Water use by whitebark pine and subalpine fir: Potential consequences of fire exclusion in the northern Rocky Mountains

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 (A_L:A_S) over twice as high. Potential consequences of high A_L:A_S 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 A_L:A_S 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 (g_s = 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; g_s = 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 (Q_L = 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 A_L:A_S 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.