Relationships between Arceuthobium americanum and the structure of Pinus contorta var. murrayana stands in central Oregon

We examined the importance of the dwarf mistletoe Arceuthobiumamericanum in determining the size-class structure ofPinuscontorta var. murrayana forests in the centralOregon pumice zone. We considered the interrelationships among A.americanum abundance, P. contorta populationstructure, soil type, site topography, the density of competing tree and shrubspecies, and understory woody litter. We found that A.americanum was associated with reduced live basal area(P < 0.001) and increased bole diameter variability(P < 0.001) of P. contorta, andalso with an increase in the density of saplings and seedlings(P < 0.001) at the expense of dominant andtall-intermediate trees. Saplings and seedlings were over 15 times as dense inheavily infested stands as in uninfested stands. Reflecting this shift towardsmaller size classes, each unit increase in stand-level dwarf mistletoe rating(DMR) reduced live basal area by 1.32 m2/ha and reducedthe density of dominant-sized trees by 45 trees/ha. The significant reductionsin live basal area and the density of dominant trees contrast with otherstudiesconducted in P. contorta stands containing A.americanum. However, DMR uniquely explained less than 25% of thetotal site-level variance in all size class variables except sapling density(36%), and varied in importance relative to interspecific competition betweenPinus ponderosa and P. contorta, theeffects of abiotic factors, and intercohort dynamics in determining thestructureof P. contorta stands across the study site. DMR uniquelyexplained less of the variance in live basal area (12%) than abiotic factors(28%) and P. ponderosa density (16%), and only 15% ofvariability in the density of the dominant P. contortasizeclass. Furthermore, the relative effects of DMR on specific P.contorta size classes varied across patch- and stand-level spatialscales. We conclude that the relative importance of the effects of A.americanum on specific P. contorta size classesdepends on spatial scale, and should be interpreted in relation to patchdynamics, site-level topoedaphic variability, and interspecific competitionamong the dominant tree species present.