Woody encroachment reduces nutrient limitation and promotes soil carbon sequestration

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.ISSN:2045-775

Cocoa agroforestry systems versus monocultures: A multi-dimensional meta-analysis

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

Mycorrhizal fungi-mediated uptake of tree-derived nitrogen by maize in smallholder farms

Trees within farmers’ fields can enhance systems’ longer-term productivity, for example, via nutrient amelioration, which is indispensable to attain sustainable agroecosystems. While arbuscular mycorrhizal fungi (AMF) are known to improve plant access to soil nutrients, the potential of AMF to mediate nutrient uptake of tree-derived nitrogen (N) by crops from beyond the crops’ rooting zones is unclear. We hypothesized that AMF quantitatively contribute to the crop uptake of tree-derived N. We set up root- and AMF-exclusion and control plots around faidherbia trees (Faidherbia albida) and used the 15N natural abundance technique to determine the magnitude of AMF-mediated uptake of tree-derived N by maize from beyond its rooting zone in smallholder fields. We further tested whether AMF-mediated N uptake decreases with distance from tree. We show that within one cropping season, maize obtained approximately 35 kg ha–1 biologically fixed N from faidherbia. One-third of tree-derived N in maize leaves was attributed to AMF-mediated N uptake from beyond the maize rooting zone and two-thirds to N from tree leaf litter, regardless of distance from tree. As hypothesized, maize grown close (1 m) to faidherbia obtained significantly more tree-derived N than that at farther distances (4 and 5 m). Thus, the faidherbia–AMF association can enhance agroecosystem functioning.ISSN:2398-962

Facilitative or competitive effects of woody plants on understorey vegetation depend on N-fixation, canopy shape and rainfall

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. 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. 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.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

Soil fertility and Theobroma cacao growth and productivity under commonly intercropped shade-tree species in Sulawesi, Indonesia

Background and aims: Trade-offs between ecological benefits and potential yield and growth reductions associated with the inclusion of shade trees in cocoa agroforests remain poorly understood. In this study we investigate interactions between shade and cocoa trees in cocoa agroforests in terms of soil fertility and cocoa productivity. Methods: We quantified the effects of individual shade trees from 11 commonly intercropped species on cocoa growth (aboveground biomass) and yield and soil fertility indicators (total soil carbon, nitrogen, phosphorus contents and soil aggregation) at field sites in Southeast Sulawesi, Indonesia. Results: Shade trees had a net positive effect on soil fertility in cocoa agroforests, with a 6% increase in soil carbon, a 4% in soil nitrogen and a 24% increase in mean weight diameter\ua0(used as an indicator for median soil aggregate size), under shade tree canopies compared to open areas. We found that shade trees had a net negative effect on cocoa tree growth and no net effect on cocoa yields. We were not able to link costs versus benefits with specific shade tree traits, but nevertheless observed significant differences between shade tree species. G. sepium (gliricidia) had significantly positive effects on yields, soil carbon and aggregation. N. lappaceum (rambutan) and D. zibethinus (durian) had significantly positive effects on soil carbon and nitrogen contents and on aggregation, but not on yields. Conclusions: Our findings confirm the potential for soil improvements under shade trees and suggest that the inclusion of individual shade trees does not always constitute a direct trade-off for farmers in terms of yield losses

Mycorrhizal fungi-mediated uptake of tree-derived nutrients by crops – the role of tree-maintained versus crop-associated fungal mycelia

ISSN:0038-0717ISSN:1879-342

Cocoa plantations are associated with deforestation in Côte d’Ivoire and Ghana

Côte d’Ivoire and Ghana, the world’s largest producers of cocoa, account for two thirds of the global cocoa production. In both countries, cocoa is the primary perennial crop, providing income to almost two million farmers. Yet precise maps of the area planted with cocoa are missing, hindering accurate quantification of expansion in protected areas, production and yields and limiting information available for improved sustainability governance. Here we combine cocoa plantation data with publicly available satellite imagery in a deep learning framework and create high-resolution maps of cocoa plantations for both countries, validated in situ. Our results suggest that cocoa cultivation is an underlying driver of over 37% of forest loss in protected areas in Côte d’Ivoire and over 13% in Ghana, and that official reports substantially underestimate the planted area (up to 40% in Ghana). These maps serve as a crucial building block to advance our understanding of conservation and economic development in cocoa-producing regions

Farmer perceptions of plant–soil interactions can affect adoption of sustainable management practices in cocoa agroforests : A case study from Southeast Sulawesi

Despite extensive research focused on increasing the sustainability and productivity of agricultural systems in the tropics, adoption rates of improved management solutions often remain low among smallholder farmers. To address this, we evaluated how local knowledge and perceptions influenced decision-making processes among smallholder cocoa farmers. We conducted individual semistructured interviews with 72 cocoa farmers in Southeast Sulawesi and documented local knowledge about soil fertility indicators, nutrient cycling processes, and the interactions among shade trees, cocoa trees, and soils in cocoa agroforests. We further collected data regarding farmers’ fertilizer preferences, additional income sources, and perceived barriers to improved cocoa production. We found that farmers’ understanding of biophysical interactions in Southeast Sulawesi was comprehensive, mostly accurately aligned with scientific literature, and sometimes provided additional complementary knowledge. Cocoa farmers in Southeast Sulawesi approached decision making in a holistic way, integrating personal observations, information from external sources, and socioeconomic limitations and priorities. This finding highlights the value of flexible conservation farming approaches that allow farmers to minimize trade-offs and prioritize their households’ needs. Finally, we identify a “dual” knowledge gap on the part of farmers and scientists regarding the direct benefits of shade tree inclusion for improved yields and income security. Addressing this through further research and targeted knowledge dissemination could contribute to an increase in the long-term adoption rates of more sustainable cocoa cultivation practices

Combatting Cocoa Swollen Shoot Virus Disease: What do we know?

Cocoa Swollen Shoot Virus Disease (CSSVD) is one of the major factors limiting cocoa (Theobroma cacao L.) productivity in West Africa. The only cure for CSSVD is to cut out visibly infected trees and the official eradication campaign in Ghana has cut out more than 200 million trees since 1946. 80 years of research on preventive control measures have mainly focused on resistance breeding, mild strain cross-protection (inoculation of cocoa seedlings with a mild strain of the virus to protect against the severe strain) and control of mealybug vectors. Meanwhile, diversification measures such as agroforestry (for shading) or barrier (strip) cropping have received less attention. Despite promising results, CSSVD is more prevalent in the field than ever before. The large body of knowledge on preventive control measures for CSSVD is fragmented and many publications are not easily accessible. Furthermore, the literature has never been systematically evaluated and quantitatively assessed. Hence, we consolidated this knowledge with an extensive literature review followed by meta-analysis to identify the pertinent research gaps. Out of 423 publications on CSSVD-related issues, we selected 34 studies, which contained 52 datasets on seven different preventive control measures. Results showed that resistance breeding and mild strain crossprotection may reduce CSSVD infection by 30 percent, while the potential of diversification measures (shading/agroforestry and barrier (strip) cropping) seems to be considerably higher (40 and 85 percent, respectively). However, there is a lack of evidence because of a low number of studies about diversification measures, indicating that our results have to be interpreted with care and calling for more research in this area. Future testing is needed to evaluate the efficacy of barrier (strip) cropping to reduce CSSVD, and address the effect of shade on CSSVD symptom severity. Furthermore, the practical relevance of different preventive control measures for farmers needs to be assessed, and shade should be considered in current breeding programs for CSSVD resistance