The influence of structural, functional, and species diversity to temporal stability of productivity and efficiency of resource use in a tropical tree diversity experiment
Final Report Abstract
Increasing evidence suggests that mixed planted forests can provide multiple ecosystem services at a higher level than monocultures. Importantly, diverse tree plantations are often reported to be more productive but also more resilient against disturbances than monocultures. Large-scale forest restoration efforts are under way globally to increase forest coverage. Such efforts are often focused on tropical regions, where extensive forest areas have been degraded or lost in the recent past. In that regard, increasing diversity in planted forests including restoration plantings might be a promising strategy to provide multiple ecosystem services at a higher level and increase resilience against disturbances. However, a major obstacle for establishing mixed plantations with native tree species is that in the tropics, we know much less about native tree species when compared to many exotic ones. This project aimed to fill this knowledge gap. In this project, we analyzed several tree-ring based indicators of tree performance derived from stem disks from ‘Sardinilla’ in Panama, the oldest planted tree diversity experiment in the tropics. Wood biomass production was calculated based on annually-resolved basal area increment series at 1.3 m stem height and wood density measurements. Nutrient acquisition capacity and use efficiency was quantified on the basis of concentrations of elements in tree rings. Finally, water-use efficiency was examined by investigating the isotopic composition (δ13C and δ18O) in tree-rings. The overall objective of the proposed project was to assess the effects of different aspects of neighborhood diversity on the biomass productivity and resource use dynamics (water and nutrients) of trees over time. In addition, we aimed at disentangling the effects of species diversity from those of structural diversity on functioning of trees growing under controlled, experimental conditions. We hypothesized that biomass productivity and use efficiency of resources (water and nutrients) increases with increasing diversity (species and structural) of tree neighborhoods and over time. In addition, we expected that structural diversity and species richness independently and directly influence the productivity and resource use of individual trees. Finally, we hypothesized that trees growing in species-rich neighborhoods would exhibit higher wood biomass production and water and nutrient resource use efficiency compared to monospecific neighborhoods even under extreme climatic conditions. Overall, we observed higher above-ground wood biomass increment (AWBI) and nutrient acquisition capacity in species-rich compared to monospecific tree neighborhoods, suggesting strong complementarity effects in mixtures. On average, overyielding in species-rich neighborhoods increased over time, indicating a progressive intensification of diversity effects. Similarly, nutrient use efficiency increased over time for trees growing in mixed species neighborhoods. Species diversity had a strong total effect on AWBI by directly enhancing annual biomass production, but also, indirectly, via positively influencing structural diversity. Structural diversity had a direct positive effect on AWBI, but this effect became gradually weaker over time. Importantly, the superior performance (biomass production, nutrient acquisition capacity, nutrient and water use efficiency) of trees growing in species-rich compared to monospecific neighborhoods was maintained or even increased under extremely dry conditions most likely due to complementary use of nutrients and water. These results add to a growing body of evidence suggesting that mixed species forests do not only function at a higher level compared to monocultures, but most importantly have a greater ability to maintain their performance under stressful environmental conditions, including extreme drought.
