Precipitation on land versus distance from the ocean: Evidence for a forest pump of atmospheric moisture

Flux of oceanic moisture brought inland by winds has been conventionally considered as a geophysical parameter practically unaffected by vegetation; accordingly, models predict only slight post-deforestation precipitation reductions. Here we show that the dependence of annual precipitation on distance x from the ocean differs markedly between the world's forested and non-forested continent-scale regions. In the non-forested regions precipitation declines exponentially with distance from the ocean with an established global mean e-folding length of l ～ 600 km. In contrast, in the forest-covered regions precipitation does not decrease or even grow along several thousand kilometers inland. Using a novel physical mechanism involving the non-equilibrium distribution of atmospheric water vapor it is explained how the high transpiration fluxes developed by forests enable them to pump atmospheric moisture from the ocean to any distance inland to compensate for the gravitational runoff of water. Our results indicate that forest cover plays a major role in the atmospheric circulation and water cycling on land. This suggests a good potential for forest-mediated solutions of the global desertification and water security problems.     

On the forest cover–water yield debate: from demand‐ to supply‐side thinking

Several major articles from the past decade and beyond conclude the impact of reforestation or afforestation on water yield is negative: additional forest cover will reduce and removing forests will raise downstream water availability. A second group of authors argue the opposite: planting additional forests should raise downstream water availability and intensify the hydrologic cycle. Obtaining supporting evidence for this second group of authors has been more difficult due to the larger scales at which the positive effects of forests on the water cycle may be seen. We argue that forest cover is inextricably linked to precipitation. Forest‐driven evapotranspiration removed from a particular catchment contributes to the availability of atmospheric moisture vapor and its cross‐continental transport, raising the likelihood of precipitation events and increasing water yield, in particular in continental interiors more distant from oceans. Seasonal relationships heighten the importance of this phenomenon. We review the arguments from different scales and perspectives. This clarifies the generally beneficial relationship between forest cover and the intensity of the hydrologic cycle. While evidence supports both sides of the argument – trees can reduce runoff at the small catchment scale – at larger scales, trees are more clearly linked to increased precipitation and water availability. Progressive deforestation, land conversion from forest to agriculture and urbanization have potentially negative consequences for global precipitation, prompting us to think of forest ecosystems as global public goods. Policy‐making attempts to measure product water footprints, estimate the value of ecosystem services, promote afforestation, develop drought mitigation strategies and otherwise manage land use must consider the linkage of forests to the supply of precipitation.     

Deep-rooted vegetation, Amazonian deforestation, and climate: results from a modelling study

The depth of the root system controls the maximum amount of soil water that can be transpired by the vegetation into the atmosphere during dry periods. Water uptake from deep soil layers has been found to contribute significantly to the dry season transpiration at some sites in Amazonia and it has been estimated that large parts of the evergreen forests in Amazonia depend on deep roots to survive the dry season. Thus, the presence of deep roots might provide a significant source of atmospheric moisture during the dry season, and one which would be affected by deforestation. We investigate the role of deep-rooted vegetation and its removal in the context of Amazonian deforestation using an atmospheric General Circulation Model (GCM). A distribution of deep roots is obtained by a numerical optimization approach. The simulated climate with the use of the calculated deep roots substantially improves the seasonal characteristics of the GCM. Three additional simulations are then conducted in order to isolate the effect of rooting depth reduction from other parameter changes associated with large-scale deforestation. Most of the climatic effects occur during the dry season and are attributed to the reduction of rooting depth. Dry periods are found to last longer, being more intense with drier and warmer air, while the wet season remains fairly unchanged. The implications of these climatic effects for the re-establishment of the natural evergreen forest are discussed.     

