Quercus rubra (northern red oak) is predicted to migrate into northern hardwood U.S. forests with climate change, and its spread is potentially tied to shifting seasonality and canopy gaps created by invasive pests, such as the hemlock woolly adelgid. Previous studies have identified light as a crucial variable for the survival of Q. rubra seedlings, while other studies have found nutrient availability, local competition, negative density dependence, or initial seedling size to be important variables. Quantifying the survival of Q. rubra at the northern edge of its range offers a window into the future makeup of the forest.
The objective of this study was to measure the influence of environmental factors on survival of Q. rubra seedlings and to compare rates of Q. rubra survival at the center and northern edge of its range. At the Hubbard Brook Experimental Forest in Woodstock, NH we measured surrounding shrub cover, mature tree community, and substrate as well as light availability during summers of 2019 and 2020. The environmental variables influence microsite conditions, while seedling location within the valley provides inference into the distribution dynamics of seed dispersal. We combined these new data with long-term seedling survival monitoring that began in 2011.
Results/Conclusions
We found higher rates of Q. rubra seedling survival in the understory at Hubbard Brook (5-year survival rate ~55%) compared to similar studies from the Harvard Forest in central Massachusetts at the center of the Q. rubra range (5-year survival rate ~20%). Within plots at Hubbard Brook, we found higher survival of seedlings further west into the valley which could be explained by the greater distance from their parent trees and/or larger acorns being dispersed longer distances by animals. Despite the consistent low light available in the understory of hemlock dominated areas (3-8% canopy opening), we found substantial rates of Q. rubra seedling survival. Additional research in the summer 2020 field season will quantify the relationship between more variable microsite conditions in hardwood-dominated forest stands and rates of Q. rubra seedling survival for comparison. We will also assess the role of stand density and the understory shrub community on Q. rubra survival at Hubbard Brook. This study provides important context for understanding Q. rubra seedling survival in relation to both local environmental factors and overall light availability at the northern edge of its species range.