My previous research has involved characterizing disturbance regimes of Colorado forests.
Jarvis, D., Kulakowski, D. Long-term history and synchrony of mountain pine beetle outbreaks
A continental-scale outbreak of mountain pine beetle (MPB) has affected millions of hectares of forest in the western U.S. and Canada over the past 15 years. The magnitude of this outbreak is widely believed to be unprecedented. But surprisingly, there is a paucity of research that has examined long-term and broad-scale history of MPB outbreaks. Such research is necessary to provide context for current and future MPB activity. In the present study we used tree-ring methods along with satellite imagery to document the history of MPB outbreaks in western Colorado, USA and then compared our results to documented outbreaks across western North America. Satellite imagery and Geographic Information System (GIS) layers of several-hundred year old lodgepole pine forest were used to identify potential sample sites in western Colorado. At 10 of these sites where evidence of past MPB outbreak (dead lodgepole pine with visible MPB galleries and blue-stain fungus) was present, tree cores were collected from dead host and adjacent living host and non-host (Engelmann spruce and subalpine fir) canopy and sub-canopy trees. Synchronous mortality of host trees and coincident growth releases (≥two-fold growth increase sustained for at least 10 years) of living host and non-host species indicated cross-site canopy disturbance events initiating in ca.1760s, 1780s, 1820s-1830s, 1860s, 1910s, 1960s, and 1980s likely caused by MPB outbreak. These data provide the longest history of MPB outbreaks in the Southern Rockies available to date. Furthermore, the outbreak dates from this study coincide with documented and reconstructed MPB outbreaks in British Columbia, Colorado, Idaho, Montana, Wyoming, and Utah. Together, these results suggest that broad-scale synchrony of MPB outbreaks may not be anomalous in western pine forests.
Johnston, R.J., Jarvis, D., Wallmo, K., Lew, D.K. Characterizing Large Scale Spatial Pattern in Nonuse Willingness to Pay: An Application to Threatened and Endangered Marine Species
This project demonstrates a suite of analytic methods that may be used to characterize otherwise undetectable spatial heterogeneity stated preference willingness to pay (WTP). We emphasize flexible methods applicable to large scale analysis with diffuse policy impacts and uncertainty regarding the appropriate scales over which spatial patterns should be evaluated. Illustrated methods include spatial interpolation and multi-scale analysis of hot/cold spots using local indicators of spatial association. An application to threatened and endangered marine species illustrates the empirical findings which emerge. Relevant findings include previously unobserved, large scale clustering of nonuse WTP estimates that appears at multiple scales of analysis.
Kulakowski, D., Jarvis, D. 2013. Low severity fires increase susceptibility of lodgepole pine to mountain pine beetle outbreaks in Colorado. Forest Ecology and Management. 289: 544-550
Both fire and insect outbreaks are critical components of many forest ecosystems and understanding the two-way interactions between these disturbance types is an important goal for researchers, resource managers, and policy makers. Much recent research has focused on understanding the effects of outbreaks on subsequent fires, but the effects of fires on subsequent outbreaks are also important in shaping ecosystem dynamics. In the current study we examined how low-severity fires influence susceptibility of lodgepole pine to mountain pine beetle (MPB). We examined 607 lodgepole pine trees in stands that were affected by low severity fire in 2002 and subsequent MPB outbreak in Routt National Forest, Colorado. For each tree we recorded effect by fire (no visible effect; visible effect – i.e. charring), recent effect by MPB (no visible effect; infestation – i.e. presence of MPB entry or exit holes; or mortality), dbh, and age (based on increment core samples). Tree diameter (dbh) was the most important factor in determining susceptibility to MPB such that larger trees were more susceptible to MPB. But once dbh was taken into account, trees that were charred were more likely to have been attacked and killed by MPB. Previous work has found that stand-replacing fires reduce susceptibility of lodgepole pine stands to MPB in these ecosystems. The current results highlight the fact that fires that are below a high threshold of severity and instead injure or otherwise weaken trees, may increase susceptibility to MPB. Fire-affected lodgepole may act as vectors for the spread of outbreaks during moderate outbreak conditions or as refuges during endemic population phases. It is important to consider how low-severity fires, including prescribed burns, may increase forest susceptibility to outbreaks at local to landscape scales.
