With a special emphasis on vineyards, which have higher wood C stocks than annual cropping systems, the paper considers the relative contributions of different land use regimes in contributing to carbon storage. Following an explanation of field methods, data analysis, and research findings, we examine some of the opportunities and limitations facing growers in the current policy climate for maintaining or increasing carbon stocks. A discussion then considers how policies and incentives could be amended to encourage greater participation in activities that promote the management of agricultural land as a multifunctional landscape—that is, a landscape in which production is only one of the valued outputs alongside carbon sequestration, soil quality, biodiversity protection, and other ecosystem services.The assessment of carbon stocks for Fetzer/Bonterra Vineyards was conducted on five ranches scattered across the Russian River valley near the town of Hopland in Mendocino County, California . Fetzer grows organic grapes for its Bonterra label on these ranches, which vary in topography, size, area under cultivation, and the number of different habitat types present. In total, the ranches comprise about 1,150 hectares of land,hydroponic nft channel roughly 30 percent of which is in vine cultivation, 35 percent in forested land, and most of the remainder in grassland.
The non‐cultivated habitat of the study area varied from oak woodland and mixed hardwood forests, to chaparral and grasslands in drier sites, as well as a distinct riparian vegetation surrounding waterways. Most of the non‐vineyard, non‐riparian forested land was mixed hardwood forest, dominated by a variety of oak species and interspersed with madrone , bay , buckeye , and the occasional Douglas fir . Several woody shrub species were found in the under story of the forest lands, including manzanita , ceonothus , and toyon , at times forming pure stands of chaparral shrubs where soils and exposure presented the ideal conditions. The riparian areas were characterized by a different suite of hardwood species, including maples , alders , cottonwoods , ash , and willows . In total, 29 woody species were recorded on the five ranches. In vineyard tracts, carbon was estimated for only the above ground woody portion of the vines, which are grown as a monoculture, planted in parallel rows that were also approximately two meters apart. Using a sampling regime based on a geographic information system , carbon was measured in soil from 44 pits that were 1 meter deep, and in above ground woody biomass, from 93 vegetation plots. The sampling points were located according to a representative set of sites on each of seven different habitat types. To estimate the carbon stored in a given tract, the average biomass per vine, calculated using allometric equations4 developed with age and main stem diameter, was multiplied by the number of vines in each tract . Similarly, field measurements combined with published allometric equations for native woody species were used to estimate woody biomass for the non‐vine species sampled on 10 x 30 m plots in forest lands. Soil carbon was estimated after combustion analysis of samples taken from the soil pits that were located in forested and vineyard lands. The data for the vegetation samples were integrated into a GIS together with remotely sensed imagery using a cluster analysis technique to produce a general land cover classification map with seven categories.
The amount of carbon stored on a given hectare of land was thus estimated to be a function of its habitat classification and the carbon values for the samples in that category . Soil carbon was estimated in a similar way, but samples were compared to existing soil maps from the national Soil Survey Geographic Database . These distribution maps, along with the sampled carbon values, were used to extrapolate carbon across the landscape to give both per hectare estimates of carbon stocks and total carbon estimates per ranch. The results of the study show two main conclusions with respect to carbon stocks : that per hectare, the top meter of soil holds substantially more carbon than the above ground woody vegetation, ranging from 5 times more in forests to 50 times more in vineyards, on average; and forest lands store more carbon in both soil and above ground woody vegetation than vineyards. On average, forested wild lands had 45 percent more total C/hectare than vineyards. That is, there are approximately12 times more above ground woody carbon and 6 percent more soil carbon per unit area in wild lands than in vineyards. Among wild land vegetation types, valley riparian habitats had the highest carbon stocks, and most of the carbon came from soil . This is most likely due to long‐term upland erosion, and subsequent deposition of organic material along the floodplains of the Russian River and its tributaries. The upland vegetation types had more variability in soil carbon stocks, but closed‐canopy mixed hardwood forest made the greatest contribution to C stocks .
