Including dynamics in the equation: Tree growth rates and host specificity of vascular epiphytes

1. The forest canopy is home to a rich biota. One salient feature are the
dynamics of the habitat-building trees, which are growing and eventually
vanishing. Tree species strongly differ in growth rates, final size and
longevity. Nevertheless, these inherent dynamics have been a blind spot in
studies on host specificity of vascular epiphytes (vascular plants
dwelling on trees without parasitizing them) – not least because tree
growth rates and longevity are usually unknown in highly diverse tropical
forests. The present study aims at tackling this blind spot. 2. We
compared epiphyte abundances (> 23,000 individuals) found on 285
individuals of four focal tree species in a lowland moist forest in
Panama. Data on repeated dbh censuses from a permanent tree plot provided
the unique opportunity to estimate the age of our sampled trees. We
compared the relative importance of tree longevity for host biases with
that of other host tree characteristics, namely microclimatic conditions
and bark acidity, rugosity and stability. 3. The studied tree species
differ in host quality and epiphyte species partly differ in host
preferences. The conclusions concerning relative host tree quality depend
hugely on whether or not different tree growth rates are considered.
Comparing these conclusions allows important insights into the role of
tree longevity in shaping epiphyte communities. Relating tree trait
differences to the observed distributions of epiphytes among the focal
tree species shows how the simultaneous action of various tree
characteristics causes host biases. 4. Synthesis. This study highlights
the substantial but, up to now, hidden role of different tree growth rates
for host tree specificity of vascular epiphytes. Future investigations
need to consider this possibly confounding factor adequately to avoid
spurious conclusions.