The role of ndividual tree nutrient demand and tree functional types in tropical rainforest nutrient limitation

Background/Question/Methods

Lowland tropical rainforests are one of the most productive ecosystems on the planet, but their future as carbon sinks during times of global climate change is partly uncertain due to an incomplete understanding of soil nutrient use by tropical trees.  We use the perspective of individual trees and species specific traits to re-approach the question of lowland tropical rainforest nutrient limitation that is predominantly studied from the ecosystem level with inconsistent results.  We focus on how temporary peaks in demand for nutrients during tree height growth and competition for light can lead to nutrient shortages in a seemingly nutrient rich ecosystem, especially in terms of nitrogen.  This study focuses on nine tree species of different functional types at La Selva Biological Station in Costa Rica.  Growth and nutrient demand of saplings were measured with varying light levels of forest gap and closed canopy environments and a nitrogen, phosphorus, potassium, and micronutrient fertilization experiment.  We studied sapling sized individuals because they are engaged in a crucial competition for light as they grow in height to establish a spot in the canopy for reproduction. 

Results/Conclusions

Using a combination of statistical approaches including maximum likelihood methods, we have found a weak but distinguishable fertilization effect with evidence that it differs across species and functional types.  Of particular concern was whether the strength of this response was influenced by either light or by the interaction of light and species or growth characteristics.  We will discuss evidence both for and against such effects in our data.  This study emphasizes the importance of nutrient demand during gap phase succession on species evolutionary success, forest composition, and ultimately the role of lowland tropical rainforests in the global carbon cycle.