Water use and carbon exchange of red oak- and eastern hemlock-dominated forests in the northeastern USA: implications for
ecosystem-level effects of hemlock woolly adelgid
Julian L. Hadley (1, 2), Paul S. Kuzeja (1), Michael J. Daley (3), Nathan G. Phillips (3), Thomas Mulcahy (4) and Safina Singh (5)
1. Harvard University, Harvard Forest, 324 N. Main Street, Petersham, MA 01366, USA / 2. Corresponding author () / 3. Department of Geography and Environment, Boston University, 675 Commonwealth Avenue, Boston, MA 02215, USA / 4. The Cadmus Group, 57 Water Street, Watertown, MA 02472, USA / 5. Department of Environmental Engineering, University of Massachusetts, Amherst, MA 01003, USA / Received February 1, 2007; accepted June 9, 2007; published online February 1, 2008
Summary
Water use and carbon exchange of a red oak-dominated (Quercus rubra L.) forest and an eastern hemlock-dominated (Tsuga canadensis L.) forest, each located within the Harvard Forest in north-central Massachusetts, were measured for 2 years by the eddy
flux method. Water use by the red oak forest reached 4 mm day–1, compared to a maximum of 2 mm day–1 by the eastern hemlock forest. Maximal carbon (C) uptake rate was also higher in the red oak forest than in the eastern hemlock
forest (about 25 versus 15 µmol m–2 s–1). Sap flux measurements indicated that transpiration of red oak, and also of black birch (Betula lenta L.), which frequently replaces eastern hemlock killed by hemlock woolly adelgid (Adelges tsugae Annand.), were almost twice that of eastern hemlock.
Despite the difference between species in maximum summertime C assimilation rate, annual C storage of the eastern hemlock
forest almost equaled that of the red oak forest because of net C uptake by eastern hemlock during unusually warm fall and
spring weather, and a near-zero C balance during the winter. Thus, the effect on C storage of replacing eastern hemlock forest
with a forest dominated by deciduous species is unclear. Carbon storage by eastern hemlock forests during fall, winter and
spring is likely to increase in the event of climate warming, although this may be offset by C loss during hotter summers.
Our results indicate that, although forest water use will decrease immediately following eastern hemlock mortality due to
the hemlock woolly adelgid, the replacement of eastern hemlock by deciduous species such as red oak will likely increase summertime
water use over current rates in areas where hemlock is a major forest species.
