Same-species neighbours press harder on growth as forests mature
Tree growth became more sensitive to same-species neighbours as Canadian post-fire forests matured in a peer-reviewed observational study. Recruitment and survival followed different patterns. Measurements from permanent plots distinguish these processes, helping describe how forest diversity changes without establishing which biological mechanism underlies the observed neighbour effects.
Science··Morning
Growth changed as post-fire stands matured
Same-species neighbours became relatively more suppressive for tree growth as post-fire forests matured in Manitoba, Canada. The transition appeared after roughly 60 years of stand development in a single peer-reviewed observational study. Younger stands showed comparatively weaker suppression from neighbours of the same species. Recruitment and survival did not show an equivalent age trend, separating the growth result from the other processes through which forests change.[1]
Permanent plots followed trees across 25 years
The observations spanned 25 years, following spatial changes within 148 permanent forest plots. Each plot covered 500 square metres and contained at least two tree species. The selected stands had regenerated naturally after fire without later management intervention. At five-year censuses, the team noted growth, deaths and new stems reaching the size threshold, recording changes in the same forest areas over successive visits.[1]
Neighbour comparisons accounted for their size and distance as well as whether they belonged to the same species. A stronger negative effect from same-species neighbours is called conspecific negative density dependence. New recruits entered the measured population on reaching the height threshold of 1.5 metres; they were already beyond initial seedling emergence. To limit edge effects, the team selected plot locations distant from open areas, defining the kinds of forest structure sampled.[1]
Rare and common species showed different pressures
Abundance had a more consistent relationship with density effects than the traits tested. For rare species, same-species crowding weighed more heavily on growth and recruitment. The survival effect was stronger among common species. Local species richness increased with stand age. These differences distinguish a species’ response during growth from its chances of surviving or joining the measured tree population, even within the same forest.[1]
The observational design leaves several possible mechanisms unresolved, including competition for resources and pressure from natural enemies, alongside other biological feedbacks. They concern a boreal system with relatively few species, rather than establishing the same relationships in tropical forests. Aggregated analytical data and code were shared, while the raw inventory data remain restricted by third-party ownership. The available measurements describe process-specific patterns during naturally recovering forest succession.[1]
