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Global warming and early spring

Logo https://crowtherlab.pageflow.io/interplay-of-leaf-out-drivers-and-limited-advance-of-spring-onset-under-future-climate-conditions

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Trees blooming in February and leaves coming out in early March...This can be observed more and more often in the Northern Hemisphere, but does this really mean that spring is starting earlier? In plant phenology - the study of plants' seasonal phenomena - the start of spring in temperate regions is defined as the day when enough heat has accumulated to initiate leaf-out and flowering of plants. In recent decades, spring has indeed been occurring earlier due to climate warming. These shifts have direct consequences for all organisms relying on spring temperature to calibrate their internal clock, from migrating birds to budding trees. With early spring onset, these species can find themselves caught up in a premature chain of events with direct consequences for ecosystems at a global scale.
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In temperate regions, the rise of temperatures in spring plays an important role in signalling billions of trees to start emerging leaves. But, other factors are suggested to play a role in driving spring leaf-out, too. Trees in temperate forests may sense the length of daylight and the period of cold weather preceding spring (winter chilling). However, the exact interplay between these different factors and their relative importance for leaf emergence in natural conditions has been unclear. So, to what extent do these parameters interact to regulate the timing of leaf emergence? And what are the consequences for forest dynamics?
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Trees take in carbon through their leaves. So, for each km2 of temperate forest, one day of earlier spring leaf-out results in trees absorbing an extra 4.5 tonnes of carbon. Observations show that climate warming trends over the past decades have led to an earlier arrival of spring leaf emergence. Depending on species and location, the shift was 3 to 8 days for every 1⁰C increase in spring air temperature. According to previous estimates, this earlier spring arrival could lead temperate forests to absorb an extra 0.8 Gt of carbon per year through the end of the century. However, this calculation assumes that leaf-out is only dependent on the accumulated warming happening during spring.

Our study aimed to consider all drivers of leaf emergence, untangle their relations, and produce a revised estimate of carbon uptake due to early leafing of temperate trees.



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Using observations from a citizen science program (Pan European Phenology Project), we extracted data of leaf-out from 9 different tree species in more than 4,000 sites resulting in a compilation of more than 700,000 phenological observations. Using temperature records from the Climatic Research Unit (CRU) and National Center for Atmospheric Research (NCAR), we calculated the spring day-length, winter chilling and the timing of spring onset and used these newly calculated parameters to develop our full phenological model.

To study the influence of each ecological parameter under future climate, we extrapolated the timing of spring leaf-out until 2100 using our newly developed full model and two climate warming scenarios (RCP 4.5 and 8.5).



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By considering spring temperature increases, winter chilling and the timing of spring onset (day-length), our so-called full-model untangles the relationship between these different drivers of leaf-out. Our analysis shows that, in all studied species, both winter chilling and day-length have a negative effect on the accumulated warming required to trigger the emergence of leaves. Therefore, the predicted early leafing response to global warming is greatly reduced when considering all drivers of spring leaf-out.

Leafing was previously predicted in vegetation models to advance by about a month by the end of the 21st century but our full-model, considering all drivers, predicted that the advance would be limited to only two weeks.



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Previous models solely driven by spring warming estimated that under a “business as usual” climate-scenario the early spring arrival would allow temperate trees to absorb an extra 0.8 Gt of carbon per year, resulting in a cumulative extra 37 Gt of carbon uptake over the rest of the century. In contrast, our full-model, considering all drivers of leaf emergence, estimates that temperate forests will only absorb an extra 0.2 Gt of carbon per year. This reduces the previous estimation by 25Gt, and predicts that earlier spring arrival will only contribute to an extra 12 Gt of carbon uptake through this century.
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Altogether our study shows that, across all nine tested species, winter chilling and the timing of spring onset have consistent negative effects on the accumulated warming required to leaf-out. As such, although spring warming is likely to increase over the rest of the century, reductions in winter chilling and the timing of spring onset are likely to constrain the advance in spring leaf emergence over the rest of the century.

Thus, although we can already observe an advance in spring arrival in our gardens, it is not expected to exceed two weeks, on average, over the rest of the century. Altogether, this means that, over that period, the total increase in carbon uptake by trees due to the early spring onset will be limited and not be as high as previously estimated (12 Gt vs. 37 Gt).

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While our study does not assess the physiological mechanisms behind each of the leaf-out drivers, it does demonstrate the complex relationships that exist between warming, cooling, and the timing of spring onset to impact when leaves emerge. Plants in temperate regions may have evolved this system as a safety strategy to prevent their young leaves from the damage of frost. However, as frost risk increases due to climate change, these safeguards may not be as effective in the future as they once were.


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Our full-model, including all three drivers, also allows us to bridge the gap between specialized phenological models and global vegetation models. Indeed, our simple regression model can easily be incorporated into large-scale vegetation models to project future terrestrial vegetation carbon dynamics. Thus, improving the overall accuracy of future climate projections.
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