Precipitation regime controls bryosphere carbon cycling similarly across contrasting ecosystems

In arctic and boreal ecosystems, ground bryophytes play an important role in regulating carbon (C) exchange between vast belowground C stores and the atmosphere. Climate is changing particularly fast in these high-latitude regions, but it is unclear how altered precipitation regimes will affect C dynamics in the bryosphere (i.e. the ground moss layer including senesced moss, litter and associated biota) and the closely associated upper humus layer, and how these effects will vary across contrasting environmental conditions. Here, we set up a greenhouse experiment in which mesocosms were assembled containing samples of the bryosphere, dominated by the feather moss Hylocomium splendens, and the upper humus layer, that were collected from across a boreal forest chronosequence in northern Sweden which varies strongly in nutrient availability, productivity and soil biota. We tested the effect of variation in precipitation volume and frequency on CO₂ exchange and dissolved organic carbon (DOC) export, and on moss growth. As expected, reduced precipitation volume and frequency lowered net CO₂ efflux, DOC export and moss growth. However, by regulating moisture, the lower bryosphere and humus layers often mediated how precipitation volume and frequency interacted to drive C dynamics. For example, less frequent precipitation reduced moss growth only when precipitation volume was low. When volume was high, high moisture content of the humus layer helped avoid moss desiccation. Variation in precipitation regime affected C cycling consistently in samples collected across the chronosequence, despite large environmental variation along the sequence. This suggests that the bryosphere exerts a strong buffering effect on environmental variation at the forest floor, which leads to similar responses of C cycling to external perturbations across highly contrasting ecosystems. As such, our study indicates that projected increases in droughts and ground evapotranspiration in high-latitude regions resulting from climate change will consistently reduce C losses from moss-dominated ecosystems.     

Physiological and molecular diversity of feather moss associative N2-fixing cyanobacteria

Cyanobacteria colonizing the feather moss Pleurozium schreberi were isolated from moss samples collected in northern Sweden and subjected to physiological and molecular characterization. Morphological studies of isolated and moss-associated cyanobacteria were carried out by light microscopy. Molecular tools were used for cyanobacteria identification, and a reconstitution experiment of the association between non-associative mosses and cyanobacteria was conducted. The influence of temperature on N2 fixation in the different cyanobacterial isolates and the influence of light and temperature on N2-fixation rates in the moss were studied using the acetylene reduction assay. Two different cyanobacteria were effectively isolated from P. schreberi: Nostoc sp. and Calothrix sp. A third genus, Stigonema sp. was identified by microscopy, but could not be isolated. The Nostoc sp. was found to fix N2 at lower temperatures than Calothrix sp. Nostoc sp. and Stigonema sp. were the predominant cyanobacteria colonizing the moss. The attempt to reconstitute the association between the moss and cyanobacteria was successful. The two isolated genera of cyanobacteria in feather moss samples collected in northern Sweden differ in their temperature optima, which may have important ecological implications.     

The effect of altered macroclimate on N-fixation by boreal feather mosses

Plant productivity is predicted to increase in boreal forests owing to climate change, but this may depend on whether N inputs from biological N-fixation also increases. We evaluated how alteration of climatic factors affects N input from a widespread boreal N-fixer, i.e. cyanobacteria associated with the feather moss Pleurozium schreberi. In each of 10 forest stands in northern Sweden, we established climate-change plots, including a control (ambient climate) plot and three plots experiencing a +2°C temperature increase, an approximately threefold reduction in precipitation frequency, and either 0.07, 0.29 or 1.16 times normal summer precipitation. We monitored N-fixation in these plots five times between 2007 and 2009, and three times in 2010 after climate treatments ended to assess their recovery. Warmer temperatures combined with less frequent precipitation reduced feather moss moisture content and N-fixation rates regardless of total precipitation. After climate treatments ended, recovery of N-fixation rates occurred on the scale of weeks to months, suggesting resilience of N-fixation to changes in climatic conditions. These results suggest that modelling of biological N-inputs in boreal forests should emphasize precipitation frequency and evaporative water loss in conjunction with elevated temperature rather than absolute changes in mean precipitation.     

