Variation in root morphology of epiphytic orchids along small-scale and large-scale moisture gradients

Root morphology is expected to respond to environmental conditions. Two earlier studies reported predictable changes in the structure of the velamen radicum (common in monocotyledons) along rainfall gradients, which was expected from its assumed role in plant-water-relations. The present study expanded on this idea by analysing nine root traits that can be linked to root function, including velamen structure, along two moisture gradients of different spatial scales: 1) along the vertical gradient of a lowland forest in central Panama and 2) along a strong regional rainfall gradient in western Panama. All studied traits (e.g. velamen width, number of xylem poles, lignification of the exodermis) of the 45 orchid species showed substantial intraspecific and interspecific variation, but none of this within forest) or very little of this (regional gradient) was related to the gradients. Only the community weighted means of velamen width, stele width and xylem pole number varied significantly with rainfall, with substantial effect sizes only found in the latter two traits. Our results clearly disagree with those of earlier publications, which did not include species abundance and/or rigorous statistics in their analyses. This discrepancy highlights that our understanding of the link between velamen form and function is still rudimentary.