Shifts in the temperature‐sensitive periods for spring phenology in European beech and pedunculate oak clones across latitudes and over recent decades

Spring phenology of temperate trees has advanced worldwide in response to global warming. However, increasing temperatures may not necessarily lead to further phenological advance, especially in the warmer latitudes because of insufficient chilling and/or shorter day length. Determining the start of the forcing phase, that is, when buds are able to respond to warmer temperatures in spring, is therefore crucial to predict how phenology will change in the future. In this study, we used 4,056 leaf‐out date observations during the period 1969–2017 for clones of European beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) planted in 63 sites covering a large latitudinal gradient (from Portugal ~41°N to Norway ~63°N) at the International Phenological Gardens in order to (a) evaluate how the sensitivity periods to forcing and chilling have changed with climate warming, and (b) test whether consistent patterns occur along biogeographical gradients, that is, from colder to warmer environments. Partial least squares regressions suggest that the length of the forcing period has been extended over the recent decades with climate warming in the colder latitudes but has been shortened in the warmer latitudes for both species, with a more pronounced shift for beech. We attribute the lengthening of the forcing period in the colder latitudes to earlier opportunities with temperatures that can promote bud development. In contrast, at warmer or oceanic climates, the beginning of the forcing period has been delayed, possibly due to insufficient chilling. However, in spite of a later beginning of the forcing period, spring phenology has continued to advance at these areas due to a faster satisfaction of heat requirements induced by climate warming. Overall, our results support that ongoing climate warming will have different effects on the spring phenology of forest trees across latitudes due to the interactions between chilling, forcing and photoperiod.     

Decadal vegetation changes in a northern peatland, greenhouse gas fluxes and net radiative forcing

Thawing permafrost in the sub‐Arctic has implications for the physical stability and biological dynamics of peatland ecosystems. This study provides an analysis of how permafrost thawing and subsequent vegetation changes in a sub‐Arctic Swedish mire have changed the net exchange of greenhouse gases, carbon dioxide (CO₂) and CH₄ over the past three decades. Images of the mire (ca. 17 ha) and surroundings taken with film sensitive in the visible and the near infrared portion of the spectrum, [i.e. colour infrared (CIR) aerial photographs from 1970 and 2000] were used. The results show that during this period the area covered by hummock vegetation decreased by more than 11% and became replaced by wet‐growing plant communities. The overall net uptake of C in the vegetation and the release of C by heterotrophic respiration might have increased resulting in increases in both the growing season atmospheric CO₂ sink function with about 16% and the CH₄ emissions with 22%. Calculating the flux as CO₂ equivalents show that the mire in 2000 has a 47% greater radiative forcing on the atmosphere using a 100‐year time horizon. Northern peatlands in areas with thawing sporadic or discontinuous permafrost are likely to act as larger greenhouse gas sources over the growing season today than a few decades ago because of increased CH₄ emissions.     

Climatic information recorded in stable carbon isotopes in tree rings of Cryptomeria fortunei,Tianmu Mountain,China

The Cryptomeria fortunei (CF) tree-ring δ¹³C_p series, which was collected from the West Tianmu Mountain forestland (30°20′ N, 119°26′ E), located in the north-west of Zhejiang Province, China, belonging to the northern margin of the mid-subtropical region of Eastern China, were determined based on cross-dated tree-ring age. There was a significant decline in the δ¹³C_p series occurring from 1685 to 1985, more especially from 1835 to 1985 in response to increasing atmospheric CO₂ concentrations and decreasing atmospheric δ¹³C_a. To reduce the noise and enhance the climatic signals, we compared the polynomial function with the correction method developed by McCarroll and Loader (2004) to remove the low-frequency variation in the raw tree ring δ¹³C_p series (defined as the δ¹³C_poly series, δ¹³C_cor series, respectively), and found the most suited correction method was the correction method developed by McCarroll and Loader (2004) in our study area. High-frequency correlation analysis between the δ¹³C_cor series and many meteorological parameters recorded by Xian Rending weather station revealed that the current August–September mean maximum temperature and previous year mean minimum and mean maximum temperature (P < 0.005) most strongly influenced tree ring δ¹³C_p discrimination from 1956 to 1985, and the strongest temperature signal captured was the current August–September mean maximum temperature (r = 0.54, P < 0.005). Mainly on this basis, the varied history of current August–September mean maximum temperatures in the West Tianmu Mountain area were reconstructed from 1685 to 1985. The reconstructed maximum temperatures revealed a slight warming trend and showed close correlation with the climatic fluctuations of the Little Ice Age cold period before 1900 as well as the 20th century warm period after 1900. It also better corresponded with some climate events recorded in historical records. Spectrum analysis showed that in the reconstructed series there was quasi-periodicity of 66.7 yr, 21.1 yr, 3.2 yr, 2.3 yr and 2.0 yr. These cycles coincided with the “torque effect” variation of planets and the geocentric convergence, and changes in solar activity and irradiance, as well as the “Quasi-biennial oscillation” (QBO). This indicated that the δ¹³C_p chronology of tree rings in West Tianmu Mountain showed a good record of the sun's activities, the change in the sun radiation and ENSO events.     

