Water Article Summaries

Evapotranspiration – A Driving Force in Landscape Sustainability, Eiseltová 2012

Vegetation cover cools Earth when it intercepts the sun’s energy. This is not just by providing shade, but also through evapotranspiration, which is how plants regulate their own internal temperatures.

For a plant … transpiration^[5] is a necessity by which a plant maintains its inner environment within the limit of optimal temperatures. And at the level of landscape, evapotranspiration is the most efficient air conditioning system developed by nature [Eiseltova 2012:10].

The water in plant tissues contains the sun’s energy in the form of latent heat, which is released from plants through evapotranspiration. In the absence of water, solar energy reaching Earth becomes sensible heat – the heat we can feel and measure in rising temperatures.

Without water, the energy of the incoming radiation is transformed into sensible heat and the local area becomes overheated during the day and likewise far cooler at night (as is well known from desert areas, with differences between day and night temperatures typically exceeding 50°C). Water-saturated landscapes provide much more stable environments than do dry terrestrial systems. In landscapes with water – abundant aquatic ecosystems, wetlands and soils with high water retention capacity – about 80% of incoming solar energy is stored as latent heat of water vapour via evapotranspiration, whilst in de-watered landscapes (with a low-water retention capacity) the vast majority of solar energy is transformed into sensible heat (Pokorný et al. 2010b) [Eiseltova 2012: 307].

With respect to landscape management for sustainability, the authors introduce the idea of a “dissipative-ecological-unit,” meaning “the smallest functional unit that is capable of forming internalized cycles of matter and water while dissipating energy” [Eiseltova 2012: 312]. This term emphasizes the importance of small, local water cycles, which occur naturally in undisturbed ecosystems, resulting in “an efficient local resource economy and … relatively even temperatures and moisture conditions” [Eiseltova 2012: 312].

In catchments with a well-developed vegetation cover, water and matter are bound to short-circuited cycles and losses are minimal. In contrast, the increased clearance of forest, exposure of bare land, and drainage of agricultural land have accelerated matter losses from catchments [Eiseltova 2012: 11].

There is an urgent need that agricultural research focuses on how to close water cycles^[6] in the landscape and the development of farming systems with a more vertically-layered vegetation structure keeping water and lower temperatures during a sunny day^[7] [Eiseltova 2012: 324].

The water cycle is akin to the ‘bloodstream’ of the biosphere. Returning water to the landscape and restoring more natural vegetation cover is the only way to restore landscape sustainability. More attention in present-day science needs to be devoted to the study of the role of vegetation in the water cycle and climate amelioration. Restoration of a more natural vegetation cover over the landscape seems to be the only way forward.

 

Based on our current scientific knowledge, we can propose two criteria for assessing sustainable land management. These criteria are: the efficiency of an ecosystem to recycle water and matter, and its efficiency to dissipate solar energy. Land managers can substantially contribute to the restoration of the water cycle, climate amelioration and reduction of irreversible matter [soil and nutrient] losses with river water flows to the sea.

It is in the interest of society as a whole that land managers (farmers, foresters) be rewarded for their actions towards sustainable management of their land. Suitable tools to assess the achievements of individual land managers with respect to sustainable management of their land are: (1) continuous monitoring of conductivity – a measure of dissolved load – and flow rates in streams in order to estimate matter losses; and (2) the regular evaluation of satellite thermal channel images to assess temperature damping, i.e., the effectiveness of land use to dissipate solar energy. Restoration of natural ‘cooling structures’ – vegetation with its evapotranspiration and condensation-induced water circulation – is essential to renew landscape sustainability [Eiseltova 2012: 325].

New climate solutions, water cycles and the soil carbon sponge, Jehne 2018

Regenerating the soil carbon sponge is our greatest point of leverage for salvaging the planet from the point of existential climate crisis. “Sponge” refers to the quality of a biologically active soil with high organic matter content to have lots of pore space for water absorption. Jehne states that every additional gram of soil carbon allows the soil to hold 8 additional grams of water. He emphasizes the soil sponge concept because it is the driver of healthy ecosystems, and also within our control to repair and regenerate.