Revealing Invisible Water: Moisture Recycling as an Ecosystem Service

An ecosystem service is a benefit derived by humanity that can be traced back to an ecological process. Although ecosystem services related to surface water have been thoroughly described, the relationship between atmospheric water and ecosystem services has been mostly neglected, and perhaps misunderstood. Recent advances in land-atmosphere modeling have revealed the importance of terrestrial ecosystems for moisture recycling. In this paper, we analyze the extent to which vegetation sustains the supply of atmospheric moisture and precipitation for downwind beneficiaries, globally. We simulate land-surface evaporation with a global hydrology model and track changes to moisture recycling using an atmospheric moisture budget model, and we define vegetation-regulated moisture recycling as the difference in moisture recycling between current vegetation and a hypothetical desert world. Our results show that nearly a fifth of annual average precipitation falling on land is from vegetation-regulated moisture recycling, but the global variability is large, with many places receiving nearly half their precipitation from this ecosystem service. The largest potential impacts for changes to this ecosystem service are land-use changes across temperate regions in North America and Russia. Likewise, in semi-arid regions reliant on rainfed agricultural production, land-use change that even modestly reduces evaporation and subsequent precipitation, could significantly affect human well-being. We also present a regional case study in the Mato Grosso region of Brazil, where we identify the specific moisture recycling ecosystem services associated with the vegetation in Mato Grosso. We find that Mato Grosso vegetation regulates some internal precipitation, with a diffuse region of benefit downwind, primarily to the south and east, including the La Plata River basin and the megacities of Sao Paulo and Rio de Janeiro. We synthesize our global and regional results into a generalized framework for describing moisture recycling as an ecosystem service. We conclude that future work ought to disentangle whether and how this vegetation-regulated moisture recycling interacts with other ecosystem services, so that trade-offs can be assessed in a comprehensive and sustainable manner.     

Evaluation of ecosystem dynamics, plant geography and terrestrial carbon cycling in the LPJ dynamic global vegetation model

The Lund–Potsdam–Jena Dynamic Global Vegetation Model (LPJ) combines process‐based, large‐scale representations of terrestrial vegetation dynamics and land‐atmosphere carbon and water exchanges in a modular framework. Features include feedback through canopy conductance between photosynthesis and transpiration and interactive coupling between these ‘fast’ processes and other ecosystem processes including resource competition, tissue turnover, population dynamics, soil organic matter and litter dynamics and fire disturbance. Ten plants functional types (PFTs) are differentiated by physiological, morphological, phenological, bioclimatic and fire‐response attributes. Resource competition and differential responses to fire between PFTs influence their relative fractional cover from year to year. Photosynthesis, evapotranspiration and soil water dynamics are modelled on a daily time step, while vegetation structure and PFT population densities are updated annually. Simulations have been made over the industrial period both for specific sites where field measurements were available for model evaluation, and globally on a 0.5°° × 0.5°° grid. Modelled vegetation patterns are consistent with observations, including remotely sensed vegetation structure and phenology. Seasonal cycles of net ecosystem exchange and soil moisture compare well with local measurements. Global carbon exchange fields used as input to an atmospheric tracer transport model (TM2) provided a good fit to observed seasonal cycles of CO₂ concentration at all latitudes. Simulated inter‐annual variability of the global terrestrial carbon balance is in phase with and comparable in amplitude to observed variability in the growth rate of atmospheric CO₂. Global terrestrial carbon and water cycle parameters (pool sizes and fluxes) lie within their accepted ranges. The model is being used to study past, present and future terrestrial ecosystem dynamics, biochemical and biophysical interactions between ecosystems and the atmosphere, and as a component of coupled Earth system models.     

云雾山自然保护区环境因素对土壤水分空间分布的影响

选取黄土高原云雾山自然保护区天然草地,研究分析了地形因素、植被类型、降雨和封育措施对土壤水分的影响,研究结果表明:在0～100 cm土壤,单点季节平均土壤含水量的空间变化主要受坡向、坡位、群落类型和封育措施的影响;在深层土壤100～300cm,相对海拔、坡位、坡向、群落类型和封育措施在控制土壤水分的再分布中具有重要作用.不同的环境因素对土壤水分空间分布影响的季节变化存在明显差异,坡度、相对海拔、坡位与土壤湿度的关系依赖于前期降雨量,土壤湿度和坡度、海拔、坡位和封育措施的相关性一般随前期降雨的增加而增大.群落类型对土壤湿度的影响与群落盖度和蒸腾速率有关,在植被盖度差异较大的5月份和蒸腾速率差异较大的7月份影响显著.坡向对土壤湿度的影响依赖于太阳辐射的变化,在太阳辐射较强烈的7月份差异显著.由于长期自然封育,草地覆盖度不断增加,其蓄水保水能力增强,对土壤水分的调节能力得到提高,从而能在一定程度上改善土壤水分条件.     

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.     

