Managing Trade-Offs in Coffee Agroforests

Abstract

Agroforestry systems provide multiple ecosystem services and are refuges for diverse flora and fauna. Coffee is one of the main tropical agroforestry crops and of global economic importance. Over recent decades, coffee agroforestry systems have been increasingly intensified by the replacement of native shade tree diversity with one or few, often non-native, tree species. Such reduction in tree diversity might have implications for ecosystem services, such as nutrient cycling, soil fertility, pest control, and coffee production, and ultimately negative effects for the systems’ resilience to climate change. The overall aim of this thesis is to investigate the direct and indirect effects of tree diversity reduction on ecosystem services, to understand the arising management trade-offs for Coffea canephora production in the face of climate change. We therefore studied 25 agroforestry systems in Kodagu, India, across a broad rainfall and management gradient, which experience a gradual transformation of native diverse shade tree canopy to Grevillea robusta dominated shade. Climate change is expected to alter the intense monsoon rainfalls during the short, wet season by an increase in the east and a reduction in the west. The results show that the transformation towards G. robusta dominated shade has not only highly negative effects for tree diversity but also for coffee production and quality. Both coffee production and quality were lower under G. robusta dominating shade. This resulted mainly from the increased development of single-seeded fruits (pea-beans), likely a result from reduced pollination success, and higher infestation rates of the coffee berry borer, Hypothenemus hampei. This suggests that pest control and pollination services are reduced in monospecific agroforestry systems. Grevillea robusta shade reduced nutrient cycling and long-term soil fertility. In contrast to native trees, G. robusta leaf litter shedding was more dependent on the annual rainfall, which reduced litterfall in wet sites to negligible amounts. Low litter quality and reduced decomposition rates characterize G. robusta dominated agroforests, and this led to low soil fertility and a decline in soil carbon contents, particularly in wet sites. The specific characteristic of G. robusta, being a Proteaceae, reduced phosphor cycling by increasing the immobilization of the same. Although, we did not find evidence for increased nutrient limitation in C. canephora leaves under G. robusta shade, the loss of soil carbon reduces soil fertility and nutrient availability over the long-term, which will likely demand higher nutrient inputs in the future. Microclimate and soil moisture was negatively altered under G. robusta monospecific shade compared to native diverse agroforests. Low soil carbon was one of the main drivers for lower soil moisture under G. robusta, which increased nutrient leaching. This, together with higher relative air humidity, reduced coffee production in monospecific shade. While high monsoon rainfall reduced coffee production in all agroforestry types, throughfall reduction benefitted coffee plants less under G. robusta dominated shade, in which high nutrient leaching resulted in a lower final berry-set. In conclusion, we provide strong evidence that there are no trade-offs, but synergies between tree diversity, coffee production and resilience to climate change. A shade cover dominated by G. robusta affects ecosystem resilience by causing: (1) increased pest attacks likely due to a loss of complex multi-trophic interactions; (2) reduced nutrient cycling and loss of soil carbon, reducing soil fertility in the long-term; (3) reduction of soil moisture and increase of nutrient leaching likely via runoff; (4) increased relative humidity potentially due to higher transpiration of the fast-growing G. robusta. These characteristics of G. robusta dominance negatively affect coffee production and quality, and reduce the agroforestry systems’ resilience to climate change.