A Guest Post by Thomas Veblen

One of the joys of working at a world-class university is that there are world-class scholars all around. Professor Thomas Veblen of our Geography department is among them. He graciously responded to a request for his views on beetles, climate, forest management and a few other topics. You can read his guest posting below.

Glad to see that our work on fire history and bark beetle outbreaks in Colorado is reaching a broader audience.I’ll try to respond to some of the questions raised, but for folks who want to be informed on the current state of the science on these issues reading of our two outreach reports is recommended (Romme et al. 2006 and Bentz et al. 2009, cited below).

The short answers to questions or issues raised are:

1. Does our published work on beetle outbreaks or fire history evaluate if warming in Colorado can be attributed to greenhouse gases?

-- No. That is beyond the scope of our work.

2. Has our published research evaluated if recent warming in the subalpine zone (which is the primary cause of current beetle outbreaks) is unique?

--I assume this question pertains to the past 100 years or so of instrumental climate data and to multi-decadal time scales. We have relied on climate station records available on websites some of which have been included in recent climate impact reports (e.g. Saunders, Ray et al.).Climate stations in the area of the MPB outbreak in northern central Colorado show strong warming trends in maximum and minimum temperatures during all seasons when analyzed over the past 50 to 100 years.

3. Don’t dead fuels ignite more easily than live fuels?

--Yes, but that is of relatively minor importance in the context of the weather that drives fires in the Colorado subalpine zone. Our point is that fire history research shows that the years during which large areas (i.e. many 10s of thousands of hectares) have burned in subalpine forests in Colorado are dependent on infrequent, extreme drought. During those drought events, any increased fire hazard associated with beetle-killed trees is a minor contribution to the fire risk determined by weather conditions. During extreme drought, fuel type (dead or alive) is a poor predictor of fire spread during the rare climate events under which vast areas of forest have burned in the subalpine zone documented in our tree-ring fire history studies.

Our published studies of fire behavior (frequency, extent and severity) after the extensive 1940s spruce beetle outbreak in northwestern Colorado and after the late 1990s mountain pine beetle outbreak in north-central Colorado do not show a significant increase in any of these fire behavior parameters in beetle-killed forests compared to green forests. Again, this is because fire is strongly controlled by exceptional climate in Colorado’s subalpine zone.

4. Questions were implied about the role of past fire suppression in contributing to either current fire hazard or current bark beetle outbreaks.

--The answers to these questions vary with forest type and elevation zone, and are discussed in detail in Romme et al. 2006. Most lodgepole pine stands in north-central Colorado were in “age-susceptible” conditions for mountain pine beetle outbreak in the late 1990s because they originated after widespread fires during the second half of the 19^th century associated with warm-dry climate conditions linked to major climate drivers reflected in sea surface temperatures and teleconnections to Colorado. The effectiveness of post-1910 fire suppression is much debated for Colorado subalpine forests, but there clearly is a strong relationship of 19^th century burning to current stand ages that are susceptible to beetle attack. Likewise, for current fire hazard in lodgepole pine and spruce-fir forests fire suppression is a minor (but probably not negligible in some areas) factor relative to weather in creating high fire potential in these fire-prone ecosystems.

5. Comments are made pro and con logging of beetle-killed trees.

--Decisions to thin forests in order to reduce fuels need to be directed to particular projects with specific objectives, and they must consider many social values beyond the scope of research on fire hazard in relation to beetle kill. Some scenarios for proactive management are discussed in the Romme et al. 2006 report. I stress that thinning trees in lodgepole forests close to homes is prudent, regardless of whether the trees are alive or dead. Projects aimed at reducing fire hazard through thinning of forests in remote areas would be of questionable value, and, in fact I don’t believe land managers are recommending thinning in remote areas.

Longer answers can be found in the outreach reports and articles cited below (full citations are listed at http://www.colorado.edu/geography/biogeography/publications.html.

The attribution of current bark beetle outbreaks across western North America, from Alaska (spruce beetle) to the Southwest, to recent warming is based on both field studies and experimental work on weather influences on beetle populations and tree resistance to infestations (references in the Bentz and Romme reports cited below).There has been some attribution to broad climate drivers such as the Arctic Oscillation, but none of the studies on beetle activity attempt to distinguish between natural sources of climate variation and the contribution from greenhouse gases. Obviously, that is beyond the scope of what ecological studies can do.

Our published work on mountain pine beetle and spruce beetle outbreaks in Colorado does not attempt to attribute warming or beetle outbreaks to either natural sources of climate variation or to AGW. We are developing longer records of mountain pine beetle and spruce beetle outbreaks (using tree rings) to statistically link past outbreaks to climate drivers such as ENSO, PDO and the Atlantic Multi-decadal Oscillation which are teleconnected to wildfire activity in Colorado.

As one of the blog comments noted, it is statistically difficult to show trends in rare events like years of large fires in Colorado’s subalpine forests in relation to a warming trend over 50 to 100 years. However, based on a very large data set covering the western U.S., Westerling et al. (2006 Science) show a strong statistical association of increased area burned since the mid-1980s with increased temperatures and a longer fire season. The authors of that report note that even in regions that experienced relatively little 20^th century fire suppression (e.g. northern Rockies) there is an increase in area burn correlated with warming.

For fire history in subalpine forests (i.e. spruce, subalpine fir, and lodgepole pine) in Colorado our work based on tree-ring reconstruction of fire years shows strong statistical linkages to ENSO, PDO and AMO over the past several centuries (Sibold & Veblen 2006, and Schoennagel et al. 2007). These linkages of wildfire activity (and drought) are replicated by independent data sets for ponderosa pine fires in Colorado (Sherriff and Veblen 2008) and generally across western North America (Kitzberger et al. 2007). The goal of our research has been to understand how variability in climate (including late 20^th century warming) has affected bark beetle outbreaks, wildfire activity, and non-beetle related tree mortality (Bigler et al. 2007, vanMantgem et al. 2009 Science). Supporting or refuting AGW is beyond the scope of our work, but our findings support the view that recent warming is having a major impact on these ecological processes in Colorado.

Citations: Romme, W.H., J. Clement, J. Hicke, D. Kulakowski, L.H. MacDonald, T.L. Schoennagel, and T.T. Veblen. 2006. Recent forest insect outbreaks and fire risk in Colorado forests: A brief synthesis of relevant research. Colorado Forest Restoration Institute, Report, 24 pp. Fort Collins, CO. PDF here

Bentz B, Allen CD, Ayres M, Berg E, Carroll A, Hansen M, Hicke J, Joyce L, Logan J, McMahon J, Macfarlane J, Munson S, Negrón J, Paine T, Powell J, Raffa K, Régnière J, Reid M, Romme W, Seybold S, Six D, Tomback D, Vandygriff J, Veblen T, White M, Witcosky J, Wood D (2009). Bark Beetle Outbreaks in Western North America: Causes and Consequences. University of Utah Press, ISBN 978-0-87480965-7. 42 p. Copies can be ordered from here for $4.95.