Abstract
Abstract 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.
Highlights
Tropical rainforests are responsible for over 40% of global terrestrial photosynthesis (Beer, Reichstein, Tomelleri, & Ciais, 2010) and a large fraction (20%–50%) of this productivity is invested in production and maintenance of the forest canopy (Malhi, Doughty, Goldsmith, & Metcalfe, 2015)
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
Leaf and reproductive litterfall are highly seasonal within tropical humid forests and likely to be strongly influenced by intra- and inter-annual variability in climate, potentially leading to high variance over the short term (Chave et al, 2010; Girardin, Malhi, Doughty, & Metcalfe, 2016; Wagner et al, 2016)
Summary
Tropical rainforests are responsible for over 40% of global terrestrial photosynthesis (Beer, Reichstein, Tomelleri, & Ciais, 2010) and a large fraction (20%–50%) of this productivity is invested in production and maintenance of the forest canopy (Malhi, Doughty, Goldsmith, & Metcalfe, 2015). We separate the litterfall into leaves, reproductive parts and twigs, and use a long- running (14 years) large-scale (1 ha) soil moisture reduction experiment to contrast unmodified natural forest with long-term drought-stressed forest Using these data, we examine: (1) the response to the initial (i.e. short-term) effect of a 50% reduction in canopy throughfall; (2) how the imposed soil moisture deficit alters long-term relationships among canopy production and seasonal and inter-annual growth and climate variables; and (3) whether current evidence indicates a long- term shift characterised by the forest re-stabilising a new functional state or whether there is a continuous change in litterfall following 15 years of soil drought stress
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