By Audrey Oh
Bigger doesn’t always mean better. At least, not for the humble Lobarian lichen. These flowery green growths are epiphytes, which means they take to other plants or objects as their home. Many of these lichen species have developed a deep-rooted bond with a number of tree species, and if anything should happen to their gracious host, the lichen are in trouble as well.
Concerned by the clear-felling practices of logging industries, conservationists have come up with methods to efficiently preserve the lichen by retaining some of the host trees in the clear-felling area. The question boils down to whether single large or several small, or more succinctly, SLOSS, reserves of trees are better at sustaining the lichen colonies.
Scientists from the Norwegian Forest and Landscape Institute and the University of Bergen decided to settle the debate by zeroing in on the relationship between tree size and lichen colonization behaviors. Aspen trees found in the coastal region of Western Norway are home to 25 species of Lobarian lichen. Contrary to previous studies’ findings, Fride Schei and her fellow researchers found that the size of the host aspen did not matter in the proportion of colonized trees, lichen density, and lichen species richness.
Forerunners of the SLOSS debate argued that larger trees were the way to go in the retention strategy. Previous studies all seemed to support this idea, which all indicated that larger trees housed a greater variety of lichen species. In their paper to be featured in the December edition of Biological Conservation, Schei and her colleagues, point out that a number of different factors can explain this pattern.
First of all, a larger tree means more space for lichen to colonize. Simple as that. Now let’s consider the fact that the older the tree, the more time it’s had to increase in size. Once we establish this link, we can see that age-related changes to the tree’s structural and chemical integrity, such as in bark pH and thickness, can help a lichen thalli, or lichen body, attach with ease. Lastly, the longer the tree’s been around, the more encounters with different lichen it’s bound to have.
The Norwegian scientists played things differently from previous studies, which measured the number of thalli on each tree. Instead, they focused on the density of lichen species per unit area of the tree bark. First, they confirmed that there was indeed a linear relationship between aspen tree age and size in each sampling site, and found, as expected, that the proportion of colonized trees increased as tree diameter also increased.
However, the density of thalli generally remained the same for trees of all sizes within each site, as did the species richness. This means retaining several small trees in a clear-felling area can be just as, if not more effective as a single large tree in lichen conservation.
Depending on the habitat conditions, this can be great news. Isolated, large trees have a higher tendency to topple in the wind. Keeping smaller, younger trees means they’ll be around longer to give the lichen enough time to settle and disperse.
However, just because the lichen don’t mind a cozier size doesn’t mean other organisms don’t either. Woodpeckers prefer aspens larger than a certain size to establish their nests in, and polypore fungi much prefer large, dead trees to their small, spry counterparts. The scientists recommend a mixture of large and small trees to yield the best possible results. In the quest to find the best, yet practical, conservation strategy, assessing and working in harmony with the available habitat conditions is just good practice.
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/>