Perspectives on Palaearctic and Nearctic bird migration; comparisons and overview of life-history and ecology of migrant passerines

In this paper we compare ecological attributes of tropical migrant passerines from the Nearctic and western Palaearctic, focusing particularly on habitat association patterns during both breeding and wintering seasons. Three regions were compared: Europe, western and eastern North America. Breeding bird census data from 32 studies (each including at least four stages of forest succession) were used to assess the association patterns of breeding habitats among tropical migrants. For each species we calculated an index of habitat diversity and habitat preference.
Tropical migrants preferred earlier successional stages than other birds in Europe. The opposite was true in eastern North America. In eastern North America, tropical migrants tended to be associated with a smaller range of serai stages than other passerine species. In their winter quarters, Palaearctic migrants live primarily in open habitats, such as savannas, whereas eastern Nearctic migrants make more frequent use of evergreen forests. Migrants from western North America show the greatest match between breeding and wintering habitats.
We relate the results to the taxonomy and probable history of contemporary avifaunas and vegetation formations of the Old and New World. Taxonomically, tropical migrants from different parts of the Holarctic are less closely related to each other than residents and short-distance migrants. Tropical and temperate avifaunas are more closely related to each other in the New World than in the Old World. Conservation implications of the between-continent differences are briefly discussed.     

Parental and genetic factors rather than nesting forest patch size affect physiological traits of offspring in an area‐sensitive passerine: an experimental study

Our understanding of the effects of habitat loss on individual performance is limited due to a lack of experimental studies that take the potential genetic and parental effects producing phenotypic variation into consideration. To assess the relative role of habitat loss on offspring phenotype while controlling for the confounding effects of genetic and parental variation we performed a partial cross‐fostering experiment using the Eurasian Treecreeper Certhia familiaris. The experiment was carried out by swapping half the nestlings in a brood between small and large nesting forest patches to determine the effect of nesting forest patch size on five nestling traits reflecting morphological size, body condition, physiological stress and inflammation status. There was no effect of nesting forest patch size on the offspring traits examined. Instead, we found evidence of genetic and early parental effects on all traits except inflammation status, as well as parental effects after cross‐fostering for all of the measured offspring traits. Our results suggest that genetic and parental effects should be taken into account when making inferences about species’ responses to habitat loss.     

Tracing carbon and oxygen isotope signals from newly assimilated sugars in the leaves to the tree-ring archive

The analysis of δ ¹³C and δ ¹⁸O in tree-ring archives offers retrospective insights into environmental conditions and ecophysiological processes. While photosynthetic carbon isotope discrimination and evaporative oxygen isotope enrichment are well understood, we lack information on how the isotope signal is altered by downstream metabolic processes.
In Pinus sylvestris , we traced the isotopic signals from their origin in the leaf water ( δ ¹⁸O) or the newly assimilated carbon ( δ ¹³C), via phloem sugars to the tree-ring, over a time-scale that ranges from hours to a growing season.
Seasonally, variable ¹³C enrichment of sugars related to phloem loading and transport did lead to uncoupling between δ ¹³C in the tree-ring, and the c _i/ c _a ratio at the leaf level. In contrast, the oxygen isotope signal was transferred from the leaf water to the tree-ring with an expected enrichment of 27‰, with time-lags of approximately 2 weeks and with a 40% exchange between organic oxygen and xylem water oxygen during cellulose synthesis.
This integrated overview of the fate of carbon and oxygen isotope signals within the model tree species P. sylvestris provides a novel physiological basis for the interpretation of δ ¹³C and δ ¹⁸O in tree-ring ecology.     

Size dependence of composition, photosynthesis and growth in the colony-forming freshwater ciliate, Ophrydium versatile

1. Ophrydium versatile is a symbiotic ciliate which forms gelatinous colonies up to several centimetres in diameter in transparent temperate lakes. The ciliates are evenly spaced at the colony surface and constitute a small proportion of the surface area (7%) and volume (3.1%) of the colony, but a large proportion of organic carbon (74%) and nitrogen content (82%) (exemplified for 1 cm³ colonies). The majority of the colony volume is formed by the jelly. The biomass proportion of ciliates scales inversely with colony size, following the decline of surface area to colony volume. The largest colonies found in Danish lakes in early summer contain almost 1 million ciliates, and assuming they derive from a single ciliate undergoing exponential division, they need twenty generations and, presumably, almost a year to reach maximum size. 2. The ciliates contain numerous symbiotic zoochlorellae that constitute about 10% of ciliate volume and more than half of the carbon content. Zoochlorellae dominate oxygen metabolism of the assemblage, resulting in low light compensation points, a large diel photosynthetic surplus, and a marked dependence on light for sustained growth and ciliate metabolism. Estimated gross photosynthesis (7ng C ciliate^?1 day^?1) of Ophnrydium from shallow, clear waters in June greatly exceeded the estimated carbon contained in filtered bacteria and small algae (1.9ng C cilicate^?1 day^?1). Nitrogen and phosphorus content of the prey, however, may provide the main nutrient source consistent with the correspondence between mass-specific rates of nutrient uptake and measured relative growth rates (average 0.067 day^?1, generation time 10 days). 3. The large Ophrydium colonies require increased allocation of photosynthetic carbohydrates with increasing colony size to maintain the jelly. The large colonies tend to become gas-filled, floating, mechanically destroyed and their ciliate inhabitants abandon them as swarmers. Colony formation, however, should offer protection against predators which may be more important for the natural abundance than the costs of growing in a colony.     

