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Alnus maritima (Marsh.) Muhl. ex Nutt. is a large shrub or small tree with potential for use in managed landscapes. Because the three subspecies of A. maritima are indigenous only to areas with mild winter temperatures (USDA hardiness zones 7a and 7b), knowledge of their cold acclimation and cold hardiness is vital if they are to be used where winters are more harsh. Phenology and depth of cold hardiness were assessed by collecting stem samples seven times from 25 Sept. 2000 to 23 Apr. 2001, subjecting the samples to cold temperature ramping, and determining the lowest survival temperature (LST) via the tissue discoloration method. Samples were collected from indigenous plants of the three subspecies and from plants growing in a common garden near Ames, Iowa (USDA zone 5a). Results indicated that some plants from all three subspecies can survive midwinter extremes as low as -80 °C; that plants grown in Ames achieved a greater depth of cold hardiness for most of the winter and were more uniform in cold hardiness than plants growing in warmer native sites; and that the three subspecies did not differ in phenology or depth of cold acclimation. Results of field trials with plots of 150 plants each installed in three northern hardiness zones (USDA zones 5a, 4a, and 3a) supported these conclusions by showing survival of all 450 plants. We resolved differences among subspecies by rating the percentage of stem tissue survival for each plant in the field plots. Subspecies maritima, from the northernmost provenance (the Delmarva Peninsula), showed the least stem death across all three plots (3.9% tissue death), followed by subsp. georgiensis from northwestern Georgia (10% tissue death), and subsp. oklahomensis from southern Oklahoma (12.8% tissue death). Our results suggest that low temperatures should not limit the use of A. maritima in areas as harsh as USDA zone 3a. Selections based on cold hardiness may allow the use of A. maritima in areas with even colder winters.

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Alnus maritima (Marsh.) Muhl. ex Nutt. is a rare woody plant species that exists as three subspecies found in widely disjunct locations in the United States. Although there is a growing interest in the phytogeography, ecology, conservation, and landscape potential of this species, the phylogeny of A. maritima has not yet been resolved by using molecular methods. We have combined a relatively new method of genome fingerprinting, ISSR-PCR, and the automated imaging capabilities of GeneScan technology to investigate the molecular systematics of A. maritima. Based on the molecular evidence from 108 ISSR loci, we confirm that the three disjunct populations of A. maritima have diverged sufficiently to be classified as subspecies. Our molecular phylogeny of the three subspecies of A. maritima agreed in topology with a phylogeny produced from morphological data and showed that subsp. oklahomensis is the most distinct of the three subspecies and was the first to diverge. The simultaneous analysis of molecular and morphological data provides a detailed and balanced phylogeny reconstruction for the three subspecies. Our results support the theory that A. maritima originated in Asia, migrated into North America across the Bering land bridge, and was established over a large range in the New World before being forced into its present meager disjunct distribution.

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Genotypic variation and horticultural potential of Alnus maritima [Marsh.] Nutt. (seaside alder), a large shrub or small tree found naturally in only three small, disjunct populations, have not been studied. We examined effects of population of origin and environment on seed germination and growth and morphology of seedlings. The first experiment showed that 6 weeks of cold stratification optimized germination of half-sibling seeds from Oklahoma at 73.2%. When this treatment was applied to multiple groups of half-siblings from all populations in a second experiment, seeds from Oklahoma had a higher germination percentage (55.0%) than seeds from Georgia (31.4%) and the Delmarva Peninsula (14.7%). In a third experiment, morphology and growth of multiple groups of half-siblings from all three populations were compared in one environment. Leaves of seedlings from Oklahoma were longer (12.8 cm) and more narrow (2.15 length to width ratio) than leaves of seedlings from Georgia (12.0 cm long; ratio = 1.76) and the Delmarva Peninsula (11.6 cm long; ratio = 1.86). Seedlings from Oklahoma and Georgia accumulated dry weight at higher rates (181 and 160 mg·d-1, respectively) than seedlings from Delmarva (130 mg·d-1), while seedlings from Oklahoma and Delmarva were more densely foliated (0.72 and 0.64 leaves and lateral shoots per centimeter of primary stem, respectively) than those from Georgia (0.46 per cm). These differences indicate genetic divergence among the three disjunct populations and the potential to exploit genetic variation to select horticulturally superior A. maritima for use in managed landscapes.

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Selection of sugar maples (Acer saccharum Marsh.) and black maples (Acer saccharum Marsh. ssp. nigrum Desm. or Acer nigrum Michx. f.) that will be more resilient than existing cultivars in managed landscapes could be facilitated by defining relationships between geographic origin and foliar traits critical to leaf function. We examined variation in leaf morphology and anatomy of both taxa, known collectively as hard maples, near 43 °N latitude and tested for relationships between foliar traits and the longitude of origin from 70 ° to 94 °W longitude. Leaves exposed to direct solar radiation were sampled from up to 20 trees indigenous at each of 42 sites during 1995 and 1996. All leaves from east of 75.84 °W and from 92.73 °W and further west expressed morphological characters associated with sugar maple and black maple, respectively; leaves with intermediate traits were found between these two longitudes. Leaves from 90 ° to 94 °W had the highest surface area due to increases in the areas of middle and proximal portions of laminae. Up to 1162 trichomes/cm2 were present on the abaxial surface of laminae from west of 85 °W, while laminae from further east were glabrous or had ≤300 trichomes/cm2. Laminae from western habitats also had relatively high stomatal frequency, and stomatal apertures of laminae west of 91 °W were particularly narrow. Longitude did not affect specific weight and thickness of laminae, which averaged 5.5 mg·cm-2 and 90 μm, respectively. Principal component analysis of laminar traits showed existence of two clusters. A large group dominated by data from trees in New England also contained data from trees as far west as ≈93 °W longitude; data for trees further west were clustered separately. Although phenotypic continua were defined, laminae west of 93 °W were distinct, which suggests trees selected there may function differently in managed landscapes than trees selected from native populations further east.

