The effects of dwarf mistletoe, witches' brooms, stand structure, and site characteristics on the crown architecture of lodgepole pine in Oregon

Abstract

We investigated the importance of lodgepole pine dwarf mistletoe (Arceuthobium americanum Nutt.) in determining the height to crown top (HCT), height to crown base (HCB), and live crown ratio (LCR) of 2025 lodgepole pine (Pinus contorta var. murrayana (Grev. & Balf.) Engelm.) growing over a 24-km2 study site in central Oregon. We compared the effects of infection and associated witches' brooms with those of site topography, soil type, shrub cover, stand density, and the abundance of mature ponderosa pine (Pinus ponderosa Dougl. ex P. Laws. & C. Laws). using multiple regression and path analysis. The density of dominant-size P. contorta was consistently the most important factor influencing HCT, HCB, and LCR across the study site. In dense stands, trees tended to have elevated crown bases due to self-pruning and, hence, lower values of LCR. Dwarf mistletoe and related witches' brooms uniquely explained 6.9% of the variance in LCR, which was close to that of dominant P. contorta (7.1%) and more than that of soil type (3.0%), but explained only 2.6% of the variance in HCB, which was less than that of dominant P. contorta (6.5%) and soil type (4.6%). Regression models suggest that heavily infected trees should be 18% shorter and have crown bases 37% lower than uninfected trees, while moderately infected trees should have an LCR over 20% larger than that of uninfected and heavily infected trees. We also found that the largest 25 heavily infected trees sampled were approximately 19% shorter and 11–13% smaller in diameter than the largest 25 uninfected trees. The results suggest that dwarf mistletoe can be an important factor in determining the crown dimensions of P. contorta but that these effects may be interpreted only in the context of site characteristics and stand structure.

The differential effects of dwarf mistletoe infection and broom abundance on the radial growth of managed ponderosa pine

“…Radial growth of moderately infected trees declines in the presence of light brooming, while heavily broomed trees show a growth decline only among severely infected individuals. These results suggest stressed trees are less likely to support broom growth, which is consistent with earlier observations that broom abundance is negatively correlated with stand density (Godfree et al, 2002). Severely infected trees may be unable to spare the energy to generate brooms, and cases of light brooming under heavy infection may be the result of this energy trade-off.…”

Effects of Dwarf Mistletoe on Stand Structure of Lodgepole Pine Forests 21-28 Years Post-Mountain Pine Beetle Epidemic in Central Oregon

“…In addition to growth loss, moderate to severe infection by A. americanum often induces host deformities by forming dense branch masses called witches brooms (Figure 1), which concentrate biomass in infected branches and act as nutrient sinks [32]. Although not all dwarf mistletoe species induce witches brooms as a symptom of infection, the individual structure of lodgepole pine crowns infected with A. americanum is significantly impacted via this mechanism, leading to shorter live crowns, which are skewed to the lower strata of the canopy [33]. Furthermore, decreases in average lodgepole pine diameter and height, particularly within larger size classes, have been observed with increased dwarf mistletoe infection levels [34], [35], [36], [37].…”

Anther and pollen development in the lodgepole pine dwarf mistletoe (Arceuthobium americanum) staminate flower

“…latifolia Engelm. ex S. Watson) in western North America (Godfree et al 2002). The plant is a small yellowish to olive-green herbaceous angiosperm consisting of aerial shoots with whorled branching (Hawksworth and Wiens 1996) connected to a network of cortical strands and radial sinkers that penetrate the coniferous host (Wilson and Calvin 1996).…”