Fire History in the Colorado Rockies

Transcription

1 Fire History in the Colorado Rockies Brief overview of fire regimes in different forest ecosystem types Relationship of wildfire activity to climate variability Effects of fire exclusion and fire suppression policy Implications for ecological restoration and fire hazard mitigation

2 Paradigm Shift in Understanding Wildfire Threats in the Western U.S. Old view: the SW Ponderosa pine fire suppression and fuels buildup model (typical of HFRA, National Fire Plan c. 2003) Formerly frequent surface fires. Suppression created unnaturally dense stands, unnatural potential for severe fire and current forest health problems Management implications: restore and mitigate through thinning New view: Only limited applicability of SW PP model to particular ecosystems Prior to any effects of suppression wildfires were intense (high severity) and occurred at irregular but longer intervals than in the SW PP model Dense stands naturally develop after severe fires Fire and forest structure fluctuate widely at multi-decadal time scales due to strong climatic variation Management implications: adapt to naturally high fire threat

3 Major fire years of 2000 and 2002 in the western U.S. triggered legislative and policy initiatives

4 Healthy Forests Restoration Act of 2003 Today, the forests and rangelands of the West have become unnaturally dense, and ecosystem health has suffered significantly. When coupled with seasonal droughts, these unhealthy forests, overloaded with fuels, are vulnerable to unnaturally severe wildfires. Healthy Forest Initiative Objective: Reduce hazardous fuels, to restore forests and to mitigate the threat of fire to life and property.

5 Basis for Forest Health Initiatives Fire Suppression

6 Basis for Forest Health Initiatives Fire Suppression Catastrophic wildfires harm people, property, and the environment

7 Southwestern Ponderosa Pine Forests Photos by W.R. Mattoon, F.R. Herman and F. Biondi

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9 Let s do some critical thinking about HFRA Identify the premises or assumptions about fire history and fire effects in HFRA Critically evaluate those premises for a particular place (i.e. site-specific information)

10 Healthy Forests Restoration Act of 2003 Premise: Fire suppression has led to unnaturally dense forests and severe fire hazard across the West.

11 Healthy Forests Restoration Act of 2003 Premise: Fire suppression has led to unnaturally dense forests and severe fire hazard across the West. This is true only for some forest types.

12 Healthy Forests Restoration Act of 2003 Premise: Fire suppression has led to unnaturally dense forests and severe fire hazard across the West. This is true only for some forest types. Fire suppression has led to unnaturally severe outbreaks of forest pests.

13 Healthy Forests Restoration Act of 2003 Premise: Fire suppression has led to unnaturally dense forests and severe fire hazard across the West. This is true only for some forest types. Fire suppression has led to unnaturally severe outbreaks of forest pests. This needs to be examined for particular forest types and specific areas.

14 Healthy Forests Restoration Act of 2003 Premise: Management (thinning and Rx fire) is necessary to restore forests and reduce the threat of fire and improve forest health.

15 Healthy Forests Restoration Act of 2003 Premise: Management (thinning and Rx fire) is necessary to restore forests and reduce the threat of fire and improve health. Forest restoration is necessary only in some forest types not all forest types are unnaturally dense today. Reducing fire risk and restoring forests may require different management actions.

16 Historic Range of Variability Studies Examine Key Premises for Specific Forest Ecosystem Types and Specific Areas How different are modern fire regimes from historic fire regimes? Did high-severity fires occur prior to any effects of fire exclusion? How different are historic and modern patterns of outbreaks of forest pest insects? What is the role of climatic variation?

