COS 51-4 - CANCELLED - Long-term impacts of hemlock woolly adelgid progression on ecosystem processes at Harvard Forest

Abstract: Disturbances caused by forest insect pests are widespread and potentially interact with climate change to shift ecosystem processes. However, studies that have characterized long-term ecosystem-scale responses with multiple years of pre-disturbance data are rare. Within a tract of forest dominated by eastern hemlock (Tsuga canadensis) at the Harvard Forest in Petersham, MA, long-term measurements since 2004 have captured shifts in ecosystem processes before, during, and after invasion by the hemlock woolly adelgid (HWA; Adelges tsugae). Hemlock woolly adelgid arrived in 2009 and began to create persistent stress in 2013, permitting comparison across time periods of pre-disturbance (2004-2009), initial invasion (2010-2015), and long-term changes and recovery in ecosystem processes with ongoing HWA presence (2016-2022). One of the most important functional shifts within the decline of hemlock due to HWA is the conversion from an evergreen-dominated forest to an increasingly deciduous forest. This shift in forest composition impacts ecosystem dynamics during the growing season due to the slower physiological rates of hemlock in comparison with deciduous trees. It also has a strong potential to shift the seasonality of ecosystem carbon, water, and energy flux with the replacement of “low and slow” hemlock ecosystem rates with faster and more seasonally punctuated early successional deciduous dynamics.

Since 2004, measurements have captured ecosystem-scale carbon dioxide and water flux with eddy covariance towers along with measurements of leaf area index and litterfall, subcanopy microclimate, soil respiration, tree growth rates and mortality in hemlock and a nearby red oak (Quercus rubra) site. At the hemlock site there has been a long-term decline in spring hemlock leaf area index (-0.178 m2/m2 per year, 2012-2021) and an increase in the fraction of incoming photosynthetically active radiation (fPAR) reaching the understory (0.00831 per year, 2009-2021). Before HWA, the subcanopy at the hemlock site was cooler (in reference to the above-canopy temperature) in both spring and summer than the red oak site. But since 2012 the hemlock subcanopy has become consistently warmer in spring, and the hemlock subcanopy summer temperature has become warmer than that of the red oak site. Documented shifts in ecosystem structure and composition due to long-term HWA stress suggest a concurrent shift in energy, carbon and water fluxes at the ecosystem scale.