Annual cyclicity in high-resolution stable carbon and oxygen isotope ratios in the wood of the mangrove tree Rhizophora mucronata

In the present study, the high‐resolution stable carbon (¹³C/¹²C) and oxygen (¹⁸O/¹⁶O) isotope ratio profiles in the wood of the mangrove Rhizophora mucronata Lam., a tropical tree species lacking distinct growth rings, were investigated. Variations of both isotope ratios revealed a remarkable annual cyclicity with lowest values occurring at the latewood/earlywood boundary (April–May) and highest values during the transition from earlywood to latewood (October–November). Based on the current knowledge of the physiology of this mangrove species, as well as on the current literature available on high‐resolution profiles of stable isotope ratios in tree rings, possible driving forces responsible for this seasonal pattern are discussed. The annual cyclicity, together with a conspicuous isotope pattern appearing in the El‐Niño year 1997, promises great potential for tropical dendrochronology.     

Long tree‐ring chronologies reveal 20th century increases in water‐use efficiency but no enhancement of tree growth at five Iberian pine forests

We investigated the tree growth and physiological response of five pine forest stands in relation to changes in atmospheric CO₂ concentration (c_a) and climate in the Iberian Peninsula using annually resolved width and δ¹³C tree‐ring chronologies since ad 1600. ¹³C discrimination (Δ≈c_i/c_a), leaf intercellular CO₂ concentration (c_i) and intrinsic water‐use efficiency (iWUE) were inferred from δ¹³C values. The most pronounced changes were observed during the second half of the 20th century, and differed between stands. Three sites kept a constant c_i/c_a ratio, leading to significant c_i and iWUE increases (active response to c_a); whereas a significant increase in c_i/c_a resulted in the lowest iWUE increase of all stands at a relict Pinus uncinata forest site (passive response to c_a). A significant decrease in c_i/c_a led to the greatest iWUE improvement at the northwestern site. We tested the climatic signal strength registered in the δ¹³C series after removing the low‐frequency trends due to the physiological responses to increasing c_a. We found stronger correlations with temperature during the growing season, demonstrating that the physiological response to c_a changes modulated δ¹³C and masked the climate signal. Since 1970 higher δ¹³C values revealed iWUE improvements at all the sites exceeding values expected by an active response to the c_a increase alone. These patterns were related to upward trends in temperatures, indicating that other factors are reinforcing stomatal closure in these forests. Narrower rings during the second half of the 20th century than in previous centuries were observed at four sites and after 1970 at all sites, providing no evidence for a possible CO₂‘fertilization’ effect on growth. The iWUE improvements found for all the forests, reflecting both a c_a rise and warmer conditions, seem to be insufficient to compensate for the negative effects of the increasing water limitation on growth.     

Beyond CO2-fixation by Rubisco – an interpretation of 13C/12C variations in tree rings from novel intra-seasonal studies on broad-leaf trees

Evidence is presented for a very specific, seasonally recurring tri‐phase carbon isotope pattern in tree rings of broad‐leaf deciduous tree species. It is derived from highly resolved intra‐annual measurements of ¹³C/¹²C ratios of wood and cellulose from tree rings of Fagus sylvatica, Populus nigra, Quercus petraea and Morus alba. Investigations on δ¹³C from buds and leaves of Fagus sylvatica revealed a similar tri‐phase δ¹³C pattern. At the very beginning of a growing season, the δ¹³C trend of tree rings and foliage shows a marked increase of up to 5‰. The maximum δ¹³C‐value of each vegetation period always occurs in young heterotrophic leaves shortly after bud burst and persistently in the early wood of each tree ring, when growth depends on carbon reserves. Thereafter, δ¹³C profiles represent the autotrophic stage of the leaves, which show different patterns of variation, by and large characterized by a decline. The minimum δ¹³C‐value always shows up in the late wood of each tree ring. At the very end of each tree ring δ¹³C‐values start rising again. This increase in δ¹³C marks the gradual switch‐over to storage‐dependent growth and can also be observed in senescent leaves. Seasonal changes of more than 4‰ were measured, whereas contiguous δ¹³C values rarely differed from each other by more than 0.3‰. This tri‐phase pattern cannot be explained by the common model of carbon isotope fractionation during photosynthesis. It appears to be primarily an indication of seasonal changes in down‐stream processes of the carbohydrate metabolism. Environmental influences on the carbon isotope fractionation during photosynthesis are presumably of secondary importance and expressed by certain peculiarities showing up during the autotrophic phase, i.e. the mid‐section of the seasonal δ¹³C pattern.     

Fusarium culmorum Infection of Barley Seedlings: Correlation between Aggressiveness and Deoxynivalenol Content

Fusarium culmorum is a serious plant pathogen, especially on cereals. The production of deoxynivalenol (DON) by F. culmorum is believed to play a role in pathogenesis. This relationship has been almost exclusively studied in connection with head blight. The present paper reports the first finding of DON in cereal seedlings infected with F. culmorum . A pathogenicity test was performed, including 70 isolates of this pathogen from different sites within northern and central Europe. All isolates caused disease on barley seedlings. For 15 isolates with varying aggressiveness, the DON content in the 19-day-old-barley seedlings was determined. There was a significant correlation between DON concentration and disease index. The aggressiveness of two outlying isolates with very low DON production is discussed. The results indicate that for F. culmorum isolates of the DON chemotype, production of this toxin influences the aggressiveness of the isolates towards barley seedlings.