1. Tropical forest mortality is controlled by biotic and abiotic processes, but how these processes interact to determine forest structure is not well understood. Using long-term demography data from permanent forest plots at the Paracou Tropical Forest Research Station in French Guiana, we analyzed the relative influence of competition and climate on tree mortality. We found that self-thinning is evident at the stand level, and is associated with clumped mortality at smaller scales (< 2 m) and regular spacing of living trees at intermediate (2.5-7.5 m) scales. A competition index based on spatial clustering of dead trees was used to build predictive mortality models, which also accounted for climate interactions.
2. The model that most closely fitted observations included both the competition index and climatic variables, with climate-only and competition-only models performing worse than the full model. There was strong evidence for size-specific mortality, with highest mortality for small and very large trees, as well as sensitivity of trees to drought, especially when temperatures were high, and when soils were water saturated. The effect of the competition index was more complex than expected a priori: a higher CI index was associated with lower mortality odds, which we hypothesize is caused by gap-phase dynamics, but there was also evidence for competition-induced mortality at very high index values.
3. The strong signature of competition as a control over mortality at the stand and individual scales confirms that it plays a very important role in determining tropical forest structure. The complexity of the competition-mortality relationship and its interaction with climate indicates that a thorough consideration of the scale of analysis is needed when inferring the role of competition in tropical forests, but demonstrates that climate-only mortality models can be significantly improved by including competition effects, even when ignoring species-specific effects.
4. Synthesis Empirical models such as the one developed here can help constrain and improve process-based vegetation models, serving both as a benchmark and as a means to disentangle mortality processes. Tropical ecosystem dynamic models would benefit greatly from explicitly considering the role of competition in stand development and self-thinning while modeling demography, as well as its interaction with climate.
Data for self-thinning analysis
Contains data for mean aboveground biomass per individual and tree density for each plot.
Data for spatial analysis
Contains geographic coordinates, year, and plot code for each tree, as well as a status code and whether the tree is dead or alive.
Data for mortality model
Contains data for each individual tree used for mortality modeling: whether the tree is dead/alive, its DBH, the temperature and precipitation in the preceding wet and dry seasons, and the Competition Index using radii of 5 and 10 meters.
Funding provided by: National Science Foundation Crossref Funder Registry ID: http://dx.doi.org/10.13039/100000001 Award Number: DGE-1143953