The canopy cover of the Pinus contorta forests of west-central Alberta, dictates colonization of the forest floor ground layer. This dynamic ground layer is a mosaic of feather mosses and reindeer lichens in a system driven by disturbance. In this project, anthropogenic was used to control canopy cover change and study its effects on the ground layer. Timber companies selectively mechanically thinned sections producing three experimental areas with uncut controls. Data were collected from 182, 6.5 m2 plots located in the four thinning areas. Six general areas of inquiry were posed around determining ground layer responses to canopy opening: 1) vegetation, 2) plant species richness, 3) plant abundance, 4) diaspore availability, 5) environmental limiting factors, 6) moss and lichen establishment. In 1997, three timber companies were involved in selective tree removal at three different stand percentages (20/40/60%), however, these were not consistent when measured in 2005. This variation in operational logging along with changes over the seven year time period, resulted in strong disparity for each of the thinning regimes. Percent canopy cover change for all thinned plots was ranked and three new groups created; least, moderate, most canopy change. These new groups formed the basis for the data presented in Chapter 3. The three thinning groups had little to no effect on species richness, but overall showed a small decrease from measurements taken pre-harvest. Numbers of locally rare species were similar to pre-harvest levels, but there were some gains and losses of species between pre-harvest and seven years post-harvest. In contrast to the lack of change in diversity, the abundance of dominant species and major vegetation components underwent dramatic changes. Dominant species of both vascular plants and bryophytes decreased with increasing canopy openness, with vascular plants being gradually affected while mosses were more affected at less intense canopy opening. Abundances of lichens showed no change. When measuring limiting factors (biotic and abiotic) for ground layer mosses and lichens, diaspores (spores and fragments) were plentiful in all stands, but differed in abundance at the micro-scale. The position of feather mosses and reindeer lichen in the forest floor mosaic appears to be due to an intermingling of environmental influences (at both the meso- and micro-scale). With less environmental constraints on lichens and the widespread availability of lichen fragments, lichens are more tolerant to the conditions evoked by thinning than are mosses. Mosses are more restricted by environmental conditions and have more constrained diaspore dispersal than lichens. Thus, mosses are more limited both by diaspore dispersal and by harsh environmental conditions in open canopy habitats. Whereas relative humidity (RH) did not differ at the stand level, moss dominated areas had higher RH no matter where they occurred, lichen-dominated areas did not--suggesting the moss occurrence is at least partially controlled by micro-scale level factors. Moss establishment is effected by the "ghosts" of past events and substrates. Mosses are widespread in formerly moss-dominated areas that contain organic substrates and high canopy cover. Lichen establishment is limited in previously moss-dominated areas. Species interactions weighed more heavily on moss establishment than on that of lichens. Therefore, the effects of canopy change on the ground layer are variable corresponding to moss decreases, but not lichens. Seven years post-harvest species diversity is unchanged, but vegetation, as a whole, has been affected.

Abstract Previous reports on the effects of enhanced UV-B radiation on bryophytes have been equivocal. This study shows that mosses not only respond to enhanced UV-B, but they are affected by changes in ambient radiation. The studies were conducted with two model species common in northern environments; red-stemmed feather moss (Pleurozium schreberi) and juniper haircap moss (Polytrichum juniperinum). Both species showed high concentrations of methanol-extractable UV-absorbing compounds (UACs) with high spring-time and early-summer UV, whereas in P. juniperinum, the concentration was affected by early-summer drought. The UACs of P. juniperinum increased again towards autumn suggesting a role in winter hardening. The (spring-time) cell wall-bound UV screen was important to both species. The fundamental adaptation of P. juniperinum to open and exposed environments was reflected in relatively higher concentrations of total UACs compared to P. schreberi. The enhanced UV-B experiments in situ were conducted over two years in Oulu and six years at the FUVIRC site in Sodankylä. Some of the effects of UV-B were seen within the first years of the experiments, or even within hours, while others were observed after several years. Five or six years of enhanced UV-B treatment increased the methanol-extractable UACs of P. schreberi and decreased the green shoot growth of P. juniperinum. The immediate light environment was proposed to have an impact on the varying UAC concentrations. Some mitigating effects of UV-A were observed as well. Off-site measured, reconstructed and modelled UV radiation data was used for comparisons of light environment in situ, or when performing a reconstructive research with historical samples. The environmental sample banks can provide a useful tool to study past environmental conditions, and even reconstruct past radiation levels. It was shown in this study that UACs in P. schreberi and P. juniperinum have fundamental roles as UV-B screens in the cell walls, but there is also a variable response with the soluble fraction that reacts and adapts to the changes in UV radiation. The responses to increasing UV-B radiation vary in magnitude and in time. As P. schreberi and P. juniperinum possess circumboreal and cosmopolitan distributions, the effects of UV-B on these species and consequently on ecosystems has a broad application.