Interactive Effects of Fuels Reduction and Large Herbivores on Shrub
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Forest Ecology and Management 463 (2020) 118031 Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco Interactive effects of fuels reduction and large herbivores on shrub assemblages in dry conifer forests of the interior west, USA T ⁎ Dallas Hall Defreesa, Joshua P. Averetta, Michael J. Wisdomb, Bryan A. Endressa,c, a Eastern Oregon Agricultural Research Center – Union Station, Department of Animal and Rangeland Sciences, Oregon State University, Union, OR 97883, USA b Pacific Northwest Research Station, USDA Forest Service, 1401 Gekeler Lane, La Grande, OR 97850, USA c Eastern Oregon Agricultural and Natural Resource Program, Oregon State University, La Grande, OR 97850, USA ABSTRACT Deciduous shrubs are widely distributed throughout temperate and boreal conifer forests and influence a wide range of ecological processes and forest resources. In the interior western U.S., many deciduous shrubs are highly preferred forage by wild (elk, Cervus canadensis; deer, Odocoileus spp.) and domestic (cattle) ungulates which can influence shrub abundance, composition, structural characteristics, and related ecological processes and interactions. Stand disturbances and silvicultural practices can also affect shrub assemblages and managers in the interior western U.S. are increasingly implementing fuels reduction treatments such as stand thinning and prescribed fire to reduce fuel loads caused by more than a century of fire suppression. We evaluated the effects of ungulate herbivory and fuels reduction, alone and in concert, on deciduous shrub assemblages in coniferous dry forests of the interior west. We measured shrub richness, diversity, height, abundance and community composition in forest stands that underwent fuels reduction 15–17 years earlier, compared to untreated stands where no silvicultural treatments have occurred in over 50 years. Within each stand type, we also measured shrub assemblages in stands with and without ungulate herbivory. Shrub richness, diversity, frequency and height all declined in stands subjected to either fuels reduction treatments or herbivory; effects were most pronounced under the combined effect of fuels reduction and herbivory. Fuels reduction and herbivory also resulted in significant differences in shrub abundance and assemblage composition. Fuels reduction in dry forests with abundant ungulates may contribute to suppressed, more homogenous shrub communities. These effects may result in unintended impacts or alterations to important ecosystem processes and forest resources. Our results highlight the importance of considering responses of forest resources with low economic value, such as shrubs, in forest management activities. 1. Introduction cattle, shrubs are an important forage resource, particularly in late summer through winter, following senescence of herbaceous vegetation Deciduous woody species (shrubs) are a common and integral (Canon et al., 1987; Clark et al., 2013; Hegland and Rydgren, 2016). component of the understory of most conifer forests, and their abun- Under certain conditions, ungulate herbivory can substantially re- dance, richness and structure can influence a wide range of ecological duce or eliminate the functional presence of highly preferred species, processes and forest resources (Johnson and Clausnitzer, 1992; Moser leaving behind fewer, less palatable shrubs on the landscape (Endress and Witmer, 2000; Hagar, 2007a; Kleintjes Neff et al., 2007; Endress et al., 2012; Frerker et al., 2013; Endress et al., 2016; Lilleeng et al., et al., 2012; Lilleeng et al., 2016). Deciduous shrubs and broadleaved 2016). Consequently, strong herbivore pressure is considered a major trees make up a large proportion of plant diversity of temperate and contributor to the decline of shrubs in temperate and boreal forests boreal conifer forests, and their presence can influence soil fertility and (Frerker et al., 2013; Endress et al., 2016; Hegland and Rydgren, 2016), nutrient cycling with the addition of high-quality litter to the soil as resulting in changes to species composition and forest dynamics well as nitrogen fixation by some species (Hagar, 2007a). Shrubs also (Woodward et al.1994; Baker et al., 1997; White et al., 1998; Lilleeng increase structural heterogeneity and complexity in the understory and et al., 2016; Kolstad et al., 2018). Ungulates may also affect the lower canopy layers that promote high biodiversity (Niemi et al., 1998; structural complexity of forest stands by altering shrub architecture and Hagar, 2007a, 2007b; Rooney, 2001; Roberson et al., 2016). They limiting heights below the browse line (height below which shrubs support diverse food webs for insects, soil