| Summary: | A comparative study of resource-use efficiency traits was made between two spruce species which grow across an interacting resource gradient in northern British Columbia. Near Fort Nelson BC, black spruce (Picea mariana (Moench) Voss) grows in the muskegs which are characteristically high in soil water content, and are extremely low in nutrient availability. In contrast, white spruce (Picea glauca (Mill.) B.S.P.) grows on drier sites which are not as limited in nutrient availability. Mixed stands containing these two species have not been reported in this area, suggesting strong selection pressures. Five contiguous populations of black and white spruce were sampled from the Fort Nelson region and grown in a growth chamber under varying water and nitrogen regimes. It was hypothesized that genetically based differences in character means, plasticity, and/or trade-offs associated with adaptation to nutrient or water limitation may contribute to maintaining these species’ unique distribution patterns. The results showed that black spruce was significantly less N-stress sensitive, exhibited greater phenotypic plasticity in nitrogen-use efficiency (NUE), and had a greater specific N absorption rate under high N conditions than white spruce. White spruce was more water-use efficient (as measured by carbon isotopic composition), more drought-tolerant, and allocated more biomass to roots than black spruce. When exposed to simultaneous limitations of N and water, both species exhibited intermediate levels of NUE and water-use efficiency (WUE), and hence were forced to utilize each resource with suboptimal efficiency. It was also found that, despite recent theories, a high relative degree of drought-tolerance did not trade-off with WUE or net photosynthesis rate (A) under well watered conditions, and that higher WUE was not associated with low A under ample or stressed water supply. On the other hand, specific N absorption rate and A were negatively correlated as expected. Water-use efficiency and NUE did not trade-off intrinsically (i.e., within treatments) for either species, but did trade-off plastically (i.e., across treatments). The species exhibited different slopes for the plastic WUE vs. NUE trade-off; this genotype by environment interaction effect may reflect specialization to their respective habitats. Two main conclusions were derived: 1) certain trade-offs (such as WUE vs. A, droughttolerance vs. A, WUE vs. drought-tolerance, WUE and drought-tolerance vs. root allocation, and WUE vs. NUE) may not be central to maintaining the distributions of black and white spruce, and thus may not represent costs, in all situations, to habitat specialization; and 2) physiological and morphological differences in black and white spruce may, in some instances, cancel at the wholeplant level, but could nevertheless have significant ecological consequences. Finally, the ecophysiological differences found between the two species imply that black spruce may be restricted to muskegs in areas near the southern border of the boreal forest because of an inability to compete on water-limited upland sites. Forestry, Faculty of Graduate
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