Warmer spring temperatures in temperate deciduous forests advance the timing of tree growth but have little effect on annual woody productivity

As the climate changes, warmer spring temperatures are causing earlier leaf-out1–6 and commencement of net carbon dioxide (CO2) sequestration2,4 in temperate deciduous forests, resulting in a tendency towards increased growing season length1,4,5,7–9 and annual CO2 uptake2,4,10–14. However, less is known about how spring temperatures affect tree stem growth, which sequesters carbon (C) in wood that has a long residence time in the ecosystem15,16. Using dendrometer band measurements from 463 trees across two forests, we show that warmer spring temperatures shifted the woody growth of deciduous trees earlier but had no consistent effect on peak growing season length, maximum daily growth rates, or annual growth. The latter finding was confirmed on the centennial scale by 207 tree-ring chronologies from 108 forests across eastern North America, where annual growth was far more sensitive to temperatures during the peak growing season than in the spring. These findings imply that extra CO2 uptake in years with warmer springs10–12 is not allocated to long-lived woody biomass, where it could have a substantial and lasting impact on the forest C balance. Rather, contradicting current projections from global C cycle models2,3,17,18, our empirical results imply that warming spring temperatures are unlikely to increase the woody productivity or strengthen the CO2 sink of temperate deciduous forests.

North Atlantic Oscillation seesaw effect in leaf morphological records from dwarf birch shrubs in Greenland and Finland

The North Atlantic Oscillation (NAO) determines wind speed and direction, seasonal heat, moisture transport, storm tracks, cloudiness and sea-ice cover through atmospheric mass balance shifts between the Arctic and the subtropical Atlantic. The NAO is characterized by the typical, yet insufficiently understood, seesaw pattern of warmer winter and spring temperatures over Scandinavia and cooler temperatures over Greenland during the positive phase of the NAO, and vice versa during the negative phase. We tested the potential to reconstruct NAO variation beyond the meteorological record through the application of a microphenological proxy. We measured the Undulation Index (UI) in Betula nana epidermal cells from herbarium leaf samples and fossil peat fragments dating back to 1865—exceeding most meteorological records in the Arctic—to estimate imprints of spring thermal properties and NAO in Greenland and Finland. We found negative relations between Greenland UI and late winter, spring and early summer NAO, and mostly positive, but not significant, relations between Finland UI and NAO in years with pronounced NAO expression. The direction of the UI response in this common circumpolar species is, therefore, likely in line with the NAO seesaw effect, with leaf development response to NAO fluctuations in northern Europe opposing the response in Greenland and vice versa. Increased knowledge of the UI response to climate may contribute to understanding ecological properties of key Arctic species, whilst additionally providing a proxy for NAO dynamics.

Empirical modelling of snow cover duration patterns in complex terrains of Italy

Snow cover duration is a crucial climate change indicator. However, measurements of days with snow cover on the ground (DSG) are limited, especially in complex terrains, and existing measurements are fragmentary and cover only relatively short time periods. Here, we provide observational and modelling evidence that it is possible to produce reliable time-series of DSG for Italy based on instrumental measurements, and historical documentary data derived from various sources, from a limited set of stations and areas in the central-southern Apennines (CSA) of Italy. The adopted modelling approach reveals that DSG estimates in most settings in Italy can be driven by climate factors occurring in the CSA. Taking into account spatial scale-dependence, a parsimonious model was developed by incorporating elevation, winter and spring temperatures, a large-scale circulation index (the Atlantic Multidecadal Variability, AMV) and a snow-severity index, with in situ DSG data, based on a core snow cover dataset covering 97 years (88% coverage in the 1907–2018 period and the rest, discontinuously from 1683 to 1895, from historical data of the Benevento station). The model was validated on the basis of the identification of contemporary snow cover patterns and historical evidence of summer snow cover in high massifs. Beyond the CSA, validation obtained across terrains of varying complexity in both the northern and southern sectors of the peninsula indicate that the model holds potential for applications in a broad range of geographical settings and climatic situations of Italy. This article advances the study of past, present and future DSG changes in the central Mediterranean region.

