Size class structure and tree dispersion patterns in old-growth cedar-hemlock forests of the northern Rocky Mountains...

Tree population size structures and dispersion patterns were studied using stem maps in three old-growth western hemlock (Tsuga heterophylla Sarg.)-western redcedar (Thuja plicata Donn.) stands in the Rocky Mountains of northern Idaho and adjacent Washington. The two species were codominant in one stand, hemlock dominated the second, and cedar the third. Shade intolerant species such as western white pine (Pinus monticola Dougl.) were present only as canopy individuals. Hemlock, but not cedar, was well represented in size classes with dbh less than 20 cm. Althrough large individuals of both species have substantial influences on soil properties beneath them, forming distinct cedar or hemlock patches, nearest neighbor analyses did not indicate that these patches influence tree recruitment patterns. Juvenile trees were generally found in monospecific groups and their location was most dependent on rotting wood and other safe sites. Aggregation decreased as size class increased for both species in all cases except for cedar in the mixed stand, where the largest size class was aggregated. Aggregation of large cedars suggests that proximity to conspecifics increases survivorship among cedar in mixed stands. This may be due to the formation and maintainance of soil patches favoring a cedar nutrient cycling regime.