17 Montane zone: focus on ponderosa pine and Douglas-fir

18 Fire history in the montane zone Has fire suppression altered current and future fire behavior?

19 How have humans altered fire regimes in Colorado forests? Native Americans Euro-American settlers ( ) Modern fire exclusion Lower montane Increased frequency Minor influence on extent Increase but strongly associated with climate Major reduction in frequency and extent Upper montane Increased frequency Minor influence on extent Increase but strongly associated with climate Moderate reduction in frequency and extent Subalpine Not significantly in extent Increase but strongly associated with climate Moderate reduction in frequency but minor reduction in extent

20 Fire history studies based on: post-fire cohorts fire-scarred trees Crossdated firescar dates on ponderosa pine

21 Montane zone (ponderosa pine) fire history in the northern Front Range: Percent Sites Scarred Historic Fire Regime Fire Exclusion Sherriff and Veblen 2006, Year Sample Depth Sherriff & Veblen 2006, 2007: 58 sample sites, m 789 fire-scar cross sections > 3500 tree cores for cohort age, death dates, growth releases

22 Different historic fire regimes in ponderosa pine-dominated ecosystems of the upper versus lower montane zones 7,700 5,600 Ponderosa, Douglas-fir <5500 Lodgepole, spruce, fir

23 Tree age structures show increased tree seedling survival as fire frequency declined. Open woodland to dense forest conversion fits the fuels buildup model. Number of trees % Trees scarred Tree Establishment Dates Site 15 Plots: Ponderosa pine Fire history (55 ha; 1900m) Increased stand density following decline in fire frequency Year Sample depth

24 Lower Montane Zone Repeat photography and tree ages document conversion from open to dense stands. Fire suppression has facilitated conversion to dense stands in this forest type. Veblen and Lorenz 1991

25 1905

26 Lower Montane Zone (low elevation ponderosa forests) These forests are denser than historically. With higher fuel loads, the risk of crown fire has increased. Thinning and Rx fire can reduce densities. Such management will restore these forests and reduce the risk of crown fire.

27 Low elevation ponderosa pine zone fits the fuels buildup model. Nearly all of this habitat type is within the wildland urban interface.

28 Upper montane zone: Ponderosa pine Douglas fir Mixed conifer 9,300 8,000 Ponderosa, Douglas-fir <5500 Lodgepole, spruce, fir

29 Montane zone fires included large stand-replacing fires long before any effects of fire suppression on fuels. 1905

30 Much lower fire frequencies in upper montane PP forests Site 18: Upper Montane PP (search area =450 acres; elevation 8100 ft) % Trees scarred Sample depth % Trees scarred Site 15: Lower Montane (search area = 132 acres; elevation = 6230 ft) Sample depth Year

31 Upper montane zone was characterized by severe fires resulting in post-fire cohorts and growth releases of surviving trees. Infrequent surface fires had only a minor role. Number of trees HIGH ELEVATION PONDEROSA PINE AND DOUGLAS-FIR Site 18: Douglas fir Pre-1650 estimated ages: 1 Site 18: Ponderosa pine Pre-1650 estimated ages: 16 Does not fit the fuels build up model. Ring Width (cm) Tree Tree Tree Note lack of young Df Trees scarred Fire history (191 ha; 2484 m) Year Sample depth

32 Upper Montane Zone: Historic vs modern fire regimes Dense stands today mainly reflect severe fires in 1800s. High severity fires were part of the HRV. See: Sherriff and Veblen, 2006, J. Veg. Sci. Veblen and Lorenz 1991

33 But what are the locations of the low-severity (higher frequency) versus the variable-severity (lower frequency) historic fire regimes? Logistic regression related fire frequency classes to environmental predictors and to map of probability surfaces Predictor variables: elevation, aspect, slope steepness, slope position, distance to grassland, distance to ravine, cover type Response variables: High fire frequency Low and variable fire frequency Sherriff & Veblen, 2007 Ecosystems. Spatially-explicit reconstruction of historical fire regimes in ponderosa pine

34 Gradient of historic fire regime types can be recognized in the ponderosa pine zone 1. High frequency return intervals < 30 yrs low severity (lack of tree mortality) 2. Low and variable fire frequency return intervals > 30 yrs moderate to high severity (canopy tree death) From: Sherriff and Veblen 2007, Ecosystems

35 Predicted area of high fire frequency class for 60,875 ha study area on east slope, AR NF; 75% of the area is dominated by Pipo (IRI data)

36 The premise that the fire regime of ponderosa pine forests formerly consisted of frequent, low-severity fires is supported for < 20% of the ponderosa pine zone in this c. 60,000 ha study area centered on Boulder County

37 Fire history in the Front Range montane zone Has fire suppression altered stand structure (tree population age structures) compared to 19 th century? Are today s forests unnaturally dense due to fire suppression? Has Douglas-fir invaded stands formerly dominated purely by ponderosa pine? And for each question, where in the landscape?

