With the help of satellite data, researchers from laboratories in France(1), the UK, Japan and Russia have completed the accurate and large-scale mapping of leaf appearance dates in boreal forests. Their work has revealed a remarkable trend towards earlier foliation, which occurred between 1987 and 1990, over a large part of northern Eurasia, caused by the unprecedented increase in spring temperatures since 1921. By comparing these results with the previous studies available, they were able to reconstruct the foliation trend over the whole 20th century. Their work, published the journal Global Change Biology, enables the effects of global warming on these forests to be measured.
Phenology studies the climate-dependent variations of seasonal phenomena of plant life. In this study, the researchers particularly focused on the date of leaf appearance in boreal forests. In the northern hemisphere at high latitudes, foliation depends essentially on temperature. It is, therefore, considered direct evidence of the warming of the climate observed during the 20th century, which is especially marked in these regions. In addition, it also sets the pace for exchanges of carbon between vegetation and the atmosphere, which have an impact on the climate.
Before 1982, two techniques were used to analyze temporal variations in foliation: modeling based on meteorological measurements, which is not very precise, and measurements in the field, which make for a more precise analysis, but only at the local level. 1982 saw the beginning of observations from space (remote sensing), which are necessary for this type of study. Since then, a considerable amount of research based on these observations has shown that leaves in deciduous forests in boreal regions have tended to appear earlier and earlier due to warming. However, these measurements only provided rough trends, which were averaged out considerably over space and time.
In their work published in Global Change Biology, the researchers refined their interpretation of satellite data(2), especially by taking into account the effect of snow on the radiometric signal(3). They also calibrated their model for leaf appearance based on temperature, thus enabling remote sensing observations and modeling to be in close agreement.
They were able to show that foliation had generally occurred at an increasingly early date from 1982 until the present, on average around 5 days earlier for the Eurasian boreal forest. Variations in leaf appearance dates since 1982 have not been linear over time, and have not been identical for the whole of boreal Eurasia: the trend to increasingly early foliation dates accelerated between 1987 and 1990, and was more marked in Central Siberia.
Besides improved methods, the novel feature of this work consisted in studying foliation trends over the whole 20th century, by comparing the new results with those for the period before 1982 (with the help of field and modeling data).
The major trend observed in Central Siberia was connected to two events:
Earlier this century, boreal Eurasia was affected by other periods of warming during which leaves appeared earlier and earlier (for instance from 1936 to 1944 in Central Siberia and on several occasions in Western Russia), as well as cooler periods which gradually led to increasingly late leaf appearance (especially between 1945 and 1960 in central and eastern Siberia. However these trends always occurred on a local or regional scale.
The recent trend towards increasingly early foliation observed by remote sensing is therefore striking when compared to similar events which have occurred since 1921, in that it has simultaneously affected the greater part of boreal Eurasia and has given rise to the earliest leaf appearance dates in the region for almost a century. With regard to global warming, it shows that there has been a large-scale increase of spring temperatures during this period.
1) Center for the study of the biosphere from space (Centre d'études spatiales de la biosphère) (CESBIO/OMP, CNRS, Université Toulouse 3, IRD, CNES), Laboratory for Environmental Geophysics and Glaciology (Laboratoire de glaciologie et géophysique de l'environnement) (LGGE/OSUG, CNRS and Université Grenoble 1), JAMSTEC-Frontier research center for global change (Yokohama, Japan), Centre for terrestrial carbon dynamics (Sheffield, GB), University of Sheffield (Sheffield, GB) and Komarov institute of botany (St Petersburg, Russia)
2) Data from CNES's SPOT VEGETATION satellite and NOAA's AVHRR satellite.
3) Snow melt can be erroneously interpreted as the beginning of foliation.
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