Shade tolerance influences the degree of intraspecific trait coordination and variation in temperate tree seedlings

Background/Question/Methods

Intraspecific coordination and variation in traits determines how a species can respond to its environment and is therefore important to understand species response to climate change. Shade tolerance is a life history strategy that varies between species and could potentially influence the degree of intraspecific variation and coordination in species because more shade tolerant species are expected to experience a wider array of environmental conditions throughout their lifespan – i.e., full shade as a seedling, partial shade as a sapling, and full sun as an adult - than are shade-intolerant species, which can only survive throughout their life in full sun. We investigated this hypothesis using four tree species commonly found throughout northeastern temperate forests that represent a spectrum of shade tolerance: Prunus serotina (shade intolerant), Quercus rubra (moderately shade tolerant), Acer rubrum (moderate shade tolerant), and A. saccharum (shade tolerant). We collected seedlings across eight sites in northern and southern Michigan, USA and measured two aboveground (specific leaf area and leaf N) and four belowground (specific root length, root tissue density, root N, and root diameter) traits to answer the question: does shade tolerance influence the degree to which a species above- and belowground traits coordinate and vary intraspecifically?

Results/Conclusions

Trait coordination at the intraspecific scale varied by degree of shade tolerance, with all moderately shade tolerant (A. rubrum and Q. rubra) and shade intolerant (P. serotina) species displaying higher levels of coordination between above- and belowground traits than the shade tolerant species (A. saccharum). For aboveground traits, variation in specific leaf area was significantly lower for the shade intolerant species compared to the other species, while leaf nitrogen content variation was highest in the shade intolerant species. Belowground, only variation in root diameter differed between groups with the shade intolerant species having significantly higher variation than all other species. Our results indicate that the shade tolerance of a species influences the coordination and variation of above- and belowground traits in seedlings. Specifically, coordination increases as shade tolerance decreases potentially indicating less phenotypic plasticity and adaptability of more shade intolerant species to shifts in the environment due to climate change.