IngentaConnect Floods, fire, and ice: disturbance ecology of riparian cottonwood...

Abstract:

Cottonwoods are poplar trees that are well adapted to dynamic riparian, or streamside, zones throughout the Northern Hemisphere. Here we assess the influences of three prominent physical disturbances, floods, fire, and ice, on cottonwood population ecology. We emphasize cottonwoods along rivers from the “Crown of the Continent”, the central Rocky Mountain zone around the Canada- United States border, where five Populus species overlap and four hybridize. Moderate to major floods scour banks and deposit bars, creating barren and moist colonization sites that are essential for cottonwood seedling recruitment. Floods also scarify shallow roots, thus promoting clonal suckering, especially for the section Tacamahaca species: narrowleaf cottonwood (Populusangustifolia James), balsam poplar (Populusbalsamifera L.), and black cottonwood (Populustrichocarpa Torr. & A.Gray). Fire would naturally be less frequent in some riparian zones because of the moist conditions and firebreaks provided by the streams, but with human use, floodplain forest fires have probably increased. Following fire, regrowth through clonal root and shoot suckers can be prolific for the Tacamahaca species, but is limited for the section Aigeiros, prairie cottonwood (Populusdeltoides Bartr.). River ice, and especially ice drives that accompany winter or spring break-up, provide powerful riparian disturbances that have often been neglected. Ice drives generate barren sites for seedling colonization, shear shoots, and scarify roots promoting shoot and root suckering, and sever branches, enabling dispersive clonal branch propagation. Following studies along many regional rivers, we conclude that: (i) riparian cottonwoods are tolerant of, and dependent upon, occasional physical disturbance for population rejuvenation; (ii) differing disturbance responses contribute to niche differentiation across the Populus species; (iii) different disturbances enable varied spatial and temporal patterns of cottonwood establishment, including fringe, general, and patch recruitment; and (iv) natural disturbance regimes probably favor native cottonwoods and disfavor some invasive, woody plants. River damming and flow regulation often attempt to attenuate flood and ice disturbance, a management objective that may hinder the perpetuation of native floodplain forests. We recommend that river resource managers seek to allow flood and ice disturbance, and additionally, fire may provide a managed disturbance that could rejuvenate overmature cottonwood groves along some regulated rivers.

Dans l'ensemble de l'hémisphère nord, on retrouve des peupliers deltoïdes bien adaptés à la dynamique riveraine, ou aux bords des cours d'eau. Les auteurs évaluent les influences de trois perturbations physiques prédominantes, les inondations, le feu et la glace, sur l'écologie des populations de peupliers deltoïdes. Ils s'intéresse surtout aux peupliers deltoïdes le long des rivières venant de la «couronne du continent», soit la région du centre des Montagnes Rocheuses, au confluent de la bordure du Canada et des Etats-Unis, où co-existent cinq espèces dont quatre forment des hybrides. Les inondations de modérées à fortes écorchent les rives et déposent des bancs de sable, créant ainsi les sites de colonisation dénudés et humides, essentiels au recrutement des plantules de ces espèces. L'inondation déterre également les racines superficielles, favorisant ainsi les rejets clonaux, surtout dans la section des Tacamahaca; le peuplier à feuilles étroites (Populus angustifolia James), le peuplier baumier (Populus balsamifera L.) et le peuplier noir (Populus trichocarpa Torr. & A. Gray). Certaines zones riveraines, par leurs conditions humides et leur effet de coupe-feu ne favorisent vraisemblablement pas beaucoup l'incendie, mais avec la présence des humains, l'incidence des feux dans la plaine d'inondation a probablement augmenté. Suite au feu, la repousse par rejets racinaires et caulinaires peut favoriser les espèces de Tacamahaca, mais à un moindre degré celles de la section Aigeros, du Populus deltoides Batr. Les glaces, et surtout celles qui accompagnent la débâcle d'hiver ou du printemps, produisent de puissantes perturbations riveraines qu'on a souvent négligées.

Floods, fire, and ice: disturbance ecology of riparian cottonwoods