This article is an Invited Commentary on Stephenson et al. (2024). This commentary attempts to provide broader context of the research within the body of literature on species loss and ecosystem functioning and highlights its relevance to conservation and global change.

An increasingly important goal of federal land managers in seasonally dry forests of the western US is restoring forest resilience. In this study, we quantified the degree to which a thinning treatment in a dry forest of eastern Oregon restored aspects of forest resilience by focusing on key functional attributes of our study system. First, we measured several physiological responses of overstory trees that are associated with resilience, including radial growth, resin production, abundance of non-structural carbohydrates (NSC), and leaf area. Second, we investigated understory vegetation responses including species diversity, composition, and cover by growth form that influence fire behavior and resilience to disturbance. We found that tree radial growth was greater in trees in thinned stands beginning three years post-treatment. The abundance of key chemical compounds found in trees, including resin, starch, total NSC and sucrose did not differ between treatments; however, abundance of glucose plus fructose was lower in treated stands, suggesting mobilization and use of carbon reserves for foliar and wood production following thinning. We observed an increase in species richness and diversity within thinned stands three to four years after thinning, primarily due to the release of forbs and reestablishment of graminoids following ground disturbance. Here, we demonstrate that elements of forest resilience can be restored in dry forest systems via selective thinning to promote historical forest structure. In forests where thinning reduces stand density, vigorous overstory trees and increased herbaceous cover can help facilitate the re-establishment of low intensity surface fire regimes that maintain stable and persistent vegetative states. Understanding the ecological effects of fuel reduction treatments allows land managers to assess potential forest resiliency and adapt future treatments based on the observed results of previous activities.

1. The value of non-commodity ecosystem services provided by forests is widely recognized, but intensive forest management practices are increasing, with uncertain consequences for a multitude of these services. Quantitative relationships among biodiversity conservation, timber production and other ecosystem services remain poorly understood, especially during the early-successional period of intensively managed forestlands. 2. We manipulated management intensity in regenerating forest plantations to test the prediction that treatments aimed at maximizing timber production decrease biodiversity conservation and non-timber services. We measured species richness of 3 taxonomic groups and 13 proxies for provisioning, cultural and regulating services within stands randomly assigned to one of the three herbicide application intensities or an untreated control. 3. Herbicides increased allocation of net primary production to crop trees, increasing projected timber volume and revenues at 40- and 60-year harvest ages. Commonly used herbicide prescriptions reduced culturally valued plants by 71%, wild-ungulate forage by 41%, avian richness by 20% and pollinator floral resources by 42%, the latter being associated with 38% fewer pollinator species. However, agriculturally valued bumblebees, pollination of blueberries, avian-mediated arthropod control, wild ungulate observations and regulation services tied to forest productivity appeared unaffected by increasing management intensity and timber production. 4. Species richness and flora-provided services in young forest plantations exhibited strong trade-offs with projected timber production, whereas post-treatment vegetation regeneration and site-level variation likely maintained a range of other services. Although vegetation recovery is important for supporting wildlife and some ecosystem services on industrial forestlands, it is unlikely that any single prescription can optimize both timber and non-timber benefits to society across managed forest landscapes. Instead, producing different services in discrete portions of the landscape may be necessary. 5. Synthesis and applications. We tested the effects of intensive forest management via herbicides on ecosystem services and found that biodiversity responses and services from early-successional vegetation trade-off against timber production. A number of services appeared to be compatible with timber production, although no single prescription optimized the full range of services. Stand-level biodiversity conservation and a variety of services could potentially be provided by treatment skips and less-intensive management on productive sites, although it is unlikely that all services can be optimized without landscape-level planning.

Understanding the degree to which management interacts with ecological processes to drive the structuring of biodiversity remains a central challenge in ecology. For instance, the effects of herbivores on post-disturbance vegetation development may depend on the degree to which anthropogenic interventions have altered plant communities. We hypothesized that by simplifying plant species composition at local scales, intensive forest management practices should amplify herbivore-mediated changes in early-successional vegetation development and diversity at multiple levels of biological observation (i.e., alpha, beta, and gamma diversity). To test this hypothesis, we implemented a six-year experiment that manipulated post-timber harvest vegetation establishment via a gradient in vegetation management intensity (i.e., herbicides), and quantified compositional changes with and without wild-ungulate herbivores. Increasingly intensive herbicide treatments suppressed broadleaf vegetation and reduced plant diversity at alpha and gamma levels, while the effects of wild-ungulates were most evident in measures of community change and beta diversity for a commonly applied treatment type. Following the broad-spectrum and follow-up herbaceous treatment combination, herbivory contributed to the suppression of vegetation cover, slowing plant community development by 21%, and resulting in 27% less among-plot heterogeneity (i.e., lower beta diversity). We found no effects of herbivory alpha diversity, but the divergence in communities subjected to herbivory resulted in disparate community composition and lower gamma diversity. Herbicide and herbivore-mediated alterations in community composition was due the suppression of broadleaf species and subsequent release of common herbivory-tolerant herbaceous species. Although herbivory also slowed vegetation development without herbicides, the high diversity and abundance of broadleaf forages likely buffered untreated sites from the herbivore-mediated alterations of plant composition at alpha, beta, and gamma levels. The variation in effects of herbicide and herbivory in this experiment highlight the importance of testing for interactions between natural and anthropogenic agents on vegetation development and quantifying diversity responses beyond local scales. These results also suggest that large herbivores may contribute to management-induced simplification of young forests, with potential implications for successional processes and regional biodiversity conservation.

