The new galaxy evolution paradigm revealed by the Herschel surveys

The Herschel Space Observatory has revealed a very different galaxyscape from that shown by optical surveys, which presents a challenge for galaxy-evolution models. The Herschel surveys reveal (1) that there was rapid galaxy evolution in the very recent past and (2) that galaxies lie on a a single Galaxy Sequence (GS) rather than a star-forming ‘main sequence’ and a separate region of ‘passive’ or ‘red-and-dead’ galaxies. The form of the GS is now clearer because far-infrared surveys such as the Herschel ATLAS pick up a population of optically-red star-forming galaxies that would have been classified as passive using most optical criteria. The space-density of this population is at least as high as the traditional star-forming population. By stacking spectra of H-ATLAS galaxies over the redshift range 0.001 < z < 0.4, we show that the galaxies responsible for the rapid low-redshift evolution have high stellar masses, high star-formation rates but, even several billion years in the past, old stellar populations— they are thus likely to be relatively recent ancestors of early-type galaxies in the Universe today. The form of the GS is inconsistent with rapid quenching models and neither the analytic bathtub model nor the hydrodynamical EAGLE simulation can reproduce the rapid cosmic evolution. We propose a new gentler model of galaxy evolution that can explain the new Herschel results and other key properties of the galaxy population

Data from: Rapid increases in forest understory diversity and productivity following a mountain pine beetle (Dendroctonus ponderosae) outbreak in pine forests

The current unprecedented outbreak of mountain pine beetle (Dendroctonus ponderosae) in lodgepole pine (Pinus contorta) forests of western Canada has resulted in a landscape consisting of a mosaic of forest stands at different stages of mortality. Within forest stands, understory communities are the reservoir of the majority of plant species diversity and influence the composition of future forests in response to disturbance. Although changes to stand composition following beetle outbreaks are well documented, information on immediate responses of forest understory plant communities is limited. The objective of this study was to examine the effects of D. ponderosae-induced tree mortality on initial changes in diversity and productivity of understory plant communities. We established a total of 110 1-m2 plots across eleven mature lodgepole pine forests to measure changes in understory diversity and productivity as a function of tree mortality and below ground resource availability across multiple years. Overall, understory community diversity and productivity increased across the gradient of increased tree mortality. Richness of herbaceous perennials increased with tree mortality as well as soil moisture and nutrient levels. In contrast, the diversity of woody perennials did not change across the gradient of tree mortality. Understory vegetation, namely herbaceous perennials, showed an immediate response to improved growing conditions caused by increases in tree mortality. How this increased pulse in understory richness and productivity affects future forest trajectories in a novel system is unknown

pec_et_al_understory_diversity_environment

pec_et_al_understory_diversity_environmen

Ranked relative importance of variables associated with evenness of herbaceous and woody understory showing model estimates of slope and variance.

<p>The most likely explanatory variables are shown in bold (based on model-average estimate being different from zero when the confidence interval excludes zero).</p><p>Ranked relative importance of variables associated with evenness of herbaceous and woody understory showing model estimates of slope and variance.</p

Ranked relative importance of variables associated with the productivity, richness, and evenness of the understory plant community showing model estimates of slope and variance.

<p>The most likely explanatory variables are shown in bold (based on model-average estimate being different from zero when the confidence interval excludes zero).</p><p>Ranked relative importance of variables associated with the productivity, richness, and evenness of the understory plant community showing model estimates of slope and variance.</p

Candidate models used for inference on the productivity, richness and evenness of understory plant community responses to <i>Dendroctonus ponderosae</i>-induced tree mortality, light, soil moisture, and nutrients.

<p><i>Notes</i>: The most likely models (<i>w</i>_i>0.90; difference in evidence ratio>2.7) are shown in <b>bold</b>. AIC_C = Akaike’s Information Criterion corrected, ΔAIC_C = difference between AIC_Ci and AIC_{C best model}, <i>w</i>_i = Akaike weight, and evidence ratio = <i>w</i>_{j best model} / <i>w</i>_i.</p><p>Candidate models used for inference on the productivity, richness and evenness of understory plant community responses to <i>Dendroctonus ponderosae</i>-induced tree mortality, light, soil moisture, and nutrients.</p

Model-averaged coefficients and 95% confidence intervals of <i>Dendroctonus ponderosae</i>-induced tree mortality, percent soil moisture, soil nutrients and light predicting understory plant community biomass in 2012 and 2013.

<p>Model-averaged coefficients and 95% confidence intervals of <i>Dendroctonus ponderosae</i>-induced tree mortality, percent soil moisture, soil nutrients and light predicting understory plant community biomass in 2012 and 2013.</p

Model-averaged coefficients and 95% confidence intervals of <i>Dendroctonus ponderosae</i>-induced tree mortality, percent soil moisture, soil nutrients and light predicting evenness of understory woody perennial species in 2012 and 2013.

<p>Model-averaged coefficients and 95% confidence intervals of <i>Dendroctonus ponderosae</i>-induced tree mortality, percent soil moisture, soil nutrients and light predicting evenness of understory woody perennial species in 2012 and 2013.</p

Ranked relative importance of variables associated with richness of herbaceous and woody understory showing model estimates of slope and variance.

<p>The most likely explanatory variables are shown in bold (based on model-average estimate being different from zero when the confidence interval excludes zero).</p><p>Ranked relative importance of variables associated with richness of herbaceous and woody understory showing model estimates of slope and variance.</p

Candidate models used for inference on the evenness of herbaceous and woody understory response to <i>Dendroctonus ponderosae</i>-induced tree mortality, light, soil moisture, and nutrients.

<p><i>Notes</i>: The most likely models (<i>w</i>_i>0.90; difference in evidence ratio>2.7) are shown in <b>bold</b>. AIC_C = Akaike’s Information Criterion corrected, ΔAIC_C = difference between AIC_Ci and AIC_{C best model}, <i>w</i>_i = Akaike weight, and evidence ratio = <i>w</i>_{j best model} / <i>w</i>_i.</p><p>Candidate models used for inference on the evenness of herbaceous and woody understory response to <i>Dendroctonus ponderosae</i>-induced tree mortality, light, soil moisture, and nutrients.</p