Showing papers by "Sven Jonasson published in 1995"

Spatial variation in high-latitude methane flux along a transect across Siberian and European tundra environments

Abstract: This paper reports results from a transect of methane flux measurements across tundra environments in Siberia and the European Arctic during July and August 1994. Overall, mean CH4 emission was 2.3±0.7 mg m−2 day−1 for the mesic tundra sites and 46.8±5.9 for the wet habitats with large intersite variability. The general scale of emissions was somewhat low compared to assumptions made about them in global methane budgets and models. In particular, the mesic tundra fluxes were lower than what would be expected based on data from similar environments in North America, and there are indications that these environments may be significant atmospheric methane consumers. Consistent consumption rates in combination with the large expanse of dry/mesic tundra environments suggest that it may be necessary to incorporate a high-latitude soil sink term in global methane budgets. However, the wet tundra emissions found between 67° and 77°N in this study were consistently higher than recent findings in comparable environments at much lower latitudes (50°–55°N). High northern latitudes therefore represent a very important player in the global methane budget. When compared across both mesic and wet sites, methane emission increased with increasing soil organic content, soil temperature, and soil moisture. The relationship between soil temperature and methane flux at the wet sites alone was highly significant, and the flux also increased with increasing soil moisture and organic content. No correlations were found between flux and the measured environmental parameters at the mesic sites when treated separately.

Arctic Terrestrial Ecosystems and Environmental Change [and Discussion]

Abstract: The impacts of environmental change on Arctic terrestrial ecosystems are complex and difficult to predict because of the many interactions which exist within ecosystems and between several concurrently changing environmental variables. However, some general predictions can be made. (i) In the sub-Arctic, subtle shifts in plant community composition with occasional losses of plant species are more likely than immigration of exotic species. In the high Arctic, colonization of bare ground can proceed and there are likely to be shifts in ecotypes. Major shifts in vegetation zones, such as the advance of the boreal forest, are likely to be slow and species specific responses will result in different assemblages of species in plant communities in the longer term. All changes in community structure, apart from species removal by direct extreme weather conditions (e.g. drought) will be slow because of the slow growth, low levels of fecundity and slow migration rates of plant species over large latitudinal ranges. (ii) Mobile mammals and birds can probably adjust to changes in the distribution of their food plants or prey in the Arctic, but vertebrate and invertebrate herbivores may face problems with changes in the quality of their food plants. Non-migratory animals could be severely affected by altered winter snow conditions which affect availability of food and shelter. (iii) Increases in primary production are uncertain and depend mainly upon the responses of soil microbial decomposer activity to changes in soil temperature, moisture and plant litter quality. Assumptions that climate warming will lead to warmer soils and increased nutrient availability to sustain higher productivity are uncertain as greater biomass may lead to reduced soil temperatures through insulation effects and increased nutrients released may be immobilized by soil microorganisms. (iv) Changes in environmental conditions are themselves often uncertain. There is particular doubt about changes in precipitation, growing season length, cloudiness and UV-B radiation levels while such environmental changes are likely to vary in magnitude and direction between different regions of the Arctic. (v) The large populations and circumpolar distributions typical of Arctic biota lead to a strong buffering of changes in biodiversity. Perhaps the greatest threats to Arctic biota will be imposed by the degradation of permafrost which may lead to either waterlogging or drought depending upon precipitation regimes.

Arctic terrestrial ecosystems and environmental change

Abstract: The impacts of environmental change on Arctic terrestrial ecosystems are complex and difficult to predict because of the many interactions which exist within ecosystems and between several concurrently changing environmental variables. However, some general predictions can be made. (i) In the sub-Arctic, subtle shifts in plant community composition with occasional losses of plant species are more likely than immigration of exotic species. In the high Arctic, colonization of bare ground can proceed and there are likely to be shifts in ecotypes. Major shifts in vegetation zones, such as the advance of the boreal forest, are likely to be slow and species specific responses will result in different assemblages of species in plant communities in the longer term. All changes in community structure, apart from species removal by direct extreme weather conditions (e.g. drought) will be slow because of the slow growth, low levels of fecundity and slow migration rates of plant species over large latitudinal ranges. (ii) Mobile mammals and birds can probably adjust to changes in the distribution of their food plants or prey in the Arctic, but vertebrate and invertebrate herbivores may face problems with changes in the quality of their food plants. Non-migratory animals could be severely affected by altered winter snow conditions which affect availability of food and shelter. (iii) Increases in primary production are uncertain and depend mainly upon the responses of soil microbial decomposer activity to changes in soil temperature, moisture and plant litter quality. Assumptions that climate warming will lead to warmer soils and increased nutrient availability to sustain higher productivity are uncertain as greater biomass may lead to reduced soil temperatures through insulation effects and increased nutrients released may be immobilized by soil microorganisms. (iv) Changes in environmental conditions are themselves often uncertain. There is particular doubt about changes in precipitation, growing season length, cloudiness and UV-B radiation levels while such environmental changes are likely to vary in magnitude and direction between different regions of the Arctic. (v) The large populations and circumpolar distributions typical of Arctic biota lead to a strong buffering of changes in biodiversity. Perhaps the greatest threats to Arctic biota will be imposed by the degradation of permafrost which may lead to either waterlogging or drought depending upon precipitation regimes.