Jehne explains that an average of 342 W/m2 of incident solar radiation enters the troposphere while just 339 W/m2 is reflected back into space due to the greenhouse effect [Jehne 2018: 19:00 min]. This leaves a continuous energy balance of 3 W/m2 heating up the planet. Of the solar radiation returning to space, 24% is released through latent heat fluxes from evapotranspiration [Jehne 2018: 1:34:15]. However, due to land use change, there is 50% less transpiration on Earth than there was some 8,000 years ago. Jehne estimates that increasing transpiration by only 5% would be enough to offset the 3 W/m2 surplus solar energy [Jehne 2018: 1:34:50].

Increasing transpiration is achieved by increasing vegetation cover, which in turn is achieved by regenerating the soil sponge. Jehne explains that conventional agriculture has employed techniques (such as burning, cultivating/tilling, applying fertilizer and pesticides, and use of irrigation and fallow) that quickly oxidize the carbon fixed by plants through photosynthesis. By contrast, regenerative agriculture builds up the soil carbon sponge by facilitating the ecological processes that create stable soil carbon and limit organic matter breakdown.

In addition to the cooling effect from the latent heat flux, transpiration also provides the moisture needed for cloud formation. Jehne states that a 2% increase in cloud cover, given its high albedo, is also enough to reflect the excess 3 W/m2 solar radiation that is otherwise absorbed on Earth [Jehne 2018: 1:39:25]. Furthermore, bacteria released from ecosystems serve as the most effective precipitation nuclei^[8] for making rain.

Continental-scale consequences of tree die-offs in North America: identifying where forest loss matters most, Swann 2018

Vegetation cover affects the amount of solar energy a land area absorbs and/or releases, thus altering local temperatures and precipitation. Plants regulate local temperatures through shading, albedo and evapotranspiration, which releases latent^[9] heat.

The ability of a surface to shed energy through latent or sensible heat is key to determining that surface’s temperature – shifts in the relative balance between the two can lead to increases in surface temperatures (where sensible heat is relatively higher) or decreases (where latent heat is relatively higher) [Swann 2018: 2].

This study shows that changes in vegetation cover in a given place affect not only the local climate, but also the climate system at a continental scale. The results are temperature and precipitation changes in remote parts of the continent relative to where the tree loss occurred, leading to changes in ecosystem productivity in those remote parts. This phenomenon is called ‘ecoclimate teleconnections.’

Plants profoundly influence local climate by controlling the exchange of energy and water with the atmosphere. Changes in and/or losses of plant type or plant functioning can alter the local climate, but also potentially large scale climate by modifying atmospheric circulation. … the potentially global impact of plant cover change on other ecosystems as communicated by the atmosphere has been under-appreciated and is only beginning to be evaluated [Swann 2018: 2].

Researchers simulated tree die-offs in their model by replacing all trees in a given domain^[10] with grass.

Domain-scale tree loss led to changes in local (within same domain) surface properties and fluxes including albedo and evapotranspiration. These changes in surface properties modified local surface climate (e.g., precipitation and temperature), as well as impacted atmospheric circulation. The atmospheric circulation response connects the direct forcing of tree loss on the local atmosphere to other regions, impacting climate and thus resulting in altered Gross Primary Productivity (GPP) across North America [Swann 2018: 3-4].

Furthermore, the severity of the remote effects of tree loss depends not only on the scale of the tree loss, but also on the location of the tree loss. The study found, for example, that tree loss in an area covering most of California had greater effect on GPP in other parts of the continent than did tree loss of a similar scale elsewhere.

Thus, in addition to the magnitude of forest loss, the location of forest loss plays an outsized role in determining the continental scale impact [Swann 2018: 6].

Trees, forests and water: cool insights for a hot world, Ellison 2017^[11] 

​This paper takes the innovative and paradigm-shifting position that carbon is not the primary consideration in climate; rather, water should be the central focus, integrated with carbon and energy cycles:

Forest-driven water and energy cycles are poorly integrated into regional, national, continental and global decision-making on climate change adaptation, mitigation, land use and water management. This constrains humanity’s ability to protect our planet’s climate and life-sustaining functions. The substantial body of research we review reveals that forest, water and energy interactions provide the foundations for carbon storage, for cooling terrestrial surfaces and for distributing water resources. Forests and trees must be recognized as prime regulators within the water, energy and carbon cycles. If these functions are ignored, planners will be unable to assess, adapt to or mitigate the impacts of changing land cover and climate. Our call to action targets a reversal of paradigms, from a carbon-centric model to one that treats the hydrologic and climate-cooling effects of trees and forests as the first order of priority. For reasons of sustainability, carbon storage must remain a secondary, though valuable, by-product. The effects of tree cover on climate at local, regional and continental scales offer benefits that demand wider recognition. The forest- and tree-centered research insights we review and analyze provide a knowledge-base for improving plans, policies and actions. Our understanding of how trees and forests influence water, energy and carbon cycles has important implications, both for the structure of planning, management and governance institutions, as well as for how trees and forests might be used to improve sustainability, adaptation and mitigation efforts [Ellison 2017: Abstract].

Our call to action targets a reversal of paradigms, from a carbon-centric model to one that treats the hydrologic and climate-cooling effects of trees and forests as the first order of priority.  [Ellison 2017: Abstract].

Biotic pump of atmospheric moisture as driver of the hydrological cycle on land, Makarieva and Gorshkov 2007^[12]

​The authors examine ecological and geophysical principles to explain how land far inland away from the ocean can remain moist, given that gravity continuously pulls surface and groundwater into the ocean over time.

All freshwater on land originates in the ocean from which it has evaporated, is carried on air flux, and precipitates over the land. Coastal regions benefit from this cycle by their proximity to the ocean, yet in the absence of natural forests in coastal regions precipitation weakens as distance from the ocean increases, leaving inland areas arid. The authors propose the concept of a biotic pump to explain how large continents can be sufficiently moist deep into the interior, and abundant with rivers and lakes.

Air and moisture are pulled horizontally by evapotranspiration over coastal forests. When water vapor from plants condenses, it creates a partial vacuum that pulls water evaporating from the ocean into the continental interior which results in forest rains. By contrast, deserts are unable to pull in ocean evaporation ​because they lack evaporative force.

Therefore, ongoing deforestation, especially coastal deforestation on a large scale, threatens to cut off rain to the interiors of Earth’s continents, thereby creating new deserts. The Amazonian rainforest is the prime example: Deforestation of the eastern coast of South America has led to changes in the rainforest that is resulting in drying and desertification of the interior, with unprecedented fires and loss of rivers. Historically, Australia’s interior became a desert around the time the first humans arrived on the continent, and the authors speculate that early coastal deforestation was the cause. On the other hand, restoring natural coastal forests can also restore inland water cycles and reverse desertification.

This article illustrates the importance of biological relationships that are ecologically complex and poorly understood. It highlights the significance of the precautionary principle in assessing what we don’t know when altering ecological processes, and taking preventive action in the face of uncertainty.

How Forests Attract Rain: An Examination of a New Hypothesis, Sheil and Murdiyarso 2009

Highlighting the significance of Makarieva and Gorshkov’s “biotic pump” hypothesis (above), Sheil and Murdiyarso explain it in layman’s terms in this article for the benefit of a broader public, and examine its validity. They point out that the biotic pump hypothesis offers an explanation for a question not otherwise resolved in conventional climate theory.

Conventional theory offers no clear explanation for how flat lowlands in continental interiors maintain wet climates. Makarieva and Gorshkov show that if only “conventional mechanisms” (including [rain] recycling) apply, then precipitation should decrease exponentially with distance from the oceans. Researchers have previously puzzled over a missing mechanism to account for observed precipitation patterns (Eltahir 1998) [Sheil & Murdiyarso 2009: 342].

They explain the biotic pump hypothesis and how it resolves the puzzle:

Air currents near Earth’s surface flow to where pressure is lowest. According to Makarieva and Gorshkov, these are the areas that possess the highest evaporation rates. In equatorial climates, forests maintain higher evaporation rates than other cover types, including open water. Thus, forests draw in moist air from elsewhere; the larger the forest area, the greater the volumes of moist air drawn in. This additional moisture rises and condenses in turn, generating a positive feedback in which a large proportion of the water condensing as clouds over wet areas is drawn in from elsewhere. The drivers (solar radiation) and basic thermodynamic concepts and relationships are the same as in conventional models, thus most behaviors are identical— the difference lies in how condensation is incorporated.

