Demographic responses of rare forest plants to multiple stressors: the role of deer, invasive species and nutrients

Abstract

Summary1. Forest ecosystems in eastern North America face multiple threats or stressors including plant and animal invasions and increased white-tailed deer (Odocoileus virginianus) herbivory. While each stressor may have independent detrimental effects on native biota, stressors often co-occur and are likely to have interactive effects. Despite recognition that concurrent processes drive plant demographic responses, few studies evaluate independent and combined effect of stressors. 2. Using a network of 12 sites that varied in non-native plant cover and introduced earthworm density and biomass, we experimentally assessed effects of deer exclusion (30 9 30 m paired plots), slug exclusion and nutrient addition on survival, growth and fecundity of four rare forest understorey plant species (Aristolochia serpentaria L., Agrimonia rostellata Wallr., Carex retroflexa Muhl. ex Willd and Trillium erectum L). 3. We found that single and combined effects of stressors were species-specific and varied according to plant stage and demographic parameter. Interactions were prevalent among all studied stressors and, for most cases, did not follow predicted responses. 4. We found detrimental deer herbivory effects on reproductive A. rostellata and non-consumptive effects on A. serpentaria and T. erectum. Negative deer effects follow underlying predictions, and override effects of other stressors, even when other concurrent processes are at play. 5. Contrary to expectations, we did not find negative effects of non-native plants. Earthworms had positive effects on A. rostellata and C. retroflexa (especially when deer were excluded), but negative effects on T. erectum. Slug effects were dependent on other stressors, especially on interactions with non-native plants and earthworms. Nutrient addition had a negative effect on survival of A. serpentaria and T. erectum, but positive effects on C. retroflexa and T. erectum growth. 6. Synthesis. We found prevalent but unpredictable interactions among all study factors and plant species. Negative direct and indirect deer effects overrode impacts of all other stressors we investigated. A multifactor approach is critical to predict plant responses to concurrent environmental forces. Assessment of combined effects should form an essential component of subsequent research on plant demography and management of declining species.

Management of invasive Phragmites australis in the Adirondacks: a cautionary tale about prospects of eradication

“…However, deciding which plant species to target for control is difficult, especially when multiple species co-occur and each is associated with or facilitated by other biotic or abiotic stressors (Simberloff and Von Holle 1999;Bertness et al 2002;Minchinton and Bertness 2003;Nuzzo et al 2009;Fisichelli et al 2013;Kuebbing et al 2013;Dávalos et al 2014Dávalos et al , 2015Craven et al 2016). An abundance of prioritization schemes exist, largely developed by scientists, to help land managers make better informed decisions that focus control efforts (Robertson et al 2003;Fox and Gordon 2009;Downey et al 2010;Esler et al 2010b;Darin et al 2011;Barney et al 2013;Abella et al 2015;Adams and Setterfield 2015;Lindenmayer et al 2015).…”

Single and interactive effects of deer and earthworms on non-native plants

“…Deer are major drivers in our study system, affecting native flora directly through herbivory (Dávalos et al, 2014) and indirectly through facilitation of earthworm (Dávalos et al, 2015) and non-native plant populations. Invasive earthworms and invasive plants are also linked, such that our target non-native plant species (A. petiolata, B. thunbergii and M. vimineum) occur at significant abundances only at sites with high earthworm density.…”

“…Target non-natives are also positively associated with earthworms (Nuzzo et al, 2009;Whitfeld et al, 2014) and, in fact, rarely occur at high densities at sites with low earthworm abundance (personal observation). Considering that these stressors co-occur and that mounting evidence indicates that populations and impacts of invasive plants, earthworms and deer are characterized by complex non-additive interactions (Waller and Maas, 2013;Dávalos et al, 2014;Flory and Bauer, 2014) it is paramount to quantify their combined effects.…”

Independent Effects of Invasive Shrubs and Deer Herbivory on Plant Community Dynamics

“…Herder) was controlled, planted jewelweed (Impatiens capensis Meerb) was larger and produced more fruits when protected from herbivory [37]. In contrast, growth, survival, and fecundity of several native herbaceous species did not differ between sites with and without a complex of invasive species (shrub/forb/graminoid), but growth of taller plants was negatively impacted by deer herbivory [39]. Exclusion of deer herbivory, but not removal of Japanese honeysuckle (Lonicera japonica Thunb.…”

“…As noted above, few studies have examined whether the effects of invasive shrubs and deer herbivory on plant communities are synergistic, additive, or antagonistic; and those studies were limited to two years [37,[39][40][41]. Our general hypothesis was that the cover dynamics of some plant guilds (defined below) would not be independent of deer herbivory and levels of invasive shrub control.…”