Sitka willow (Salix sitchensis) is a foundational species that has recently become common on primary successional sites of Mount St. Helens Pumice Plain, a 15 km2 landscape formed and sterilized by pyroclastic flows in 1980. Willow is the only common colonizing species with substantial vertical structure, providing habitat for bird and small mammal and affecting ecosystem processes through high annual litter input to soils impoverished of organic matter. Individual plants tend to be much smaller in drier upland areas than riparian zones, possibly due to episodic summer drought stress. Among reproductive willow plants in this landscape, females outnumber males roughly 2:1 in both upland and riparian habitat. We investigate whether 1) secondary sexual dimorphism (SSD) in gas exchange rates and water use efficiency (WUE) of juvenile willow in wet and dry habitat could be a contributing factor to female bias in mature plants and 2) how drought stress impacts juvenile willow survivorship, size, and physiology. In 2007, we planted 120 willow cuttings of known gender in upland habitat on the Pumice Plain. Whips were continuously watered for two seasons (2008-2009) to allow establishment. In 2010 and 2011 we performed a dry-down experiment on a subset of these willows and measured gas exchange rates using a Licor 6400. We measured survival at the end of the establishment phase and the end of the experiment, as well as plant size annually from 2009-2011.
Gender and the dry-down treatment had an additive effect on photosynthetic rate and conductance, and a marginal interactive effect on WUE of experimental willows in 2011, but neither of these factors affected survivorship at the end of the establishment phase, or their size and growth rate at the end of the experiment. In 2011, watered plants had almost a twofold increase in photosynthetic rates and threefold increase in conductance over ambient plants while male plants had 14% higher photosynthetic rates and 20% higher conductance than females. These results suggest that drought can have a large impact on carbon acquisition by juvenile willows and that annual variation in soil moisture and temperature will largely dictate whether certain years are stressful for willows in upland areas. Gender differences confirm the presence of SSD in S. sitchensis but do not support the hypothesis that SSD causes differential mortality in juvenile willow contributing to the large female bias at the landscape level.