Repeatability of dispersal behaviour in a common dwarf spider: evidence for different mechanisms behind short‐ and long‐distance dispersal

Abstract 1. The response of dispersal towards evolution largely depends on its heritability for which upper limits are determined by the trait’s repeatability. 2. In the Linyphiid spider E. atra, we were able to separate long‐ and short‐distance dispersal behaviours (respectively ballooning and rappelling) under laboratory conditions. By performing repeated behavioural trials for females, we show that average dispersal trait values decrease with increasing testing days. By comparing mated and unmated individuals during two periods (before and after mating for the mated group, and the same two periods for the unmated group), we show that mating has no effect on the mean displayed dispersal behaviour or its within‐individual variation. Repeatabilities were high and consistent for ballooning motivation, but not for rappelling. 3. Ballooning motivation can be regarded as highly individual‐specific behaviour, while general pre‐dispersal and rappelling behaviours showed more individual variation. Such difference in repeatability between long‐ and short‐distance dispersal suggests that short‐ and long‐distance dispersal events are triggered by different ecological and evolutionary mechanisms.     

Analysis of Transient Changes in Fluid Exudation from Isolated Maize Roots

It was found that the fluid exudation rate from isolated maizeroots rapidly decreases when the external concentration of varioussolutes is increased; thereafter a relatively slow increaseto a new value occurs. An analysis of this transient phenomenonhas been achieved on the basis that there is an active transportof salt (KCl) into a compartment within the root. Further, ithas been assumed that fluid exudation is created by a net osmoticwater flow into this compartment. Our analysis of the experimentaldata indicates that the volume of the compartment has a similarmagnitude to the estimated total volume of the xylem vesselsin the root. The transient curve obtained with some solutes (methanol, magnesiumsulphate, mannitol, and raffinose) were significantly differentfrom the usual response. An attempt to apply an appropriate diffusion equation to theinitial phase of the transient curves showed that the time courseof this phase may be interpreted as being controlled by therate of solute diffusion towards an osmotic barrier within theroot.     

Water Transport in Isolated Maize Roots

A simple model has been devised which predicts the concentration,C^x_s, of salt (e.g. KCl) in the exudate from isolated roots asa function of the salt concentration, C⁰_s, in the medium. Thechief assumption, made in deriving the relationship betweenC^x_s and C⁰₅, is that the exudation of water, J, from the rootsconsists of two components (one being osmotic, Ø, inorigin and the other, Ø⁰, flowing in the absence of anosmotic gradient). The exudation of salt, J_s, calculated asJ C^x_s, was found to be dependent on C⁰_s. Our investigationson maize roots were concerned with estimations of L_p and Ø^vand determinations of C^x_s as a function of C⁰_s. Satisfactoryagreement between prediction and experiment was found in thesepreliminary studies. It is considered that water movement inisolated roots can be explained by a simple osmotic model withthe additional possibility that a relatively small non-osmoticwater flow occurs.     

Impact of past and present land‐management on the C‐balance of a grassland in the Swiss Alps

Grasslands cover about 40% of the ice‐free global terrestrial surface, but their quantitative importance in global carbon exchange with the atmosphere is still highly uncertain, and thus their potential for carbon sequestration remains speculative. Here, we report on CO₂ exchange of an extensively used mountain hay meadow and pasture in the Swiss pre‐Alps on high‐organic soils (7–45% C by mass) over a 3‐year period (18 May 2002–20 September 2005), including the European summer 2003 heat‐wave period. During all 3 years, the ecosystem was a net source of CO₂ (116–256 g C m^?2 yr^?1). Harvests and grazing cows (mostly via C export in milk) further increased these C losses, which were estimated at 355 g C m^?2 yr^?1 during 2003 (95% confidence interval 257–454 g C m^?2 yr^?1). Although annual carbon losses varied considerably among years, the CO₂ budget during summer 2003 was not very different from the other two summers. However, and much more importantly, the winter that followed the warm summer of 2003 observed a significantly higher carbon loss when there was snow (133±6 g C m^?2) than under comparable conditions during the other two winters (73±5 and 70±4 g C m^?2, respectively). The continued annual C losses can most likely be attributed to the long‐term effects of drainage and peat exploitation that began 119 years ago, with the last significant drainage activities during the Second World War around 1940. The most realistic estimate based on depth profiles of ash content after combustion suggests that there is an 500–910 g C m^?2 yr^?1 loss associated with the decomposition of organic matter. Our results clearly suggest that putting efforts into preserving still existing carbon stocks may be more successful than attempts to increase sequestration rates in such high‐organic mountain grassland soils.     

Other factors than apoplastic ascorbate contribute to the differential ozone tolerance of two clones of Trifolium repens L.

Apoplastic reactive oxygen intermediates, which are formed during the exposure of a higher plant to ozone (O₃), have been proposed to be detoxified by apoplastic ascorbate (ASC). An investigation to determine whether the differential sensitivity of two white clover clones (Trifolium repens L. cv Regal) to O₃ was related with their levels of ASC, glutathione derivatives or with the total antioxidative capacity. In contrast to what might be expected, the sensitive clone of white clover (NC-S) constitutively showed a 72% higher concentration of apoplastic ASC compared to the O₃-tolerant clone (NC-R). Furthermore, NC-S also showed a higher redox status of apoplastic ASC. These results indicate that higher ASC levels in the apoplast of NC-S are not sufficient to induce a higher O₃ tolerance. The redox status, but not the absolute concentration of homoglutathione in the symplast was found to be constitutively higher in NC-R than in NC-S. It is not clear, however, whether homoglutathione is a direct cause of the differential O₃ detoxification capacity of both clones. Total antioxidative capacity measurements ruled out the contribution of other low-molecular antioxidants to the relative tolerance of NC-R. It was concluded that elevated apoplastic ASC levels can not always be sufficient to render a plant O₃ tolerant.