© 2005 Heron Publishing—Victoria, Canada
Physiological responses of ponderosa pine in western Montana to thinning, prescribed fire and burning season
Anna Sala (1, 2), Gregory D. Peters (1), Lorna R. McIntyre (1) and Michael G. Harrington (3)
1. Division of Biological Sciences, The University of Montana, Missoula, MT 59812, USA / 2. Corresponding author (sala@mso.umt.edu) / 3. Fire Sciences Laboratory, USFS Rocky Mountain Research Station, P.O. Box 8089, 5775 Highway 10, Missoula, MT 59807, USA / Received May 9, 2003; accepted August 28, 2004; published online January 4, 2005
Summary
Low-elevation ponderosa pine (Pinus ponderosa Dougl. ex. Laws.) forests of the northern Rocky Mountains historically experienced frequent low-intensity fires that maintained
open uneven-aged stands. A century of fire exclusion has contributed to denser ponderosa pine forests with greater competition
for resources, higher tree stress and greater risk of insect attack and stand-destroying fire. Active management intended
to restore a semblance of the more sustainable historic stand structure and composition includes selective thinning and prescribed
fire. However, little is known about the relative effects of these management practices on the physiological performance of
ponderosa pine. We measured soil water and nitrogen availability, physiological performance and wood radial increment of second
growth ponderosa pine trees at the Lick Creek Experimental Site in the Bitterroot National Forest, Montana, 8 and 9 years
after the application of four treatments: thinning only; thinning followed by prescribed fire in the spring; thinning followed
by prescribed fire in the fall; and untreated controls. Volumetric soil water content and resin capsule ammonium did not differ
among treatments. Resin capsule nitrate in the control treatment was similar to that in all other treatments, although burned
treatments had lower nitrate relative to the thinned-only treatment. Trees of similar size and canopy condition in the three
thinned treatments (with and without fire) displayed higher leaf-area-based photosynthetic rate, stomatal conductance and
mid-morning leaf water potential in June and July, and higher wood radial increment relative to trees in control units. Specific
leaf area, mass-based leaf nitrogen content and carbon isotope discrimination did not vary among treatments. Our results suggest
that, despite minimal differences in soil resource availability, trees in managed units where basal area was reduced had improved
gas exchange and growth compared with trees in unmanaged units. Prescribed fire (either in the spring or in the fall) in addition
to thinning, had no measurable effect on the mid-term physiological performance and wood growth of second growth ponderosa
pine.
Keywords:
forest management, photosynthesis, Pinus ponderosa, prescribed burning, soil nitrogen, water potential.
