Streams in mediterranean regions have highly seasonal discharge patterns, with predictable torrential floods and severe droughts. In contrast, discharge is less variable in temperate regions and intermittent flow conditions are uncommon. Hydroclimatic models predict that climate change would increase frequency and severity of floods and droughts across Europe, thus increasing the proportion of streams with mediterranean characteristics in actually temperate areas. Correspondingly, understanding actual ecological differences between mediterranean and temperate streams may help to anticipate large-scale ecological impacts of climate change. Given that large-scale factors determine local community composition, we hypothesized that climatic differences between mediterranean and temperate regions should affect the taxonomic and biological trait composition in streams. We assembled the abundance of stream macroinvertebrate genera of 265 sites each from the Mediterranean Basin and from temperate Europe and linked these abundances to published information on 61 categories of 11 biological traits reflecting the potential of resilience from and resistance to disturbances. Although regional taxonomic richness was higher in the mediterranean than in the temperate region, local taxonomic richness and diversity did not significantly differ between regions. Local trait richness and diversity were significantly higher in the mediterranean region. Both local taxonomic and traitcommunity composition differed between regions, but the former varied much more than the latter, highlighting that climate change could produce large changes in the taxonomic but rather weak changes in the trait composition. The mediterranean region was characterized by macroinvertebrates with higher dispersion and colonization capabilities, suggesting that species loss in the temperate region, by extinction or northward emigration of taxa, would be compensated for by immigration of southern mediterranean taxa. Thus, climate change would likely have stronger implications for the local conservation of taxa than for the trait composition of stream macroinvertebrate communities.

Taxonomic and biological trait differences of stream macroinvertebrate communities between mediterranean and temperate regions: implications for future climatic scenarios

Question: Are plant traits more closely correlated with mean annual temperature, or with mean annual precipitation? Location: Global. Methods: We quantified the strength of the relationships between temperature and precipitation and 21 plant traits from 447,961 species-site combinations worldwide. We used meta-analysis to provide an overall answer to our question.

/pdf/which-is-a-better-predictor-of-plant-traits-temperature-or-4ci4es0swl.pdf

Which is a better predictor of plant traits: temperature or precipitation?

Quantifying how functional traits relate to environmental gradients provides insight into mechanisms governing species distributions. Here, we bring together the fi elds of species distribution modelling and functional trait ecology with hierarchical modelling by explicitly incorporating traits into a multi-species distribution model. We combined traits from the leaf-height-seed strategy scheme (specifi c leaf area (SLA), plant height and seed mass) with a distribution model for 20 eucalypt taxa in Victoria, Australia. Th e key insight of this approach is how traits modulate species responses to environmental gradients. Th e strongest link was between SLA and percent rock cover (species with low SLA had positive responses to rockiness, whereas high SLA species responded negatively to rockiness). We found evidence for complex yet potentially important interactions. For instance, the probability of species occurrence increased with rainfall and solar radiation on average yet the response varied depending on species height and SLA. Tall species were predicted to increase with rainfall and solar radiation across the range of SLA values (tall species with low SLA were especially sensitive to rainfall). Short species responded positively to rainfall and solar radiation only if they had low SLA. Th is framework readily accounts for interactions between combinations of traits and environmental variables unlike multi-step approaches. Further application of this concept will contribute to a generalized mechanistic understanding of how traits infl uence species distributions along environmental gradients, with implications for understanding the response of species to global change.

The role of functional traits in species distributions revealed through a hierarchical model

Minimum temperature is assumed to be an important driver of tree species range limits. We investigated during which period of the year trees are most vulnerable to freezing damage and whether the pressure of freezing events increases with increasing elevation. We assessed the course of freezing resistance of buds and leaves from winter to summer at the upper elevational limits of eight deciduous tree species in the Swiss Alps. By reconstructing the spring phenology of these species over the last eight decades using a thermal time model, we linked freezing resistance with long-term minimum temperature data along elevational gradients. Counter-intuitively, the pressure of freeze events does not increase with elevation, but deciduous temperate tree species exhibit a constant safety margin (5-8.5 K) against damage by spring freeze events along elevational gradients, as a result of the later flushing at higher elevation. Absolute minimum temperatures in winter and summer are unlikely to critically injure trees. Our study shows that freezing temperatures in spring are the main selective pressure controlling the timing of flushing, leading to a shorter growing season at higher elevation and potentially driving species distribution limits. Such mechanistic knowledge is important to improve predictions of tree species range limits.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.12452

European deciduous trees exhibit similar safety margins against damage by spring freeze events along elevational gradients.

