By Audrey Oh
In conservation designs for epiphytic lichen species, retention of larger host trees in clear-felling areas was thought to have a significant advantage over smaller trees in lichen colonization. A recent study conducted in Norway, however, shows that size does not matter.
Fride Schei and her colleagues from the Norwegian Forest and Landscape Institute and the University of Bergen borrowed from the theory of island biogeography to explore the effects of Single Large Or Several Small (SLOSS) reserves of host trees on epiphytic lichen distribution.
Previous studies tended to focus on the number of individuals and species per tree, leading to conclusions that single large reserves of host trees was the best strategy for preserving lichen species.
However, this new research takes into account that several different factors could have contributed to higher species density on large host trees.
Larger trees are usually a result of longer development times, which closely links tree age and size. Furthermore, structural and chemical qualities of the tree, such as bark pH, crevice depth, and thickness, can become more favorable for lichen establishment as the tree ages.
In addition, larger tree area simply means it is more likely that lichen species will colonize the tree, as does a longer time window for colonization.
As a definitive test of the SLOSS approaches, the researchers decided to examine lichen density and species richness per unit area of tree bark instead.
The study was conducted over 38 sites along the coast of Western Norway, where aspen trees are home to 25 species of epiphytic Lobarion lichen.
The forest sites were divided into young (<120 years) and old (>120 years) sites, within which host trees were categorized by diameter.
Measurements were taken for the proportion of colonized trees in relation to tree size, number of lichen thalli (lichen bodies) per aspen area, and number of lichen species per aspen area.
Using a linear mixed model with age as a fixed effect and site as a random factor, the researchers found a linear relationship between aspen tree age and size. There was a significant increase in the proportion of colonized aspen as tree diameter increased.
Meanwhile, density of lichen thalli did not increase with increasing diameter, although number of thalli did increase proportionally with diameter. Similarly, the number of species with increasing bark area was about the same for all sizes within each site.
From these results, we can see that the larger aspens did not possess any additional qualities beyond size that made them more suitable for Lobarion lichen colonization. In conservation efforts, the retention of several small host trees could be equally as effective as the retention of a single large host tree.
These findings significantly modify the retention tree debate in epiphyte conservation.
Since lichen longevity is highly dependent on the survival of the host tree itself, small, young trees with long remaining life spans should be selected for retention. Solitary large trees also have higher vulnerability to wind felling.
However, the researchers advise that a mixture of different sized trees may be the ideal strategy, to account for other organism groups that are strongly associated with larger tree sizes. Woodpeckers, for example, depend on large aspen trees for constructing their nesting holes.
Larger trees that have allowed ample time for lichen colonies to fully establish themselves may also lead to more efficient dispersal between trees.
More importantly, the researchers note, is to consider the unique distribution of trees and the configuration of the habitat in the conservation area. Given a habitat with isolated patches of similarly aged trees, the sites with larger trees should be selected as they reflect old and most likely well-colonized trees.
Source: Schei, F. H., Blom, H. H., Gjerde, I., Grytnes, J.A., Heegaard, E., Sætersdal, M. (2013). Conservation of epiphytes: Single large or several small host trees? Biological Conservation, 168, 144-151. http://dx.doi.org/10.1016/j.biocon.2013.10.001
Photograph: Vilseskogen. “lunglav (Lobaria Pulmonaria), a kind of lichen.” 19 Aug 2009. Online Image. Flickr. 04 Nov 2013. <http://www.flickr.com/photos/vilseskogen/3836101271/>