Thomas D. Stokely

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.

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.

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. 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. 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. 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. 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.

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.