Altitudinal changes in tree leaf and stem functional diversity in a semi-tropical mountain

Question: Along an altitudinal gradient of 2000 min a semi-tropical mountain, we explored the relation between tree specific diversity and community functional composition by studying variations in tree allometry, stem and leaf functional traits, and their relationship with temperature and precipitation. Location: Tequila Volcano, Jalisco, Mexico (20 degrees 48' N, 103 degrees 51' W). Methods: We surveyed tree specific diversity, five forest structural parameters and six functional traits in ten horizontal transects (50-75 m in length) located every 200 m along a 2000-m gradient (from 800 to 2800 m a. s.l.). We calculated alpha and beta diversity, and quantified the community-weighted means for wood and bark density, Huber value (sapwood to leaf area ratio), leaf area, leaf dry mass content and leaf mass per unit area. The patterns of association were explored using Pearson correlations, and summarized using PCA. Results: Alpha diversity was independent of altitude, and species turnover was almost complete between consecutive transects. Altitude (and its associated abiotic factors, temperature and precipitation) were highly correlated with functional traits. Maximum tree height, total basal area and the community-weighted mean values for leaf mass per unit area, leaf dry mass content, and the Huber values were positively correlated with altitude, and the opposite was found for the number of basal stems. Stem and leaf trait values were correlated along the altitudinal gradient. Conclusions: Altitude imposes environmental filters at the community scale that determine a high species replacement. Stem and leaf traits were correlated along the gradient; trees at higher altitudes were taller with a single stem, higher density of wood and bark, and leaves with higher leaf mass per area and dry mass content than in the low-altitude sites. These results suggest the consistency of a fast-slow acquisitive trade-off across environments, tending to promote slow acquisition and high longevity at higher altitudes.