5.1 Study sites
We selected ten forest stands across four regions in the eastern US that spanned a hydroclimatological gradient (Fig. 1, Table 1). Four of the stands were ~85-year-old temperate deciduous forest AmeriFlux sites (US-MMS, US-CWT, US-Dk2, and US-MOz) in the states of Indiana (IN), North Carolina (NC), and Missouri (MO). The gradient approach allowed us to understand how key plant hydraulic traits varied as a function of climate. Additionally, the ~85-year-old stands in IN and NC were each end-members of a chronosequence (including ~15- and ~35-year-old stands co-located within 20 km of the ~85-year-old stand). The chronosequences in NC and IN allowed us to investigate how the relationship between ΨL behavior and vulnerability to hydraulic failure varied with stand age in regions experiencing a similar climate.
Indiana chronosequence stands:
The ~85-year-old (IN 85yo) (39° 19’ 23.52”, -86° 24’ 47.16”) and ~35-year-old (IN 35yo) (39° 19’ 19.87”, -86° 28’ 51.92”) IN stands were located in Morgan-Monroe State Forest. Dominant species were A. saccharum, L. tulipifera, Q. alba, Sassafras albidum Nutt., Quercus rubra L., and denseLindera benzoin L. understory (Roman et al ., 2015). Deep silt clay loam soils characterized the sites (90–120 cm). The ~15-year-old stand (IN 15yo) (39° 13’ 10.93”, -86° 32’ 30.96”) was a nearby (<20 km) regenerating planting with similar species composition located at The Indiana Research and Teaching Preserve’s Bayles Road site. There, 5-year-old saplings of common Indiana forest tree species from local forest seed stock were planted in 2006 at a spacing of 5.25m \(\times\) 5.25m and in random 12 \(\times\)12 arrangements (Flory & Clay, 2010).