OOS 30-6 - Insights into drivers of tree species coexistence from Harvard Forest

Understanding the drivers of species coexistence is integral to advancing knowledge on how biodiversity may respond to climatic change. In forests, conspecific negative density dependence (CNDD) and niche differentiation are two mechanisms that can promote coexistence. CNDD occurs when proximity to conspecific individuals decreases performance, whereas niche differentiation promotes species coexistence via non-random sorting of species across resource gradients. The Harvard Forest (HF) in Petersham, MA is uniquely situated to provide insights into drivers of forest biodiversity because of its position at the nexus of a network of ecological observatory networks (i.e., Long Term Ecological Research [LTER] Network, National Ecological Observatory Network [NEON], Smithsonian ForestGEO Network, Ameriflux). Here we discuss insights into these two mechanisms of species coexistence that leverages the rich data streams from these co-located networks.

The Smithsonian ForestGEO plot at HF provides long-term observations to monitor trends in temperate hardwood forests to pair with manipulative experiments that can tease apart mechanisms behind those trends. For instance, observations from the HF ForestGEO plot found evidence of significant, negative CNDD for saplings in 23 of 28 species, and positive CNDD in two species. Seedling survival in a field experiment adjacent to the ForestGEO plot was also lower with increasing CNDD for three species. Conspecific adult trees tended to have a negative influence on seedlings and saplings with traits, such as ectomycorrhizal-associated species and more shade-tolerant species. This work provides evidence for CNDD linked to woody species with specific life histories and traits.

The temperate Smithsonian ForestGEO plots also provide rich data for exploring tradeoffs in life histories of woody species allowing for niche differentiation. There is strong evidence for life history tradeoffs in plants along a slow-fast continuum linked to leaf traits with the leaf economics spectrum. Recent work in tropical forests suggests there may also be an orthogonal life history axis tied to plant stature and reproductive output. Using the rich observational data from the ForestGEO network, we explore whether there is evidence for these two axes of life history variation in temperate forests. In conclusion, we consider how a synthesis of CNDD and niche differentiation frameworks integrating life history, traits, and remote sensing data from NEON and the National Aeronautics and Space Administration may enable scaling of this site-specific work to landscapes.