Forest regeneration can positively contribute to local hydrological ecosystem services: Implications for forest landscape restoration

Mark Mulligan,Bob W. Zwartendijk,H. J. (Ilja) van Meerveld,Jaona Lahitiana,Chandra P. Ghimire,Julia P. G. Jones,Luke Flory,Maafaka Ravelona

doi:10.1111/1365-2664.13836

Abstract

Abstract Governments are increasingly committing to significant forest restoration. While carbon sequestration is a major objective, the case for restoration often includes benefits to local communities. However, the impacts of forest restoration on local hydrological services (e.g. flood and erosion risk, stream flow during dry periods) are surprisingly poorly understood. Particularly limited information is available on the impacts of passive tropical forest restoration following shifting cultivation. The outcome depends on the trade‐off between the improved soil infiltration capacity (reducing overland flow and increasing soil and groundwater recharge) and greater evapotranspiration (diminishing local water availability). Using measurements from highly instrumented plots under three vegetation types in the shifting cultivation cycle in Madagascar's eastern rainforests (forest, tree fallow and degraded abandoned agricultural land), and infiltration measurements for the same vegetation types across the landscape, we explore the impacts of forest regeneration on the ecohydrological processes that underpin locally important ecosystem services. Overland flow was minimal for the tree fallow (similar to the forest) and much lower than for the degraded land, likely leading to a lower risk of erosion and flooding compared to the degraded land. Conversely, evapotranspiration losses were lower for the tree fallow than the forest, leading to a higher net recharge, likely resulting in more streamflow between rainfall events. These results demonstrate that young regenerating tropical forest vegetation can positively contribute to locally important hydrological ecosystem services. Allowing tree fallows to recover further is unlikely to further reduce the risk of overland flow but may, at least temporarily, result in less streamflow. Synthesis and applications. Encouraging natural regeneration is increasingly seen as a cost‐effective way to deliver forest landscape restoration. Our data suggest that increasing the abundance of young secondary forest in the tropics, by increasing fallow lengths in the shifting cultivation cycle, could make a positive contribution to locally important hydrological ecosystem services (specifically reducing overland flow and therefore erosion and flooding, while maintaining streamflows). Such empirical understanding is needed to inform the models used for planning forest landscape restoration to maximize benefits to local communities.

Highlights

There is enormous global interest in forest restoration, with governments around the world making high profile and ambitious commitments (Fagan et al, 2020)
These results demonstrate that young regenerating tropical forest vegetation can positively contribute to locally important hydrological ecosystem services
Carbon sequestration is an important driver of this policy interest, but it is widely recognized that forest restoration needs to be considered as part of a landscape approach which aims to improve both ecological functioning and the livelihoods of local people (Chazdon et al, 2017)

Summary

| INTRODUCTION

There is enormous global interest in forest restoration, with governments around the world making high profile and ambitious commitments (Fagan et al, 2020). Remarkably little is known about the likely impacts on the ecohydrological processes underpinning locally important hydrological ecosystem services Such understanding is critical to ensuring that forest landscape restoration benefits local people. It is vital to understand the impact of forest restoration on the local hydrological ecosystem services that affect rice production These services are the maintenance of continuous flow in streams that ensures water is available for irrigation between rainfall events (for which net recharge is an indicator), and the mitigation of streamflow peaks (for which overland flow can be an indicator) that could damage the irrigation systems associated with rice cultivation. We combine detailed plot-scale measurements for a whole year for a semi-mature forest, tree fallow and degraded abandoned agricultural land (Ghimire et al, 2020), and support our inferences with measurements of infiltration rates and maximum infiltration depths across the same spectrum of vegetation types across the landscape (Zwartendijk et al, 2020)

| MATERIALS AND METHODS

Findings

| DISCUSSION