More info for the terms: achene, avoidance, cover, density, duff, interference, litter, severity, shrubs, tree
Curlleaf mountain-mahogany reproduces predominantly by seed. Sprouting occurs to "some extent" from the trunk base following cutting, but sprouts are rare after fire [212].
Regeneration variability: Several studies indicate that the number of seedlings in a given stand is highly variable, suggesting that the necessary conditions for successful seed germination, emergence, and establishment do not co-occur regularly. Of 22 curlleaf mountain-mahogany stands in southwestern Montana, only 7 had seedlings [53]. Reproduction was considered poor in 19 curlleaf mountain-mahogany communities scattered throughout Utah. Forty-seven percent of the plots had no seedlings. Seedling density ranged from 0 to 230 seedlings/acre and averaged 37/acre. Variable seed production and/or lack of conditions favorable to germination and establishment may have contributed to poor regeneration. However, periodic failure to regenerate may not be important for this long-lived (over 700 years) plant [40,42].
Pollination: Flowers are chiefly wind pollinated, although some pollination by insects may occur [108,129,152].
Breeding system: A review reports that outcrossing is most common and selfing is limited in mountain-mahogany species [129].
Seed production: Mountain-mahogany species are reproductively mature at 10 to 15 years old [108]. Good seed crop production is irregular. Researchers suggest that good curlleaf mountain mahogany seed crops are produced at 2- to 10-year intervals [157,179]. A single tree can produce 90,000 to 100,000 seeds in a year [46]. During a 2-year study of 22 curlleaf mountain-mahogany stands in southwestern Montana, seed production was abundant in 1 year and lacking in the other [53].
Seed production was cyclic in 2 curlleaf mountain-mahogany populations in central Oregon. High seed production occurred in 3 of the 12 years the populations were monitored, and good seed production years did not coincide in the widely separated populations. During high production years, seed piles were up to 9.8 inches (25 cm) deep in "rocky pockets;" however, seed predation by an unidentified insect was severe. Nearly 100% of seeds were damaged in heavily concentrated areas. In areas where seed was scattered, 90% escaped predation, but scattered seeds made up a small proportion of the total [45,47].
Seed dispersal: Curlleaf mountain-mahogany seed is moderately heavy and predominantly wind dispersed [108,236]. The feathery seed tail aids in wind dispersal [169]. Small mammals feed on the seeds and may serve as dispersal agents as well [129,157,179].
Researchers studied seed production, seed fall, seed predation, and seed redistribution in curlleaf mountain-mahogany stands in northeastern Utah. Initially most seed was under the curlleaf mountain-mahogany canopy and the fewest seeds occurred in the open interspaces. However, differences between the quantity of seed initially deposited under curlleaf mountain-mahogany, under other shrubs, or in the open interspaces were less than differences in these microhabitats when measured the following growing season. Seed predation, which was greatest in the open interspaces and lowest under curlleaf mountain-mahogany canopies, and/or secondary seed movement by wind or snow likely affected seed distribution changes. Overall, however, seed predation was low in the study area. Seed loss was 1% when all predators were excluded, 3% when only small mammals were excluded, and 8% when seed was unprotected. Predation levels were higher in the year with lower seed production than in the year with greater seed production, suggesting that seed survival may be density dependent [169].
Seed banking: Studies on the longevity of curlleaf mountain-mahogany seed in the soil under field conditions are lacking. Reports of high predation levels [47,179] suggest that predator avoidance is a necessary 1st step in seed bank building.
Curlleaf mountain-mahogany seed viability was not compromised after 10 years of storage in an open warehouse, and some seed still germinated after 25 years of storage. Temperature extremes of -21.8 °F (-29.9 °C) and 101 °F (38.3 °C) were recorded in the warehouse over the 25-year study period. Curlleaf mountain-mahogany germination from seed stored between 2 and 10 years did not differ significantly (p<0.05). Germination was a high of 80% after 5 years of storage and a low of 63% after 3 years of storage. Seed stored for 20 years had 44% germination, and germination after 25 years of storage was 28% [195].