Makarieva and Gorshkov’s estimates, incorporating volume changes from condensation, imply that when forest cover is sufficient, enough moist air is drawn in to maintain high rainfall inside continents. The numbers now add up: thus, condensation offers a mechanism to explain why continental precipitation does not invariably decline with distance from the ocean [Sheil & Murdiyarso 2009: 342].

Commenting on the relevance of the hypothesis, the authors conclude:

Acceptance of the biotic pump would add to the values that society places on forest cover. By raising regional concerns about water, acceptance of Makarieva and Gorshkov’s biotic pump demands attention from diverse local actors, including many who may otherwise care little for maintaining forest cover [Sheil & Murdiyarso 2009: 346].

Human modification of global water vapor flows from the land surface, Gordon 2005

Human modification of the hydrological cycle has profoundly affected the flow of liquid water across the Earth’s land surface. Compared to changes to liquid water flow, alteration of water vapor flows through land-use changes has received comparatively less attention, despite compelling evidence that such alteration can influence the functioning of the Earth System.

We show that deforestation is as large a driving force as irrigation in terms of changes in the hydrological cycle. Deforestation has decreased global vapor flows from land by 4% (3,000 km3/yr), a decrease that is quantitatively as large as the increased vapor flow caused by irrigation (2,600 km3/yr). Although the net change in global vapor flows is close to zero, the spatial distributions of deforestation and irrigation are different, leading to major regional transformations of vapor-flow patterns [Gordon 2015: 7612].

A green planet versus a desert world: estimating the maximum effect of vegetation on the land surface climate, Kleidon 2000

This climate model simulation illustrates how the biosphere affects the climate system. With “maximum vegetation,” more water is absorbed in the ground, allowing for evaporation to cool the land surface while also recycling more rain. This simulation resulted in an average temperature reduction over land of 1.2C.

The authors describe their approach:

We quantify the maximum possible influence of vegetation on the global climate by conducting two extreme climate model simulations: in a first simulation (‘desert world’), values representative of a desert are used for the land surface parameters for all non-glaciated land regions. At the other extreme, a second simulation is performed (‘green planet’) in which values are used which are most beneficial for the biosphere’s productivity [Kleidon 2000: 471].

They describe the effects of maximum vegetation on the water cycle, stating that over land:

…the hydrological cycle is more active, with precipitation roughly increasing by 100%, evapotranspiration by more than 200% and the mean moisture content of the atmosphere (or precipitable water) increasing by 30%. These increases can be understood by enhanced recycling of soil water as a response of both, (i) more absorbed radiation at the surface so that more energy is available for evapotranspiration and (ii) larger soil water storage capacities (SWCs) which enhance water availability during dry periods. This increased recycling also leads to an overall decrease in continental runoff by about 25% [Kleidon 2000: 476].

Changes in the water cycle result in land surface temperature changes:

The substantial increase in evapotranspiration is associated with differences in the surface energy balance, primarily concerning the partitioning between sensible and latent heat. The latent heat flux increases by the same amount (more than 200%) as evapotranspiration and the sensible heat flux decreases to 30% of its original value. … Subsequently, the increased latent heat flux leads to more efficient cooling of the surface, resulting in temperatures reduced by 1.2 K^[13] [Kleidon 2000: 477-478].

Historical deforestation locally increased the intensity of hot days in northern mid-latitudes, LeJeune 2018

Deforestation contributes to climate change on a global scale through carbon emissions (biogeochemical effects), and on a local/regional scale through biogeophysical effects related to albedo, evapotranspiration and roughness, affecting surface energy budgets.

Here, we show that historical deforestation has led to a substantial local warming of hot days over the northern mid-latitudes – a finding that contrasts with most previous model results. Based on observation-constrained state-of-the-art climate-model experiments, we estimate that moderate reductions in tree cover in these regions have contributed at least one-third of the local present-day warming of the hottest day of the year since pre-industrial time, and were responsible for most of this warming before 1980 [LeJeune 2018: 1].

The study uses observational data to constrain the outcome of a climate model simulating the effects of deforestation on regional temperatures. The authors found that during most of the 20th century, the biogeophysical effects of deforestation were the main cause of regional temperature increases, and that by 1980 deforestation in northern mid-latitudes had declined. By that time other forcings began to take on a proportionally greater role in regional temperature increases.

