97th ESA Annual Meeting (August 5 -- 10, 2012)

Wednesday, August 8, 2012

Exhibit Hall, Oregon Convention Center

Katherine A. McCulloh, Botany, The University of Wisconsin-Madison, Madison, WI, Daniel M. Johnson, Nicholas School of the Environment, Duke University, Durham, NC, Joshua P. Petitmermet, Forest Ecosystems and Society, Oregon State University, Corvallis, OR, Brandon E. McNellis, Biology Department, University of California, Riverside, Riverside, CA, Frederick C. Meinzer, Pacific Northwest Research Station, USDA Forest Service, Corvallis, OR and Barbara Lachenbruch, Oregon State University, Department of Forest Ecosystems & Society, Corvallis, OR

Background/Question/Methods

What makes a shrub a shrub? While this question has answers from many perspectives, including evolutionary, ecological, and natural history views, the mechanistic processes that lead shrubs to stop growing below the main canopy in a forest are not clear. Although research suggests that the maximum height of trees is associated with a decline in hydraulic efficiency in the top branches, it is unknown if a similar trend is present in shrubs. To test this possibility, we measured various parameters, including anatomical features and vulnerability to embolism of stems and leaves from the bottoms to the tops of co-occurring shrubs and saplings of trees. We also measured water potentials of stems and leaves at predawn and midday. The individuals from two species of each functional group that were sampled were roughly the same height, but while this represented a near maximum height for the shrubs, the trees would have continued growing to reach the canopy at maturity.

Results/Conclusions

We found that the young trees had greater vessel diameters at a given stem diameter than the shrubs did. The young trees also had a lower average wood density than the shrubs did, and, consistent with this, were more vulnerable to drought-induced embolism than the shrubs. At all three size classes of stem compared within each species, the pressures at which 50% of the hydraulic conductivity was lost in the shrubs were more negative than in the trees. Despite having a more drought resistant xylem network, the water potentials in the leaves (both predawn and midday) and stems (midday) in the shrubs were not different from those of the trees. Our results indicate that the xylem network of the shrubs tended to be overly resistant to drought compared to that of the trees given the water potentials shrubs experienced during a normal summer. This conservative behavior would require greater investment in the xylem in shrubs than in trees, which could alter the allocation of carbon in such a way that would limit height growth.

PS 49-112 - Are shrubs short because of their hydraulic architecture? A comparison of co-occurring trees and shrubs