Post-settlement abundance, condition, and survival in a climate-stressed population of Pacific cod

The Pacific cod (Gadus macrocephalus) fishery recently collapsed in the Gulf of Alaska after a series of marine heatwaves that began in 2014. To gauge the likelihood of population recovery following these extreme warming events, we investigate potential thermal stress on age-0 cohorts through a comprehensive analysis of juvenile cod abundance, condition, growth, and survival data collected from 15 years of beach seine surveys. Abundance was strongly negatively related to ocean temperature during the egg and larval phase (winter/spring), but age-0 cod were larger in the early summer following warm winter/spring temperatures. Body condition indices suggest that warm summers may improve energetic reserves prior to the first winter; however, there was no summer temperature effect on post-settlement growth or survival. Spatial differences in abundance, condition, or growth were not detected, and density-dependent effects were either weak or positive. While the positive effects of increased summer temperatures on juvenile condition may benefit overwintering survival, they cannot compensate for high pre-settlement mortality from warming winter/spring temperatures. We conclude the critical thermal bottleneck for juvenile abundance occurs pre-settlement.

The influence of weather conditions on breeding dates of the common gull (Larus canus) in the south of Western Siberia

Aim. The purpose of this research was to find out what local weather factors influence the nesting timing of the common gull (Larus canus). Material and Methods. The time of egg laying by the common gull was determined using data obtained during regular surveys over 8 years (1996‐1998, 2002‐2003, 2006‐2008) on the islands of Lake Bolshie Chany. Weather and climatic factors were assessed using open‐access databases. Results. It was been established that the start of egg‐laying in the colony of the common gull is determined by wind strength, the number of rainy days, the associated atmospheric pressure during the second decade of April, and the air temperature – the transition date at which the average daily air temperature rose above 0°C. In years with unstable spring temperatures, a relationship was revealed between the air temperature and the intensity of egg laying by the common gulls by day. In years when temperatures rise evenly, precipitation and wind speed become the main factors.Conclusion. We conclude that the egg‐laying dates of the common gull is influenced by weather variables during the whole nesting season and not predominantly by early season variables. We also show the importance of large‐scale climatic phenomena such as the EAWR in explaining variability in timing of the nesting of the common gull in Siberia. We suggest that future studies should focus on the effects of extremes in weather variables and global climatic phenomena.

Comparison of Seasonal Flow Rate Change Indices Downstream of Three Types of Dams in Southern Quebec (Canada)

The objective of this study is to use two hydrological indices (coefficients of variation and immoderation) to analyze the impacts of dam management methods on seasonal daily flow rate change downstream of three dams: Manouane (diversion-type management method), Ouareau (natural-type management method) and Matawin (inversion-type management method). The results show that this change is far greater downstream of the Matawin dam (characterized by an inversion-type management method) than downstream of the two other dams. Moreover, downstream of the Matawin dam, this daily flow rate change increases significantly over time, while decreasing downstream of the two other dams and in natural rivers. Lastly, this change is better correlated with climate downstream of the Ouareau dam than downstream of the two other dams. It is positively correlated with winter and spring temperatures as well as summer and fall rain. Contrary commonly accepted hypothesis, this study shows that the impacts of dams generally result in an increase of the seasonal flow rate change in Quebec.

Eastern white pine and eastern hemlock growth: possible tradeoffs in response of canopy trees to climate

A warming climate and extended growing season may confer competitive advantages to temperate conifers that can photosynthesize across seasons. Whether this potential translates into increased growth is unclear, as is whether pollution could constrain growth. We examined two temperate conifers - eastern white pine (Pinus strobus L.) and eastern hemlock (Tsuga canadensis (L.) Carrière) - and analyzed associations between growth (476 trees in 23 plots) and numerous factors, including climate and pollutant deposition variables. Both species exhibited increasing growth over time and eastern white pine showed greater maximum growth. Higher spring temperatures were associated with greater growth for both species, as were higher autumnal temperatures for eastern hemlock. Negative correlations were observed with previous year (eastern hemlock) and current year (eastern white pine) summer temperatures. Spring and summer moisture availability were positively correlated with growth for eastern white pine throughout its chronology, whereas for hemlock, correlations with moisture shifted from being significant with current year’s growth to previous year’s growth over time. The growth of these temperate conifers might benefit from higher spring (both species) and fall (eastern hemlock) temperatures, though this could be offset by reductions in growth associated with hotter, drier summers.