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Information on the heat resistance of silver maple (Acer saccharinum L.) could help develop stress-resistant Freeman maples (Acer ×freemanii E. Murray). Our first objective was to determine how 26, 30, 32, 34, and 36 °C in the root zone affect growth and water relations of plants from rooted cuttings of a silver maple clone indigenous to Mississippi (33.3 °N latitude). Fresh mass increased over time for plants at all temperatures and was highest for plants with root zones at 30 °C. Quadratic regression functions predicted maximal plant dry mass, leaf surface area, and stomatal conductance at 29, 29, and 28 °C, respectively. Stem xylem water potential (ψ) during the photoperiod decreased linearly with increasing root-zone temperature from -0.83 MPa at 26 °C to -1.05 MPa at 36 °C. Our second objective was to compare six clones of silver maple from the Mississippi location with six clones from 44.4 °N latitude in Minnesota for effects of 35 °C in the root zone on plant growth, stomatal conductance, and stem ψ. Provenance and temperature main effects were significant for most dependent variables, but there were no provenance × temperature interactions. Over both provenances, plant fresh and dry mass, leaf surface area, stomatal conductance, and stem ψ during the photoperiod were higher at 29 than 35 °C. Over both temperatures, plants from Minnesota clones had higher fresh and dry mass and more leaf surface area than plants from Mississippi clones. The lack of temperature × provenance interactions suggests that ecotypic or clinal variation in heat resistance is minimal and will not be useful for identifying superior genotypes for use in interspecific crosses with red maple (Acer rubrum L.).

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Responses of five bottomland tree taxa to drought and flooding were studied to identify those adapted to urban environments. During one experiment, containerized `Franksred' red maple [Acer rubrum L. `Franksred' (trademark = Red Sunset)], sweetbay magnolia (Magnolia virginiana L.), black tupelo (Nyssa sylvatica Marsh.), bald cypress [Taxodium distichum (L.) Rich.], and pawpaw [Asimina triloba (L.) Dunal.] were treated with various irrigation regimes for up to 118 days. Net assimilation rate (NAR) and relative growth rate (RGR) were reduced more by flooding than by drought for plants of all taxa, except pawpaw, which showed similar NAR and RGR during flooding and drought. Only sweetbay magnolia and bald cypress maintained positive NAR and RGR during flooding, and sweetbay magnolia was the only taxon that did not produce significantly less leaf surface area, shoot dry mass, and root dry mass during flooding and drought. Apparent morphological mechanisms of stress resistance included an increase in specific mass of leaves (mg·cm-2) during drought for red maple and bald cypress and a 385% increase in the root: shoot mass ratio for droughted plants of pawpaw. Leaf water relations of drought- and flood-stressed `Franksred' red maple and sweetbay magnolia were determined in a second experiment. Predawn and mid-day leaf water potential (ψ) decreased with decreasing root-zone matric potential for both taxa, and transpiration rate was reduced by drought and flooding. Pressure-volume analysis showed that leaves of `Franksred' red maple responded to drought by shifting symplastic water to the apoplast. Leaves of drought-stressed sweetbay magnolia adjusted osmotically by reducing osmotic potential (ψπ) at full turgor by 0.26 MPa. Our results suggest that sweetbay magnolia and bald cypress will perform well at urban planting sites where episodes of drought and flooding regularly occur.

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Abstract

Root-zone temperature (RZT) of 15 landscape planting sites in a metropolitan area was monitored from 13 June to 5 Sept. 1985. RZT was highest at urban sites associated with city surface materials, such as asphalt and concrete. The RZT was significantly lower at suburban and woodland sites. Temperature was uniform throughout the root zone at sites along urban streets; it decreased with increasing depth at all other sites. High temperature extremes may contribute to the decline of landscape plants at urban sites.

Open Access

Abstract

Most systems used for controlling rootzone temperature (RZT) involve grouping plants in each treatment together in one temperature-controlling apparatus (3, 5). The power of experiments using systems with grouped plants is limited because the groups constitute single experimental units during data analysis. Some systems have overcome this problem, but reports may lack fabrication details (2) or indicate a limited RZT range was used (1, 4). We designed a precise, inexpensive system capable of achieving a wide range of RZT in which individual plants are discrete experimental units.

Open Access

Amur maackia (Maackia amurensis Rupr. & Maxim.) has potential as a more widely grown nursery crop, but little information is available on effects of media and nutrition on growth of containerized plants. We compared growth of seedlings in five media and determined growth responses to two fertility regimes. After 35 days, total dry mass of plants grown in 1 perlite: 1 vermiculite (by volume) or in 5 sphagnum peat: 3 perlite: 2 soil was 3.2 times the dry mass of plants grown in three soilless media that contained composted bark; and after 70 days, growth was greater in the medium with soil than in 1 perlite: 1 vermiculite. Plants grown in solution culture with N at 0.75 mm had 1.8 times the dry mass of those provided N at 3.75 mm. Form of N in solution did not affect dry mass, but N content of leaves of plants grown with >50% NH 4 + was 1.3 times as great as that of plants provided only NO 3 . Plants in containers attained maximal dry mass when fertilized with solutions containing N at 10.8 mm from NO 3 , NH 4 + , and urea or N at 7.5 mm with equal amounts of NO 3 and NH 4 + . None of the soilless media used consistently evoked growth similar to growth of plants in the soil-based medium.

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