Interannual variability,stability and resilience in UK plant communities

Plant communities are often assumed to be stable on a year-to-year basis. We present evidence that species composition in permanent quadrats changes considerably between years at a range of Environmental Change Network sites across the UK. The extent of this variability varies with habitat type. Communities associated with low disturbance levels and low agricultural inputs, particularly moorland (upland grass and heath) and bog communities, are most stable.Inter-annual variability should, therefore, be considered in designing monitoring schemes to ensure that frequency of recording is sufficient to avoid short-term fluctuations obscuring long-term trends.More diverse communities were more stable, with less species turnover between years. However, diverse communities also tended to be dominated by slow-growing, slow-reproducing plants, adapted to low nutrient conditions, identified as ‘stress tolerators’ in the Grime CSR scheme and low Ellenberg N values. Species compositional stability was more strongly correlated with these indices of plant functional types than species richness. Nevertheless, a significant effect of species richness could be identified, even after other causes of variation were accounted for.More stable communities in our study are likely to be resilient to low levels of environmental change, although they may still change, and possibly change dramatically if critical ‘tipping points’ are reached.     

Slug activity density increases seed predation independently of an urban–rural gradient

Post-dispersal seed predation by slugs is increasingly being acknowledged as a potentially important contribution to this ecosystem function. However, specific drivers of predation rates in different landscape contexts are still unknown. We assessed the role of slugs as facultative seed predators along an urban–rural gradient and identified scale-dependent anthropogenic drivers that might affect either slugs or their function as seed predators.We used a combination of seed cafeterias and pitfall traps in grassy areas of eleven schoolyards and five field margins surrounded by different land-use types to assess slug activity density and seed predation rates and quantified urbanization, habitat heterogeneity and microhabitat structure.We show that slugs are important seed predators regardless of anthropogenic land use. Their activity density was a significant predictor for seed predation but increases in woody vegetation and bare soil also increased seed predation. This indicates that the magnitude of seed predation might also be affected by slug foraging activity and feeding behaviour, which might be influenced by habitat features such as the availability of vegetation and bare soil. Our results suggest that not only assessing the activity density (as proxy for relative abundance) of seed predators but also identifying effects on their feeding patterns and behaviour might increase our mechanistic understanding of relationships between land-use changes and seed predation and their impact on cultivated and wild plant species.     

Correlation analysis of Normalized Different Vegetation Index (NDVI) difference series and climate variables in the Xilingole steppe, China from 1983 to 1999

There is a crucial need in the study of global change to understand how terrestrial ecosystems respond to the climate system. It has been demonstrated by many researches that Normalized Different Vegetation Index (NDVI) time series from remotely sensed data, which provide effective information of vegetation conditions on a large scale with highly temporal resolution, have a good relation with meteorological factors. However, few of these studies have taken the cumulative property of NDVI time series into account. In this study, NDVI difference series were proposed to replace the original NDVI time series with NDVI difference series to reappraise the relationship between NDVI and meteorological factors. As a proxy of the vegetation growing process, NDVI difference represents net primary productivity of vegetation at a certain time interval under an environment controlled by certain climatic conditions and other factors. This data replacement is helpful to eliminate the cumulative effect that exist in original NDVI time series, and thus is more appropriate to understand how climate system affects vegetation growth in a short time scale. By using the correlation analysis method, we studied the relationship between NOAA/AVHRR ten-day NDVI difference series and corresponding meteorological data from 1983 to 1999 from 11 meteorological stations located in the Xilingole steppe in Inner Mongolia. The results show that: (1) meteorological factors are found to be more significantly correlation with NDVI difference at the biomass-rising phase than that at the falling phase; (2) the relationship between NDVI difference and climate variables varies with vegetation types and vegetation communities. In a typical steppe dominated by Leymus chinensis, temperature has higher correlation with NDVI difference than precipitation does, and in a typical steppe dominated by Stipa krylovii, the correlation between temperature and NDVI difference is lower than that between precipitation and NDVI difference. In a typical steppe dominated by Stipa grandis, there is no significant difference between the two correlations. Precipitation is the key factor influencing vegetation growth in a desert steppe, and temperature has poor correlation with NDVI difference; (3) the response of NDVI difference to precipitation is fast and almost simultaneous both in a typical steppe and desert steppe, however, mean temperature exhibits a time-lag effect especially in the desert steppe and some typical steppe dominated by Stipa krylovii; (4) the relationship between NDVI difference and temperature is becoming stronger with global warming. __________ Translated from Acta Phytoecologica Sinica, 2005, 29(5): 753–765 [译自: 植物生态学报]     