Winter–spring warming in the North Atlantic during the last 2000 years: evidence from southwest Iceland

Temperature reconstructions from the Northern Hemisphere (NH) generally indicate cooling over the Holocene, which is often attributed to decreasing summer insolation. However, climate model simulations predict that rising atmospheric CO2 concentrations and the collapse of the Laurentide Ice Sheet caused mean annual warming during this epoch. This contrast could reflect a seasonal bias in temperature proxies, and particularly a lack of proxies that record cold (late fall–early spring) season temperatures, or inaccuracies in climate model predictions of NH temperature. We reconstructed winter–spring temperatures during the Common Era (i.e., the last 2000 years) using alkenones, lipids produced by Isochrysidales haptophyte algae that bloom during spring ice-out, preserved in sediments from Vestra Gíslholtsvatn (VGHV), southwest Iceland. Our record indicates that winter–spring temperatures warmed during the last 2000 years, in contrast to most NH averages. Sensitivity tests with a lake energy balance model suggest that warmer winter and spring air temperatures result in earlier ice-out dates and warmer spring lake water temperatures and therefore warming in our proxy record. Regional air temperatures are strongly influenced by sea surface temperatures during the winter and spring season. Sea surface temperatures (SSTs) respond to both changes in ocean circulation and gradual changes in insolation. We also found distinct seasonal differences in centennial-scale, cold-season temperature variations in VGHV compared to existing records of summer and annual temperatures from Iceland. Multi-decadal to centennial-scale changes in winter–spring temperatures were strongly modulated by internal climate variability and changes in regional ocean circulation, which can result in winter and spring warming in Iceland even after a major negative radiative perturbation.

Contributions of winter and spring warming to the temporal shifts of leaf unfolding

Changes in winter and spring temperatures have been widely used to explain the diverse responses of spring phenology to climate change. However, our understanding of their respective roles remain incomplete. Using >300,000 in situ observations of leaf unfolding date (LUD) in Europe, we show that the advancement of LUD since 1950 is due both to accelerated spring thermal accumulation and changes in winter chilling which explain 61% and 39% of the LUD shifts, respectively. Winter warming did not substantially retard the releasing of bud dormancy, but increased the thermal requirement to reach leaf unfolding. The increase of thermal requirement and decreased efficiency of spring warming on accelerating thermal accumulation partly explained the temporally (1950s-2010s) decreasing response of LUD to warming. Our study stresses the need to better assess the antagonistic and heterogeneous effects of winter and spring warming on leaf phenology, which is key to projection of future vegetation-climate feedbacks.

Decadal and Bi-Decadal Periodicities in Temperature of Southern Scandinavia: Manifestations of Natural Variability or Climatic Response to Solar Cycles?

Nine proxies of temperature over the last 225–300 years in Southern Fennoscandia (55–63° N) were analyzed. Seven reconstructions of the mean growing season temperatures were obtained by dendroclimatological methods. Reconstructions of spring temperatures in Stockholm and winter temperatures in Tallinn were based on historical documentary sources. It was found that significant decadal (10–13 years) and bi-decadal (22–25 years) periodicities were present in many of these series during the entire time interval. Four proxy records correlated significantly with the quasi 22-year solar cycle of Hale. Three time series correlated significantly with the quasi 11-year solar cycle of Schwabe. This can be considered as evidence of a link between decadal and bi-decadal changes in solar activity and climate in Southern Fennoscandia. On the other hand, signs of correlation differed, as well as the time shift between the solar and temperature cycles. It is difficult to explain such an intricate relationship and, thus, the physical mechanism of solar−climatic linkages remains unclear. That is why assumptions about the purely occasional appearance of correlations cannot yet be rejected. Guidelines for further research are suggested.