Attenuating the defeminization of the neonatal rat brain: Mechanisms of action of cyproterone acetate, 1,4,6-androstatriene-3,17,-dione and a synthetic progestin, R5020

Although defeminization of the rat brain appears to depend significantly on the conversion of testosterone (T) to estradiol (E₂), the antiandrogenic steroid cyproterone acetate (CA) is able to attenuate defeminization. In order to study the mechanism of action of CA on brain sexual differentiation, newborn male rats were given subcutaneous injections of this steroid on postnatal Days 2–6. When castrated on Day 70 and given estrogen and progesterone, these CA-treated males displayed elevated lordosis quotients (LQ) compared to controls. CA-treated neonatal males were also examined at the end of the drug treatment to ascertain the mechanism of drug action: (1) Serum T levels were normal; (2) Brain cell nuclear estrogen receptor occupation, estimated by an exchange assay, was reduced by ≈ 30% in the brains of the CA-treated males, although the ability of exogenous E₂ to occupy these brain estrogen receptors was not reduced. Other work has demonstrated a weak competitive effect of cyproterone on aromatization, and thus cyproterone acetate may have interfered with the conversion of T to E₂ CA also has progestogenic activity, and 5-mm capsules of a potent synthetic progestin, R5020, given to newborn male rats on Days 2–6, are shown to elevate the LQ after postnatal Day 70 to the same extent as CA. However, R5020 did not reduce estrogen receptor occupation in the neonatal male rat brain and was without effect on serum T levels in the neonatal male. Because of the implied role of T-derived estrogens in defeminization, an experiment was conducted showing that the defeminizing action of estradiol benzoate given to 3-day-old female rat pups is attenuated by the antiestrogen, CI628, and not by the potent inhibitor of aromatization, 1,4,6-androstatriene-3,17-dione (ATD). This result complements previous experiments showing that both ATD and CI628 attenuate the defeminization produced by T. Taken together, the results lend further support to a pivotal role for aromatization and for estrogen-receptor interactions in the defeminizing effects of T. The actions of progestins such as CA and R5020 in attenuating defeminization are discussed in relation to the recent demonstration of progestin receptors in the neonatal rat brain. It is concluded that CA may act by a combination of actions, both by inhibiting aromatization and by acting as a progestin.     

Variation in the effectiveness of biotic defence: the case of an opportunistic ant-plant protection mutualism

Benefits to plants in facultative ant protection mutualisms are highly variable. This allows examination of the sources of this variation and the mechanisms by which ants protect plants. We studied opportunistic interactions between ants and an extrafloral nectary-bearing vine, Dioscorea praehensilis, during 3 different years. Variation in plant protection among years was striking. Several factors affected the effectiveness of the biotic defence. Stems recently emerged from the underground tuber were self-supporting, contacting no other plants and encountering few foraging ants. Stems then became lianescent, and contact with supporting plants greatly increased ant recruitment. Both species and number of ant workers influenced the effect of ants on the major herbivore, the chrysomelid beetle Lilioceris latipennis. Protective actions included limitation of oviposition (reduction in the number of eggs laid on the plant) and predation, leading to increased larval mortality. The probability of successful predation was strongly dependent on larval size. If temporarily low ant-patrolling activity allows larvae to grow beyond a critical size, their mechanical (thick integument) or chemical (plant-derived compounds in a fecal shield) defences become more effective against ants. Secondary metabolites derived from the host plant thus appear to be important for the anti-predator mechanisms of this beetle, being necessary for its survival and reproduction on a host plant that actively recruits ants as a biotic defence against herbivores.     

Microbe-plant competition, allelopathy and arctic plants

Michelsen et al. (1995) present results of an experiment in which aqueous leaf extracts of three arctic woody plant species were found to inhibit growth and nutrient acquisition of three graminoid species, and suggested that microbial nutrient immobilisation, rather than allelopathy, was responsible for the observed trends. In doing this they also question previous work proposing that the Arctic dwarf shrub Empetrum hermaphroditum is allelopathic. We suggest that their conclusions are not unequivocally supported by their data. Firstly we indicate that the approaches used for estimating microbial nutrient immobilisation are questionable. Secondly we indicate that most of the trends that they discussed are based on data in which the treatments and controls are not significantly different for the majority of cases. Finally we respond specifically to their criticisms of previous work on E. hermaphroditum. While the question of how arctic plants interact is an interesting one, we conclude that this question cannot be answered by their data. Received: 20 November 1995 /Accepted: 10 July 1996