Differential stress tolerance of four pines (Pinaceae) across the elevation gradient of the San Bernardino Mountains, Southern California, USA1

Ecophysiological studies that investigate species sorting across environmental gradients provide insights into the mechanisms underscoring tree distribution patterns. We examined pine functional traits by measuring a suite of physiological parameters including needle water content, spectral reflectance, transpiration decline, and leaf morphology to explore pine stratification patterns across the elevation gradient of the San Bernardino Mountains in southern California. The high and low elevation species demonstrated moisture and solar radiation tolerance. The lowest elevation species, Pinus attenuata Lem., maintained low transpiration rates under drought stress and had high concentrations of photosynthetic pigments indicating high photosynthetic capacity. Pinus coulteri Don was the most stress intolerant pine as demonstrated by high transpiration rates. Although Pinus lambertiana Doug. dominated cooler north-facing slopes, its lower transpiration rates suggested that it was well adapted to moisture stress. Pinus contorta Doug. was a typical high elevation stress tolerant tree growing under high incident solar radiation, heavy snow pack, and severe winds. Our results suggest that water availability and solar radiation intensity are the major factors influencing southern California pine distributions.