Lichens are often important photosynthetic organisms in oligotrophic environments where high-quality fodder plants are rare. A strong herbivore defence and/or low nutritional quality allows the accumulation of a high lichen biomass in such areas. However, it is not known how N deposition influences lichen palatability. This study analyzes possible changes in gastropod grazing preference after 3 months simulated N deposition on 3 foliose (Lobaria scrobiculata, Platismatia glauca, and Xanthoria aureola) and 1 pendulous lichen species (Alectoria sarmentosa). Lichens were daily irrigated in the field with rainwater containing 1.625 mM NH4NO3 from June to September, equivalent to a deposition of 50 kg N·ha-1·y-1. Irrigations applied at night, morning, or noon simulated different C-gain regimes. Afterwards in the lab, we offered 2 common lichen-feeding gastropods the choice between N-fertilized thalli and control thalli irrigated with artificial rainwater. The gastropods clearly preferred the unfertilized thalli of the 3 foliose species. For the pendulous A. sarmentosa, the gastropods preferred N-enriched thalli (irrigated at night) to controls. In conclusion, N-enrichment changes the palatability of lichens in species-specific ways.
In most boreal regions snow composes a large portion of the annual precipitation. Although many boreal forest floor bryophytes depend largely on precipitation for their nitrogen (N) supply, bryophyte uptake of snow N little explored. We studied chemical forms of plant-accessible N in snowmelt, as well as the temporal dynamics of their release. In conjuction we performed a N uptake experiment using the common boreal bryophyte Hylocomium splendens. the results demonstrated that the snowmelt N pool was dominated by NO3 (86%), followed by NH4+ (11%) and amino acid N (3%), in total providing ca 0.3 kg N.ha(-1) to the forest floor vegetation. Hylocomium splendens was able to access both inorganic and organic N-15 labelled N forms (NO3-, NH4+, and glycine) applied in situ to the snow covering the moss prior to snowmelt. Across all the N forms H. splendens took up ca 24% of the snow-deposited N. Nitrate uptake exceeded that of glycine, while NH4+ uptake was intermediate, reflecting the ambient distribution of the snowmelt N pool between plant-accessible N forms.
Predictions on displacement of suitable habitats due to climate change suggest that plant species with poor colonization ability may be unable to move fast enough to match forecasted climate-induced changes in habitat distribution. However, studies on early Holocene plant migration show fast migration of many plant species that are poor colonizers today We hypothesize that warmer temperatures during the early Holocene yielded higher seed quality, contributing to explaining the fast migration. We studied how the 3 seed quality variables, seed mass, germinability, and requirements for break of seed dormancy, vary for seeds of 11 forest herb species with varying colonization capacity collected along a 1400-km latitudinal gradient. Within species, seed mass showed a positive correlation with latitude, whereas germinability was more positively correlated with temperature (growing degree hours obtained at time of seed collection). Only slow-colonizing species increased germinability with temperature, whereas only fast-colonizing species increased germinability with latitude. These interactions were only detectable when analyzing germinability of the seeds, even though this trait and seed mass were correlated. The requirement for dormancy break did not correlate with latitude or temperature. The results indicate that seed development of slow colonizers may be favoured by a warmer climate, which in turn may be important for their migration capacity.
Tree establishment is a potentially important factor affecting tree populations in alpine-treeline ecotones. Patterns of seedling establishment of Abies lasiocarpa, Pinus albicaulis, and Picea engelmannii were evaluated relative to neighbouring trees and herbs over two years and three treelines of the Rocky Mountains, USA. The greatest mortality rates were observed in seedlings that had just emerged from seed and were in their first year of growth and in seedlings that had the least amount of cover provided by trees or other landscape features that block exposure to the sky. Although herb cover promoted survivorship in microsites that were not near trees, no seedlings were detected at or above the upper limit of the treeline ecotone. Microsite tree cover was greatest for A. lasiocarpa and least for P. albicaulis seedlings, which matches predictions based on their relative photosynthetic tolerances to the bright sunlight and frequent frost that occur in exposed microsites. Interspecific differences in seedling requirements for neighbouring plant cover likely contribute to the apparent coexistence and possible interdependency of these conifers along a continuum of colonization and succession within treelines.
The relationships between grey-sided vole (Clethrionomys rufocanus) densities, levels of invertebrate herbivory on Northern willow (Salix glauca) leaves, and chemical quality of the willows was studied on 8 islands and 2 mainland sites with contrasting vole densities in northernmost Norway. These variables were measured at each of the study sites to determine the degree and nature of the effects of browsing-induced alterations in plant quality on subsequent invertebrate herbivory. The level of invertebrate herbivory was positively correlated with vole density, as were the number of leaves per shoot, leaf size, and leaf nitrogen content, while leaf C/N ratios were negatively correlated with vole density. The level of herbivory increased from > 1% on the vole-free island to < 4% on the island with the highest vole density. The plant character that explained most of the variance in the level of invertebrate herbivory was leaf size. Since the vole densities have been altered by human intervention and their numbers are largely governed by predation rather than food quality, the positive correlation between vole densities and level of invertebrate herbivory is probably due to a facilitative effect of voles on invertebrate herbivores, mediated through changes in plant chemistry. We suggest that voles affect susceptibility of willows to invertebrate herbivory both directly by winter browsing and indirectly by reducing the abundance of competing plants.