Germination: Seeds require cold moist stratification and germinate best on mineral soil [27,41]. Germination typically increases with increased cold exposure [19,45,47,107]. Dealy [46] suggests that the persistent plume on seeds curls and straightens with humidity changes and effectively drills the seed into the ground, while temperature changes, moisture, and bacteria break down the seed coat.
Several studies report the germinability of seed collected in the field. Germination was enhanced by cold stratification. Seventy-six percent of cold stratified curlleaf mountain-mahogany seed collected in southwestern Montana germinated, and 50% produced cotyledons [53]. Current-year and 2-year-old seed collected near the high desert steppe zone in central Oregon averaged 74% and 78% viability, respectively. Unstratified seeds did not germinate, and seeds stratified for 60 and 170 days in dark at 40 °F (4 °C) had germination percentages of 20% and 98%, respectively. Seeds planted in containers, left outdoors in the fall to stratify naturally averaged 24% germination. Germination started on 14 April when soil moisture levels were high and temperatures were cool [45,47].
Elevation was not an indicator of cold stratification length for seed collected from 6 populations in Utah, Idaho, and Nevada. Seeds were kept moist in blotters. Chilling treatments were at 30 °F (1 °C) then followed by 59 °F (15 °C). Without chilling, germination was 0.5%. After 4 weeks of chilling germination was 32.2%, after 6 weeks was 76.3%, after 8 weeks was 91.7%, after 10 weeks was 96.0% and after 12 weeks was 98% [107].
Numerous seed treatments were tested on seed collected in Nevada. Neither dry heat treatments nor seed tail burning, which charred the achene, enhanced curlleaf mountain-mahogany seed germination. Decreases were not reported either. Feathery plumes ignited easily, and achenes remained intact after this type of burning. Seeds rarely germinated at temperatures other than 50 to 59 °F (10-15 °C). Germination increased some with storage. Germination was 2% two months after collection and increased to 17% at 6 months after harvest. Soaking in water enhanced germination [234].
Seedling establishment/growth: Depth of litter, denseness of shade, distance to established vegetation, and severity of browsing can affect curlleaf mountain-mahogany seedling emergence, establishment, and/or growth. Often times site characteristics that are conducive to seedling emergence are less conducive to seedling establishment and/or plant maturation.
Observational and ecological studies indicate that curlleaf mountain-mahogany seedlings are sensitive to drought, frost, and interference from nonnative vegetation, especially cheatgrass (Bromus tectorum) [157,179]. Ross [166] indicates that seedling recruitment is good on disturbed sites near the California-Nevada border. In Oregon and adjacent areas, seedling establishment was rare under mature curlleaf mountain-mahogany trees, but researchers reported that seeds outside the canopy had a "good opportunity for establishment" [45].
Initial resource allocation is to the roots of developing seedlings [41]. Newly established plants typically have a deep taproot and a small crown [114]. Twenty-four seedlings grown in a greenhouse that mimicked spring conditions in central Oregon's curlleaf mountain-mahogany habitats had an average taproot extension of 1.1 feet (0.34 m) after 35 days. After 112 days, the average seedling root length was 37.2 inches (94.5 cm), and average stem diameter was 0.029 inch (0.074 cm). Plants with the largest stem diameter usually had the longer root systems, but the same was not true regarding height [47,203].
Herbivory: Browsing can result in heavy seedling mortality or suppressed growth. In 17 curlleaf mountain-mahogany stands in southern and central Idaho, there were an average of 1.9 seedlings/ft². Seedling survival through the 1st summer was 28%. Of the surviving seedlings, 25% without protection from browsing animals survived, 45% protected from big game animals survived, and 50% protected from all browsers survived. Researchers attributed substantial mortality to mountain cottontails. Four- to five-year-old plants were only 1 to 2 inches (3-5 cm) tall due to heavy browsing. Two juvenile growth forms referred to "basketball- and umbrella-shaped" resulted from the heavy browsing pressure. Basketball shrubs measured only 3 to 4 feet (0.9-1 m) tall at 60 to 70 years of age. Umbrella shrubs had 1 main stem and no low branches [173]. Dealy [44] suggests that successful seedling establishment in curlleaf mountain-mahogany/Idaho fescue habitats with heavy deer use may be a result of seedlings being undetected among bunchgrasses and in snow until they reach 6 to 12 inches (20-30 cm), when a well-developed root system and numerous branches increase curlleaf mountain-mahogany's resiliency.
