John M. Mahoney

Streamflow requirements for cottonwood seedling recruitment—An integrative model

This paper describes the ‘recruitment box,’ an integrative model that defines the stream stage patterns that enable successful establishment of riparian cottonwood seedlings. In western North America, cottonwood seed dispersal generally occurs after annual peak river flows. The receding stream exposes moist sites upon which seeds land after transport by wind and water. Germination is rapid, and initial seedling establishment is often prolific. However, the vast majority of seedlings die, primarily due to drought stress, as root growth is insufficient to maintain contact with the receding zone of moisture. Cottonwood roots grow about 0.5 to 1 cm per day or 60 to 100 cm in the first year. Along the ‘losing’ streams in semi-arid regions, the riparian water table is an almost horizontal extension from the stream stage. A capillary fringe exists above the water table and is often 30 to 40 cm in elevation, but can range from about 5 to 130 cm depending on substrate texture. The combination of root growth and capillary fringe define the successful recruitment band, which is usually from about 0.6 to 2 m in elevation above the late summer stream stage. Within this range, higher elevation establishment occurs (i) for theAigeiros cottonwoods,Populus deltoides, andP. fremontii, which grow more rapidly thanTacamahaca species and occur in warmer areas with longer growing seasons; (ii) along larger rivers that are characterized by more gradual stage fluctuations; and (iii) along streams with finer substrate. The rate of stream stage decline is also critical for seedling survival and should not exceed 2.5 cm per day. The recruitment box model is consistent with dendrochronological interpretations that moderate flood events are naturally required for cottonwood recruitment. Flood events with recurrences of about 1 in 5 to 1 in 10 years often satisfy the model and provide stream stage patterns with a gradual decline through the recruitment box. The model will facilitate analyses of the reproductive ecology of riparian cottonwoods and also permit the prescription of stream stage patterns for cottonwood seedling recruitment along dammed rivers.

Collapse of riparian poplar forests downstream from dams in western prairies : probable causes and prospects for mitigation

Although historically abundant, the riparian poplar forests of the western prairies are now endangered as a result of the damming and diversion of rivers in this region. Recent reports have described substantial declines of riparian poplar forests downstream from dams in Alberta, Canada; Montana, North Dakota, Wyoming, Colorado, and Arizona, USA. The present report analyzes the forest and hydrological conditions reported previously in order to clarify the causes of the downstream forest decline.Dams were found to contribute to forest failure by (1) reducing downstream flows and/or (2) altering flow patterns to attenuate spring flooding and/or stabilize summer flows. Reduced flows are reported to induce drought stress, which is particularly lethal to seedlings and very old poplars. The artificial moderation of spring flooding may inhibit the formation of seedbeds essential for seedling replenishment. Increased river valley development involving cattle grazing, agricultural clearing, and direct harvesting of trees also contributes to forest failure.Potential methods for mitigating the impacts of dams on downstream forests include downstream flow schedules that (1) retain occasional spring flooding, (2) taper off rather than abruptly drop downstream flow, and (3) provide adequate flows throughout the summer. Poplar forest stabilization and recovery can be further promoted by fencing to protect trees from livestock grazing and trampling, or artificial site preparation such as cultivation or scarification to encourage poplar regeneration.

Initial cottonwood seedling recruitment following the flood of the century of the Oldman River, Alberta, Canada

Following heavy rain in early June 1995, flows of the Oldman River in Alberta, Canada were the highest on record (since 1911). This ‘flood of the century’ preceded cottonwood seed release, and created suitable sites for seedling establishment. After the flood peak, the Oldman River Dam and tributary dams were operated to deliver a relatively natural and gradual river stage recession of about 2.5 to 5 cm per day. Nine research sites were established on lateral and point bars to study establishment, survival, and growth of seedlings ofPopulus angustifolia, P. balsamifera. andP. deltoides. In 1995, transects were established perpendicular to the river to the zone of mature cottonwoods; 131 quadrats were established at positions along the transects that intersected cottonwood seedling bands and revisited in 1996 and 1997. At all sites, extensive areas of seedlings occurred in 1995, and seedling bands ranged in elevation from 0.6 to 4 m above the late summer stream stage. Low elevation seedlings were removed in 1996 and 1997 by ice and water scouring; high elevation seedlings died primarily due to drought stress. Seedlings that survived through 1996 and 1997 occurred at elevations ranging from 1.7 to 3 m, but seedlings above 2.5 m grew slowly. Within the seedling bands, densities after the first season ranged from 80 to 4,000 seedlings m−2, and densities fell to about 10% and then about 5% over the second and third years, respectively. The seedlings averaged only 2 cm in height after 1995 and increased to about 8 and 25 cm in 1996 and 1997;P. deltoides seedlings were larger than those of the other species, with some reaching 1 m in 1997. Thus, a major flood enabled a massive cottonwood seedling recruitment event that commenced in the flood year. The extensive recruitment occurred along a dammed river and was probably promoted deliberately by gradual stream stage decline after the flood peak.

Response of a hybrid poplar to water table decline in different substrates

Abstract The effects of substrate texture and rate of water table decline on the growth of a Populus balsamifera x Populus deltoides hybrid were studied using rhizopods. Rooted shoot cuttings were transplanted into growth tubes filled with gravel, sand, or a 1:1 (v/v) mixture of sand and were treated with water level declines of 0, 2, 5 or 10 cm day−1. Water drained fastest from the gravel-filled tubes, intermediate in the mixture and slowest from the sand-filled tubes. The effects of rapid water table decline were most severe on plants grown in the gravel and least severe on those grown in sand. Transpiration, height, leaf number, leaf area and plant health decreased with increasing rates of water table decline and increasing gravel content in the substrate. The reduction in transpiration and plant growth indicates that rapid water table decline caused drought stress of the poplars. Root elongation was promoted in all substrates by water table decline. The results indicate that alterations to river flow that cause abrupt drops in riparian water table will retard the transpiration and growth of riparian poplars. These effects will be more severe along flood-plains with coarse substrates.