La mort des géants: Près de la moitié des grands arbres de la Sierra Nevada américaine ont péri en 2015 et 2016

Dorota Retelska Arbre, Avion, Climat, eau, Etats-Unis, Feu, Forêt, Incendie, Pluie, Sécheresse, Sol

Les forêts dans toutes les région du monde, en Amazonie et au Congo tropicaux, dans les zones tempérées, et dans les zones boréales subissent depuis quelques années des sécheresses prolongées.  Et les arbres en meurent.

En Suisse, au printemps 2019, j’ai remarqué des arbres jaunes, secs, dont certains ont succombé à la prolifération de bostryches. D’autres ont été fauchés par  les vents violents de l’hiver passé.

Aux Etats-Unis,  des scientifiques ont compté les arbres morts après les avoir filmé par avion. Ils ont ainsi étudié près de deux millions d’arbres dans la Sierra Nevada, où se trouve le parc National de Yosemite et ses séquoias millénaires. La végétation était filmée et les individus morts, jaunis, étaient comptés par un algorithme informatique.  Les forêts ont été observées entre 2009 et 2016.  Presque la moitié de grands arbres (de plus de trente mètres de haut) sont morts au cours de ces années, essentiellement entre 2014 et 2016.  Selon les auteurs de l’article, leur vulnérabilité pourrait s’expliquer par leur taille. Les arbres les plus hauts ne résistent pas aux sécheresses prolongées. Lors de celles-ci, les vaisseaux conducteurs des grands arbres, qui sont en permanence remplis d’eau, pourraient être définitivement endommagés, ce qui peut être fatal pour la plante.

Ils pourraient encore préciser si une seule espèce est touchée, ou si les arbres sont souffrent d’une maladie particulière ou seulement de la sécheresse. Des solutions différentes s’imposeraient alors. L’événement est dramatique. Ce sont probablement les plus belles forêts du monde, des cathédrales de profonde paix verte. Au sol,  la végétation pousse sur une enchevêtrement de troncs anciens. Le carbone est conservé ainsi des dizaines d’années après la chute de l’arbre.  Les végétaux les plus hauts sont souvent les plus âgés, ils sont là depuis cent ou deux mille ans, ils sont exceptionnels, irremplaçables. Ils ont survécu aux changements météorologiques survenus au cours de ces dernières centaines d’années. Ils ont succombé à l’année la plus chaude de notre l’Histoire.  On peut supposer que la météo de l’année 2016 a dépassé tout ce qu’ils ont vécu et leur a été fatale. Des études antérieures ont montré que les séquoias avaient cessé de poussé  il y a quelques années,, les sécheresses avaient arrêté leur croissance. Ils souffraient déjà des effets du changement climatique depuis plusieurs années.   Combien ont encore été touchés cette année, combien en reste-il maintenant?

Actuellement, la Californie est dévastée par de grands incendies. Des études ont montré que les feux sont favorisés par la fonte des neiges précoce, par les températures élevées et la sécheresse, ainsi que par les vents forts. La mort des arbres pourrait aussi provoquer les feux de forêts, les arbres morts et secs brûlent plus facilement. Il semble y avoir actuellement trois incendies dans la Sierra Nevada (carte).

Si ces arbres uniques, millénaires succombent vraiment à la sécheresse, je propose de les arroser  pour les sauver, et cela dès cette année, car il semblent mourir très vite. La Californie est un état riche est prospère, et ses forêts contiennent des individus de plus de deux mille ans mais sont relativement petites. Il est sûrement possible d’amener des conduites d’eau, et de mettre en place des installations qui sauveraient les arbres. Le changement climatique va continuer et s’aggraver, et sans les forêts, il progresserait plus vite encore.

Michael E. Mann estime que les feux sont provoqués par le réchauffement climatique. Il prévoit qu’à l’avenir, la Californie sera touchée par des feux plus graves, ou tout au moins plus exposée aux feux. Ils ont tout intérêt à centupler les investissements pour éviter les incendies.