Twentieth Century regional climate change during the summer in the central United States attributed to agricultural intensification, Alter 2018

Noting that “major increases in crop productivity and changes in regional climate are generally collocated in time and space over the central United States” [Alter 2018: 1587], the study tested the hypothesis that there is a causal relationship – that historical agricultural intensification has affected regional summer climate in this area.

… from 1950 to 2010, the amount of corn harvested annually in the Corn Belt increased by 400%, from 2 billion to 10 billion bushels (National Agricultural Statistics Service, 2016) [Alter 2018: 1586].

and

 

From 1910 to 1949 (pre-agricultural development, pre-DEV) to 1970–2009 (full agricultural development, full-DEV), the central United States experienced large-scale increases in rainfall of up to 35% and decreases in surface air temperature of up to 1°C during the boreal summer months of July and August, when crop water use in the Corn Belt is at its peak [Alter 2018: 1586].

The authors used a regional climate model to test their hypothesis by comparing a set of simulations where “enhanced photosynthesis over cropland [serves] as a proxy for agricultural intensification” [Alter 2018: 1589] to a control simulation with no agricultural intensification. They found that:

Over the region that has experienced significant increases in observed rainfall (region of significant change—ROSC), the mean rainfall increase is ~7% (0.20mm/d) for the simulations and ~15% (0.37mm/d) for the observations. Thus, it seems that agricultural intensification has been a major contributor to the observed increase in summer rainfall in the central United States [Alter 2018: 1589].

Strikingly, these increases in rainfall are also very consistent: Agricultural intensification enhances simulated rainfall across the aforementioned swath in the central United States during at least 62% of the 150 ensemble years (significant at the 5% level using the chi-square test). In the observational data, a similar consistency in precipitation enhancement is evident when comparing the pre-DEV and full-DEV time periods. This suggests that the changes in rainfall due to agricultural intensification are not the result of occasional increases but instead are indicative of a more systematic change in the summer rainfall regime of the central United States [Alter 2018: 1589].

This study usefully contributes evidence that vegetation cover affects local and regional climates, while drawing conclusions, however, that are not necessarily helpful to understanding how to mitigate and adapt to climate change. The study’s findings suggest that agricultural intensification can potentially mitigate local climate change effects in the future, but it is unlikely that the methods that drove agricultural intensification in the 20th Century will continue to work in a changing climate. The reason that these methods are now obsolete is that they strip the soils of the organic material and living organisms necessary for the resilience of plants, and their ability to cope with droughts, floods, heat and other challenging conditions.

The model here uses “enhanced photosynthesis” as a proxy for agricultural intensification. While the increase in yield between early and late 20th Century Corn Belt production represents an increase in photosynthesis, high-input agriculture is but one pathway to enhanced photosynthesis. Moreover, it is an extremely problematic one with respect to climate change, given the high energy costs of fertilizer, pesticides and fuel, and the damage to the soils from these practices.

Instead, a useful lesson to draw from this study is simply that enhanced photosynthesis itself can mitigate climate change regionally. In the context of agricultural production in the era of climate change, enhanced photosynthesis might best be accomplished through ecological intensification, a strategy for improving resilience within an agro-ecosystem, and thereby greater photosynthesis and more reliable crop production.

Intermediate tree cover can maximize groundwater recharge in the seasonally dry tropics, Ilstedt 2016

Responding to a common belief that trees lower groundwater infiltration due to transpiration, and a contrasting view that trees increase groundwater infiltration by increasing organic matter and soil porosity, these authors test an “optimum tree cover theory.”

They find that “intermediate” tree cover maximizes groundwater recharge in the tropics, resulting in a 2-14% increase in total annual water input from rainfall. However, the tree species used in this study consume more water compared to many other tree species in the semi-arid tropics. Therefore, the results here may be conservative in terms of the potential of trees to increase groundwater recharge. Furthermore, the study doesn’t consider the potential effects of greater transpiration from increased tree cover on local rainfall patterns.

Critical impact of vegetation physiology on the continental hydrologic cycle in response to increasing CO2, Lemordant 2018

This study finds that the physiological response of plants to increased atmospheric CO2 affects the global hydrological cycle even more than does the greenhouse effect and changes in precipitation. The authors conclude:

This highlights the key role of vegetation in controlling future terrestrial hydrologic response and emphasizes that the carbon and water cycles are intimately coupled over land [LeMordant 2018: 1].

