Overstory Structure and Surface Cover Dynamics in the Decade Following the Hayman Fire, Colorado

Abstract

Abstract:The 2002 Hayman Fire burned with mixed-severity across a 400-ha dry conifer study site in Colorado, USA, where overstory tree and surface cover attributes had been recently measured on 20 0.1-ha permanent plots. We remeasured these plots repeatedly during the first post-fire decade to examine how the attributes changed through time and whether changes were influenced by fire severity. We found that most attributes were temporally dynamic and that fire severity shaped their dynamics. For example, low-severity plots experienced a modest reduction in live overstory density due to both immediate and delayed tree mortality, and no change in live overstory basal area through time; in contrast, high-severity plots experienced an immediate and total loss of live overstory density and basal area. Large snag density in low-severity plots did not vary temporally because snag recruitment balanced snag loss; however, in high-severity plots large snag density increased markedly immediately post-fire and then declined by about half by post-fire year ten as snags fell. Mineral soil cover increased modestly immediately post-fire in low-severity plots and substantially immediately post-fire in high-severity plots, but changed little in ensuing years for either severity class. By incorporating pre-fire and repeatedly-measured post-fire data for a range of severities, our study uniquely contributes to the current understanding of wildfire effects in dry conifer forests and should be of interest to managers, researchers, and others.

Forest structure and regeneration responses 15 years after wildfire in a ponderosa pine and mixed-conifer ecotone, Arizona, USA

“…Although fire severity classification reportedly described the variability in first-order fire effects at this site (see Cocke et al 2005a), secondary tree mortality was indistinguishable among the burned (low, moderate, high) severity classes from 2002 to 2016, and ranged from 38 to 42% for all species combined. In low-severity plots, secondary tree mortality ranged from 26 to 62%, which may be high compared with patterns of secondary mortality reported for low-severity wildfire areas in western US dry forests elsewhere (Keyser et al 2008;Fornwalt et al 2018). In addition, tree mortality on unburned plots from 2002 to 2016 was 26%, double the initial mortality (2000 to 2002) one year following the Leroux Fire (Fig.…”

Fire severity and changing composition of forest understory plant communities

“…Furthermore, differences in pre‐fire forest structure may have been related to differences in eventual fire severity and in pre‐fire plant community composition. In particular, plots that eventually burned at low severity had a tendency to have increased bare ground cover and reduced litter cover, tree density, basal area and canopy cover compared to plots that eventually burned at moderate or high severity, although these differences were rarely significant (Appendix ) and differences in weather also likely played a strong role in eventual fire severity (Fornwalt et al., ). However, notably, the considerable variation in the pre‐fire ratio of cool‐mesic to warm‐xeric taxa (Figure a) is well explained by variation in pre‐fire canopy cover, with higher canopy cover plots associated with significantly higher ratios of cool‐mesic to warm‐xeric taxa (Figure ).…”

“…Supporting this inference, we showed that extinctions of cool‐mesic lineages were most pronounced in high‐severity burned areas (Figure ), which had complete canopy removal at the plot scale. Although our post‐fire timeframe of 10 years may limit our ability to infer longer‐term community dynamics, we believe that the changes we observed in canopy structure and understory diversity should persist for the near future due in large part to the slow growth rates of surviving and establishing trees (Fornwalt et al., ; Malone et al., ).…”

“…Plots where ≤50% of the pre‐fire live overstorey trees were killed and where tree crown and organic surface material consumption were generally slight were categorized as burning with low severity (i.e., burned by light surface fire; ten plots). This 50% threshold was used to accommodate mortality of the smallest size class (0–10 cm DBH); mortality of all trees >10 cm DBH in the low‐severity plots did not exceed 20%, a more typical low‐severity threshold, and accordingly basal area mortality in the low‐severity plots was very low at <12% (Fornwalt, Stevens‐Rumann, & Collins, ). Moderate‐severity plots experienced >50% mortality of live overstorey trees and had modest tree crown and organic surface material consumption (i.e., burned by moderate to severe surface fire; six plots).…”

Soil Nitrogen Leaching in Logged Beetle‐Killed Forests and Implications for Riparian Fuel Reduction

“…Trees surviving within beetle‐infested pine stands grew faster (Romme et al, 1986) and nutrient uptake by residual, live trees limited NO 3 –N leaching in simulated beetle outbreaks in Wyoming pine forests (Parsons et al, 1994). In this study, there were 3000 to 4500 trees ha −1 (0–200 cm tall) on uncut hillslopes (Supplemental Table S1) and dense understory plant cover (Supplemental Table S2), similar to conditions seen in nearby beetle‐infested forests (Fornwalt et al, 2018; Rhoades et al, 2018). Greater light and soil resource availability beneath dead overstory trees stimulate understory plants (Hawkins et al, 2013; Pec et al, 2015) and regenerating trees (Pelz et al, 2018), and this compensatory growth and nutrient demand appears sufficient to limit N losses.…”