Adaptation of Sugar Maple Populations Along Altitudinal Gradients: Photosynthesis, Respiration, and Specific Leaf Weight

Abstract

Sugar maple seeds were collected from populations spaced along two altitudinal gradients in the White Mountains of New Hampshire. When grown in a uniform environment, progeny of stands less than 0.8 km apart differed significantly in photosynthesis, respiration, and leaf characteristics, despite a lack of physical barriers to gene migration. Sugar maple is a long-lived (200–300 yr) species with continuous distribution, but adaptive adjustment along the altitudinal gradient has occurred in only 8,000 yr, the time since colonization of the White Mountains in the wake of glacial melting. Photosynthesis was highest in progeny from high-altitude populations, representing the species' ecological margin. High-altitude populations also had the lowest specific leaf weight (SWL), the ratio of leaf weight to leaf area, providing a highly cost-effective photosynthetic system, probably the result of natural selection in a short growing season. Respiration rates were also highest in populations native to high altitudes and constitute the cost of maintaining the photosynthetic machinery at high capacity. Photosynthesis tended toward a minimum and SLW to a maximum at mid-elevations. There were parallel patterns on both gradients, suggesting parallel evolution. There were no differences among sugar maple populations in photosynthetic response to temperature, in contrast to observations on balsam fir in the same locality.