Subalpine Forests: Wildfire, Bark Beetle Outbreaks, and Recent Climate Variation

Transcription

1 Subalpine Forests: Wildfire, Bark Beetle Outbreaks, and Recent Climate Variation Focus on ecological causes and consequences of trends in fire and bark beetle outbreaks in northern Colorado subalpine forests? Societal responses? (more in Geog. 4430) Suppression Mitigation Adaptation

2 The Broader Context: increasing wildfire in the West Relative importance of climate vs. fuel types/quantities? Annually fire tracks warmer springs and longer summer dry seasons Earlier snow melt Longer fire seasons Strongest increase in subalpine forests where fire suppression would have had the least impact Westerling et al Science

3 Broad-scale Role of Climate in MPB Outbreaks Warming Accelerates beetle life cycle and population growth Reduces cold-induced mortality Beetle range expansions Drought decreases tree resistance Research and Consensus Logan et al Assessing the effects of changing climate on mountain pine beetle dynamics. USDA For. Serv. GTR-RM-262 Logan and Powell Ghost forests, global warming, and the mountain pine beetle. Amer. Entomol. Safranyik, L. and B. Wilson The mountain pine beetle: A synthesis of biology, management, and impacts on lodgepole pine. Natural Resources Canada. Bentz et al Bark beetle outbreaks in western North America: Causes and consequences. Utah State Univ. Press

4 Modeled Future trends Expanded climate suitability for MPB Hicke et al JGR, Regniere and Bentz 2007 J Ins Physiol Canadian For. Serv Risk assessment MPB boreal and eastern pine

5 Potential consequences and feedbacks that require predictive modeling and adaptation Shift in MPB-killed forests from carbon sinks to carbon sources Kurz et al. Nature BC emissions equivalent to c. 70% from fire in boreal forests per year Increased fire will result in further carbon emissions to the atmosphere? Will bark beetle kills result in increased wildfire activity and result in further carbon emissions to the atmosphere?

6 Bark beetle and wildfire interactions With bark beetles, one size does not, fit all -- Bentz et al The factors influencing bark beetle outbreak vary depending on the species of bark beetle, geographic region, and host tree species. Historical precedence for current outbreak extents? Effects of fire suppression on stand susceptibility? Consequences of beetle kill for fire hazard and risk?

7 Since 1996 > 1.5 million acres of lodgepole killed by MPB in Colorado

8 Perfect storm of effects of fire exclusion promoting MPB creating unprecedented fire hazard -- Rocky Mountain News "I look at Colorado as the Katrina of the West" in terms of inadequate federal disaster preparation for wildfire in beetle-killed forests. --Sen. Ken Salazar, July Beetlemania in Colorado -- Newsweek, July 2008

9 For northern Colorado subalpine forests 1. How unusual are the current MPB and spruce beetle (SB) outbreaks in comparison to outbreaks of the past several centuries? 2. What is the role of climate variability in the current bark beetle outbreaks? 3. What has been the role of climatic variation in past episodes of widespread severe fires? 4. How has 20 th century fire suppression affected the hazard and risk of fire and beetle outbreak? 5. Following previous bark beetle outbreaks, did wildfire increase in frequency, extent or severity?

10 1. How unusual are the current MPB and spruce beetle (SB) outbreaks in comparison to outbreaks of the past several centuries?

11 1940s SB outbreak affected most spruce-fir in White River, southern Routt, and Grand Mesa NFs. In White River NF in the 1940s > 90% of spruce timber volume killed

12 Tree-ring reconstructions show spruce beetle outbreaks affected even larger areas during the second half of the 19 th century

13 Is the current MPB outbreak unprecedented in northern Colorado? No published tree-ring records of MPB in Colorado but 19 th century drought and 16 th century mega-drought? A.D. Hopkins (1909) described enormous beetle kills for southern Rockies in general (including Colorado) FS and RMNP reports of widespread MPB but much less than current outbreak*: 1930s (Ponderosa pine, RMNP) 1950s (Ponderosa pine, Front Range) 1970s (Ponderosa pine, Boulder County) 1980s (Lodgepole pine, Grand County) Lodgepole forests do not show extensive post-beetle cohorts during 20 th century * Veblen and Donnegan HRV.

