Degradation of coastal forests and associated wetland habitats by excessive flooding and saltwater intrusion is a global problem, and may become even more so if predicted climate changes and consequent rises in sea level occur. In the United States, there’s been great concern about the degradation of the entire Mississippi River Delta biotic system, much of which can be traced to manmade changes in the nature and flow of the Mississippi river. One example of this degradation is the loss of coastal forests south of New Orleans, which has left this city more vulnerable than ever to the impact of hurricanes. (Allen 1992; Earles 1975; Krauss et al. 1999)
These circumstances make it increasingly important to identify, select, and even improve tree species that have some innate tolerances to flooding and salinity. Such trees will be valuable for restoring degraded coastal areas as well as for urban landscapes and other greening projects. For this reason, we are particularly interested in Taxodium distichum.
Taxodium Taxa
Of all native swamp forest tree species in the southern United States, Taxodium distichum (baldcypress) has long been recognized as being among the most tolerant to flooding and salinity. This long-lived and generally pest-free deciduous conifer is a popular landscape tree in many parts of the world. Once established, Taxodium is tolerant of flooding, salt, alkalinity, and strong winds.
The precise nomenclature for Taxodium remains a matter of some debate. Once considered three species—T. distichum (baldcypress), T. ascendens (pondcypress), and T. mucronatum (Montezuma cypress)—we believe there’s enough consensus in recent literature to list Taxodium distichum as a single species with three botanical varieties:
- Taxodium distichum (L.) Rich. var. distichum (baldcypress)
- Taxodium distichum imbricarium (Nutt.) Croom (pondcypress)
- Taxodium distichum mexicanum (Carriere Gordon) (Montezuma cypress) (Arnold and Denny 2007)
While baldcypress and pondcypress natural ranges overlap in many areas across the South, the commingling of baldcypress and Montezuma cypress natural ranges is less apparent. Hardin (1971) was the first to speculate on the nature of intermediates where baldcypress and pondcypress ranges overlap. The same is perhaps true for baldcypress communities in central and southwestern Texas. They are often Montezuma cypress-like, leading many to believe this is the result of natural introgression present between baldcypress and Montezuma cypress in this transitional zone.
Baldcypress is native to much of the southeastern United States, from Delaware to Texas and inland up the Mississippi River to southern Indiana. It occurs mainly along rivers with alluvial flood deposits. Baldcypress is a durable conifer particularly well adapted to wetland habitats. It is easy to grow from seed and is relatively free of pests and diseases. The tree is modestly to highly resistant to cercosporidium needle blight and tolerates compacted soils and low-oxygen or swampy soil conditions. It stands strong in the face of hurricanes, is amazingly long lived (1,000+ years) and, with time, can become quite large (70+ feet [21+ meters] tall). Baldcypress produces knees (pneumatophores), which are considered a negative in most landscaping situations since they can interfere with routine maintenance such as lawn mowing. While their exact function is unknown, knees may contribute substantially to wind throw resistance (Conner et al. 2002).
Baldcypress in the western part of its range (central and western Texas) is generally more salt and alkalinity tolerant, and is less prone to produce knees than baldcypress from more eastern sources. East Texas genotypes of Taxodium planted in San Antonio, Texas, where soils are highly alkaline, often turn chlorotic and perform poorly. As with pond cypress and baldcypress, botanists and horticulturists speculate that baldcypress in central to western Texas are perhaps commingled with Montezuma cypress and represent transitional genetics (Lickey and Walker 2002).
Pondcypress occurs in the southern portion of the range of baldcypress and only on the southeastern coastal plain from North Carolina into Louisiana. While southeast Texas is not normally included as part of the pondcypress natural range, an approximately 1,200-year-old pondcypress at Shangri La Gardens, Orange, Texas, appears to broaden the range. Pondcypress occurs in blackwater rivers, ponds, bayous, and swamps, usually without alluvial flood deposits. Pondcypress is relatively easy to distinguish by its feathery foliage, which is ascendant, rather than more splayed and flat as in baldcypress, but this may not always be consistent. Landscapers often use pondcypress as a specimen, particularly when moist soil conditions exist and a smaller stature (40+ feet [12+ meters]) is desired.