Kulakowski, D., Matthews, C., Jarvis, D., Veblen, T.T. 2013. Compounded disturbances in subalpine forests in western Colorado favor future dominance by quaking aspen (Populus tremuloides). Journal of Vegetation Science. 24: 168-176
As the extent, magnitude and/or frequency of various forest disturbances are increasing due to climate change, it is becoming increasingly likely that forests may be affected by more than one type of disturbance in short succession. We studied how compounded disturbances and pre-fire composition influence post-fire tree regeneration. Specifically, do compounded disturbances reduce overall regeneration and favour initial dominance of species that regenerate vegetatively? The study region was affected by a severe outbreak of Dendroctonus rufipennis in the 1940s, a severe wind storm in 1997 and severe fires in 2002. Permanent plots to monitor regeneration were established in 2003 and were re-measured in 2004, 2005 and 2010. Plots were located in stands that varied in long-term disturbance history (stands that originated following fires in 1879 or 1880 vs older stands), recent disturbance history (fire only; outbreak then fire; blowdown then fire) and pre-fire forest dominance (Populus tremuloides, Pinus contorta, or Picea engelmannii–Abies lasiocarpa). Combined density of regeneration of all tree species was highest in stands dominated by P. tremuloides prior to the 2002 fires. In P. contorta stands that were affected only by the 2002 fires, regeneration density was higher in stands that were younger prior to the fire (those that originated in the 1880s), in which cone serotiny is more prevalent, than in older stands (those that originated >200 yr ago). However, the advantage of relatively young P. contorta stands to regenerate following fire was inhibited by compounded disturbances of wind and then fire. Similarly, following compounded disturbances the combined density of conifer seedlings of all species was lower than following only fire. In contrast, the density of P. tremuloides was not lower following compounded disturbances than following fire only, and was higher than that of other species. Pre-fire forest composition and disturbance history influence the abundance and composition of post-fire regeneration. Compounded disturbances generally reduce the regeneration of conifers, which regenerate exclusively from seed, and appear to favour initial stand dominance by P. tremuloides, which regenerates both sexually and asexually. Such differential effects may thereby alter trajectories of post-fire regeneration. As P. tremuloides are less susceptible than conifers to fires, bark beetle outbreaks and wind disturbances, increased dominance by P. tremuloides may contribute to a negative feedback that may diminish the probability and/or severity of future disturbances and thus increase overall forest ecosystem resiliency.
Vanderhoof, M., Williams, CA, Shuai, Y, Jarvis, D, Kulakowski, D, Masek, J (2013). "Albedo induced radiative forcing from mountain pine beetle outbreaks in forests, south-central Rocky Mountains: magnitude, persistence, and relation to outbreak severity." BioGeosciences Discuss10(7): 11935-11968.
Mountain pine beetle (MPB) outbreaks in North America are widespread and have potentially-persistent impacts on forest albedo and associated radiative forcing. This study utilized multiple datasets, both current and historical, within lodgepole pine stands in the south-central Rocky Mountains to quantify the full radiative forcing impact of outbreak events for decades after outbreak (0 to 60 yr) and the role of outbreak severity in determining that impact. Change in annual albedo and radiative forcing peaked at 14–20 yr post-outbreak (0.06 ± 0.006 and −0.8 ± 0.1 W m−2, respectively) and recovered to pre-outbreak levels by 30–40 yr post-outbreak. Change in albedo was significant in all four seasons, but strongest in winter with the increased visibility of snow (radiative cooling of −1.6 ± 0.2 W m−2, −3.0 ± 0.4 W m−2, and −1.6 ± 0.2 W m−2 for 2–13 yr, 14–20 yr and 20–30 yr post-outbreak, respectively). Change in winter albedo and radiative forcing also increased with outbreak severity (percent tree mortality). Persistence of albedo effects are seen as a function of the growth rate and species composition of surviving trees, and the establishment and growth of both understory herbaceous vegetation and tree species, all of which may vary with outbreak severity. The establishment and persistence of deciduous trees was found to increase the temporal persistence of albedo effects. MPB induced changes to radiative forcing may have feedbacks for regional temperature and precipitation, which could impact future MPB outbreaks dynamics.
