Document: JEN-3-99-145

Superior fitness of hybrid cottonwoods along a western stream: Ecological and conservation implications.

SCHWEITZER, J.A.*

Northern Arizona University, Flagstaff AZ 86011 USA ¹

Abstract:
Using natural surveys, common gardens and hand-pollinated crosses we tested the hypothesis that hybrid cottonwoods produce as many viable offspring as the pure parentals. We found that hybrid cottonwoods do not support the tension zone, mosaic and bounded superiority models of hybrid fitness. The fitness of hybrid cottonwoods is equal to or greater than at least one of the parental species under all conditions. While Fremont genotypes (Populus fremontii) allocate more resources to their seeds than narrowleaf genotypes (P. angustifolia) or their hybrids, they are not more successful in creating viable offspring. Both narrowleaf and hybrid cottonwoods regularly produce ramets, which the Fremont genotypes do not generally produce. By combining data on multiple parameters of reproduction, we found that narrowleaf and hybrid genotypes can produce offspring without the specific environmental conditions that cottonwood seeds and seedlings require. Because > 90% of germinated seeds of all cottonwood tree types in nature perish while almost no connected ramets die, successful production of ramets, at least in the short-term, make hybrids more fit than Fremont cottonwood and at least as fit as the narrowleaf cottonwood. Cottonwood hybrid zones, which naturally occur wherever two or more species co-exist, are common along drainages throughout the western U.S., can be up to 1000km in length, and are thought to be very old. The demonstrated fitness of these trees is important to the conservation and restoration of riparian zones as hybrid trees support a higher biodiversity of arthropods and bird species than either pure species.

Keywords: riparian zones

This abstract is being presented at: 8:30 AM in session:
Oral Session #2: Conservation Ecology.