Staghorn lichen is found encrusting the trunks of red and white firs (Abies sp.) throughout the Sierra Nevada Mountains, but is absent from the trunks of pines (Pinus sp.). The distribution of staghorn lichen on fir is not uniform. Staghorn lichen is absent at heights below approximately two meters, and appears to be unevenly distributed laterally around the trunks of trees. To determine if Staghorn lichen is more abundant on particular sides of fir trees, or more abundant on red than on white fir, we examined the abundance of Staghorn lichen on fir trees at Yuba Pass, California on June 24, 1999.

We designed and executed two sampling programs to determine if the abundance of Staghorn lichen was a function of aspect, tree size, tree species, or light levels. We used these two sampling designs in parallel to examine the merits of each design in an experimental context. One sampling program was a "between subjects design" in which each of the sampled trees was observed only on one of four aspects (northeast, southeast, southwest, or northwest). The other sampling program was a "within subjects design" in which each of the sampled trees was observed on all four levels of aspect. For each tree we also measured its circumference at 1.5 m, identified the species, and measured the incident photosynthetically active radiation (PAR) at the sampled aspects. A total of 20 individual A. magnifica and 20 A. concolor were sampled. Twenty-four of these trees were used for the between subjects analysis and 16 were used for the within subjects analysis. Circumference was measured with a 30 m tape and PAR was estimated using a quantum sensor on a LI1600 Diffusive Porometer. Lichen abundance was estimated as the average of three visual estimates from a circular sample quadrat located placed on the tree trunk at the sampled aspect at approximately 3 m above the ground surface.

Analysis of variance of the data collected in the between subjects design indicates that after adjusting for the effects of tree size and light level, the abundance of Staghorn Lichen differs between aspects (F_3,14 = 5.499, p = 0.010) and between host tree species (F_1,14 = 4.651, p = 0.049), but there is no interaction between aspect and species. Averaging across tree species, Staghorn lichen was more abundant on the western sides of the trees and averaging across aspects, it was more abundant on Red than on white fir (Figure 1).

Analysis of variance of the data collected in the within subjects design indicates that after adjusting for tree size, the abundance of Staghorn Lichen does not differ between aspect (F_3,39 = 1.419, p = 0.252 ), but is greater on red than on white fir (F_1,13 = 6.268 , p = 0.026 ). Averaging across species there was no difference in lichen abundance among the four aspects, but averaging across aspects Staghorn lichen was more abundant on red than on white fir (Figure 2).

The results of the two sampling designs are qualitatively similar - both show higher lichen abundance on red than on white fir and both show higher abundance on the west sides of trees. However, the within subjects design did not yield a statistically significant effect of aspect and the abundance of Staghorn Lichen on the northeast side of Red Fir trees was considerably higher in the within subjects analysis. Nevertheless, the lack of an effect of aspect in the within subjects design does not arise from a lack of statistical power, rather it arises because estimates of lichen abundance on two Red Fir trees were very high on the northeast aspect.

An additional set of samples would be useful in confirming or denying the effect of aspect detected in the between subjects design.

Why is Staghorn Lichen more abundant on Red than on White Fir? Why does Staghorn Lichen appear to be more abundant of the west than on the east sides of trees? These two questions are suggested by the results of the data collection and analyses reported here. However, to answer these questions would require manipulative experiments. The texture of Red Fir bark or the chemistry of its exudates may be more conducive for the establishment of lichen than that of White Fir. One might apply the exudates of the bark of each tree species to lichens established on other media to tests for chemical interactions between host tree and lichen. One could count the number of newly established lichens over a fixed time interval on each sections of the bark of each tree species to test the hypothesis that bark texture effects establishment. The effects of aspect are likely to be mediated by some aspect of microclimate. Exposure of bark sections with lichens to various climate regimes in controlled environmental chambers or in the field would permit one to examine this hypothesis in detail.