PARENT SESSION
Contributed Oral Session 140: Forest Photosynthesis and Water Relations
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 520 B, Level 5, Palais des congrès de Montréal

Douglas-fir vs. western hemlock: Is hydraulic architecture a constraint on release from suppression?

Renninger, Heidi^*,1, Gartner, Barbara ¹, Meinzer, Frederick^{1, 2}, ¹ College of Forestry, Corvallis, Or, USA² USDA Forest Service, Corvallis, Or, USA

ABSTRACT- As forest management patterns in Oregon shift from even- to uneven-aged systems, it becomes increasingly important to understand how understory trees will react to sudden changes in the growing environment when the upper canopy is removed, releasing trees from suppression. We compared hydraulic architecture and growth of Douglas-fir (Pseudotsuga menziesii), a shade intolerant species, with that of western hemlock (Tsuga heterophylla), a shade tolerant species, to determine if hydraulic architecture limits the release of Douglas-fir. The study was conducted in two sites, both containing an overstory of Douglas-fir and an understory of 10-20-year old Douglas-fir and western hemlock individuals. In 1998 one site was thinned to release suppressed trees whereas the other site remained unthinned and contained trees that were still suppressed. In 2004, we studied 10 suppressed and 10 released trees of each species in each site. We determined relative height growth (height growth in five years/initial height), sapwood specific conductivity (ks) of the basal sapwood, leaf area, and sapwood area, then calculated leaf-specific conductivity (kl). Relative height growth of released Douglas-fir was 130% greater than suppressed Douglas-fir, whereas relative height growth of released western hemlock was only 65% greater than suppressed western hemlock (1.25 m m^-1 vs 0.54, and 1.79 vs 1.08, respectively). However released Douglas-fir had 22% lower kl than suppressed Douglas-fir, whereas the kl of released and suppressed western hemlock were not significantly different (1.16 x 10^-4 kg m^-1 s^-1 MPa^-1 vs. 1.48 x 10^-4, and 1.09 x 10^-4 vs 1.06 x 10^-4, respectively). Therefore, hydraulic architecture of suppressed Douglas-fir does not appear to limit its ability to release from suppression, but may become a limiting factor after release.

Key words: hydraulic architecture, release from suppression, Douglas-fir, western hemlock