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Survival and Growth of Cyanolichen Transplants in Douglas-Fir Forest Canopies

1998; American Bryological and Lichenological Society; Volume: 101; Issue: 1 Linguagem: Inglês

10.1639/0007-2745(1998)101[20

1938-4378

Stephen C. Sillett, Bruce McCune,

Botany and Plant Ecology Studies

In the Oregon Cascades, epiphytic cyanolichens are abundant in old-growth forest canopies, but they accumulate very slowly in young forests. We evaluated whether epiphytic cyanolichens require old growth and/or thick, underlying moss mats to achieve normal rates of growth and mortality. We transplanted over one thousand mature thalli of two old-growth associated species (Lobaria oregana and Pseudocyphellaria rainierensis) into the crowns of Douglasfir trees in thirteen forest stands representing four age classes: old-growth (400-700 yr), mature (140-150 yr), young (35-40 yr), and recent clearcut. Wooden racks were used instead of trees in the clearcuts. Half of the cyanolichen thalli were transplanted onto thick moss mats and half were transplanted onto bare bark. After one year, both species grew at least as well in younger forests as they did in old growth (20 to 30% increase in mass), but growth rates were significantly lower in clearcuts. Mortality rates were very low (<10%) in young, mature, and old-growth forests, but high (50 to 90%) in clearcuts. Pseudocyphellaria rainierensis grew significantly better on moss than on bare bark (30 vs 23% increase in mass). Since epiphytic cyanolichens can survive and grow in a broad range of forest age classes, silvicultural treatments that facilitate their colonization of young forests have great conservation potential. Epiphyte biomass increases slowly during succession in temperate forests (Esseen et al. 1996; Lesica et al. 1991; McCune 1993; Neitlich 1993; Rose 1992; Selva 1994). Old-growth forests support much higher epiphyte loads than younger, managed forests. In the Douglas-fir forests of Oregon's western Cascades, the most striking epiphytic difference between old-growth and managed forests involves cyanolichens. These nitrogen-fixing lichens dominate the old-growth forest canopy (Pike et al. 1975; Sillett 1995a), but are scarce or absent in younger forests (McCune 1993; Neitlich 1993; Spies 1991). Many epiphyte species, including several endemic cyanolichens, are seldom if ever, found in forests less than a century old (Rosentreter 1995; Sillett & Neitlich 1996). Conservation of old growth-associated epiphytes is becoming an important consideration in the Pacific Northwest, where most old-growth forest habitat has been destroyed by logging (Norse 1990). A recent emphasis on ecosystem management (Swanson & Franklin 1992) has stimulated efforts to promote epiphytes in managed forests. Recovery of epiphytic cyanolichens is particularly important because of their potential contributions to forest productivity. Nitrogen fixed by epiphytic cyanolichens can represent an important nutrient input in humid old-growth forests (Denison 1979; Pike 1978; Sollins et al. 1980). Epiphytes also constitute a significant part of biodiversity. For example, the number of lichens and bryophyte species often equals or exceeds the number of vascular plant species in old-growth forests (e.g., Lesica et al. 1991). The importance of epiphytic lichens has been recognized under a comprehensive new ecosystem management plan (FEMAT); populations of many oldgrowth associated species must now be monitored and protected (Rosentreter 1995). Reasons for the slow accumulation of epiphytic lichens in forests are poorly understood. Unsuitable microclimates in younger forests is one of several possible explanations (Sillett & Neitlich 1996). Microclimatic differences between old-growth and younger forests presumably arise from structural differences such as the frequency and size of gaps, abundance of standing dead material, and complexity of canopy architecture. Unlike more open, structurally complex old-growth forests, much of the canopy in dense, young, even-aged Douglas-fir forests, is shaded and sheltered from direct sunlight and precipitation (Kuiper 1988; Spies & Franklin 1991; Van Pelt & North 1996). Furthermore, bryophytes develop in thick mats on branches in large Douglas-fir trees, and many epiphytic cyanolichens are closely associated with these mats (Sillett 1995a). Perhaps these water storing mats ameliorate temperature and moisture fluctuations of epiphytes in their immediate vicinity, making these microhabitats more suitable to desiccation sensitive species (Lawrey 1991). 0007-2745/98/20-31$1.35/0 This content downloaded from 207.46.13.119 on Wed, 30 Mar 2016 05:39:46 UTC All use subject to http://about.jstor.org/terms 1998] SILLETT & McCUNE: CYANOLICHEN TRANSPLANTS 21 TABLE 1. Characteristics of study sites in the Willamette National Forest in western Oregon. Trunk diameter (dbh) and height are listed for the two Douglas-fir trees climbed in each stand. Stand number Age class Elevation (m) Estimated age Tree dbh (m) Tree height (m) 1 old growth 610 700 2.77, 2.08 68.0, 66.0 2 old growth 830 560 1.84, 1.65 82.5, 78.0 3 old growth 490 450 1.56, 1.41 74.0, 66.5 4 old growth 590 400 1.62, 1.31 71.0, 67.5 5 mature 890 150 0.87, 0.76 48.5, 49.5 6 mature 560 140 1.30, 1.26 59.5, 62.5 7 mature 650 140 1.02, 0.93 54.5, 48.0 8 young 780 40 0.40, 0.38 25.0, 23.5 9 young 760 37 0.46, 0.38 25.5, 20.0 10 young 500 36 0.33, 0.30 25.0, 26.0 11 clearcut 670 12 clearcut 460 13 clearcut 790 This study evaluates one explanation for the slow development of this critical component of Douglasfir forest--that epiphytic cyanolichens require the environment created by the structure of old-growth forest canopies. We also consider the possible effects of moss mats on cyanolichen growth and mortality in the forest canopy. Two cyanolichen species