Canopy light and the distribution of hemlock dwarf mistletoe (Arceuthobium tsugense (Rosendahl) G.N. Jones subsp. tsugense) aerial shoots in an old-growth Douglas-fir/western hemlock forest
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Hemispherical photography was used to quantify the relationship between canopy light and the distribution of hemlock dwarf mistletoe (Arceuthobium tsugense subsp. tsugense) aerial shoots in an old-growth Douglas fir/western hemlock (Pseudotsuga menziesii/Tsuga heterophylla) forest to determine if aerial shoots only occur in higher light environments in the upper canopy. The Wind River Canopy Crane provided 3-dimensional access by lowering a gondola into gaps between trees and stopping at 5 m intervals and sampling all trees around the gap at that height. A total of 89 dwarf mistletoe infections in live branches were sampled on 14 trees from 18 to 60 m height. Forty-one infections had no aerial shoots whereas 48 had aerial shoots. All infections above 50 m had shoots, while all infections below 30 m (except one) had none. There were no aerial shoots at infections exposed to estimated insolation (calculated as the sum of indirect and direct site factors weighted for diffuse and direct shortwave insolation) of 1000 MJ m-2yr-1, while all infections exposed to insolation of above 3200 MJ m-2yr-1 had aerial shoots. Height and light were highly correlated, but between 30 and 50 m the light environment became especially heterogeneous, with a 50% probability of aerial shoots occurring at 40 m, or at 2200 MJ m-2yr-1. A complex of biotic and abiotic factors may account for the correlation of high light and aerial shoot occurrence in the field, because laboratory studies have shown that dwarf mistletoe produces the most aerial shoots in low light and at high temperature. In this tall, multi-layered canopy, the source of the seed rain from western hemlock dwarf mistletoe was above the bulk of the western hemlock foliage, perhaps another explanation for the fast spread and intensification of mistletoe in old-growth forests.