Climate variability slows evolutionary responses of Colias butterflies to recent climate change

How does recent climate warming and climate variability alter fitness, phenotypic selection and evolution in natural populations? We combine biophysical, demographic and evolutionary models with recent climate data to address this question for the subalpine and alpine butterfly, Colias meadii, in the southern Rocky Mountains. We focus on predicting patterns of selection and evolution for a key thermoregulatory trait, melanin (solar absorptivity) on the posterior ventral hindwings, which affects patterns of body temperature, flight activity, adult and egg survival, and reproductive success in Colias. Both mean annual summer temperatures and thermal variability within summers have increased during the past 60 years at subalpine and alpine sites. At the subalpine site, predicted directional selection on wing absorptivity has shifted from generally positive (favouring increased wing melanin) to generally negative during the past 60 years, but there is substantial variation among years in the predicted magnitude and direction of selection and the optimal absorptivity. The predicted magnitude of directional selection at the alpine site declined during the past 60 years and varies substantially among years, but selection has generally been positive at this site. Predicted evolutionary responses to mean climate warming at the subalpine site since 1980 is small, because of the variability in selection and asymmetry of the fitness function. At both sites, the predicted effects of adaptive evolution on mean population fitness are much smaller than the fluctuations in mean fitness due to climate variability among years. Our analyses suggest that variation in climate within and among years may strongly limit evolutionary responses of ectotherms to mean climate warming in these habitats.     

A Temporal Mechanism for Generating the Phase Precession of Hippocampal Place Cells

The phase relationship between the activity of hippocampal place cells and the hippocampal theta rhythm systematically precesses as the animal runs through the region in an environment called the place field of the cell. We present a minimal biophysical model of the phase precession of place cells in region CA3 of the hippocampus. The model describes the dynamics of two coupled point neurons—namely, a pyramidal cell and an interneuron, the latter of which is driven by a pacemaker input. Outside of the place field, the network displays a stable, background firing pattern that is locked to the theta rhythm. The pacemaker input drives the interneuron, which in turn activates the pyramidal cell. A single stimulus to the pyramidal cell from the dentate gyrus, simulating entrance into the place field, reorganizes the functional roles of the cells in the network for a number of cycles of the theta rhythm. In the reorganized network, the pyramidal cell drives the interneuron at a higher frequency than the theta frequency, thus causing a systematic precession relative to the theta input. The frequency of the pyramidal cell can vary to account for changes in the animal's running speed. The transient dynamics end after up to 360 degrees of phase precession when the pacemaker input to the interneuron occurs at a phase to return the network to the stable background firing pattern, thus signaling the end of the place field. Our model, in contrast to others, reports that phase precession is a temporally, and not spatially, controlled process. We also predict that like pyramidal cells, interneurons phase precess. Our model provides a mechanism for shutting off place cell firing after the animal has crossed the place field, and it explains the observed nearly 360 degrees of phase precession. We also describe how this model is consistent with a proposed autoassociative memory role of the CA3 region.     

