Functional trait state diversity of Andean forests in Venezuela changes with altitude

Question
It is presumed that environmental stress is more pronounced at higher altitudes in Andean forests due to lower rates of temperature-constrained processes of growth, nutrient uptake and decomposition. Because community assembly theory predicts a low variation in viable traits under conditions of strong ecological filtering, we asked if the diversity in plant traits related to productivity and reproduction would decrease upslope.

Location
Guaramacal National Park in the Andes of Venezuela (9°05-21′N, 70°-20′W).

Methods
We studied altitudinal patterns in the number of trait states of seven categorical traits found among vascular plants (DBH ≥ 2.5 cm) in 35 0.1-ha plots between 1330 and 2890 m. Information on plant traits was obtained from literature and herbaria. To test the altitudinal correlations, we permuted the plot-to-species matrix by randomly selecting species from the entire species pool (conserving the species richness and the species-abundance pattern in each plot), applying no restrictions and by creating plot-wise null assemblages of species that most likely occurred near a particular plot. In addition, the altitudinal pattern of trait states was examined using fourth-corner analysis.

Results
Both null model tests yielded similar results. The trait state diversity in fruit size decreased upslope. Plants with small fruits were mostly, but not exclusively, found at higher altitudes, whereas plants with large fruits occurred only at lower elevations. Leaf size showed more states at higher altitudes but this pattern disappeared when rare leaf size classes were down-weighted. The upslope decrease in fruit size might be explained by the relatively low tree height of the upper montane rain forests, the increased selection for large seeds at high temperatures downslope, and the upslope decrease of large-leaved taxa (in view of Corner's rule).

Conclusions
The functional diversity of important plant traits changed with altitude at Guaramacal. Altitudinal effects on plant trait responses should be controlled for in studies that monitor the degradation of functional diversity in Andean forests in relation to human influence.