Light use and water relations of three evergreen shrubs in an eastern temperate forest understory

Background/Question/Methods

Water relations and light-use parameters of three evergreen woody shrubs were characterized in a temperate forest to investigate physiological strategies for survival in the light-limited understory.  The three species represent different families and include Morella cerifera (Myricaceae), Ilex opaca (Aquifoliaceae), and Kalmia latifolia (Ericaceae).  Life history characteristics differ among these species in that Morella is a nitrogen-fixer with indeterminate growth whereas the other two species are non-fixers and display determinate growth forms. These species are sympatric in distribution; however they can exist in substantially different microenvironments.  Morella is generally found in higher light environments. At this site, Morella is not found strictly within the understory, but is beginning to fill out the edge of the understory; light availability may be limiting encroachment in the forest interior.  We wanted to determine if differences in physiological strategies would reflect differences in habitat preference among species.  Parameters measured included stem conductivity, water content of leaves, stomatal conductance, leaf water potential, fluorescence, and photosynthesis. 

Results/Conclusions

In situ loss of conductivity at mid-day was higher in Kalmia than the other two species, indicating that it is more susceptible to cavitation at comparable mid-day xylem pressure potentials.  Leaf specific conductivity of Morella was higher than that of the other two species. Stem conductivity was also higher for Morella and maximum conductivity was more than two-fold greater than that of Ilex or Kalmia.  Morella, had a much lower molar C:N ratio (21 ± 0.06) than the two non-N fixing species (~ 48), indicating a lower nitrogen use efficiency.  Water use efficiency; however, was relatively comparable among species.  Morella appears to be capable of higher light-use efficiency than either Ilex or Kalmia based on mid-day fluorescence values and it is also able to maintain higher stomatal conductance at more negative water potentials.  Differences in physiological response among species were reflected by differences in species location and microhabitat.