Sheil, D., & Bargués-Tobella, A. (2020). More trees for more water in drylands: myths and opportunities. ETFRN News.

The restoration of tree cover influences water availability. Many people—some experts too—believe incorrectly that greater tree cover has an invariably negative impact on local water availability. Where do these beliefs come from? Here we summarise the origin of these misconceptions and illustrate how tree cover can improve water availability. We have recognised the extent of these opportunities only recently, and considerable work remains, but we know enough to dismiss some myths and to highlight major opportunities to improve water security in Africa by restoring degraded landscapes with trees.

Del Campo, A. D., González-Sanchis, M., Ilstedt, U., Bargués-Tobella, A., & Ferraz, S. (2019). Dryland forests and agrosilvopastoral systems: water at the core. Unasylva 251: Forests: nature-based solutions for water, 251(1), 27.

Dryland systems occur on all continents and cover about 41 percent of the Earth’s land surface, with little variation in this figure in recent decades (Cherlet et al., 2018). Drylands differ in their moisture deficit and can be classified in four subtypes according to the United Nations Environment (UNEP) aridity index (AI)1  as dry subhumid (0.65– 0.5), semiarid (0.5–0.2), arid (0.2–0.05) or hyperarid (<0.05) (Figure 1).2  Forests and grasslands are the dominant biomes in the dry subhumid and semiarid subtypes, respectively (more than 60 percent of the subtype areas). On the other hand, the arid and hyperarid subtypes are mostly treeless (FAO, 2016) and thus beyond the scope of this article.

Based on their underlying definition (i.e. by AI), annual potential evapotranspiration (PET) in dry subhumid and semiarid lands is considerably higher than annual precipitation, with frequent meteorological droughts. These atmospheric drivers lead to low soil moisture and this, in turn, means slow tree growth and low productivity, resulting in a socio-ecological context of water scarcity. Marked rainfall seasonality, with torrential events followed by long dry periods, and the combination of high intra- and interannual variability, put such regions within the “difficult” hydrology framework, which hampers water security, sustainable development and poverty reduction (Grey and Sadoff, 2007). southern Africa, Australia, the Middle East and Central Asia (Cherlet et al., 2018). The intensification of precipitation and other climatic extremes under warmer conditions is likely to increase water scarcity and moisture deficits in drylands and beyond. Climatic constraints increase the role of soil processes and properties in the regulation and magnitude of water-related issues in drylands, especially those concerned with resource storage (e.g. soil depth, infiltrability, deep-water storage and erosion). Thus, land-use and management practices, especially nature-based solutions, are extremely important for the soil–water–productivity complex. This article uses case studies in dryland on three continents to show the importance of a water-centred approach to dryland management for increasing resilience and adaptation to climate change

Bastin, J. F., Finegold, Y., Garcia, C., Mollicone, D., Rezende, M., Routh, D., … & Crowther, T. W. (2019). The global tree restoration potential. Science, 365(6448), 76-79.

Climate change is expected to cause an increase in the global area of drylands of 10–23 percent, depending on dryland subtype, by the end of the twenty-first century, particularly in areas of North and South America, the Mediterranean,The restoration of trees remains among the most effective strategies for climate change mitigation.We mapped the global potential tree coverage to show that 4.4 billion hectares of canopy cover could exist under the current climate. Excluding existing trees and agricultural and urban areas, we found that there is room for an extra 0.9 billion hectares of canopy cover, which could store 205 gigatonnes of carbon in areas that would naturally support woodlands and forests. This highlights global tree restoration as our most effective climate change solution to date. However, climate change will alter this potential tree coverage.We estimate that if we cannot deviate from the current trajectory, the global potential canopy cover may shrink by ~223 million hectares by 2050, with the vast majority of losses occurring in the tropics. Our results highlight the opportunity of climate change mitigation through global tree restoration but also the urgent need for action.

Cunningham, S. C., Mac Nally, R., Baker, P. J., Cavagnaro, T. R., Beringer, J., Thomson, J. R., & Thompson, R. M. (2015). Balancing the environmental benefits of reforestation in agricultural regions. Perspectives in Plant Ecology, Evolution and Systematics, 17(4), 301-317.

