Most timber species/logs are susceptible to various degrees of staining during periods of high temperature and humidity -- light-colored heartwood species are obviously much more susceptible compared to dark heartwood species such as black walnut.
Furthermore, all hardwood logs are susceptible to end-checking. Use log end-sealer products like Anchorseal green wood sealer if end checking becomes problematic.
For more information, please refer to the USDA Forest Products Lab based out of Madison, WI:
http://www.fpl.fs.fed.us/documnts/usda/ah188/chapter10.pdf