Understanding how land-management intensification shapes the relationships between biodiversity, yield, and economic benefit is critical for managing natural resources. Yet, manipulative experiments that test how herbicides affect these relationships are scarce, particularly in forest ecosystems where considerable time lags exist between harvest revenue and initial investments. We assessed these relationships by combining 7 yr of biodiversity surveys (>800 taxa) and forecasts of timber yield and economic return from a replicated, large-scale experiment that manipulated herbicide application intensity in operational timber plantations. Herbicides reduced species richness across trophic groups (-18%), but responses by higher-level trophic groups were more variable (0-38% reduction) than plant responses (-40%). Financial discounting, a conventional economic method to standardize past and future cash flows, strongly modified biodiversity-revenue relationships caused by management intensity. Despite a projected 28% timber yield gain with herbicides, biodiversity-revenue trade-offs were muted when opportunity costs were high (i.e., economic discount rates ≥7%). Although herbicides can drive biodiversity-yield trade-offs, under certain conditions, financial discounting provides opportunities to reconcile biodiversity conservation with revenue.

Research hypotheses have been a cornerstone of science since before Galileo. Many have argued that inclusion of multiple hypotheses (1) encourage discovery of mechanisms, and (2) reduce bias – both features that should increase transferability and reproducibility. However, we are entering a new era of big data and highly predictive models where some argue the hypothesis is outmoded. Indeed, using a detailed literature analysis, we found prevalence of hypotheses in eco-evo research is very low (6.7-26%) and static from 1990-2015, a pattern mirrored in an extensive literature search (N=302,558 articles). Our literature review also indicates that neither grant success or citation rates were related to the inclusion of hypotheses, which may provide disincentive for hypothesis formulation. Here we confront common justifications for avoiding hypotheses and present new arguments based on benefits to the individual. Although hypotheses are not always necessary, we expect their continued and increased use will help our fields move toward greater understanding, reproducibility, prediction, and effective conservation of nature.

Abstract Research hypotheses have been a cornerstone of science since before Galileo. Many have argued that hypotheses (1) encourage discovery of mechanisms, and (2) reduce bias—both features that should increase transferability and reproducibility. However, we are entering a new era of big data and highly predictive models where some argue the hypothesis is outmoded. We hypothesized that hypothesis use has declined in ecology and evolution since the 1990s, given the substantial advancement of tools further facilitating descriptive, correlative research. Alternatively, hypothesis use may have become more frequent due to the strong recommendation by some journals and funding agencies that submissions have hypothesis statements. Using a detailed literature analysis (N = 268 articles), we found prevalence of hypotheses in eco–evo research is very low (6.7%–26%) and static from 1990–2015, a pattern mirrored in an extensive literature search (N = 302,558 articles). Our literature review also indicates that neither grant success nor citation rates were related to the inclusion of hypotheses, which may provide disincentive for hypothesis formulation. Here, we review common justifications for avoiding hypotheses and present new arguments based on benefits to the individual researcher. We argue that stating multiple alternative hypotheses increases research clarity and precision, and is more likely to address the mechanisms for observed patterns in nature. Although hypotheses are not always necessary, we expect their continued and increased use will help our fields move toward greater understanding, reproducibility, prediction, and effective conservation of nature.

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 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 plant proliferation.

