Deciduous shrubs are an integral component of mixed conifer forests of the interior Pacific Northwest by providing structural complexity, forage, and niche habitat. Currently, these species are under pressure from high levels of ungulate herbivory and forest management activities such as fuels reduction treatments. Ungulate herbivory is influential in the suppression of shrubs by maintaining low levels of abundance and shrub diversity that can eventually contribute to the alteration of species composition. Beyond herbivory, episodic disturbances such as prescribed fire and stand thinning can also influence the shrub layer and forest dynamics. Over a century of fire suppression has increased the occurrence of fuels reduction treatments across the interior Pacific Northwest. The aim of this study was to 1) evaluate the effects fuels reduction treatments as well as the effects of herbivory by wild (Cervus elaphus, Odocoileus hemionus) and domestic (Bos taurus) ungulates on shrub diversity, richness, and height; and 2) examine the relationship between the amount of protection by physical barriers (coarse woody debris and conifer trees) on shrub architecture and height.
Concerning the first objective, we measured shrub richness, frequency, diversity, and height in four treatments in the mixed conifer forests of the interior Pacific Northwest, USA: 1) ungulate herbivory in fuels treated forest stands, 2) no herbivory in fuels treated stands, 3) herbivory in untreated forest stands and 4) no herbivory in untreated forest stands. We found that ungulate herbivory decreased shrub richness, diversity, and height in both fuels reduction treated and untreated stands. We also found that fuels reduction treatments decreased overall shrub richness, diversity, and height with and without ungulate herbivory. Furthermore, the interaction of ungulate herbivory and fuels reduction treatments had compounding negative effects on shrub diversity and height.
Regarding objective two, we measured the height and architecture of highly preferred shrubs with and without herbivory in untreated forest stands. We examined how well shrubs were protected from herbivory by coarse woody debris and conifer trees, and how that protection may influence the height and architecture of the shrub. Goodness-of-fit tests and multiple linear regressions showed that shrubs exposed to herbivory with a lesser degree of physical protection had more strongly influenced growth form as evidenced by reduced height and an increased proportion of arrested individuals.
This research explored how two of the most common forest disturbances (ungulate herbivory and fuels reduction treatments) influence shrub composition and structure, and specifically how forest structural elements may harbor preferred shrub species and protect them from herbivory. Interactions between the increasing amount of acres of forest undergoing fuels reduction treatments and the high levels of ungulates within the interior Pacific Northwest may contribute to a loss in biodiversity and resources that are provided by deciduous shrubs.