14 2. What is the role of climate variability in the current bark beetle outbreaks?

15 Temperature regulates rate of development and survival of MPB in lab and field studies* Maturity and emergence at threshold temperatures Peak rates of emergence at higher temperatures Shift from two-year to one-year life cycle Cold tolerance varies with life-cycle stage Death from winter temperatures below -29 to 40 o C Temperature thresholds vary with location! Prediction of climate effects on MPB requires more on-site studies *Logan et al. 1995; Safraynik 2006; Hicke et al. 2006; Malm and Negron in progress

16 Recent warming in the Colorado subalpine zone is highly favorable to MPB population growth and survival Minimum temperatures in north central Colorado (Grand Lake, Hayden, Spicer, Steamboat Springs, Walden) From K. Wolter, NOAA, Boulder

17 Drought and moisture stress reduce tree defenses against bark beetle attack

18 3. What has been the role of climatic variation in past episodes of widespread severe fires?

19 Subalpine fire history from stand-origin mapping and fire scars: infrequent but extensive fires in LP and Spruce-Fir Kulakowski and Veblen J. Ecol. Kulakowski, Veblen and Bebi J. Biogeog. Sibold and Veblen J. Biogeog.

20 Fire history and climate variability from tree-ring records in Colorado subalpine and montane forests Tree-ring Reconstructions of Sea Surface Temperature Anomalies : ENSO (Niño 3) -- D Arrigo et al Cook et al PDO -- Cook unpublished -- D Arrigo et al AMO -- Gray et al. 2004

21 During the 20 th century, decadalscale drought in Colorado is associated with positive AMO and negative PDO -- McCabe et al and 2008 and with cool ENSO (La Niña) Drought frequency (in percent of years) for positive and negative regimes of the PDO and AMO. (A) Positive PDO, negative AMO. (B) Negative PDO, negative AMO. (C) Positive PDO, positive AMO. (D) Negative PDO, positive AMO

22 Higher probability of fire occurrence (2 to 6 fold increase) during years when AMO is positive and ENSO and PDO are negative East subalpine: Rocky Mtn N.P. Dry combination - Niño3 - PDO + AMO Wet combination + Niño3 + PDO - AMO Proportion Observed Expected All differences are significant (P < 0.001); n = 17 fires, RMNP, 17 large fires Sibold and Veblen J. Biogeog. White R and Routt, 22 large fires Schoennagel et al Ecology. Montane zone northern Front Range, 68 large fires Sherriff & Veblen Int. J. Wildland Fire.

23 At a yr time scale, AMO tends to synchronize fire in ponderosa pine forests across western North America Fire synchrony Correlation AMO Fire r = AMO (10-year spline) Black curve = fire synchrony index (ave. pairwise correlations of 241 sample areas) Blue curve = tree ring reconstruction of AMO index (Gray et al. 2004) Kitzberger et al. 2007

24 In the 1850s-1880s widespread burning in Colorado synchronized stand ages in the subalpine zone In the 1940s spruce beetle outbreak, some spruce were too small (< 70 yrs) to be susceptible Marvine Lakes (3500 ha), Veblen et al J. Ecol. North Fork White R (4600 ha), Kulakowski et al J. Biogeog. White River Plateau (695 km 2 ), Bebi et al Ecol. < 5% overlap of 1940s spruce beetle outbreak with s burns In the current MPB outbreak, lodgepole pine were in the susceptible age class (> 120 yrs)

25 Extensive burning in mid- to late-19 th century synchronized lodgepole pine age structures creating the currently mature, susceptible age structures For. Serv. data

26 4. How has 20 th century fire suppression affected the hazard and risk of fire and beetle outbreak?

