Trees, forests and water: Cool insights for a hot world

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

Explore before you restore: Incorporating complex systems thinking in ecosystem restoration

Abstract The global movement for ecosystem restoration has gained momentum in response to the Bonn Challenge (2010) and the UN Decade on Ecosystem Restoration (UNDER, 2021–2030). While several science‐based guidelines exist to aid in achieving successful restoration outcomes, significant variation remains in the outcomes of restoration projects. Some of this disparity can be attributed to unexpected responses of ecosystem components to planned interventions. Given the complex nature of ecosystems, we propose that concepts from Complex Systems Science (CSS) that are linked to non‐linearity, such as regime shifts, ecological resilience and ecological feedbacks, should be employed to help explain this variation in restoration outcomes from an ecological perspective. Our framework, Explore Before You Restore, illustrates how these concepts impact restoration outcomes by influencing degradation and recovery trajectories. Additionally, we propose incorporating CSS concepts into the typical restoration project cycle through a CSS assessment phase and suggest that the need for such assessment is explicitly included in the guidelines to improve restoration outcomes. To facilitate this inclusion and make it workable by practitioners, we describe indicators and methods available for restoration teams to answer key questions that should make up such CSS assessment. In doing so, we identify key outstanding science and policy tasks that are needed to further operationalize CSS assessment in restoration. Synthesis and applications. By illustrating how key Complex Systems Science (CSS) concepts linked to non‐linear threshold behaviour can impact restoration outcomes through influencing recovery trajectories, our framework Explore Before You Restore demonstrates the need to incorporate Complex Systems thinking in ecosystem restoration. We argue that inclusion of CSS assessment into restoration project cycles, and more broadly, into international restoration guidelines, may significantly improve restoration outcomes. </jats:p

Soil property changes over a 120-yr chronosequence from forest to agriculture in western Kenya

Much of the native forest in the highlands of western Kenya has been converted to agricultural land in order to feed the growing population, and more land is being cleared. In tropical Africa, this land use change results in progressive soil degradation, as the period of cultivation increases. Both rates and variation in infiltration, soil carbon concentration and other soil parameters are influenced by management within agricultural systems, but they have rarely been well documented in East Africa. We constructed a chronosequence for an area of western Kenya, using two native forest sites and six fields that had been converted to agriculture for up to 119 yr

Determination of land restoration potentials in the semi-arid areas of Chad using systematic monitoring and mapping techniques

The restoration of degraded lands has received increased attention in recent years and many commitments have been made as part of global and regional restoration initiatives. Well-informed policy decisions that support land restoration, require spatially explicit information on restoration potentials to guide the design and implementation of restoration interventions in the context of limited resources. This study assessed ecosystems indicators of land degradation using a systematic approach that combines field surveys and remote sensing data into a set of multi-criteria analyses to map restoration potentials in the semi-arid areas. The indicators considered were soil organic carbon, erosion prevalence, enhanced vegetation index, Normalized differences water index and the Net Primary productivity. Three classes of restoration potential were established: (1) areas not in need of immediate restoration due low degradation status, (2) areas with high potential for restoration with moderate efforts required and (3) areas in critical need of restoration and require high level of efforts. Of the total area of the study site estimated at 88,344 km2, 59,146.12 km2, or 66.94% of the theoretically recoverable area, was considered suitable for restoration, of which 38% required moderate efforts while 28% require less efforts. The recoverable areas suitable for restoration could be restored through tree planting, soil and water conservation practices, farmers managed natural regeneration, and integrated soil fertility management. These results can help to spatially identify suitable multifunctional restoration and regeneration hotspots as an efficient way to prioritize restoration interventions in the context of limited resources

Soil property changes over a 120-yr chronosequence from forest to agriculture in western Kenya

Much of the native forest in the highlands of western Kenya has been converted to agricultural land in order to feed the growing population, and more land is being cleared. In tropical Africa, this land use change results in progressive soil degradation, as the period of cultivation increases. Both rates and variation in infiltration, soil carbon concentration and other soil parameters are influenced by management within agricultural systems, but they have rarely been well documented in East Africa. We constructed a chronosequence for an area of western Kenya, using two native forest sites and six fields that had been converted to agriculture for up to 119 yr. &lt;br&gt;&lt;br&gt; We assessed changes in infiltrability (the steady-state infiltration rate), bulk density, proportion of macro- and microaggregates in soil, soil C and N concentrations, as well as the isotopic signature of soil C (&amp;delta;&lt;sup&gt;13&lt;/sup&gt;C), along the 119-yr chronosequence of conversion from natural forest to agriculture. Infiltration, soil C and N decreased within 40 yr after conversion, while bulk density increased. Median infiltration rates fell to about 15% of the initial values in the forest, and C and N concentrations dropped to around 60%, whilst the bulk density increased by 50%. Despite high spatial variability, these parameters have correlated well with time since conversion and with each other

Assessing biogeochemical and human-induced drivers of soil organic carbon to inform restoration activities in Rwanda

Land restoration is of critical importance in Rwanda, where land degradation negatively impacts crop productivity, water, food and nutrition security. We implemented the Land Degradation Surveillance Framework in Kayonza and Nyagatare districts in eastern Rwanda to assess baseline status of key soil and land health indicators, including soil organic carbon (SOC) and soil erosion prevalence. We collected 300 topsoil (0–20 cm) and 281 subsoil (20–50 cm) samples from two 100 km2 sites. We coupled the soil health indicators with vegetation structure, tree density and tree diversity assessments. Mean topsoil organic carbon was low overall, 20.9 g kg−1 in Kayonza and 17.3 g kg−1 in Nyagatare. Stable carbon isotope values (d13CV-PDB ) ranged from −15.35 to −21.34 ‰ indicating a wide range of plant communities with both C3 and C4 photosynthetic pathways. Soil carbon content decreased with increasing sand content across both sites and at both sampling depths and was lowest in croplands compared to shrubland, woodland and grasslands. Field-saturated hydraulic conductivity (Kfs) was estimated based on infiltration measurements, with a median of 76 mm h−1 in Kayonza and 62 mm h−1 in Nyagatare, respectively. Topsoil OC had a positive effect on Kfs, whereas pH, sand and compaction had negative effects. Soil erosion was highest in plots classified as woodland and shrubland. Maps of soil erosion and SOC at 30-m resolution were produced with high accuracy and showed high variability across the region. These data and analysis demonstrate the importance of systematically monitoring multiple indicators at multiple spatial scales to assess drivers of degradation and their impact on soil organic carbon dynamics

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

Water scarcity contributes to the poverty of around one-third of the world\u27s people. Despite many benefits, tree planting in dry regions is often discouraged by concerns that trees reduce water availability. Yet relevant studies from the tropics are scarce, and the impacts of intermediate tree cover remain unexplored. We developed and tested an optimum tree cover theory in which groundwater recharge is maximized at an intermediate tree density. Below this optimal tree density the benefits from any additional trees on water percolation exceed their extra water use, leading to increased groundwater recharge, while above the optimum the opposite occurs. Our results, based on groundwater budgets calibrated with measurements of drainage and transpiration in a cultivated woodland in West Africa, demonstrate that groundwater recharge was maximised at intermediate tree densities. In contrast to the prevailing view, we therefore find that moderate tree cover can increase groundwater recharge, and that tree planting and various tree management options can improve groundwater resources. We evaluate the necessary conditions for these results to hold and suggest that they are likely to be common in the seasonally dry tropics, offering potential for widespread tree establishment and increased benefits for hundreds of millions of people