A pertinent question in ecology is how forests respond to widespread canopy tree mortality. Gaps that are created when canopy-sized trees die may have dramatic and long-term effects on the composition and structure of the understory community, potentially facilitating the establishment and spread of invasive species, however there is limited information available about this. More detailed information on how native and invasive plants respond to canopy tree mortality is critical for developing effective land management strategies. From 2008 through 2011 we have monitored native and invasive woody plant species in the understory of 129 plots in southeastern Michigan. Changes in abundance and diversity were calculated for plants ranging from new seedlings to established saplings. We also measured percent canopy gap fraction at 5 to 20 points per plot using hemispherical photography. In addition, individual plants were monitored to determine absolute growth and relative growth rates in response to canopy gap fraction.
Results/Conclusions
Abundance of new invasive seedlings was variable among species and year. The most common invasive species were Celastrus orbiculatus, Rhamnus cathartica, and Frangula alnus, however relative abundance was related to soil hydrological condition (xeric, mesic, or hydric). Diversity changed slightly over time with loss of Fraxinus species and gain of other native and some invasive species. Gap fraction was highly variable within and among plots. Minimum variation within a plot was 0.42% and maximum within-plot variation was 19.4%; average variation among plots ranged from 2% to 30%. Median gap fraction decreased slightly from 2008 to 2011, suggesting that the gaps were beginning to close, but the response was variable among plots. Growth of native and invasive species were not related to canopy gap fraction, however invasive species generally had higher relative growth rates than native species.