Forest mesification: The demographic causes and consequences of maples’ rise to dominance in temperate forests

Background/Question/Methods

The abundance of maple species is increasing across the Eastern United States, presumably due to shifts in fire frequency and disturbance regimes. Increasing maple dominance has been linked to higher soil moisture and organic matter and changing species composition. Climate change models predict increased temperatures and decreased precipitation during the growing season for in this region, which may further impact species composition.

In this study, we integrate data from multiple forest types and tree life stages to answer the following questions: 1)What biotic and abiotic factors lead to maple dominance in the forest sub-canopy? 2) What are the effects of maple dominance on other tree species? 3) How will climate change impact maple demography and forest mesfication in northern forests?

Starting in 2008, we established eight research sites in a variety of forest types in Southern and Northern Michigan. We collected data on seed rain, first year seedling recruitment, second year seedling survival, canopy tree basal area, and abiotic variabiles at each site. Seed abundance and seedling recruitment were analyzed to determine variability of natural recruitment rates among sites and species. Species richness and abundance of second year seedlings were analyzed to determine the effects of maple presence on diversity. 

Results/Conclusions

In total, we counted 15,982 seeds, 1,932 first year seedlings, and 5,712 second year seedlings.  At many of our sites, red maple (Acer rubrum) and sugar maple (Acer saccharum) were the most commonly observed species in the subcanopy, the seedling layer, and as the portion of seed rain. Seedling recruitment was highest in six native species (Acer rubrum, Acer saccharum, Carya glabra, Fagus grandifolia, Prunus serotina, Tsuga canadensis). However, we found corresponding lower species richness of non-maple seedlings in areas with high maple basal area and recruitment. 

During the summer of 2012, there was an extensive drought, which impacted soil moisture at many of our sites.  This drought negatively impacted seed production and rates of seedling recruitment. The largest decrease in seedling recruitment following the drought period of 2012 was found to be in Acer rubrum.  If the frequency of extreme weather droughts increases, as is predicted for this region, it has the potential to reduce the demographic advantages which maples experience at younger life stages and halt forest mesification.