黑河流域高寒草甸生态系统水分收支及蒸散发拆分研究

水分收支是对水循环要素降水、蒸发蒸腾、径流以及土壤贮储水量变化等的定量刻画,对水资源的可持续开发及利用至关重要。基于黑河流域阿柔观测站2014和2015年水文气象观测数据,运用水量平衡理论,定量的评估了高寒草甸生态系统的水分收支动态,并结合双源模型对高寒草甸生态系统蒸散发(植被蒸腾和土壤蒸发)进行拆分及评价。研究结果表明(1)在生长季(5—9月)植被蒸腾是高寒草甸生态系统主要的耗水形式,2014和2015年生长季平均蒸散比(T/ET)分别为0.74和0.79;(2)土壤水分的剧烈变化主要发生在0—40 cm处,且受冻融过程影响显著;(3)在降水较多的年份(2014)高寒草甸生态系统水分收支基本平衡,且不受冻融影响的月份(6—9)有地表径流产生约42 mm;在正常年份(2015),生态系统呈现水分亏缺,亏缺量约为134 mm,6—9月约亏缺26 mm;(4)模型估算蒸散发(ET)与实测蒸散发具有很好的一致性,相关系数可达0.90,敏感性分析表明模型输入变量对蒸散发(ET)及蒸散比(T/ET)产生的误差较小,双源模型可以很好地实现对高寒草甸生态系统蒸散发(ET)的拆分。     

黄河源区不同类型冻土土壤水分入渗特性

冻土土壤水分运动由于受到冻融过程的影响而显示其独特性,而目前对于不同类型冻土土壤水分入渗特性尚缺乏足够的认识。为此,以黄河源区康穷小盆地多下坡年冻土和上坡季节冻土区为例,结合季节降雨变化,基于大气降水、冻土土壤水分、冻结层上水等野外监测数据分析,采用HYDRUS-1D软件冻融模块进行土壤水分入渗模拟,对比分析了融化期多年冻土和季节冻土土壤水分运移过程的差异性,研究结果表明：①在快速融化阶段,降雨以地表径流为主,表层土壤水分含量增加,土壤下渗有限,冻结层上水位上升幅度较小;在稳定融化阶段,土壤水分含量增加,土壤水分下渗增强,受冻土层阻隔影响,多年冻土区冻结层上水水位上升幅度较大,季节冻土区土壤水分则以深层渗漏或侧向流动为主。②受到降雨强度、土壤质地、蒸散发、植被覆盖等因素的影响,降雨损失主要以地表径流为主,下坡各层土壤水分随冻结土壤融化自上而下逐渐增加并达到饱和状态,但上坡表层土壤不易达到饱水状态。③区域河流贯穿融区地下水发育,导致上坡冻结层上水位小幅度上升,下坡冻结层上水位的变化除受到降雨入渗的影响外,还受到融区地下水的影响,引起下坡冻结层上水位的快速上升。研究结果有助于深入了解全球气候变化背景下的冻土退化及其水文效应,进而为定量评估流域水资源脆弱性与区域生态敏感性提供科学依据。     

Contribution of Tibetan Plateau ecosystems to local and remote precipitation through moisture recycling

The ecosystems of the Tibetan Plateau (TP) provide multiple important ecosystem services that benefit both local populations and those beyond, such as through climate regulation services on precipitation for East Asia and China. However, the precipitation regulation service of the TP ecosystems for supplying moisture and maintaining precipitation is yet to be evaluated. In this study, we used the moisture recycling framework and a moisture tracking model to quantify the precipitation regulation services of TP ecosystems for their contribution to precipitation. We found TP ecosystems contributed substantially to local and downwind precipitation, with a contribution of 221 mm/year for the TP and neighboring areas through evapotranspiration (ET) (104 mm/year through transpiration), declined to <10 mm/year for eastern China and other surrounding countries. Among ecosystem types, grassland contributed most to precipitation, followed by barren and snow lands, forests, and shrublands. In terms of seasonality, precipitation contribution from TP ecosystems was greater in summer months than in non-summer months for western China, while the opposite was true for eastern China—although the magnitude was much smaller. Over the past two decades, the significant ET increases in TP translated to a widespread increase in precipitation contribution for TP and downwind beneficiary regions from 2000 to 2020. Our study provides a quantitative way to understand the precipitation regulation services of TP ecosystems through moisture recycling, substantiating their key role to maintain precipitation and the water cycle for downwind regions—effectively acting as an ecological safeguard that could be perceived by the public.     

