Impacts of American beech sapling density on herbaceous plant diversity

Background/Question/Methods

While the impacts of beech bark disease on adult beech trees is well studied, little is known about the indirect impacts of this disease on other plants in the northern hardwood forests, particularly on herbaceous plant diversity. Mechanisms by which beech impacts other species include both the dense shade under the sapling thickets, as well as the increased longevity of beech leaf litter relative to other leaf litter in the forest floor.  While the overstory trees in northern hardwood forests tend to represent just a handful of species, herbaceous diversity is much more varied and typically makes up much of the biodiversity of these systems.  To investigate the effects of beech sapling density on herbaceous plant density and diversity, a removal experiment was conducted at Hubbard Brook Experimental Forest in central New Hampshire, USA.  Twenty 10m x 10m plots were selected for uniformity, and then the understory beech were removed from half of these plots.  All herbaceous plants in the permanently marked center of the plot (5m x 5m) were then identified and counted every two years for 14 years.

Results/Conclusions

There was no difference among plots in terms of initial herbaceous plant density or diversity (P = 0.33; P = 0.40).  Due to the differential light regime that spring ephemeral plants experience, I separated the species that emerge before canopy leaf-out from all other species and analyzed them separately.  The removal of the beech did not appear to affect the spring ephemeral species density (P = 0.37) or diversity (P = 0.89), however, the other species were significantly different by treatment both in terms of density (P = 0.008) and diversity (P = 0.032). Changes in diversity were correlated with both changes in light (as indicated by LAI) and also with changes in forest floor Oi depth. In general, shade tolerant species increased in control plots, while more light-flexible or light-loving species declined, which may indicate that light availability plays the stronger role in determining understory herbaceous biodiversity in this area.   The density of beech saplings in the understory does appear to impact the herbaceous plant diversity, and as the forest is continually recovering and being reinfected from beech bark disease, the impacts may result in a shifting mosaic of patches with reduced diversity on a landscape level.