Responses to mineral nutrient availability and heterogeneity in physiologically integrated sedges from contrasting habitats.

Clonal plants from poor habitats benefit less from morphologically plastic responses to heterogeneity than plants from more productive sites. In addition, physiological integration has been suggested to either increase or decrease the foraging efficiency of clonal plants. We tested the capacity for biomass production and morphological response in two closely related, rhizomatous species from habitats that differ in resource availability, Carex arenaria (from poor sand dunes) and C. disticha (from nutrient-richer, moister habitats). We expected lower total biomass production and reduced morphological plasticity in C. arenaria, and that both species would produce more ramets in high nutrient patches, either in response to signals transported through physiological integration, or by locally determined responses to nutrient availability. To investigate mineral nutrient heterogeneity, plants were grown in boxes divided into two compartments with homogeneous or heterogeneous supply of high (H) or low (L) nutrient levels, resulting in four treatments, H-H, H-L, L-H and L-L. Both C. arenaria and C. disticha produced similar biomass in high nutrient treatments. C. disticha responded to high nutrients by increased biomass production and branching of the young parts and by altering root:shoot ratio and rhizome lengths, while C. arenaria showed localised responses to high nutrients in terms of local biomass and branch production in high nutrient patches. The results demonstrated that although it has a conservative morphology, C. arenaria responded to nutrient heterogeneity through morphological plasticity. An analysis of costs and benefits of integration on biomass production showed that young ramets of both species benefited significantly from physiological integration, but no corresponding costs were found. This suggests that plants from resource-poor but dynamic habitats like sand dunes respond morphologically to high nutrient patches. The two species responded to nutrient heterogeneity in different traits, and this is discussed in terms of local and distant signalling of plant status.

Uptake capacity of amino acids by ten grasses and forbs in relation to soil acidity and nitrogen availability.

Uptake capacity of organic nitrogen was studied in solution experiments on eight grasses and two forbs growing in acid soils with relatively high nitrogen mineralisation in southern Sweden. Uptake of a mixture of amino acids (alanine, glutamine, glycine), that varied between 1.6 and 6.3 µmol g(-1) dw root h(-1), could not be explained by soil data from the species' field distributions (pH, total carbon and nitrogen, potential net mineralisation of ammonium and nitrate). The ratio between organic and inorganic nitrogen (methylamine) uptake was <0.05 for the forbs, higher for the grasses with a maximum of 1.42 for Deschampsia flexuosa. The ratio was negatively correlated with measures related to soil acidity (Ellenberg's R-value, soil nitrate and total carbon) but not, as hypothesised, with the total amount of mineralised nitrogen. The total demand on nitrogen by all components of the ecosystem would probably have described the extent to which competition among and between plants and microbes induced nitrogen limitation. In a methodological study two grasses were exposed to pH 3.8, 4.5 and 6.0 and to 50, 100 and 250 µmol l(-1) of three amino acids. Uptake was also compared between intact plants and excised roots. The treatment response varied considerably between the species which stresses the importance of studying intact plants at field-relevant pH and concentrations.

Modelling long-term cation supply in acidified forest stands.

A dynamic soil chemistry model was used to explain the observed decrease in soil base saturation between 1949 and 1984 at three stands in southern Sweden. The results show that acid deposition has caused soil acidification. The model, SAFE (Soil Acidification in Forest Ecosystems), includes the fundamental physical processes such as leaching and accumulation, and chemical processes such as cation exchange, mineral weathering, nutrient uptake and solute equilibrium reactions. The sources and sinks of base cations in the soil system were quantified, showing that weathering, deposition of base cations and depletion of exchangeable base cations supply cations to the soil solution in similar amounts in the upper 1 m during the acidification phase. This demonstrates that budget studies alone cannot be used to distinguish between long-term capacity to resist acidification, represented by weathering, from short-term buffering caused by cation exchange.

Reversibility of stemflow-induced soil acidification in Swedish beech forest.

Stemwater running down the trunks of beech (Fagus sylvatica L.) has an acidifying effect on soil near the stem. The deposition of acidifying substances may be two to four times higher close to the stem compared to in the stand in general. To study reversibility of soil acidification, 72 stumps of beech trees were chosen from five different year classes of felling (3, 6, 9, 14-15, 25-30), representing the years when stemflow ceased to affect the soil. The H(+) concentration (pH KCl) in the topsoil (0-5 cm) differed between the distances 10-30 cm and 230-250 cm from the stumps, the soil close to the stem being more acid. The longer the time since felling the smaller the differences in H(+) concentration became. This reduction in soil acidity amounted to ca. 50% after 15 years, and only small further changes occurred over the next ten years. This indicates that soil may not recover fully from acidification, or does so at a rather slow rate after the initial 15 years of recovery.

Long-term changes in pH of forest soils in southern Sweden.

Changes in pH in 22 forest soils (Fagus, Carpinus, Quercus, Picea) and one Calluna health from southern Sweden were examined in 1984 by repeating studies originally made in 1949-1970. The topsoil had become more acid in 1984 with pH decreases in the old profiles of more than 1.0 in less acid soils and of 0.5 in acid soils. Younger profiles also showed pH decreases, although of a smaller magnitude. Soil pH had, on the whole, declined, and acid deposition is probably a main contributor. Acidification has also occurred in deeper horizons, most markedly in 30-35-year-old horizons. Podsols had largest pH decreases in deeper horizons, and grey-brown soils in the upper profile.