Inhibition of growth, and effects on nutrient uptake of arctic graminoids by leaf extracts - allelopathy or resource competition between plants and microbes?

Abstract: Previous research has shown that plant extracts, e.g. from boreal dwarf shrubs and trees, can cause reduced growth of neighbouring plants: an effect known as allelopathy. To examine whether arctic and subarctic plants could also be affected by leaching of phytochemicals, we added extracts from the commonly occurring arctic dwarf shrubs Cassiope tetragona and Empetrum hermaphroditum, and from mountain birch, Betula pubescens ssp. tortuosa to three graminoid species, Carex bigelowii, Festuca vivipara and Luzula arcuata, grown in previously sterilized or non-sterilized arctic soils. The graminoids in non-sterilized soil grew more slowly than those in sterilized soil. Excised roots of the plants in non-sterilized soil had higher uptake rate of labelled P than those in sterilized soil, demonstrating larger nutrient deficiency. The difference in growth rate was probably caused by higher nutrient availability for plants in soils in which the microbial biomass was killed after soil sterilization. The dwarf shrub extracts contained low amounts of inorganic N and P and medium high amounts of carbohydrates. Betula extracts contained somewhat higher levels of N and much higher levels of P and carbohydrates. Addition of leaf extracts to the strongly nutrient limited graminoids in non-sterilized soil tended to reduce growth, whereas in the less nutrient limited sterilized soil it caused strong growth decline. Furthermore, the N and P uptake by excised roots of plants grown in both types of soil was high if extracts from the dwarf shrubs (with low P and N concentrations) had been added, whereas the P uptake declined but the N uptake increased after addition of the P-rich Betula extract. In contrast to the adverse extract effects on plants, soil microbial respiration and soil fungal biomass (ergosterol) was generally stimulated, most strongly after addition of the Betula extract. Although we cannot exclude the possibility that the reduced plant growth and the concomitant stimulation of microbial activity were caused by phytochemicals, we believe that this was more likely due to labile carbon in the extracts which stimulated microbial biomass and activity. As a result microbial uptake increased, thereby depleting the plant available pool of N and P, or, for the P-rich Betula extract, depleting soil inorganic N alone, to the extent of reducing plant growth. This chain of events is supported by the negative correlation between plant growth and sugar content in the three added extracts, and the positive correlation between microbial activity, fungal biomass production and sugar content, and are known reactions when labile carbon is added to nutrient deficient soils.

Little Ice Age temperature estimated by growth and flowering differences between subfossil and extant shoots of Cassiope tetragona, an arctic heather

Abstract: Little Ice Age temperature estimated by growth and flowering differences between subfossil and extant shoots of Cassiope tetragona, an arctic heather

Resource Allocation in Relation to Leaf Retention Time of the Wintergreen Rhododendron Lapponicum

Abstract: Extended leaf longevity has been explained both as an adaptation to conserve nutrients and to increase seasonal or life-time carbon gain. The relative importance of these possible adaptations was investigated in the wintergreen dwarf shrub Rhododendron lap- ponicum by analyses of responses of growth and nutrient allocation to early and late spring defoliation and to natural reduction of leaf biomass following flowering. Removal of old leaves at the time of budbreak had no effect on biomass accumulation and nutrient pools of the new shoots 1 mo later, although the defoliation prevented the transport of resorbed nutrients from old leaves to the new shoots. This transport accounted for by far the largest part of the increase in shoot N and P in undefoliated controls. Extra nutrients were not transported from stems and roots to compensate for the lost import from old leaves. Hence, defoliated plants compensated for the losses solely by increasing nutrient uptake. In contrast, defoliation 3 wk earlier caused a 29% reduction of annual shoot growth and a proportional decline of nutrient pools in the shoots, while the nutrient concentrations, reflecting the shoot nutrient status, did not decline. Almost identical responses were ob- served in unmanipulated plants the year after heavy flowering. Reproductive branches do not form leaves, resulting in a decreasing photosynthetic capacity and a reduction of resource storage and allocation to next year's leaf cohort. Although early and late spring defoliation caused equal losses of mobile nutrients, only the early defoliation decreased growth, suggesting that it was the loss of carbon rather than nutrients that affected growth. This supports earlier observations that the extended retention time of leaves in wintergreen and evergreen species plays a greater role in improving the carbon balance than in supplying the new leaves with nutrients.

