Plant Ecology

To investigate how the composition of wetland communities changes over time in response to altered light regimes, experimental communities of five Carex and four grass species were subjected to artificial shading (continuous or seasonal) in a three-year field experiment. Shoot number and size was measured after six weeks, and shoot biomass was harvested five times during the experiment. Communities were initially dominated by three grass species in all treatments, but subsequently, the Carex species increased and reached dominance in the control plots, whereas grasses remained dominant in the shaded plots. Shading had no effect on the biomass of communities or of single species in the first year. In the second year, community biomass was still unaffected, but shading reduced the biomass of three Carex species and also reduced species diversity. In the third year, shading reduced community biomass and all Carex species, but not species diversity. The greater shade tolerance of the grasses could not be explained by differences in morphological plasticity: after six weeks of growth all species had increased shoot height in response to shade by 40–70%. Grasses were hardly more plastic than Carex species. We propose that the long-term success of the Carex species in full light was due to a high allocation of biomass to belowground parts, which may have reduced losses caused by repeated harvesting of shoots (a simulation of management in productive wet meadows). Shading probably caused the Carex plants to change their allocation pattern, and thus prevented their progressive increase.

The response of forest floor vegetation and regeneration of major treespecies to deer exclusion in a riparian forest were studied for 3 years withtheinteractive effects of natural disturbances. At the start of this study, deerdensity had quickly increased to a fairly high level (29–31 individualskm−2) during the last decade and had severely reduced theamount of forest floor vegetation in the study area. Dwarf bamboos, which weredominant before, had almost diminished from the forest floor. During the studyperiod, aboveground biomass increased steadily but species diversity did notchange much in the exclosures. Outside the exclosures, the seedlings of alltreespecies were damaged greatly by deer browsing, especially the taller ones. Deerbrowsing had little effect on the emergence of tree seedlings, but deertrampling might have accelerated emergence indirectly by disturbing the soilsurface for some species. Differences in plant responses were also observed fordeer browsing and the presence of dwarf bamboo that strongly inhibits therecruitment of tree seedlings. The nine major species were classified intothreegroups according to the response of seedlings to deer browsing and disturbance.Group 1 consists of the species whose seedling survival is affected bybrowsing,but seldom by disturbances (Phellodendron amurense,Kalopanax pictus, Quercus crispulaandMalus toringo). Groups 2 and 3 consist of species adaptedto tree-fall disturbance (Betula spp.) and ripariandisturbance (Alnus hirsuta var.sibirica, Ulmus davidiana var.japonica, Populus maximowiczii andSalix sachalinensis), respectively, and seedling survivalof these two groups is principally affected by light conditions. The effect ofdeer browsing on seedling survival and growth was greater for Group 2 than forGroup 3.

Our study aimed at identifying mechanisms that permit the long-termpersistence of Sarcopoterium spinosum, a dwarf-shrub thatdominates large areas of hilly landscape in eastern Mediterranean countries.Data were obtained from 16 1×1 m quadrats distributed amongwell-established S. spinosum communities in Israel thathadnot been burned or otherwise disturbed for well over 20 years. We sampled threecommunities that represent different climatic and habitat conditions andanalysed the growth rings in the root crowns of 604 uprooted plants, todetermine the age structures of the sampled stands. Many older'shrubs’ that appeared to be individual plantswere in fact clusters of ramets with their root crowns concentrated in a verysmall area. The morphology of these ramets indicates that they arose by rootingof peripheral stems. Seedlings and young plants were rare. Ramets seldomexceeded 17 years of age, although a few were 18–21 years old and one had34 growth rings. The average ramet age on the different sites varied between 9and 11 years. The ramets appeared to separate from the mother plant when theywere older than 7 years, after which they had an estimated average half-life ofca. 2.8 years. Young shrubs grew rapidly in height, but their basal cover waslow; it increased when they were much older, presumably as the older plantsdied. Consequently, three or four cohorts of older ramets between 12 and 17years old often dominated stand cover. The continuing profuse production ofseeds even where seedlingrecruitment is normally rare could be related to the relatively short averagelife-span of the ramets. We conclude that where resources are irregularlydistributed and establishment sites are fully occupied by perennial species,localised phalanx-type clonal proliferation can reduce the risk of genetextinction. On favourable sites this strategy confers upon S.spinosum genets the tenacity required to hold off replacement byother species and to dominate large areas of the landscape for much longer thanthe average life span of individual ramets.

Over the last few decades, shrub species have expanded rapidly in open tundra environments due to climate change. Previous experimental studies in this environment have shown that nutrient addition and, to a lesser extent, warming, had positive effects on shrub growth. However, the response of shrub species in open forested ecosystems such as lichen woodland is still largely unknown. The main objective of this study was to evaluate the performance of Betula glandulosa (Michx., dwarf birch) seedlings subjected to direct (warmer temperature, increased precipitation) and indirect (increased nutrient availability) effects of climate change in a lichen woodland (25 % tree cover). The study took place 10 km south of the subarctic treeline in western Québec (Canada). At the end of the second growth season, seedling leaf, woody stem and root biomass along with leaf area had increased significantly in response to nutrient addition. Moreover, seedlings exposed to nutrient addition had greater nitrogen, phosphorus and potassium concentrations in their leaves. Warming treatment also had a significant but weaker impact on leaf and woody stem biomass, while increase in precipitation had only a slight impact on seedling root biomass. Our results indicate that B. glandulosa response to simulated changes in the abiotic environment is similar to that observed in open tundra, suggesting that this species could also become more widespread in the forested ecosystems of the forest–tundra ecotone.