Reforestation is an important tool for reducing or reversing biodiversity loss and mitigating climate change. However, there are many potential compromises between the structural (biodiversity) and functional (carbon sequestration and water yield) effects of reforestation, which can be affected by decisions on spatial design and establishment of plantings. We review the environmental responses to reforestation and show that manipulating the configuration of plantings (location, size, species mix and tree density) increases a range of environmental benefits. More extensive tree plantings (>10 ha) provide more habitat, and greater improvements to carbon and water cycling. Planting a mixture of native trees and shrubs is best for biodiversity, while traditional plantation species, generally non-native species, sequester C faster. Tree density can be manipulated at planting or during early development to accelerate structural maturity and to manage water yields. A diversity of habitats will be created by planting in a variety of landscape positions and by emulating the patchy distribution of forest types, which characterized many regions prior to extensive landscape transformation. Areas with shallow aquifers can be planted to reduce water pollution or avoided to maintain water yields. Reforestation should be used to build forest networks that are surrounded by low-intensity land use and that provide links within regions and between biomes. While there are adequate models for C sequestration and changes in water yields after reforestation, the quantitative understanding of changes in habitat resources and species composition is more limited. Development of spatial and temporal modelling platforms based on empirical models of structural and functional outcomes of reforestation is essential for deciding how to reconfigure agricultural regions. To build such platforms, we must quantify: (a) the influence of previous land uses, establishment methods, species mixes and interactions with adjacent land uses on environmental (particularly biodiversity) out- comes of reforestation and (b) the ways in which responses measured at the level of individual plantings scale up to watersheds and regions. Models based on this information will help widespread reforestation for carbon sequestration to improve native biodiversity, nutrient cycling and water balance at regional scales.

Suding, K. N. (2011). Toward an era of restoration in ecology: successes, failures, and opportunities ahead. Annual review of ecology, evolution, and systematics, 42.

As an inevitable consequence of increased environmental degradation and anticipated future environmental change, societal demand for ecosystem restoration is rapidly increasing. Here, I evaluate successes and failures in restoration, how science is informing these efforts, and ways to better address decision-making and policy needs. Despite the multitude of restoration projects and wide agreement that evaluation is a key to future progress, comprehensive evaluations are rare. Based on the limited available information, restoration outcomes vary widely. Cases of complete recovery are frequently characterized by the persistence of species and abiotic processes that permit natural regeneration. Incomplete recovery is often attributed to a mixture of local and landscape constraints, including shifts in species distributions and legacies of past land use. Lastly, strong species feedbacks and regional shifts in species pools and climate can result in little to no recovery. More forward-looking paradigms, such as enhancing ecosystem services and increasing resilience to future change, are exciting new directions that need more assessment. Increased evidence-based evaluation and cross-disciplinary knowledge transfer will better inform a wide range of critical restoration issues such as how to prioritize sites and interventions, include uncertainty in decision making, incorporate temporal and spatial dependencies, and standardize outcome assessments. As environmental policy increasingly embraces restoration, the opportunities have never been greater.

Meli, P., Martínez‐Ramos, M., Rey‐Benayas, J. M., & Carabias, J. (2014). Combining ecological, social and technical criteria to select species for forest restoration. Applied vegetation science, 17(4), 744-753.

Question: How to evaluate and integrate relevant ecological, social and technical criteria to select species to be introduced in restoration projects of highly diverse ecosystems such as tropical riparian forests.

Location: Riparian forest, Marques de Comillas municipality, southeast Mexico (16°54′N, 92°05′W).

Methods: We proposed a ‘species selection index’ (SSI) using five independent criteria related to ecological, social and technical information. SSI targeted species that (1) are important in the reference forest; (2) are less likely to establish following disturbance; (3) are not specific to a particular habitat; (4) are socially accepted; and (5) their propagation requires a reasonable time and financial investment. SSI may range between zero and 50, with higher values meaning higher potential for restoration purposes.

Results: Out of a local pool of 97 species, we identified 30 target tree species that together represented >60% of total importance value index in the reference riparian forests. SSI averaged 28.3  1.0 over the studied species, suggesting that species with high values are not frequent. For 20 species, reintroduction by means of active forest restoration was deemed necessary. Species that established through natural regeneration, following secondary regrowth, had lower social value among local farmers. Nearly half of the identified species showed technical constraints for easy propagation and seeding.

Conclusions: The proposed procedure is useful for selecting species to initiate forest restoration projects and of other woody ecosystems that harbour high biodiversity, and is suitable for several stakeholders interested in restoration.

