Snagfall the first decade after severe bark beetle infestation of high‐elevation forests in Colorado, USA

Abstract

The persistence and fall rate of snags (standing dead trees) generated during bark beetle outbreaks have consequences for the behavior, effects, and suppression of potential wildfires, hazard tree and timber salvage operations, wildlife habitat, and numerous ecosystem processes. However, post‐beetle snagfall dynamics are poorly understood in most forest types. We tagged standing live and dead lodgepole pine (Pinus contorta), subalpine fir (Abies lasiocarpa), and Engelmann spruce (Picea engelmannii), including beetle‐killed pine snags following the peak of a recent mountain pine bark beetle outbreak in watersheds at the Fraser Experimental Forest in northcentral Colorado and sampled snagfall 10 and 12 years later. Bark beetle attacks began in 2003, peaked by 2006, and killed 78% of overstory lodgepole pine in 133 plots distributed across a range of stand and site conditions. Of those snags, only 17% fell between 2007 and 2018. Most snags broke at ground level, due to butt rot, and were oriented downhill. In contrast, snags that tipped up or snapped off above the ground were oriented with the prevailing winds. Equal numbers of snags fell singly and in multiple‐tree groups, and equal numbers remained elevated rather than in contact with the ground. Lodgepole pine snagfall was 1.6‐times higher on steep slopes (>40%) where dead pine density was higher, compared to flatter sites. Based on our findings and previous research, we estimate that one‐half the beetle‐killed lodgepole pine in high‐elevation forests such as those at Fraser may fall within 15–20 yr of beetle infestation, but that some pine snags are likely to persist for decades longer. Post‐outbreak snagfall dynamics create a multiple‐decade legacy of bark beetle outbreaks that will persist longer in high‐elevation compared to lower‐elevation forests.

Fuels change quickly after California drought and bark beetle outbreaks with implications for potential fire behavior and emissions

“…3). No red fir had fallen by the end of the study period, which may not only be due to species specific differences in snag longevity, but the colder climate, and thus slower decay rate of these higher elevation mixed conifer forests (Rhoades et al 2020). Less tree mortality in this forest type may have also contributed to this pattern, as the likelihood of snags falling and breaking other snags was lower.…”

“…We attribute this to the speed with which snags fell over four years of monitoring, particularly for pine species (Fig. 3), which fell faster than, for example, mountain pine beetle-killed Rocky Mountain lodgepole pine (Audley et al 2021;Rhoades et al 2020) but at similar rates to beetle-killed ponderosa pine in the southwest U.S. (Chambers and Mast 2014;Hoffman et al 2011). Landram et al (2000) reported snag fall rates in northern California mixed-conifer forests and found that an average of 45% of ponderosa pine snags had fallen by 5 years since death, which is similar to our observation (40%).…”

Twenty years of drought‐mediated change in snag populations in mixed‐conifer and ponderosa pine forests in Northern Arizona

“…Snag loss rates also may have been influenced by bark beetle activity, especially later in the study. Snags resulting from bark beetle infestations appear to fall relatively quickly (Chambers and Mast 2014;Rhoades et al 2020), and this also may have contributed to the increase in snag loss rates observed over time in this study (see Fig. 1c and d).…”

Temperature and Tree Size Explain the Mean Time to Fall of Dead Standing Trees across Large Scales

“…We aggregated six-hourly CRUNCEPv7 data (1901-2016; [117]) with a resolution of 0.5 • × 0.5 • over the standardized reference period, 1981-2010 (CRUclim), and over the observation period of each study if start and end year were reported (CRUobs; np = 56; Figure A1). We treated extensions of the observation periods beyond the temporal coverage of CRUNCEPv7 as missing data (np = 1; [100]) and used CRUclim for studies with location coordinates but missing information about the observation period (np = 9). We used bioclimatic data of different standardized reference periods of MAT and MAP from CHELSA (Climatologies at high resolution for the Earth's land surface areas) aggregated for 1981-2010 at 30 arc seconds [118,119] and WorldClim2 at 10 m and 30 arc seconds over the standardized reference period, 1970-2000 [120].…”