Process-evaluation of forest aerosol-cloud-climate feedback shows clear evidence from observations and large uncertainty in models.

Natural aerosol feedbacks are expected to become more important in the future, as anthropogenic aerosol emissions decrease due to air quality policy. One such feedback is initiated by the increase in biogenic volatile organic compound (BVOC) emissions with higher temperatures, leading to higher secondary organic aerosol (SOA) production and a cooling of the surface via impacts on cloud radiative properties. Motivated by the considerable spread in feedback strength in Earth System Models (ESMs), we here use two long-term observational datasets from boreal and tropical forests, together with satellite data, for a process-based evaluation of the BVOC-aerosol-cloud feedback in four ESMs. The model evaluation shows that the weakest modelled feedback estimates can likely be excluded, but highlights compensating errors making it difficult to draw conclusions of the strongest estimates. Overall, the method of evaluating along process chains shows promise in pin-pointing sources of uncertainty and constraining modelled aerosol feedbacks.

Pre-copulatory choices drive post-copulatory decisions: mechanisms of female control shift across different life stages.

BACKGROUND: The 'wallflower' hypothesis proposes females mate indiscriminately to avoid reproductive delays. Post-copulatory mechanisms may then allow 'trading up', favouring paternity of future mates. We tested links between pre- and post-copulatory choice in Latrodectus geometricus female spiders paired sequentially with two males. These females copulate as adults or as subadults and store sperm in paired spermathecae. Choosy adults have a higher risk of delays to reproduction than subadults. RESULTS: We predicted low pre-copulatory, but high post-copulatory choice at first matings for adults and the opposite for subadults. At second matings, we expected all females would prefer males superior to their first. We found all females mated indiscriminately at their first pairing, but in contrast to subadults, adults usually allowed only a single insertion (leaving one of their paired spermatheca empty); a mechanism of post-copulatory choosiness. Adult-mated females were more likely to remate than subadult-mated females when they became adults, showing a preference for larger males, while subadult-mated females tended to prefer males of greater size-corrected mass. CONCLUSIONS: Our results show that the 'wallflower' effect and 'trading up' tactics can be utilized at different life stages, allowing females to employ choice even if rejecting males is costly.

Chemistry-driven changes strongly influence climate forcing from vegetation emissions.

Biogenic volatile organic compounds (BVOCs) affect climate via changes to aerosols, aerosol-cloud interactions (ACI), ozone and methane. BVOCs exhibit dependence on climate (causing a feedback) and land use but there remains uncertainty in their net climatic impact. One factor is the description of BVOC chemistry. Here, using the earth-system model UKESM1, we quantify chemistry's influence by comparing the response to doubling BVOC emissions in the pre-industrial with standard and state-of-science chemistry. The net forcing (feedback) is positive: ozone and methane increases and ACI changes outweigh enhanced aerosol scattering. Contrary to prior studies, the ACI response is driven by cloud droplet number concentration (CDNC) reductions from suppression of gas-phase SO2 oxidation. With state-of-science chemistry the feedback is 43% smaller as lower oxidant depletion yields smaller methane increases and CDNC decreases. This illustrates chemistry's significant influence on BVOC's climatic impact and the more complex pathways by which BVOCs influence climate than currently recognised.

Tree species richness and diversity predicts the magnitude of urban heat island mitigation effects of greenspaces.

The Urban Heat Island Effect (UHIE) is a widely recognised phenomenon that profoundly affects the quality of life for urban citizens. Urban greenspace can help mitigate the UHIE, but the characteristics that determine the extent to which any given greenspace can cool an urban area are not well understood. A key characteristic is likely to be the properties of trees that are found in a greenspace. Here, we explore the sensitivity of the strength of the cooling effect to tree community structure for greenspaces in Changzhou, China. Land surface temperatures were retrieved from Landsat 7 ETM+ and Landsat 8 TIRS and were used to evaluate the temperature drop amplitude (TDA) and cooling range (CR) of 15 greenspaces across each of the four seasons. Tree community structure of the greenspaces was investigated using 156 sample plots across the 15 greenspaces. We found that a number of plant community structure indicators of greenspaces have a significant impact on the strength of the cooling effect. The Shannon-Wiener diversity index, tree species richness and tree canopy coverage of greenspaces are all positively correlated with the magnitude of the temperature drop amplitude, with the strength of their influence varying seasonally. We also find that mean crown width is positively correlated with cooling range in summer and autumn, while greenspace tree density is negatively correlated with cooling range in winter. Our findings improve understanding of the relationship between plant community structure and the cooling effect of greenspaces. In particular, we highlight the important role that tree species diversity provides for mitigating the UHIE, and suggest that if planners wish to improve the role of urban greenspaces in cooling cities, they should include a higher diversity of trees species.

