Forest carbon and solar siting

Almost every utility-scale solar project in Western Massachusetts faces the same hard question. The site has trees on it. The trees are storing carbon. Solar generation displaces fossil generation. Which net benefit is larger, and over what time horizon?

This is the question our group has spent the most time on. The short answer is that the trade is real, the numbers matter, and most public discussion gets the framing wrong in one direction or the other.

The case against forest conversion

A typical Massachusetts forest holds something like 220 to 290 tonnes of carbon dioxide equivalent per acre across trees, deadwood, litter, and soil, and old forest holds 370 to 440 (Catanzaro and D’Amato 2019). Clear the trees, and much of the carbon in the biomass escapes over the years that follow, though soil carbon, roughly a third of the stock in our common forest types, largely stays in the ground. The site stops sequestering. The replacement forest, if it ever comes back, takes decades to recover.

The case for accepting some forest conversion

A dense Massachusetts ground-mount array produces something like 350 to 400 megawatt-hours per year for each acre of fenced panels (Bolinger and Bolinger 2022). The whole site, with its buffers and access roads, covers more land, so the yield per acre of the full project is lower. On the 2024 New England grid, each panel acre displaces roughly 95 to 165 tonnes of carbon dioxide per year, depending on whether the displaced generation is valued at the grid average or at the fossil margin (ISO-NE 2024 emission rates), and the figure declines as the grid gets cleaner.

The math says the deployment benefit overtakes the conversion debt within roughly five to fifteen years on most sites, depending on the carbon stock of the forest, how much land is cleared beyond the panels, and the carbon intensity of the grid over time. After that, every additional year is net positive.

What this does and does not settle

The math says forest-to-solar conversion is usually a net climate gain on a thirty-year horizon. It does not say the conversion is the right choice for any particular site. Sites that are already cleared, degraded, or built-on score better on every measure that matters. Brownfields, parking lots, rooftops, and former agricultural land come first.

Chapter 239 requires large projects to complete a Site Suitability Assessment that puts this hierarchy on paper. How well it works in practice is an open question.