 

With increasing [CO2] at the leaf surface, the density of stomata at the leaf surface is decreased and their individual opening is reduced and therefore less water is transpired per unit leaf area. In other words, leaf-level water use efficiency increases, potentially increasing surface soil moisture and runoff. On the other hand, leaf biomass tends to also increase with increasing [CO2] … generating a larger evaporative surface that can partly offset the reduction in stomatal conductance and negate the soil water savings. Our objective is therefore to quantify how such plant [CO2] effects influence future hydrological variable responses compared with radiative effects ––the atmospheric impact of the “greenhouse effect.” Radiative effects impact precipitation, i.e., water supply, and evaporative demand, through increase in radiation, temperature, and atmospheric dryness as estimated by the vapor pressure deficit (VPD), i.e., saturation minus actual vapor pressure [LeMordant 2018: 1].

 

Our study illustrates how deeply the physiological effects [on vegetation] due to increasing atmospheric [CO2] impact the continental water cycle. Contrary to previous wisdom, changes in precipitation and radiation [greenhouse effect] do not play the primary role in future drying and moistening in most regions. Rather, biosphere physiological effects and related biosphere–atmosphere interactions are key for predicting future continental water stress as represented by ET [evapotranspiration], long-term runoff, EF, or leaf area index. In turn, vegetation water stress largely regulates land carbon uptake, further emphasizing how tightly the future carbon and water cycles are coupled so that they cannot be evaluated in isolation [LeMordant 2018: 5].

Weakening of Indian summer monsoon rainfall due to changes in land use land cover, Paul 2016

The Indian summer monsoon rainfall has decreased since 1950, and several hypotheses have been proposed to explain why. Most of these hypotheses involving weakening temperature gradients over the continent. This study explores the potential link between a weakening monsoon and widespread land use land cover (LULC) change from woody savanna to cropland in recent decades. Citing earlier studies, the authors note that:

Precipitation resulting from local land surface Evapotranspiration (ET) is known as recycled precipitation. Pathak et al. found that evapotranspiration from land surface vegetation plays a major role during the end of a monsoon. They observed that, during the initial phase of a monsoon, oceanic sources play a major role, and the soil is recharged with moisture. However, during the latter half of a summer monsoon (August and September), land surface ET increases as recycled precipitation increases, a pattern that is more prominent in the Ganga Basin and Northeast India. This recycled precipitation accounts for approximately 20–25% of the rainfall in North India (Ganga Basin) and Northeast India during August and September. Hence, deforestation associated with changes in LULC may affect ET and may significantly affect monsoon rainfall [Paul 2016: 1-2].

Summarizing their own study, the authors conclude:

Here, we performed a sensitivity analysis to quantify the impacts of large-scale conversion from woody savannah to crop land in India on monsoon precipitation. We found such a change results in decreased ET and subsequently a decrease in recycled precipitation leading to a decline in monsoon precipitation. This decline is similar in extent to the observed recent decadal weakening of monsoon precipitation. However, other reasons may account for this observed weakening, such as the warming of Indian Ocean SST [sea surface temperature] [Paul 2016: 5-6].

To better clarify causality of the weakening monsoon, given other potential factors, the authors propose that:

The future scope of this present work is to perform detection and attribution studies for potential declines of Indian monsoons with model runs forced with SST warming only, aerosol forcing only, LULC changes only and all controlling factors together [Paul 2016: 6].

Tropical reforestation and climate change: beyond carbon, Locatelli 2015

When managed with both climate adaptation and mitigation in mind, tropical reforestation (TR) can serve multiple synergistic functions. TR mitigates regional and global climate change, not only by sequestering carbon but also through biophysical cooling (via evapotranspiration), by recycling rainfall regionally, and by reducing pressure on old growth forests.

Furthermore, TR helps local communities adapt to climate change by recharging stream flow in the dry season, reducing the severity of floods, protecting slopes against landslides and, through mangrove reforestation, reducing the impact of coastal storms and waves. Reforestation also creates livelihood opportunities through the sustainable harvest of forest products, and creates shelter and habitat for species vulnerable to climate change. However, to achieve this broad range of benefits, “reforestation practices should be designed to avoid the implementation of one strategy (mitigation or adaptation) to the detriment of the other.” Arguing for the application of what they term “climate-smart reforestation,” the authors recommend the following:

The challenge for climate-smart reforestation is to implement an effective combination of approaches to meet all three objectives: societal adaptation, climate mitigation, and ecological resilience [Locatelli 2015: 4].

