Tag Archives: Trees

Trees and water: don’t underestimate the connection

By: Douglas Sheil, Norwegian University of Life Sciences
This blog was originally posted in CIFOR Forest News

Trees have extraordinary powers, especially when it comes to water. But such powers must be wielded with care.

  Lake Bam, in the Centre-Nord region a hundred kilometers from Ouagadougou, is undergoing enormous environmental challenges such as silting, drastic reduction of aquatic life and conflicts of interest the 28,000 people living from this lake see their livelihoods threatened, Burkina Faso. Photo by Ollivier Girard/CIFOR

Trees have extraordinary powers. They provide shade, cool the local climate, draw carbon dioxide from the air, and can repair and replicate themselves while running on little more than sunlight and rainwater (Pokorný 2018). They also contribute numerous goods and services like fruit, wood and soil improvement with a wide choice of species and varieties suitable for different needs and conditions. But such powers should be wielded with care.

On the 5th of July 2019 Science published an article by Jean-François Bastin and colleagues titled “The global tree restoration potential”. In it, they explain how, without displacing agriculture or settlements, there is enough space to expand the world’s tree cover by one-third or around one billion hectares. Such increased forest would eventually reduce atmospheric carbon by about a quarter. A lot could be said about this proposition, much of it supportive. But in a brief comment piece just published in Science, colleagues and I highlight some reservations along with some even bigger opportunities. We focus on water.

The idea that the protection and restoration of tree cover could improve the climate while providing other benefits is well established. Indeed, there have been numerous international programs based on this including REDD “Reducing Emissions from Deforestation and Degradation”, the Bonn Challenge, which seeks to reforest and restore degraded land, as well as various related programs.

So what is new here?

Well, what Bastin et al. have done is estimate the scale of this opportunity and the contribution that restoring tree cover could make. For example, they list such estimates country by country as a “scientific evaluation” with relation to restoration targets specified under the Bonn Challenge. Under these targets, and those specified by the New York Declaration on Forests, an impressive list of countries (59) have undertaken to end deforestation and to restore 350 million hectares of land by 2030. They note that several of these countries have committed to restoring an area that “exceeds the total area that is available for restoration”. They note how these results “reinforce the need for better country-level forest accounting”.

Yet there is a paradox lurking within these claims. The authors state that their estimates are not “future projections of potential forest extent”. So what are they?

Aerial view of the Amazon rainforest and river, near Manaus, the capital of the Brazilian state of Amazonas. Brazil. Photo by Neil Palmer/CIAT

In brief, their assessment represents an estimate of potential tree cover assuming current environmental conditions and no influence or modifications arising from the trees themselves. But large-scale changes in tree cover would modify these conditions.

Trees and forests influence the availability of water and water influences the degree to which a landscape can support trees. While current tree cover reflects current conditions, any assessment of the prospects for large-scale changes in tree cover must account for how these changes will influence those conditions. Potential tree cover should reflect the conditions that would exist with that tree cover.

This may seem esoteric, which may explain why it was not raised in the extensive media coverage, but these details matter. They matter a lot.

Access to adequate fresh water is a key development challenge and is central to the United Nations Sustainable Development Goals. Around half a billion people suffer insufficient fresh water year-round while many more face seasonal scarcity. Such shortages cause hardship and are widely believed to play an increasing role in the complex of issues that increase the likelihood of conflict and migration. With relatively fixed fresh water resources and a growing population, the global fresh water resources per person are declining.

As we highlight in our comment, trees influence the availability of water both locally and regionally. Neglecting these influences undermines the value of the estimates and renders them near meaningless. This affects both the technical aspects of the estimates—the variables used to predict tree cover would change, and more importantly, the wider implications for people and life on the planet.

Tree cover influences water availability through a range of processes and mechanisms. Only some of these are well understood. But we know enough to know there will be impacts.

Impacts can be negative. Where trees use a lot of water this can accentuate local water scarcity. There are many examples where dense plantations have caused a decline in local stream flows and depleted groundwater when compared to open lands. This is crucial, but far from being the whole story.

Impacts can also be positive. This has been shown by studies in Burkina Faso where landscapes with some tree cover captured several times more water than otherwise comparable tree-free landscapes. In this case, the costs of increased water use are more than compensated by the increased soil infiltration and moisture storage. Trees and forest also provide water vapour and condensation nuclei (the particles that promote cloud formation) that can contribute to rainfall elsewhere. Thus, it is clear that tree cover supports rainfall downwind—and many people depend on such rainfall.

The power of such recycling suggests that if tree cover in drylands can be expanded in the right manner, it can generate increased rainfall, thus opening the opportunity to increase regional moisture and land able to support trees and forests. In addition, an exciting new theory, the Biotic Pump, suggests that forest cover plays a fundamental role in generating the winds that carry moisture into continents. This theory conforms with observations in the Amazon region concerning how rainfall relates to changes in air pressure, and how forest derived moisture controls the monsoon. In effect, we could develop a system that waters itself and thereby regreens the world’s deserts. We could, for example, imagine returning a much wetter climate to the Sahel of Africa or to Western Australia.

