Temperature and vegetation effects on soil organic carbon quality along a forested mean annual temperature gradient in North America

Both climate and plant species are hypothesized to influence soil organic carbon (SOC) quality, but accurate prediction of how SOC process rates respond to global change will require an improved understanding of how SOC quality varies with mean annual temperature (MAT) and forest type. We investigated SOC quality in paired hardwood and pine stands growing in coarse textured soils located along a 22 °C gradient in MAT. To do this, we conducted 80‐day incubation experiments at 10 and 30 °C to quantify SOC decomposition rates, which we used to kinetically define SOC quality. We used these experiments to test the hypotheses that SOC quality decreases with MAT, and that SOC quality is higher under pine than hardwood tree species. We found that both SOC quantity and quality decreased with increasing MAT. During the 30 °C incubation, temperature sensitivity (Q₁₀) values were strongly and positively related to SOC decomposition rates, indicating that substrate supply can influence temperature responsiveness of SOC decomposition rates. For a limited number of dates, Q₁₀ was negatively related to MAT. Soil chemical properties could not explain observed patterns in soil quality. Soil pH and cation exchange capacity (CEC) both declined with increasing MAT, and soil C quality was positively related to pH but negatively related to CEC. Clay mineralogy of soils also could not explain patterns of SOC quality as complex (2 : 1), high CEC clay minerals occurred in cold climate soils while warm climate soils were dominated by simpler (1 : 1), low CEC clay minerals. While hardwood sites contained more SOC than pine sites, with differences declining with MAT, clay content was also higher in hardwood soils. In contrast, there was no difference in SOC quality between pine and hardwood soils. Overall, these findings indicate that SOC quantity and quality may both decrease in response to global warming, despite long‐term changes in soil chemistry and mineralogy that favor decomposition.     

Future atmospheric CO2 leads to delayed autumnal senescence

Growing seasons are getting longer, a phenomenon partially explained by increasing global temperatures. Recent reports suggest that a strong correlation exists between warming and advances in spring phenology but that a weaker correlation is evident between warming and autumnal events implying that other factors may be influencing the timing of autumnal phenology. Using freely rooted, field‐grown Populus in two Free Air CO₂ Enrichment Experiments (AspenFACE and PopFACE), we present evidence from two continents and over 2 years that increasing atmospheric CO₂ acts directly to delay autumnal leaf coloration and leaf fall. In an atmosphere enriched in CO₂ (by ～45% of the current atmospheric concentration to 550 ppm) the end of season decline in canopy normalized difference vegetation index (NDVI) – a commonly used global index for vegetation greenness – was significantly delayed, indicating a greener autumnal canopy, relative to that in ambient CO₂. This was supported by a significant delay in the decline of autumnal canopy leaf area index in elevated as compared with ambient CO₂, and a significantly smaller decline in end of season leaf chlorophyll content. Leaf level photosynthetic activity and carbon uptake in elevated CO₂ during the senescence period was also enhanced compared with ambient CO₂. The findings reveal a direct effect of rising atmospheric CO₂, independent of temperature in delaying autumnal senescence for Populus, an important deciduous forest tree with implications for forest productivity and adaptation to a future high CO₂ world.     

Spatial distribution of communal nests in a colonial breeding bird: benefits without costs?

Abstract The spatial organization of individuals, or groups of individuals, within a population can provide valuable information about social organization and population dynamics. We analysed the spatial distribution of nests of the sociable weaver (Philetairus socius) on two farms in the Kalahari. Sociable weavers build large communal nests on big savannah trees, forming a pattern of trees with and without nests. We used two spatial statistics, Ripley's K and the pair correlation function, to describe characteristics of the point patterns over a range of distances. (i) At distances of 200 and 300 m, communal nests were clustered. (ii) At distances greater than 1000 m, communal nests were regularly distributed. These findings are independent of the spatial distribution of trees. Furthermore, we used Moran's I to analyse spatial autocorrelation of nest sizes. We expected negative autocorrelation because of competition between nests. But on both farms there was no significant autocorrelation of nest sizes for any distance class. The regular distribution observed at larger distances may indicate competition and/or territoriality among different nests, but the lack of spatial autocorrelation between nest sizes suggests that these interactions may happen between nest clusters rather than between single nests. This was confirmed by significant clustering of nests on small scales. We thus suggest, that colonies of P. socius consist of several nests on adjacent trees forming a cluster of subcolonies. The question why sociable weavers establish subcolonies instead of adding more chambers to the natal nest, could not simply be answered by limitation of nesting space. We suggest a strategy to avoid costs due to increasing colony size.     

