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

<div><p>Lodgepole pine (<i>Pinus contorta</i>) forests are widely distributed throughout North America and are subject to mountain pine beetle (<i>Dendroctonus ponderosae</i>) epidemics, which have caused mortality over millions of hectares of mature trees in recent decades. Mountain pine beetle is known to influence stand structure, and has the ability to impact many forest processes. Dwarf mistletoe (<i>Arceuthobium americanum</i>) also influences stand structure and occurs frequently in post-mountain pine beetle epidemic lodgepole pine forests. Few studies have incorporated both disturbances simultaneously although they co-occur frequently on the landscape. The aim of this study is to investigate the stand structure of lodgepole pine forests 21–28 years after a mountain pine beetle epidemic with varying levels of dwarf mistletoe infection in the Deschutes National Forest in central Oregon. We compared stand density, stand basal area, canopy volume, proportion of the stand in dominant/codominant, intermediate, and suppressed cohorts, average height and average diameter of each cohort, across the range of dwarf mistletoe ratings to address differences in stand structure. We found strong evidence of a decrease in canopy volume, suppressed cohort height, and dominant/codominant cohort diameter with increasing stand-level dwarf mistletoe rating. There was strong evidence that as dwarf mistletoe rating increases, proportion of the stand in the dominant/codominant cohort decreases while proportion of the stand in the suppressed cohort increases. Structural differences associated with variable dwarf mistletoe severity create heterogeneity in this forest type and may have a significant influence on stand productivity and the resistance and resilience of these stands to future biotic and abiotic disturbances. Our findings show that it is imperative to incorporate dwarf mistletoe when studying stand productivity and ecosystem recovery processes in lodgepole pine forests because of its potential to influence stand structure.</p></div

Linear mixed models of log cohort diameter on BIC preferred explanatory variables.

<p>Scatterplots of backtransformed linear mixed models of A) natural logarithm of cohort diameter of dominant lodgepole pine (cm) on dwarf mistletoe rating (DMR) with stand density fixed at its mean, B) natural logarithm of cohort diameter of intermediate lodgepole pine (cm) on stand density and C) natural logarithm of cohort diameter of suppressed lodgepole pine on DMR with 95% confidence intervals.</p

Generalized linear mixed models of proportion of lodgepole pine in cohorts on BIC preferred explanatory variables.

<p>Scatterplots of generalized linear mixed models of A) proportion of lodgepole pine in the dominant cohort on dwarf mistletoe rating (DMR), B) proportion of lodgepole pine in the intermediate cohort on stand density and C) proportion of lodgepole pine in the suppressed cohort on DMR with 95% confidence intervals.</p

Linear mixed models of stand attributes on dwarf mistletoe rating.

<p>Scatterplots of linear mixed models of A) stand density (stems/ha), B) stand basal area (m²/ha), and C) backtransformed natural logarithm of canopy volume (m³) on dwarf mistletoe rating (DMR) with 95% confidence intervals.</p

Study area map.

<p>Deschutes National Forest, Oregon boundary with sampled stands and lodgepole pine plant associations.</p

Forest structures of lodgepole pine stands 21–28 years after a mountain pine beetle epidemic.

<p>Pictured are stands A) without dwarf mistletoe and B) with severe dwarf mistletoe and high levels of witches’ brooming.</p

Characteristics of 39 lodgepole pine plots within 13 stands in the Deschutes National Forest, Oregon.

<p>Note: Data are calculated from measures of trees DBH >/ = 5.0 cm. DMR = stand-level dwarf mistletoe rating; MPB = mountain pine beetle. Productivity class was determined by previously developed plant associations for the area. MPB Mortality class was determined using Aerial Detection Survey cumulative mortality data where L = 5–15 trees/acre, M = 15–25 trees/acre, H = 25–36 trees/acre killed by mountain pine beetle over the time period of the epidemic.</p><p>Characteristics of 39 lodgepole pine plots within 13 stands in the Deschutes National Forest, Oregon.</p