Individual hydraulic response strategies of four Mediterranean-type shrubs to long-term dry season nitrogen deposition

Background/Question/Methods

Anthropogenic nitrogen (N) deposition represents a significant N input for many semi-arid ecosystems, including southern California.  N deposition can alter plant communities, resulting in species composition shifts and reduced diversity.  On a smaller scale, increased N availability can strongly affect photosynthesis.  Few studies have investigated how changes in N availability affect plant hydraulic properties, including hydraulic conductivity and vulnerability to cavitation.

Experimental plots were established in a completely randomized design at Santa Margarita Ecological Reserve (SMER) and at Sky Oaks Field Station (SOFS) in southern California, USA, in 2003.  Each site has eight 10 x 10 m plots, four of which are fertilized with 50 kg N ha^-1 yr^-1 in a single application during the dry season and four of which are un-manipulated control plots.  The dominant species at SMER are Artemisia californica and Salvia mellifera, and at SOFS are Adenostoma fasciculatum and Ceanothus greggii.  In 2013-14, we measured stem hydraulic conductivity (K_h), xylem vulnerability to cavitation (P₅₀), wood density, specific leaf area (SLA), predawn and midday water potentials (Ψ_predawn and Ψ_midday), leaf hydraulic conductance (K_leaf), and gas exchange including light saturated photosynthetic rate per area and mass (A_area and A_mass) and transpiration (E).

Results/Conclusions

Each species had an individual response strategy to long-term N addition.  Of the four study species, Artemisia californica was particularly responsive, as N addition increased stem hydraulic conductivity and transpiration, and decreased wood density and specific leaf area.  N addition also increased transpiration in C. greggii.  Adenostoma fasciculatum was more vulnerable to xylem cavitation in N plots than in control plots.  July predawn xylem water potentials were less negative in N plots for all species except A. fasciculatum, but there was no difference in water potentials between N and control plots at midday, February predawn, nor February midday.  There were no differences in leaf hydraulic conductance, light saturated photosynthetic rate per area or mass between N and control plots for any species.  This Mediterranean-type ecosystem receives most precipitation during the winter months and is subject to an annual seasonal water deficit, which may limit the range of plant responses to increased N availability.