Shock and stabilisation following long-term drought in tropical forest from 15 years of litterfall dynamics

Litterfall dynamics in tropical forests are a good indicator of overall tropical forest function, indicative of carbon invested in both photosynthesising tissues and reproductive organs such as flowers and fruits. These dynamics are sensitive to changes in climate, such as drought, but little is known about the long-term responses of tropical forest litterfall dynamics to extended drought stress. We present a 15-year dataset of litterfall (leaf, flower and fruit, and twigs) from the world's only long-running drought experiment in tropical forest. This dataset comprises one of the longest published litterfall time series in natural forest, which allows the long-term effects of drought on forest reproduction and canopy investment to be explored. Over the first 4 years of the experiment, the experimental soil moisture deficit created only a small decline in total litterfall and leaf fall (12% and 13%, respectively), but a very strong initial decline in reproductive litterfall (flowers and fruits) of 54%. This loss of flowering and fruiting was accompanied by a de-coupling of all litterfall patterns from seasonal climate variables. However, following >10 years of the experimental drought, flower and fruiting re-stabilised at levels greater than in the control plot, despite high tree mortality in the drought plot. Litterfall relationships with atmospheric drivers were re-established alongside a strong new apparent trade-off between litterfall and tree growth. Synthesis. We demonstrate that this tropical forest went through an initial shock response during the first 4 years of intense drought, where reproductive effort was arrested and seasonal litterfall patterns were lost. However, following >10 years of experimental drought, this system appears to be re-stabilising at a new functional state where reproduction is substantially elevated on a per tree basis; and there is a new strong trade-off between investment in canopy production and wood production.