Montezuma cypress should probably be named Moctezuma cypress because by all accounts it has the name of the fifth Aztec King, Moctezuma (1466–1520), whose reign included the first contact between the Mesoamerican civilization and Europeans. It is popular in Mexico among pre-Hispanic cultures and is widely planted in public parks and gardens in most major cities in Mexico. A Montezuma cypress near Oaxaca, Mexico, the famous “Árbol del Tule,” features a trunk over 56 feet (17 meters) in diameter and is estimated to be over 2,500 years old.
Montezuma cypress is native to Mexico (in 27 of the 32 states), some areas of Guatemala, the tip of South Texas, and, perhaps, a few populations in New Mexico. It typically grows next to water sources such as creeks, rivers, lakes, and ponds and performs better in deep loamy soils than in volcanic soils where firs, pines, and oaks are found. While it will grow in a hot tropical climate, it does not perform best there.
Montezuma cypress differs from baldcypress and pondcypress in several ways: it is substantially evergreen, produces smaller seeds, never produces distinct knees, is generally more tolerant of salt and alkaline soils, and is less tolerant of extended flooding. At Stephen F. Austin State University Gardens in Nacogdoches, Texas, Montezuma cypress forces new growth early in the spring and continues to grow late into the fall. Observations of Montezuma cypress in USDA plant hardiness zone 8 (average annual minimum temperature 10 to 20°F [-12 to -7°C]) and lower suggest that there may be hardiness and winter damage issues, particularly with trees derived from lowland, subtropical Mexican genotypes. This may be a seed source provenance problem, and there is good reason to believe that Montezuma cypress can be grown much further north if the proper genotypes are selected as seed sources.
Montezuma cypress is not usually considered a superior landscape tree in the southern United States since it often fails to form a strong central leader and is generally more susceptible to Cercosporidium needle blight than baldcypress, especially when grown in humid areas. However, there are exceptions, and further breeding and selection may bring better choices. At Stephen F. Austin State University Gardens there are several Montezuma cypress specimens worth noting, including one that survived the December 23, 1989 freeze (0°F [-17.8°C]) with no damage. Over the years, Montezuma cypress has withstood droughts of considerable magnitude at Stephen F. Austin State University Gardens. In fact, we note that Montezuma cypress can shed almost all its foliage in a summer drought, yet it will push new growth when rain or irrigation finally returns. Montezuma cypress kept in a high state of vigor often keeps foliage through mid-winter.
In Mexico, Montezuma cypress is much appreciated, but little genetic improvement has been undertaken. Coauthor Teobaldo Eguiluz-Piedra is supervising a large planting of genotypes near Texcoco, Mexico, that includes ten provenances. While just in the first year, there are already apparent differences in foliage color, tree form, growth rate, and branching characteristics. In Mexico, Montezuma cypress is considered quite variable from one provenance to another and nursery conditions can greatly impact growth rate and form. The Viveros Genfor nursery in Texcoco has grown Montezuma cypress for the last twenty years and reports that it requires no more water than ash, oaks, or other conifers, contrary to what might be expected from Montezuma cypress’s natural preference for a riparian habitat. Most of the nursery’s propagation is by seed collected from mature trees that are more than 500 years old. Viveros Genfor is also cloning the oldest Montezuma cypress trees nearby using juvenile tissue from rooted cuttings with a modest success rate.
