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Alnus maritima (Marsh.) Muhl. ex Nutt. is a large shrub or small tree with potential for use in managed land- scapes. 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 subspe- cies 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 fi eld 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 fi eld 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. Alnus maritima (seaside alder) is a rare woody species that occurs naturally as three subspecies from widely disjunct provenances in the United States. Alnus maritima subsp. oklahomensis Schrader & Graves is found naturally in only two counties in south-central Oklahoma, A. maritima subsp. georgiensis Schrader & Graves is isolated to one county in northwestern Georgia, and A. maritima subsp. maritima is found only on the Delmarva Peninsula in four counties in Maryland and two counties in Delaware (Schrader and Graves, 2002). There is a growing interest in the use of A. maritima in managed landscapes, but the proper geographical range for the use of this species has not been established. The natural distribution of A. maritima is limited to areas with relatively mild winter temperatures. Subspecies oklahomensis and maritima occur in USDA hardiness zone 7a, where the mean minimum temperature is -15 to -18 °C, and subsp. georgiensis occurs in zone 7b with a mean minimum of -12 to -15 °C (Agriculture Research Service, 1990). Although it appears that a lack of cold hardiness has limited A. maritima from areas with harsher winters, the present distribution of the species may be only a remnant of a much larger range that included areas as far north as Washington and Idaho (Furlow, 1979; Schrader and Graves, 2000). While Alnus is among the most cold hardy genera in the world (Vezina et al., 1997), little information is available on A. maritima. Therefore, our overall goal was to model the phenology and cold acclimation of this species. The peculiar natural distribution of A. maritima provided a unique situation for the study of plant cold hardiness. Their isolation and extreme geographic disjunction suggest that the three subspecies of A. maritima may differ in phenology and depth of cold acclimation, therefore it is possible that genotypes from one of the subspecies may be more suitable for use in colder areas. We examined the cold hardiness of plants from all three subspecies growing both in natural environments and in fi eld plots we established in Iowa and Min- nesota. Our objectives were to characterize the phenology of winter dormancy, cold acclimation, and return of active shoot growth; to determine the effects of colder climates on the cold hardiness of A. maritima; to compare the cold hardiness of A. maritima with that of four other species of Alnus; and to establish the safe geographical range for A. maritima in managed landscapes as defi ned by plant cold hardiness. Our results will be used to develop guidelines for proper use of A. maritima in managed landscapes and will help guide plant breeding and selection efforts. They also provide new clues concerning the origin of the disjunct distribution of A. maritima.

Phenology and Depth of Cold Acclimation in the Three Subspecies of Alnus maritima