Landscape controls of CH4 fluxes in a catchment of the forest tundra ecotone in northern Siberia

Terrestrial ecosystems in northern high latitudes exchange large amounts of methane (CH₄) with the atmosphere. Climate warming could have a great impact on CH₄ exchange, in particular in regions where degradation of permafrost is induced. In order to improve the understanding of the present and future methane dynamics in permafrost regions, we studied CH₄ fluxes of typical landscape structures in a small catchment in the forest tundra ecotone in northern Siberia. Gas fluxes were measured using a closed‐chamber technique from August to November 2003 and from August 2006 to July 2007 on tree‐covered mineral soils with and without permafrost, on a frozen bog plateau, and on a thermokarst pond. For areal integration of the CH₄ fluxes, we combined field observations and classification of functional landscape structures based on a high‐resolution Quickbird satellite image. All mineral soils were net sinks of atmospheric CH₄. The magnitude of annual CH₄ uptake was higher for soils without permafrost (1.19 kg CH₄ ha⁻¹ yr⁻¹) than for soils with permafrost (0.37 kg CH₄ ha⁻¹ yr⁻¹). In well‐drained soils, significant CH₄ uptake occurred even after the onset of ground frost. Bog plateaux, which stored large amounts of frozen organic carbon, were also a net sink of atmospheric CH₄ (0.38 kg CH₄ ha⁻¹ yr⁻¹). Thermokarst ponds, which developed from permafrost collapse in bog plateaux, were hot spots of CH₄ emission (approximately 200 kg CH₄ ha⁻¹ yr⁻¹). Despite the low area coverage of thermokarst ponds (only 2.1% of the total catchment area), emissions from these sites resulted in a mean catchment CH₄ emission of 3.8 kg CH₄ ha⁻¹ yr⁻¹. Export of dissolved CH₄ with stream water was insignificant. The results suggest that mineral soils and bog plateaux in this region will respond differently to increasing temperatures and associated permafrost degradation. Net uptake of atmospheric CH₄ in mineral soils is expected to gradually increase with increasing active layer depth and soil drainage. Changes in bog plateaux will probably be much more rapid and drastic. Permafrost collapse in frozen bog plateaux would result in high CH₄ emissions that act as positive feedback to climate warming.     

Sympatric occurrence of diverged mtDNA lineages of Pityogenes chalcographus (Coleoptera, Scolytinae) in Europe

Differentiation among the European populations of the six-toothed spruce bark beetle Pityogenes chalcographus was observed for the first time in the 1970s as mating males from Northern Europe with females from Central Europe led to a significant decline in fecundity. Morphological examination revealed that P. chalcographus can be separated into two European races. Here, we investigated the genetic background of this separation by analysing 39 populations ( n  = 695), sequencing almost the complete cytochrome oxidase I gene (1543 bp) and applying single-strand conformation polymorphism. Phylogenetic analysis of 58 haplotypes yielded three major clades with a maximum sequence divergence of 2.33%, indicating that the demographic events took place in the late Pleistocene. These results support the hypothesis of allopatric divergence of the mtDNA lineages, which postglacially came into sympatric existence in Europe. However, as a result of partial crossing incompatibility the diverged lineages retained their genetic identity during postglacial times.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 331–340.     

Late Oligocene mesotheriids (Mammalia, Notoungulata) from Salla and Lacayani (Bolivia): implications for basal mesotheriid phylogeny and distribution

The oldest known mesotheriids are referred to the genus Trachytherus , a taxon traditionally included in its own subfamily 'Trachytheriinae'. The ontogeny, morphological variation and systematics of the genus Trachytherus are still poorly understood and the monophyly of this taxon is questionable. For the first time, a large collection of basal mesotheriids (belonging to the genus Trachytherus ) yielded by the Bolivian Salla beds (Deseadan, late Oligocene) and previously referred to T. spegazzinianus was included in a large-scale analysis. This has led to the recognition of a unique new species in the locality of Salla-Luribay, distinct from T. spegazzinianus : Trachytherus alloxus sp. nov . Skulls of this species are described and cheekteeth variation range throughout ontogeny is discussed. These new data are incorporated in a parsimony analysis of basal mesotheriids. The most probable hypothesis of a clade T. spegazzinianus -mesotheriines is discussed. The distinction of the new Salla species from the Patagonian species T. spegazzinianus confirms the faunal differences observed between these Bolivian and Patagonian Deseadan localities. Moreover, the assignment to Trachytherus spegazzinianus of mesotheriid remains from the Bolivian Lacayani deposits complicates the problem of these faunal differences. Further systematic investigations on the Lacayani fauna and on the age of the Patagonian localities are needed.  © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society , 2008, 152 , 153–200.     

Microsatellite loci for population and behavioural studies of Horsfield's bronze‐cuckoo (Chalcites basalis: Aves)

Eight polymorphic microsatellite loci were identified for Horsfield's bronze‐cuckoo (Chalcites basalis). These include seven newly isolated loci from cuckoo genomic libraries enriched for GA and GAAA repeat‐containing clones. These loci have a mean expected heterozygosity of 0.71, a mean number of alleles of 13.8 and a combined exclusion probability (one parent known) of 0.9999. Two loci (Cba01 and Cba07) showed a significant deficiency of heterozygotes and may therefore have null alleles, although this effect could be the result of nonrandom population sampling.