Functional differentiation among tree species in wet and dry tropical forests: its importance for species coexistence

Researcher: Lourens Poorter, Post-doctorado
Institution: Wageningen University, The Netherlands
Study Location: La Chonta and INPA
Email: Lourens.Poorter@wur.nl

Introduction
Tropical rain forests are characterized by a high species diversity. In Bolivia rain forests up to 120 tree species may be found in a single hectare. The maintenance of such a high biodiversity is, amongst others, possible through niche differentiation.

Niche differentiation occurs when species have different life history traits, and exploit limiting resources in different ways. Trade-offs amongst species traits may lead to species coexistence if a given trait is adaptive in one environment, but not adaptive in another environment. Insight in the life-history trade-offs that drive the coexistence of the different functional groups is far from complete, but pertinent to our understanding of forest functioning.

In the tropics changes in community structure, composition and functioning are driven to a large extent by variation in rainfall. Species coexistence in dry and wet forest communities will be governed by different life history trade-offs. Species coexistence in dry forest may be determined by the trade-off to tolerate drought and compete for water. In wet forest there might be a trade-off to tolerate shade and to compete for light. With an increase in rainfall there is a shift from competition for water to competition for light. It has been hypothesised that there is a trade-off between the drought tolerance and shade tolerance of the species, but the current evidence is far from conclusive .

Aims
This study focuses on the two major environmental gradients in tropical forest; light and rainfall. Fifty tree species from a tropical Bolivian dry forest and fifty species from a moist forest are screened for a large number of functional plant traits related to shade and drought tolerance. The association of co-evolved plant traits will be shown using a principal component analysis. Trait corrleations reveal the main avenues of specialisation of plants, highlight their functional differences, and enable to predict plant responses to environmental change.

Based on their traits, the species will be classified into functional groups, that respond in a similar way to the environment. The functional grouping of trees is very important for tropical forest conservation, management, and climate change mitigation efforts. The classification of the wealth of tree species into a limited number of functional groups allows to model forest communities, and predict species responses to different management scenarios.

A sneak preview …
The study is one year underway, and I would like to share with you some preliminary results. In figure one, I analysed the correlations amongst 27 traits life history traits of 50 wet forest tree species, using a principle component analysis. The first axis represents a gradient in light demand, with the most shade tolerant species at the left, and the most light demanding species at the right. Light demanding species (at the right) are characterised by a high light environment, and fast sapling and tree growth rates, and leaves with a large leaf area per unit leaf mass (SLA ; specific leaf area). This fast growth comes at the expense of an increased mortality. Shade tolerant species (at the left) are characterised by a high survival. They realise this by making long-lived, well-defended, tough leaves with a high leaf dry matter content. At the same time they make dense wood which protects them against pathogens. The second axis represent a gradient in adult stature and drought tolerance. Large tree species (at the top) are growing in the forest canopy. They have large heights and diameter, and because of their exposed conditions they tend to be deciduous in the dry season, during which they produce mainly wind-dispersed fruits. Understory species need to complete rapidly their life cycle, and have therefore a small size. Because of a lack of wind in the understory, their seeds are dispersed by specialised animal dispersers. Interestingly, the species that dominate the exposed (and therefore dry) canopy of wet forests are the same ones that are common in dry forests. These preliminary results suggest that there are two axes of specialisation in wet forest (height and light), and that there is no trade-off between shade tolerance (first axis) and drought tolerance (second axis).