Gradient analysis of forests of the Sangre de Cristo Range, Colorado

Interrelationships between dominant compositional and environmental gradients were studied using 186 forest stands sampled on the east slope of the Sangre de Cristo Range, Colorado. Detrended correspondence analysis and detrended canonical correspondence analysis were used to analyze indirect and direct gradients, respectively. The dominant compositional gradient was strongly correlated with elevation. Increasing elevation was associated with decreasing soil pH and percent base saturation, and increasing total soil N. Ordination of stands stratified by elevation showed the major compositional gradient within each elevation class was strongly correlated with a topographic moisture index. Most soil variables correlated with this gradient in low and high elevation classes. In mid-elevation forests, a third compositional gradient correlated with soil pH, percent base saturation, and potential solar radiation. In these forests, Pinus contorta and Pseudotsuga menziesii were associated with acidic soils on north-facing slopes, whereas Populus tremuloides and Abies concolor were associated with base-rich soils on south-facing slopes. Ordination axes accounted for least variation in comparatively young mid-elevation forests. A plausible explanation is that the mid-elevation forests represent an unpredictable stage in forest development where competition has not yet had sufficient time to sort species along environmental gradients.