5.3 Characterizing midday ΨL regulation
Periodic midday ΨL measurements (10:00–16:00 local
time) were compiled from a dataset of over 1600 observations collected
throughout the growing seasons of 2011–2017. On each measurement day,
one to five samples were collected from one to three trees per species
from the upper third of the canopy. After excision, ΨLwas measured using a pressure chamber (PMS Instruments, Corvallis, OR,
USA) (Turner, 1988) immediately in the field, or after being transferred
back to the lab in a humidified bag stored in a chest cooler. All
together, we made 704, 178, and 757 ΨL observations ofL. tulipifera , A. saccharum , and Q. alba ,
respectively. The number of ΨL observations and sampling
days varied across the stands, but ΨL was measured on
4–51 different days at each specific stand. Moreover, these days were
distributed across the growing season, including sampling at the
beginning (June) and end (September) to permit observation throughout
dynamic seasonal changes of moisture conditions.
While drought response behavior is commonly described along an isohydric
spectrum (e.g., \(\partial\Psi_{L}/\partial\Psi_{S}),\) often a species’
relative degree of isohydricity is inconsistent from one stand to the
next (Martínez-Vilatla & Garcia-Forner, 2017). For example,
Roman et al . (2015) reported\(\partial\Psi_{L}/\partial\Psi_{S}\) for Quercus spp. in
southern Indiana was 1.31 during a severe regional drought. Bahariet al . (1985) observed that \(\partial\Psi_{L}/\partial\Psi_{S}\)was 0.28 for Q. alba in the Missouri Ozarks, while a decade
later, Loewenstein & Pallardy (1998) observed that\(\partial\Psi_{L}/\partial\Psi_{S}\) was 0.61 for Q. alba in the
same region. These inconsistent results likely reflect the fact that the
degree of isohydricity, defined
as\(\ \partial\Psi_{L}/\partial\Psi_{S}\), is complicated by
environmental interactions (Hochberg et al ., 2018), including
site-to-site variations in D (Novick et al ., 2019), or
when the magnitude of soil water deficit during the sampling period is
insufficient to capture stress responses (Martínez-Vilatla &
Garcia-Forner, 2017). This is particularly challenging in mesic
ecosystems with frequently saturated soils (e.g., NC_W chronosequence
stands), where the seasonal variation of midday ΨLpatterns may be driven by stomatal sensitivity to D rather than
declines in soil moisture (Novick et al ., 2019). To overcome
these challenges for this study, the degree of isohydricity was
characterized by quantifying seasonal midday ΨLvariability with the assumption that more anisohydric trees, which more
readily allow ΨL to drop in response to soil moisture
decline and/or increasing D , will have a greater seasonal range
of ΨL than more isohydric trees.To minimize error associated with uncharacteristic behavior during
spring leaf out and fall senescence, ΨL data used for
this analysis were constrained to a period of relatively stationary leaf
area index (days of year 150–270).