Spatial and ecological analysis of red fir decline in California using FIA data

Red fir (Abies magnifica) is a high elevation conifer generally growing between an altitude of 1,400 and 2,700 meters. In California, red fir grows in the Sierra Nevada, the Klamath Mountains, the eastern edges of the northern Californian Coast Ranges, and in the southern Cascades. Red fir commonly grows in pure stands and is often found in association with white fir (Abies concolor), lodgepole pine (Pinus contorta) or Jeffrey pine (Pinus Jeffreyi). Red fir is present in popular recreational areas including Yosemite, Kings Canyon, Sequoia and Lassen Volcanic national parks as well as the Mt. Shasta area and the Lake Tahoe region. Increasing and higher-than-expected red fir mortality and decline over the past five years has been observed in the central Sierra Nevada. This mortality and decline is seen as being caused by a complex interaction of biotic, anthropogenic and abiotic factors. The abiotic factors include drought, climate change (especially decreased snowpack), and the effects of changing fire regimes. The key anthropogenic factors are air pollution and forest management. The biotic factors include red fir dwarf mistletoe (Arceuthobium abietinum f. sp. magnificae), Annosus root disease (Heterobasidion annosum), Cytospora canker (Cytospora abietis), and the fir engraver beetle (Scolytus ventralis). Using USFS Forest Inventory & Analysis (FIA) plots at a density of one every 5.47 km (3.4 miles) across California allowed for the first stand-level analysis of the entire red fir distribution zone in California. The results show that mortality is increasing in red fir, which suggests that at least a short term decline is occurring. The rate at which mortality is increasing varies, depending on which analysis approach is used and decreases if recently burned plots are removed from the analysis. At the individual tree level, red fir mortality (all size classes) is occurring at an annual rate of 2.64%. Red fir dwarf mistletoe is the most significant factor in red fir mortality and decline based on our field observations and statistical analysis. There is a clear visual and statistical difference in forest health between areas that possess red fir dwarf mistletoe and those that don't. This remains true even when stands are heavily stocked and Annosus root disease is present, suggesting that Annosus root disease (which is common throughout the red fir distribution range) is greatly exacerbated by the presence of red fir dwarf mistletoe. Cytospora canker is associated with red fir dwarf mistletoe, and is likely a significant reason why the red fir dwarf mistletoe impacts red fir forest health so significantly. The only variation in spatial pattern of red fir mortality in California is an area of very low red fir mortality around Mt. Shasta (where red fir dwarf mistletoes does not occur). There was not a consistent correlation between red fir mortality and drought stress. The amount of fir engraver activity in California is not well characterized making it difficult to assess its' role in red fir mortality

Cross-scale interaction of host tree size and climatic water deficit governs bark beetle-induced tree mortality

The recent Californian hot drought (2012&ndash;2016) precipitated unprecedented ponderosa pine (Pinus ponderosa) mortality, largely attributable to the western pine beetle (Dendroctonus brevicomis; WPB). Broad-scale climate conditions can directly shape tree mortality patterns, but mortality rates respond non-linearly to climate when local-scale forest characteristics influence the behavior of tree-killing bark beetles (e.g., WPB). To test for these cross-scale interactions, we conduct aerial drone surveys at 32 sites along a gradient of climatic water deficit (CWD) spanning 350&thinsp;km of latitude and 1000&thinsp;m of elevation in WPB-impacted Sierra Nevada forests. We map, measure, and classify over 450,000 trees within 9 km2, validating measurements with coincident field plots. We find greater size, proportion, and density of ponderosa pine (the WPB host) increase host mortality rates, as does greater CWD. Critically, we find a CWD/host size interaction such that larger trees amplify host mortality rates in hot/dry sites. Management strategies for climate change adaptation should consider how bark beetle disturbances can depend on cross-scale interactions, which challenge our ability to predict and understand patterns of tree mortality. The 2012&ndash;2016 drought and western pine beetle outbreaks caused unprecedented mortality of ponderosa pine in the Sierra Nevada, California. Here, the authors analyse drone-based data from almost half a million trees and find an interaction between host size and climatic water deficit, with higher mortality for large trees in dry, warm conditions but not in cooler or wetter conditions.</p

Tree Mortality following Thinning and Prescribed Burning in Central Oregon, U.S.

We examined causes and levels of tree mortality one year after thinning and prescribed burning was completed in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests at Pringle Falls Experimental Forest, Oregon, U.S. Four blocks of five experimental units (N = 20) were established. One of each of five treatments was assigned to each experimental unit in each block. Treatments included thinning from below to the upper management zone (UMZ) for the dominant plant association based on stand density index values for ponderosa pine followed by mastication and prescribed burning: (1) 50% UMZ (low density stand), (2) 75% UMZ (medium density stand), (3) 75% UMZ Gap, which involved a regeneration cut, (4) 100% UMZ (high density stand), and (5) an untreated control (high density stand). Experimental units were thinned in 2011 (block 4), 2012 (block 2), and 2013 (blocks 1 and 3); masticated within one year; and prescribed burned two years after thinning (2013–2015). A total of 395,053 trees was inventoried, of which 1.1% (4436) died. Significantly higher levels of tree mortality occurred on 100 UMZ (3.1%) than the untreated control (0.05%). Mortality was attributed to prescribed fire (2706), several species of bark beetles (Coleoptera: Curculionidae) (1592), unknown factors (136), windfall (1 tree), and western gall rust (1 tree). Among bark beetles, tree mortality was attributed to western pine beetle (Dendroctonus brevicomis LeConte) (881 trees), pine engraver (Ips pini (Say)) (385 trees), fir engraver (Scolytus ventralis LeConte) (304 trees), mountain pine beetle (D. ponderosae Hopkins) (20 trees), Ips emarginatus (LeConte) (1 tree), and Pityogenes spp. (1 tree)

Tree Mortality following Thinning and Prescribed Burning in Central Oregon, U.S.

