Integrated demographic strategies are more strongly associated with variation in conspecific density dependence than single traits in tropical tree seedlings

Conspecific negative density dependence (CNDD) is considered a key
mechanism shaping species diversity in plant communities. However, species
vary widely in CNDD strength, and the main ecological drivers of this
variation remain unclear. We tested whether interspecific variation in
CNDD is linked to (i) species’ demographic strategies related to
growth–survival and stature–recruitment trade-offs, (ii) functional
traits, including wood density, seed mass, maximum height, and four leaf
traits, and (iii) species’ relative abundance. Using 18 years of seedling
mortality data (145,768 individuals, 260 tree species) from a tropical
moist forest in Panama and a robust modeling framework that accounts for
potential biases due to non-linearities and variation in baseline
mortality, we calculated species-specific estimates of  CNDD that
quantify the effect of conspecific relative to heterospecific neighbors
and tested for relationships between CNDD strength and demographic
strategies, functional traits, and relative abundance. CNDD strength
varied widely across species and was significantly related to species
location along both the growth-survival and stature-recruitment trade-off
axes. Fast-growing species and tall, long-lived pioneers exhibited
stronger sensitivity to CNDD compared to slow-growing and short-lived
species, respectively. With the exception of wood density, single
functional traits were not significantly associated with CNDD strength.
Rare species experienced stronger CNDD than common species. These findings
underscore the importance of life-history strategies over isolated traits
in shaping density-dependent effects, suggesting that CNDD variation could
reinforce niche differences and influence community composition and
species coexistence in tropical forests.   Additionally,
our results confirm that rare species experience stronger CNDD than common
species at the seedling stage, suggesting that CNDD may constrain species
abundances and shape tropical tree diversity. Our results show that, with
the exception of wood density, single traits were weak predictors of CNDD
strength, while integrated demographic strategies captured meaningful
variation. CNDD is strongest in rare, fast-growing, and long-lived pioneer
species, suggesting that biotic interactions at early life stages align
with key life-history trade-offs. Studies that assess and integrate links
between CNDD and demographic trade-offs across multiple life stages are
needed to understand the combined impact of these mechanisms on tree
communities.