Agroforestry - the practice of growing trees and crops on the same land - has recently been suggested as a potential tool for adaptation to climate change. There are many mechanisms by which trees on farmland could help to ameliorate climatic stresses: moderation of microclimate, increased soil water-holding capacity, increased infiltration, reduced runoff, complementary use of water resources, and diversification of production risk. However, these benefits remain largely speculative, with very few controlled experiments to demonstrate them. Furthermore, little is known about how agroforestry systems themselves might be adversely affected by climatic stress, especially during the vulnerable stage of seedling establishment.

This dissertation attempts to address some of these questions with regard to "fertilizer tree" agroforestry systems in Malawi. In these systems, maize is intercropped with fast-growing leguminous trees, and biomass from the trees is regularly incorporated into the soil to increase crop yields. Though fertilizer trees are demonstrably effective under normal climatic conditions in southern Africa, their drought tolerance has not been systematically examined. For this three-year project (2008-2011), a novel and low-cost rain exclusion shelter design was used to impose an artificial drought on several Gliricidia sepium and Tephrosia candida fertilizer tree systems at Makoka Agricultural Research Station (15º31'S, 35º13'E) in southern Malawi.

In the first experiment, a mature Gliricidia-maize intercropping system was subjected to complete rain exclusion from maize anthesis to maize harvest during the 2009-2010 and 2010-2011 growing seasons. Monoculture maize was used as a control. The drought dramatically decreased maize yields in both years (by 61% and 39%, respectively), with approximately similar decreases in both Gliricidia and monoculture plots. This implies that Gliricidia does not protect maize from drought conditions, but neither does it exacerbate the effects of drought by competing with maize for water. Whether in the presence or absence of drought, maize yields in Gliricidia plots were higher than in monoculture plots. Gliricidia had no effect on soil moisture and had a slight cooling effect on microclimate at the end of the growing season.

In the second experiment, a similar drought - compounded with the additional stress of late planting - was imposed upon newly established fertilizer tree systems of three types: Gliricidia-maize intercropping, Tephrosia-maize relay intercropping, and Tephrosia improved fallows. Seedling survival, growth, and biomass production were monitored for signs of adverse effects. Results varied by species: Gliricidia growth was retarded by drought and late planting, but Gliricidia survival remained near 100%, and its biomass at the end of the growing season was not affected. By contrast, Tephrosia maintained constant height growth under these stresses, but its survival decreased, which translated into lower maize yields the following year. However, all three fertilizer tree systems demonstrated good overall drought resilience and conferred maize yield benefits even under suboptimal conditions.

A modeling experiment was conducted to compliment the above field experiments. Output from global and regional climate models was applied to long-term data sets on weather and maize yield at this location. The results suggested that for the next several decades, current precipitation variability in southern Malawi will likely pose a greater risk to maize production than will long-term precipitation trends caused by global climate change. However, higher maximum temperatures have the potential to reduce maize yields within the next several decades.

The field experiments described herein suggest that fertilizer tree systems can improve maize production even under conditions of drought stress. Current climate variability and future climate change should pose no obstacle to their adoption in Malawi. Although this project did not find evidence that fertilizer trees directly protect the maize crop from drought, their beneficial effects on food security and farm income may indirectly help to cushion farmers against climate variability.

The performance of agroforestry systems under future climate is a topic that demands much more research, both descriptive and theoretical (including controlled experiments, simulation models, and meta-analyses). Furthermore, climate manipulation experiments in the developing world are an underutilized but potentially important tool for understanding the effect of climate change on subsistence agricultural systems and developing climate-resilient alternatives. It is hoped that the methods and outcomes of this dissertation represent a small step toward closing both of those knowledge gaps.