17 research outputs found
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Interactive effects of silvicultural herbicides and cervid herbivory on early seral plant communities of the northern Oregon Coast Range
Intensive forest management (IFM, dense conifer plantings and herbicide applications) may alter the characteristics of early seral plant communities that function as major habitat resources for a host of wildlife species, including cervid herbivores such as Cervus elaphus and Odocoileus hemionus. Such large herbivores can also substantially affect plant community characteristics and succession, especially in disturbed early seral habitats. I hypothesized that the effect of cervid herbivory on early seral plant communities is mediated by the effect of silvicultural herbicide treatments. If that is the case, intensively treated stands with low plant cover and diversity should be most susceptible to herbivory, as cervids are less selective and herbivory impacts are highly concentrated where forage has been diminished.
To test this hypothesis, I experimentally established paired 225 m² cervid Exclusion and Open-Herbivory treatment plots in 28, 12-15 ha early seral plantation stands throughout the northern Oregon Coast Range, USA, representing a gradient in IFM. The gradient included three herbicide treatments and a no-spray Control applied at the stand scale and replicated using a randomized complete block design. I compared estimates of cover, height and diversity for entire plant communities and specific functional groups among herbivory and herbicide treatments using mixed-effects models with a blocked split-plot design.
I found convincing evidence that the effect of herbivory was mediated by herbicide treatment. No-spray Control stands were too vigorous, diverse and rich with native perennial herb and deciduous shrub forage to be substantially impacted by cervid herbivory. The herbaceous specific, Light herbicide treatment reduced Shannon diversity and the cover and richness of native-perennial herbs, releasing deciduous shrub height growth where cervids where excluded. Highly selective herbivory suppressed the shrub height response by 20.5 cm, increasing the abundance and richness of introduced herb species. The broad spectrum, Moderate herbicide treatment reduced diversity, forage cover and diminished the cover and richness of deciduous shrubs and native-perennial herbs, favoring the dominance of introduced-ruderal herbs. Herbivory in the Moderate treatment reduced total cover by 17.7 percent cover, moderate-quality forage cover by 13.2 percent cover and native perennial herb richness by 1.5 species, while suppressing the cover of introduced-ruderal herbs by 4.58 percent cover and reducing the height of ferns and introduced-perennial herbs by 19.9 and 17.3 cm, respectively. Plant communities subject to the Heavy treatment were the most depauperate of all and herbivory exacerbated the effect of this treatment on native-perennial herbs only. Average height of dominant vegetation was consistently lower with cervid access across all stands, especially with Moderate herbicide treatment.
My results provide evidence that by reducing diversity and the abundance of native forage species, herbicide treatments altered herbivory selectivity and pressure. The alteration of herbivory pressure in turn influenced the outcome of herbicide treatments, resulting in an exacerbated effect with Moderate and Heavy treatments. The effect of herbivory in controlling vegetation in managed stands may have positive implications for conifer seedling growth, constituting a possible 'ecosystem service' by cervids in the Pacific Northwest. This 'service' may be to the detriment of biodiversity and other early seral associates when coupled with common intensive forest management practices
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Tradeoffs and Synergies among Wild Ungulates, Early Successional Plant Communities and Management Intensification in Forest Plantations of the Oregon Coast Range
As the global demand for natural resources increases, more land will be intensively managed for the production of commodities such as timber, with potential consequences to biodiversity, ecological functioning and ecosystem services provided to society. Although there is strong consensus that intensive land management practices can negatively affect biodiversity, less well understood are the ways that management and biodiversity-associated processes interact. In managed forest systems, wild ungulates have caused shifts in forest succession by selectively browsing regenerating trees and have promoted exotic species proliferation by negatively affecting native biodiversity. However, few studies have addressed the ways that management intensification alters the effects of wild ungulate herbivores on plant community assembly, plantation development and native species conservation.
To address this knowledge gap, I used data from a six-year experimental dataset to test hypotheses pertaining to the interactive effects of forest management intensity and herbivory by both black-tailed deer (Odocoileous hemionus colombianus) and Roosevelt elk (Cervus elaphus roosevelti) in the Oregon Coast Range, USA. Specifically, the experiment manipulated early forest plantation management intensity via herbicides and deer and elk access to vegetation via exclosures. The six-year dataset included annual measurements of plant species and crop tree responses to herbivory and herbicide and the documentation of ungulate foraging via camera traps. In Chapter 1, I present the theory, management history and initial findings from my Master’s thesis that helped formulate the hypotheses for the dissertation, which are specifically addressed in Chapters 2-4.
For Chapter 2, I hypothesized that by simplifying community composition at local scales, intensive forest management practices cause plant communities to be more susceptible to herbivore-mediated changes across time and space. This hypothesis relates to the initial findings of my Master’s thesis, which provided evidence that the initial effects of herbicide on plant communities were mediated by the effect of ungulate herbivory (Stokely 2014). To test this hypothesis, I used six years of plant community data to test the interactive effects of herbivory and herbicide treatments on the changes in plant community composition, functional group associations and diversity quantified at multiple scales of species assembly. I found evidence that selective foraging by deer and elk slowed developmental changes in community composition, resulting in divergent plant communities, but the strength and direction of the effect depended on management intensity. The effect of herbivory was strongest in a commonly used management regime that initially suppressed plant diversity and native broadleaf assemblages, thereby promoting the abundance of exotic herbaceous species. Selective foraging suppressed the succession of broadleaf species and further promoted the homogeneity of herbaceous assemblages among stands, resulting in lower beta and gamma diversity.
