• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

GENERAL BOTANICAL CHARACTERISTICS:

Narrowleaf cottonwood is a native deciduous tree with a slender crown [45]. It attains heights up to 60 feet (20 m) [91]; trunks can exceed 30 inches (76 cm) in diameter [57]. The bark of narrowleaf cottonwood is smooth when young, becoming furrowed with age [,39]. Narrowleaf cottonwood is dioecious [10,11,24]. Flowers are borne in catkins, with male flowers smaller than female flowers. Seeds are very small and have cotton tufts [11].

Narrowleaf cottonwood is reported as a facultative wetland species tolerant of frequent and prolonged flooding [39]. It is not drought resistant [31] and is extremely vulnerable to drought induced xylem cavitation [88].

A life span of 100 to 200 years is not uncommon [5].

RAUNKIAER [71] LIFE FORM:

Phanerophyte

REGENERATION PROCESSES:

Narrowleaf cottonwood shows both asexual and sexual regeneration. Channel narrowing and redistribution of sediment are the 2 main fluvial processes providing suitable narrowleaf cottonwood establishment sites [62].

Sexual: Pollen is wind dispersed with fertilization occurring within 24 hours of pollination. Subsequent seed maturation is temperature dependent, generally requiring 3 to 6 weeks. Seed dispersal through wind and water is aided by fluffy cottonlike hairs [11].

Narrowleaf cottonwood seeds possess a small window of viability [33], generally 2 days [57]. Seeds readily germinate when deposited in favorable environments. Successful germination and establishment is dependent upon a suite of abiotic and biotic conditions, with insolation and moisture availability the most limiting factors. Seedlings require wet alluvium in full sunlight [11,18,57].  Narrowleaf cottonwood seedlings are poor competitors in vegetated sites because lack of endosperm food reserves limits successful establishment in shaded areas [11]. Baker [3] found narrowleaf cottonwood seedlings uncommon in stands older than 45 to 60 years. Seedlings are intolerant of dry conditions [11,18] and are most vulnerable during summer drought [11,54,72] before roots reach late-season alluvial water tables [11].

Asexual: Narrowleaf cottonwood sprouts from the root crown and roots [11,33,77]. Sprouting is often linked to disturbance that scarifies stems and/or roots [33]. Narrowleaf cottonwood also sprouts from branch and root fragments when adequate sediment deposition is present [33,54]. Vegetative propagation is a dominant mode of reproduction within foothill rivers and relatively clear streams with coarse substrates [73], promoting colonization of bare gravel bars [39]. Steep gradients, coarse streambeds, and constrained channels also promote clonal regeneration [33]. 

Cloning is important in narrowleaf cottonwood gallery maintenance. Male and female trees have similar clonal abilities. In southern Alberta, Gom and Rood [33] found cloning was important to narrowleaf cottonwood population structure and expansion. Ramet spacing ranged from inches to 164 feet (50 m) depending upon mechanism of clonal propagation. Shoot sprouting produced tightly clustered clones around buried root crowns. Root suckers produced wider spacing than shoot sprouting. Beavers influenced clonal propagation, perpetuating clonal expansion and encouraging regeneration near stream channels. 

Schier and Cambell [77] found the majority of root suckers occurred from suppressed buds embedded in periderm of undisturbed roots after death or injury of aboveground parts. Roots with damaged vascular cambium were most likely to sprout.

Branch fragments provide an important mode of asexual propagation. Beavers, high winds, heavy rain, hail, snow, and flooding all produce branch fragments capable of regeneration. Branches have the ability to impede stream flow and trap sediment for localized deposition, and may promote narrowleaf cottonwood establishment in areas lacking unvegetated sediment deposits [54].

SITE CHARACTERISTICS:

Narrowleaf cottonwood is common to alluvial benches of high plains to low mountains [46]. It occurs on a wide range of soil textures [39], inhabiting loamy sand to sandy loam with high percentages of coarse fragments [44], and gravel bars [39].

Regional: Narrowleaf cottonwood is rare in California where it is restricted to mid-elevation streamsides [45]. It is common to drainages greater than 6,000 feet (1828 m) in Arizona [6]. Narrow benches, small streams, and floodplains of larger streams are common sites in central Colorado [44]. In Montana, narrowleaf cottonwood is most abundant on medium to coarse-textured, well-drained soils [39]. Mid- to high-elevation streambanks or canyon bottoms are common sites in Utah [51].

The elevational ranges for narrowleaf cottonwood in several states are listed below:

Arizona	6,200 to 7,700 feet (1,900-2,350 m) [28]
California	3,937 to 5,905 feet (1,200-1,800 m) [45]
Colorado	5,000 to 8,000 feet (1,900-2,450 m) [42]
Eastern Idaho	< 6,700 feet (2,000 m) [9]
Southern Idaho	5,000 to 6,000 feet (1,500-1,800 m) [95]
New Mexico	6,200 to 7,700 feet (1,900-2,350 m) [28]
Texas	> 3,500 feet (1,000 m) [19]
Northern Utah	5,000 to 6,000 feet (1,500-1,800 m) [95]
Southern Utah	6,200 to 8,000 feet (1,900-2,450 m) [67]
Western Wyoming	< 6,700 feet (2,000 m) [9]

SUCCESSIONAL STATUS:

Narrowleaf cottonwood is a rapidly growing pioneer species, showing natural stand replacement adjacent to riparian systems with undisturbed hydrology. However, narrowleaf cottonwood shows slower growth rates than the majority of cottonwood species, requiring a longer time for adequate establishment  [81].

Although successional events in communities dominated by narrowleaf cottonwood are different between sites, common attributes are shared. Narrowleaf cottonwood colonizes moist, barren, newly deposited alluvium exposed to full sunlight. Shade intolerance, and lack of successful sexual regeneration under its own canopy prevent narrowleaf cottonwood communities from achieving climax status. In Montana Hansen and others [40] observed asexual sprouting within narrowleaf cottonwood communities, but sustained population maintenance was limited. They considered eventual replacement of narrowleaf cottonwood by later stages inevitable. Merigliano [62] provides a detailed outline of narrowleaf cottonwood ecology and other associated riparian species of the South Fork Snake River, Idaho. Successional trends in narrowleaf cottonwood communities are provided by Hall and Hansen [38] for southern and eastern Idaho.

SEASONAL DEVELOPMENT:

Phenological development of narrowleaf cottonwood is directly related to annual hydrologic events [72]. Flowering and pollination generally coincide with springtime peaks in river flow [11]. Catkins appear before leaf emergence [57] on twigs of the previous year [23] from specialized buds of preformed inflorescences [24]. Male flowers appear earlier than females with both occurring approximately 1 to 2 weeks prior to leaf initiation. Seed dispersal generally coincides with annual declines in river flow [11].

In general, flowering occurs May to June with leaf fall taking place from mid-September to October [34]. Leaf primordia are formed in the fall. New leaves continue to form throughout the growing season [24].

Related categories for SPECIES: Populus angustifolia | Narrowleaf Cottonwood