PARENT SESSION
Oral Session #55: Fire Ecology. Presiding: M. Moritz.
Wednesday, August 8, 2001. 1:00 PM to 5:00 PM. Hall of Ideas H.
Management of fire-excluded ponderosa pine stands increases soil water availability.
SALA, ANNA1, CAREY, EILEEN1, 1
ABSTRACT- Since the early 1900's, fire exclusion in the interior northwest has enhanced the successional replacement of open ponderosa pine stands (Pinus ponderosa) to dense stands of interior Douglas-fir (Pseudotsuga menziesii var glauca). We tested the hypothesis that increases in stand density and canopy cover associated with this replacement result in increased stand-level water use, reduced soil water availability and increased individual-level water stress. During the summer of 1998, we measured sap flow and needle water potential of ponderosa pines and Douglas-firs in fire-excluded stands and of ponderosa pine in open stands managed to simulate frequent fires. We also measured soil moisture in the two stand types. During the early part of the summer, water use of Douglas fir in fire-excluded stands was substantial. Consequently, soil water availability in these stands decreased as the summer proceeded to values significantly lower than those in open stands managed to simulate frequent fires. By July, ponderosa pines in open, managed stands exhibited higher sapwood relative water content and sapwood area-based sap flow relative to trees in denser stands, although predawn xylem water potential did not differ between ponderosa pines from open and denser stands. Overall, our results indicate that the incursion of Douglas-fir into fire excluded ponderosa pine stands results in a measurable depletion of water in upper soil layers. Consequently, trees in denser fire-excluded stands have less water available for transpiration relative to trees in open stands.
KEY WORDS: fire-exclusion, Pinus ponderosa, Pseudotsuga menziesii, sap-flow