For Chapter 3, I hypothesized that wild ungulates provide an ecosystem service in forest plantations when selective browsing of non-crop vegetation reduces competition and increases crop-tree growth, but the direction and magnitude of the service should vary by management intensity at stand and landscape scales. This hypothesis relates to initial findings from an early manuscript of the study (Stokely et al. 2018), which revealed that deer and elk improved crop-tree survival at Lighter management intensities. To test the hypothesis, I used a five-year dataset of Douglas-fir (Pseudotsuga menziesii) volume, stand-scale broadleaf cover (i.e., angiosperm shrubs and trees) and landscape-scale broadleaf habitat measurements as well as deer and elk camera trap detections to assess the prevalence of foraging ungulates in plantations. I found evidence that deer and elk provided an ecosystem service in intensively managed plantations by further suppressing the development of broadleaf species, thereby promoting crop-tree growth. Alternatively, deer and elk negatively affected crop-tree growth in less intensively managed stands where crop-trees were unable to outgrow competition and browsing. The ecosystem service increased with the prevalence of deer and elk in intensively managed stands, which was positively associated with both stand and landscape-scale abundance of broadleaf habitat. This finding indicates that surrounding broadleaf habitat is an important factor in providing a source of herbivores to plantations, which serve to further suppress stand-scale broadleaf regeneration and provide the ecosystem service we observed.
For Chapter 4, I hypothesized that by suppressing native plant species, wild ungulates and management intensification synergistically promote the proliferation of exotic species. This hypothesis relates to initial findings from the Master’s thesis, which revealed that herbicides and herbivory promoted exotics at certain management intensities (Stokely 2014). To test this hypothesis I used a six-year dataset of native and exotic species responses to management intensity and herbivore exclusion. I found evidence that by initially reducing the diversity and abundance of native species, management intensification caused native species to be more susceptible to herbivory, which further promoted the proliferation of exotics.
The findings of Chapters 1-3 indicate that by initially suppressing plant diversity, intensive management practices (i.e., herbicide treatment) altered early successional plant community assembly, promoting the abundance of exotic herbaceous species. The altered composition of heavily managed plantations created susceptibility to the effects of herbivory, which further suppressed native broadleaf succession and lengthened the duration of the herbaceous stage. By suppressing broadleaf species, deer and elk provided an ecosystem service to plantations, increasing both the survival and volume growth of Douglas-fir trees. Despite apparent browse damage in those stands, the lack of competition allowed crop trees to accrue biomass and eventually release above regenerating competitors and browsing herbivores. When broadleaf species were retained at lighter treatments, the combined effects of browsing and competition inhibited the release of crop trees above competitors and browsing deer and elk. The positive ecosystem service provided by deer and elk in intensively managed plantations had negative consequences for native early successional species, further promoting exotic species proliferation.
The findings presented in this dissertation further support for the need for ecological research to account for multiple interacting disturbances and ecological processes. Additionally, the scale at which plant community responses are quantified and perceived effects of management are considered may provide alternative interpretations of the effects of interacting disturbances. In Chapter 5, I present an overview and synthesis of these findings and discuss the mechanisms responsible for our observations as well as management implications and recommendations
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Deer-mediated ecosystem service vs. disservice depends on forest management intensity - Dataset
This dataset consists of planted crop-tree growth metrics (Pseudotsuga menziesii), non-crop tree vegetation metrics, and foraging data for black-tailed deer (Odocoileus hemionus columbianus) and Roosevelt elk (Cervus canadensis rooseveli) collected from the Intensive Forest Management experiment, Oregon Coast Range, USA, 2011-2016. The objective of the experiment was to quantify the effects of silvicultural herbicide treatments on biodiversity and ecosystem functions.MANUSCRIPT ABSTRACT
As global terrestrial biodiversity declines via land-use intensification, society has placed increasing value on non-commercialspecies as providers of ecosystem services. Yet, many deer species and non-crop plants are perceived negatively when they decrease crop productivity, leading to reduced economic gains and human-wildlife conflict. We hypothesized that deer provide an ecosystem service in forest plantations by controlling competition and promoting crop-tree growth, although the effects of herbivory may depend on forest management intensity. If management negatively affects foraging habitat at local and landscape scales, then we expected browsing to shift to less-palatable crop trees. To test these hypotheses, we established a 5-year experiment that manipulated early forest management intensity via herbicide treatments and access of two deer species to vegetation via exclosures. Contrary to our hypothesis, deer provided an ecosystem service at high management intensities and a disservice occurred with low-intensity management. Crop-tree growth and survival was greatest
when herbivory and herbicides suppressed broadleaf regeneration. In contrast, crop-tree growth was lowest when broadleaf vegetation was retained and crop-trees were subject to both browse damage and competition. We found a positive, yet variable, association between deer detections and stand- and landscape-scale broadleaf habitat, and despite initial reductions in forage, herbivory pressure was similar among management intensities. When broadleaf vegetation was suppressed by herbicides and herbivory, selection of herbaceous forage by deer intensified, likely aiding in the service. Overall, our findings indicate that the effects of vegetation management for promoting timber production are highly dependent on the presence of large herbivores.
