Trait patterns of epiphytes compared to other plant life-forms along a tropical elevation gradient

Abstract

Compared to other plant life-forms, epiphytes remain understudied. Understanding the responses of epiphytes to changing environmental conditions is necessary to predict changes in ecosystem functioning especially in subtropical and tropical regions. We investigated the functional traits of epiphytes along a large elevation gradient on Mount Kilimanjaro, Tanzania. We measured traits of co-occurring trees and terrestrial non-tree life-forms and compared changes in community-weighted means (CWM) of traits and trait spread, the range of observed trait values. We chose traits linked to growth and persistence: leaf area, specific leaf area, leaf dry matter content, stem specific density, plant height, leaf carbon, leaf nitrogen and leaf phosphorus. For most traits, differences in CWM between life-forms exceeded differences within life-forms along the elevation gradient. Many CWM showed linear changes with elevation, but no response and unimodal patterns were also frequent. This was best explained by temperature, or a combination of temperature with precipitation or humidity, indicating effects of these factors on the distribution of epiphytic and non-epiphytic species. Trait spread did not change with elevation in nearly half of the traits, but hump-shaped patterns were also common, probably a result of weaker environmental filtering in the gradient centre. The magnitude of trait spread, that is, the variability between species of the same life-form within communities, was highest for terrestrial non-trees (TNT). Excluding ferns from the analyses lead to marked differences in trait patterns for epiphytes, as ferns made up 59% of the epiphytic species, while playing a minor role in the other groups. The observed differences can be explained by a dichotomy in epiphytic life strategies, with tough-leaved xerotolerant species on one side and succulent soft-leaved species on the other. However, the influence of phylogeny was lower than expected from the taxonomic composition of the three life-form groups. Our results emphasize that environmental constraints act upon functional traits of epiphytes, trees and TNT. The differences in trait expressions, arguably adaptations of the different life-forms, need to be taken into account in conservation contexts as well as when modelling the effects of global change on ecosystems. A plain language summary is available for this article.