Local and regional determinants of vascular epiphyte mortality in the Andean mountains of Colombia

Abstract

1. We present the first large-scale assessment of vascular epiphyte mortality in the neotropics. Our goals were to explore the primary types of vascular epiphyte death and to identify local and regional determinants of epiphyte mortality in natural forests located 60 to 2900 m a.s.l. in the Colombian Andes.2. Based on two consecutive annual surveys, we followed the fate of 4247 epiphytes to estimate the epiphyte mortality rate on 116 host trees at nine sites. A logistic regression analysis for proportional data with a binomial distribution of the error was applied to determine the probability of epiphyte death in relation to local and regional explanatory variables. 3. The overall epiphyte mortality rate was 7.5 AE 1.1% year À1 (mean AE standard error). Nonmechanical factors, such as desiccation, accounted for a mortality rate of 1.9 AE 0.3% year À1 . Mechanical factors, such as falling branches, accounted for a mortality rate of 5.6 AE 1.1% year À1 . According to generalized linear modelling analyses, both local and regional factors played key roles in determining epiphyte mortality. The actual evapotranspiration (regional factor) and the mean epiphyte attachment height (local factor) were both consistently positively associated with the probability of epiphyte death. Additional variables identified as possible determinants of the epiphyte mortality were the temperature seasonality, annual temperature range, the height and number of branches of the tree and the abundance of large trees (DBH ≥10 cm). 4. Synthesis. The recorded high mortality rate indicates that natural epiphyte assemblages must be highly dynamic to avoid local extinction of species. Our study identifies actual evapotranspiration as an important driver of epiphyte mortality, and we highlight its importance in determining the fate tropical epiphyte communities may experience if evapotranspiration increases due to climate change. We hope our study addresses the paucity of research on non-tree growth forms, typically ignored in vegetation dynamics, and encourages their inclusion in future studies that investigate the function of tropical ecosystems.

Functional composition of epiphyte communities in the Colombian Andes

“…For the surveys, we haphazardly selected 35 host trees of the following six size classes within each of the 10 1-ha tree plots: 10 trees with a diameter at breast height (DBH) >30 cm and 25 trees in each of five smaller DBH classes (5)(6)(7)(8)(9)(10). In most cases, epiphytes were photographed for subsequent analyses and identification (see Zuleta et al 2016). In most cases, epiphytes were photographed for subsequent analyses and identification (see Zuleta et al 2016).…”

“…Because the availability of water and light to epiphytes is largely determined by their location within the forest strata (Allen et al 1972, Parker 1995, we used the aboveground relative height of attachment of each individual epiphyte (H) as a surrogate of microclimatic variation at the local scale (Petter et al 2016, Zuleta et al 2016. Relative H was calculated as the ratio between the observed epiphyte height of attachment and the maximum tree height measured in each plot.…”

“…The vascular epiphytes on each host tree were mapped by accessing the plants by climbing, using ladders and/or binoculars. In most cases, epiphytes were photographed for subsequent analyses and identification (see Zuleta et al 2016). We defined epiphyte individuals as all independent stems or closely packed groups of conspecifics that were spatially separated and distinguishable from other stems/clumps (Sanford 1968).…”

“…The lack of direct access to some epiphytes, and the obligate use of binoculars from the ground in some cases, may have introduced error and caused us to sometimes underestimate total epiphyte species abundance and richness, especially in the upper canopy (Flores-Palacios and Garc ıa-Franco 2001, Gradstein et al 2003). Given limitations, our results most accurately reflect patterns for understory to midcanopy epiphytes (Zuleta et al 2016). The height of epiphyte attachment (H) was then standardized with regard to the maximum tree height at each site in order to minimize effects from changing canopy heights between sites.…”

“…Documenting the ways that epiphytic species adapt and partition resources at local to regional scales (Zuleta et al 2016) can shed new light on plant adaptive strategies and improve our understanding of the mechanisms maintaining the high diversity of epiphytes that epitomizes much of the tropics (K€ uper et al 2004(K€ uper et al , Kr€ omer et al 2005. Furthermore, understanding how both microclimatic (e.g., vertical variation in light availability; Petter et al 2016, Zuleta et al 2016) and macroclimatic (e.g., rainfall and temperature) variables influence the distribution and functional composition of epiphytes at different spatial scales will increase our ability to predict their responses to global warming and other environmental changes Bader 2009, Hsu et al 2012).…”