Stewart B. Rood

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.

Managing river flows to restore floodplain forests

River damming has dramatic environmental impacts and while changes due to reservoir flooding are immediate, downstream impacts are more spatially extensive. Downstream environments are influenced by the pattern of flow regulation, which also provides an opportunity for mitigation. We discuss impacts downstream from dams and recent case studies where collaborative efforts with dam operators have led to the recovery of more natural flow regimes. These restoration programs, in Nevada, USA, and Alberta, Canada, focused on the recovery of flow patterns during high flow years, because these are critical for riparian vegetation and sufficient water is available for both economic commitments and environmental needs. The restoration flows were developed using the “Recruitment Box Model”, which recommends high spring flows and then gradual flow decline for seedling survival. These flow regimes enabled extensive recruitment of cottonwoods and willows along previously impoverished reaches, and resulted in improvements to river and floodplain environments. Such restoration successes demonstrate how instream flow management can act as a broadly applicable tool for the restoration of floodplain forests.

Activation Tagging of a Dominant Gibberellin Catabolism Gene (GA 2-oxidase) from Poplar That Regulates Tree Stature

We identified a dwarf transgenic hybrid poplar (Populus tremula × Populus alba) after screening of 627 independent activation-tagged transgenic lines in tissue culture, greenhouse, and field environments. The cause of the phenotype was a hyperactivated gene encoding GA 2-oxidase (GA2ox), the major gibberellin (GA) catabolic enzyme in plants. The mutation resulted from insertion of a strong transcriptional enhancer near the transcription start site. Overexpression of the poplar GA2ox gene (PtaGA2ox1) caused hyperaccumulation of mRNA transcripts, quantitative shifts in the spectrum of GAs, and similarity in phenotype to transgenic poplars that overexpress a bean (Phaseolus coccineus) GA2ox gene. The poplar PtaGA2ox1 sequence was most closely related to PsGA2ox2 from pea (Pisum sativum) and two poorly known GA2oxs from Arabidopsis (AtGA2ox4 and AtGA2ox5). The dwarf phenotype was reversible through gibberellic acid application to the shoot apex. Transgenic approaches to producing semidwarf trees for use in arboriculture, horticulture, and forestry could have significant economic and environmental benefits, including altered fiber and fruit production, greater ease of management, and reduced risk of spread in wild populations.

Flows for Floodplain Forests: A Successful Riparian Restoration

Abstract Throughout the 20th century, the Truckee River that flows from Lake Tahoe into the Nevada desert was progressively dammed and dewatered, which led to the collapse of its aquatic and riparian ecosystems. The federal designation of the endemic cui-ui sucker (Chasmistes cujus) as endangered prompted a restoration program in the 1980s aimed at increasing spring flows to permit fish spawning. These flows did promote cui-ui reproduction, as well as an unanticipated benefit, the extensive seedling recruitment of Fremont cottonwood (Populus fremontii) and sandbar willow (Salix exigua). Recruitment was scattered in 1983 but extensive in 1987, when the hydrograph satisfied the riparian recruitment box model that had been developed for other rivers. That model was subsequently applied to develop flow prescriptions that were implemented from 1995 through 2000 and enabled further seedling establishment. The woodland recovery produced broad ecosystem benefits, as evidenced by the return by 1998 of 10 of 19 riparian bird species whose populations had been locally extirpated or had declined severely between 1868 and 1980. The dramatic partial recovery along this severely degraded desert river offers promise that the use of instream flow regulation can promote ecosystem restoration along other dammed rivers worldwide.

