Variation in Carbon Isotope Discrimination and Photosynthetic Gas Exchange Among Populations of Pseudotsuga menziesii and Pinus ponderosa in Different Environments

Abstract

Seedlings representing 25 populations of Pseudotsuga menziesii and 26 populations of Pinus ponderosa were grown in a common garden in Moscow, ID, USA. The seeds were collected across the natural distribution of each species, at altitudes ranging from 170 to 2774 m above sea level, latitudes from 33 °N to 53 °N, and longitudes from 105 °W to 124 °W. Lipid-free seeds from mother trees and leaf tissue from the 2-year-old progeny were analysed. The design enabled us not only to measure genetically determined variation in carbon isotope discrimination (Δ) and gas exchange characteristics but also to compare performance in the common garden and in situ. In the common garden, significant population variation in Δ, gas exchange and specific leaf area was detected among seedlings of Pseudotsuga menziesii. Coastal, low-altitude genotypes had significantly lower Δ than interior, high-altitude genotypes. In Pinus ponderosa, populations varied only in specific leaf area. These broadly distributed sympatric species differ in genetic structure with respect to gas-exchange characteristics. In the common garden, high Δ of both species was associated with high stomatal conductance relative to photosynthetic rate. Specific leaf area, although strongly correlated with Δ, varied in the wrong direction to explain variation in Δ. In situ Δ was correlated with both altitude and vapour pressure deficit (VPD) in Pseudotsuga menziesii (r 2 =0.42, P=0002 and r 2 =025, P 0.77 and r 2 =0.06, P>0.21 for altitude and VPD, respectively). Foliage Δ from the progeny grown in the common garden was significantly correlated, in both species, with Δ of the in situ maternal photosynthate in the seeds; however, the correlation was negative in Pseudotsuga menziesii and positive in Pinus ponderosa. The negative correlation indicates a strong acclimatory response, perhaps to VPD, in Pseudotsuga menziesii. Altitudinal decreases in intercellular CO 2 partial pressures inferred from isotopic data were insufficient to compensate for increased VPD. Photosynthetic water-use efficiency (net photosynthesis/transpiration) was estimated to decrease by two- to fourfold from sea level to 2800m altitude within the distribution limits of these species.