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

There are many calls to consider how feasibility will constrain the implementation of NCS [28–30], but few studies that directly address feasibility from the landowner or smallholder farmer’s perspective [31, 32] or expert perspectives at larger geographical scales [33–35]. The need for more information on sequestration rates for NCS activities and the long-term climate mitigation potential is also a high research priority [36]. To reach the 2030 goal, the chapter could implement NCS on conservation easements. This is challenging because of the need to have multiple landowners grant permission to implement NCS. Wetland restoration and cover cropping on agricultural lands were the most promising activities on easements, but some question whether NCS activities on agricultural lands will scale-up given it will require large numbers of individual owners to implement and maintain the activities over multiple decades [37]. Additionally, there are likely tradeoffs with agricul- tural production that could limit adoption of wetland restoration and other NCS. There is a need for more information on the feasibility of implementing NCS on easements, specifically the acceptability of different NCS and potential tradeoffs for landowners as these are likely be different than the acceptability and tradeoffs found for The Nature Conservancy. This study focused on restoration and agricultural management NCS activities on current properties but did not account for the impact of land protection, which is likely The Nature

Conservancy’s biggest contribution to climate mitigation [5, 6]. By preventing land conversion and implementing changes to forest management by default when setting easement terms for industrial timberland, The Nature Conservancy is protecting biodiversity and carbon stocks. The Conservation Fund is claiming the carbon credit for changes to forest management at the Garcia River Forest as the landowner, so The Nature Conservancy does not count this activity towards it CO2e reduction goal. However, future land deals where The Nature Conservancy holds the fee title and is the landowner could provide an opportunity for not only avoided con- version of carbon rich habitat but also other high impact NCS like changes to forest management. Changes to forest management on the Garcia River Forest easement was the NCS and prop- erty with the highest mitigation impact, more than three times the potential of all other NCS activities combined and contributes substantially to the state’s reduction goal. This finding matches past studies that found changes to forest management had the largest mitigation potential in California [3] and Oregon [4]. This highlights the potential for the two largest pri- vate landowners in California, both industrial timber operators, to have a disproportionate impact on carbon mitigation if they implement or continue to implement changes to forest management that are additive. However, Marvin et al. [5] found that if you include avoided loss of soil carbon storage, then land conservation becomes the NCS with the largest mitigation

potential in California. The biggest impact the organization can have will likely be from land protection first, changing forest or agricultural management second, and restoring ecosystems third [6]. Most of the reduction potential from restoration, agricultural management, and clean energy transition was concentrated in a single property. At Staten Island there was overlap in the area identified for potential wetland restoration and agricultural management, represent- ing a tradeoff if only one activity was pursued at the expense of the other. By choosing to implement a mixture of some wetland restoration and some conversion of corn to rice, The Nature Conservancy did not maximize climate mitigation but balanced the need to support sandhill cranes that feed on the leftover corn post-harvest, maintain a working farm, and sequester carbon in the restored wetland and converted rice fields. Implementing wetland res- toration only at Staten would provide four times greater climate mitigation than the current mix of activities, but wetland restoration would have eliminated the leftover corn supply for the cranes conflicting with the conservation objectives defined for the property. 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 10 / 14