Canopy light and the distribution of hemlock dwarf mistletoe (Arceuthobiumtsugense [Rosendahl] G.N. Jones subsp tsugense) aerial shoots in an old-growth Douglas-fir/western hemlock forest

Hemispherical photography was used to quantify the relationship between can opy light and the distribution of hemlock dwarf mistletoe (Arceuthobium tsu gense (Rosendahl) G.N. Jones subsp. tsugense) aerial shoots in an old-growt h Douglas-fir/western hemlock forest to determine if aerial shoots only occ ur in higher light environments in the upper canopy. The Wind River Canopy Crane provided three-dimensional access by lowering a gondola into gaps bet ween 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 branch es were sampled on 14 trees from 18 to 60 m. Forty-one infections had no ae rial shoots whereas 48 had aerial shoots. All infections above 50 m had sho ots, while all infections below 30 m (except one) had none. There were no a erial shoots at infections exposed to estimated insolation (yearly insolati on = diffuse light * indirect site factor + direct light * direct site fact or) of 1,000 MJ m(-2)yr(-1), while all infections above 3,200 MJ m(-2)yr(-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 2,200 MJ m(-2)yr(-1) . A complex of biotic and abiotic factors may account for the correlation o f high light and aerial shoot occurrence in the field because laboratory st udies have shown dwarf mistletoe produces the most aerial shoots in low lig ht and high temperature. In this tall, multi-layered canopy, the source of the seed rain from western hemlock dwarf mistletoe was above the bulk of th e western hemlock foliage, perhaps another explanation for the fast spread and intensification of mistletoe in old-growth forests.