Annual row crops dominate agriculture around the world and have considerable negative environmental impacts, including significant greenhouse gas emissions. Transformative land-use solutions are necessary to mitigate climate change and restore critical ecosystem services. Alley cropping (AC)-the integration of trees with crops-is an agroforestry practice that has been studied as a transformative, multifunctional land-use solution. In the temperate zone, AC has strong potential for climate change mitigation through direct emissions reductions and increases in land-use efficiency via overyielding compared to trees and crops grown separately. In addition, AC provides climate change adaptation potential and ecological benefits by buffering alley crops to weather extremes, diversifying income to hedge financial risk, increasing biodiversity, reducing soil erosion, and improving nutrient- and water-use efficiency. The scope of temperate AC research and application has been largely limited to simple systems that combine one timber tree species with an annual grain. We propose two frontiers in temperate AC that expand this scope and could transform its climate-related benefits: (i) diversification via woody polyculture and (ii) expanded use of tree crops for food and fodder. While AC is ready now for implementation on marginal lands, we discuss key considerations that could enhance the scalability of the two proposed frontiers and catalyze widespread adoption.

1. Agricultural producers, academics and policy-makers are increasingly interested in multifunctional tree crop systems as a solution for maintaining ecosystem services and producing food. The US Midwest is emerging as a hotbed of such systems in the temperate North in the form of farm-scale woody perennial polyculture enterprises, but they are currently only a tiny fraction of the landscape. Understanding how such approaches might be scaled up, thus, requires learning from the farmers that are at the forefront of the transition of land to woody perennial polyculture to answer a range of questions: What unique management knowledge is being implemented by farmers to manage complexity on multiple scales? What key challenges have farmers faced? And what values and motivations underpin these fledgling efforts? 2. From 13 interviews with 18 midwestern perennial polyculture farmers, we found that they largely used a small portion of their farm's land for their perennial enterprises, and did not earn a large portion of their income from them, though this was projected to increase as trees matured. Through experimentation, innovation and farmer networks, the farmers had amassed unique adaptive management expertise for balancing diverse crops and livestock within multifunctional tree crop systems over time and space, an area largely absent from mainstream agricultural science and policy. The barriers these farmers report facing are largely economic rather than biophysical, involving access to capital, insurance, mid-sized markets and regional processing infrastructure, as well as government programmes mismatched with perenniality.3. Cross-cutting these topics, farmers sought to fulfil values anchored in their relationships to land, to the community or to both. The values of long-termism, learning and sharing, diversity, stewardship and care of farmland, connection to nature and wildlife, self-sustenance, other-sustenance and eudaimonia were embodied and expressed in farmer decisions from the practical to the personal. 4. Economic and agrarian policy, as well as programme development for multifunctional tree crop systems, should (a) be designed to align with farmer's values and motivations and (b) take advantage of their expert management and systems | 181 People and Nature KREITZMAN ET Al.

Given the massive and growing environmental impacts of conventional agriculture, humanity needs new methods for growing food that not only meet dietary needs but also provide for multiple ecosystem functions with potential benefits to people and biodiversity. Concepts from ecology and complex adaptive systems suggest that persistent structural heterogeneity and functional diversity are key for supporting biodiversity, ecosystem services, and resilience, but these concepts have not been extensively applied in agriculture, which is still dominated by annual monocropping systems. Perennial agriculture seems to embody these ecological concepts-particularly perennial polycultures that combine a variety of long-lived woody perennial food crops with continuous ground cover. However, our understanding of the benefits and trade-offs of such systems compared to conventional agriculture is limited, especially in temperate climates. Here, we provide a systematic and comprehensive study of the ecological attributes of 14 woody perennial polyculture farm fields to conventional annual and hay fields in the U.S. Midwest, one of the most industrialized food-producing regions in the world. We found that perennial fields had (1) more diverse soil fungal, invertebrate, plant, and bird communities but found no difference in soil bacterial communities; (2) less compacted soil; (3) denser ground cover; (4) more active carbon, organic carbon, and nitrogen and the same available phosphorus in the top layer of soil; and (5) more species of predatory, detritivorous, and herbivorous insects, and approximately fourfold higher abundance of herbivorous insects. Food production from the oldest half of perennial fields was only 14.7% of a regional corn/soy rotation by weight, but increased with the age of the perennial field and had high nutritional diversity. Together, these findings indicate that woody perennial polyculture fields in the U.S. Midwest are characterized by higher biodiversity and ecosystem functions than adjacent conventional fields. Woody perennial polycultures in the temperate north might play a role in the transition of agricultural landscapes toward sustaining both people and nature over the long term.

Agroforestry represents a solution to land degradation by agriculture, but social barriers to wider application of agroforestry persist. More than half of all cropland in the USA is leased rather than owner-operated, and the short terms of most leases preclude agroforestry. Given insufficient research on tenure models appropriate for agroforestry in the USA, the primary objective of this study was to identify examples of farmers practicing agroforestry on land they do not own. We conducted interviews with these farmers, and, in several cases, with landowners, in order to document their tenure arrangements. In some cases, additional parties also played a role, such as farmland investors, a farmer operating an integrated enterprise, and non-profit organizations or public agencies. Our findings include eleven case studies involving diverse entities and forms of cooperation in multi-party agroforestry (MA). MA generally emerged from shared objectives and intensive planning. MA appears to be adaptable to private, investor, institutional, and public landowners, as well as beginning farmers and others seeking land access without ownership. We identify limitations and strategies for further research and development of MA.

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