When does emerald ash borer (Agrilus planipennis) cause changes in light regimes and shifts in species composition?

Background/Question/Methods

Emerald ash borer (EAB) (Agrilus planipennis), an introduced insect pest, has killed millions of ash trees in the Midwest and is spreading rapidly.  The effects of EAB on forest ecosystems are being studied through a collaborative research program between the US Forest Service and the Ohio State University.  We are monitoring the decline and mortality of >4500 ash trees and saplings, as well as changes in understory light availability, the responses of both native and non-native plant species, changes in species composition and forest structure, and effects on other organisms and ecosystem processes in over 250 monitoring plots in forests in Ohio and Michigan, representing a gradient of EAB infestation duration.  Yearly monitoring began in 2004 and is continuing.  The plots are located in forest stands representing different ages and habitat types to include all five ash tree species native to the region: pumpkin ash (Fraxinus profunda) and black ash (Fraxinus nigra) are found in swamp ecosystems, green ash (Fraxinus pennsylvanica) is typical of floodplains, white ash (Fraxinus americana) inhabits upland forests, and blue ash (Fraxinus quadrangulata) has an affinity for calcium-rich areas.

Results/Conclusions

Our data and modeling shows that ash stands may progress from healthy to nearly 100% mortality within a period of five to six years.  In ash-dominated forests with few midstory trees of other species, understory light increases gradually over this time period.  In forests where ash is mixed with other species, or where a well-developed midstory of other tree species exists, the canopy gaps formed by dying ash trees are rapidly filled as the other trees grow, and sometimes understory light remains relatively low and constant during ash mortality.  Once the dead ash trees begin to fall, knocking down smaller trees, gaps may open.  Increases in understory light affects both native and invasive plants in these ecosystems, and species composition may shift toward species poised to take advantage of light regime changes.  We have identified 14 species of invasive plants in the monitoring plots, with at least one invasive plant species present in most plots.  Initial cover of invasive species was low in most plots, and individual invasive species distributions were associated with habitat, geography, and land use history.