Black widows as plastic wallflowers: female choosiness increases with indicators of high mate availability in a natural population.

Female choice is an important driver of sexual selection, but can be costly, particularly when choosy females risk remaining unmated or experience delays to reproduction. Thus, females should reduce choosiness when mate encounter rates are low. We asked whether choosiness is affected by social context, which may provide reliable information about the local availability of mates. This has been demonstrated in the lab, but rarely under natural conditions. We studied western black widow spiders (Latrodectus hesperus) in the field, placing experimental final-instar immature females so they were either 'isolated' or 'clustered' near naturally occurring conspecifics (≥10 m or ≤1 m, respectively, from a microhabitat occupied by at least one other female). Upon maturity, females in both treatments were visited by similar numbers of males, but clustered females were visited by males earlier and in more rapid succession than isolated females, confirming that proximity to conspecifics reduces the risk of remaining unmated. As predicted, isolated females were less choosy in staged mating trials, neither rejecting males nor engaging in pre-copulatory cannibalism, in contrast to clustered females. These results demonstrate that exposure of females to natural variation in demography in the field can alter choosiness of adults. Thus, female behaviour in response to cues of local population density can affect the intensity of sexual selection on males in the wild.

Male black widows parasitize mate-searching effort of rivals to find females faster.

Mate-searching success is a critical precursor to mating, but there is a dearth of research on traits and tactics that confer a competitive advantage in finding potential mates. Theory and available empirical evidence suggest that males locate mates using mate-attraction signals produced by receptive females (personal information) and avoid inadvertently produced cues from rival males (social information) that indicate a female has probably already mated. Here, we show that western black widow males use both kinds of information to find females efficiently, parasitizing the searching effort of rivals in a way that guarantees competition over mating after reaching a female's web. This tactic may be adaptive because female receptivity is transient, and we show that (i) mate searching is risky (88% mortality) and (ii) a strongly male-biased operational sex ratio (from 1.2 : 1 to more than 10 : 1) makes competition inevitable. Males with access to rivals' silk trails moved at higher speeds than those with only personal information, and located females even when personal information was unreliable or absent. We show that following rivals can increase the potential for sexual selection on females as well as males and argue it may be more widespread in nature than is currently realized.

Oxidation products of biogenic emissions contribute to nucleation of atmospheric particles.

Atmospheric new-particle formation affects climate and is one of the least understood atmospheric aerosol processes. The complexity and variability of the atmosphere has hindered elucidation of the fundamental mechanism of new-particle formation from gaseous precursors. We show, in experiments performed with the CLOUD (Cosmics Leaving Outdoor Droplets) chamber at CERN, that sulfuric acid and oxidized organic vapors at atmospheric concentrations reproduce particle nucleation rates observed in the lower atmosphere. The experiments reveal a nucleation mechanism involving the formation of clusters containing sulfuric acid and oxidized organic molecules from the very first step. Inclusion of this mechanism in a global aerosol model yields a photochemically and biologically driven seasonal cycle of particle concentrations in the continental boundary layer, in good agreement with observations.

Prefrontal cortical involvement in verbal encoding strategies.

The lateral prefrontal cortex is critical for the control and organization of information in working memory. In certain situations, effective reorganization can attenuate task difficulty, suggesting a dissociation between lateral prefrontal activity and basic memory demand. In a verbal working memory task, we investigated the enhancement of performance that occurs when incoming information can be reorganized into higher-level groups or chunks. In the fMRI scanner, volunteers heard and repeated a sequence of digits. Mathematically structured sequences, encouraging 'chunking', were compared with unstructured, random sequences. Though structured sequences were easier to remember, fMRI showed increased lateral prefrontal activation for these sequences. Specifically, both the dorsolateral and ventrolateral prefrontal cortices were activated preferentially for the structured sequences during encoding. When visual stimuli that can be chunked using spatial structure are used, similar results are observed. These results demonstrate that cognitively less demanding tasks may elicit greater lateral prefrontal recruitment. Thus, the lateral prefrontal cortex appears to play a general role in strategically recoding information from memory, in order to optimize performance.