Spatial and population characteristics of dwarf mistletoe infected trees in an old-growth Douglas-fir – western hemlock forest

Abstract

We investigated the distribution and severity of trees infected with western hemlock dwarf mistletoe (Arceuthobium tsugense (Rosendahl) G.N. Jones subsp. tsugense) in an old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) -western hemlock (Tsuga heterophylla (Raf.) Sarg.) forest. With the use of Hawksworth six-class dwarf mistletoe rating system, infection status was assessed for 3516 hemlock and true firs ≥5 cm diameter on a 12-ha stem-mapped plot located in the Cascade Mountains of southwest Washington State. Within the plot, 33% of the area had some level of infection and 25% (719) of western hemlocks, 2.2% (12) of Pacific silver fir (Abies amabilis (Dougl.) Forbes), and 29% (2) of noble fir (Abies procera Rehd.) trees were infected. Infected trees are larger than uninfected trees, on average, and within the infected tree population, the severely infected trees averaged larger than lightly infected trees. Abundant dwarf mistletoe in larger trees definitely positions the dwarf mistletoe population for future spread. Ripley's K analysis indicates a negative association between infected and uninfected hemlock trees, confirming that the infected trees form distinct dwarf mistletoe infection centers. The infection centers are actively spreading at their margins, which was confirmed by nearest neighbor analysis. Heavily infected trees had a negative association with uninfected trees, while lightly infected trees had a positive association with uninfected trees.Résumé : Les auteurs ont étudié la distribution des arbres infectés par le faux-gui de la pruche (Arceuthobium tsugense (Rosendahl) G.N. Jones subsp. tsugense) et la sévérité de la maladie dans une forêt ancienne de Douglas (Pseudotsuga menziesii (Mirb.) Franco) et de pruche de l'Ouest (Tsuga heterophylla (Raf.) Sarg.). Le degré d'infection, basé sur le système d'évaluation à six classes de Hawksworth, a été évalué pour 3516 pruches et espèces du genre Abies ≥5 cm de diamètre dans une placette cartographiée de 12 ha située dans la chaîne des Cascades, au sud-ouest de l'État de Washington. À l'intérieur de la parcelle, 33 % de la superficie était infectée et 25 % (719) des pruches de l'Ouest, 2,2 % (12) des sapins gracieux (Abies amabilis (Dougl.) Forbes) et 29 % (2) des sapins nobles (Abies procera Rehd.) étaient infectés. Les arbres infectés étaient en moyenne plus gros que les arbres non infectés et parmi les arbres infectés, les arbres sévèrement infectés étaient plus gros que les arbres légèrement infectés. L'abondance de faux-gui dans les plus gros arbres favorisera la dispersion future de la population de faux-gui. L'analyse du K de Ripley indique qu'il y a une relation négative entre les pruches infectées et non infectées confirmant que les arbres infectés constituent des centres d'infection distincts du faux-gui. Les centres d'infection s'étendent activement à la marge tel que confirmé par l'analyse du plus proche voisin. Les arbres sévèrement infectés avaient une relation négative avec les arbres non infectés alors que les arbres légèrement ...

Growth of hartweg’s pine (<em>Pinus hartwegii</em>) parasitized by two dwarf mistletoe species (<em>Arceuthobium</em> spp.)

“…With respect to crown spread size susceptibility, some patterns agree with what was expected: most of the uninfected trees were the smallest ones and MDMR increased with tree size (Shaw et al, 2005). Young trees are rarely parasitized because they cannot provide enough resources to the parasite and the consequences for them are mostly lethal (Press, 1995); however, a larger tree represents a better resource because it has a larger surface for seed reception (Arriaga et al, 1988;Hernández-Benítez et al, 2005) and can bear a larger load of parasites declining in health but not in mortality (Musselman and Press, 1995;Hawksworth and Wiens, 1996).…”

“…The stand structure is strongly associated with dwarf mistletoe dynamics, where it has been shown that the crown volume and size of the dominant cohort decreases with infection intensity (Shaw et al, 2005;Agne et al, 2014). These hemiparasitic plants form a complex rootlike structure, called haustorium, which is the organ of contact with the hosts' xylem and phloem .…”

Mistletoes: Pathology, Systematics, Ecology, and Management

“…Most notable, and well studied, in this regard are the dwarf mistletoes in conifers of western North America (198). Severe dwarf mistletoe infestations alter forest stand structure by creating gaps due to mistletoe-associated mortality and branch and tree-top dieback (92), creating patchiness in the stand due to the clumped nature of dwarf mistletoe infection centers (197), and reorganizing vertical canopy structure so that more foliage is concentrated in the lower canopy (76,77). Succession is influenced by mortality of host species and the interaction of dwarf mistletoes, fire, and fire suppression (92,197,203).…”

Interactions between hemiparasitic plants and their hosts

“…46 Dwarf-mistletoes (genus Arceuthobium) that are considered to be dependent on host-derived organic carbon (based on gasometric measurements; 47,48 no detailed carbon budget studies are available) tend to perform better on vigorous host that are in a good physiological state. 48,49 Nonetheless, even these highly heterotrophic species were demonstrated to produce more aerial shoots under high irradiance which is however not likely to be underpinned by a causal dependence of parasite performance on the light level. It is more probable that the dwarf-mistletoe is indirectly positively affected by increased photosynthetic capacity of highly irradiated branch or by elevated temperature.…”