However, as most policies consider the three objectives of climate-smart reforestation separately, they often overlook possible trade-offs and synergies. For example, reforestation projects managed with a carbon purpose could have detrimental consequences on water availability in the semi-arid tropics (Trabucco et al. 2008) or on biodiversity (O’Connor 2008). By contrast, reforestation that is explicitly climate-smart uses a multi-objective planning focus that enables different objectives to reinforce each other so that their interactions produce synergies rather than trade-offs. For example, tree regeneration in Tanzania under the Ngitili resource management system achieves carbon storage together with improved watershed conservation and greater provision of natural resources (water, food, and fodder) for livelihoods (Duguma et al. 2014). A proposed adaptation project in Colombia aims to reforest with flood-resistant native tree species to reduce flood impacts on downstream communities (UNDP 2012). A project in Costa Rica is testing different mixes of species and silvicultural practices to reduce vulnerability to storms and fires while also achieving carbon storage (Locatelli et al. 2011) [Locatelli 2015: 4-5].

This article underscores a key concept of this compendium – that functioning ecosystems (whether old growth or restored forests, for example) provide multiple, interwoven functions that support human and biodiverse life by regulating local, regional and global climate conditions.           

Water-retention potential of Europe’s forests: A European overview to support natural water-retention measures, European Environment Agency (EEA) 2015

The importance of water retention (the rainfall absorbed or used within an ecosystem) for mitigating flood and drought conditions and contributing to clean drinking water, for example, has been increasingly recognized in Europe in the past decade. Along with wetland preservation, better agriculture practices and other measures, preserving and re-growing forests are seen as key to enhanced natural water retention. Forests cover a third of Europe, and:

can soak up excess rainwater, preventing run-offs and damage from flooding. By releasing water in the dry season, forests can help to provide clean water and mitigate the effects of droughts [EEA 2015: 6].

In recognition of the important water management role of forests and other natural ecosystems, new policy instruments have proposed Natural Water-Retention Measures (NWRMs).

Natural Water-Retention Measures (NWRMs) are defined as ‘measures to protect and manage water resources and to address water-related challenges by restoring or maintaining ecosystems, natural features and characteristics of water bodies using natural means and processes’ (European Commission and Directorate-General for the Environment 2014). … The main focus is to enhance and preserve the water retention capacity of aquifers, soil and ecosystems and improve their status [EEA 2015: 9].

This EEA study found that:

In water-basins where the forest cover is 30%, water retention is 25% higher than in basins where the forest cover is only 10%. In basins where the forest cover is 70%, water retention is 50% higher than in basins where the forest cover is only 10%. … Coniferous forests in general retain 10% more water than broadleaved forests or mixed forests [EEA 2015: 5].

Why Climate Change Makes Riparian Restoration More Important than Ever: Recommendations for Practice and Research, Seavy 2009

Riparian^[14] ecosystems are naturally resilient, provide linear habitat connectivity, link aquatic and terrestrial ecosystems, and create thermal refugia for wildlife: all characteristics that can contribute to ecological adaptation to climate change [Seavy 2009: 330].

Arguing for the restoration of riparian areas because of their ecological significance and inherent resilience, these authors articulate the importance of both surface and groundwater – protected within a biodiverse ecosystem – for its cooling effect.

Because riparian areas have higher water content than surrounding upland areas, they absorb heat and buffer organisms against extreme temperatures (Naiman et al. 2000). During previous periods of climate change, riparian areas served as refugia because they provided microclimates that protected plant biodiversity (Bakker 1984, Meave and Kellman 1994). Riparian vegetation can maintain cooler water temperatures by shading water from sunlight (Sridhar et al. 2004, Cassie 2006) and the infusion of cold groundwater into warmer surface waters creates and maintains pockets of cool water (Chu et al. 2008). Thus, riparian areas provide thermal refugia for animals with thermoregulatory limitations [Seavy 2009: 332].