Kulakowski, D., Jarvis, D., Veblen, T.T., Smith, J. 2012. Stand-replacing fires reduce susceptibility of lodgepole pine to mountain pine beetle outbreaks in Colorado. Journal of Biogeography. 39(11): 2052-2060
As climate change is increasing the frequency, severity and extent of wildfire and bark beetle outbreaks, it is important to understand how these disturbances interact to affect ecological patterns and processes, including susceptibility to subsequent disturbances. Stand-replacing fires and outbreaks of mountain pine beetle (MPB), Dendroctonus ponderosae, are both important disturbances in the lodgepole pine, Pinus contorta, forests of the Rocky Mountains. In the current study we investigated how time since the last stand-replacing fire affects the susceptibility of the stand to MPB outbreaks in these forests. We hypothesized that at a stand-scale, young post-fire stands (< c. 100–150 years old) are less susceptible to past and current MPB outbreaks than are older stands. Location Colorado, USA. We used dendroecological methods to reconstruct stand-origin dates and the history of outbreaks in 23 lodgepole pine stands. The relatively narrow range of establishment dates among the oldest trees in most sampled stands suggested that these stands originated after stand-replacing or partially stand-replacing fires over the past three centuries. Stands were affected by MPB outbreaks in the 1940s/1950s, 1980s and 2000s/2010s. Susceptibility to outbreaks generally increased with stand age (i.e. time since the last stand-replacing fire). However, this reduced susceptibility of younger post-fire stands was most pronounced for the 1940s/1950s outbreak, less so for the 1980s outbreak, and did not hold true for the 2000s/2010s outbreak. Younger stands may not have been less susceptible to the most recent outbreak because: (1) after stands reach a threshold age of > 100–150 years, stand age does not affect susceptibility to outbreaks, or (2) the high intensity of the most recent outbreak reduces the importance of pre-disturbance conditions for susceptibility to disturbance. If the warm and dry conditions that contribute to MPB outbreaks concurrently increase the frequency and/or extent of severe fires, they may thereby mitigate the otherwise increased landscape-scale susceptibility to outbreaks. Potential increases in severe fires driven by warm and dry climatic trends may lead to a negative feedback by making lodgepole pine stands less susceptible to future MPB outbreaks.
Kulakowski, D., Jarvis, D. 2011. The influence of mountain pine beetle outbreaks and drought on severe wildfires in northwestern Colorado and southern Wyoming: A look at the past century. Forest Ecology and Management. 262(9):1686-1696.
Outbreaks of bark beetles and drought both lead to concerns about increased fire risk, but the relative importance of these two factors is the subject of much debate. We examined how mountain pine beetle (MPB) outbreaks and drought have contributed to the fire regime of lodgepole pine forests in northwestern Colorado and adjacent areas of southern Wyoming over the past century. We used dendroecological methods to reconstruct the pre-fire history of MPB outbreaks in twenty lodgepole pine stands that had burned between 1939 and 2006 and in 20 nearby lodgepole pine stands that were otherwise similar but that had not burned. Our data represent c. 80% of all large fires that had occurred in lodgepole pine forests in this study area over the past century. We also compared Palmer Drought Severity Index (PDSI) and actual evapotranspiration (AET) values between fire years and non-fire years. Burned stands were no more likely to have been affected by outbreak prior to fires than were nearby unburned stands. However, PDSI and AET values were both lower during fire years than during non-fire years. This work indicates that climate has been more important than outbreaks to the fire regime of lodgepole pine forests in this region over the past century. Indeed, we found no detectable increase in the occurrence of high-severity fires following MPB outbreaks. Dry conditions, rather than changes in fuels associated with outbreaks, appear to be most limiting to the occurrence of severe fires in these forests.
Hart, S.J., Eisenhart, K.S., Jarvis, D., Kulakowski, D., Veblen, T.T. In press. Drought induces spruce beetle (Dendroctonus rufipennis) outbreaks across northwestern Colorado. Ecology.
This study examines influences of climate variability on spruce beetle (Dendroctonus rufipennis) outbreak across NW Colorado during the CE 1650-2011 period. Periods of broad-scale outbreak reconstructed using documentary records and tree-rings were dated to 1843 to 1860, 1882-1889, 1931-1957, and 2004-2010. Periods of outbreak were compared with seasonal temperature, precipitation, vapor pressure deficit (VPD), the Palmer Drought Severity Index (PDSI), and indices of ocean-atmosphere oscillation that include the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). Classification trees showed that outbreaks can be predicted most successfully from above average annual AMO values and above average summer VPD values, indicators of drought across Colorado. Notably, we find that spruce beetle outbreaks appear to be predicted best by interannual to multidecadal variability in drought, not by temperature alone. This finding may imply that spruce beetle outbreaks are triggered by decreases in host tree defenses, which are hypothesized to occur with drought stress. Given the persistence of the AMO, the shift to a positive AMO phase in the late 1990s is likely to promote continued spruce beetle disturbance.