Indirect vegetation–soil moisture feedback with application to Holocene North Africa climate1

Using a fully coupled climate–terrestrial ecosystem model, we demonstrate explicitly that an initial perturbation on vegetation induces not only a direct positive vegetation feedback, but also a significant indirect vegetation–soil moisture feedback. The indirect feedback is generated through either fractional cover change or soil moisture depletion. Both indirect feedback mechanisms are triggered by a vegetation perturbation, but involve subsequent effects of soil moisture and evaporation, indirectly. An increase in vegetation tends to reduce bare‐ground evaporation through either the area reduction in bare ground or the depletion of soil moisture; the reduced evaporation may then counter the initial plant transpiration, favoring a negative net vegetation feedback. Furthermore, grasses are more effective in inducing the indirect vegetation–soil feedbacks, because of their limited plant evapotranspiration and shallower roots that tend to change surface soil moisture, and, in turn, evaporation, effectively. In comparison, trees favor a direct positive vegetation feedback due to their strong plant transpiration on subsurface soil moisture as well as a lower albedo.     

气候变化背景下全球陆地干湿变化研究综述

气候变化背景下,全球水循环加剧,出现了大气变干与植被变绿等陆地干湿变化趋势的解耦现象,旱区面积变化也存在争议。为回答上述问题,在梳理常见干湿指标变化趋势与驱动因素的基础上,根据指标变化方向对其进行归类,然后从机理角度解析影响不同指标趋势耦合或解耦的关键要素,并提出未来干湿变化研究展望。结果表明,气候变化背景下,饱和水汽压差、干燥度指数和土壤水分指标显著变干,植被绿度和生产力显著变湿(增加),降水、径流、陆地水储量和其他复合指标区域分异明显、但整体趋势不显著。二氧化碳浓度增加、气温升高和土地利用变化是导致不同指标趋势分异的重要因素,不同指标的趋势分异也解释了旱区面积评估在不同维度上的差异。未来研究中应开展干湿变化的综合评估,其综合性主要体现在以下四个方面：1)关注大气-生态-水文多维度评估;2)解析自然与人类双重压力下,不同维度要素间的关联、互馈过程,及其对系统干湿演变的促进、限制与调节作用;3)重视干湿演变程中的极端灾害事件和空间上以旱区为代表的气候变化敏感性区域;4)构建以脆弱性评估与适应性治理为核心的气候变化应对路径。     

黄土高原沟壑区小流域水分环境演变研究

以南小河沟为例,评价了水土保持工程措施、生物措施和生产力水平提高过程对流域水分环境条件的影响．结果发现南小河沟流域水环境在近５０年内发生了很大变化,坡面年总汇流量由治理前的１５．８３ｍｍ逐步减小为５．１６ｍｍ和现在的３．６６ｍｍ,未来１０年可能减到２．８６ｍｍ;塬面产流量却因庭院和道路面积增加以及建筑风格提高而逐步增大;坡地和沟谷的产流量逐步减小．同时生产力水平的提高进一步加强了水分小循环,削弱了水文大循环,改变了流域水均衡要素的定量分配     

An exceptional role for flowering plant physiology in the expansion of tropical rainforests and biodiversity

Movement of water from soil to atmosphere by plant transpiration can feed precipitation, but is limited by the hydraulic capacities of plants, which have not been uniform through time. The flowering plants that dominate modern vegetation possess transpiration capacities that are dramatically higher than any other plants, living or extinct. Transpiration operates at the level of the leaf, however, and how the impact of this physiological revolution scales up to the landscape and larger environment remains unclear. Here, climate modelling demonstrates that angiosperms help ensure aseasonally high levels of precipitation in the modern tropics. Most strikingly, replacement of angiosperm with non-angiosperm vegetation would result in a hotter, drier and more seasonal Amazon basin, decreasing the overall area of ever-wet rainforest by 80 per cent. Thus, flowering plant ecological dominance has strongly altered climate and the global hydrological cycle. Because tropical biodiversity is closely tied to precipitation and rainforest area, angiosperm climate modification may have promoted diversification of the angiosperms themselves, as well as radiations of diverse vertebrate and invertebrate animal lineages and of epiphytic plants. Their exceptional potential for environmental modification may have contributed to divergent responses to similar climates and global perturbations, like mass extinctions, before and after angiosperm evolution.     

