Pollination of Coffea canephora in relation to local and regional agroforestry management.
Abstract
Pollination of crops depends on local agricultural management and the quality of adjacent habitats. Lowland coffee Coffea canephora, is an important tropical cash crop. Fruit set depends on cross-pollination by bees, so inadequate pollination leads to reduced yield. In this study we analyse the relationship between bee pollinators, fruit set in coffee, and the local and regional agroforestry systems to identify the optimal conditions for pollinators. We analysed the abundance and species composition of coffee flower-visiting bees in 15 agroforestry systems differing in distance to forest (important for wood-nesting species), light intensity (important for ground-nesting species), blossom cover of coffee and noncoffee-flowering plants, and species richness of flowering plants (as pollen and nectar resources) in Central Sulawesi (Indonesia). We examined which factors were most important for optimal pollination success. We carried out bagged and open pollination experiments in each agroforestry system, to measure the pollination efficiency of 15 bee species. The number of social bee species decreased with distance to forest, whereas the number of solitary bee species increased with light intensity (less shade) and greater quantities of blossoms. Fruit set of open pollinated flowers (as opposed to manually cross-pollinated flowers) increased with the diversity and abundance of flower-visiting bees. In the agroforestry systems studied, a bee community of 20 species or more led to a higher fruit set (95%) than a species-poor bee community of six species (70% fruit set). Pollination activity by members of the species-rich solitary bee assemblage led to higher levels of fruit set than that arising from pollination activity by members of the more abundant social bee assemblage. Synthesis and applications. A species-rich and abundant bee assemblage will facilitate high pollination success in lowland coffee. This will increase fruit set and coffee yield. Farmers can encourage different species of bees through simple management measures such as growing coffee in shade beneath a variety of trees; by pruning trees to increase levels of sunlight and numbers of flowering herbs; and by increasing the availability of nesting sites for solitary bees. Weed control and the use of herbicides should be kept to a minimum so that a diverse nectar and pollen resource is available to bees throughout the year. Natural forests and forest fragments should be preserved in the vicinity of coffee agroforestry systems (<500 m) so that forest-nesting social bees can travel easily to the coffee fields to pollinate the flowers.