Environmental influences on seedling growth of cottonwood species following a major flood

Abstract

A major flood in June 1995 along most streams in southern Alberta (AB) and southeastern British Columbia (BC), Canada, permitted the comparison of natural seedling establishment of different cottonwood species across different environments. Nine study sites were established along a 340-km corridor from BC, over the Rocky Mountain Continental Divide, and onto the foothills and then prairies of AB. Four native cottonwood species occurred from BC to AB: the black cottonwood, Populus trichocarpa Torr. and Gray, balsam poplar, P. balsamifera L., and narrow-leaf cottonwood, P. angustifolia James, of section Tacamahaca, and the prairie cottonwood, Populus deltoides Bartr. ex Marsh, of section Aigeiros. Cottonwood seedlings of the 1995 cohort were monitored from 1995 through 1998 in quadrats along riparian transects. The study confirmed that a major flood enabled extensive cottonwood recruitment along mountain, foothills, and prairie river reaches and revealed that both, environment and species influenced cottonwood seedling growth in situ. Across the sites, seedling heights varied ten-fold and were closely negatively correlated with site elevation (1995: n = 9, r 2 = 0.93; 1997: n = 4, r 2 = 0.92). The increased growth at lower elevations was probably associated with warmer temperatures and a longer growing season; seedling growth was positively correlated with the accumulation of growing degree days of nearby weather stations (1995: n = 6, r 2 = 0.88). Growth rate was also influenced by species as height varied up to three-fold across species at sites where species co-occurred. P. deltoides seedlings grew fastest followed by P. trichocarpa/P. balsamifera and intersectional hybrids and, finally, P. angustifolia and intrasectional hybrids. The superior seedling growth of P. deltoides is consistent with its life history and distribution. P. deltoides occurs along prairie river reaches with warmer and drier climates and higher-order streams with finer substrate textures; these physical conditions would favor seedling recruitment. The alternate environmental conditions of foothillls and mountain regions probably encourage clonal (asexual) recruitment that may supplement seedling recruitment, particularly for the Tacamahaca species.