PS 43-153 - Tree seedling growth rates are mostly unresponsive to temperate-boreal climate and canopy openness gradients in northeastern United States

Abstract: While we know that the structure, function, and distribution of forest ecosystems will continue to shift in response to climate change, the mechanisms for these shifts remain largely untested. The boreal-temperate forest ecotone, particularly at the ground layer, is especially sensitive to climate and may display responses earliest because the adjacent forest types can shift to alternate stable states based on changing climate, disturbance regimes, or land use. Canopy gaps are a key disturbance in both temperate and boreal forests, and could facilitate upward expansion of temperate forests as a result of increased light availability and temperature within gaps. However, it is not clear whether such shifts are related to growth differences in tree seedling stage or other demographic processes.

The objectives of this study were to determine whether tree seedling growth rates can reveal varying responses to elevation (and therefore climate) among two temperate and two boreal species that comprise the boreal-temperate forest community, and whether canopy gaps interactively affect these growth rate responses across the elevational gradient.

Our approach to addressing these objectives was to measure tree seedling growth rates across elevational transects on 10 study mountains across the Northeastern US. Abies balsamea, Picea rubens, Acer saccharum, and Fagus grandifolia were selected based on relative dominance and abundance. We measured height, diameter, and approximated ages using annual ring counts to determine growth rates. We used mixed-effect linear models to analyze the interaction between elevation and canopy gaps by comparing annual growth rates in understory and gap environments along the elevational gradient.

Of the four species examined, only A. balsaminea growth rate was significantly affected by elevation, as height growth rate increased with elevation (p < 0.005), possibly reflecting a species-specific temperature optima at higher elevations. This species’ height growth also experienced an interactive effect between elevation and light environment (p < 0.05). Surprisingly, none of the other three study species experienced significant main or interactive effects of elevation and light environment on growth rates.

Our results suggested tree seedling growth rates were not likely the primary mechanism of species range shifts along climate gradients, and gaps did not appear to facilitate higher growth rates for most species. Rather, effects of elevation and gaps may play out on other demographic processes, such as seedling recruitment, establishment, and survival, and may be affected by competitive feedbacks such as shading from other ground layer vegetation.