Water dynamics in coffee agroforestry systems of Costa Rica

During 5 years, two studies were undertaken to monitor water dynamics in various coffee agroforestry systems (AFS) in two regions of Costa Rica with contrasting ecological conditions. From 2001 to 2003, three coffee AFS (with shade provided by either Erythrina poeppigiana or Terminalia ivorensis or Eucalyptus deglupta) and a full sun coffee monoculture were monitored in the suboptimal conditions of the Southern region of Perez Zélédon characterized by a low altitude (~700 m), hot climate (~24°C), high annual rainfall (>3000 mm) and rather long dry season of four months. From 2003 to 2005, one coffee AFS (with shade provided by Inga densiflora) and a full sun monoculture (MC) were monitored in the optimal conditions of the central valley characterized by high altitude (~1200 m), cool climate (~22°C), high annual rainfall (~2300 mm) and a dry season of 2.5 months. Coffee transpired less in AFS than in MC on a leaf area basis due to the buffering effects of shade on the micro-climate and hence a lower evaporative demand in AFS compared to MC. Associated shade trees increased the overall transpiration, decreased runoff, increased rainfall interception in AFS and ultimately reduced drainage in comparison to MC. Due to high annual rainfall pattern (> 2500 mm) in both sites, there was no evidence that water use by shade trees decreased soil water availability for coffee and hence limited its transpiration in the dry season. Furthermore, there was evidence that deep soil water and/or capillary rise contributed to transpiration of coffee and shade tree during the dry season. Coffee stomatal conductance decreased above values 1.5-2.0 kPa of leaf VPD and hence coffee transpiration rate that could not keep up with the evaporative demand along the day in both systems during the dry season. The present studies increased our understanding on coffee and shade tree transpiration during the entire production cycle. Modeling allowed us to simulate water partition between components (transpiration, runoff, soil water content & drainage) of a coffee system. Furthermore, this work allows us to calculate the amount of water drainage and hence provides a sound basis for calculation of nitrate or other nutrient leaching out the coffee system into the sub-soil and aquifers. (Résumé d'auteur