Patterns and mechanisms of epiphytic macrolichen and abundance, diversity and composition in boreal forest

Abstract

Epiphytes are important components of the forest ecosystem, but the mechanisms that control epiphyte diversity are not clear. Epiphytes are sensitive to disturbance, but their responses to stand-replacing fire are poorly understood. Furthermore, despite increasing rates of logging in boreal forests, there is lack of understanding of the potential effects of logging relative to wildfire on epiphytic lichen abundance, diversity, and composition. The focus of this thesis was to: (1) identify the mechanisms that regulate epiphytic species diversity; (2) examine the responses of epiphytic macrolichen abundance, diversity, and composition to stand-replacing fire and multiple successional pathways of the canopy tree layer; (3) examine the temporal dynamics of epiphytic macrolichen cover, richness, and composition as affected by disturbance origin, time since disturbance, and forest type; and (4) examine the influence of time since disturbance and host tree species on the species dynamics of epiphytic macrolichens in the central boreal forest of Canada. A comprehensive literature review was conducted to explore possible mechanisms that regulate epiphytic species diversity and to develop a mechanistic framework to guide investigations of epiphyte assemblages. Six putative mechanisms of epiphyte species diversity were identified, and the extent to which the mechanisms interact was evaluated in a conceptual model. The mechanisms include constrained dispersal, slow growth rate, substrate availability, host tree mortality, disturbance, and global climate change. They are identified as inherent, local- and stand-level, and landscape-level mechanisms. The mechanisms are interrelated and the linkages between them were elaborated. Future studies should test these mechanisms over broad spatial and temporal scales. The effects of time since wildfire and overstory composition on the dynamics of epiphytic lichens were examined in a retrospective study of 51 stands of conifer, mixed- wood, and broadleaf overstory boreal forest stands ranging from 7 to 209 years since fire. Total lichen cover continuously increased with stand age for all overstory types, and mixed-wood and conifer stands had higher total lichen cover than broadleaf stands in all age classes except similarly low cover in stands ¡Ü 15 years old for all overstory types. Lichen species richness reached peaks in 98- or 146-year-old stands, and mixed-wood stands had higher lichen richness than broadleaf and conifer stands at 98 years old, but not at other age classes. Multivariate analysis indicated that lichen communities were compositionally distinct for all age classes and overstory types. The results demonstrate that epiphytic lichen communities show continued changes with time since disturbance that may span decades to centuries. Also, epiphytic lichens show a strong association with overstory composition with higher diversity in mixed-woods than conifer and broadleaf stands at canopy transition stage. The temporal dynamics of epiphytic macrolichen cover, richness, and composition as affected by disturbance origin, time since disturbance, and forest type was compared for logging vs. wildfire over 33-year chronosequences in Ontario, Canada. Epiphytic macrolichens had not recovered 7 years after fire or logging, but their cover and richness thereafter increased with stand age for all overstory types. Within the same age and overstory, post-logged stands had lower or similar macrolichen cover and richness than post-fire stands, except for 33-year-old mixed-wood stands. More pronounced was the compositional difference of epiphytic lichens among overstory types, stand ages, and disturbance origins. The results demonstrate that epiphytic macrolichen abundance and diversity increase with stand age and lichen species composition is strongly influenced by overstory tree species composition. Furthermore, logging produces epiphytic macrolichen communities different from fire. The different effects of logging vs. fire could be a result of different regeneration density, genetic diversity of trees, and soil nutrient availability and stoichiometry. Epiphytic macrolichen species in post-fire successional stands were examined in relation to forest stand age (7 to 209 years since last fire disturbance) and host tree species (jack pine Pinus banksiana, trembling aspen Populus tremuloides, paper birch Betula papyrifera, black spruce Picea mariana, and balsam fir Abies balsamea) in the central boreal forest. Recruitment of epiphytic lichen species after fire increased with time since fire, with new recruits at every stage. The occurrence of individual epiphytic macrolichen species was strongly influenced by time since fire and host tree species. Some lichen species, most of which reproduce asexually, colonized early on in the stand initiation stage whereas others appeared to establish only in the mature and old-growth stands. Frequency of occurrence of some epiphytic lichen species on the host trees increased with time since fire, whereas others decreased. Multivariate analyses indicated significantly different epiphytic lichen species composition on host trees at each stand age. The majority of epiphytic lichen species appeared to be generalist species with occurrence on all tree species, except for a few species that were found exclusively on aspen. In summary, epiphyte species diversity is regulated by multiple, interacting mechanisms that operate at local, stand-level, and landscape level. Epiphytic macrolichen abundance and diversity increase with stand age and lichen species composition is strongly influenced by overstory tree species composition. Epiphytic macrolichen communities in logged stands differ from in post-fire stands. Epiphytic lichens show habitat preferences, but the majority of species are rarely host-specific.