Direct Phenological Responses but Later Growth Stimulation upon Spring and Summer/Autumn Warming of Prunus spinosa L. in a Common Garden Environment

Future predictions of forest ecosystem responses are a challenge, as global temperatures will further rise in the coming decades at an unprecedented rate. The effect of elevated temperature on growth performance and phenology of three Prunus spinosa L. provenances (originating from Belgium, Spain, and Sweden) in a common garden environment was investigated. One-year-old seedlings were grown in greenhouse conditions and exposed to ambient and elevated temperatures in the spring (on average 5.6 °C difference) and in the late summer/autumn of 2018 (on average 1.9 °C difference), while they were kept hydrated, in a factorial design. In the following years, all plants experienced the same growing conditions. Bud burst, leaf senescence, height, and diameter growth were recorded. Height and radial growth were not affected in the year of the treatments (2018) but were enhanced the year after (2019), whereas phenological responses depended on the temperature treatments in the year of the treatments (2018) with little carry-over effects in the succeeding years. Spring warming enhanced more height growth in the succeeding year, whereas summer/autumn warming stimulated more radial growth. Spring warming advanced bud burst and shortened the leaf opening process whereas summer/autumn warming delayed leaf senescence and enlarged the duration of this phenophase. These results can help predict the putative shifts in species composition of future forests and woody landscape elements.

The timing of spring warming shapes reproductive effort in a warm-water fish: the role of mismatches between hepatic and gonadal processes

Spring-spawning fishes native to northern environments rely on both increasing temperature and lengthening photoperiod to cue reproduction and may thus be particularly sensitive to rapid warming earlier in the year while day lengths remain short. We investigated the reproductive response of pumpkinseed sunfish Lepomis gibbosus to spring warming commencing at a range of day lengths (9 – 15 hours), corresponding to various calendar days (January 10 – May 22). In both the laboratory and field, both male and female fish that experienced early warming while day lengths were <11 hours: 1) failed to initiate reproductive preparation in the liver before gonad development began, and 2) had reduced reproductive allocation. Analysis of published data on temperate fishes suggested that liver development prior to gonad development is widespread across warm-, cool-, and cold-water thermal guilds, though the precise phenology of liver relative to gonad development appears to vary widely among species. Together, our results point toward dampened reproductive preparation as a novel mechanism mediating reduced reproductive output in both warm- and cool-water fish following earlier spring warming.

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.

The core of the Baltic CIL: shall we introduce the Bornholm Intermediate Water?

&lt;p&gt;Cold Intermediate Layer (CIL) is apparent in the thermohaline structure of the Baltic Sea every year, typically from April to December. Within the CIL, water temperature, salinity, oxygen content, and other parameters are highly inhomogeneous in vertical, reflecting a complicated process of its formation. The core of the CIL (the layer of the coldest waters) has its T,S-index) allowing to identify the south-western part of the sea as the source of these waters. At the beginning of spring warming, a combination of environmental factors favors the subduction of the cold surface waters into the intermediate layers of the Baltic Proper, where they adjust to the density field, making up the coldest layer right above the permanent pycnocline.&lt;/p&gt;&lt;p&gt;For spring 2006, CTD measurements from 2 expeditions of research vessels &amp;#8220;Professor Shtokman&amp;#8221; of the Shirshov Institute of Oceanology and &amp;#8220;Gauss&amp;#8221; of Leibniz Institute for Baltic Sea Research in Warnem&amp;#252;nde (IOW) were analyzed, along with the CTD measurements from ICES open database, and meteorological information. Remote sensing data provide observations of the abrupt transformation of SST field in the Bornholm Basin in early spring 2006, when the coldest surface water occurred within the coastal zones and its temperature was close to or below the temperature of maximum density (Tmd). The beginning of spring warming in the region and further heating of the cold surface water from temperature below the Tmd induce horizontal exchange, which favors the penetration of winter-cold (1.1&amp;#8211;2.1 &amp;#176;C) surface waters of moderate salinity (7.6-8.1) into the intermediate layers in March. This water was observed in the Gdansk and Gotland basins in April-May 2006 as the core of the CIL. On the basis of vertical T,S-profiles and T,S-diagrams, the range of parameters of the CIL core waters in spring 2006 was determined (T: 1.4&amp;#8211;2.1 &amp;#176;C; S: 7.6&amp;#8211;8.1), which corresponds to the upper mixed layer in the vicinity of the Bornholm Island in March, 2006. Since this relation has already been confirmed for other years, and having in mind the importance of the process of the CIL formation for the entire Baltic Sea conveyor belt, we suggest to term waters of the CIL core as the Bornholm Intermediate Waters (BIW). Obviously, the T,S-index of the BIW shall vary from year to year, reflecting the severity of the past winter and the conditions of the particular spring. However, the BIW location right above the pycnocline, the lowest (for the current year) temperature, and its characteristic salinity of 7.6-8.1 seem to be repeatedly confirmed by field observations in the Baltic Proper in spring.&lt;/p&gt;&lt;p&gt;Investigations are supported by the Russian Foundation for Basic Research, grant No. 19-05-00717 (in part of the data analysis) and the State Assignment No. 0149-2019-0013 (in part of satellite data collecting and processing).&lt;/p&gt;