Long-term development of transition hardwood and Pinus strobus - Quercus mixedwood forests with implications for future adaptation and mitigation potential

Abstract Uncertainty about global environmental change has led to increased emphasis on the climate adaptation and mitigation potential of forests. Given the linkages between adaptive capacity and ecosystem complexity, this increased emphasis has motivated evaluations of the compositional, functional, and structural conditions characterizing a given forest ecosystem in the context of future stressors; however, less is known about how these conditions develop over time or vary between secondary forests shaped by a history of land use. To address this need, we capitalize on a 69-year field experiment to examine the long-term structural and compositional dynamics of transition ecotone hardwood (forests containing species from both southern and northern communities) and Pinus strobus-Quercus mixedwood (stands characterized by hardwood and softwood species mixtures) forests, communities which are commonly found in the Northeastern US. As expected, we observed a general increase in biomass over time and increasing density and structural complexity, with live aboveground biomass greatest in structurally complex mixedwood systems dominated by large diameter P. strobus. While all forests trended toward greater shade tolerance with increasing stand age, the functional identities of hardwood stands were stable over time compared to mixedwoods, which were more transient and ultimately generated trait profiles resembling hardwood-dominated stands. Ingrowth on all sites favored shade-tolerant Fagus grandifolia and Tsuga canadensis, which have lower future climate compatibility and adaptability compared to overstory trees, with a noticeable absence of P. strobus and Quercus spp. regeneration. Although all forest types exhibited some conditions that foster adaptation potential, the compositional stability of hardwood-dominated systems highlight the capacity of these stands to maintain comparable levels of adaptive capacity into the future. Conversely, given that P. strobus-Quercus mixedwoods examined are largely an artifact of land use, the natural successional patterns of these forests may lead to a reduction in these mixed species communities and a depreciation of associated climate benefits without silvicultural intervention to favor recruitment of these constituent species.