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

undeveloped uplands and levees in existing wetlands that were either projected to be inun- dated from sea level rise or were adjacent to vulnerable habitats [22]. For freshwater and tidal wetland restoration, we determined whether the property was cultivated or pasture using the same vegetation dataset used for agroforestry, cover cropping, and riparian restoration [15]. Freshwater wetland restoration covered 4,785 ha and tidal wetland restoration covered 594 ha, both types of restoration applied primarily to cultivated agricultural fields and secondarily to pastures. Rice best management practices was identified in rice fields and covered 351 ha. We assumed that the NCS activities were not mutually exclusive, meaning they could co- occur at the same location. This assumption does not hold for seven properties where wetland restoration overlapped spatially with agricultural management activities: agroforestry, compost application, cover cropping, and rice best management practices. To understand the impact of the overestimate, we calculated what the total CO2e reduction would be for the seven proper- ties if the greater reduction activity, wetland restoration or agricultural NCS only (combined total of all agricultural activities) was implemented and compared this to the total reduction with the overlapping wetland and agricultural activities. We compiled information on CO2e sequestration rates for all feasible NCS activities using a systematic literature review following the PSALSA approach [23]. We searched Google

Scholar from 2015, when the state implemented a Greenhouse Gas Reduction Fund to imple- ment and study NCS, to 2023 using specific search strings (S3 Table). To refine our search terms, we first did a pilot search. Articles that were in peer-reviewed journals, in English, accessible, specific to California, similar to the NCS activity descriptions, and primary research were included in the screening process. The initial studies found were reduced based on the screening criteria and we evaluated the first fifty results as the studies beyond that number did not match any of the inclusion criteria. Two activities, agroforestry and rice best management practices, had no California specific studies so we relied on meta-analysis from other states. We removed woodland restoration as we did not find any studies on the change in carbon sequestration or emissions with oak restoration in the literature, even outside of California. We recorded the sequestration rates from the studies, calculated the mean value, and used the range of mean values from individual studies to represent the uncertainty. For the activities with only one study, we used the standard error or 95% confidence interval to represent the uncertainty. We converted Mg C to CO2e using a conversion factor of 3.67, the molecular ratio of carbon dioxide to carbon (44 g mole-1CO2) / (12 g mole-1CO). We combined carbon sequestration in biomass and soils when they were reported separately, and only included empirical values [24]. For studies that reported total carbon sequestration over a multi-year

period we calculated the mean annual rate assuming a linear increase. While carbon accumula- tion shows a sigmoidal growth over time for riparian restoration [25], the increase over the first fifty years for ‘riparian woodlands,’ the type of riparian vegetation in this study, was linear. We calculated the CO2e reduction (MTCO2e year –1) rate by multiplying the sequestration rate (MTCO2e ha–1 year –1) by the area of each NCS activity (ha) on individual properties. For ongoing NCS activities at Staten Island we calculated CO2e reduction using the mean sequestra- tion rate for cover cropping and cultivated to managed wetland restoration from the literature review. We assumed the mean sequestration rate annually did not decrease or saturate from 2020–2030. Changes to forest management does begin to result in saturation of mitigation at 25 years [1] and would apply to the Garcia River Forest property after 2032. The other activities all reach saturation after more than forty years, beyond the 2030 goal period [1]. Clean energy solutions to reduce emissions was based on data from the consumption of electricity and propane to power buildings and water pumps on The Nature Conservancy fee- owned, conservation easements, offices, and lands where The Nature Conservancy has a 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 5 / 14