Forest canopy epiphytes and microclimate mutually interact and influence each other. These studies quantify microclimate gradients of air temperature and vapor pressure deficit (VPD) along the vertical profile of an old-growth mixed-conifer forest in the southern Sierra Nevada, and examine their influences on the structure and composition of arboreal macroepiphyte communities. Temperature and relative humidity were recorded for a year: (1) along a two-dimensional grid of 24 data-loggers to assess diurnal and seasonal changes in vertical and horizontal microclimate with distance from a perennial stream, and (2) by 60 data-loggers arrayed in trees to compare vertical microclimate among four forest thinning treatments: thinning from the overstory, thinning from the understory, an un-thinned control, and an un-thinned riparian environment. Results suggest a dynamic zone of limited riparian influence on microclimate (< 5.0 m vertically, < 7.5 m horizontally) that fluctuates diurnally and seasonally. Microclimate change was greater vertically above the stream than horizontally away from the stream. Such a steep vertical gradient likely influences arboreal epiphyte communities. Thinning treatments had more extreme summer daily ranges of temperature and VPD than the control. Understory thinning did not alter the mean or range of microclimate as much as overstory thinning. Riparian microclimate had lower minimums and means, and greater daily ranges of temperatures and VPDs than the control. Results suggest that thinning canopy overstory increases understory microclimatic extremes and variability compared to thinning from below or not thinning.

Epiphyte abundances were estimated in fifty white fir, red fir, incense cedar, Jeffrey pine, and sugar pine trees from ground-level to their tops. The vertical gradient of bark pH within tree species was also measured. Mosses had a strong positive association with white fir and proximity to perennial water. Bark pH distinguished presence/absence of moss and composition of macrolichens among tree species. Within-species community structure was best explained by a gradient of increasing VPD with height. Bryoria abundance had strong positive associations with red fir, proximity to streams, and decreasing VPD. To minimize VPDs and foster epiphytes, overstory canopy should be left largely undisturbed and a heterogeneous mix of mature tree species should be retained, especially red fir, in the narrow riparian zone.