Four Years of Epiphyte Colonization in Douglas-fir Forest Canopies

In 1995, we installed surface-sterilized, rough-barked and smooth-barked tree branches in clearcuts, young forests, and old growth. Half of the experimental branches were inoculated with propagules of the epiphytic cyanolichen, Lobaria oregana. In 1997, we concluded that L. oregana was associated with old-growth Douglas-fir forests because of dispersal limitation; addition of L. oregana propagules resulted in a marked increase in establishment rates. In 1999, we revisited the experiment to determine whether other epiphytes had colonized the tree branches. We also checked to see if the 1997 results of the experiment persisted. A total of 26 epiphyte genera (nine bryophytes and 17 lichens) had colonized the branches. Lichen colonization was generally fastest in clearcuts. Colonization by alectorioid lichens was rapid in both clearcuts and old growth but slow in young forests. In contrast, bryophyte colonization was relatively rapid in all age classes. Epiphyte colonization was generally more rapid on smooth bark than on rough bark, although Cladonia was more frequent on rough-barked branches. Bryophytes, cyanolichens, and Sphaerophorus globosus were more frequent on inoculated branches than on control branches, implying that the L. oregana propagule mixture used in 1995 was contaminated with other epiphytes. Like L. oregana, these species may also be dispersal-limited. The number of established L. oregana thalli in clearcuts and young stands decreased from 1997 to 1999, but the number of thalli remained relatively stable in old growth. After four years, established L. oregana thalli were larger in clearcuts than in either young stands or old growth. Overall, the fourth-year results of our experiment confirm the importance of dispersal limitation as the cause of old-growth association in L. oregana in western Oregon. The importance of propagule availability to plant community development has been known for many years (Gleason 1926). However, it was difficult to believe that dispersal limitations could result in the huge epiphytic differences between old growth and younger forests in the Pacific Northwest. The much higher epiphyte biomass and diversity of old growth has usually been attributed to microclimatic and structural differences between old and young forests (e.g., Franklin et al. 1981; Lesica et al. 1991). In 1995, we began experiments to contrast the relative importance of propagule supply versus habitat in controlling the development of Lobaria oregana (Tuck.) Müll. Arg. populations. We chose this cyanolichen for its massive accumulation and renown as the dominant epiphytic lichen in oldgrowth Douglas-fir forests of western Oregon (McCune 1993; Pike et al. 1975; Sillett 1995). In 1997, we found that L. oregana can establish and grow in a wide range of habitats, including clearcuts and young forests (Sillett et al. 2000). Dispersal limitations result in very slow development of L. oregana populations after clearcutting in this region. Are other epiphytes also dispersal-limited? The diversity of reproductive strategies in lichens led us to believe that many, if not most, species would be more successful than L. oregana in long distance dispersal. Sorediate species and those producing abundant ascospores should be able to colonize new substrates more easily than L. oregana with its relatively large, heavy lobules. Experimental branches, which were initially devoid of epiphytes, were placed in a variety of habitats for long-term study. This allowed us to measure natural rates of colonization in these habitats. During re-examination of our experimental branches in 1999, we also checked to see if the 1997 results of the L. oregana dispersal experiment persisted for a longer period of time. 662 [VOL. 103 THE BRYOLOGIST