spruce‐fir conifer forests have a lower optimum temperature for photosynthesis, making them more susceptible to temperature stress than more tolerant hardwood species, potentially giving hardwoods a competitive advantage in the future. However, some hardwood forests are projected to experience seasonal water stress, despite anticipated increases in precipitation, due to the higher temperatures, earlier loss of snow packs, longer growing seasons, and associated water deficits. Considering future CO₂ effects on WUE in the model alleviated water stress across all sites. Modeled streamflow responses were highly variable, with some sites showing significant increases in annual water yield, while others showed decreases. This variability in streamflow responses poses a challenge to water resource management in the northeastern United States. Our analyses suggest that dominant vegetation type and soil type are important attributes in determining future hydrological responses to climate change.