All studies of remnant trees and lichens to date point to the same conclusion: remnant trees are important for fostering epiphytes in young stands. The evidence includes correlations between remnant trees and old-growth components of lichen biodiversity (Neitlich & McCune 1997, Peterson & McCune 2001b, Rosso et al., 2000b), higher biomass of old-growth associated species in mature stands with remnants, as compared with adjoining stands without remnants (Peck & McCune 1997a), comparable lichen biomass in uncut and partially cut Abies-Picea stands in northern Idaho and southeastern BC (Rominger et al. 1994), and a demonstration of the importance of dispersal limitations to Lobaria oregana, the cyanolichen achieving very high biomass in old-growth (Sillett et al. 2000, 2000b).
Studies from outside the PNW have drawn similar conclusions. Hazell and Gustafsson (1999) studying Populus tremula forests in Sweden concluded "...retained trees can form biodiversity links during forest succession after final harvest and that they are beneficial to at least some of the species considered to be sensitive to forest operations." Gauslaa et al. (2001) concur, qualified by statements that (1) the effects of retention trees may vary substantially, depending solar exposure and (2) some species may be particularly sensitive on edges and isolated remnant trees.
Young stands with remnant trees typically have lower biomass of epiphytes than comparable mature and old-growth stands (Price and Hochachka 2001; Berryman and McCune 2001b). We therefore expect that the functional contributions of lichens will be substantially less in a landscape matrix of young stands with old-tree retention than in a landscape of predominantly mature and old-growth stands.
Not all remnant trees are equal. The practice of cutting or blasting tops out of remnant trees may promote cavity-nesting animals, but it effectively destroys the remnant tree's function as a source of propagules for lichens and bryophytes.
Although retention cuts as prescribed by the NW Forest Plan will provide much better than clearcuts for the long-term maintenance of old-growth associates, a system of reserves of old forests is necessary to provide for certain rare, old-growth associates. An example with Nephroma occultum illustrates this point (Rosso et al. 2000):
"Maintenance of the N. occultum population within the Cobble Knob area will require the maintenance of an old-growth stand. Placing Cobble Knob in a normal harvest rotation is likely to eventually eliminate N. occultum from the stand, even with green-tree retention. As the older trees are eliminated over time but not replaced, the N. occultum population would likely also be eliminated. For the species to maintain itself with a regime of green-tree retention overlaid on a 100-yr rotation, the rate of colonization of the younger cohort must equal or exceed the rate of attrition of the retention trees. If, for example, we assume under a "best-case" scenario:
- a 40 ha stand with 25 large trees/ha retained (approximately equal to a 100 acre stand with 10 large live trees/acre retained)
- presence of Nephroma occultum on one-fourth of the retention trees
- 100% survival of N. occultum on all retention trees on which it occurs at the time of cutting (again, a best-case assumption which may not be met in all areas)
- an average survival rate of 99% of the retention trees per year (retention trees have been observed to be prone to windthrow),
then about 91 trees bearing N. occultum will survive a single rotation:
N = 0.99^100 * 40 * 25/4
N = 0.366 * 250 (100-yr survival rate * trees with N. occultum)
N = 91 trees with N. occultum
Long-term maintenance of the population of N. occultum with this scenario would therefore require that it would have re-established on approximately 159 trees (250 - 91 = 159 trees) in the new cohort by the end of the first rotation. At present there is no evidence to suggest that N. occultum is capable of this rate of re-establishment. To the contrary, the sporadic distribution of N. occultum even in old-growth forests, and its even more sporadic occurrence in young stands, suggest that the species is limited by poor dispersal or establishment. Although N. occultum may have a high survival rate on retention trees (Sillett & Goslin 1999), in our study area, cutting with green tree retention will not likely perpetuate the species in the long term."
A related question is whether retention trees should be aggregated or dispersed.
