Light Acclimation by Laurel Oak Seedlings
MATERIALS
AND METHODS
To determine the ability of laurel oak seedlings to acclimate to the changing light environment, photosynthetic light response curves were determined with a LICOR 6400 portable photosynthesis system, using the "LIGHT CURVE" automatic program with an artificial red/blue LED light source. Measurements were made at light levels of 75, 150, 300, 600, 900, and 1200 µmol/m2/s during 1999 and 2000. In 2000, seedlings existing in full sun conditions in a clearcut site were also measured at 1500 and 1800 µmol/m2/s. The leaf chamber environment was maintained at 400 µmol CO2/mol and 30 °C. Physiological responses measured include:
| • net photosynthesis | • transpiration |
| • stomatal conductance | • internal CO2 concentration |
To verify the light conditions of the seedlings, the ambient photosynthetically
active radiation (PAR) was measured at the time of the photosynthesis
measurements, and hemispherical photographs were taken over the seedling
plots.
LIGHT ENVIRONMENT
The
inherent variability of light across a site is large regardless of the
presence of canopy gaps. In 1999, after gaps were enlarged, mean openness
of the gaps in the two sites were similar (14.0 vs. 13.7), although
the range in individual gap openness was broad. The most open gap was
2.5x more open than the least open gap (19.8 vs. 7.1). There was also
considerable overlap between some of the more open non-gap areas and
gaps that had not been opened very fully. In 2000, two years after the
gaps were enlarged, some trees which were girdled still had not died
or fully lost their canopy.
Measurement of PAR by the LICOR 6400 generally
confirmed the light environments shown above in the hemispherical photographs.
Mean light intensity in the non-gap plots was 67 µmol/m2/s, while
that in the gaps was 185 µmol/m2/s.
