Epiphytic lichen communities on mountain beech, Nothofagus solandri var. cliffortioides in Craigieburn Forest Park, South Island, New Zealand

Abstract

Understanding epiphytic lichen communities is critical to further our knowledge about the structure and functions of forest ecosystems. New Zealand has a uniquely diverse lichen flora yet information is lacking on nearly all aspects of epiphytic communities. The ecology, distribution and relative roles of factors and mechanisms that influence epiphytic lichen communities in mountain beech Nothofagus solandri var. cliffortioides forests of in Craigieburn Forest, New Zealand were investigated. The epiphyte communities on mountain beech trees are dynamic systems that have a mosaic structure. Lichens were the predominant epiphytes in all aged stands (25, 40, 140, 265 yrs), and across entire trees, with a striking biodiversity of 91 species. The lichen community structure was highly variable across aged stands in that lichen species richness decreased with stand age (range 20 -9 species per m²) and also significantly from southern to northern aspects along trunks. Lichens were most abundant in intermediate aged (140 yrs) trees. Successional processes were clear driving factors for observed differences. The abundance of crustose lichens significantly decreased with age stand, foliose lichen abundance was greatest in intermediate aged stands, whereas squamulose lichens, cyanolichens and bryophytes abundance significantly increased with age stand. Fruticose and usnic acid-lichens were present throughout succession and showed consistent patterns across aged stands. These findings suggest that these groups require different habitat conditions provided by particular aged trees. Most epiphytes abundance patterns were significantly affected by interactions of age stand and aspect or age stand and vertical position on the trunk. Measurements of abiotic and biotic environmental variables that may affect community structure showed that humidity, light transmittance and bark morphology (texture & pH) significantly differed between tree ages (25yrs &140yrs) whereas canopy density (average 13% canopy openness) and cool temperatures (average 5.3°C) did not. Younger trees had significantly higher humidity (average 87.2%), more light transmitted (average 19.42%), and smoother and more acidic bark (pH 3.62) compared to older trees. Relative humidity was highly variable throughout a day, between 4-100%. Vertical position on the trunk was the most influential factor for epiphyte distribution. The abundance of few epiphyte groups decreased (cyanolichens), some epiphyte abundance patterns increased (fruticose, usnic acid-lichens), while other groups exhibited a patchy distribution (foliose, crustose and sooty molds) up the tree trunk. Specific epiphyte groups abundance patterns were also significantly related to transmitted light (crustose, fruticose, cyanolichens), canopy openness (cyanolichens, usnic acid- lichens, atranorin-lichens), tree height (crustose, cyanolichens), bark texture and bark pH .The abundance of crustose, fruticose, usnic acid-lichens and sooty molds were the exceptions and not related to bark pH. Epiphyte distribution patterns demonstrate that particular groups have adaptations for ecological niches at different vertical regions. The examination of entire trees revealed clear differences in community structure between trunks and branches. Cyanolichens, bryophytes and sooty molds abundance was higher on trunk areas with intermediate bark whereas fruticose and usnic acid-lichens abundance was higher on branches and smoother bark. Lichen richness and abundance was highest at 8 - 12m trunk height and greater on secondary branches and twigs. All lichens, especially species with usnic acid significantly increased up the tree and only cyanolichen abundance decreased. This pattern demonstrates photobiont versatility of lichens within trees and suggests adaptation to environmental conditions by secondary compounds. Epiphytic lichen communities were clearly changed by disturbance caused by the removal of selected species. Removal experiments at 1.5m on 140year-old trunks using species with distinct abundance patterns and different expansion patterns, Menegazzia pertransita and Pseudocyphellaria colensoi showed that they are able to successfully re-establish after disturbance within the same tree suggesting that these species are not limited by dispersal at this scale. Some epiphyte species (sooty molds, liverworts, P. colensoi, M. pertransita, P. glaba and Usnea spp.) showed strong competitive abilities through rapid expansion into neighbouring removal areas measured as high growth rates, and an ability to overgrow existing species. These results clearly highlight that multiple processes such as dispersal, expansion abilities, competition and facilitation are important mechanisms in structuring the lichen communities. Biomass of the cyanolichens P. colensoi and P. faveolata significantly increased with stand age (0.43-5.9 kgha⁻¹) emphasizing their potential contribution to the lichen community in older forests. Transplant experiments were used to examine annual growth and possible limiting factors of species distribution. P. colensoi and P. faveolata showed mean annual growth rates between 8%-13% independent of aged stands or vertical positions on the trunk. The exceptions were transplants on 140yr old trees where growth was significantly higher on lower positions of the trunk. P. colensoi showed higher annual growth when the thallus had some level stress or mortality which was indicated by discoloration of the thallus, while P. faveolata showed greater growth at intermediate levels of herbivory when the cortex of the thallus was eaten away. Although P. colensoi and P. faveolata thalli can survive and grow across a broad range of conditions and habitats both species are limited in that their propagules require intermediate bark, that is less acidic (pH 4.5), high humidity (mean >65%RH), and low light conditions [transmittance (mean 17.33%) and canopy openness (mean 12.67%)], that favour the growth of cyanobacteria. Mountain beech trees in Craigieburn comprise diverse and structurally complex assemblages of lichens that are shaped by numerous factors and processes. This research has increased our knowledge of lichen communities, and the relationship of the component species (ranging from morphological character, photobionts and secondary compounds) with abiotic and biotic variables at various spatial scales. Further research is required to make better decisions about management and restoration of these forest communities. This is especially true as these forest ecosystems are threatened with increasing risk from climate change, loss of habitat due to modified landscapes and threats from exotic invasive species.