Snow depth, soil freezing, and fluxes of carbon dioxide, nitrous oxide and methane in a northern hardwood forest

Soil–atmosphere fluxes of trace gases (especially nitrous oxide (N₂O)) can be significant during winter and at snowmelt. We investigated the effects of decreases in snow cover on soil freezing and trace gas fluxes at the Hubbard Brook Experimental Forest, a northern hardwood forest in New Hampshire, USA. We manipulated snow depth by shoveling to induce soil freezing, and measured fluxes of N₂O, methane (CH₄) and carbon dioxide (CO₂) in field chambers monthly (bi-weekly at snowmelt) in stands dominated by sugar maple or yellow birch. The snow manipulation and measurements were carried out in two winters (1997/1998 and 1998/1999) and measurements continued through 2000. Fluxes of CO₂ and CH₄ showed a strong seasonal pattern, with low rates in winter, but N₂O fluxes did not show strong seasonal variation. The snow manipulation induced soil freezing, increased N₂O flux and decreased CH₄ uptake in both treatment years, especially during winter. Annual N₂O fluxes in sugar maple treatment plots were 207 and 99 mg N m⁻² yr⁻¹ in 1998 and 1999 vs. 105 and 42 in reference plots. Tree species had no effect on N₂O or CO₂ fluxes, but CH₄ uptake was higher in plots dominated by yellow birch than in plots dominated by sugar maple. Our results suggest that winter fluxes of N₂O are important and that winter climate change that decreases snow cover will increase soil:atmosphere N₂O fluxes from northern hardwood forests.     

Microsatellites identify depredated waterfowl remains from glaucous gull stomachs

Prey remains can provide valuable sources of information regarding causes of predation and the species composition of a predator's diet. Unfortunately, the highly degraded state of many prey samples from gastrointestinal tracts often precludes unambiguous identification. We describe a procedure by which PCR amplification of taxonomically informative microsatellite loci were used to identify species of waterfowl predated by glaucous gulls ( Larus hyperboreus ). We found that one microsatellite locus unambiguously distinguished between species of the subfamily Anserinae (whistling ducks, geese and swans) and those of the subfamily Anatidae (all other ducks). An additional locus distinguished the remains of all geese and swan species known to nest on the Yukon–Kuskokwim delta in western Alaska. The study focused on two waterfowl species which have experienced precipitous declines in population numbers: emperor geese ( Chen canagica ) and spectacled eiders ( Somateria fischeri ). No evidence of predation on spectacled eiders was observed. Twenty-six percent of all glaucous gull stomachs examined contained the remains of juvenile emperor geese.     

Collembola foraging responses to interacting fungi

Abstract 1. Temperate woodland Collembola feed predominantly on plant detritus and fungi. They exhibit preferences for different fungi reflecting resource value in terms of Collembola growth and reproduction. 2. Studies of fungal grazing by Collembola have focused on non‐interacting fungi and have used one genetic isolate as representative of an entire fungal species. This study examines the effects of fungal interactions on Collembola behaviour, and elucidates differences arising from the use of genetically distinct fungal isolates. 3. Folsomia candida were added to microcosms in which paired combinations of the fungi, Hypholoma fasciculare (four isolates), Phallus impudicus, Phanerochetae velutina and Resinicium bicolor (two isolates), were interacting. Collembola movement and mortality was recorded for 26 days. A constant preference for one fungal mycelium over the other was shown by Collembola in 12 of the 36 interactions studied. Lowest Collembola mortality was normally recorded on the preferred mycelium. 4. In 11 of the remaining 24 interactions, Collembola preference switched from one mycelium to the other; the greatest number of dead Collembola was recorded on the final preference. 5. Collembola behaviour on different fungal isolates was broadly similar, although in one species’ combination a change in fungal isolate resulted in a total reversal of Collembola preference. The implications of these results for decomposer food webs are considered.     

The demography of coarse wood in north temperate lakes

1. Exchange of material across habitat boundaries is a key process in riparian zones. The movement of coarse wood from lakeshore forests to the littoral zone, where it provides habitat for aquatic organisms, is not well understood. In 2003, we resampled coarse wood within the littoral zone of four lakes in Northern Wisconsin (U.S.A.), that had originally been surveyed in 1996, to quantify the spatial arrangement of littoral coarse wood and estimate input, loss and movement rates.
2. All four lakes had a clumped pattern of littoral coarse wood, and the locations of clusters were similar in both census years. Littoral coarse wood was more abundant than expected on moderate to steep slopes, on southern shorelines, and in areas with sparse residential development.
3. All four lakes had a net accumulation of coarse wood; rates of wood input ranged from 0.5 to 1.9 logs km⁻¹ year⁻¹, i.e. there was more wood in 2003 than 1996. Movement rates of tagged logs varied 14-fold among lakes, with a maximum in one lake of 42% of logs recovered in 2003 more than 20 m from their 1996 location. Median distance moved ranged from 22 to 323 m among the four lakes.
4. Areas of persistently high wood density may be keystone habitats whose presence enables the persistence of populations of certain aquatic organisms. Conservation of locations with high wood density may be important to maintain target densities of coarse wood of lakes with human development.     

