As much as a quarter of the world’s land surface is degraded, in Africa this even reaches up to 65% which seriously undermines sustainable development. Reaching halfway the UN decade of ecosystem restoration (2021-2030), restoration efforts are being scaled up. This often involves planting trees because trees stabilize soil, improve microclimate and increase biomass. However, large-scale tree plantations have shown high tree mortality, with reports of 90 per cent seedling mortality after 10 years. Fortunately, there is an alternative: natural regeneration (NR) is the process by which vegetation grows back from seeds or tree stumps. NR can effectively restore important ecosystem functions and biodiversity at lower costs and higher success rates compared to tree planting. It has therefore been advocated as a low-cost nature-based solution to upscale restoration efforts. Yet, we is currently hampered by our limited understanding of what species regenerate where, and what restoration outcomes may be achieved within which timeframe. Within REACT research project we aim to understand how, when and where NR can be used to restore degraded lands.
Predicting NR requires to understand its underlying mechanisms. For this we build on ecological theories proposing that species performance is predicable based on species properties. These properties are called ‘functional traits’ and the conditions that affect species performance are referred to as ‘assembly filters’. The filter is used as an analogy because it separates species that pass through the maze (i.e. species that occur because they were successful to arrive, establish, grow and survive under given conditions) and species that are held back by the filter (i.e. species that are absent or dead). In the current study we distinguish four types of assembly filters that are relevant in social-ecological landscapes: The dispersal filter affects seed arrival, and is shaped by the presence and abundance of adult trees that provide seeds and by availability of disperser agents (e.g. monkeys and birds). The environmental filter (eg light availability, nutrients in the soil) affects germination, survival and growth of trees. While NR has been mainly studied by ecologists, here we take an explicit social-ecological approach and thus add two explicitly social filters: the land use filter (eg land use strategies and management practices) shapes which species survive and thrive when the land is also used for agricultural production. Such integration of naturally-regenerating trees with agricultural use, a form of agroforestry, is called farmer managed natural regeneration (FMNR). Under FMNR NR is intentionally combined with crops or livestock, and farmers select and promote desired trees, for instance by pruning and active protection; which we call the human filter. As such we take an interdisciplinary approach to understanding natural regeneration, a topic that has been tackled mainly from a disciplinary (ecology) perspective.
Within REACT research we take a social-ecological approach to how, when, and where NR contributes to restoration, which we will use to evaluate the upscaling potential now and in the future.
We start with the ‘how’, aiming to mechanistically understand how social filters (land use, tree selection) and ecological filters (dispersal, environment) determine natural regeneration and its restoration outcomes, then we address the ‘when’, evaluating under what conditions is natural regeneration suitable for restoration, to finalize with the ‘where’, predicting the potential for natural regeneration-based restoration across Sub-Saharan Africa, now and in the future.