Taxodium Cultivars
While most Taxodium plants sold in the United States are seedlings, there are a number of cultivars available, primarily of baldcypress. Mostly available as grafted trees through specialty nurseries, baldcypress cultivars vary in form, ultimate size, and foliage color. For over twenty years, Stephen F. Austin State University Gardens has acquired a wide array of cultivars from specialty nurseries, arboretum and botanical garden collections, and private conifer enthusiasts. Baldcypress cultivars at the Gardens include ‘Sofine’ (Autumn Gold™), ‘Pendens’, ‘Mickelson’ (Shawnee Brave™), ‘Fastigiata’, ‘Contorta’, ‘Secrest’, ‘Hurley Park’, ‘Peve Minaret’, ‘Peve Yellow’, ‘Jim’s Little Guy’, ‘Cody’s Feathers’ (synonym ‘Wooster Broom’), ‘Cave Hill’, ‘Cascade Falls’, and ‘Falling Water’. Only two pondcypress cultivars are listed—‘Prairie Sentinel’ and ‘J.B.’—and two cultivars of Montezuma cypress, the mounding weeper ‘McClaren Falls’ and modestly weeping ‘Sentido’, can also be seen in the collection. In addition to cultivars, Stephen F. Austin State University Gardens has numerous specimens of baldcypress, pondcypress, and Montezuma cypress from a wide range of documented provenances.
Taxodium Hybrids
Controlled Taxodium hybridization (crosses between botanical varieties of Taxodium distichum) can combine the best characteristics of superior parents and allow for selection of superior clones from the progeny. Much hybridization work has occurred at the Nanjing Botanical Garden, where selection criteria for controlled cross and open pollinated seed crops include growth rate, salinity and alkalinity tolerance, flooding tolerance, Cercosporidium needle blight resistance, form, and ease of cutting propagation. In several studies in China, Taxodium hybrids often demonstrated improvements in growth rate, salt tolerance, form, and vigor.
One Taxodium hybrid was given the cultivar name ‘Nanjing Beauty’ and was cooperatively introduced in 2004 by Nanjing Botanical Garden and Stephen F. Austin State University Gardens. A baldcypress × Montezuma cypress cross, this clone was originally selected in 1988 from the breeding work of Professor Chen Yong Hui at the Nanjing Botanical Garden. Chen and others report that the selection’s attributes include 159% faster growth than baldcypress, longer foliage retention in fall and early winter, and no knees. It also tolerates alkaline soils and fairly high salt concentrations. Cuttings root at good percentages and the clone is commercially available in China. ‘Nanjing Beauty’ is currently under evaluation in over 30 locations in the southern United States and is offered by several nurseries across the South.
Additional crosses made at the Nanjing Botanical Garden in 1992 used pollen from a superior selection of Montezuma cypress applied to female flowers of ‘Nanjing Beauty’ (then known as selection T302). Fifteen selections were made in 1995, with the main characteristics for selection being fast growth rate, dark green leaf color during the growing season, and red-orange leaf color in the autumn. Several of these clonal selections are now widely used in China. Additional Montezuma cypress × baldcypress hybrids have been selected, including four “merit” clones that have been verified by molecular identification, and tested in the field for salt tolerance, growth rate, form, etc. (http://sfagardens.sfasu.edu/images/files/Documents/PDF%20Archive/dissertation%2010-02-2011.pdf). These selections are rapidly multiplied by cutting propagation, with high rooting percentages the norm. Acceptance of “merit” clones by the industry in China indicates that more and more Taxodium cultivars will enter the commercial market in the future. With great potential for use as timber, energy biomass, carbon sinks, and water conservation forests, Taxodium hybrids can be widely used for urban and rural greening, shelterbelts for farmland, and forests for coastal areas in southeastern China.
Asexual Propagation of Taxodium
For superior Taxodium clones to make a substantial impact on nursery numbers, it is important to propagate asexually. While grafting is common (especially for ornamental cultivars such as those with dwarf or weeping forms), it is expensive. Cutting propagation of Taxodium is generally reported as difficult, but rooting success is influenced by genotype, the physiological age of the clone, rooting hormones, substrate, and the vigor of the cutting wood. (Pezeshki and DeLaune 1994; St. Hilaire 2003; Zhou 2008).
