© 2001 Heron Publishing—Victoria, Canada
Water use by whitebark pine and subalpine fir: potential consequences of fire exclusion in the northern Rocky Mountains
Anna Sala (1), Eileen V. Carey (1, 2), Robert E. Keane (3) and Ragan M. Callaway (1)
1. Division of Biological Sciences, The University of Montana, Missoula, MT 59812, USA / 2. Department of Forest Resources, University of Minnesota, St. Paul, MN 55108, USA / 3. USDA Forest Service, Rocky Mountain Research Station, Intermountain Fire Sciences Laboratory, Missoula, MT 59807, USA / Received August 9, 2000
Summary
In subalpine forests of the northern Rocky Mountains, fire exclusion has contributed to large-scale shifts from early-successional
whitebark pine (Pinus albicaulis Engelm.) to late-successional subalpine fir (Abies lasiocarpa (Hook.) Nutt.), a species assumed to be more shade tolerant than whitebark pine and with leaf to sapwood area ratios (AL:AS) over twice as high. Potential consequences of high AL:AS for subalpine fir include reduced light availability and, if hydraulic sufficiency is maintained, increased whole-tree water
use. We measured instantaneous gas exchange, carbon isotope ratios and sap flow of whitebark pine and subalpine fir trees
of different sizes in the Sapphire Mountains of western Montana to determine: (1) whether species-specific differences in
gas exchange are related to their assumed relative shade tolerance and (2) how differences in AL:AS affect leaf- and whole-tree water use. Whitebark pine exhibited higher photosynthetic rates (A = 10.9 μmol m–2 s–1 ± 1.1 SE), transpiration rates (E = 3.8 mmol m–2 s–1 ± 0.7 SE), stomatal conductance (gs = 166.4 mmol m–2 s–1 ± 5.3 SE) and carbon isotope ratios (δ13C = –25.5‰ ± 0.2 SE) than subalpine fir (A = 5.7 μmol m–2 s–1 ± 0.9 SE; E = 1.4 mmol m–2 s–1 ± 0.3 SE; gs = 63.4 mmol m–2 s–1 ± 1.2 SE, δ13C = –26.2 ‰ ± 0.2 SE; P < 0.01 in all cases). Because subalpine fir had lower leaf-area-based sap flow than whitebark pine (QL = 0.33 kg m–2 day–1 ± 0.03 SE and 0.76 kg m–2 day–1 ± 0.06 SE, respectively; P < 0.001), the higher AL:AS in subalpine fir did not result in direct proportional increases in whole-tree water use, although large subalpine firs used
more water than large whitebark pines. The linear relationships between tree size and daily water use (r2 = 0.94 and 0.97 for whitebark pine and subalpine fir, respectively) developed at the Sapphire Mountains site were applied
to trees of known size classes measured in 12 natural subalpine stands in the Bob Marshall Wilderness Complex (western Montana)
ranging from 67 to 458 years old. Results indicated that the potential for subalpine forests to lose water by transpiration
increases as succession proceeds and subalpine fir recruits into whitebark pine stands.
Keywords:
Abies lasiocarpa, biomass allocation, Pinus albicaulis, sap flow, shade tolerance, subalpine conifers, succession, water relations.
