DOI: 10.25921/xctt-na52
title: NOAA/WDS Paleoclimatology - Southern Sierra Nevada 60 Year Tree Ring Carbon Isotope and Resin Duct Data

description: Tree mortality associated with drought and concurrent bark beetle outbreaks is expected to increase with further climate change. When these two types of disturbance occur in concert it complicates our ability to accurately predict future forest mortality. The recent extreme California USA drought and bark beetle outbreaks resulted in extensive tree mortality and provides a unique opportunity to examine questions of why some trees die while others survive these co-occurring disturbances. We use plot-level data combined with a three-proxy tree-level approach using radial growth, carbon isotopes, and resin duct metrics to evaluate 1) whether variability in stand structure, tree growth or size, carbon isotope discrimination, or defenses precede mortality, 2) how relationships between these proxies differ for surviving and now-dead trees, and 3) whether generalizable risk factors for tree mortality exist across pinyon pine (Pinus monophylla), ponderosa pine (P. ponderosa), white fir (Abies concolor), and incense cedar (Calocedrus decurrens) affected by the combination of drought and beetle outbreaks. We find that risk factors associated with mortality differ between species, and that few generalizable patterns exist when bark beetle outbreaks occur in concert with a particularly long, hot drought. We see evidence that both long-term differences in physiology and shorter-term beetle-related selection and variability in defenses influence mortality susceptibility for ponderosa pine, whereas beetle dynamics may play a more prominent role in mortality patterns for white fir and pinyon pine. In contrast, incense cedar mortality appears to be attributable to long-term effects of growth suppression. Risk factors that predispose some trees to drought and beetle-related mortality likely reflect species-specific strategies for dealing with these particular disturbance types. The combined influence of beetles and drought necessitates the consideration of multiple, species-specific risk factors to more accurately model forest mortality in the face of similar extreme events more likely under future climates.
description: Tree ring carbon isotope (d13C) and resin duct data from trees in southern California (Los Padres and Sierra National Forests) that died and those that survived the 2012-2016 California drought. Samples were analyzed for delta 13C from 4 tree species (ABCO-Abies concolor, CADE-Calocedrus decurrens, PIPO-Pinus ponderosa, and PIMO-Pinus monophylla). Resin duct data were measured on the two Pinus species. Total ring width measurements are archived separately in the International Tree-Ring Data Bank, Site Codes CA699-CA702. Axial resin duct data for trees in southern California that died (D) and those that survived (L) the 2012-2016 California drought. Resin ducts were measured on dendrochronologically-dated tree rings derived from 5 mm-wide increment cores. Resin duct metrics measured for each annual ring include: total duct area (mm2) (i.e. the sum of all area consisting of resin ducts within an annual ring), mean duct size (mm2) (i.e. mean resin duct size within an annual ring), and duct production (i.e. count of total number of resin ducts within an annual ring). These metrics have not been standardized by ring area. To do so, divide values by ring area (provided). For a complete description of resin duct variables and suggested terminology, see Hood, S., and A. Sala. 2015. Ponderosa pine resin defenses and growth: metrics matter. Tree Physiology 35:1223-1235.