Multiple Browsers Structure Tree Recruitment in Logged Temperate Forests

Abstract

Historical extirpations have resulted in depauperate large herbivore assemblages in many northern forests. In eastern North America, most forests are inhabited by a single wild ungulate species, white-tailed deer (Odocoileus virginianus), and relationships between deer densities and impacts on forest regeneration are correspondingly well documented. Recent recolonizations by moose (Alces americanus) in northeastern regions complicate established deer density thresholds and predictions of browsing impacts on forest dynamics because size and foraging differences between the two animals suggest a lack of functional redundancy. We asked to what extent low densities of deer + moose would structure forest communities differently from that of low densities of deer in recently logged patch cuts of Massachusetts, USA. In each site, a randomized block with three treatment levels of large herbivores–no-ungulates (full exclosure), deer (partial exclosure), and deer + moose (control) was established. After 6–7 years, deer + moose reduced stem densities and basal area by 2-3-fold, Prunus pensylvanica and Quercus spp. recruitment by 3–6 fold, and species richness by 1.7 species (19%). In contrast, in the partial exclosures, deer had non-significant effects on stem density, basal area, and species composition, but significantly reduced species richness by 2.5 species on average (28%). Deer browsing in the partial exclosure was more selective than deer + moose browsing together, perhaps contributing to the decline in species richness in the former treatment and the lack of additional decline in the latter. Moose used the control plots at roughly the same frequency as deer (as determined by remote camera traps), suggesting that the much larger moose was the dominant browser species in terms of animal biomass in these cuts. A lack of functional redundancy with respect to foraging behavior between sympatric large herbivores may explain combined browsing effects that were both large and complex.

Cool as a moose: How can browsing counteract climate warming effects across boreal forest ecosystems?

“…However, as we expected (Hypothesis 2), indirect negative moose effects via reduced height also play a role. For preferred forage species, it is possible that moose herbivory creates “browsing traps” (Staver and Bond 2014, Faison et al 2016, Churski et al 2017, Olofsson and Post 2018), from which trees are unable to escape (Kolstad et al 2018 a ) and thus the growth rates are permanently low, as smaller trees grow slower than taller ones.…”

Ungulate herbivores as drivers of aspen recruitment and understory composition throughout arid montane landscapes

“…Herbivory by wild and domestic ungulates can be a major driver of forest health and regeneration for a wide range of tree species throughout the world (Bernes et al, 2018; Beschta & Ripple, 2009; Côté et al, 2004; Danell et al, 2003; Redick & Jacobs, 2020). Numerous studies have documented that browsing can cause reduced tree recruitment (Bradshaw & Waller, 2016; Faison et al, 2016), large‐scale changes in forest demographics (Davis et al, 2011), and shifts in the structure and composition of understory plant communities (Martin et al, 2011; Tanentzap et al, 2011). In an era of unprecedented global change, ungulate browsing may have especially severe consequences for forests, as many tree species are already suffering declines due to a variety of anthropogenic stressors, including drought and high temperatures (Allen et al, 2010; Anderegg et al, 2015; van Mantgem et al, 2009), altered fire regimes (Cohen et al, 2016), and novel pests and pathogens (Kautz et al, 2017; Sturrock et al, 2011; Weed et al, 2013).…”

Long‐term and interactive effects of different mammalian consumers on growth, survival, and recruitment of dominant tree species

“…Understanding the patterns of tree decline and their drivers is of great importance given that forests, woodlands, and savannas cover 30% of the world's terrestrial landscapes (FAO, 2006) and have major influences on ecological systems, including carbon sequestration, water purification, and habitat provisioning (Trumbore et al, 2015). Although many factors have been implicated as drivers of tree decline-either acting individually or in combination-mammalian consumers are increasingly recognized as factors that can reduce tree recruitment in many forests of the world (Bradshaw & Waller, 2016;Davis, Tyler, & Mahall, 2011;Faison, DeStefano, Foster, Rapp, & Compton, 2016;Long, Pendergast, & Carson, 2007). Due to intensifying human land use and declines in large carnivore populations, many ungulates have expanded their ranges and/or increased in abundance worldwide in recent decades, and are frequently implicated in the reduced performance and abundance of dominant trees in many forested ecosystems (Beschta & Ripple, 2009;Martin, Arcese, & Scheerder, 2011;Ripple & Beschta, 2007b;Russell, Zippin, & Fowler, 2001;Takatsuki, 2009;Tanentzap et al, 2011).…”

“…Although multiple mammalian consumer taxa commonly co-occur in the same system, studies typically address the effects of either only a single consumer species on focal plant populations and communities or all consumer species together (Faison et al, 2016). Given the practical constraints of conducting large-scale experiments in the field, focusing on individual species is understandable.…”