Young trees generally root with greater ease than older trees. Coauthor Yin Yunlong reports that the original plant of ‘Nanjing Beauty’, selected in 1988, has over time become more difficult to root, a condition attributed to chronological and physiological age factors.
To counter lower rooting percentages, a strict protocol for achieving cutting propagation success has been developed. Small well-rooted liners are field planted at close spacing and grown for one year, with trees often reaching 3.3 to 6.6 feet (1 to 2 meters) in the first growing season. Then, in that first winter, they are cut back to 1 foot (0.3 meters) tall. These pollarded trees produce vigorous upright shoots in the spring. Cuttings are collected in early summer and one upright shoot is left on the stock tree to grow for the rest of the season into a straight tree, 6.6 feet (2 meters) tall, ready for sale in the winter. Yin Yunlong notes that collecting cuttings from upright shoots produces upright growing plants of better form than trees produced from cuttings taken from side branches, a technique to avoid the problem of plagiotropic growth.
Early summer cuttings are rooted under part shade to sun, using intermittent mist and a well drained mix in deep rooting beds. While rooting hormones are utilized, cutting wood quality and maintaining good turgor are recognized as critical factors for high rooting percentages (80+%). Four cutting trials in 2006 at Stephen F. Austin State University Gardens indicated that a high concentration of K-IBA (5,000 to 10,000 ppm) improved rooting as did slightly wounding the basal portion of the stem. Other studies indicate better rooting with hormones, very well drained substrates like perlite, and no wounding (Zhou 2007, King et al. 2011).
Improving Taxodium
Several Taxodium germplasm collections exist in the southern United States but they remain relatively unexploited. In addition to the Taxodium collection at Stephen F. Austin State University Gardens, Dr. Donald L. Rockwood, University of Florida, Gainesville, Florida, manages a large planting of varied genotypes, many of which serve as seed sources for superior seedlings, with plantings that target tolerance of fly ash, salinity, or polluted soils. Dr. Ken W. Krauss, at the United States Geological Survey, National Wetlands Research Center, Lafayette, Louisiana, is collecting seed from survivor trees in the Mississippi Delta that have been exposed to increasing inundation and salt surges (Krauss et al. 2000; Conner and Inabinette 2005). By cruising the massive “ghost cypress forests” (large stands of dead or declining baldcypress) of the southern delta, individual survivor trees can be found that perhaps have good resistance to subsidence and high salinity. Their progeny may offer promise for reforestation projects in marginal sites, and the opportunity for selecting superior clones is immense. Finally, Dr. Mike Arnold, Texas A & M University, College Station, Texas, has planted a large collection of baldcypress genotypes from across the South; the collection includes central and western Texas provenances, as well as a collection of Montezuma cypress from Mexico and southern Texas (McDonald et al. 2008)
Citation: Creech, D., Zhou, L., Yunlong, Y., and Eguiluz-Piedra, T. 2011. Can Taxodium be improved? Arnoldia, 69(2): 11–20.
Taxodium has many positive environmental attributes as a wetland species and as a landscape plant. It is fortunate that there is such great diversity available in the baldcypress, pondcypress, and Montezuma cypress gene pool, since with great diversity comes great opportunity for selection. No doubt superior Taxodium clones can be found in the progeny from controlled cross and open pollinated seeds. Improvements in salt and alkalinity tolerance, growth rate, resistance to Cercosporidium needle blight, drought resistance, and form could be expected from a breeding program. In the United States and Mexico, where Taxodium is used primarily as an ornamental, the market for improved Taxodium cultivars is relatively small in comparison to China, where Taxodium has a huge market built on hundreds of “greening” companies vying for government contracts. Millions of trees are needed for a wide array of development projects: large gardens and parks, highways, railroad lines, canal edges, and the coastal windbreak forest project. We have much to gain by connecting the native Taxodium germplasm resources in the United States and Mexico with the many Taxodium improvement projects under way in China.
Literature
Allen, J. A. 1992. Cypress-tupelo swamp restoration in southern Louisiana. Restoration Management Notes 10: 188–189.
