Tree core samples were collected in 2017-18 from eight common
northeastern U.S. tree species which have some degree of shade
tolerance in the sapling stage. Sampled stands were all naturally
regenerated, fairly mature secondary forests with a closed canopy and
no evidence of stand-scale disturbance in the past 30 years. To reduce
variation in water status unrelated to tree size, we sampled in flat
or mid-slope positions, avoiding microsites with unusually high or low
soil moisture.
Our sampling methods were adapted from Brienen et al. (2017). Across
all size classes present in each stand, we sampled trees of
representative canopy position, health, and vigor for their size. We
avoided sampling trees with evidence of a substantial recent change in
canopy position. Some samples were collected in or near single-tree
natural canopy gaps, but we avoided forest edges and multi-tree gaps.
Sapwood samples were collected with a 4.3 mm-diameter increment borer,
to a depth of at most 5 cm. One sample was collected per tree, at
approximately 30 cm height, to minimize the effects of sampling on
timber value and hydraulic function. The height of sampling on parts
of the bole below well all live branches is not thought to
systematically affect wood delta13Cp (Leavitt, 2010).
Diameter at breast height and canopy position were recorded for each
sampled tree. The height of each sampled tree was measured from at
least two vantage points using a Haglof Vertex IV hypsometer. Crown
illumination categories were adapted from Clark and Clark (1992), with
a greater focus placed on diffuse light below the canopy, due to the
latitude of our study sites. These categories reflect judgements about
summer mid-day illumination conditions, regardless of the season in
which sampling occurred.
In stands where we could find them, saplings (defined here as any tree
< 4 cm DBH but at least 5 years old) were also included in the
analysis, by collecting 5-year old twig segments from the upper,
middle, and lower third of the crown. Twig segments were aged by
counting terminal bud scars. Bark was removed before drying, and
lengths proportional to the total 2013 growth of each twig segment
were composited for processing.
For most species, additional twig samples were taken opportunistically
from saplings growing nearby in fully illuminated conditions (in
recent clearcuts, utility rights-of-way, and clearings around
meteorological stations). These samples are intended to help separate
the direct effect of size from those of canopy position, and to
represent the initial growth conditions of trees that regenerated
following canopy-removing disturbance. A small number of young
saplings were also sampled in a recent strip-cut partial harvest at
the Jones site, to provide comparable data for saplings with an
intermediate light environment (no canopy directly overhead, but
nearby trees > 20 m tall providing shade for much of the day
regardless).
After air-drying samples, growth rings from 2013-17 were identified,
measured on a sliding-stage micrometer under 10-40X magnification, and
separated with a razor blade from bark and older wood. Core and twig
samples were extracted to alpha-cellulose using a Jayme-Wise procedure
adapted from Leavitt and Danzer (1993) by Gregg and Brooks (2003).
Homogenized samples were analyzed for delta13C on an Isoprime IRMS at
the University of New Hampshire Instrumentation Center. Median
delta13C precision (difference between duplicates) of 15 samples
analyzed in duplicate was 0.03 per mil (maximum 0.23 per mil).
We calculated 13C discrimination and intrinsic water use efficiency
for each sample following Farquhar et al. (1982, 1989) and McCarroll
and Loader (2004), without attempting to account for
post-photosynthetic C fractionation. For these calculations, we
assumed that the atmospheric CO2 concentration was 401 ppm and the
delta13C of the atmosphere was -8.44 per mil, which are the mean
values observed at Mauna Loa in 2015 (Keeling et al., 2017; Scripps
CO2 Program, 2019).
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A., Bottrell, S., Callaghan, M., Heaton, T., Helama, S., Helle, G.,
Leng, M.J., Mielikainen, K., Oinonen, M., Timonen, M., 2017. Tree
height strongly affects estimates of water-use efficiency responses to
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and alpha-cellulose extraction. US EPA Integrated Stable Isotope
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Scripps CO2 Program, 2019. Scripps Institution of Oceanography [WWW
Document]. URL
http://scrippsco2.ucsd.edu/data/atmospheric_co2/mlo.html