Effect of canopy position on germination and seedling survival of epiphytic bromeliads in a Mexican humid montane forest.
OBJECTIVE
Seeds of epiphytes must land on branches with suitable substrates and microclimates to germinate and for the resulting seedlings to survive. It is important to understand the fate of seeds and seedlings in order to model populations, but this is often neglected when only established plants are included in analyses.


METHODS
The seeds of five bromeliad species were exposed to different canopy positions in a Mexican montane forest, and germination and early seedling survival were recorded. Additionally, the survival of naturally dispersed seedlings was monitored in a census over 2.5 years. Survival analysis, a procedure rarely used in plant ecology, was used to study the influence of branch characteristics and light on germination and seedling survival in natural and experimental populations.


RESULTS
Experimental germination percentages ranged from 7.2 % in Tillandsia deppeana to 33.7 % in T. juncea, but the seeds of T. multicaulis largely failed to germinate. Twenty months after exposure between 3.5 and 9.4 % of the seedlings were still alive. There was no evidence that canopy position affected the probability of germination, but time to germination was shorter in less exposed canopy positions indicating that higher humidity accelerates germination. More experimental seedlings survived when canopy openness was high, whereas survival in census-seedlings was influenced by moss cover. While mortality decreased steadily with age in juveniles of the atmospheric Tillandsia, in the more mesomorphic Catopsis sessiliflora mortality increased dramatically in the dry season.


CONCLUSIONS
Seedling mortality, rather than the failure to germinate, accounts for the differential distribution of epiphytes within the canopy studied. With few safe sites to germinate and high seedling mortality, changes of local climate may affect epiphyte populations primarily through their seedling stage.
Winkler
Manuela
M
Institute of Botany, Department of Integrative Biology, University of Natural Resources and Applied Life Sciences, Gregor-Mendel-Str. 33, A-1180 Vienna, Austria. mwinkler@edv1.boku.ac.at
Hülber
Karl
K
Hietz
Peter
P
D004777:Environment; D018525:Germination / Q000502:physiology*; D006813:Humidity; D008027:Light; D008800:Mexico; D018547:Plant Stems / Q000502:physiology; D011156:Population Density; D031232:Tillandsia / Q000254:growth & development / Q000502:physiology*; D014197:Trees