Susceptibility to Successful Bark Beetle Attack
Air pollution exposure (O3 and N deposition) increases tree susceptibility to drought stress, and drought stress increases tree susceptibility to successful beetle attack. Dunning (1928) was one of the first to report a relationship between drought conditions and increased levels of tree mortality caused by western bark beetles. In the west, bark beetles reach epidemic proportions after 2 to 3 years of drought. The correlation between beetle attacks and climate can be diffuse because bark beetles may delay or prolong the exact time of tree mortality. However, mortality tends to increase in multiyear droughts, particularly in highly dense stands or those with pre-existing damage or stress. Figure 1 indicates that sequences of 2 to 3 years of drought have occurred 9 times (Table: List of drought years) in the last 120 years. We can document five bark beetle epidemics associated with those sequences.
Beetles are opportunists that attack trees in a weakened state. With only a few exceptions, either the host tree is killed by the colonizing bark beetles or the host resistance of the tree kills the attacking adults. To kill a tree, large numbers of bark beetles must successfully colonize it in a relatively short period of time (Paine and others 1984, Paine and others 1997). However, fewer beetles may be sufficient to kill a compromised tree (Paine and others 1984). The bark beetles most commonly responsible for tree mortality in the western San Bernardino Mountains are western pine beetle (Dendroctonus brevicomis) and mountain pine beetle (D. ponderosae). Western pine beetle can produce up to 4 generations in a year in southern California, where the mild thermoclimate permits populations of this species to expand rapidly when an abundance of susceptible host material is available for colonization.
Eggs are laid in the inner bark and the larvae excavate galleries. Pupation occurs in either the inner or outer bark, depending on the species of beetle. Adults emerge from the larval host tree and search for susceptible hosts. Healthy pines and firs respond by exuding pitch, which either pitches out the adults or blocks their progress. Resin production impedes bark beetle attack both physically and chemically. Oleoresin pressure, caused by turgidity of cells lining the resin ducts, forces preformed resin to the site of injury or invasion and results in a flushing action. The cell turgidity is derived from the transpirational stream, so if the tree is under moisture stress, the cells become increasingly flaccid, the resin pressure is reduced, and the effectiveness of the preformed resistance is compromised (Vite 1961). In weak trees with reduced resin pressure, the adults are able to initiate colonization and produce specific pheromones that attract other colonizing adults. Pheromone production ceases when the host tree ceases resin flow, (i.e., when the tree dies) (Raffa and Berryman 1983).
Severe drought and other stresses also reduce the photosynthetic capacity of trees and the levels of carbohydrates used for growth, defense, and tissue repair. This can have significant impact on the ability of the tree to induce an effective response to invasion (Paine and Stephen 1987a, b). Drought-stressed trees are also known to have elevated levels of free, translocatable proteins (Lei and others 2006), which are produced to generally increase cell osmoticum. We conjecture that increased bole carbohydrate content as a result of O3 exposure + N deposition + drought and elevation of protein levels in response to drought enhance beetle fecundity. Pollutant-exposed trees may thus be primed for successful bark beetle attack.
The forest had been recently thinned early in the late 19th century by commercial logging, so we would not expect to observe an epidemic beetle infestation in immature, low-density stands despite the drought stress experienced in the late 1920s (Minnich and others 1995). Human population in the Los Angeles Basin significantly increased after World War II, but air pollution levels were not quantified (or reconstructable) until 1963. From 1963 through 1980, peak 1-hour O3 concentrations averaged 250 to 425 ppb (Lee and others 2003). From 1980 on, peak 1- hour O3 concentrations were still high (>250 ppb), but cumulative O3 exposures over the growing season began to decline. Through strong regulatory controls, O3 concentrations declined further to tens of occurrences to only isolated occurrences of hourly concentrations exceeding 95 ppb from the mid-1990s to present. Throughout this time, N deposition continued to accumulate, and drought stress was exacerbated by chronic, if not acute, O3 exposure.
The most extreme drought recorded in 250 years was experienced in the hydrologic year 2002 (10/1/01 through 09/30/02) after 3 years of chronic drought. Results of 3 years of chronic drought (1999-2001) and extreme drought (2002) are shown in a sequence of imagery taken at 5 km above the forest canopy at the most polluted site in the western San Bernardino Mountains (Figure 2). After the chronic drought, bark beetles attacked, and there were clusters of tree mortality. However, the average stand tree mortality rate was near 0 percent. After the chronic and severe drought, tree mortality (primarily ponderosa pine) due to both bark beetle and drought increased to approximately 5 percent at the stand level. In the spring following the wet winter, bark beetle populations reached epidemic proportions, and 42 percent of the stand had died (ponderosa pine, white fir, and sugar pine). The stand was at high risk for an intense fire with high litter layers, high numbers of standing dead trees, and exacerbated drought stress. In autumn of that year, the Old Fire swept through the stand. Interestingly, not all of the red trees (standing dead trees with needles retained) were consumed in a crown fire because the highly dense understory was not in contact with the lower branches of the 100+ year- old-trees—the effects of O3 exposure, N deposition, and drought had promoted lower branch abscission (Miller and others 1996b), so that the lowest branches were attached at 60’ or greater. Trees are still dying from bark beetle at this site (as of 7/06), but the rate of change is now statistically undetectable.
Encyclopedia ID: p3353