Arnold, M. A. and G. C. Denny. 2007. Taxonomy and nomenclature of Baldcypress, Pondcypress, and Montezuma Cypress: One, two, or three species? HortTechnology 17(1): 125–127.
Conner, W. H., I. Mihalia, and J. Wolfe. 2002. Tree community structure and changes from 1987–1999 in three Louisiana and three South Carolina forested wetlands. Wetlands 22: 58–70.
Conner, W. H., and L. W. Inabinette. 2005. Identification of salt tolerant baldcypress (Taxodium distichum (L.) Rich) for planting in coastal areas. New Forests 29: 305–312.
Craig, N. J., R. E. Turner, and J. W. Day, Jr. 1979. Land loss in coastal Louisiana (USA). Environmental Management 3: 133–144.
Daniels, R. C. 1992. Sea-level rise on the South Carolina coast: two case studies for 2100. Journal of Coastal Restoration 8: 56–70.
Hardin, J. W. 1971. Studies of the Southeastern United States Flora. II. The Gymnosperms. Journal of the Elisa Mitchell Scientific Society 87: 43–50.
King, A., M. A. Arnold, D. F. Welsh, and W. Todd Watson. 2011. Substrates, Wounding, and Growth Regulator Concentrations Alter Adventitious Rooting of Baldcypress Cuttings. HortScience 46(10): 1387–1393.
Krauss, K. W, J. L. Chamber, J. A. Allen, D. M. Soileau, Jr., and A. S. DeBoier. 2000. Growth and nutrition of baldcypress families planted under varying salinity regimes in Louisiana, USA. Journal of Coastal Restoration 16: 153–163.
Krauss, K. W., R. J. Howard, and T. W. Doyle. 2009. Is there evidence of adaptation to tidal flooding in saplings of baldcypress subjected to different salinity regimes? Environmental and Experimental Botany 67: 118–126.
Lickey, E. B., and G. L. Walker. 2002. Population genetic structure of baldcypress (Taxodium distichum (L.) Rich. var. distichum) and Pondcypress (T. distichum var. imbricarium (Nuttall) Croom): Biogeographic and Taxonomic Implications. Southeastern Naturalist 1(2): 131–148.
McDonald, G. V., G. C. Denny, M. A. Arnold, D. L. Bryan, and L. Barnes. 2008. Comparative Canopy Damage among Provenances of Baldcypress Associated with the Presence of Cercosporidium sequoia (Ellis and Everth.) W.A. Baker and Partridge. HortScience 43: 1703–1705.
Pezeshki, S. R. and R. D. DeLaune. 1994. Rooting of baldcypress cuttings. New Forests 8(4): 381–386.
St. Hilaire, R. 2003. Propagation of Taxodium mucronatum from softwood cuttings. Desert Plants 19(1): 29–30.
Zhou, L. 2007. Salt tolerance, propagation and provenance evaluation of Taxodium as landscape and coastal wetland tree. M.S. Thesis, Department of Agriculture, Stephen F. Austin State Univ., Nacogdoches, TX.
Introduction of merit Taxodium clones of the Nanjing Botanical Garden http://sfagardens.sfasu. edu/UserFiles/File/PLANTS/Taxodium%20 breeding%20brochure%20feb%2020100.pdf
For a more extensive literature review, access the MS thesis and PhD dissertation of coauthor Lijing Zhou under “Arboretum” then “Links” on the Stephen F. Austin State University Gardens website http://sfagardens.sfasu.edu
David Creech is Director of the Stephen F. Austin State University Gardens in Nacogdoches, Texas; Lijing Zhou is a Graduate Research Assistant at Stephen F. Austin State University; Yin Yunlong is a Professor at the Institute of Botany in the Chinese Academy of Sciences and Jiangsu Province, Nanjing Botanical Garden, Nanjing, China; and Teobaldo Eguiluz-Piedra is a forest geneticist and owner of Viveros Genfor nursery in Texcoco, Mexico.
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