Craine S. I. and Orians C. M. (Tufts University, Medford, Mass.). Effects of flooding on pitch pine (Pinus rigida Mill.) growth and survivorship. J. Torrey Bot. Soc. 133(2): 289–296. 2006.—Pitch pines (Pinus rigida Mill.) colonize the shores of Cape Cod coastal plain ponds in periods of low water and, once established, inhibit the growth of resident herbaceous species, including rare endemics. The effects of flooding on P. rigida may vary with plant age and may be affected by previous exposure to flooded conditions. To determine the effects of flooding on pitch pine survivorship, we performed three sets of experiments. First, we examined the effects of relative flood depth—root flooding versus total submersion—on three-month-old seedlings. Second, we determined whether prior exposure to root flooding enhanced subsequent survivorship in flooded conditions one year later. Lastly, we compared the effects of root flooding on pines of different ages—three months, 15 months, and five years. We found that three-month-old P. rigida succumbed to total submersion in two to four weeks but tolerated root flooding for up to one year. Fifteen-month-old seedlings that had survived eight weeks of root flooding one year earlier were significantly more flood-tolerant than members of the same cohort that had never been flooded. Fifteen-month-old seedlings, as well as five-year-old saplings, suffered significantly more mortality after four months of root flooding than did three-month-old seedlings. The ability of young P. rigida, normally an upland plant, to survive in waterlogged soil appears to be due, in part, to morphologic modifications acquired in response to flooded conditions, including hypertrophy of lenticels on the stem and plagiotropism. Both these acquired modifications are known to help oxygenate root tissues through internal channels, avoiding cellular anoxia despite reduced oxygen availability in the rhizosphere. Overall, our results suggest that periodic flooding has historically played a role in preventing the colonization of P. rigida on coastal plain pond shores and that reduced flooding intensity may lead to more frequent encroachment of P. rigida in this habitat.