The influence of thinning at age eight on the photosynthetic rate, needle conductance, xylem water potential, water use efficiency and
growth of loblolly pine trees was studied during their ninth and tenth growing seasons. Litterfall dynamics were monitored throughout the
study period as well.
At the end of the second post-thinning growing season, trees in
thinned plots had greater stem diameters, greater live crown ratios,
greater live crown diameters and greater average volumes and basal
areas than did trees in control plots.
Significant physiological changes due to thinning were observed
only in the lower crowns where needles were found to adapt to the
sudden increase in available light by adjusting their physiology to
resemble that of sun needles. Lower crown photosynthesis and needle conductance in thinned plots were more similar to upper crown values
for these parameters than to lower crown, control plot values.
Foliage produced during the current year and measured after full
elongation had a higher average photosynthetic rate, needle conductance
and lower average water potential than did foliage in its second
growing season during both 1988 and 1989. Additionally, the water use
efficiency of current-year foliage was significantly greater than that
of the second-year foliage.
In both thinned and control plots, significant diameter growth and
photosynthesis continued throughout the dormant season although at a
slower rate than during the growing season. Litterfall timing did not
differ between thinned and control plots, nor did the amount of
litterfall per unit basal area during the first year following
thinning. Partial second-year data suggest that subsequent litterfall
amounts will mirror the greater average crown size in thinned plots.
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