The ecological value of unmanaged early successional habitat created by disturbances has received considerable research attention. Less well studied is the capacity of tree plantations to contribute to local and regional biological diversity while subsidizing conservation strategies. We present a conceptual framework for describing variability in early successional forests and illustrate how the spatial pattern of this habitat may vary as a function of productivity, land ownership, and disturbance in the Pacific Northwest, USA. We posit that plantations will only increase in ecological and economic importance as the world addresses conservation and resource production challenges associated with climate change, altered disturbance regimes, increasing rates of global wood consumption, and evolving social, legal, and economic expectations. Researchers and managers can foster collaborative frameworks to encourage provisioning of non-timber services on private land, while prioritizing non-timber benefits of early successional habitat on public land to optimize biodiversity conservation and ecosystem services provided in managed forest landscapes.

By regulating populations of herbivores, predators can indirectly influence plant production. However, the factors influencing the strength of this type of trophic cascade are still unclear. We hypothesized that changes to plant community structure would affect the number of avian predators, thereby mediating cascade strength. Using a 4-yr, blocked, split-plot experiment, we independently manipulated both predators (birds) and plants in an early seral managed forest system in western Oregon, USA, and measured abundance across three trophic levels. We applied herbicides, as a surrogate for land-use intensification, to recently clear-cut stands to establish an experimental gradient in plant abundance and species richness, and excluded birds using 28, 225 m(2) exclosures. In total, we counted and identified 94,738 arthropods of 141 families in paired control and bird exclosure plots. On average, insectivorous birds reduced arthropod abundance by 16% and plant damage by 14%, and some well-known pests (e.g., Adelges cooleyi) of crop trees (mostly Pseudotsuga menziesii) in our system were reduced by as much as 30%. However, this effect did not translate into a trophic cascade that increased crop-tree growth in the presence of birds. We experimentally reduced plant abundance and diversity by 67% and 55%, respectively, in the most intensive herbicide treatment in relation to untreated controls, but reduced vegetative resources did not change the strength of the direct effect of birds on arthropods or the indirect effect of birds on plants.

As global terrestrial biodiversity declines via land-use change, society has placed increasing value on non-commercial species 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 nutritious 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.

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-m2 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.

Intensive forest management (IFM) promises to help satisfy increasing global demand for wood but may come at the cost of local reductions to forest biodiversity. IFM often reduces early seral plant diversity as a result of efforts to eliminate plant competition with crop trees. If diversity is a function of bottom-up drivers, theory predicts that specialists at lower trophic levels (e.g., insect herbivores) should be particularly sensitive to reductions in plant diversity. We conducted a stand-level experiment to test bottom-up controls on moth community structure, as mediated by degrees of forest management intensity. Using a dataset of 12,003 moths representing 316 moth species, moth richness decreased only slightly, if at all, as herbicide intensity increased (P = 0.062); the moderate treatment, which is most commonly applied in the northwestern USA, was estimated to have 4.72 (±2.14 SE, P = 0.039) fewer species than the control. Structural equation modeling revealed strong support for an effect of herbicide on plant abundance, which influenced plant species richness and subsequently moth species richness. Moth species richness was associated with plant species richness and followed a power law function (z = 0.42, P = 0.006), which is surprisingly consistent with a recent large-scale experiment in agricultural systems, and provides support for bottom-up drivers of moth community structure. Moth abundance was not influenced by the direct effects of silvicultural herbicide treatments. Site-level effects and variation in pre-harvest vegetation communities resulted in residual broadleaf and herbaceous vegetation in even the most intensive treatment. Even at low densities, these residual deciduous and herbaceous plants supported higher than expected moth abundance and richness. We conclude that forest management practices that retain early seral vegetation diversity are the most likely to conserve moth communities.

Components of biodiversity in intensively managed forest stands may be reduced in comparison to naturally regenerated stands. Use of herbicides to suppress herbaceous and woody plant species that compete with planted seedlings has been implicated in negative impacts. We designed a large-scale experimental study to test the influence of intensive forest management on the abundance of early seral bird species in the Oregon Coast Range, US. Experimental applications consisted of 'Intensive' (i.e., heavy use of herbicides), 'Moderate' and 'Light' treatments, as well as controls with no herbicide application. In relation to the control, abundance of six out of thirteen bird species was significantly reduced in at least one of the three treatments. Leaf-gleaning insectivorous birds were more negatively affected by heavier herbicide treatments in general than bird species with other foraging behavior. Long-term bird population trends, derived from the Breeding Bird Survey, were correlated with the effect of intensive treatment; species more negatively associated with intensive treatments at the stand scale, were more likely to be in decline across the Pacific Northwest, US. Our results also indicate that reducing intensity of herbicide applications has positive effects on early seral bird abundance during the first 2 years of stand growth - particularly those species exhibiting negative population trends. To balance biodiversity conservation and timber production, research examining the tradeoffs between reduced application of herbicide and tree growth is required. (C) 2013 Elsevier B.V. All rights reserved.