PS 34-232 - Tree growth and physiological activity following forest restoration treatments and extreme drought in a Sierra Nevada mixed-conifer forest, USA

Abstract: Fire exclusion in frequent-fire forests in western North America has disrupted fundamental ecological processes, leaving these forests vulnerable to drought, insects and disease, and increased fire severity. Prescribed burning and mechanical thinning are widely used to reduce fuels and restore ecosystem properties. Numerous studies a have examined the effects of restoration treatments on residual tree growth, but inferences from observed changes in growth would be strengthen by linking growth to underlying physiology. In this study, we used tree-rings to quantify growth and stable-carbon isotope responses of trees to prescribed burning, mechanical thinning, and extreme drought the Teakettle Experimental Forest, a historically frequent fire mixed-conifer forest in the southern Sierra Nevada of California. Burning and thinning treatments were implemented in 2000-2001. We had three primary questions. First, what were the growth and physiological responses to thinning and prescribed burning treatments, and did these responses differ between predrought (2003-2007) and drought (2012-2016) time periods? Second, what tree-level attributes and environmental conditions influenced growth and physiological responses to treatments and drought? Third, did treatments alter the sensitivity of growth and physiological activity to climate?

Thinning increased residual tree growth in both pre-drought and drought time periods, but prescribed burning did not affect growth in either time period. Growth response also varied by species, with greater growth responses for sugar pine and Jeffrey pine (Pinus lambertiana and P. Jeffreyi) versus white fir (Abies concolor) and incense-cedar (Calocedrus decurrens), but species growth responses did not differ by treatment or time period. Carbon isotope discrimination (Δ¹³C) decreased with thinning and/or prescribed burning. Carbon isotope discrimination also declined for sugar pine versus all other species, but this species difference only occurred in unburned treatments and during drought. Higher growth responses and lower carbon isotope discrimination were associated with smaller tree size and low neighborhood basal area, while topographic conditions became important during drought. Both growth and carbon isotope discrimination were more sensitive to climate after treatments. Linking growth and carbon isotope discrimination indicates that thinning reduced competition, allowing trees to continue to fix carbon as new stem growth, even during extreme drought conditions. Carbon isotope discrimination data suggests additional physiological effects after prescribed burning, along with species specific effects of treatments and drought that were otherwise not found in growth data alone.