Summary
A black oak savanna in central Wisconsin was analyzed in detail, including canopy trees, understory and tree seedlings. Although many authors have held that savanna in this region is a subclimax maintained by fire, this stand, unburned for many years, shows a high degree of stability and very little trend toward the postulated increase in trees. Of the 25 most frequent understory species, 80% have higher affinity for prairie or other very open habitats than for forest. Although oak seedlings had 72% frequency in 1 m2 quadrats, their growth is exceedingly slow, while dieback and mortality are very high; few survive to 1 m in height and saplings are nearly absent. The tree canopy at 40% is low in relation to B. A., probably due to frequent wind and lightning damage. The inhibition of oak seedlings which prevents succession toward forest appears to be caused by a combination of frost, drouth, sandy soil of low water retaining capacity, interspecific competition and allelopathy; seedlings in an adjacent old field grow approximately 3 times faster, with little injury or mortality. Light grazing in early years of settlement may have allowed a temporary increase in tree establishment, since over 2/3 of the present canopy trees appear to be in one age group of 105–110 years.
On the basis of this evidence we propose that at least some savanna sites in this region are naturally stable communities maintained by a complex interaction of the local climate, edaphic factors, and the existing community; fire is not an essential factor.
Résumé
La Savane à Chêne noir du Wisconsin central est analysée en détail, comprenant arbres, arbustes et semis d'arbres. Beaucoup d'auteurs considèrent cette Savane dûe à l'incendie. Notre groupement pas soumis à l'incendie depuis beaucoup d'années montre un haut degré de stabilité.
Des 25 espèces les plus fréquentes du sous-bois 80% montrent davantage d'affinité avec la prairie où d'autres stations ouvertes qu'avec la forêt. Sur une surface de 1 m2 les semis deQuercus ont une fréquence de 72%. Ils poussent très lentement et la mortalité est très grande, la hauteur de la strate arbustive, influencée par les vents fréquents est faible.
L'évolution très lente des semis de Chêne semble causée par une combinaison de froid, sêcheresse, sol sablonneux, d'une faible capacité de retenu d'eau, concurrence interspécifique.
Dans un vieux champ, les semis poussent à peu près 3 fois plus vite et montrent une plus faible mortalité. Plus de 2/3 des individus de la strate arborescente ont 105–110 ans.
Tenant compte de ces faits, nous pouvons admettre qu'au moins certaines Savanes de cette région sont des unités naturelles, stables, maintenues dans cet état par une interaction du climat local, des facteurs édaphiques et de la végétation existante.
Le feu n'est pas un facteur essentiel.
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Whitford, P.B., Whitford, K. Savanna in central Wisconsin, U.S.A.. Plant Ecol 23, 77–87 (1971). https://doi.org/10.1007/BF02326654
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DOI: https://doi.org/10.1007/BF02326654