Methods description is from Reinmann et al. 2023, Frontiers in Forests and Global Change.
This research was conducted at Black Rock Forest, which is a 1,550-ha research forest located in Cornwall, New York (41°24’ N, 74°01’ W) within the Hudson Highlands Ecoregion. The topography is mountainous with elevations ranging from 110 m to 450 m above mean sea level and forest structure, composition, and recruitment dynamics driven by natural processes. Mean annual precipitation is 1,200 mm evenly distributed throughout the year and air temperatures are strongly seasonal ranging from a mean of -2.7 °C in January to a mean of 23.4 °C in July. The forests are dominated by oak trees (Quercus sp.) with red oak (Q. rubra) comprising the highest proportion of total tree biomass (Schuster et al. 2008). Red maple (Acer rubrum) is increasing in abundance in the canopy and is the most common tree species in the understory (Schuster et al. 2008). Black Rock Forest is within the Hudson Highlands Ecoregion (58i), which spans an approximately 8,000 km2 area from northern New Jersey northeast through southern New York that is dominated by temperate deciduous forest.
In 2018, we established three long-term monitoring plots spanning an elevation range from 300m to 420m at Black Rock Forest. Each plot was centered on three mature red oak trees and two (n = 1 plot) or three (n = 2 plots) mature red maple trees in the canopy (hereafter ‘core trees’). Within each plot, all of the core canopy trees were within 30m of one another.
During March, 2020 (i.e., before the defoliating freeze event), one Meter Group D1 manual dendrometer band (Meter Group, Pullman, Oregon) was installed at breast height on each core red oak and red maple tree in each plot (n = 9 total red oak trees; n = 8 total red maple trees) to quantify temporal patterns in radial tree growth. We recorded dendrometer band measurements bi-weekly from March through October during 2020 and 2021, but growth generally ceased by the end of September. Radial growth measurements were converted to Basal Area Increment (BAI) from:
BAI=(π*r^2 )_t1- (π*r^2 )_t0
where r is the tree radius at the most recent measurement (t1) or the initial measurement (t0).
Following the 2020 freeze event, we selected an additional two canopy red oak trees and two canopy red maple trees in each plot for sampling canopy foliage for leaf-level physiological measurements. Canopy sampling was conducted on separate trees from those with dendrometers to avoid any artifacts of adding disturbance to the trees (i.e., frequent foliar sampling) with dendrometer bands. Using a shotgun, we excised sunlit twigs < 1cm in diameter from the canopy of each of the two red oak trees and two red maple trees designated for canopy sampling in each plot. During both 2020 and 2021, we collected canopy samples once during the early growing season (May 29 to June 7; hereafter ‘Early’), once during the middle growing season (June 16 to July 7, hereafter ‘Mid’), once during the late growing season (August 6 to August 26, hereafter ‘Late’), and once at the end of the growing season (September 11 to September 20, hereafter ‘End’). For a given year and sampling interval, all trees were sampled within a seven-day period. Each excised twig was cut in water generally within three minutes of sampling to maintain the transpiration stream. In a few instances, a twig got hung up as it fell to the ground and it took 5 to 10 minutes to get it into water. However, we have found that for the oak and maple trees in our study site twigs left out of water can maintain stomatal conductance and assimilation rates following excision for at least ten minutes (i.e., the duration of our test). Twigs were kept immersed in water until after leaf-level physiological measurements were made (i.e., within 60 minutes of being excised). All oak leaves sampled were second cohort leaves.
We used a LiCor 6800 portable photosynthesis system with the Multiphase Fluorometer chamber (2 cm2 leaf cuvette; LiCor Bioscience, Lincoln, Nebraska, USA) to measure leaf-level photosynthetic capacity (‘Amax’) and stomatal conductance (‘gsw’) on 2-3 leaves per tree (from the excised branches) on each sampling date. Measurements were made under light saturating conditions (1400 mols m-2 s-1) at ~60% relative humidity, carbon dioxide concentrations of 410 ppm, and air temperatures of 24 to 26 °C. Intrinsic water use efficiency (‘WUE’) was calculated from:
WUE=A/gsw
