Session 17 Ecophysiology

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Evolutionary changes in correlations among functional traits in <i>Ceanothus</i> in response to Mediterranean conditions

<i>Ceanothus</i> is a widely speciated genus with two sections that differ markedly in size, patterns of biomass allocation and reproductive strategies. On greenhouse-grown seedlings we tested whether divergence occurred in single traits or in suites of traits, and whether taxonomic affinity or the ability to fix nitrogen (N) influenced the relationships among functional traits. Species of the subgenus <i>Cerastes</i> differed from species in the subgenus <i>Ceanothus</i> in the following characteristics: reproduction by seed rather than primarily resprouting, high rates of photosynthesis and high stomatal conductance, thick leaves, low root allocation, and high leaf allocation. Correlations of traits across the entire genus showed positive correlations among traits that maximize photosynthesis and tradeoffs between root allocation and carbon gain patterns frequently observed in other broad taxonomic comparisons. Trait correlations differed between the two subgenera (divergence in allocation-photosynthesis tradeoffs in <i>Ceanothus</i> and divergence of growth-related traits in <i>Cerastes</i>). Similarly, N-fixers, which were distributed broadly between the two subgenera, differed in trait correlations (primarily among traits related to photosynthesis) from non-fixers (primarily among traits related to growth and allocation). These results indicate that 1) divergence in genus <i>Ceanothus</i> was associated with changes in entire suite of traits, rather than independent changes in individual traits and 2) evolution occurring under different environmental or nutritional circumstances alters the suites of traits exhibited by plants

Benefactor and allelopathic shrub species have different effects on the soil microbial community along an environmental severity gradient

Patches where shrubs have either positive or negative effects on their understory plant community are common in arid ecosystems. The intensity and balance of these effects change along environmental severity gradients but, despite the major role of soil microbes in plant interactions, little is known about the differences among soil microbial communities under these species and their possible influence on such contrasting shrub effects. We hypothesized that microbial communities associated to benefactor and allelopathic shrubs would differ among them and that differences would increase with environmental severity. To test these hypotheses we characterized soil microbial biomass, activity and community composition under a benefactor shrub species, Retama sphaerocarpa, an allelopathic shrub species, Thymus hyemalis, and in bare soil among plants (gaps) at three sites along an environmental severity gradient. Shrubs promoted an increase in soil bacterial diversity, being bacterial communities associated to benefactor shrubs, allelopathic shrubs and gaps different in composition. Microbial enzymatic activity and biomass increased under shrubs and under more mesic conditions; nonetheless, they were highest under benefactor shrubs at the most arid site and under allelopathic shrubs at the less severe site. Compared to gaps, the presence of shrubs induced changes in microbial activity and community composition that were larger at the most severe site than at the less severe site. Along the gradient, benefactor shrubs enhanced the abundance of bacterial groups involved in organic matter decomposition and N fixation as well as plant pathogens, which could contribute to Retama's outstanding positive effects on understory plant biomass and diversity. Plant patches mitigate the effects of extreme conditions on associated plant and soil microbial communities and promote soil biodiversity and ecosystem functioning in arid ecosystems, with shrubs actively selecting for specific microbial groups in their understory

Linking patterns and processes in alpine plant communities: A global study

Predictable relationships among patterns, processes, and properties of plant communities are crucial for developing meaningful conceptual models in community ecology. We studied such relationships in 18 plant communities spread throughout nine Northern Hemisphere high-mountain subalpine and alpine meadow systems and found linear and curvilinear correlative links among temperature, precipitation, productivity, plant interactions, spatial pattern, and richness. We found that sites with comparatively mild climates have greater plant biomass, and at these sites strong competition corresponds with overdispersed distribution of plants, reducing intraspecific patchiness and in turn increasing local richness. Sites with cold climates have little biomass, and at these sites a high proportion of species benefit from strong facilitative effects of neighbors, leading to an aggregated distribution of plants. Sites with intermediate, or relatively moderate climates are intermediate in biomass, and at these sites interactions are weak (or competition may be counterbalanced by facilitation), corresponding with a nearly random distribution of plants. At these sites species richness is lower than average. We propose that the relationship between interspecific spatial pattern and community richness reflects niche differentiation and/or construction, which allows for the coexistence of more species than would be possible with random, unstructured spatial distributions. Discovering the mechanisms that drive the relationships described here would further link functional and structural components of plant communities and enhance the predictive capability of community ecology

Soil microbial community under a nurse-plant species changes in composition, biomass and activity as the nurse grows

Shrubs in semiarid ecosystems facilitate the establishment of other plant species under their canopies and promote changes in the understory plant communities as they grow. To better understand whether plant community dynamics are linked to changes in soil microbes, we characterized soil microbial communities in gaps (open spaces) and under Retama sphaerocarpa shrubs of three different size classes. Different methodological approaches including pyrosequencing of 16S rDNA, phospholipid-fatty acid (PLFA) analysis and microbial activity indicators were combined to characterize both the structure and function of soil microbial communities in the different treatments. Soil microbial communities under small shrubs showed higher microbial biomass and activity, as well as different relative abundance of several bacterial groups, than communities in gaps. Shrubs were associated with a higher relative abundance of Bacteroidetes, Betaproteobacteria and Gammaproteobacteria in detriment of Actinobacteria and Firmicutes without changes in overall bacterial diversity. Soil microbial community changed with shrub size, being most different in gaps and under the canopy of large shrubs. We suggest that changes in composition and function of soil microbial communities may promote the increase in the understory plant growth and species richness previously reported for this shrub species. Our data emphasize the importance of plant soil interactions in defining the structure and composition of both plant and soil microbial communities and their impact on ecosystem functioning