Incorporating crown densities improves predictions of species’ growth strategies

Background/Question/Methods

Species differences in their potential growth rates are an important dimension of their functional niche space. Fast-growing species occupy high light environments, dominating gaps created after disturbance events, while slow-growing species persist in low light environments. These dynamics promote species coexistence in heterogenous light environments and are particularly important for earlier ontogenetic stages. Previous functional trait analyses have shown that fast-growing species have lower wood densities (WD), indicating lower construction costs, but that species’ leaf photosynthetic capacities, theoretically a critical component of plant growth, were poor predictors of growth.This weak relationship might be improved by incorporating the allocation context of leaves. Two species with different photosynthetic capacities may achieve similar net growth rates if one allocates more biomass to leaves. In this study, we explore the influence of leaf area index (LAI, a measure of crown densities) on plant growth. Additionally, we explore how incorporating multiple dimensions of plant traits (leaf photosynthetic capacity, leaf allocation, and construction costs) affects our ability to explain the variability in sapling growth rates for a tree community in Michigan. We hypothesize that LAI positively correlates with growth, and that incorporating multiple trait dimensions improves our prediction of species potential growth rates.

Results/Conclusions

Incorporating LAI significantly improved our predictions of species growth rates, and models incorporating multiple dimensions of plant traits performed better. None of the trait dimensions on their own were significant predictors of growth. Instead, models incorporating leaf photosynthetic capacity, LAI, and WD performed better than those using only leaf photosynthetic capacity and WD. Additionally, LAI had higher interspecific variation than WD and other leaf traits, and was independent of plant size.Our models show that species with denser crowns, lighter wood, and more conservative leaves (higher leaf mass per area, LMA, and lower leaf nitrogen content, Nmass) had higher potential growth rates. Crown and wood densities correlated with growth as expected, but relationships with leaf photosynthetic capacities were counterintuitive. It is possible that slow-growing species in this winter deciduous forest may invest in higher photosynthetic leaves to maximize carbon gain within each growing season before dropping their leaves, while fast-growing species mitigate desiccation risk in high light environments by building thicker leaves. Despite some counterintuitive trends, overall, our results suggest that leaf allocation context is an important but often unexamined component of species’ growth strategies.