Drivers of habitat partitioning among three Quercus species along a hydrologic gradient

A critical process that allows multiple, similar species to coexist in an ecological community is their ability to partition local habitat gradients. The mechanisms that underlie this separation at local scales may include niche differences associated with their biogeographic history, differences in ecological function associated with the degree of shared ancestry and trait-based performance differences, which may be related to spatial or temporal variation in habitat. In this study we measured traits related to water-use, growth and stress tolerance in mature trees and seedlings of three oak species (Quercus alba L., Quercus falcata Michx. and Quercus palustris Münchh). which co-occur in temperate forests across the eastern USA but tend to be found in contrasting hydrologic environments. The three species showed significant differences in their local distributions along a hydrologic gradient. We tested three possible mechanisms that influence their contrasting local environmental distributions and promote their long-term co-existence: (i) differences in their climatic distributions across a broad geographic range, (ii) differences in functional traits related to water use, drought tolerance and growth and (iii) contrasting responses to temporal variation in water availability. We identified key differences between the species in both their range-wide climatic distributions (especially aridity index and mean annual temperature) and physiological traits in mature trees and seedlings, including daily water loss, hydraulic conductance, stress responses, growth rate and biomass allocation. Taken together, these differences explain the habitat partitioning that allows three closely related species to co-occur locally.

Contrasting dynamics and environmental controls of dispersed bacteria along a hydrologic gradient

Freshwater bacterioplankton communities are influenced by the transport of bacteria from the surrounding terrestrial environments. It has been shown that, although most of these dispersed bacteria gradually disappear along the hydrologic continuum, some can thrive in aquatic systems and become dominant, leading to a gradual succession of communities. Here we aimed at exploring the environmental factors driving the structure of such contrasting bacterial populations as well as their functional properties. Using Illumina sequencing of the 16S rRNA gene, we characterized the taxonomic composition of bacterioplankton communities from 10 streams and rivers in Québec spanning the whole hydrologic continuum (river Strahler order 0 to 7), which were sampled in two occasions. With the aim to understand the fate and controls of the transported bacteria, among the taxa present at the origin of the hydrologic gradient (i.e., in the smallest headwater streams) we identified two types of dynamics: i) ‘Tourist’ taxa, which were those that decreased in abundance from the headwaters towards the largest rivers, and ii) ‘Seed’ taxa, those that increased their abundances along the hydrologic continuum. Communities changed gradually from the fast-flowing headwater streams dominated by ‘Tourist’ taxa (ca. 95% of the sequences) towards the largest rivers (Strahler order 4-7) where ‘Seed’ taxa comprised up to 80% of community sequences. Variation in taxonomic composition of the communities dominated by ‘Tourist’ taxa in streams seemed related to different degree of terrestrial inputs, whereas compositional changes in ‘Seed’ communities in the large rivers were linked to differences in autochthonous processes. Finally, the two types of communities differed significantly in their metabolic potential assessed through Biolog Ecoplates. All this suggests that hydrologic transport modulates the gradual replacement of two contrasting population types subjected to different environmental controls and with different metabolic potentials. Moreover, we show that the separate exploration of the two pools of taxa allows unveiling environmental drivers and processes operating on them that remain hidden if explored at the whole community level.

Kaltbach Cave (Siebenhengste, Switzerland): Phantom of the Sandstone?

Jama Kaltbach je razvita v Hochgantskem eocenskem peščenjaku na območju Siebenghengste v Švici. Rezultat ponovne izmere jame je precej večja dolžina, nov načrt jame in številna opažanja, ki kažejo na to, da jama ni nastala z »normalno speleogenezo«, ampak je t.i. fantomska jama. Fantomske jame nastajajo pri delnem preperevanju nečistega apnenca v toplih klimah, v pogojih nizkega hidravličnega gradienta. Ob povečanju gradienta, voda izpere netopni preostanek, pri čemer nastane jama. Članek obravnava geomorfološke značilnosti jam, ki nam omogočajo prepoznavati fantomske jame. Kaltbach cave is developed within the Eocene Hohgant sandstone in the Siebenhengste area in Switzerland. A remapping project of the cave resulted in a huge increase in length. It also produced a complete, updated map and longitudinal section. The cave’s morphology does not fitwiththe“normal”speleogenesis:itisaso-calledphantom cave. Phantoms are created by differential weathering of impure limestone under a preferably warm climate and a very low hydrologic gradient. Once the gradient steepens, the undissolved residual sediments are piped out; the “cave” manifests itself. The paper discusses the geomorphological features that permit to recognize the phantom caves.