Denny, Geoffrey Carlile (2009-05). Evaluation of selected provenances of taxodium distichum for drought, alkalinity and salinity tolerance.. Doctoral Dissertation.
Thesis
Taxodium distichum (L.) Rich. is a widely adaptable, long-lived tree species for landscape use. It is tolerant of substantial soil salt levels, but tends to defoliate in periods of extended or severe drought, when leaves come into contact with salty irrigation water, and tends to develop chlorosis on high pH soils. The purpose of this research was to identify provenances which may yield genotypes tolerant of these stresses. The appropriate name for baldcypress is Taxodium distichum (L.) Rich. var. distichum, for pondcypress is T. distichum var. imbricarium (Nutt.) Croom, and for Montezuma cypress is T. distichum var. mexicanum Gordon. A germination study of T. distichum var. mexicanum revealed that if immediate germination of ripe seed is desired, then the best treatments are a citric acid soak and hot water baths, however, if seeds can be stratified, then no pre-germination seed treatment is needed. Citric acid scarification and hot water baths produced the best germination. Stratification hastened germination rates and cumulative mean germination percentages. Stratification for 45 d appears to be sufficient, although for the best pre-germination treatments stratification requirements were less pronounced. Greenhouse screening studies of open-pollinated families for drought tolerance show genotypes from eastern localities were less tolerant than western genotypes. Taxodium distichum likely relies on both drought avoidance and drought tolerance strategies to deal with drought stress. A field screening for alkalinity tolerance showed a strong geographic component to the variation in tolerance of alkaline soils. When selecting plant material for an alkaline site, genotypes from Mexico and south Texas should be preferred, followed by central Texas genotypes. Greenhouse salinity screening showed that most genotypes tolerate moderate levels of soil salts, but at high soil salinities the tolerance appears to be highly genotype-dependent, rather than having a strong geographic pattern. Field evaluations demonstrated that T. distichum var. mexicana grew more rapidly on three Texas sites than the other varieties. These evaluations also suggest that when selecting plant material for an alkaline or xeric site, Mexican and south Texas genotypes should be preferred, followed by central Texas genotypes. Cold tolerance was not determined north of USDA hardiness zone 8.