So how can we avoid the negatives and promote the positives of increased tree cover? We don’t yet know the optimal way. Likely we may not even agree what “optimal” implies. My personal view is that, if we emphasise the protection, expansion and restoration of natural vegetation that can regenerate and maintain itself (rather than industrial plantations), the positives are generally more likely. The rationale is that nature has evolved effective systems for distributing and maintaining water. These are the systems that kept the world green and productive long before people got involved. (Such restoration is what Bastin and colleagues are suggesting, though much of the media attention discussed “tree planting” more generally as if this is equivalent—it isn’t).

 General View of the Brazilian Amazon. Photo by Neil Palmer/CIAT

But there are plenty of good reasons to promote tree cover even in productive landscapes and to identify how we might green large areas of our planet. The potential to bring more water into currently arid regions seems a real opportunity. We can also look for ways to ensure that plantations, where justified, are developed without wider environmental costs. Natural systems can provide both template and inspiration.

But it remains true that negative impacts can still result, especially as what may be optimal at a continental scale may not be ideal at more restricted scales, and patches of regenerating forest may deplete local water even if it boosts rain downwind. When tree cover does boost groundwater in arid regions there can be additional challenges if this raises salt within the soil profile.

Looking beyond water there is no shortage of additional concerns. For example, we need to ensure people benefit, we need to protect key grasslands and we need to ask why the tree cover was depleted in the first place.

There are many good reasons to protect and restore tree cover and other natural vegetation—wherever and to the degree that that is possible. There are also plenty of good reasons to promote agroforestry and to encourage even scattered tree cover where that is possible within productive landscapes.

Our point is that there will be wider impacts than those on atmospheric carbon alone. Many impacts are likely to be positive, increasing greenness, stabilising rainfall, and reducing biodiversity losses. But widespread tree planting can also cause harm, displacing people and biodiversity and contributing to water scarcity.

The power of trees is often underestimated—it is a transformative power with capacity to achieve great good and great harm. Please use it wisely.

Original Science Article:

Bastin, J.F. et al. 2019. “The global tree restoration potential”, Science, Vol. 365, Issue 6448, pp. 76-79, DOI: 10.1126/science.aax0848 

Comment letter to Bastin et al.:

Sheil, D. et al. 2019. “Forest restoration: Transformative trees”, Science, Vol. 366, Issue 6463, pp. 316-317, DOI: 10.1126/science.aay7309 

Bastin et al. response:

Bastin, J.F. et al. 2019. “Forest restoration: Transformative trees-Response”, Science, Vol. 366, Issue 6463, pp. 317, DOI: 10.1126/science.aaz2148 

Restoring degraded tropical landscapes with trees

By: Aida Bargues Tobella, Postdoctor at the Department of Forest Ecology and Management; Tropical Forestry and Land Use Management Unit 

Land degradation is a major problem in the tropics. Such degradation entails a decline in the capacity of the land to produce and provide ecosystem goods and services, with negative impacts for human livelihoods, food security and the environment at large. 

Land degradation is a widespread phenomenon across the tropics. The Nyando River Basin (Western Kenya) is a regional erosion hotspot and one of the main sources of sediment and phosphorous into Lake Victoria. Photo: Aida Bargues Tobella

The establishment of trees on degraded lands is considered a fundamental tool in landscape restoration. Establishing trees is about more than just planting trees, and can include assisted natural regeneration (ANR) of forests, farmer-managed natural regeneration or direct seeding among other techniques. Similarly, the concept of landscape restoration is not limited to re-establishing lost forests and should be seen on a broader perspective, taking into consideration, for instance, the incorporation of trees into farming systems (agroforestry).

Faidherbia albida is a popular agroforestry tree which generates numerous provisioning and regulating ecosystem services. Photo: Aida Bargues Tobella

The potential benefits from tree-based restoration include enhanced water quality, biodiversity, carbon sequestration, soil fertility, and food and nutrition security. But, how much do we know about tree-based restoration? What are the trade-offs and synergies among ecosystem services from trees? What management practices and tree traits contribute most to promote specific ecosystem services? As we enter the UN Decade on Ecosystem Restoration, answering such questions is pressing. 

Sesbania sesban improved fallows have a great potential to restore soil fertility and increase crop yields. Photo: Aida Bargues Tobella

In the research group on Tropical Forestry and Land Use Management at the Department of Forest Ecology and Management in SLU, we work towards advancing our understanding of tree-based restoration of degraded landscapes in the tropics. Currently, we have projects in six countries across the global tropics: Malaysia, Thailand, Kenya, Burkina Faso, Tanzania and Mozambique.

