Climate mitigation potential of natural climate solutions and clean energy on The Nature Conservancy properties in California, USA

Delta will be registered under the American Carbon Registry (ACR581) in 2025 for crop con- version from corn to rice (1,671 ha) and wetland restoration (176 ha) which are being imple- mented from 2019–2025 with an average reduction of 11,180 MTCO2e year-1 once fully implemented [9]. In this study, we assessed the CO2e reduction potential from realistic ecosystem and infra- structure-based consideration of feasible NCS activities and transitions to clean energy on The Nature Conservancy fee and easement lands in California. We included transitions to clean energy on these lands plus offices because phasing out the use of fossil fuels is a primary cli- mate mitigation strategy, and like NCS can be implemented by a landowner. We compared the total CO2e reduction from the potential NCS and clean energy on fee lands where TNC is the landowner to the three ongoing NCS projects, the chapter’s 2030 reduction goal, and the state’s ambitious reduction goal of 2 million MTCO2e year-1 from 2020–2030 for natural and working lands [10]. Based on fee land ownership only, The Nature Conservancy is the fourth largest private landowner in the state, behind two industrial timber companies and The Wild- land Conservancy, another non-profit land conservation organization. By downscaling the potential climate mitigation to a single, large landowner, the contribution that the chapter can make to their 2030 goal, and tradeoffs that can limit the carbon mitigation potential become clear. This approach could be duplicated by other landowners to determine their individual

climate mitigation potential based on NCS or clean energy transitions. Methods We surveyed fifteen stewardship and science staff from the chapter to determine the feasibility of eight NCS activities in California. We presented a photo of each NCS activity along with a brief description of the activity and asked if that staff thought it made sense to implement this activity on some of The Nature Conservancy’s fee and conservation easement lands (S1 Fig). Participants scored their response from 1–5; (1) strongly disagree, (2) disagree, (3) neither agree nor disagree, (4) agree, and (5) strongly agree. We calculated the mean score and response rate for each NCS activity (S1 Table). Based on the feasibility survey and follow-up conversations with staff, we eliminated compost application which was deemed to be unsuitable because it could negatively impact biodiversity by introducing non-native seeds. Past studies are mixed on the impact of compost application on biodiversity finding it can promote invasive species [11], not affect plant diversity [12], or have been inconclusive on impacts to attributes of plant diversity important for biodiversity conservation [13]. Given the uncertainty and what one survey respondent said was a “very controversial practice” there is a need for additional study on the impact of this NCS on biodiversity. Another respondent noted there is almost always plastic trash in compost that could have a negative impact on sen- sitive species. The Nature Conservancy, California chapter’s position has been that compost

could be appropriate for highly disturbed lands such as agricultural fields, but more research is needed for application in other areas. We identified properties where the feasible NCS activities could be applied using multiple sources. We relied on past studies [5, 14] to determine where agroforestry, cover cropping, and riparian restoration applied (Table 1), and then used a vegetation dataset [15] to calculate the total possible area for each activity on each property (Table 2). Marvin et al. [5, 14] modeled NCS under different climate change projections and implementation rates: agroforestry was implemented on ~3,198 ha year-1 (7% of all agricultural land annually), cover cropping on ~69,000 ha year-1 (three times the recent 2001–2014 annual rate of cover cropping), and ripar- ian restoration on ~2,000 ha year-1 (25% greater than the annual rate of total potential riparian restoration in the Central Valley). We used data from these two studies for the 2020–2050 PLOS ONE Climate mitigation potential of natural climate solutions and clean energy: The nature conservancy, California PLOS ONE | https://doi.org/10.1371/journal.pone.0311195 October 21, 2024 3 / 14