MCCUNE, BRUCE. Department of Botany, Oregon State University, Corvallis, OR 97331-2902. - Lichen transplant results pose a treetop paradox.
Three groups of students designed and executed lichen transplant
studies in an old-growth Pseudotsuga-Tsuga forest at the Wind
River Canopy Crane in the southern Washington. Each team focused on a
particular question or hypothesis of their own choosing. This paper
does not address their specific hypotheses, but rather summarizes
differences in growth with respect to height in the canopy. Four
species of macrolichens grew much faster in middle to upper canopy
positions than in the lower half of the canopy, based on 300
transplants. This held true for species that peak in the upper canopy
(Letharia vulpina and Usnea filipendula) as well as
those with peak abundance in mid canopy (Lobaria oregana and
Lobaria pulmonaria). Lichens lost weight or died below the
light transition zone. These results can be mimicked by a simple
model of lichen net photosynthesis (Ps) as a function of cumulative
leaf area (CLA) – leaf area above a certain level in the canopy. CLA
influences both light and moisture regimes. We hypothesize that gross
Ps, integrated over a year, increases to a plateau with canopy height,
while respiration decreases linearly with canopy height. The
dependence of CLA on height in the canopy will vary a lot from forest
to forest and is likely to be an extremely important variable in
controlling the epiphyte profile. The paradox is that some species
(Lobaria oregana and Lobaria pulmonaria) grow faster at
higher levels in the canopy than in their stratum of peak abundance.
Possible resolutions of the paradox invoke competition, uncommon
extreme environmental events (such as extreme high or low
temperatures), and dispersal limitations.
Key words: canopy structure, Letharia, lichens, Lobaria, transplant experiment, Usnea