Natural and Human-Caused Disturbances
Work on ecological restoration in my lab focuses primarily on investigating how disturbance (natural or human-caused) affects ecosystems and whether restoration treatments are efficacious in returning systems to a state of pre-disturbance condition and function. Most work on restoration in my lab focuses on bark beetles, fungi, fire, and invasive exotic plants.
Whitebark pine is a five-needle pine that dominates many western North American subalpine forests. Whitebark pine is considered a keystone species. In high elevation zones where it occurs, its loss would result in a permanent lowering of the treeline, negative impacts on snow retention and hydrological processes, and the loss or reduction of many plant and animal species, including the endangered grizzly bear.
Whitebark pine is in rapid decline across much of its range, with some areas already experiencing as much as 90% mortality. This decline is due to three factors. The most important one is white pine blister rust, an exotic disease. Another important factor is the mountain pine beetle, a native insect that kills only scattered trees during endemic periods, but which can cause high levels or even complete mortality of mature whitebark pine over large areas during outbreaks. The mountain pine beetle prefers the large, mature cone-bearing trees upon which regeneration is dependent. With the already considerable loss of large numbers of the pines due to blister rust in many stands, the mountain pine beetle accelerates stand decline directly by killing mature trees, and indirectly through its impact on reproduction. Finally, fire suppression has allowed the encroachment of shade-tolerant trees, primarily subalpine fir, into many stands of shade-intolerant whitebark pine contributing to its decline. Stands choked with subalpine fir provide few suitable sites for germination of whitebark pine seeds, growth of seedlings, or for seed caching by Clark’s nutcrackers (birds), the primary agents of dispersal of whitebark pine.
Very little is known about the dynamics and interactions of insects and pathogens associated with whitebark pine. We investigated whether white pine blister rust severity, tree diameter, bark and phloem thickness, and sapwood moisture content, influence the preference of the mountain pine beetle for individual trees of whitebark and lodgepole pine. We found a significant relationship between blister rust severity and attack of trees by the beetle with trees exhibiting greater blister rust severity being more likely to be attacked by the beetle. Sapwood moisture content was significantly negatively correlated with blister rust severity indicating that as severity increases there is an increasingly negative effect on water relations within the tree. Sapwood moisture content was significantly lower in whitebark pine than in lodgepole pine at sites with beetle activity suggesting that there may be an interaction between blister rust severity and drought stress that in turn may affect beetle preference. Diameter and bark and phloem thickness did not appear to influence beetle preference for individual trees. Our results indicate that as blister rust spreads in whitebark pine stands across the west, this may translate to increasing levels of beetle-caused mortality.
We also investigated how proposed restoration treatments including stand replacement fire, underburning, thinning (removal of lodgepole pine and subalpine fir), and nutcracker openings (small clearcuts to provide areas for seed caching by birds, leaving only whitebark pine saplings) affect bark beetle populations in the short term (sites monitored for one year before and for four years after treatment). Bark beetle activity (killed trees) remained low throughout the study, however, there was an increase in plots that were burned and in plots that were cut for nutcracker openings in the year immediately after treatment. Our results indicate that if burning or cutting treatments are implemented during periods of high beetle activity, mitigation measures for beetles may need to be taken. Future work is planned to look at long term effects.
We recently discovered evidence that temperature plays a key role in determining the relative abundance of two mutualistic fungi associated with an economically and ecologically important bark beetle, Dendroctonus ponderosae (the mountain pine beetle). The beetle appears to require the two fungi for successful development and reproduction. The symbiotic fungi possess different optimal temperature ranges. These differences determine which fungus is vectored by dispersing host beetles as temperatures fluctuate over a season. Grosmannia clavigera is the predominant fungus carried by dispersing beetles during cool periods but decreases in prevalence as daily maximum temperatures approach 25C, and becomes extremely rare when temperatures reach or exceed 32C. In contrast, Ophiostoma montium increases in prevalence as temperatures approach 25C, and becomes the predominant symbiont dispersed when temperatures reach or exceed 32C. The possession of different optimal growth temperatures may facilitate the stable coexistence of the two fungi by supporting growth of each fungus at different times minimizing direct competition. Furthermore, the beetle may reduce its risk of being left without a required symbiont by exploiting not one, but two symbionts, whose combined growth optima span a wide range of environmental conditions. The possession of multiple symbionts with different temperature tolerances may allow the beetle to occupy highly variable habitats over a wide geographic range. Such temperature-driven symbiont shifts are likely to have major consequences for both the host and its symbionts under current temperature regimes, as well as those predicted to occur due to climate change. We are currently modeling how temperature may affect the distribution of both the host beetle and the symbiotic fungi under temperatures predicted to occur in the next fifty years due to climate change.
Many forests in North America now have altered structures and compositions due to decades of fire suppression. These changes have affected fire behavior and impact, altered biotic communities, and have led, in some cases, to increased problems with insects and pathogens. Many treatments have been proposed and are being implemented to restore stands to pre-fire suppression conditions, but few of these have been monitored after implementation to determine if they actually achieve their desired goals. As part of the large national study on fire and fire surrogate treatments, a large interdisciplinary team of scientists from UM and the USFS Fire Lab in Missoula, MT, conducted a study at Lubrecht Experimental forest assessing how thinning only, thinning and burning, burning only, or no treatment affects fire suppressed mixed ponderosa pine and Douglas-fir forests. My team focused on the effects of these treatments on insects (primarily bark beetles) and pathogens (primarily root disease and dwarf mistletoe). The study ran for five years and data are currently being analyzed. Results will be posted on this website in the near future.
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