COS 229-3 - Adaptive Silviculture for Climate Change: Physiological Performance of Future-Adapted Seedlings to Drought

Background/Question/Methods

Climate regimes are changing at a faster rate than natural tree migration has historically been able to track—generating significant lags in natural migration of tree species. These shifts are expected to lead to vulnerabilities in climate adaptability and potential losses in key ecosystem functions. To mitigate these challenges, the Adaptive Silviculture for Climate Change network (ASCC)—an internationally co-produced network informed by local and regional expertise—operationalized varying silviculture adaptation strategies at Dartmouth College’s Second College Grant in northern NH, USA. Among these strategies are treatments designed to evaluate the efficacy of transitioning forest composition using assisted migration. For our study, we assessed the physiological performance of future-adapted species used in assisted migration, both those predicted to experience a population expansion or range expansion, to determine the filtering effects of extreme events on their establishment in novel environments. We measured seedlings planted in 2016 in one-acre gaps—specifically Betula lenta (black birch), Castanea dentata (American chestnut), and Quercus rubra (red oak)—to monitor gas exchange (A_max) and water potential (∆Ψ=Ψ_pd- Ψ_md MPA) to determine species limitations and adaptive capacity throughout the growing season (June-August) and during a moderate severity drought.

Results/Conclusions

During the sampling period, a severe drought occurred during July followed by a moderate severity drought in August. In response to these precipitation declines in July and August, B. lenta—an isohydric species selected for range expansion at the site—significantly reduced A_max (p = 0.002). Additionally, both B. lenta and C. dentata had significantly lower A_max compared to the anisohydric species—Q. rubra—during these periods of drought in July and August. Q. rubra experienced its highest photosynthetic rates during a moderate severity drought in August (13.2 μmol m⁻² sec⁻¹). C. dentata displayed moderately anisohydric behavior in its response throughout the growing season, with A_max consistently falling in the range between 6.1 μmol m⁻² sec⁻¹ and 7.6 μmol m⁻² sec⁻¹ during the sampling period. Although there were no significant intraspecies differences in ∆Ψ(Ψ_pd- Ψ_md MPA), ∆Ψ was highest during periods of moderate to severe drought for all species. From these results, drought may be an important filter in determining which assisted migration species survive over time in areas where adaptive silviculture is applied, with B. lenta being more sensitive to periods of reduced precipitation compared to species such as C. dentata and Q. rubra.