Melo, F. P., Pinto, S. R., Brancalion, P. H., Castro, P. S., Rodrigues, R. R., Aronson, J., & Tabarelli, M. (2013). Priority setting for scaling-up tropical forest restoration projects: Early lessons from the Atlantic Forest Restoration Pact. Environmental Science & Policy, 33, 395-404.

Ongoing conversion of tropical forests makes it urgent to invest in ecological restoration on grand scales in order to promote biodiversity conservation and ecosystem services. The 4-year old Atlantic Forest Restoration Pact (AFRP) aims to restore 15,000,000 ha of tropical forest in 40 years. The approaches and lessons learned appear transferable, and could help achieve the global restoration targets. Fundamental prerequisites for success include: effective technology undergoing continuous improvement, ongoing teaching, outreach and capacity-building efforts, presence of local intelligentsia, maintaining a clear and transparent legal environment, and presence of effective economic instruments and incen- tives for landowners. These prerequisites can be achieved by expanding and strengthening the network of stakeholders both in public and private forums that must be aware of macro- economic and social/cultural shifts and trends which may provide opportunities and impose constraints to further restoration activities. Finally, environmental regulations imposing habitat protection and restoration are usually beyond individual land-owners’ possibilities and level of interest. Therefore, forest restoration, even in a biodiversity hotspot, must be approached as a potentially sustainable economic activity. Otherwise, private landowners, and most other stakeholders, will not persevere.

Lawson, S.S. and Michler, C.H., 2014. Afforestation, restoration and regeneration – Not all trees are created equal. Journal of Forestry Research, 25(1): 3−20 DOI 10.1007/s11676-014-0426-5.

Undulations in weather patterns have caused climate shifts of increased frequency and duration around the world. The need for additional research and model data on this pressing problem has resulted in a plethora of research groups examining a particular tree species or biome for negative effects of climate change. This review aims to (1) collect and merge recent research data on regeneration within old- and new-growth forests, (2) highlight and expand upon selected topics for additional discussion, and (3) report how shade tolerance, drought tolerance, and inherent plasticity affect tree growth and development. Although shade and drought tolerance have been well studied by a number of research groups, this review reveals that in-depth analysis of a single or a few species in a given area will not generate the data required to implement a successful regeneration plan. Studies using historical accounts of previous species composition, information regarding site seasonality, species competition, and individual responses to drought and shade are needed to (1) develop best management plans and (2) ensure future modeling experiments are focused on a greater variety of species using more innovative methods to evaluate climate change effects.

Holl, K. D. (2017). Restoring tropical forests from the bottom up. Science, 355(6324), 455-456.

Recent initiatives at regional, national, and global scales have called for unprecedented levels of forest restoration to counteract decades of rapid deforestation (1, 2). Thus far, 30 countries have committed to restore 91 million hectares (ha) of deforested landscapes, an area the size of Venezuela, by 2020; at the 2014 United Nations Climate Summit, a global target of 350 million ha was set for 2030 (1). These bold targets are motivated by diverse goals, including conserving biodiversity, sequestering carbon, improving the water supply, and sustaining human livelihoods (2, 3). How can these challenging targets be met, given competing land uses and limited funds for restoration?

Chazdon, R. L., Brancalion, P. H. S., Lamb, D., Laestadius, L., Calmon, M., & Kumar, C. (2017). A policy-driven knowledge agenda for global forest and landscape restoration. Conserv Lett 10: 125–132.

The global restoration movement is gaining momentum. International and national leaders are demonstrating unparalleled political will for achieving ambitious targets. However, the knowledge base for implementing large-scale forest and landscape restoration (FLR) needs further development. Besides application of scientific and local knowledge, a broad understanding of the social, economic, and environmental context in which this knowledge is being applied is also needed. To address knowledge gaps and guide implementation of FLR at local to global scales we propose a knowledge creation agenda that we derive from emerging policy goals. We present a holistic approach that addresses food security, ecosystem services, and livelihoods, and that supports implementation by a wide array of actors from farmers and municipalities to corporations and state agencies. Our knowledge creation agenda is based on six broad policy goals, with several associated knowledge gaps for each goal. We recognize that this agenda is simply a starting point and will surely evolve and become more locally focused as the concept of FLR gains ground and as multiple groups of stakeholders engage in the long-term process of restoring functionality and value to ecosystems and landscapes around the world.