The influence of structural, functional, and species diversity to temporal stability of productivity and efficiency of resource use in a tropical tree diversity experiment

Deforestation, degradation and climate change are pushing tropical forest ecosystems beyond their limits. These ecosystems are not only biodiversity hotspots, but also play a key role in regulating the terrestrial carbon and hydrological cycle and therefore help to mitigate the negative consequences of climate change. Some of the ecosystem services and functions of tropical forests can be also fulfilled by diverse tree plantations. However, a major obstacle for establishing mixed plantations with native tree species is that in the tropics, much less is known about native tree species compared to many exotic ones.

Results from existing studies on the relationship between biodiversity and ecosystem functioning support the hypothesis that forest ecosystem functions are promoted by different aspects of biodiversity and that mixtures may have a higher stability against a range of stressors and disturbances than monocultures. Yet, these results are based on snapshots in time, i.e. from inventories, or from short-term analyses of relatively young experimental plantations. Nevertheless, the effect of tree diversity on productivity can change as a function of temporal or spatial gradients in the availability of resources such as water or nutrients.

Accordingly, the overall objective of this project is to determine whether there is a stabilizing effect of neighbourhood diversity on the productivity and resource use dynamics (water and nutrients) of trees over time. In addition, we aim at disentangling the effects of species diversity from those of structural and functional diversity on ecosystem functioning for trees growing under controlled, experimental conditions.

In this project, we will study the oldest tree diversity experiment (‘Sardinilla’, Panama https://treedivnet.ugent.be/ExpSardinilla.html) after its final harvest after 16 years of growth. Several indicators of tree performance will be calculated at the level of individual year-rings. Wood biomass production will be calculated based on annually-resolved basal area increment series at 1.3 m stem height and wood density measurements. Water-use efficiency will be examined by investigating time-series of isotopic wood composition (δ13C and δ18O). Finally, nutrient acquisition capacity will be quantified on the basis of concentrations of immobile nutrients such as Ca in tree rings.

Results from this work will be highly relevant for the large afforestation efforts that are intended for tropical and subtropical regions (see for example the Bonn Challenge: http://www.bonnchallenge.org).