Streamside trees: responses of male, female and hybrid cottonwoods to flooding.

Cottonwoods, riparian poplars, are dioecious and prior studies have indicated that female poplars and willows can be more abundant than males in low-elevation zones, which are occasionally flooded. We investigated the response to flooding of clonal saplings of 12 male and 9 female narrowleaf cottonwoods (Populus angustifolia) grown for 15 weeks in a greenhouse, along with three females of a co-occurring native hybrid (Populus × jackii = Populus deltoides × Populus balsamifera). Three water-level treatments were provided, with substrate inundation as the flood treatment. In the non-flooded condition, the hybrids produced about four-fold more dry weight (DW) than the narrowleaf cottonwoods (P < 0.01). In both cottonwood taxa, flooding reduced stem height and DW, root and leaf area and weight, leaf chlorophyll and stomatal conductance (all P < 0.01). Inundation increased the foliar carbon-to-nitrogen ratio (+11%; P < 0.05) but did not significantly alter leaf water potential (mean -1.5 MPa), or foliar δ(13)C, which was lower in P. angustifolia (-32.8‰) than P. × jackii (-31.5‰; P < 0.05). Water level influenced the root distribution as roots were sparse in the saturated substrate and abundant in the capillary fringe above. The male and female P. angustifolia genotypes grew similarly with the favorable water levels, but the males tended to be more inhibited by flooding. Sapling DW of males was reduced by 56% compared with a 44% reduction for females (P = 0.1), and there were similar lower reductions for leaf, stem and root DW in females. These results demonstrate the inundation response of floodplain trees and suggest relative flood tolerance as: P. angustifolia female > P. angustifolia male > P. × jackii female. This indicates that narrowleaf cottonwoods are relatively flood tolerant and suggests that females are more flood tolerant than males. We propose the concept of 'strategic positioning', whereby the seed-producing females could be better adapted to naturally flooded, low-elevation streamside zones where seedling recruitment generally occurs.