Carbon-Water Tradeoffs in Old-Growth and Young Forests of the Pacific Northwest

Despite much interest in relationships among carbon and water in forests, few studies assess how carbon accumulation scales with water use in forested watersheds with varied histories. This study quantified tree growth, water use efficiency, and carbon-water tradeoffs of young versus mature/old-growth forest in three small (13–22 ha) watersheds in the H.J. Andrews Experimental Forest, Oregon, USA. To quantify and scale carbon-water tradeoffs from trees to watersheds, tree-ring records and greenness and wetness indices from remote sensing were combined with long-term vegetation, climate, and streamflow data from young forest watersheds (trees ∼45 years of age) and from a mature/old-growth forest watershed (trees 150–500 years of age). Biomass production was closely related to water use; water use efficiency (basal area increment per unit of evapotranspiration) was lower; and carbon-water tradeoffs were steeper in young forest plantations compared with old-growth forest for which the tree growth record begins in the 1850s. Greenness and wetness indices from Landsat imagery were not significant predictors of streamflow or tree growth over the period 1984 to 2017, and soil C and N did not differ significantly among watersheds. Multiple lines of evidence show that mature and old-growth forest watersheds store and accumulate more carbon, are more drought resistant, and better sustain water availability compared to young forests. These results provide a basis for reconstructions and predictions that are potentially broadly applicable, because first-order watersheds occupy 80%–90% of large river basins and study watersheds are representative of forest history in the Pacific Northwest region.