We currently have on-going research projects in six different countries across the global tropics

Rainforest degradation and restoration

The group has been doing research on rainforest degradation and restoration in Borneo for over 35 years. The INIKEA Sow-a-Seed rainforest restoration project in the Malaysian state of Sabah is a collaboration between the Sabah Foundation, SLU and the Swedish furniture company IKEA andit is unique in that it is one of the largest and most successful tropical rainforest restoration projects in the world. Since the startof the project in 1998, we have planted more than three millionseedlings, consisting of ca. 80 different indigenous tree species, and roughly14,000 ha of forest have been restored with assisted natural regeneration and enrichment plantings. 

In connection with the project, we have established a number of scientific experiments: 

  • In the SUAS experiment, established already in 1992, we aim to develop silvicultural methods that make management of natural forests environmentally and economically sustainable.
  •  In our three different species/genetic common gardens we seek to advance the present lack of knowledge on the economic and environmental values of indigenous species. Here we also study the importance of genetic variation in traits within and among species.
  •  In the Rainforest Restoration Experiment,we have established 84 plots in various forest types to evaluate where each of our four different approaches of restoration is most appropriate; 1) Passive protection; 2) ANR; 3) ANR with line planting and 4) ANR with gap-cluster planting.
  • In our permanent sampling plots inside the restoration area and surrounding landscape of large-scale oil palm and industrial tree plantations as well as undisturbed protected forests, we are evaluating ecosystem values, such as economic value, carbon sequestration, water quality and biodiversity among these land-use systems. 

These long-term forest management experiments in northern Borneo provide many opportunities for research. In the project Balancing production and ecosystem services from degraded tropical rainforests to aid the transition to a more sustainable bio-based economy, we are using data from these experiments to quantifybiomass production and a range of ecosystem services across multiple spatial and temporal scales. Using a multi-disciplinary approach, including aspects of economics, social science, silviculture, plant ecophysiology, ecology, human health,and biogeochemistry, we aim to identify sustainable management practices that can maximize the production of raw materials while at the same time minimizing adverseenvironmental impacts. Using this holistic approach, the overall objective is to obtain and communicate novel information to scientists, private, and government stakeholders about trade-offs between biomass production and ecosystem services to aid the transition to a sustainable bio-based economy.

Rainforest vulnerability to climatic water stress

The frequency and intensity of drought events are predicted to increase in tropical monsoon forests of Southeast Asia, ecosystems that are known to be biodiversity hotspots and a persistent carbon sink in the global carbon cycle. Such increases could drive rapid and large-scale shifts in forest structure and species composition as well as cause dramatic decreases in the amount of carbon stored by these tropical forests. We have recently started a research project thatbrings together scientists from Thailand, France,and Sweden, to assess the vulnerability of mature and secondary forests to climatic water stress. Such information is crucial to more accurately predicted how future climate change wouldaffect the cycling of carbon and water in tropical forested ecosystems. 

Trees and water in African tropical drylands

Another leading research topic of the group is how we can use trees to improve soil and water resources in African tropical drylands. Our previous research in the seasonally dry tropics indicates that an intermediate tree cover can maximize groundwater recharge, which is contrary to the predominant scientific view that more trees always lead to less water. But, under what specific conditions can more trees improve groundwater recharge? Together with scientists from the World Agroforestry Centre (ICRAF), the Norwegian University of Life Sciences and Wageningen University, we are evaluating the extent of the optimum tree cover theoryacross African tropical drylands. To do this, we are primarily using data from the network of Land Degradation Surveillance Framework (LDSF) sites, which is hosted at ICRAF. To date, the LDSF has been employed in over 200 sites across the global tropics and therefore constitutes a unique dataset to test this theory. The overall aim of the project is to provide evidence to inform better land-use policies in African tropical drylands and identify management options that can increase groundwater resources. 

LDSF field campaing in Embu county, Kenya. Photo: Aida Bargues Tobella
LDSF field campaing in Makueni county, Kenya. Photo: Aida Bargues Tobella

Courses

Are you interested in these questios and want to learn more about tropical forestry and land-use management? At the moment we offer two courses within this field:

This year’s MSc course on Sustainable Forestry and Land-se Management in the tropics included a one- week field trip to Mozambique. Photo: Rosa Goodman
Participants of the course “Forest Management Forest Management, Land Use Change and Ecosystem Services in Degraded Tropical Landscapes” had the opportunity to visit the INIKEA Sow-a-Seed restoration project in northern Borneo. Photo: Niles Hasselquist

Who we are

Ulrik Ilstedt, associate professor; ulrik.ilstedt@slu.se
Gert Nyberg, associate professor; gert.nyberg@slu.se
Niles Hasselquist, associate professor; niles.hasselquist@slu.se
Rosa Goodman,associate senior lecturer; rosa.goodman@slu.se
Aida Bargues Tobella, postdoc;  aida.bargues.tobella@slu.se
Daniel Lussetti, postdoc; daniel.lusetti@slu.se