38 Has fire suppression resulted in a high proportion of young trees in the montane landscape? Mean % of tree establishment dates (n = 3200; 23 sites) Pre % Settlement ( ) 55% Fire suppression (post-1920) 13%

39 Has there been a large increase in tree establishment following fire suppression as predicted by the SW Pipo model? Percent of Tree Establishment for Different Time Periods Average percent of tree establishment pre pre pre Pipo Psme Other Lower Montane Zone Mid-Montane Zone Upper Montane Zone m m Mean n = % 9 sites of tree establishment n dates = 5 sites (n = 3200) Pre % Settlement ( ) 55% Fire suppression (post-1920) 13% m n = 9 sites

40 Boulder Upper Montane Zone: New data collected in year smoothing of a regional moisture index (dotted line, where smaller values are drier, axis inverted) and the proportion of total tree establishment (solid line) during the period. The area shaded in grey highlights a multidecadal period of drought from , during which 53% of the fires from the period burned. From Schoennagel, Sherriff, and Veblen unpublished m.s.

41 How does this inform public discussions of management issues? Risk of high severity fire is a natural feature of most of the Pipo zone. A relatively small proportion of that risk is attributable to fuel accumulation due to fire suppression. Identification of where thinning can simultaneously achieve goals of restoration and fire risk reduction.

42 Discuss: Platt et al Are wildfire mitigation and restoration of historic forest structure compatible? Annals AAG 96:

43 Scenarios of fire mitigation and restoration of historic forest structures in relation to land ownership in Boulder County Platt, Veblen & Sherriff, 2006, Annals AAG Fireline intensity under weather scenario (FLAMap) Historic fire regimes Land ownership In montane zone both mitigation and restoration can be achieved on: 41% of Boulder Open Space and Mtn Parks 31% of Private Land 28% of BLM Land 18% of ARNF Land

44 Wildfire Behavior Model: Areas of potential high fire hazard suitable for fuels treatments Historical Fire Model: Areas of high to moderate fire frequency (< 40 year fire intervals) suitable for ecological restoration R. Platt, T. Veblen and R. Sherriff, 2006

45 Percent of Land Use Lands suitable for both restoration and fire hazard mitigation occur in higher percentages on private land and Open Space. If restoration were limited to areas of certain high fire frequency (i.e. not including the moderate fire frequency) the area where both goals could be achieved would by much smaller. 50% 45% 40% 35% 30% 25% 20% 15% 10% 5% 0% Open space BLM Private Forest Service Both Goals Mitigation Only Low Hazard

46 Summary and some solutions Feasibility of Fire Risk Reduction Forest Restoration Lower montane Yes Yes Thinning Rx Fire Upper montane Uncertain Limited areas Subalpine No No Alternative strategies

47 Alternative strategies for reducing fire risk: Managing the 2.5 acres surrounding homes has the greatest impact on whether it will burn. House Materials and Configuration Defensible Space Easy Access Zoning and Restricting Development

48 Housing density affects fire risk From: Western Futures

49 Housing density affects fire risk Fire risk (risk to people and property) is not simply a function of fuels and forest management, but also where people live. With projected population growth in the Front Range, fire risk will increase. From: Western Futures

50 Conclusion For most forest types in Colorado, the primary control on fire occurrence is climate fuel quantities are secondary. Forest restoration and fire risk reduction may or may not be accomplished by the same management actions. Different forest types require different management approaches. Thinning and prescribed fire may be effective in some forests (Lower Montane) but ineffective in others (Subalpine). Fire risk reduction is indeed a function of forest density but also of housing density and configuration in the Wildland-Urban Interface.