No abstract

During the past century, the biomass of woody species has increased in many grassland and savanna ecosystems. As many of these species fix nitrogen symbiotically, they may alter not only soil nitrogen (N) conditions but also those of phosphorus (P). We studied the N-fixing shrub Dichrostachys cinerea in a mesic savanna in Zambia, quantifying its effects upon pools of soil N, P, and carbon (C), and availabilities of N and P. We also evaluated whether these effects induced feedbacks upon the growth of understory vegetation and encroaching shrubs. Dichrostachys cinerea shrubs increased total N and P pools, as well as resin-adsorbed N and soil extractable P in the top 10-cm soil. Shrubs and understory grasses differed in their foliar N and P concentrations along gradients of increasing encroachment, suggesting that they obtained these nutrients in different ways. Thus, grasses probably obtained them mainly from the surface upper soil layers, whereas the shrubs may acquire N through symbiotic fixation and probably obtain some of their P from deeper soil layers. The storage of soil C increased significantly under D. cinerea and was apparently not limited by shortages of either N or P. We conclude that the shrub D. cinerea does not create a negative feedback loop by inducing P-limiting conditions, probably because it can obtain P from deeper soil layers. Furthermore, C sequestration is not limited by a shortage of N, so that mesic savanna encroached by this species could represent a C sink for several decades.We studied the effects of woody encroachment on soil N, P, and C pools, and availabilities of N and P to Dichrostachys cinerea shrubs and to the understory vegetation. Both N and P pools in the soil increased along gradients of shrub age and cover, suggesting that N fixation by D. cinerea did not reduce the P supply. This in turn suggests that continued growth and carbon sequestration in this mesic savanna ecosystems are unlikely to be constrained by nutrient limitation and could represent a C sink for several decades.

Summary 1.A recent meta-analysis suggested that differences in rainfall are a cause of variation in tree-grass interactions in savannas, with trees facilitating growth of understorey grasses in low-rainfall areas, but competing with them under higher rainfall. We hypothesized that this effect of rainfall upon understorey productivity is modified by differences in the growth form of the woody plants (i.e. the height of the lower canopy) or by their capacity to fix nitrogen. 2. We performed a meta-analysis of the effects of woody plants on understorey productivity, incorporating canopy height and N-fixation, and their interaction with rainfall. 3. N-fixing woody plants enhanced understorey productivity, whereas non-fixers had a neutral or negative effect, depending on high or low canopy, respectively. We found a strong negative correlation between rainfall and the degree to which trees enhanced understorey productivity, but only for trees with a high canopy. 4. Synthesis. The effect of woody plants on understorey productivity depends not only on rainfall, but also on their growth form and their capacity to fix N. Facilitation occurs mostly when woody plants ameliorate both water and nitrogen conditions. However, a low canopy suppresses understorey vegetation by competing for light, regardless of water and nutrient relations.

Scientific knowledge, societal debates, and industry commitments around sustainable cocoa are increasing. Cocoa agroforestry systems are supposed to improve the sustainability of cocoa production. However, their combined agronomic, ecological, and socio-economic performance compared to monocultures is still largely unknown. Here we present a meta-analysis of 52 articles that directly compared cocoa agroforestry systems and monocultures. Using an inductive, multi-dimensional approach, we analyzed the differences in cocoa and total system yield, economic performance, soil chemical and physical properties, incidence of pests and diseases, potential for climate change mitigation and adaptation, and biodiversity conservation. Cocoa agroforestry systems outcompeted monocultures in most indicators. Cocoa yields in agroforestry systems were 25% lower than in monocultures, but total system yields were about ten times higher, contributing to food security and diversified incomes. This finding was supported by a similar profitability of both production systems. Cocoa agroforestry contributed to climate change mitigation by storing 2.5 times more carbon and to adaptation by lowering mean temperatures and buffering temperature extremes. We found no significant differences in relation to the main soil parameters. The effect of the type of production system on disease incidence depended on the fungal species. The few available studies comparing biodiversity showed a higher biodiversity in cocoa agroforestry systems. Increased and specific knowledge on local tree selections and local socio-economic and environmental conditions, as well as building and enabling alternative markets for agroforestry products, could contribute to further adoption and sustainability of cocoa agroforestry systems.

A B S T R A C TAgroforestry is often promoted as a sustainable agricultural practice that can ameliorate causes of declining yields, such as soil degradation. However, despite the often-stated potential of agroforestry, quantitative data on the benefits of shade trees are limited to relatively few cropping systems, particularly maize and coffee. Furthermore, agroforests are not cost-free and the benefits of agroforests might not be sufficient to outweigh these costs in all cropping systems or environments. Here we quantify costs and benefits of agroforests for cocoa production in Ghana, West Africa. Specifically, we quantified the ability of shade trees to increase soil carbon stocks and soil fertility (i.e. total soil carbon, nitrogen and phosphorus, available phosphorus and potassium, cation exchange capacity, soil aggregation, pH, and foliar nitrogen and phosphorus concentrations), and investigate if these benefits are sufficient to outweigh the negative effects of shade trees on cocoa growth and yields. We measured cocoa yields, soil fertility and carbon-sequestration under individual shade trees, and in 30 × 30 m plots that were distributed along a gradient of shade-tree cover (plot-scale). We found localized positive effects of individual shade trees on soil carbon and nitrogen content, as well as soil aggregation. However, we found no evidence for positive effects of agroforests via improved soil fertility or carbonsequestration with increasing shade-tree cover at the plot scale, a scale that more closely matches the scale at which agroforests are managed. Cocoa growth was lower under individual shade trees and decreased with increasing shade-tree cover in plots, and cocoa yields also decreased with increasing shade-tree cover. Our results indicate that the benefits of agroforestry for soil fertility and carbon sequestration in cocoa cultivation systems might not be as extensive as believed, and may not be sufficient to compensate for short-term costs to production.