Dendrochronological assessment of the time since death of dead wood in an old growth Magellan's beech forest,Navarino Island (Chile)

The present study investigated the relationship between time since death and the morphological characteristics of fallen dead trees in a Nothofagus betuloides forest stand located on the island of Navarino (Chile). In this unmanaged forest, there were 399 m³ ha^?1 of dead wood, which represented about half of the living tree volume. At the investigation site, 18 living trees were selected and increment cores were collected from them to build master ring‐width chronologies. Cross sections were also collected from 48 fallen dead trees. The samples collected were then assigned to observable decay classes and their death date was determined dendrochronologically. Cross‐dating techniques were used and it was found that the fallen dead trees cross‐dated significantly with standard chronologies. A year of death was successfully determined for 75% of the sampled fallen dead trees. However, this study demonstrated that, in the standard classification, the transition rate from one class of decay to another was highly variable. Furthermore, the inconsistencies found in the decay rates of the fallen dead trees demonstrated that the existing decay classification schemes were unsuitable for this type of forest stand and that the relationship between qualitatively assessed decay classes and the time since death of trees in this extreme environment was rather weak. In addition, the analysis of the time since death, in this old growth forest, was indicative of the persistence of dead wood on the forest floor in austral cold ecosystems and of its contribution to long‐term carbon storage.     

Understorey environments influence functional diversity in tank‐bromeliad ecosystems

1. A substantial fraction of the freshwater available in neotropical forests is impounded within the rosettes of bromeliads that form aquatic islands in a terrestrial matrix. The ecosystem functioning of bromeliads is known to be influenced by the composition of the contained community but it is not clear whether bromeliad food webs remain functionally similar against a background of variation in the understorey environment. 2. We considered a broad range of environmental conditions, including incident light and incoming litter, and quantified the distribution of a very wide range of freshwater organisms (from viruses to macroinvertebrates) to determine the factors that influence the functional structure of bromeliad food webs in samples taken from 171 tank‐bromeliads. 3. We observed a gradient of detritus‐based to algal‐based food webs from the understorey to the overstorey. Algae, rotifers and collector and predatory invertebrates dominated bromeliad food webs in exposed areas, whereas filter‐feeding insects had their highest densities in shaded forest areas. Viruses, bacteria and fungi showed no clear density patterns. Detritus decomposition is mainly due to microbial activity in understorey bromeliads where filter feeders are the main consumers of microbial and particulate organic matter (POM). Algal biomass may exceed bacterial biomass in sun‐exposed bromeliads where amounts of detritus were lower but functional diversity was highest. 4. Our results provide evidence that tank‐bromeliads, which grow in a broad range of ecological conditions, promote aquatic food web diversity in neotropical forests. Moreover, although bromeliad ecosystems have been categorised as detritus‐based systems in the literature, we show that algal production can support a non‐detrital food web in these systems.     

Low specificity and nested subset structure characterize mycorrhizal associations in five closely related species of the genus Orchis

Most orchid species rely on mycorrhizae to complete their life cycle. Despite a growing body of literature identifying orchid mycorrhizal associations, the nature and specificity of the association between orchid species and mycorrhizal fungi remains largely an open question. Nonetheless, better insights into these obligate plant–fungus associations are indispensable for understanding the biology and conservation of orchid populations. To investigate orchid mycorrhizal associations in five species of the genus Orchis (O. anthropophora, O. mascula, O. militaris, O. purpurea, and O. simia), we developed internal transcribed spacer‐based DNA arrays from extensive clone library sequence data sets, enabling rapid and simultaneous detection of a wide range of basidiomycetous mycorrhizal fungi. A low degree of specificity was observed, with two orchid species associating with nine different fungal partners. Phylogenetic analysis revealed that the majority of Orchis mycorrhizal fungi are members of the Tulasnellaceae, but in some plants, members of the Thelephoraceae, Cortinariaceae and Ceratobasidiaceae were also found. In all species except one (O. mascula), individual plants associated with more than one fungus simultaneously, and in some cases, associations with ≥3 mycorrhizal fungi at the same time were identified. Nestedness analysis showed that orchid mycorrhizal associations were significantly nested, suggesting asymmetric specialization and a dense core of interactions created by symmetric interactions between generalist species. Our results add support to the growing literature that multiple associations may be common among orchids. Low specificity or preference for a widespread fungal symbiont may partly explain the wide distribution of the investigated species.     

Isolation of 12 polymorphic microsatellite loci in the phytopathogenic fungus Alternaria brassicicola

Twelve polymorphic microsatellite markers were isolated from the phytopathogenic fungus Alternaria brassicicola, the causal agent of black spot of crucifers. An enrichment protocol was used to isolate microsatellite loci, which were then analysed in a collection of 46 isolates sampled from seven different countries. The number of alleles detected in 12 loci ranged from two to 10 (mean 3.5). Investigation of cross‐species amplifications showed that the designed primers were specific to A. brassicicola.     

Morphological divergence patterns among populations of Poecilia vivipara (Teleostei Poeciliidae): test of an ecomorphological paradigm

The structure of body size and shape divergence among populations of Poecilia vivipara inhabiting quaternary lagoons in South-eastern Brazil was studied. This species is abundant throughout an environmental gradient formed by water salinity differences. The salinity gradient influences the habitat structure (presence of macrophytes) and the fish community (presence of large predators). Size and shape variation within and among populations was quantified by geometric morphometrics and analysed by indirect and direct gradient ordinations, using salinity and geography as a framework. Morphological divergence was associated with the salinity gradient. The evolutionary allometries observed were independent of within-group static allometries. Sexually dimorphic patterns were observed in size variation and within-population allometries. Specimens from freshwater (higher predation) sites presented smaller sizes, relatively longer caudal regions, lower anterior regions and a ventrally displaced eye. These features are consistent with an ecomorphological paradigm for aquatic organisms from populations subject to intense predation. A process of directional selection is postulated as the most likely force driving diversification among P. vivipara populations.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 799–812.