The drivers of avian‐haemosporidian prevalence in tropical lowland forests of New Guinea in three dimensions

Haemosporidians are among the most common parasites of birds and often negatively impact host fitness. A multitude of biotic and abiotic factors influence these associations, but the magnitude of these factors can differ by spatial scales (i.e., local, regional and global). Consequently, to better understand global and regional drivers of avian‐haemosporidian associations, it is key to investigate these associations at smaller (local) spatial scales. Thus, here, we explore the effect of abiotic variables (e.g., temperature, forest structure, and anthropogenic disturbances) on haemosporidian prevalence and host–parasite networks on a horizontal spatial scale, comparing four fragmented forests and five localities within a continuous forest in Papua New Guinea. Additionally, we investigate if prevalence and host–parasite networks differ between the canopy and the understory (vertical stratification) in one forest patch. We found that the majority of Haemosporidian infections were caused by the genus Haemoproteus and that avian‐haemosporidian networks were more specialized in continuous forests. At the community level, only forest greenness was negatively associated with Haemoproteus infections, while the effects of abiotic variables on parasite prevalence differed between bird species. Haemoproteus prevalence levels were significantly higher in the canopy, and an opposite trend was observed for Plasmodium. This implies that birds experience distinct parasite pressures depending on the stratum they inhabit, likely driven by vector community differences. These three‐dimensional spatial analyses of avian‐haemosporidians at horizontal and vertical scales suggest that the effect of abiotic variables on haemosporidian infections are species specific, so that factors influencing community‐level infections are primarily driven by host community composition.     

Regional heterogeneities in the production of uric acid from adenosine in the bivascularly perfused rat liver

The heterogeneity of the liver parenchyma in relation to uric acid production from adenosine was investigated using the bivascularly perfused rat liver in the anterograde and retrograde modes. Adenosine was infused in livers from fed rats during 20 min at four different concentrations (20, 50, 100 and 200 M) according to four experimental protocols as follows: (A) anterograde perfusion, with adenosine infusion into the portal vein; (B) anterograde perfusion, with adenosine in the hepatic artery, (C) retrograde perfusion, with adenosine in the hepatic vein; (D) retrograde perfusion, with adenosine in the hepatic artery. With protocols A, B, and D uric acid production from adenosine was always characterized by initial bursts followed by progressive decreases toward smaller steady-states. With protocol C the initial burst was present only when 200 M adenosine was infused. The initial bursts in uric acid production were accompanied by simultaneous increases in the ratio of uric acid production/adenosine uptake rate. These initial bursts are thus representing increments in the production of uric acid that are not corresponded by similar increments in the metabolic uptake rates of adenosine. Global analysis of uric acid production revealed that the final steady-state rates were approximately equal for all infusion rates with protocols A, B and C, but smaller with protocol D. This difference, however, can be explained in terms of the differences in accessible cellular spaces, which are much smaller when protocol D is employed. When the analysis was performed in terms of the extra amounts of uric acid produced during the infusion of adenosine, where the initial bursts are also taken into account, different dose-response curves were found for each experimental protocol. These differences cannot be explained in terms of the accessible cell spaces and they are likely to reflect regional heterogeneities. From the various dose-response curves and from the known characteristics of the microcirculation of the rat liver it can be concluded that the initial bursts in uric acid production are generated in periportal hepatocytes. The reason for this heterogeneity could be related to the metabolic effects of adenosine, especially to oxygen uptake inhibition, which is likely to produce changes in the ATP/AMP ratios.     

