CORRELATED POPULATION DIFFERENCES IN DRY-MATTER ACCUMULATION, ALLOCATION, AND WATER-USE EFFICIENCY IN 3 SYMPATRIC CONIFER SPECIES

Populations of three sympatric conifer species, Douglas-fir (Pseudotsu ga menziesii [Mirb.] Franco), ponderosa pine (Pinus ponderosa Laws.), and western larch (Larix occidentalis Nutt.), were studied for growth, water-use efficiency, and dry matter allocation. Seedlings from seed collected from across the respective natural distributions were grown under two watering regimes. In addition, trees at age 12 for western l arch and 20 for ponderosa pine were sampled from experimental planting s. Seedling growth was measured in terms of total dry matter, abovegro und dry matter, height, diameter, and relative growth rate; tree growt h in the field was measured in terms of height. Dry matter allocation was measured in terms of leaf weight ratio and root weight ratio. Wate r-use efficiency was measured in terms of biomass production per unit water loss and stable carbon isotope discrimination. Douglas-fir popul ations differed in all measured variables under both watering regimes (all P less than or equal to 0.04); correlations between measurements under well-watered and water-stressed conditions were high (0.69 less than or equal to r less than or equal to 0.99). Ponderosa pine populat ions differed consistently in leaf weight ratio and root weight ratio (all P less than or equal to 0.02), less consistently in growth (0.00 < P less than or equal to 0.35); and only leaf weight ratio and height were significantly correlated between watering regimes (0.82 less tha n or equal to r less than or equal to 0.85). Western larch populations differed inconsistently in dry matter allocation (0.01 less than or e qual to P less than or equal to 0.15) and growth (0.00 < P less than o r equal to 0.82); however, population differences in total dry matter, aboveground dry matter, relative growth rate, and leaf weight ratio w ere significantly correlated between watering regimes (0.56 less than or equal to r less than or equal to 0.75). Generally speaking, total d ry matter was correlated with water-use efficiency (WUE) but not alloc ation in western larch, with allocation but not WUE in ponderosa pine, and with both allocation and WUE in Douglas-fir. Similarly, tree heig ht was significantly correlated with carbon isotope discrimination (De lta) in Douglas-fir and in 12-yr-old trees of western larch, but not p onderosa pine. These results suggest that water-use efficiency correla ted with growth much more consistently than dry matter allocation. The simplicity of measuring Delta and its stability in this and other stu dies lead us to suggest that carbon isotope discrimination may be used as a marker for selection and breeding programs to improve growth for Douglas-fir and western larch.