Mark,
I still don't think you need a reference, but using the search terms "slope
aspect sun" in Web of Knowledge, I was able to quickly come up with this
potentially useful reference:
Geiger, R. J. (1965) The Climate Near the Ground. Cambridge, MA: Harvard
University Press. (cited in Huang et al. 2008. Modeling monthly
near-surface air temperature from solar radiation and lapse rate:
Application over complex terrain in Yellowstone National Park. Physical
Geography 29(2): 158-178.)
The Huang et al. study is not about boulders, so Geiger's book might be more
relevant to hills and mountains than to boulders. I've found another study
with a promising title (using "lichen boulder* sun*"), but I can't get it
online to look into it: vandenBoom et al. 1996. The lichen flora of
megalithic monuments in the Netherlands. Nova Hedwigia 62(1-2): 91-104.
The abstract concludes:
The most important abiotic factors correlating with the lichen
vegetation appear to be the exposure to sunlight, the pressure of recreation
and the exposure to wind. Sufficient measures
should be taken to minimize the negative effects of the
increased treegrowth, and to protect some vulnerable monuments against
intensive recreation."
I hope that helps.
Jim Crants