Aim

Determining the relative influence of niche-based and neutral processes in driving the spatial turnover of community composition is a central challenge in community ecology. Spatial patterns of functional turnover, or functional beta diversity, may capture important signals of niche-based assembly processes, but these patterns have not been quantified for communities across broad geographic and environmental gradients. Here, we analyse continental-scale patterns of species and functional beta diversity in relation to space and the environment to assess the relative importance of niche-based and neutral community assembly mechanisms.



Location

Eastern North America.



Methods

We use a continental-scale forest plot dataset and functional trait data to quantify spatial patterns of species and functional beta diversity. We use redundancy analysis-based variance partitioning to evaluate the influence of space, soil and climate on beta-diversity metrics. We use a null model approach to test for non-random functional beta diversity given the observed patterns of species turnover across spatial scales.



Results

Species and functional beta diversity increased with increasing geographic distance (i.e. distance decay of community similarity). Results of variance partitioning analysis show that species and functional beta diversity were spatially structured and significantly related to environmental, particularly climatic, variation. Results of null model analysis show that functional beta diversity was lower than expected based on species turnover at fine scales (  1800 km).



Main conclusions

The observed patterns of functional beta diversity support a niche-based model of community assembly, driven by the deterministic filtering of species across environmental gradients based on their functional traits.

Functional beta‐diversity patterns reveal deterministic community assembly processes in eastern North American trees

Aim To determine whether latitudinal and longitudinal gradients in seed mass are related to variation in climatic features including temperature, solar radiation and rainfall



Location  Australia



Methods  Seed mass was estimated from over 1600 provenances covering the latitudinal and longitudinal extents of 34 perennial Glycine taxa in Australia Climatic data were obtained from ANUCLIM 51 for collection locations based on long-term meteorological records across Australia These climatic data were subject to principal components analysis to extract three components as climatic indices Generalized linear models were used in three separate sets of analyses to evaluate whether seed mass–latitude and seed mass–longitude relationships persisted after taking climatic variation into account First, relationships were examined across species in analyses that did not explicitly consider phylogenetic relationships Secondly, phylogenetic regressions were performed to examine patterns of correlated evolutionary change throughout the Glycine phylogeny Within-species analysis was also performed to examine consistency across different taxonomic levels



Results  Geographical variation in seed mass among species was related primarily to temperature and solar radiation, while rainfall was much less influential upon seed mass Partialing out the influence of temperature and solar radiation in models resulted in the disappearance of significant seed mass–latitude and seed mass–longitude relationships Patterns within species were generally consistent with patterns among species However, in several species, factors additional to these climatic variables may contribute to the origin and maintenance of geographical gradients in seed mass, as significant seed mass–latitude and seed mass–longitude relationships remained after controlling for the influence of climatic variables



Main conclusions  Our empirical results support the hypotheses that (1) seed mass is larger at low latitudes and in the interior of the Australian continent due to increased metabolic costs at high temperatures, and that (2) higher levels of solar radiation result in an increase in the availability of photosynthate, which in turn leads to an increase in biomass for the production of large seeds In effect, our findings show that greater energy is available precisely where needed, that is, where high temperatures require large seed mass on the basis of metabolic requirements

Geographical gradients in seed mass in relation to climate

In the Northern Hemisphere, a latitudinal gradient of increasing seed size towards the equator has been well documented. Because of a paucity of studies from the Southern Hemisphere, however, the global generality of this latitudinal gradient in seed size is unknown. This study investigated variation in seed size in relation to latitude among and within perennial Glycine species across Australia. Seed size was estimated from over 1500 provenances covering the latitudinal, longitudinal and altitudinal extents of 37 taxa within the subgenus. In order to ensure that any observed latitudinal gradient in seed size existed independently of two other major geographic variables, longitude and altitude, we controlled for their influence via the use of general linear models. Among species, a significant negative relationship emerged between seed size and latitude when latitude was considered on its own and after accounting for the influence of longitude and altitude in models. For Australian populations of the subgenus Glycine, mean species seed size increased by 4.23% with each shift of one degree of latitude towards the equator and increased significantly along an east–west cline across the continent, by 2.25% with each degree of longitude. This latter relationship was obtained both when longitude was considered on its own and after controlling for the influence of latitude and altitude in models. Patterns of seed size variation with latitude and longitude within species mirrored patterns among species, although there were some notable exceptions. Altitude was significantly related to seed size among species only after removing the linear effects of latitude and longitude, when the trend was for lowland species to have smaller seeds. In contrast to the interspecific pattern, some intraspecific negative trends were found within G. canescens and G. cyrtoloba, which accords with Baker's hypothesis for an inverse relation between seed size and altitude. We discuss the relationships between seed size and the three geographic variables in the subgenus Glycine, with respect to environmental factors that could account for the trends detected.

A. H. D. Brown

Papers

Geographical gradients in seed mass in relation to climate

Geographic gradients in seed size among and within perennial Australian Glycine species