Site characteristics: A number of studies report on the importance of shallow litter depths and open canopies in the successful emergence and establishment of curlleaf mountain-mahogany seedlings. While seeds emerged successfully in deep litter substrates, establishment was rare. Establishment was more successful on low litter or bare mineral soil substrates outside of the curlleaf mountain-mahogany canopies.
Curlleaf mountain-mahogany seedlings were most common in the deep duff and litter (>5 inches (13 cm)) under and near mature trees in south-central Utah. However, seedlings in the deep duff lacked a main taproot, and fewer seedlings established in the deep litter than in the open sites with shallow litter layers (<4 inches (10 cm)). Four- to five-year-old seedlings were 4 to 10 inches (10-30 cm) tall, while 20-year-old seedlings were 2 to 3 feet (0.6-0.9 m) tall [207]. Regeneration was low in closed-canopy curlleaf mountain-mahogany stands in Idaho, Montana, and Nevada. Shading, litter, and absence of bare mineral soil were suggested as possible barriers to successful seedling establishment. No seedlings over 2 years old were found on sites with litter depths greater than 0.25 inch (0.63 cm). Open-canopy stands were "regenerating at a sustained rate" [71].
In northeastern Utah's Cache National Forest, survival of 6-week-old seedlings in open interspaces, under big sagebrush, and beneath curlleaf mountain-mahogany was compared. Survival was low in the interspaces in a dry year, but good in a wet year, especially when herbivory was excluded. Findings stressed the importance of interactions between microhabitat and environmental conditions in seedling establishment [90]. In the same area, researchers found that emergence was significantly (p<0.10) better on soils without litter than those with litter. However, seedling survival was significantly greater on sites with litter than those without. Emergence was best in the open interspaces, with sparse litter and adequate light and temperatures. Establishment was best in the litter under curlleaf mountain-mahogany trees. Researchers thought that seedlings experienced moderated temperatures, increased moisture, and some herbivore protection under the canopy. Findings suggested that seedling establishment would be successful in the open interspaces when conditions were wet and herbivory was low. However, seedlings that established under trees may not reach maturity [91,92].
Twenty-five curlleaf mountain-mahogany stands were studied in western and central Nevada. Seedling production ranged from zero to abundant. Reproduction was abundant only on sites with an open canopy. Poor seedling establishment and survival in old (>500 years) stands with high curlleaf mountain-mahogany cover and crown volume was likely due to seedling root development. In deep litter, roots rarely extended into the mineral soil, and seedlings dried out and died. Under a closed canopy, seedlings rarely grew beyond 0.3 inch (7 mm) in basal diameter. Some curlleaf mountain-mahogany plants that were only a few inches tall were aged to over 30 years. Researchers predicted that the suppressed juvenile plants in the overstory would come to occupy gaps created in the overstory through mortality [175,176,177].
Significantly (p<0.05) more current-year seedlings occurred in the curlleaf mountain-mahogany stands, but significantly more established seedlings and juveniles occurred in nearby sagebrush (Artemisia spp.) communities in the Shoshone Range in Nevada. Fewer immature curlleaf mountain-mahogany plants (5) were found in curlleaf mountain-mahogany stands than in big sagebrush communities (21), but differences were not significant. Curlleaf mountain-mahogany seedlings in curlleaf mountain-mahogany stands had extensive root growth up to 6 inches (20 cm), but growth was largely lateral in litter layer. Rarely did roots reach the mineral soil [178].
Asexual regeneration: Curlleaf mountain-mahogany asexual regeneration is limited. Sprouting may occur after "light" fires [71], but typically sprouts do not live beyond the 2nd or 3rd postfire year [145]. Plants in southwestern Montana that were approximately 15 feet (4.5 m) tall and pruned to 3 foot (1 m) heights sprouted new growth when at least 1 stem was left on the cut plants. Cut stems ranged from 2 to 5.9 inches (5-15 cm) in diameter. Increase of annual biomass on uncut plants was140 g/plant and on cut plants was 436 g/plant [151].
