Responses of Southeast Alaska Understory Species to Variation in Light and Simulated Herbivory

Hanley, Thomas A.; Barnard, Jeffrey C. 2014. Responses of southeast Alaska understory species to variation in light and simulated herbivory. Res. Pap. PNW-RP-599. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 22 p. Responses to variation in light intensity, simulated herbivory by clipping, and their interaction were studied over three seasons in a factorial experimental design. Six major species of southeast Alaska forest understories were studied, each as a separate experiment: bunchberry, Cornus canadensis L.; threeleaf foamflower, Tiarella trifoliata L.; oval-leaf blueberry, Vaccinium ovalifolium Sm.; salmonberry, Rubus spectabilis Pursh; devilsclub, Oplopanax horridus (Sm.) Miq.; and western hemlock, Tsuga heterophylla (Raf.) Sarg. Seedlings of each species were grown in plastic grow pots under three intensities of light in an outdoor garden experiment—full sun in the open (100 percent incident light) and under two natural forest canopies, red alder (Alnus rubra Bong.; about 17 percent incident light) and younggrowth conifers (about 1.7 percent incident light). Plants within each light treatment were subjected to simulated herbivory by clipping the current year’s growth at the end of the first and second growing season at three intensities of clipping—0-percent removal, 50-percent removal, and 100-percent removal (75-percent removal for western hemlock). At the end of the third growing season, mean percentage survival was determined and total biomass (both belowand aboveground ovendry weight) of each surviving plant was measured. Factorial analyses of variance, conducted at the α level of 0.10, indicated that for survival, light was a significant main effect for only three of the six species, and clipping was a significant main effect for only one of the species (salmonberry, which also was the only species having a significant light by clipping interaction effect). Treatment effects were much stronger for mean plant size, however, with light having a significant main effect on all six species, clipping having a significant main effect on all but one species (devilsclub), and three of the six species having significant light by clipping interaction effects. Overall, the results emphasized the predominant importance of light in determining plant responses to herbivory, although the light by clipping interaction effect was not as strong as hypothesized.