Cannabis agriculture is a multi-billion dollar industry in the United States that is changing rapidly with policy liberalization. Anecdotal observations fuel speculation about associated environmental impacts, and there is an urgent need for systematic empirical research. An example from Humboldt County California, a principal cannabis-producing region, involved digitizing 4428 grow sites in 60 watersheds with Google Earth imagery. Grows were clustered, suggesting disproportionate impacts in ecologically important locales. Sixty-eight percent of grows were >500 m from developed roads, suggesting risk of landscape fragmentation. Twenty-two percent were on steep slopes, suggesting risk of erosion, sedimentation, and landslides. Five percent were <100 m from threatened fish habitat, and the estimated 297 954 plants would consume an estimated 700 000 m 3 of water, suggesting risk of stream impacts. The extent and magnitude of cannabis agriculture documented in our study demands that it be regulated and researched on par with conventional agriculture.

On agricultural frontiers, minimal regulation and potential windfall profits drive opportunistic land use that often results in environmental damage. Cannabis, an increasingly decriminalized agricultural commodity in many places throughout the world, may now be creating new agricultural frontiers. We examined how cannabis frontiers have boomed in northern California, one of the United States' leading production areas. From 2012-2016 cannabis farms increased in number by 58%, cannabis plants increased by 183%, and the total area under cultivation increased by 91%. Growth in number of sites (80%), as well as in site size (56% per site) contributed to the observed expansion. Cannabis expansion took place in areas of high environmental sensitivity, including 80%-116% increases in cultivation sites near high-quality habitat for threatened and endangered salmonid fish species. Production increased by 40% on steep slopes, sites more than doubled near public lands, and increased by 44% in remote locations far from paved roads. Cannabis farm abandonment was modest, and driven primarily by farm size, not location within sensitive environments. To address policy and institutions for environmental protection, we examined state budget allocations for cannabis regulatory programs. These increased six-fold between 2012-2016 but remained very low relative to other regulatory programs. Production may expand on frontiers elsewhere in the world, and our results warn that without careful policy and institutional development these frontiers may pose environmental threats, even in locations with otherwise robust environmental laws and regulatory institutions.

In many countries around the globe, private freehold lands cover large areas. Conservation on these private lands, next to statutory protected areas, promises to play a critical role in efforts for reaching internationally agreed environmental protection targets. Lying at the heart of an emerging land system science, in which ecology, economics, geography, psychology, and other social sciences interact, private land conservation is reflecting the intertwined and multiscalar processes of our rapidly transforming world. Situated at this disciplinary meeting point, private land conservation invites a great breadth of approaches and cross-disciplinary work that offer deep insights into social and environmental change, often from surprising angles. Although many questions remain in private land conservation, we can now build on a large body of recent high-quality studies as we push this field forward in both research and practice. The Special Feature "Private Land Conservation-Landowner Motives, Policies, and Outcomes of Conservation Measures in Unprotected Landscapes" brings together contributions that explore the diversity of recent advances in private land conservation science. As an introduction to this Special Feature, first we are reviewing recent dynamics in important social-ecological drivers with bearing on private land conservation science. We go on to introduce the individual contributions to this Special Feature and then examine common themes as they are emerging from these papers, including the need for flexibility in conservation approaches, pursuit of community cobenefits of conservation, increasing consideration of environmental justice questions, and acknowledgment of the importance of social psychology in shaping private land conservation. We conclude with identification of knowledge gaps and recommendations for future research, as we advance from diagnostics to normative work in private land conservation science.

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Early assessment of environmental impacts from emerging agriculture is a scientific challenge that – if poorly executed – limits sound policy making. Here, we present an approach for evaluating and forecasting the effects of emerging and expanding land uses on natural habitats. By analyzing landscape change per unit area, the effects of emerging land uses can be compared with established benchmarks. We apply this approach to study forest fragmentation in northern California, comparing the effects of cannabis agriculture – a booming commodity worldwide that affects diverse ecosystems – to those of timber harvest from 2000 to 2013. We found that although timber has greater impacts on the landscape overall, cannabis leads to far greater changes in key metrics on a per‐unit‐area basis. Thus, despite its small current land‐use footprint, if changes are not made in the spatial pattern of its expansion, the boom in cannabis agriculture will likely create substantial threats to the surrounding environment. Future research, land management, and agricultural policy must account for these threats.

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