A numerical study on impact of crop canopy on mesoscale climate

The impact of well watered mesoscale wheat planted on the mesoscale boundary layer structures of midlatitude arid area has been investigated by using a mesoscale biophysical meteorological model. The investigation indicates that mesoscale perturbations in temperature and specific humidity over crop area from the adjacent dry, bare soil, caused by the transpiration from the crop canopy and evaporation from underlying humid soil, result in a horizontal pressure gradient. A mesoscale circulation is forced by the pressure perturbation with a wind speed of about 5 m/s directing from the crop canopy to the bare soil in the lower boundary layer. In the daytime, the boundary layer structure over a complex terrain is determined by the interactions between upslope flow circulations and the circulations mentioned above when wheat crop canopies are located on plain and plateau. The impact of crop canopy scale on this thermally forced mesoscale circulation is also investigated.     

Influence of short- and long-term climatic cycles on floristic change across the Eocene–Oligocene boundary in the Ebro Basin (Catalonia,Spain)

The Eocene–Oligocene transition (EOT) climatic turnover is modelled in the Ebro Basin using CLAMP and analysing the Sarral (Priabonian) and the Cervera (Rupelian) floras. The results show a drop of temperature and an increase in seasonality and precipitation. The changes in temperature and seasonality follow the trend described for the EOT in southern Europe; however, the increase in precipitation is the opposite of what would be expected. This increase might be related to the stratigraphic location of the Sarral Priabonian leaf bed within a dry stage of a precession cycle, whereas the Cervera Rupelian leaf bed would be located within the wet stage of a similar cycle. CLAMP combined with sedimentology, taphonomy and palaeoecology reveals that precession cycles would produce a shift in the habitat of certain plants during the EOT, with the Lauraceae being limited to riparian communities during the Priabonian of Sarral to grow in small laurisilvas during the Rupelian of Cervera.     

Using Biophysical Models to Improve Survey Efficiency for Cryptic Ectotherms

Inefficiencies in monitoring programs waste resources. Ideally, we would predict when and where target species are most detectable and place our effort accordingly. Statistical models can generate predictor functions relating survey conditions to detectability but are phenomenological; they do not incorporate biological constraints and so using them to predict into unsampled time and space is risky. Biophysical ecology allows us to place constraints on detection by identifying abiotic conditions in which the target species cannot be present. We show how such constraint can be incorporated into standard detection models. We use the striped legless lizard (Delma impar), a threatened cryptic species, in southeastern Australia, as a case study, using fortnightly monitoring data collected between June 2016 to May 2017. These lizards are monitored by searching under tiles placed in arrays; we used a biophysical model to simulate the thermal microclimate under tiles and combined this with the striped legless lizard's thermal physiology to predict when tiles could be used. When compared against a large monitoring dataset, lizards were rarely observed at times the model predicted they should not be present, but the model also over-predicted presences. A statistical occupancy model showed that much of this over-prediction is explained by a generally low detection probability, and a seasonal trend in detection beyond that captured in the biophysical model. We then replaced the temperature covariates of detection in the statistical model with our biophysical prediction (a categorical variable). The resulting model explained almost the same amount of variance in detections as the original model but using a single variable that captured biological constraints. The biophysical model allowed us to place mechanistic constraints on detection, but it still needed to be informed by observed aspects of the species' behavior that were not influenced by temperature. Hybrid statistical-biophysical models such as ours offer a powerful tool for forecasting optimal survey conditions for a wide array of species. © 2020 The Wildlife Society.     

Whale killers: Prevalence and ecological implications of killer whale predation on humpback whale calves off Western Australia

Reports of killer whales (Orcinus orca) preying on large whales have been relatively rare, and the ecological significance of these attacks is controversial. Here we report on numerous observations of killer whales preying on neonate humpback whales (Megaptera novaeangliae) off Western Australia (WA) based on reports we compiled and our own observations. Attacking killer whales included at least 19 individuals from three stable social groupings in a highly connected local population; 22 separate attacks with known outcomes resulted in at least 14 (64%) kills of humpback calves. We satellite‐tagged an adult female killer whale and followed her group on the water for 20.3 h over six separate days. During that time, they attacked eight humpback calves, and from the seven known outcomes, at least three calves (43%) were killed. Overall, our observations suggest that humpback calves are a predictable, plentiful, and readily taken prey source for killer whales and scavenging sharks off WA for at least 5 mo/yr. Humpback “escorts” vigorously assisted mothers in protecting their calves from attacking killer whales (and a white shark, Carcharodon carcharias). This expands the purported role of escorts in humpback whale social interactions, although it is not clear how this behavior is adaptive for the escorts.