Allometric relationships of frequently used shade tree species in cacao agroforestry systems in Sulawesi, Indonesia

Shade trees play an important role within agroforestry systems by influencing radiation and wind regimes as well as nutrient and hydrological cycling. However, there is a lack of quantitative assessments of their functions. One of the reasons is the rare information on structural characteristics of shade tree species. Therefore, the aim of this study is to provide basic information on the structure of frequently used shade tree species for the implementation of models simulating the ecosystem processes in agroforestry systems. The investigation of the shade trees was conducted at two cacao agroforestry sites on Sulawesi, Indonesia. The measurements of the main structural parameters: diameter at breast height, tree height, trunk height, crown length and crown radius were carried out for the shade tree species Aleurites moluccana, Cocos nucifera and Gliricidia sepium. For data collection, the National Forest Inventory Field Manual Template by FAO (2004) was applied. Based on this information allometric functions were derived for the correspondent shade tree species. The best significant relationships were obtained for the height-crown length relationship of the dicotyledonous tree species’ A. moluccana and G. sepium with a coefficient of determination r² = 0.925 and r² = 0.738, respectively, and the height-crown length relationship of the monocotyledonous palm C. nucifera with r² = 0.663. The transferability tests ‘analysis of covariance’ and ‘homogeneity of slopes’ have shown that the obtained allometric functions are also applicable to other cacao agroforestry systems of the region.