Shade trees in cacao agroforestry systems: influence on roots and net primary production

Huge areas of natural forests are rapidly cleared worldwide to make way for agricultural cultivation systems. This has a substantial impact on the functioning of ecosystems, as well as on carbon cycling and hence effects global climate change. Currently, Indonesia has one of the highest deforestation rates worldwide and on the island of Sulawesi, where this study was carried out, substantial tracts of primary forest are being cleared in order to predominantly cultivate cacao (Theobroma cacao L.), which made Sulawesi the biggest producer of cacao within Indonesia. Cacao is an understory rainforest species and was traditionally planted beneath thinned primary forest, but naturally occurring shade trees are increasingly being replaced by planted shade trees, such as fast growing nitrogen-fixing or other commercially valuable species. Nowadays, shade trees are often completely or partly removed as the cacao tree matures, because farmers fear that above- and belowground competition for light, water and nutrients will diminish bean yield. Not only do diverse agroforestry systems harbor higher biodiversity and enhance carbon sequestration and soil fertility, they also provide microclimatic benefits such as increased humidity and buffer temperature extremes, thus also improving drought resistance. However, not much is known about the effects these intensifications have on already existing cultivation systems. The present thesis aims to compare cacao agroforests with different shade intensities, focusing on above- and belowground biomass, carbon stocks and net primary production, as well as on vertical root distribution, fine root dynamics and the hydraulic-anatomical architecture of cacao and shade trees. Shade tree cover increased from: 1) non-shaded cacao monoculture (‘Cacao-mono’) to 2) planted single-shade by the nitrogen fixing Gliricidia sepium (Jacq.) Kunth (‘Cacao-Gliricidia’) to 3) a multi-species shade tree layer (‘Cacao-multi’). The general hypotheses we wanted to verify were that i) above- and belowground biomass and NPP, as well as the related C stock and C sequestration increase with increasing shade tree cover, that ii) cacao bean yield is negatively affected by an increasing shade tree cover, that iii) standing fine root biomass, production and turnover increase with increasing shade tree cover, that iv) there is vertical root segregation between cacao and shade trees and that v) aboveground productivity is positively related to vessel size and hydraulic conductivity. In order to test these general hypotheses, this dissertation is subdivided into three studies. In the first study (Chapter 2), the three cultivation systems were compared with respect to above- and belowground biomass and the corresponding carbon (C) stocks, as well as above- and belowground net primary production (NPP) and the associated C sequestration. As hypothesized, total above- and belowground biomass and NPP increased from 23 to 124 Mg ha-1 and 20 to 38 Mg ha-1 yr-1, respectively, as well as the corresponding C stocks and C sequestration increased significantly (from 11 to 57 Mg C ha-1 and 9 to 18 Mg C ha-1 yr-1, respectively) with increasing shade tree abundance and diversity. In contrast to the commonly held assumption, however, we did not detect a significant reduction in cacao bean production, although cocoa bean yield per individual tree slightly decreased, which was, however, not statistically significant. The second study (Chapter 3) addressed the influence of shade trees on vertical rooting patterns and fine root dynamics of cacao and shade trees. Biomass of fine, large and coarse roots increased with shade tree diversity and abundance. The highest standing fine root biomass values in the Cacao-mono and the Cacao-Gliricidia sites were located in the upper 20 cm and decreased with increasing soil depth, although Gliricidia-shaded plots showed a significantly higher fine root biomass in deeper soil layers compared to the Cacao-mono sites. The results of the deuterium stable isotope analysis revealed that cacao trees obtained water predominantly between 40 and 60 cm soil depth, while Gliricidia primarily took up water from deeper soil depths indicating vertical root distribution. In the Cacao-multi stands, the highest standing fine root biomass was located in the subsoil. In the Cacao-multi system, we have a clear indication that cacao and shade trees use the same depth interval for soil water uptake indicating a displacement effect as cacao roots were found in deeper soil layers. Contrasting to what was expected, no significant influence of shade trees on total fine root productivity could be proven. Annual production of cacao fine roots in the total soil profile was highest in the Cacao-mono plots (167 g m-2 yr-1) and decreased slightly with increasing shade tree abundance and diversity (133 and 113 g m-2 yr-1 in Cacao-Gliricidia and Cacao-multi, respectively). Shade tree species in both the Cacao-Gliricidia (56 g m-2 yr-1) and the Cacao-multi plots (40 g m-2 yr-1) had a significantly lower annual fine root production as compared to cacao. In the third study (Chapter 4), the interrelationship between sapwood area and specific hydraulic conductivity of root, stem and branch xylem tissue with wood anatomical traits along the water flow path across six common cacao agroforestry tree species with different biogeographical origins (perhumid vs. drought-tolerant) were examined. Drought-adapted species showed divergent patterns of hydraulic conductivity, vessel density and relative vessel lumen area between root, stem and branch wood compared to perhumid forest species. Wood density showed no relationship to specific conductivity. In general, aboveground growth performance was better predicted by specific hydraulic conductivity than by foliar traits and wood density. Overall, the results show that there is vertical root segregation between cacao and Gliricidia shade trees, but there seems to be a displacement effect of cacao roots to deeper soil layers when growing under a diverse shade tree layer. However, elevated specific root area and length and associated thinner cacao fine roots in the Cacao-multi stands may indeed compensate for the decrease in fine root biomass. Although shade trees may compete with the cacao trees for resources, there are several positive ecosystem services provided by shade trees which are likely to compensate for possible negative effects. Furthermore, additional income from carbon payments and from shade tree products as well as lower labor and input costs makes cacao production less susceptible to highly fluctuating prices. The present study proved that smallholder agroecosystems with diverse shade tree cover offer the opportunity to combine high yield, high biodiversity, and high carbon sequestration and thus may help to reduce tropical deforestation and mitigate global climate change.