Branch sacrifice: cavitation-associated drought adaptation of riparian cottonwoods

Abstract In their native riparian zones (floodplains), Populus deltoides (prairie cottonwood) and P. fremontii (Fremont cottonwood) commonly experience substantial branch die-back. These trees occur in semi-arid areas of North America and unexpectedly given the dry regions, they are exceptionally vulnerable to xylem cavitation, drought-induced air embolism of xylem vessels. We propose that the vulnerability to cavitation and branch die-back are physiologically linked; drought-induced cavitation underlies branch die-back that reduces transpirational demand enabling the remaining shoot to maintain a favorable water balance. This proposal follows field observation along various western North American rivers as precocious branch senescence, the yellowing and death of leaves on particular branches during mid- to late summer, was common for P. deltoides and P. fremontii during hot and dry periods of low stream-flow. Branches displaying precocious senescence were subsequently dead the following year. The proposed association between cavitation, precocious senescence and branch die-back is also supported by experiments involving external pressurization of branches to about 2.5 MPa with a branch collar or through an adjacent cut-branch. The treatments induced xylem cavitation and increased leaf diffusive resistance (stomatal closure) that was followed by leaf senescence and branch death of P. deltoides. P. trichocarpa (black cottonwood) appeared to be less affected by the pressurization treatment and this species as well P. angustifolia (narrowleaf cottonwood) and P. balsamifera (balsam poplar) seldom display the patchy summer branch senescence typical of P. deltoides and P. fremontii. ’Branch sacrifice’ describes this cavitation-associated senescence and branch die-back that may provide a drought adaptation for the prairie and Fremont cottonwoods.

COMPARATIVE TOLERANCES OF RIPARIAN WILLOWS AND COTTONWOODS TO WATER-TABLE DECLINE

Cottonwoods (Populus sp.) and willows (Salix sp.) generally dominate riparian landscapes across western North America. To investigate their relative tolerances to water-table decline, rooted shoot cuttings (saplings) of two willows, Salix exigua and S. drummondiana, and two cottonwoods, Populus angustifolia, and P. balsamifera, were grown in rhizopods, controlled growth devices that allow water-table manipulation. Water-table-decline rates of 0 to 12 cm/d were applied and plant growth and survival were monitored. In a second study, seedlings of S. exigua, S. lutea, P. balsamifera, and P. deltoides were grown in rhizopods and exposed to water-table-decline rates of 0 to 8 cm/d. For saplings and seedlings of both genera, gradual declines of 1 or 2 cm/d promoted root elongation and often promoted shoot growth relative to the constant water-table treatment (0 cm/d). More abrupt declines (> 2 cm/d) reduced growth and survival, and there were some differences in sensitivity within as well as across the two genera. Thus, the willow and cottonwood saplings were similarly affected by abrupt water-table decline, but willow seedlings were slightly more vulnerable than cottonwood seedlings. It is recommended that gradual stream stage recession along regulated rivers of about 2.5 cm/d in the late spring would encourage the recruitment of cottonwood seedlings, but subsequently, more gradual recession of about 1 cm/d through the lower elevational streamside zones would enable recruitment of S. exigua and other willows.

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.

Transgenic modification of gai or rgl1 causes dwarfing and alters gibberellins, root growth, and metabolite profiles in Populus

In Arabidopsis and other plants, gibberellin (GA)-regulated responses are mediated by proteins including GAI, RGA and RGL1-3 that contain a functional DELLA domain. Through transgenic modification, we found that DELLA-less versions of GAI (gai) and RGL1 (rgl1) in a Populus tree have profound, dominant effects on phenotype, producing pleiotropic changes in morphology and metabolic profiles. Shoots were dwarfed, likely via constitutive repression of GA-induced elongation, whereas root growth was promoted two- to threefold in vitro. Applied GA3 inhibited adventitious root production in wild-type poplar, but gai/rgl1 poplars were unaffected by the inhibition. The concentrations of bioactive GA1 and GA4 in leaves of gai- and rgl1-expressing plants increased 12- to 64-fold, while the C19 precursors of GA1 (GA53, GA44 and GA19) decreased three- to ninefold, consistent with feedback regulation of GA 20-oxidase in the transgenic plants. The transgenic modifications elicited significant metabolic changes. In roots, metabolic profiling suggested increased respiration as a possible mechanism of the increased root growth. In leaves, we found metabolite changes suggesting reduced carbon flux through the lignin biosynthetic pathway and a shift towards allocation of secondary storage and defense metabolites, including various phenols, phenolic glucosides, and phenolic acid conjugates.