Inter- and intra-population variation in seedling performance of Rio Grande cottonwood under low and high salinity

The Rio Grande watershed ecosystem has been increasingly threatened since the construction of dams and severe channelization in the past century. Rio Grande cottonwood (Populus deltoides var. wislizenii (S. Wats.) Eckenw.) has been suffering stress and high mortality rates with decreased water availability and increased salinity levels. Genetic variation in salt tolerance has been documented in adult cottonwoods, and we hypothesized that these traits might be heritable. This potential heritable genetic variation in seedling offspring might be advantageous in reforestation efforts along the Rio Grande. We screened four New Mexican Rio Grande populations for seedling genotypes that might be salt tolerant and correlated seedling performance under both high- and low-salt treatments with the physiological performance of their open-pollinated family. For all populations, we found significant stunting effects of high salinity on mean leaf size, plant height, total plant mass, root mass, and shoot mass, with no effects on chlorophyll content (as measured by a Minolta SPAD-502 meter) or root/shoot ratio. Although there were no significant differences between the four populations, there were highly significant differences between open-pollinated families within each site. In addition, at one site (San Antonio), genetically based open-pollinated family physiology, as measured in a common garden, was significantly correlated with seedling performance, especially under lowsalt conditions. This indicates these traits are heritable, and adult salt tolerance may convey an advantage in offspring establishment under high-salt conditions.