Resistance to water stress of Alnus maritima: intraspecific variation and comparisons to other alders

Alnus maritima (seaside alder) is a rare and threatened woody species that occurs naturally as three disjunct subspecies. While its limited numbers and distribution have prompted investigations into the cause of its rarity, the aesthetic appeal and apparent flood resistance of A. maritima have led to a strong interest in its horticultural potential. The occurrence of all three subspecies of A. maritima exclusively in waterlogged soils in nature suggests that water availability may limit both the success of plants in the wild and the potential for their use in landscapes. We conducted three experiments to assess how A. maritima would respond to a wide range of root-zone moisture contents. First, we examined foliar gas exchange and growth of one clone during drought and flooding. Plants survived, grew, and avoided damage to and abscission of leaves during seven treatments that ranged from complete root-zone inundation to severe drought. Partial flooding and slight drought maximized plant dry weight, while total flooding and mild, moderate, and severe drought limited growth. High rates of photosynthesis were sustained except during severe drought. Next, we examined genotypic variation in drought resistance among the subspecies of A. maritima and compared the resistance of A. maritima to that of four other species of Alnus. Only subtle differences were detected among subspecies in how deficit irrigation influenced the physiology and development of plants. Of the five species evaluated, A. maritima and Alnus nepalensis most strongly expressed indicators of drought resistance. Lastly, a field trial with 270 seedlings of subspecies oklahomensis confirmed that A. maritima can be grown on soils with moisture contents ranging from dry (≈10% water by volume) to saturated. We conclude that, even though A. maritima thrives when a portion of its root zone is constantly inundated, it is more drought tolerant than some other species indigenous to well drained soils. The drought resistance of A. maritima can be attributed in part to its continued high rate of photosynthesis, fine control of stomatal conductance, increased specific leaf weight, and increased root:shoot ratio under conditions of drought. Considering the natural habitat to which A. maritima is restricted, our data illustrate that the niche a species occupies in nature does not necessarily represent how adapted the species may be to dissimilar environmental conditions. We also conclude that A. maritima and A.