For vineyard tracts, the age of the vines explained much of the variation in above ground C stocks and wood biomass. But even the largest vines contained only about one‐fourth of the wood biomass per hectare of the adjacent wooded wild lands.Planning ahead for climate change in agricultural landscapes involves more than crop management for reducing GHG emissions and coping with uncertain temperatures and precipitation . Land managers must have the capacity to respond to unforeseen change in natural resources as well. Integration of forest, other natural habitat and vegetation types, and agricultural ecosystems into complex landscapes is increasingly viewed as a way to increase the provision of multiple ecosystem services, including carbon storage, pest management, nutrient retention, erosion control, and water quality . Complex landscapes that are rich in biodiversity help to “keep options open” for alternative future management, even if such a strategy appears inefficient and sub-optimal in the present tense . In a mosaic approach for vineyard management, management objectives allow topography and habitat variability to determine the amount and configuration of vine tracts and wild lands. It was the landscape variability that also presented the greatest challenges to modeling carbon in this study. Combining GIS‐ and field‐based approaches was a useful way to sample and analyze vegetation‐based habitat types for their carbon stocks across the landscape . Variability in tree species composition and distribution within habitat types, as well as the many soil types showed the necessity of refined models to address heterogeneity for assessing C stocks. Two specific improvements would make for greater accuracy in future woody plant carbon estimates: a more comprehensive set of allometric equations for extrapolating above ground woody biomass of California tree species from field measurements such as diameter at breast height ; and understanding which environmental variables best explain the variation in above ground woody biomass and developing relevant procedures so carbon stocks can be accurately estimated on specific land holdings. Methods for improving the estimation of carbon stocks will be necessary if regulating bodies make carbon accounting mandatory or provide incentives for maximizing C storage. At present, however, most regulation is focused on emissions, such as AB 32. The U.S. Government has also taken an emissions control approach, such as when the U.S. Environmental Protection Agency declared carbon dioxide and five other GHGs to be air pollutants subject to regulation in 2009. In California, viable voluntary carbon offset projects must qualify for one of three categories: reforestation; improved forest management; or avoided conversion. This means that forest cover must increase by planting or management techniques, or land owners must demonstrate that forested land is at risk for conversion, and therefore its protection meets the requirement of additionality . In California in 2006, transportation, energy production,nft growing system and industry accounted for more than 80 percent of annual GHG emissions; whereas, agriculture collectively contributed only 6 percent . At first glance, it seems that paying for carbon storage on croplands in California would have only a small effect on reducing total GHG emissions and that high transaction costs would discourage any such policy, given the thousands of farms in the state. But marginal lands, remnant natural vegetation, and restored ecosystems within agricultural landscapes could potentially account for substantial carbon benefits in California, based on the results of this study, as well as provide a host of additional environmental benefits not measured here. At present, woody plants in agricultural landscapes are not eligible for carbon offsets in California’s forest protocol .
This situation deserves further recognition, not only to retain an important set of carbon stocks by avoided deforestation, but because incentives for managing a vineyard/wild land mosaic contribute toward other ecosystem services, such as threatened or endangered species habitat protection , water quality and storage capacity, soil erosion, and nutrient run‐off control. The importance of maintaining forest lands has been a major issue for scientists and policy makers concerned with global warming. Efforts to develop incentives to reduce deforestation have produced global campaigns like the United Nations REDD and REDD‐plus programs , as well as specific efforts to slow deforestation in key tropical forest biomes . A crucial issue in the global deforestation debate is the renewed recognition for the importance of forested lands that exist in agricultural landscapes that are not formally considered forests . In conclusion, for complex landscapes, high resolution spatial modeling is challenging and requires accurate characterization of the landscape by vegetation type, physical structure, sufficient sampling, and allometric equations that relate tree species to the landscape. While remote sensing techniques may improve the accuracy of carbon estimation, climate change policy in California shows a lack of focus on storage compared to emissions, and on agriculture compared to other sectors. These oversights may lead to missed opportunities for maximizing ecosystem services, including carbon storage, as well as for encouraging better farm stewardship and habitat conservation. Many types of agricultural landscapes have some fraction of their land out of production and in forests or other forms of conserved habitat. Yet this land is generally not being counted in carbon accounting protocols such as AB 32. As a result, land owners are not being recognized or rewarded for the role they are playing in storing carbon in forested lands. Furthermore, if such rewards or incentive programs did exist, it is highly likely that many producers would take an active role in reforesting parts of their land that is not in production, or planting hedgerows or other vegetation, in order to qualify for these programs.The Global Warming Solutions Act of 2006 has been a catalyst for greenhouse gas mitigation and has generated awareness of climate change adaptation in California’s agricultural sector. While agriculture accounts for only 6 percent of the state’s total GHG emissions, it has the potential to play a significant role in statewide mitigation efforts through the sequestration of carbon in soils and plant biomass and as a source of feed stock for renewable energy generation . Agricultural residues are a major, and largely untapped, renewable energy source for California . Statewide estimates suggest that the potential feed stock from agricultural residues is over 8.8 million tons of dry biomass per year . Various biomass‐derived fuels can be used to partially displace fossil fuel consumption and facilitate GHG mitigation. Since agriculture tends to be particularly vulnerable to climate change, market volatility, and urban development, some have argued that on‐farm energy generation using agricultural residues is a key way to link mitigation and adaptation to generate profitable co‐benefits .In short, mitigation strategies that integrate renewable energy sources into farm operations can themselves be viewed as an adaptation in response to climate change regulation. California’s net metering laws were established in 1995. They currently allow wind, solar, and some biogas installations to be connected to the energy grid through net metering accounts, provided they meet certain energy output and pollution‐control requirements . With the passage of AB 920,5 which became effective in 2011, residences, farms and businesses with renewable energy installations can also sell excess power back into the grid. In contrast, state policies have not previously allowed projects which generate electricity on‐farm from crop residues to participate in these net metering programs. However, the Renewable Energy Equity Act 6 which has been passed by the State Legislature, makes all forms of renewable energy eligible for California’s net metering program.