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).