Growing on trees – it’s a
matter of life or death
The vascular plant species that I most usually see growing on trees in
the UK
is the fern Polypodium vulgare. Indeed, there are some fine examples of
this on trees near my house. However, in some ecosystems, such as the humid
montane forests of Mexico,
epiphytic vascular plants are much commoner and it was in this habitat that Winkler et al. (Vienna, Austria, pp. 1039–1047) investigated germination and seedling establishment of five epiphytic
bromeliad species. Seeds ‘were tied to… woody twigs… with a thin thread’. The
twigs were themselves tied to branches of forest trees at various positions in
the canopy. Overall, less than 20 % of the seeds germinated. Canopy position
did not affect germination percentage but it did affect the speed of
germination: germination was faster in the inner crown than in the outer crown.
This was attributed to the higher humidity in the inner crown. These epiphyte
seeds are small, which aids both dispersal and the ability to lodge in bark
crevices, but which mitigates against survival because
of small nutrient reserves. Thus, seedling establishment is a very critical
period. This is well illustrated by the data: after 1 year, only about 20 % of
the seedlings had survived. Interestingly, it was those in the outer crown that
were more likely to survive. Another factor affecting seedling survival was
canopy structure, with open canopies being more favourable than closed
canopies. In wild populations, survival of two species, Catopsis sessiliflora
and Tillandsia deppeana, was favoured by bryophyte cover on
the branches (presumably because this was associated with moister conditions)
but this was a negative factor for T. multicaulis. Overall, in both
experimental and wild populations it was clear that seedling establishment is a
critical phase and that microclimate within the habitat has a major influence
on this.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
