Hydraulic compensation in northern Rocky Mountain conifers: does successional position and life history matter?
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As trees grow tall and the resistance of the hydraulic pathway increases, water supply to foliage may decrease forcing stomata to close and CO2 uptake to decline. Several structural (e.g. biomass allocation) and physiological adjustments, however, may partially or fully compensate for such hydraulic constraints and prevent limitations on CO2 uptake and growth. The degree to which trees compensate for hydraulic constraints as they grow tall may depend on the costs and benefits associated with hydraulic compensation according to their ecology and life history. Because later successional Rocky Mountain conifers are more shade tolerant, optimization of CO2 uptake as trees grow tall and shade increases may confer greater benefits than in earlier successional species. If so, higher compensation for hydraulic constraints is expected in later successional species relative to co-occurring earlier successional species. I have examined height-related changes of crown stomatal conductance on a leaf area basis (G LA) and leaf to sapwood ratios (A L:A S) for five conifer species in the northern Rocky Mountains. Species were arranged in pairs, each pair consisting of an early and late successional species. For high elevations I used, respectively, whitebark pine (Pinus albicaulis) and subalpine fir (Abies lasiocarpa); for mid-elevations, western larch (Larix occidentalis) and Douglas-fir (Pseudotsuga menziesii); for lower elevations, ponderosa pine (Pinus ponderosa) and Douglas-fir. A L:A S either decreased (subalpine fir, ponderosa pine), remained constant (Douglas-fir, western larch) or increased (whitebark pine) with tree height. As hypothesized, earlier successional species (ponderosa pine, whitebark pine and western larch) exhibited significantly stronger decreases of G LA with tree height relative to their later successional pairs (Douglas-fir and subalpine fir), which fully compensated for height-related hydraulic constraints on G LA. A life history approach that takes into account the optimization of size- and species-specific ecological functions may also help researchers better understand biomass allocation and hydraulic function in trees.
KeywordsCrown conductance Leaf to sapwood ratio Reproduction Sap flow Tree height
This research was supported by grants from the USDA Forest Service, Rocky Mountain Research Station, INT-95086-RJVA to R. Callaway and A. Sala, USDA NRIGCP 95-37101-1683 to R. Callaway and USDA NRIGP 97-35106-5061 to A. Sala. I am particularly indebted to R. Callaway for his enthusiasm, generosity with the data and helpful comments on the manuscript. This work could not have been possible without the help of E. Carey, R. Keane and S. Arno. I am very thankful to L. Ashley, E. Aschehoug, J. Bickley, T. Byker, T. Carlson, A. Carroll, K. Corwall, W. Foster, G. Hammon, T. Laboski, S. Mincemoyer, J. Moran, T. Palm, K. Simonin, J. Smith and G. Thelen for their invaluable help in the field and in the laboratory. Thanks to C. Fiedler, B. Rich, V. Applegate and F. Mauss for their help during site selection and to B. Geils, J. Schloss, and E. DeLucia for the loan of data loggers. J. Maron, E. Crone and two reviewers provided very helpful comments on earlier versions of this manuscript.
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