Conservation of water cycle on land via restoration of natural closed-canopy forests: implications for regional landscape planning

Investigating the role of forests for maintenance of the water cycle on land is critically important in the current situation of rapid global elimination of the natural vegetation cover. In this paper we contribute to the on-going discussion of the issue with two aspects. (1) Theoretical consideration of the water cycle on land reveals the importance of correct identification of independent and dependent terms in the water budget with respect to changing vegetation cover for understanding possible scenarios of water cycle change under anthropogenic impact. An important controlling influence of the vegetation cover is imposed on the outgoing fluxes of atmospheric moisture A ⁻ from land to the ocean, which is maximized in deserts and minimized in forested areas, while the dependencies for runoff and precipitation are the reverse. (2) Physical mechanisms allowing for efficient water retention and minimization of A ⁻ in forest ecosystems are investigated. Atmospheric water vapor is in aerostatic equilibrium when the temperature lapse rate is less than G = 1.9 K km⁻¹ and out of aerostatic equilibrium when G > 1.9 K km⁻¹. In the former case there are no vertical upward fluxes of the evaporated water. It is shown that the temperature profiles developed under the closed canopies of natural forests keep water vapor in aerostatic equilibrium preventing soil moisture loss to A ⁻ , in contrast to the situation in open ecosystems like grasslands. The analyzed evidence allows one to conclude that an intensive water cycle on land can be restored after recovery of natural, self-sustained closed canopy ecosystems on continent-wide areas.     

Warming Reduces Carbon Losses from Grassland Exposed to Elevated Atmospheric Carbon Dioxide

The flux of carbon dioxide (CO₂) between terrestrial ecosystems and the atmosphere may ameliorate or exacerbate climate change, depending on the relative responses of ecosystem photosynthesis and respiration to warming temperatures, rising atmospheric CO₂, and altered precipitation. The combined effect of these global change factors is especially uncertain because of their potential for interactions and indirectly mediated conditions such as soil moisture. Here, we present observations of CO₂ fluxes from a multi-factor experiment in semi-arid grassland that suggests a potentially strong climate – carbon cycle feedback under combined elevated [CO₂] and warming. Elevated [CO₂] alone, and in combination with warming, enhanced ecosystem respiration to a greater extent than photosynthesis, resulting in net C loss over four years. The effect of warming was to reduce respiration especially during years of below-average precipitation, by partially offsetting the effect of elevated [CO₂] on soil moisture and C cycling. Carbon losses were explained partly by stimulated decomposition of soil organic matter with elevated [CO₂]. The climate – carbon cycle feedback observed in this semiarid grassland was mediated by soil water content, which was reduced by warming and increased by elevated [CO₂]. Ecosystem models should incorporate direct and indirect effects of climate change on soil water content in order to accurately predict terrestrial feedbacks and long-term storage of C in soil.     

Coupling microscale vegetation–soil water and macroscale vegetation–precipitation feedbacks in semiarid ecosystems

At macroscale, land–atmosphere exchange of energy and water in semiarid zones such as the Sahel constitutes a strong positive feedback between vegetation density and precipitation. At microscale, however, additional positive feedbacks between hydrology and vegetation such as increase of infiltration due to increase of vegetation, have been reported and have a large impact on vegetation distribution and spatial pattern formation. If both macroscale and microscale positive feedbacks are present in the same region, it is reasonable to assume that these feedback mechanisms are connected. In this study, we develop and analyse a soil‐vegetation‐atmosphere model coupling large‐scale evapotranspiration–precipitation feedback with a model of microscale vegetation–hydrology feedback to study the integration of these nonlinearities at disparate scales. From our results, two important conclusions can be drawn: (1) it is important to account for spatially explicit vegetation dynamics at the microscale in climate models (the strength of the precipitation feedback increased up to 35% by accounting for these microscale dynamics); (2) studies on resilience of ecosystems to climate change should always be cast within a framework of possible large‐scale atmospheric feedback mechanism (substantial changes in vegetation resilience resulted from incorporating macroscale precipitation feedback). Analysis of full‐coupled modelling shows that both type of feedbacks markedly influence each other and that they should both be accounted for in climate change models.     

