Shaun Russell

Bio: Shaun Russell is an academic researcher from University of Namibia. The author has contributed to research in topics: Tundra & Bryophyte. The author has an hindex of 1, co-authored 1 publications receiving 32 citations.

Impacts of increased nitrogen supply on high Arctic heath: the importance of bryophytes and phosphorus availability.

Abstract: Summary • This study investigates effects of nitrogen and phosphorus on high Arctic heath vegetation, particularly bryophytes. • Heath communities received factorial combinations of nitrogen (0, 10 and 50 kg ha−1 yr−1) and phosphorus (0 and 5 kg ha−1 yr−1) in five applications per growing season, for 8 yr. • Nitrogen decreased lichen cover but did not affect cover of any other functional type. However, just 10 kg ha−1 yr−1 increased the proportion of physiologically active bryophte shoots, and decreased their nitrate assimilation capacity. Phosphorus had greater effects, and the combination of both nutrients altered species composition. Individual bryophyte species displayed contrasting responses to fertilization, suggesting that they should not be grouped as a single functional type. • The ‘critical load’ of nitrogen for Arctic heath lies below 10 kg ha−1 yr−1. Nitrogen and phosphorus are colimiting in this sytem, so the critical load of nitrogen will be lower where phosphorus availability is greater. Responses of vegetation to any increase in net mineralisation due to soil warming will depend on the ratio in which nitrogen and phosphorus availabilities increase. The effects of nutrient enhancement are very persistent.

Responses of subarctic-alpine plant communities to simulated environmental change : Biodiversity of bryophytes, lichens, and vascular plants

Abstract: The predicted changes in climate over the next 50 years are expected to be most pronounced in arctic and subarctic regions. In the present study, we examine the responses of a subarctic-alpine rich meadow and poor heath community to factorial manipulations of temperature and nutrient treatments. Specifically, we address response to the treatments in terms of biodiversity and relative cover of the bryophyte, lichen and vascular plant communities. We point out that the responses differ among mosses, lichens, vascular plants, and communities, and this will probably cause shifts in the dominance of both bottom layer and canopy layer species. It is important to note that the decrease in cover and species number of the bottom layer mainly occurred due to a decline in mosses; in contrast, lichen cover increased in all treatments in both communities. Climate change may thus cause a shift in the bottom layer from being dominated by mosses, to become dominated by lichens.

Estimating global carbon uptake by lichens and bryophytes with a process-based model

Abstract: Lichens and bryophytes are abundant globally and they may even form the dominant autotrophs in (sub)polar ecosystems, in deserts and at high altitudes. Moreover, they can be found in large amounts as epiphytes in old-growth forests. Here, we present the first process-based model which estimates the net carbon uptake by these organisms at the global scale, thus assessing their significance for biogeochemical cycles. The model uses gridded climate data and key properties of the habitat (e.g. disturbance intervals) to predict processes which control net carbon uptake, namely photosynthesis, respiration, water uptake and evaporation. It relies on equations used in many dynamical vegetation models, which are combined with concepts specific to lichens and bryophytes, such as poikilohydry or the effect of water content on CO 2 diffusivity. To incorporate the great functional variation of lichens and bryophytes at the global scale, the model parameters are characterised by broad ranges of possible values instead of a single, globally uniform value. The predicted terrestrial net uptake of 0.34 to 3.3 Gt yr −1 of carbon and global patterns of productivity are in accordance with empirically-derived estimates. Considering that the assimilated carbon can be invested in processes such as weathering or nitrogen fixation, lichens and bryophytes may play a significant role in biogeochemical cycles.

Growth responses of Polytrichum commune and Hylocomium splendens to simulated environmental change in the sub-arctic

Abstract: summary Total moss cover and the growth responses of the two dominant bryophytes within a sub-arctic heath ecosystem, Polytrichum commune Hedw, and Hylocomium splendens (Hedw.) Br. Eur., were investigated in response to simulated environmental change. Temperature, water supply and nutrients (NPK fertilizer) were increased in a factorial field study in Swedish Lapland. After 3 yr of treatment, total moss caver on fertilized plots was found to be less than 50% of that on unfertilized plots. H. splendens made a smaller contribution to total cover, and also had fewer shoots on the fertilizer treated plots. The relative contribution of P. commune to the bryophyte community was greater on the fertilized plots, although total shoot numbers of P. commune were not greater. There was, however, a larger number of dead shoots of P. commune on the fertilized plots. Total moss cover, relative contribution to moss cover by P. commune and H. splendens, and the numbers of shoots of these two species, were unresponsive to both the temperature and water treatments. The growth of the current year's segment of H. splendens was retarded by both the fertilizer and temperature treatments. Furthermore, the dry weight of the physiologically active shoot was found to be significantly altered by all three treatments. The current year's growth of P. commune showed increases in d. wt and morphological parameters in response to the fertilizer treatment. There were also interactive effects of both the fertilizer and temperature treatments with the water treatment on the current year's stem length increment.

Relationships between productivity, number of shoots and number of species in bryophytes and vascular plants

Abstract: Summary 1 We measured species density, biomass and shoot density for both bryophytes and vascular plants in 90 small plots in 18 calcareous fens. In addition, we recorded leaf area index and litter mass of vascular plants. Our goals were: (a) to compare the relationship between biomass and species density for the two taxonomic groups, (b) to test whether biomass and species density of bryophytes and vascular plants are related to their shoot density, and (c) to assess the degree to which biomass, shoot and species density of bryophytes are correlated with characteristics of the vascular plant layer. 2 For bryophytes there was a positive linear relationship between biomass and species density. Vascular plant species density was not related to biomass. Furthermore, bryophyte biomass and species density were linearly and positively related to bryophyte shoot density. For vascular plants, only biomass but not species density was related to shoot density. 3 We concluded that a bryophyte favourability gradient existed along which biomass and shoot and species density increased. This gradient was attributed to positive interactions within dense bryophyte stands, high clonal fragmentation, absence of competitive hierarchies and to the limited ability of larger bryophyte species to replace small species along this favourability gradient. 4 Since species density for vascular plants varied independently from biomass and shoot density, there was no such favourability gradient as for bryophytes. Large size variation, predominantly negative interactions between species, and clonal integration of species (e.g. tussock-forming grasses and sedges) may be responsible for the different behaviour of the two taxonomic groups. 5 Bryophyte favourability decreased with increasing vascular plant biomass. Concerning light availability, we found highest bryophyte favourability at intermediate levels where the combination of radiation and moisture seems to be optimal for bryophytes. No relationship was found between bryophyte favourability and vascular plant shoot density and litter mass. 6 The negative relationship between bryophyte favourability and vascular plant biomass is important for bryophyte conservation. Stands of low vascular plant production are those with the potential for highest species richness, and should therefore receive conservation priority.