Dwarf mistletoes (Arceuthobium species) are arboreal, hemiparasitic plants of conifers that can change the structure and function of the tree crown. Hemlock dwarf mistletoe (Arceuthobium tsugense subsp. tsugense) principally parasitizes western hemlock (Tsuga heterophylla) and effects 10.8% of all western hemlock trees in Oregon, USA. In this study, we climbed 16 western hemlock trees (age 97–321 years, height 33–54.7 m) across a gradient of infection (0%–100% of branches infected) and measured occurrence of all dwarf mistletoe infections, dwarf mistletoe caused deformities, foliage, branch and crown metrics, and sapwood area. We then modelled over 25 different response variables using linear and generalized linear models with three metrics of severity as explanatory variables: total infection incidence, proportion of all live branches infected, and proportion of all live, infected branches with 33 per cent or more foliage distal to infection. A strong effect of dwarf mistletoe intensification was the reduction of branch foliage and an increase in the proportional amount of foliage distal to infections, with severely infected trees having the majority of foliage distal to infections. Increasing severity led to an apparent crown compaction as crown volumes decreased and became increasingly comprised of deformities. Sapwood area was unrelated to infection severity. Branch length and diameters were unrelated to increasing infection severity despite severely infected branches supporting 1–70 infections. The most severely infected tree had 3,615 individual plants in the crown. Our results suggested that shifts in crown structure and branch deformation, foliage amount, and foliage distal to infection, reflected a likely reduction of capacity for tree growth that coincided with a hypothesized increase in resource demand by dwarf mistletoe plants as infection severity intensified.

中文翻译：

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矮槲寄生（（虫，粘菌科）对西铁杉（Tsuga heterophylla）树冠的转化

矮小槲寄生（Arceuthobium种）是针叶树的树状半寄生植物，可以改变树冠的结构和功能。铁杉矮槲寄生（油杉寄生属tsugense亚种。tsugense）主要寄生于西部铁杉（铁杉太子参），并影响美国俄勒冈州所有西部铁杉的10.8％。在这项研究中，我们攀登了16个西部铁杉树（年龄97-321岁，身高33-54.7 m），跨感染梯度（被感染树枝的0％–100％），并测量了所有矮小的槲寄生感染的发生，畸形，枝叶，枝冠度和边材面积。然后，我们使用线性和广义线性模型对25种不同的响应变量进行建模，并使用三个严重性指标作为解释变量：总感染发生率，所有被感染活枝的比例以及所有被感染活枝的比例（远端叶的百分比为33％或更多）感染。矮槲寄生集约化的强大作用是减少枝叶并增加感染远端叶的比例，带有严重感染的树木，其中大部分叶子远离感染。随着牙冠体积减小并越来越多地由畸形组成，严重程度的提高导致牙冠的表观压实。边材面积与感染的严重程度无关。尽管严重感染的分支支持1–70次感染，但分支的长度和直径与感染的严重程度无关。感染最严重的树冠上有3,615株植物。我们的研究结果表明，冠状结构的变化和枝条变形，枝叶数量以及感染远端的枝叶的变化反映了树木生长能力的降低，这与矮小槲寄生植物随着感染严重程度的提高而推测的资源需求增加相吻合。随着牙冠体积减小并越来越多地由畸形组成，严重程度的提高导致牙冠的表观压实。边材面积与感染的严重程度无关。尽管严重感染的分支支持1–70次感染，但分支的长度和直径与感染的严重程度无关。感染最严重的树冠上有3,615株植物。我们的研究结果表明，冠状结构的变化和枝条变形，枝叶数量以及感染远端的枝叶的变化反映了树木生长能力的降低，这与矮小槲寄生植物随着感染严重程度的提高而推测的资源需求增加相吻合。随着牙冠体积减小并越来越多地由畸形组成，严重程度的提高导致牙冠的表观压实。边材面积与感染的严重程度无关。尽管严重感染的分支支持1–70次感染，但分支的长度和直径与感染的严重程度无关。感染最严重的树冠上有3,615株植物。我们的研究结果表明，冠状结构的变化和枝条变形，枝叶数量以及感染远端的枝叶的变化反映了树木生长能力的降低，这与矮小槲寄生植物随着感染严重程度的提高而推测的资源需求增加相吻合。边材面积与感染的严重程度无关。尽管严重感染的分支支持1–70次感染，但分支的长度和直径与感染的严重程度无关。感染最严重的树冠上有3,615株植物。我们的研究结果表明，冠状结构的变化和枝条变形，枝叶数量以及感染远端的枝叶的变化反映了树木生长能力的降低，这与矮小槲寄生植物随着感染严重程度的提高而推测的资源需求增加相吻合。边材面积与感染的严重程度无关。分支的长度和直径与感染的严重程度无关，尽管严重感染的分支支持1–70次感染。感染最严重的树冠上有3,615株植物。我们的研究结果表明，冠状结构的变化和枝条变形，枝叶数量以及感染远端的枝叶的变化反映了树木生长能力的降低，这与矮小槲寄生植物随着感染严重程度的提高而推测的资源需求增加相吻合。