Linking hydraulic properties, canopy structure, and light use to shrub expansion

Background/Question/Methods

Shrubs have a unique growth form which allows formation of a dense vertical array of leaves through the canopy while minimizing self-shading.  The increased growth rate, leaf area index (LAI), and water use of shrubs relative to grasslands indicates hydraulic adaptations are of particular importance when considering woody encroachment into grasslands. In temperate habitats, broadleaf shrub species that are invasive or have expansive niche qualities in open grasslands should display high hydraulic efficiency and high photosynthetic potential at the cost of less cavitation resistance.  For our study, hydraulic measurements included in situ stem hydraulic conductivity (k_Hi), maximum hydraulic conductivity (k_{H max}), specific conductivity (k_s), leaf specific conductivity (k_L), midday percent loss of conductivity (PLC), and leaf and stem xylem pressure potential (Ψ_leaf and Ψ_xylem, respectively).  In order to estimate photosynthetic yield, light-adapted rapid light curves were measured on fully expanded, sunlit leaves using a pulse amplitude modulated leaf fluorometer. Canopy variables such as bifurcation (i.e. branching) ratio, leaf area, and leaf area per primary branch were also investigated.

Results/Conclusions

Shrub species that have shown expansive tendancies into open grasslands in eastern temperate environments (e.g. Elaeagnus umbellata and Morella cerifera) displayed higher hydraulic efficiency (2.8 ± 0.2 and 4.0 ± 0.6kg s^-1 MPa^-1 m^-1, respectively) than those that are not expansive (0.70 ± 0.1, 0.73 ± 0.1, and 0.73 ± 0.2 kg s^-1 MPa^-1 m^-1 for Clethra alnifolia, Iva frutescens, and Kalmia latifolia, respectively).  A strong positive relationship was found between k_s and electron transport rate (ETR) at high light levels for measurements taken during summer months.  Leaf specific conductivity was also higher for expansive species.  Bifurcation ratios were higher for E. umbellata and M. cerifera compared to non-expanders, suggesting an architecture that maximizes LAI and light capture.  Expansive species also displayed higher vulnerability to drought-induced cavitation than non-expansive species suggesting a reliance on fairly regular intervals of rainfall, which is characteristic of temperate zones.  Periods of prolonged drought would have dramatic consequences for the expansive species mentioned here because they would significantly reduce hydraulic conductivity and increase the number of embolized vessels, potentially leading to catastrophic hydraulic failure and ultimately reducing rate of expansion.