Synthesis and applications: Although deer are thought to reduce crop productivity in many systems, we found that herbivory switched from reducing crop tree growth where non-crop vegetation was retained, to promoting crop tree growth when both herbivory and herbicides suppressed competing vegetation. However, the provision of this ecosystem service is likely contingent on the amount of forage available in the landscape and subsequent foraging pressure. We conclude that nature’s capacity to provide ecosystem services depends on the intensity of management at local and landscape scales.Keywords: Elk (Cervus canadensis roosevelti); Deer (Odocoileus hemionus columbianus); Douglas-fir (Pseudotsuga menziesii
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Synergistic effects of wild ungulates and management intensification suppress native plants and promote exotics - Dataset
This dataset consists of native and exotic plant species metrics, including cover, richness, relative abundance and responses to the exclusion of wild ungulates and herbivory. The data were used for the manuscript: "Synergistic effects of wild ungulates and management intensification suppress native plants and promote exotics", published in the journal Forest Ecology and Management. We used mixed-effects models in the R Statistical Package to test the interactive effects of herbicide and herbivory on native and exotic species for six years following timber harvest.MANUSCRIPT ABSTRACT
In managed forest landscapes, conflicts among wood production, ungulates, native biodiversity, and exotic species are common. As humanity allocates more land as intensively managed plantations, these conflicts may become more severe. For instance, native ungulates have been implicated in the loss of native plant diversity, declines in timber revenues and the spread of exotic species in many forest systems, yet the synergistic effects of management and ungulate herbivory are not well understood. We hypothesized that herbicide and herbivore-induced suppression of native forage species promotes the release of exotic species in young forest plantations. Further, we expected herbivory and the retention of native forage via less intensive management would have negative consequences for reforestation objectives. To test this hypothesis, we conducted an experiment that manipulated management intensity (via herbicides) and access of two deer species to vegetation (via exclosures) on 28 operational Douglas-fir plantations of the Pacific Northwest, USA. We recorded yearly plant species cover estimates and tested the effects of herbivory and herbicides on native and exotic plants during the first six years of plantation establishment. Heavier herbicide treatments reduced the cover and diversity of native plants, and increased the cover of exotics over time, particularly that of exotic herbaceous species. Regardless of herbicide intensity, herbivores had reduced the cover of native forage species. In a treatment that represents operational standards applied to over 2.5 million ha in the region, the suppression of native species by deer corresponded with an increase in exotic species cover by 23 percent, resulting in equitable abundance of native and exotic plants. As expected, Douglas-fir growth was suppressed when herbivores were present and vegetation was left untreated by herbicides, although the presence of deer promoted both crop-tree growth and the relative abundance of exotics in our most intensive treatment, presumably due to the added suppression of native competitors. Our findings suggest that wild ungulates amplify
management-driven shifts toward exotic species in intensively managed forest plantations. Exotic plant forage status and adaptations to frequent disturbance seem to be key mechanisms for the synergistic effects we observed. Without herbicides, diverse assemblages of native species are buffered from herbivory and exotic species proliferation, with potential tradeoffs for timber production. Our results highlight the role of management intensity in modifying the interactions among exotic plants and native herbivores, contributing to a mechanistic understanding of the role of native biodiversity in regulating exotic species spread. Our data provide further support for growing evidence that synergistic effects of multiple drivers can facilitate exotic proliferation
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Wild ungulates compound herbicide-mediated simplification of early-successional plant communities in forest plantations - Dataset
This dataset consists of vascular plant species cover estimates collected from 12, 1x1-m quadrats, nested within 15x15-m plots that excluded and were open to wild ungulate access. The wild ungulate exclusion experiment was conducted in young forest plantations (<6 years old), each randomly assigned to one of 3 silvicultural herbicide treatments or an untreated control, collected from 2011-2016 in the Oregon Coast Range, USA. Data were used in the analyses presented in Stokely et al. (2021).
Stokely, T. D., U. Kormann, M. G. Betts. 2021. Wild ungulates compound herbicide-mediated simplification of early successional plant communities in forest plantations. Forest Ecology and Management. 494:1-13. https://doi.org/10.1016/j.foreco.2021.11927
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Dataset: Experimental evaluation of herbicide use on biodiversity, ecosystem services and timber production tradeoffs in forest plantations.
This dataset includes species richness of native plants, pollinators and birds as well as 13 proxies for ecosystem services representing timber production, culturally-valued plants and animals, and regulating services tied to forest productivity. The data were collected from an experiment conducted in the Oregon Coast Range USA that sought to quantify the effects of herbicides on species richness and ecosystem services in young Douglas-fir plantations (Pseudotsuga menziesii), including the relative tradeoffs between timber production and non-timber ecosystem services