Herbicides and herbivory interact to drive plant community and crop‐tree establishment

Abstract

Land management practices often directly alter vegetation structure and composition, but the degree to which ecological processes such as herbivory interact with management to influence biodiversity is less well understood. We hypothesized that large herbivores compound the effects of intensive forest management on early seral plant communities and plantation establishment (i.e., tree survival and growth), and the degree of such effects is dependent on the intensity of management practices. We established 225-m wild-ungulate (deer and elk) exclosures, nested within a manipulated gradient of management intensity (no-herbicide control, light herbicide, moderate herbicide, and intensive herbicide treatments), replicated at the scale of whole harvest units (10-19 ha). Vegetation structure, composition, and crop-tree responses to herbivory varied across the gradient of herbicide application during the first two years of stand establishment, with herbivory effects most evident at light and moderate herbicide treatments. In the moderate herbicide treatment, which approximates management applied to >2.5 million hectares in the Pacific Northwest, United States, foraging by deer and elk resulted in simplified, low-cover plant communities more closely resembling the intensive herbicide treatment. Herbivory further suppressed the growth of competing vegetation in the light herbicide treatment, improving crop-tree survival, and providing early evidence of an ecosystem service. By changing community composition and vegetation structure, intensive forest management alters foraging selectivity and subsequent plant-herbivore interactions; initial shifts in early seral communities are likely to influence understory plant communities and tree growth in later stages of forest development.

Deer‐mediated ecosystem service versus disservice depends on forest management intensity

“…When broadleaf competition was untreated, high levels of competition likely limited the ability of crop trees to grow above competition and browsing. With Light herbicide applications, deer aided in promoting crop‐tree survival by controlling broadleaf growth, consistent with preliminary findings presented in Stokely et al (). However, deer also browsed crop trees most frequently in the Light treatment, resulting in reduced height growth.…”

“…Alternatively, patches of abundant forage may increase the prevalence of browsers and thus browse damage to crop trees (Beguin et al, ). Stokely, Verschuyl, Hagar, and Betts () reported that with low levels of vegetation management in the Pacific Northwest (PNW), USA, deer provided an ecosystem service by reducing competition from deciduous shrubs, thereby improving initial crop‐tree survival.…”

Do birds help trees grow? An experimental study of the effects of land‐use intensification on avian trophic cascades

“…Forest plantations here are dominated by nursery‐stock Douglas‐fir ( Pseudotsuga menziesii ), along with other naturally regenerating conifers, deciduous trees, shrubs, and herbs. See Stokely et al (2018) for a detailed description of the study area.…”

Ecological consequences of compound disturbances in forest ecosystems: a systematic review

“…Droughts consisted of discrete periods of exceptionally dry conditions, triggered by anomalously high temperatures and low moisture availability (Peters et al 2011, Keith et al 2012. Intensive forest management (IFM; sensu Stokely et al 2018) was used to describe clear-cut harvesting, experimental manipulation, and other related silvicultural entries (e.g., replanting and herbicide application). Moths consisted of a Lymantria monacha outbreak (Bottero et al 2013), and successive Epirrita autumnata and Operophtera brumata outbreaks (Karlsen et al 2013).…”

“…Salvage logging was conducted after wind and before fire, after wind without subsequent fire, and after fire. Intensive forest management (IFM; sensu Stokely et al 2018) was used to describe clear-cut harvesting, experimental manipulation, and other related silvicultural entries (e.g., replanting and herbicide application). A psyllid insect (Cardiaspina spp.)…”