平茬对半干旱黄土丘陵区柠条林地土壤水分的影响

半干旱黄土丘陵区多年生柠条人工林地发生土壤旱化,研究柠条林平茬对土壤水分影响对于防治土壤旱化具有重要意义。采用中子仪测定土壤水分,对未平茬和平茬柠条林地土壤水分进行测定,分析了平茬对土壤水分的影响。结果表明:未平茬和平茬柠条林地降雨补给量(R₁,R₂)同降雨量(P)显著正相关(P<0.05)。定义降雨耗损量(林冠截留量和地表径流之和)占降雨量的百分比为降雨耗损率,未平茬林地降雨损耗率(L₁)和平茬柠条林地降雨损耗率(L₂)分别与其降雨前土壤表层(0-20 cm)含水量(S₁,S₂)呈明显指数关系(P<0.05):L₁=2.54exp(0.22S₁),L₂=2.40exp(0.27S₂),表层含水量相同时,平茬林地降雨损耗率明显高于未平茬林地。平茬后,林地降雨最大入渗深度减小,土壤水分利用深度减小;短时间内(2个月左右)林地20-160 cm含水量增加,之后平茬林地土壤含水量与未平茬林地土壤含水量接近;丰水年和丰水年后的第一年,平茬林地含水量低于未平茬林地,0-400 cm土壤储水量比未平茬林地最多低45.9 mm。平茬后200-400 cm土层土壤水分有少量增加,但是0-200 cm土层土壤含水量损失更严重。平茬3a后,平茬对柠条林地土壤水分的影响减弱。     

Water use and yield of bioenergy poplar in future climates: modelling the interactive effects of elevated atmospheric CO2 and climate on productivity and water use

Vegetation exerts large control on global biogeochemical cycles through the processes of photosynthesis and transpiration that exchange CO₂ and water between the land and the atmosphere. Increasing atmospheric CO₂ concentrations exert direct effects on vegetation through enhanced photosynthesis and reduced stomatal conductance, and indirect effects through changes in climatic variables that drive these processes. How these direct and indirect CO₂ impacts interact with each other to affect plant productivity and water use has not been explicitly analysed and remains unclear, yet is important to fully understand the response of the global carbon cycle to future climate change. Here, we use a set of factorial modelling experiments to quantify the direct and indirect impacts of atmospheric CO₂ and their interaction on yield and water use in bioenergy short rotation coppice poplar, in addition to quantifying the impact of other environmental drivers such as soil type. We use the JULES land‐surface model forced with a ten‐member ensemble of projected climate change for 2100 with atmospheric CO₂ concentrations representative of the A1B emissions scenario. We show that the simulated response of plant productivity to future climate change was nonadditive in JULES, however this nonadditivity was not apparent for plant transpiration. The responses of both growth and transpiration under all experimental scenarios were highly variable between sites, highlighting the complexity of interactions between direct physiological CO₂ effects and indirect climate effects. As a result, no general pattern explaining the response of bioenergy poplar water use and yield to future climate change could be discerned across sites. This study suggests attempts to infer future climate change impacts on the land biosphere from studies that force with either the direct or indirect CO₂ effects in isolation from each other may lead to incorrect conclusions in terms of both the direction and magnitude of plant response to future climate change.     

Effects of afforestation on water yield: a global synthesis with implications for policy

Carbon sequestration programs, including afforestation and reforestation, are gaining attention globally and will alter many ecosystem processes, including water yield. Some previous analyses have addressed deforestation and water yield, while the effects of afforestation on water yield have been considered for some regions. However, to our knowledge no systematic global analysis of the effects of afforestation on water yield has been undertaken. To assess and predict these effects globally, we analyzed 26 catchment data sets with 504 observations, including annual runoff and low flow. We examined changes in the context of several variables, including original vegetation type, plantation species, plantation age, and mean annual precipitation (MAP). All of these variables should be useful for understanding and modeling the effects of afforestation on water yield. We found that annual runoff was reduced on average by 44% (±3%) and 31% (±2%) when grasslands and shrublands were afforested, respectively. Eucalypts had a larger impact than other tree species in afforested grasslands (P=0.002), reducing runoff (90) by 75% (±10%), compared with a 40% (±3%) average decrease with pines. Runoff losses increased significantly with plantation age for at least 20 years after planting, whether expressed as absolute changes (mm) or as a proportion of predicted runoff (%) (P<0.001). For grasslands, absolute reductions in annual runoff were greatest at wetter sites, but proportional reductions were significantly larger in drier sites (P<0.01 and P<0.001, respectively). Afforestation effects on low flow were similar to those on total annual flow, but proportional reductions were even larger for low flow (P<0.001). These results clearly demonstrate that reductions in runoff can be expected following afforestation of grasslands and shrublands and may be most severe in drier regions. Our results suggest that, in a region where natural runoff is less than 10% of MAP, afforestation should result in a complete loss of runoff; where natural runoff is 30% of precipitation, it will likely be cut by half or more when trees are planted. The possibility that afforestation could cause or intensify water shortages in many locations is a tradeoff that should be explicitly addressed in carbon sequestration programs.