Spatial and temporal changes in salt marsh distribution in the Dee estuary,NW England,determined from aerial photographs

Vegetation changes in salt marsh communities of the Dee estuary, northwest England, were analysed with a combination of remote sensing techniques using data dating back to the 1950s. The distribution of communities in 1997 was classified using Airborne Thematic Mapper data and used to develop a methodology for the analysis of black and white photographs of the marsh. These methods were then applied retrogressively to a time sequence of monochrome photographs running from 1955 to 1975. At the apex of the salt marshes on the English shore of the Dee estuary, the marsh expanded dramatically to 1975, and consisted predominantly of pioneer and low marsh vegetation types. Between 1975 and 1997, however, there was only a slight increase in salt marsh area, but with an increase in mid and high marsh vegetation, replacing pioneer marsh. In a second area of the salt marsh on the English shore, a different pattern of salt marsh expansion was observed. The area occupied by marsh continued to increase right up to 1997, with extensive pioneer vegetation suggesting a process of continuing expansion. However, the pattern of marsh colonisation appeared to be different in 1997 compared to 1975. The significance of the changes in salt marsh distribution within the Dee estuary are discussed in relation to the historical pattern of salt marsh colonisation, the importance of Spartina anglica in the process and the implications for strategic management of the estuarine resources.     

Vegetation of the Lago de Sanabria area (NW Iberia) since the end of the Pleistocene: a palaeoecological reconstruction on the basis of two new pollen sequences

Various pollen sequences from lacustrine deposits close to Lago de Sanabria (NW Iberia) have for several decades been a key source of information for palaeoenvironmental reconstructions of SW Europe, though their interpretation has been the subject of some controversy. Here we present two new pollen sequences obtained from this area, and a new palaeoenvironmental reconstruction of the region. The available pollen data reach back to before 18,000 b.p., a period of very harsh climate with seasonal (non continuous) sedimentation and a landscape characterised by herbaceous formations dominated by Gramineae and Artemisia, and scrub formations dominated by Ericaceae and Cistaceae. Subsequently sedimentation became continuous, and various regional forest expansions are apparent. At a local level, the first forest expansion began about 12,000 b.p., when Betula pollen reached 70% followed by peaks in Pinus sylvestris-type (>80%) and Quercus robur-type (40%). The Younger Dryas saw a retreat of woodland formations in the area around the lake, with broadleaved deciduous woodland (largely oak) retreating at mid and low altitudes, but with pine woodland persisting in more sheltered sites. The climatic improvement in the Early Holocene favoured re-expansion of woodland, dominated by Pinus sylvestris-type at higher and Quercus robur and Q. pyrenaica at lower altitudes, until anthropogenic deforestation commenced around 4,000 b.p. The disappearance of natural pine woodlands in this region is probably largely attributable to human interference.     

Relative performance of clumped vs. experimentally isolated plants in a South African winter-rainfall desert community

We conducted a factorial removal experiment in the arid strandveld onthe west coast of South Africa to test the hypothesis that perennial specieswith a preference for occurring in multi-species clumps shouldderive benefits from their association into clumps. Contrary to our hypothesis,we obtained evidence of competition for water in the clumped non-succulentspecies studied in the form of depressed water potentials. We were not able todetect any effect on leaf water contents associated with isolation, suggestingthat clumped plants are able to compensate physiologically in response tocompetitive stress. We also found that isolating individuals had no effect(positive or negative) on branch growth. Finally, we showed that isolatingindividuals exposed them to a far greater risk of damage by wind or animals. Inlight of these results we conclude that the spatial arrangement of plants inthis community does suggest a situation where the benefits associated withoccurring in clumps exceed any competitive costs.     

A study on relations of vegetation,climate and soils in Shanxi province,China

Relationships of vegetation, climate and soils in Shanxi plateau wereanalyzed by use of Canonical Correspondence Analysis (CCA). Shanxi province,located at 34°35–40°43 N, 110°15–114°33 E, was divided into a series of rectangular districts of30 latitude by 20 longitude. Areas of vegetation formations and soil types ineach district were measured carefully using fine grain on the vegetation andsoil maps of Shanxi. Climatic data were mean values of 25 years records in eachdistrict. Three data matrices of climate, vegetation and soil were combined byCCA. The results showed that the distribution of vegetation is closely relatedto the variety of climates and to soils distribution.