Global change in tropical forests

Enormous research efforts are made to understand how plants and ecosystems respond to rising atmospheric CO 2 - concentrations, but also to other drivers of global change such as changes in climate and deposition of nitrogen. While it is mostly unknown how these have already affected tropical forest over the past decades, detecting and distinguishing the impacts of global change is most important to understand past and predict future changes. Currently tropical forests appear to act as major carbon sinks, but will not continuously do so forever. Experimental studies from other forests suggest that a fertilizing effect of CO 2 is strongest when temperature, water and nutrients are least limiting, but the complexity and size of mature tropical forests has so far prevented free air CO 2 - enrichment studies in this important biome. Alternatively, throughout the tropics repeated forest inventories, originally designed to study forest dynamics, growth and function, are used to detect and analyse long-term changes potentially related to global change. These data show evidence of increased tree growth, turnover and changing species composition, but have met considerable discussion. One problem is that measurements rarely go back more than three decades and are potentially subject to bias from initial site selection or disturbance. Annual growth rings are widely used to analyse environmental impact, including global change, on temperate trees. Tree rings have been little used in tropical trees, although a number of species do produce regular rings that can be used to analyse past trends in climate, tree physiology, and growth as a reaction to rising CO 2 levels and other factors over the past 100+ years. Tree rings appear to be the best, or indeed the only option for a range of research questions which the project will address. The main questions are if growth has increased in the past decades, and which global driver is responsible for changes in growth. We propose to sample increment cores from five long- term forest plots in Panama, India, Thailand and Australia, representing a gradient from strong to moderate seasonality of rainfall and total annual rainfall. Trees from each species will be selected to represent different ages and sizes, to be able to distinguish ontogenetic effects from long-term environmental changes. Increment cores will be analysed for ring width (growth), wood density (may be affected by CO 2 and possibly growth, and is important to relate diameter growth to biomass), cellulose 13C (indicating water use efficiency and leaf internal CO 2 concentrations), 18O, 2 H (climate signals), wood 15N and N concentration (changes in nitrogen deposition or limitation). In addition to revealing long-term trends related to global change, short-term changes in growth will show how different species respond to inter-annual changes in climate, and analysing many trees from one site provides insights in the long-term dynamics of tropical forests. The project will be an important contribution to understand how tropical forests have responded to global change in the past and are likely to do in the future and provide insight into short-term growth dynamics.

In addition to chainsaws, climate change, and rising CO2 concentrations, the project found that tropical forests are also affected by increasing deposition of nitrogen. Tropical forests have the highest biodiversity and the highest biomass in terrestrial ecosystems and play important roles in the global carbon cycle and global and regional climate. While these forests continue to be lost at alarming rates, substantial changes in tree growth, carbon uptake and species composition have also been observed in apparently undisturbed primary forests. Which of these changes are caused by human activities and which factors (changes in climate, atmospheric CO2, nitrogen deposition, hunting ...) are responsible for this, is hotly debated among scientists, but important to understand if we want to predict the future of tropical forests. In temperate forests the annual growth rings in trees have been used to analyse changes in the environment affecting the trees` growth for a long time. Some tropical trees also produce tree rings, and although these are often more difficult to work with, this project set out to make use this archive to study recent changes in tropical trees. As CO2 is essential for photosynthesis, rising concentrations of CO2 in the atmosphere (resulting from fossil fuel burning and the loss of forests) might result in increased plant growth. CO2 is one of the main suspects in explaining the increase in tree growth observed in many tropical forests. Analysing trees from a forest in Thailand, we found that although the increase in CO2 did have a positive effect on trees by increasing water use efficiency (this is the amount of biomass produced per water lost and is seen in the ratio of carbon isotopes in wood), at the same time tree growth rates declined rather than increased. The analysis of oxygen in combination with carbon isotopes suggests that in response to increasing CO2 trees in this relatively dry monsoon forest reduce water loss more than increase growth. In the Thai forest, the decrease in growth over the past decades probably resulted from climate changes resulting in increased drought stress for the trees. An element that is rarely studied in tree rings is nitrogen, whose concentrations in wood are very low. An analysis of nitrogen concentrations and isotopes in wood and leaves from three different forests found a change in the isotopes and an increase in leaf N concentrations. This is the first study showing the effect of increasing deposition of N, originating from agriculture and the burning of fossil fuels, on tropical forests. The nitrogen isotope ratios tell us that the nitrogen cycle, which is relatively closed in most ecosystems, has become more open recently with higher inputs and losses, some of which are very potent greenhouse gases contributing to global warming. In leaves, N is mostly involved in photosynthesis, the basis of plant growth. As the increasing availability of nitrogen results in increased N concentrations in leaves, this may contribute to the observed increase in tree growth in tropical forests.