 

https://www.nature.com/articles/s41467-019-12380-6

 

 

Dorota Retelska

Dorota Retelska, décrypte les nouvelles du climat. Docteure ès Sciences de l’UNIL, auteure d’Antarctique-Ouest dans le Vide, elle alerte sur les dangers du climat depuis plusieurs années. Elle est active dans plusieurs organisations de défense du climat, entre autres l’Association Climat Genève, Greenpeace, TACA, et le Collectif Climat 2020.

142 réponses à “La mort des géants: Près de la moitié des grands arbres de la Sierra Nevada américaine ont péri en 2015 et 2016”

  1. Sachant que les habitants humains de Californie sont eux aussi menacés à moyen terme de sécheresse, je ne suis pas convaincu que des systèmes d’irrigation pour amener de l’eau douce aux arbres soient leur priorité…

  2. Oui, en effet ! En plus, le sol de la Californie s’enfonce inexorablement du à un pompage excessif d’eau des nappes phréatiques. L’homme pompe les aquifères plus rapidement qu’ils ne se rechargent, c’est ce qui provoque l’affaissement des sols. En grande partie pour les besoins de l’agriculture, comme le relate cet article 11 octobre dernier.
    https://rmc.bfmtv.com/emission/objectif-terre-la-californie-s-enfonce-inexorablement-dans-le-sol-1784953.html

  3. Ce phénomène s’observe également en Europe. Les derniers étés chauds et secs ont provoqué, notamment en France, une hécatombe de hêtres et de sapins. Il est urgent, je crois, de modifier notre manière de voir la forêt et les arbres qui les habitent. Nous avons tendance à trop souvent voir les arbres comme quelque chose d’inamovible mais la réalité est que ceux-ci se déplacent et suivent les changements climatiques de génération en génération, avec plus ou moins de retard, mais jusqu’à présent de manière très efficace avec l’aide des vecteurs qui dispersent leurs graines (vent, animaux). Le grand problème actuellement est que nous avons d’une part provoqué un changement climatique d’une ampleur considérable (qui ne fait que commencer), et d’autre part morcelé les écosystèmes, empêchant la migration naturelle des espèces. La “solution” pour les sequoias n’est pas tant de les arroser (ce qui serait tout de même bien pour préserver ces forêts pendant quelques décennies encore), sinon de les semer ou de les planter ailleurs, là où les conditions actuelles et futures leur sont favorables. Les scientifiques, très réticents il y a à peine 10-15 ans, commencent à s’intéresser de plus en plus aux initiatives de “migration assistée” menées par des associations comme les “Torreya Guardians”. Sachant que les conditions climatiques futures ressembleront de plus en plus à ce qu’elles étaient à la fin de l’ère tertiaire (Pliocène), peut-être vaudrait-il la peine de se demander à quoi ressemblaient nos forêts à cette époque. L’idée de planter des cèdres de l’Atlas dans le Jura ou… des séquoias dans les Alpes ne semble dès lors pas aussi folle qu’il n’y paraît. Et il ne faudrait pas attendre un demi siècle pour commencer, sachant que les arbres que nous plantons aujourd’hui devraient être capables de supporter le climat de demain, qu’ils aurons eux l’occasion de découvrir et de subir…

  4. Technicien horticole et botaniste amateur, je pense que d’apporter l’eau n’est pas la réponse d’autant que pour être efficace il faudrait des quantités qui dépassent toutes les imaginations. Et puis à bien réfléchir ce système pourrait bien déplacer le problème (qui va t-on priver d’eau !)… De plus, nous pouvons parier que les quantités se révéleraient de toute manière insuffisantes en ne mouillant qu’une petite épaisseur de sol – Et si c’était le cas on encouragerait le végétal à privilégier le système racinaire de surface.
    On oublie assez vite que le végétal s’habitue à la facilité ( pourquoi continuer à faire vivre les racines très profondes alors que l’eau se trouve en surface ???)
    Pour appuyer cette thèse, il suffit d’observer la mortalité important des pus grands arbres dans les parcs où l’on a décidé d’interrompre l’arrosage par soucis d’économie.

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