organisms, and wildlife in- cannot escape heavy browsing pressure; Keigley et al., 2002), resulting cluding small mammals, birds, and bears by providing cover and food in hedged or ‘arrested’ shrubs (Hester et al., 2000; Riggs et al., 2000; resources (e.g., leaves, roots, berries) that differ in composition from Beschta and Ripple, 2012; Endress et al., 2016). Herbivore-related those provided by other vegetation (Berg, 2002; Hagar, 2007b; Briand suppression of the shrub layer can have cascading impacts on stand et al., 2009). For wild and domestic ungulates such as elk, deer and dynamics including: altered competitive interactions between plant ⁎ Corresponding author at: Eastern Oregon Agricultural Research Center – Union Station, Oregon State University, PO Box E, Union, OR 97883, USA. E-mail addresses: [email protected] (J.P. Averett), [email protected] (M.J. Wisdom), [email protected] (B.A. Endress). https://doi.org/10.1016/j.foreco.2020.118031 Received 23 August 2019; Received in revised form 19 February 2020; Accepted 24 February 2020 0378-1127/ © 2020 Elsevier B.V. All rights reserved. D. Hall Defrees, et al. Forest Ecology and Management 463 (2020) 118031 species (Pastor et al., 1988; Randall and Walters, 2011); decreased re- stands both fuels treated and exposed to ungulate herbivory. productive potential and establishment of shrubs (Beschta and Ripple, 2008); modified understory shade and subsequent changes to unders- 2. Methods tory herbaceous composition (Woodward et al., 1994); decreased structural complexity of the sub-canopy with related impacts to fauna 2.1. Study area (Hagar, 2007a) and invertebrates (Roberson et al., 2016; Lilleeng et al., 2018); and reduced fruit production that can have cascading effects Research was conducted within the 10,000 ha Starkey Experimental across multiple trophic levels (Ripple et al., 2014). Growth of shrub Forest and Range (SEFR) in the Blue Mountain Ecological Province of species that are highly selected in the diets of ungulates can be sup- northeast Oregon. Elevations range from 1200 m to 1500 m above sea pressed to such a degree that even low herbivore densities may be level. Precipitation averaged 510 mm annually, most of which occurred sufficient to arrest growth below the browse line (Hester et al., 2000; as winter snow or spring rain (Rowland et al., 1997). Late summer Endress et al., 2016). (July-August) is a period of predictably high temperatures, typi- Fuels reduction treatments, including mechanical thinning and cally > 30° C, and sparse precipitation, often totaling < 20 mm and prescribed fire, can also affect shrub assemblages (Willms et al., 2017). defined as drought most years (Prism Climate Group, 2014; National Mechanical thinning and prescribed fire have generally been shown to Oceanic and Atmospheric Administration, 2019). This climate supports reduce shrub abundance and cover in favor of herbaceous vegetation, at drier montane conifer forest types typical of those in the interior wes- least in the short term (Curtis et al., 1998; Willms et al., 2017). Fuels tern United States. Our research specifically focused on coniferous reduction treatments, which usually involve combinations of stand forest stands classified as dry forest potential vegetation types (Burcsu thinning and prescribed fire, are common silvicultural practices in dry et al., 2014) dominated by overstories of Douglas fir(Pseudotsuga coniferous forests of the interior western U.S. Fuels reduction is one of menziesii) and grand fir(Abies grandis). Stands also included Ponderosa the primary forest management goals of national forests in the western pine (Pinus ponderosa), western larch (Larix occidentalis), Englemann U.S., with plans for a significant increase in fuels reduction activities spruce (Picea engelmannii), and lodgepole pine (Pinus contorta) with (e.g., Reinhardt et al., 2008; USDA Forest Service, 2018) because fuel varying degrees of overstory subdominance (Franklin and Dyrness, loads in many dry conifer stands greatly exceed those of pre-European 1988). settlement, and constitute a major source of uncharacteristic stand-re- Approximately 500 cow-calf pairs graze SEFR from mid-June to placement fire events (Franklin and Agee, 2003; Hessburg et al., 2005). mid-October annually, with an additional 200 mule deer and 350 elk Despite the increase in fuels reduction efforts in recent years and plans that occupied SEFR from April-November for the last 25 years (Endress to further increase their scale and scope, little is known about how such et al., 2016). Ungulate densities for cattle (7.15/km2), mule deer (1.95/ treatments affect shrub assemblage composition and structure. km2), and elk (4.55/km2) were similar to those outside of SEFR Further complicating our understanding of shrub responses to forest (Wisdom and