samedi 16 février 2008

Le séquoia et l'hiver froid

Une autre source qui confirme que ce n'est pas le froid lui-même qui tuerait le séquoia, mais le sol gelé et sec. Avec un bon manteau de neige et des sources qui coulent même l'hiver, les chances semblent bien meilleures.

Extrait du livre
The Natural Geography of Plants, 1964, 438 pages
By Henry Allan Gleason, Arthur Cronquist

There are many trees which can stand some frost but die if the temperature goes low enough and stays low long enough. Low temperature is not always the direct cause of such winter-killing; much of it is, in fact, due to physiological drought – the plant’s inability to absorb enough water form the frozen soil to make up for what evaporates from its twigs and branches. A cold spell accompanied by drying winds when the ground is bare of snow is much more serious than a similar degree of cold with ample snow-cover and without wind. The giant sequoia withstands severe cold in its native home in the Sierra Nevada of California, where there is a deep mantle of winter snow and an ample supply of ground water. In New York state, where there may be prolonged cold spells and frozen ground without snow, the tree sooner or later succumbs. Peaches are an important commercial crop near the shores of the Great Salt Lake in Utah. A few miles away in neighboring Cache Valley, where it is just a little cooler, young peach trees may do well for several years, only to die during the first unusually severe winter.

Note that in all of the examples in the previous paragraphs it is the unusual winter which does the plants in. In all climatic factors affecting plant growth, it is likely to be the unusual extreme rather than the average condition, which sets the limits of range. Conversely, it may be only the unusual year with exceptionally favorable conditions which permits the establishment of seedlings of a species, yet this occasional good year may be enough to permit the perpetuation of trees and other long-lived plants.

The mesquite is a shrub or small tree of our southwestern deserts and formerly extended east as far as central Texas. Suppose we had tried sixty or eighty years ago to decide what set its northern and eastern boundaries. Just by observation in the field, we might have concluded that it could not migrate north into the Great Basin because of cold winters, or on the eastern side of the Rocky Mountains because of cold winters and the competition with sod-forming grasses, or straight east because of the moist climate. In recent years mesquite has begun a migration which has taken it north into Kansas and east, strange to say, across southern Louisiana and actually across the Mississippi River. We do not know or guess how much farther it may spread in another half century. It now seems that its former boundary in northern Texas may have been determined by prairie fires, and its eastern line by its intolerance of shade in forested eastern Texas and Louisiana. Prairie fires have virtually ceased, forests have been cleared, and mesquite is on the march.

3 commentaires:

kildare a dit…

Allo Antoine! Je lis ton blog depuis quelques mois et me décide à t'écrire. Moi aussi, j,ai attrapé une inquiétante piqure pour les coniferes et j'ai planté des séquoias sur le bord d'un ruisseau cet automne en septembre. Trouvant mes sequoias trop jeunes (2,5 mois) j'ai décidé de rentrer la majorité dans mon appart mais je me demande si en faisant ainsi, si je ne ruinerai pas les chances de s'adapter au froid québécois. Je demeure dans lanaudière.


kildare a dit…

ceci est un test...

kildare a dit…

ceci est un site intéressant!