Carbon isotope discrimination, water-use efficiency, growth, and mortality in a natural shrub population

In order to scale up from the ecophysiological characters of individual plants to population-level questions, we need to determine if character patterns in natural populations are stable through time, and if the characters are related to growth and survival. We investigated these questions in a 3-year study for one character, integrated water-use efficiency (WUE) as estimated by carbon isotope discrimination (δ) in a population of the Great Basin shrub, Chrysothamnus nauseosus. WUE was a conservative character for a given plant within and across seasons, and a previously documented difference between two size classes (represented by juveniles and adults) was maintained; smaller juveniles had a lower WUE than larger adults. The lower WUE of juveniles was often accompanied by higher rates of photosynthesis and stomatal conductance as compared to adults even though juveniles generally had more negative xylem pressure potentials. Although many discussions of the role of WUE in natural populations have been based on the expectation that higher WUE (lower δ) is generally associated with less growth, we found no such relation-ship for juvenile plants in this population (i.e δ was not positively correlated with height increase). In addition, juvenile plant mortality was not correlated with δ. Although there were stable patterns of WUE for plants in this population, the positive correlation between WUE and size, and the lack of a negative correlation between WUE and height growth, make it unlikely that the WUE of an individual plant will be related in a simple manner to its growth and survival in the population.