27 Role of 20 th -century fire suppression in the subalpine zone is comparatively small Effectiveness of fire suppression in subalpine zone is uncertain Long fire-free intervals are natural in the subalpine zone Surface fires were unimportant no woody tree encroachment no change in fire severity North St Vrain South Thompson East Inlet Tonahutu- North Inlet Colorado Headwaters Big Creek, Routt North Fork White River

28 5. Following previous bark beetle outbreaks, did wildfire increase in frequency, extent or severity? White River NF 1940s Spruce Beetle Kill

29 After the 1940s SB outbreak in White River N.F. fewer fires in beetle-killed forest than in the spruce-fir forests not affected by the 1940s outbreak. 303 fires between 1950 and 1990 in 2800 km 2 but small fires until 2002 Fires / km 2 from 1950 to s Beetle kill No beetle kill From: Bebi, Kulakowski & Veblen 2003, Ecology

30 During the 2002 drought, fires burned in areas where we had previously mapped disturbance history Big Fish Lake fire burned 7100 ha including: 1940s spruce beetle kill 1879 burns Beetle kill was not an important predictor of fire severity Fire severity was more strongly predicted by stand structure, cover type, presence/absence of 1879 burns, and topography than by 1940s spruce beetle kill --- Bigler, Kulakowski & Veblen 2005, Ecology

31 The 2002 Mount Zirkel fires burned 13,000 ha in areas affected by potential predictor variables: 1997 Routt Divide blowdown (10,000 ha) 19 th century and earlier fires Current spruce beetle outbreak Current mountain pine beetle outbreak Areas that were salvage logged after blowdown

32 Routt NF: Recent insect outbreaks (1998-) and 2002 fire extent and severity Recent insect outbreaks did not appear to affect severity or extent of 2002 fires. However, this should be interpreted cautiously due to potential spatial and temporal limitations of aerial detection surveys of beetle activity. Kulakowski and Veblen 2007 Ecology

33 Mt. Zirkel 2002 Fires: Pre-burn stand conditions that were good predictors of fire spread and severity were related to previous fire in 1879: cover type less in aspen than in conifers conifer stand age less in the 1879 post-fire stands than in older stands Fire extent and severity were not increased by the late 1990s SB and MPB kills. Overall fire potential may be more strongly controlled by extreme drought than by fuel structures --- Kulakowski & Veblen 2007, Ecology

34 Conclusions and Practical Implications for Colorado Subalpine Forests Climate variation has been the critical control of fires and insect outbreaks historically in northern Colorado. Past episodes of climate-induced burning have been key in creating the current landscape template of varying resistance/susceptibility to beetle outbreak wildfire use policy. Effectiveness of fuel treatments needs to consider future climate scenarios the extreme of 2002 may be the average of 2050.

35 How do these findings inform policy development? The risk of increased wildfire due to the beetle kill appears not to be as great as previously believed. In the short-term context of the bark beetle emergency this has been both negatively and positively received: resistance to complex information that may decrease public support for mitigation acceptance of information that supports more realistic public expectations about fire risk, effectiveness of fuel treatments, and limited resources for mitigation See Romme et al. 2007

36 Do bark beetle outbreaks increase risk or severity of fires in Colorado subalpine forests? Arguments in favor (fuel-driven fire behavior models): Dry fine fuels in canopy Dry coarse fuels burn intensively after treefalls Lagged, increased ladder fuels due to growth releases Uncertainties and arguments against: Short period of increased dry fine fuels in canopy Reduction in continuity of canopy fuels Live needles may burn hotter due to chemical properties Fire spread is so dependent on drought and fire-event weather that fuel conditions are unimportant Need more fire behavior modeling studies of actual beetle effects on fuel structures (e.g. Jenkins et al. 2008) initial results highly variable by initial conditions (pre-beetle structure) differences among models

37 Fire behavior model predictions based on fuels created by beetle-kill in lodgepole (Jenkins et al. 2008) Rates and intensities of surface fire increased in current epidemics due to decreased vegetative sheltering Passive crown fires (spread on ground with torching) was more likely in post-epidemic stands Active crown fires (crown-to-crown) were less likely in post-epidemic stands They stress complexity of pre-existing stand structures in assessing beetle-kill effects on fire potential.