We examined causes and levels of tree mortality one year after thinning and prescribed burning was completed in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests at Pringle Falls Experimental Forest, Oregon, U.S. Four blocks of five experimental units (N = 20) were established. One of each of five treatments was assigned to each experimental unit in each block. Treatments included thinning from below to the upper management zone (UMZ) for the dominant plant association based on stand density index values for ponderosa pine followed by mastication and prescribed burning: (1) 50% UMZ (low density stand), (2) 75% UMZ (medium density stand), (3) 75% UMZ Gap, which involved a regeneration cut, (4) 100% UMZ (high density stand), and (5) an untreated control (high density stand). Experimental units were thinned in 2011 (block 4), 2012 (block 2), and 2013 (blocks 1 and 3); masticated within one year; and prescribed burned two years after thinning (2013&ndash;2015). A total of 395,053 trees was inventoried, of which 1.1% (4436) died. Significantly higher levels of tree mortality occurred on 100 UMZ (3.1%) than the untreated control (0.05%). Mortality was attributed to prescribed fire (2706), several species of bark beetles (Coleoptera: Curculionidae) (1592), unknown factors (136), windfall (1 tree), and western gall rust (1 tree). Among bark beetles, tree mortality was attributed to western pine beetle (Dendroctonus brevicomis LeConte) (881 trees), pine engraver (Ips pini (Say)) (385 trees), fir engraver (Scolytus ventralis LeConte) (304 trees), mountain pine beetle (D. ponderosae Hopkins) (20 trees), Ips emarginatus (LeConte) (1 tree), and Pityogenes spp. (1 tree)

Cross-scale interaction of host tree size and climatic water deficit governs bark beetle-induced tree mortality

The recent Californian hot drought (2012-2016) precipitated unprecedented ponderosa pine (Pinus ponderosa) mortality, largely attributable to the western pine beetle (Dendroctonus brevicomis; WPB). Broad-scale climate conditions can directly shape tree mortality patterns, but mortality rates respond non-linearly to climate when local-scale forest characteristics influence the behavior of tree-killing bark beetles (e.g., WPB). To test for these cross-scale interactions, we conduct aerial drone surveys at 32 sites along a gradient of climatic water deficit (CWD) spanning 350 km of latitude and 1000 m of elevation in WPB-impacted Sierra Nevada forests. We map, measure, and classify over 450,000 trees within 9 km2, validating measurements with coincident field plots. We find greater size, proportion, and density of ponderosa pine (the WPB host) increase host mortality rates, as does greater CWD. Critically, we find a CWD/host size interaction such that larger trees amplify host mortality rates in hot/dry sites. Management strategies for climate change adaptation should consider how bark beetle disturbances can depend on cross-scale interactions, which challenge our ability to predict and understand patterns of tree mortality

Drone-derived data supporting "Cross-scale interaction of host tree size and climatic water deficit governs bark beetle-induced tree mortality"

Drone-derived passive reflectance imagery and derived products documenting 9 square kilometers of yellow pine/mixed-conifer forest that experienced western pine beetle-induced tree mortality during the California hot drought of 2012 to 2015 and its aftermath in the Sierra Nevada mountain range. Imagery was taken between early April, 2018 and early July, 2018. Support was provided by the USDA Forest Service Western Wildlands Environmental Threat Assessment Center

Carbon stored in live ponderosa pines in the Sierra Nevada will not return to pre-drought (2012) levels during the 21st century due to bark beetle outbreaks

Outbreaks of several bark beetle species can develop rapidly in response to drought and may result in large transfers of carbon (C) stored in live trees to C stored in dead trees (10s of Tg C yr-1 in the western U.S. alone), which over time will be released back to the atmosphere. The western pine beetle (WPB) outbreak incited by the 2012–2015 mega-drought in the Sierra Nevada, California, U.S., could portend more frequent and/or severe bark beetle outbreaks as the temperature warms and drought frequency and intensity increase in the future. However, changes in the frequency and/or severity (resultant levels of host tree mortality) of beetle outbreaks are difficult to predict as outbreaks are complex with non-linear and eruptive processes primarily driven by interactions among beetle populations, the demography of hosts and other tree species, and climate and weather. Using an insect phenology and tree defense model, we projected the future likelihood of WPB outbreaks in the Sierra Nevada with climate drivers from different Earth System Models. Our goal was to understand how host (ponderosa pine, PIPO) recovery and future warming and drought affect the frequency and severity of WPB outbreaks and their C consequences. Our projections suggested that by 2100 the C stored in live PIPO (mean: 1.98 kg C m-2, 95% CI: 1.74–2.21 kg C m-2) will not return to levels that occurred before the 2012–2015 drought (2012: ∼2.30 kg C m-2) due to future WPB outbreaks. However, differences in climate models indicate a wide range of possible WPB outbreak frequencies and severities. Our results suggest that total plot basal area is the most significant factor in the mortality rate of PIPO by WPB in any given year, followed by drought severity and temperature. High levels of host basal area, higher temperature, and extreme drought all contribute to the frequency and severity of future WPB outbreaks. While PIPO basal area may decline under increased drought and warming, limiting high-stand basal area (&gt;60 m2 ha-1) may reduce the severity of future WPB outbreaks in the Sierra Nevada