Latitudinal patterns in stabilizing density dependence of forest communities
Abstract
Numerous studies have shown reduced performance in plants that are surrounded by neighbours of the same species, a phenomenon known as conspecific negative density dependence (CNDD). A long-held ecological hypothesis posits that CNDD is more pronounced in tropical than in temperate forests, which increases community stabilization, species coexistence and the diversity of local tree species. Previous analyses supporting such a latitudinal gradient in CNDD have suffered from methodological limitations related to the use of static data. Here we present a comprehensive assessment of latitudinal CNDD patterns using dynamic mortality data to estimate species-site-specific CNDD across 23 sites. Averaged across species, we found that stabilizing CNDD was present at all except one site, but that average stabilizing CNDD was not stronger toward the tropics. However, in tropical tree communities, rare and intermediate abundant species experienced stronger stabilizing CNDD than did common species. This pattern was absent in temperate forests, which suggests that CNDD influences species abundances more strongly in tropical forests than it does in temperate ones. We also found that interspecific variation in CNDD, which might attenuate its stabilizing effect on species diversity, was high but not significantly different across latitudes. Although the consequences of these patterns for latitudinal diversity gradients are difficult to evaluate, we speculate that a more effective regulation of population abundances could translate into greater stabilization of tropical tree communities and thus contribute to the high local diversity of tropical forests.
What the paper shows and why it matters
(AI-generated)
The Janzen–Connell hypothesis seeks to explain the high diversity of tropical forests by conspecific negative density dependence (CNDD): trees do worse when many neighbours of their own species stand nearby, because specialised enemies such as herbivores and pathogens spread among such neighbours. This self-limitation should keep any one species from dominating and so let many coexist, and the hypothesis predicts that it is stronger towards the tropics. Seed and seedling experiments, which may be hard to compare across studies, found no trend with latitude. Large-scale studies of static forest inventories – single snapshots of which trees stand where – such as LaManna et al. (2017) reported a strong one, but later work attributed that pattern to statistical artefacts of such data. The problem the paper takes on is whether the average strength of CNDD really increases from temperate to tropical forests.
The study tests whether trees suffer more from neighbours of their own species in tropical than in temperate forests, using repeated censuses at 23 forest sites from the tropics to the temperate zone. It follows individual small trees from one census to the next and relates their chance of dying to the density of conspecific neighbours, a design that avoids the artefacts of static data. CNDD was present at all sites but one, yet on average it was not stronger towards the tropics, and it varied strongly between species at every latitude. Only one pattern changed with latitude: in tropical forests rare species suffered stronger CNDD than common ones, whereas temperate forests showed no such link. LaManna et al. (2017) had reported this pattern from static data; the study provides stronger evidence for it, from dynamic data that avoid the main artefacts of static analyses.
The results shift the question from how strong CNDD is to what it does. If CNDD hits rare species hardest in the tropics, it may hold species abundances in check more effectively there, even though its average strength is similar across latitudes. The paper reads this as a refined and only partly supportive version of the Janzen–Connell idea, and it states that the causal link between CNDD and rarity remains to be established.
References
- LaManna et al. (2017). Plant diversity increases with the strength of negative density dependence at the global scale. Science 356, 1389–1392. doi:10.1126/science.aam5678