Sapling ecology and management in multi-species afforestation system on degraded cropland in the Sudano-Sahelian zone of Benin

Abstract

Sub-Saharan Africa faces the multiple challenges of improving food security of the rapidly growing human population while halting the severe land degradation and coping with climate change impacts. Re- and afforestation of degraded lands have been recognized worldwide as cost-effective strategies to mitigate and adapt to adverse environmental changes. In this respect, the establishment phase of tree plantations is in particular decisive for the sustainability of forestation enterprises, since the early development determines long-term growth trajectories and potential yields. Through a combination of open-field experiments and modeling, this study aimed to improve the understanding of sapling eco-physiology and responses to silvicultural management of multipurpose tree species (MPTS) introduced on degraded cropland in the Sudano-Sahelian zone (SSZ) of Benin, West Africa. <br /> Survival, above- and belowground functional traits and biomass production of five MPTS subjected to manuring (1 kg per sapling) and/or supplemental drip irrigation (0.5 L per sapling daily) were monitored over the first 15 months after planting, covering two rainy (growing) and one dry season. Based on the relative growth rates (RGRs) observed, species were classified as (i) fast-growing (Leucaena leucocephala Lam., Jatropha curcas L. and Moringa oleifera Lam.) or (ii) slow-growing (Anacardium occidentale L. and P. biglobosa Jacq.). All species successfully established on degraded cropland, albeit with differing survival rates (67-100%). The fast-growing species had the highest survival rates (94-100%). Dry-season irrigation resulted in a 10-fold reduction in the mortality of P. biglobosa, and fertilization consistently enhanced shoot growth in all species during the growing seasons. These management interventions had either insignificant or positive effects on root growth. However, belowground biomass allocation was mediated predominantly by ontogeny, which explained 86-95% of the variation in root-shoot biomass relationships. <br /> Allometric equations based on sequential measurements of stem diameter and tree biomass (i.e., leaves, stems and roots) showed very good fits (R² > 0.93). Following the integration of these equations with field-collected climate and soil physico-chemical data in the Water, Nutrient and Light Capture in Agroforestry Systems model (WaNuLCAS), the early growth dynamics of J. curcas and M. oleifera were reproduced with acceptable accuracy and precision. Climate-growth analysis using historical climate data (1981-2016) revealed that drought (indicated by annual water deficit, length of the longest dry spell and length of the dry season), projected to increase in severity and occurrence in the SSZ, was the main climatic factor limiting sapling growth and thereby posing risks for future afforestation efforts. However, scenario analysis of rooting depth greater than the empirical values suggests that deeper rooting might be an effective adaptive trait to enhance biomass growth under extreme dry conditions, and thus reduce sapling sensitivity to drought. <br /> Overall, all five MPTS were assessed as suitable for the afforestation of degraded cropland: the fast-growing species due to the high responsiveness of aboveground traits to silvicultural management during the rainy seasons, and the slow-growing species due to their ability to maintain growth during the dry season and to adjust to the prevailing water and nutrient stress through increased resource allocation to roots. Given the investment risks for smallholder farmers in the SSZ, the vast area of degraded croplands, and the projected increase in extreme weather events, an afforestation system that integrates silvicultural management and a diversity of species able to develop deep-penetrating root systems will have the highest potential to increase the resilience of tree plantations to the climate change and deliver expected benefits. The presented findings are relevant to local and regional decisions on land restoration and forest management in the SSZ, and might be of interest for other dryland agro-ecological zones.