Effects of simulated climate change on the sexual reproductive effort of Cassiope tetragona

Abstract: Effects of simulated climate change on the sexual reproductive effort of Cassiope tetragona.

Nutrient Limitation in Rhododendron lapponicum, Fact or Artefact?

Abstract: Leaf longevity in plants can vary from weeks to a decade and several suggestions have been proposed as plausible adaptive explanations for this variation in life length (e.g. Chabot and Hicks 1982, Jonasson 1989). For instance, retention of the leaves over winter in seasonal environments could lead to a significant increase of the annual carbon fixation in some habitats where carbon fixation is constrained (Bell and Bliss 1977). Plants which keep the leaves over winter can continue to photosynthesize in late season and resume photosynthesis early the next season, or at any time when the environmental conditions are favourable for photosynthesis. Retention of the leaves over several growing seasons can also be of adaptive significance in nutrient deficient environments because the annual need for replacements of nutrients lost in shed leaves decreases as leaf longevity increases. In a recent Gikos article Karlsson (1994) reports results of a combined shading and leaf removal experiment, in which he investigated the relative significance of nutrient (N and P) and carbon supply from old leaves for the subsequent growth of the wintergreen, alpine dwarf shrub Rhododendron lapponicum. Shading of last year's leaves in early spring was intended to reduce the time for photosynthesis and the leaves' life-time carbon gain. Defoliation in early spring was supposed to give a combined effect of reduced photosynthetic carbon gain and of removal of mobile nutrients stored in the old leaves, i.e. reserves which could be used for supporting growth of the current year's leaf cohort after resorption. The treatments were done in spring at the onset of flowering but before the overwintered leaves started to senesce and the new leaf cohort developed. For non-reproductive branches Karlsson found that nutrient transport from old to new leaves was important for the subsequent growth, whereas carbon depletion by shading and defoliation affected subsequent growth little. In contrast, both the nutrient transport and the carbon fixation by old leaves were important for the subsequent growth of reproductive branches. In my opinion, Karlsson's data do not, however, give any strong argument in support for his interpretation of nutrient limited growth in non-reproductive branches for several reasons. Firstly, after one season's growth both the mean leaf area and the mean weight per unit area of the leaves tended to be lower in shaded plants, causing a mean reduction of leaf weight with about 16-17% as compared to the controls (the percentages are approximated from Karlsson's Fig. 5). Similarly, the weight of the current year's stem also showed a declining trend, resulting in a mean end-of-season decline of about 20% in leaf plus stem biomass. However, even though the differences in leaf and stem weight were large the weights were not significantly different, and Karlsson considers correctly hat he has not been able to show any effect by the shading. This leads Karlsson to the conclusion that the carbon gained from early spring photosynthesis was not a significant source of resources for the subsequent vegetative growth. In my opinion, this conclusion is doubtful because the variation in weight of both controls and shaded plants was large. The standard errors in Karlsson's Fig. 5 show that the coefficient of variation was at least 0.30-0.35 for the leaves and about 0.5, or even higher, for stem weight. Given this variation in the branches that Karlsson measured, the chosen sample size of 15 gives a very unsensitive test and can only pick up significant differences that are appreciably higher than the mean of about 20% shown for leaves plus stems in Fig. 5. Hence, Karlsson's statement (p. 196) that the effect of shading was "marginal" has, in my opinion, little biological relevance because I consider that an end-ofseason decline of growth by 20%, or less, indicates that the plants were affected by the treatment. Also, as pointed out above, it is probable that such a decline took place because all measured growth parameters (leaf size, leaf

Spatial variation in high latitude methane flux - a transect across tundra environments in Siberia and the European Arctic

Abstract: Spatial variation in high latitude methane flux - a transect across tundra environments in Siberia and the European Arctic