Novel,panzootic and hybrid genotypes of amphibian chytridiomycosis associated with the bullfrog trade

Global amphibian declines are linked with the presence of specific, highly virulent genotypes of the emerging fungal disease chytridiomycosis caused by Batrachochytrium dendrobatidis (Bd) known as the global panzootic lineage (Bd‐GPL). The global trade in amphibians for human consumption is suspected to have facilitated emergence of the disease, but evidence to support this is largely lacking. Here, we investigated the role the Lithobates catesbeianus (North American bullfrog) trade in spreading Bd genotypes by comparing strains associated with L. catesbeianus to a global panel using 36 sequenced loci from multiple chromosomal regions. Most bullfrogs were infected with Bd‐GPL genotypes, but we also detected novel, highly divergent Bd genotypes (Bd‐Brazil) from a live bullfrog in a US market and from native Brazilian anurans in the Atlantic Forest where bullfrogs are widely farmed. Sexual reproduction was also detected for the first time in Bd in the form of a hybrid genotype between the Bd‐GPL and Bd‐Brazil lineages in the Atlantic Forest. Despite the demonstration that ribosomal RNA types in Bd fail to undergo concerted evolution (over 20 sequence types may be found in a single strain), the Bd‐GPL and Bd‐Brazil lineages form largely separate clusters of related internal transcribed spacer (ITS) RNA sequences. Using ITS sequences, we then demonstrate the presence of Bd‐Brazil in Japan, primarily on invasive L. catesbeianus. The finding that Bd is capable of sexual reproduction between panzootic and endemic genotypes emphasizes the risk of international wildlife trade as a source of additional Bd epizootics owing to hybridization.     

Can long‐distance migratory birds adjust to the advancement of spring by shortening migration distance? The response of the pied flycatcher to latitudinal photoperiodic variation

Many organisms use day length as a cue for synchronizing their life cycles with seasonal changes in environmental productivity. Under rapid climate change, however, responses to day length may become maladaptive, and photo‐responsive organisms may only be able to evade increasingly unsuitable habitats if they can accommodate to a wide range of photoperiodic conditions. A previous experiment showed that the pied flycatcher, Ficedula hypoleuca, a Palaearctic‐Afrotropical migratory bird, would strongly advance the timing of spring migration and reproductive maturation if it shifted its wintering area from sub‐Saharan Africa to the Mediterranean region. However, it is unknown whether this marked response to latitudinal variation in photoperiodic conditions is continuous over the entire range of potential wintering areas, and if a shortening of migration distance would be an effective mechanism to adjust the timing of migration to rapidly changing climatic conditions. Here, we experimentally show that a moderate northward displacement of the pied flycatcher's current wintering grounds by 10° would result in a clear advancement of the termination of prenuptial moult and the initiation of spring migratory activity and gonadal growth. However, we found no further advancement under conditions simulating higher wintering latitudes, suggesting the existence of a critical photoperiodic threshold or a steep gradual response within a narrow geographical range between 10° and 20° northern latitude. Because habitat conditions in this area are deteriorating rapidly, the potential for pied flycatchers to adjust their life cycle to changing climatic conditions by shortening the migration distance may be limited in the future.     

Topographic and ungulate regulation of soil C turnover in a temperate grassland ecosystem

A major obstacle for predicting the effects of climate and land use changes on global soil carbon (C) stores is the very limited knowledge about the long timescale dynamics of the relatively stable fraction of soil C, which represents the bulk of soil C and the primary determinant of the long‐term C balance of terrestrial ecosystems. In this study, we examined how variable topo‐edaphic conditions and herds of native migratory ungulates influenced turnover of the stable pool (total minus active fraction) of soil C in grasslands of Yellowstone National Park (YNP). Soil C properties were determined for grasslands located inside and outside long‐term ungulate exclosures established 1958–1962 at seven variable topographic positions. Active C pool sizes, estimated with soil laboratory incubations, and soil radiocarbon measures were used to parameterize a process‐based model to determine turnover of the stable C pool at the sites. Stable C turnover ranged 37–653 and 89–869 years for 0–10 and 0–20 cm soils, respectively. Among ungrazed communities, there was a trend for stable soil C turnover to slow along topographic gradients of increasing soil moisture, soil C content, and shoot biomass from hilltop to slope‐bottom positions. This was likely a result of an increasing amount of support tissue resulting in greater concentrations of lignin and cellulose as shoot biomass increased down slope. In contrast, across the grazed landscape, stable C turnover sped up from hilltop to slope‐bottom positions, which was likely a consequence of grazer effects on plant species composition along the topographic gradient. These findings indicated that despite topography playing the primary role in controlling such important site characteristics as soil moisture, soil C content, and plant production in YNP grassland, the long‐term turnover of the stable C pool was determined by herbivores. The results demonstrate the important regulatory role of herbivores in controlling the C balance of this semiarid grassland ecosystem.