Functional traits better correlate with wet season than dry season water potentials in a Californian chaparral plant community

Background/Question/Methods

Plant functional traits are quantitative traits that relate to a species’ ecological role within an environment.  These traits are often coordinated, as demonstrated by the ‘leaf economic spectrum,’ seed mass to output, and others.  In this study, we examined how the functional traits of wood density and leaf mass per area are correlated with each other and with seasonal diurnal water potentials.  Water potentials are a measure of plant water status, and vary based on environmental conditions and species physiological processes. 

We measured woody density, leaf mass per area, and wet and dry season diurnal stem and leaf water potentials for 17 chaparral and coastal sage scrub species at the Santa Margarita Ecological Reserve in California.  We hypothesized that wood density and leaf mass per area would be correlated with dry season rather than wet season water potentials if these functional traits serve these species survival during annual seasonal drought.  Alternatively, we hypothesized that these functional traits would be better correlated with wet season water potentials if these traits serve to optimize resource capture when conditions are favorable.

Results/Conclusions

Woody density and leaf mass per area were positively correlated with each other.  Leaf mass area was negatively correlated with wet season predawn stem and leaf water potential and midday stem water potential.  Leaf mass per area was not correlated with any dry season water potential measurements.  Wood density was negatively correlated with wet season predawn and midday stem and leaf water potentials, and dry season midday stem water potential.  Woody density was not correlated with dry season predawn stem or leaf water potentials, or midday leaf water potential. 

Overall, our functional traits were correlated with more wet season measures of water potential than dry season water potentials.  This supports our alternative hypothesis that wood density and leaf mass per area are optimized for resource capture during favorable environmental conditions rather than survival during stressful drought conditions, when many of these chaparral and coastal sage scrub species “shut down.”