Correlated Population Differences in Dry Matter Accumulation, Allocation, and Water-Use Efficiency in Three Sympatric Conifer Species

Abstract

Abstract Populations of three sympatric conifer species, Douglas-fir (Pseudotsuga 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 larch and 20 for ponderosa pine were sampled from experimental plantings. Seedling growth was measured in terms of total dry matter, aboveground dry matter, height, diameter, and relative growth rate; tree growth in the field was measured in terms of height. Dry matter allocation was measured in terms of leaf weight ratio and root weight ratio. Water-use efficiency was measured in terms of biomass production per unit water loss and stable carbon isotope discrimination. Douglas-fir populations differed in all measured variables under both watering regimes (all P ≤ 0.04); correlations between measurements under well-watered and water-stressed conditions were high (0.69 ≤ r ≤ 0.99). Ponderosa pine populations differed consistently in leaf weight ratio and root weight ratio (all P ≤ 0.02), less consistently in growth (0.00 < P ≤ 0.35); and only leaf weight ratio and height were significantly correlated between watering regimes (0.82 ≤ r ≤ 0.85). Western larch populations differed inconsistently in dry matter allocation (0.01 ≤ P ≤ 0.15) and growth (0.00 < P ≤ 0.82); however, population differences in total dry matter, aboveground dry matter, relative growth rate, and leaf weight ratio were significantly correlated between watering regimes (0.56 ≤ r ≤ 0.75). Generally speaking, total dry matter was correlated with water-use efficiency (WUE) but not allocation in western larch, with allocation but not WUE in ponderosa pine, and with both allocation and WUE in Douglas-fir. Similarly, tree height was significantly correlated with carbon isotope discrimination (Δ) in Douglas-fir and in 12-yr-old trees of western larch, but not ponderosa pine. These results suggest that water-use efficiency correlated with growth much more consistently than dry matter allocation. The simplicity of measuring Δ and its stability in this and other studies 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 For. Sci. 42(2):242-249.