Plants in tropical dry forests face large variability in water availability due to periodic dry spells and seasonality in precipitations. Two phenological strategies are common in tropical forests to cope with this variability – the drought-deciduous strategy and the evergreen strategy. But which strategy is favored and what will happen when the climate changes? Giulia Vico at SLU and her colleagues address these questions in a paper that was recently published in Environmental Research Letters.
Tropical dry forests have dry periods that can last several months. Some trees and shrubs avoid the lack of water during the driest periods by shedding their leaves. The other phenological strategy is to stay green and endure the lack of water.
The researchers combined a stochastic eco-hydrological framework with key plant economy traits to derive the long-term mean annual net carbon gain of trees exhibiting different phenological strategies. Carbon gain is the carbon that the plants have left after the cost of growing and maintaining the leaves has been accounted for. This carbon is available to the plant for example to grow or form seeds.
Always a cost
Both phenological strategies entails costs. The plants that drop their leaves have to form new ones when the dry period ends and they will miss the opportunity to photosynthesize for a while. The evergreen species on the other hand will lose carbon dioxide through respiration when the water is scarce. They also have thicker leaves that cost more carbon to build up.
Implications for ecosystem functioning
The evergreen strategy emerged as the one leading to higher net carbon gain and more evolutionarily stable communities where wet season was long. Reductions in the length of the wet season or the total rainfall, as predicted under climate change scenarios, should promote a shift towards more drought-deciduous communities, with implications for ecosystem functioning. For example it can influence how much carbon is fixed by the forest. It could also be important for herbivores what kind of leaves are available.
The proposed framework is sufficiently simple to address with a concise parameterization broad-scale questions on tropical dry forest productivity and change. In other words, this work provides a tool for prediction of ecosystem functioning and risk assessment, suitable for areas with limited empirical data to support complex models – as often the case in the tropics.