Gas exchange measurements
Eleven years of surveys of net assimilation
(Anet ) of CO2 at light saturation
using Li-6400XT – infrared gas exchange analyzers (LICOR, Lincoln,
Nebraska, USA) were conducted in situ on randomly selected
individuals of target species (for details see Tables 3 and S2),
yielding 17,727 unique measurements. Measurements were typically
conducted in one to two weeks long campaigns from two to five times in
each growing season starting late spring and ending in early fall
(i.e., roughly mid-June to the end of September). We used the
same measurement protocol for all gas exchange measurements performed
across all years that defined their scope and constraints of
environmental conditions during which they were performed. Thus, all
measurements were performed between 09:00 hours and 16:00 hours on
foliage from the upper part of the crown, using fully expanded, healthy
leaves or current year needles, ensuring that the foliage we measured
was fully mature and acclimated to growing conditions. Over the course
of the day, we let measurement track ambient conditions for temperature,
and VPD, but to achieve saturating levels of irradiance the
Photosynthetically Active Radiation (PAR) was set to 1200 µmol
m-2 s-1 for plants grown in the open
canopy and 800 µmol m-2 s-1 for
plants grown in the closed canopy. Light levels were chosen based on a
light response curve survey performed in the first year of the study and
represent the light intensity needed to saturateAnet for all species. Relative humidity (RH) in
the leaf chamber ranged from 40 to 80% with the target goal of 60%.
The air flow rate was set to 500 µmol s-1 and the leaf
temperature and VPD (Vapor Pressure Deficit) were unconstrained due to
limitations of the instrument to control temperature and VPD under field
conditions. Across all measurements, leaf temperatures ranged from 16.5
– 35.7°C (for 90% of the data) with a mean of 27.2°C and
VPDchamber (Vapor Pressure Deficit of the air inside the
leaf chamber) ranged from 1.0 to 2.89 kPa (for the 10% and 90%
quantile respectively) and a mean of 1.89 kPa. For the other
meteorological background information related to conditions to which
plants were exposed and during which measurements were taken refer to
Tables 1 and 2 and Figures S1 and S2 that describe overall research plot
level conditions that plants experienced. The environmental conditions
under which foliage gas exchange was measured were used to parameterize
the USO model and obtain estimates of g1 .