Assessing the Carbon Consequences of Western Juniper (Juniperus occidentalis) Encroachment Across Oregon, USA

Abstract

Our ability to assess the continental impacts of woody encroachment remains compromised by the paucity of studies quantifying regional encroachment rates. This knowledge gap is especially apparent when it comes to quantifying the impact of woody encroachment on large-scale carbon dynamics. In this study, we use a combination of aerial photography from 1985–1986 and 2005 and near-annual Landsat satellite imagery over the same period to assess the rates of encroachment by western juniper, Juniperus occidentalis Hook., into the grasslands and shrublands of eastern Oregon. The approximately 20-yr Landsat reflectance trajectories identified for the juniper woodlands of eastern Oregon did not correlate well with changes in juniper crown cover over the same period, suggesting that systematic trends in reflectance are being driven by vegetation other than juniper. Using a random sample of 150 aerial photography plots, we estimate the average aboveground accumulation of carbon in undisturbed juniper woodlands to be 2.9 kg C?m22 ? yr21; about 0.20 Tg C? yr21 across all of Oregon. However, juniper
removal by cutting and or burning, occurring at a rate of ,1% yr21, counteracted regional encroachment by about 35%, bringing the net change in aboveground carbon down to 1.9 kg C?m22 ? yr21, about 0.13 Tg C? yr21 across all of Oregon. This study illustrates the capacity of woody removal, over very small areas, to offset encroachment over very large areas and cautions against scaling site-level encroachment studies over entire regions.