Search

Larches (Larix spp.), deciduous conifers, occur in the northern hemisphere in cold-temperate and boreal climates - an environment normally thought to favor evergreen tree species. We compare foliar carbon isotope discrimination (Δ), instantaneous water use efficiency, total foliar nitrogen concentration, and specific leaf area (for a subset of sites) between Larix spp. and co-occurring evergreen conifers at 20 sites throughout the natural range of larches. Except for Larix occidentalis in the xeric Intermountain West, USA, Δ is significantly (P < 0.05) greater for larches than co-occurring evergreen conifers at 77% of the sites, suggesting that larches use water less efficiently. At elevations greater than 3000 m, the Δ of Larix spp. and co-occurring conifers converge, suggesting that water is not the limiting resource. Foliar nitrogen concentration and specific leaf area are two ecophysiological characteristics that are positively correlated with high photosynthetic capacity. Foliar nitrogen concentration is significantly greater for larches than evergreen conifers at 88% of the sites and specific leaf area is approximately three times greater for larches than co-occurring conifers. Future studies should examine the potential effect that global warming may have on the distribution of larch forests because the water use efficiency of larches is commonly less than co-occurring evergreen conifers and the boreal and high-latitude environments are likely to experience the greatest climate warming.

The relationship between photosynthetic capacity (A max ) and leaf nitrogen concentration (N) among all C 3 species can be described roughly with one general equation, yet within that overall pattern species groups or individual species may have markedly different A max -N relationships. To determine whether one or several predictive, fundamental A max -N relationships exist for temperate trees we measured A max , specific leaf area (SLA) and N in 22 broad-leaved deciduous and 9 needle-leaved evergreen tree species in Wisconsin, United States. For broad-leaved deciduous trees, mass-based A max was highly correlated with leaf N (r 2 =0.75, P<0.001). For evergreen conifers, mass-based A max was also correlated with leaf N (r 2 =0.59, P<0.001) and the slope of the regression (rate of increase of A max per unit increase in N) was lower (P<0.001) by two-thirds than in the broad-leaved species (1.9 vs. 6.4 μmol CO 2 g -1 N s -1 ), consistent with predictions based on tropical rain forest trees of short vs. long leaf life-span. On an area basis, there was a strong A max -N correlation among deciduous species (r 2 =0.78, P<0.001) and no correlation (r 2 =0.03, P>0.25) in the evergreen conifers. Compared to deciduous trees at a common leaf N (mass or area basis), evergreen trees had lower A max and SLA. For all data pooled, both leaf N and A max on a mass basis were correlated (r 2 =0.6) with SLA; in contrast, area-based leaf N scaled tightly with SLA (r 2 =0.81), but area-based A max did not (r 2 =0.06) because of low A max per unit N in the evergreen conifers. Multiple regression analysis of all data pooled showed that both N (mass or area basis) and SLA were significantly (P<0.001) related to A max on mass (r 2 =0.80) and area (r 2 =0.55) bases, respectively. These results provide further evidence that A max -N relationships are fundamentally different for ecologically distinct species groups with differing suites of foliage characteristics: species with long leaf life-spans and low SLA, whether broad-leaved or needle-leaved, tend to have lower A max per unit leaf N and a lower slope and higher intercept of the A max -N relation than do species with shorter leaf life-span and higher SLA. A single global A max -N equation overestimates and underestimates A max for temperate trees at the upper and lower end of their leaf N range, respectively. Users of A max -N relationships in modeling photosynthesis in different ecosystems should appreciate the strengths and limitations of regression equations based on different species groupings.