North American temperate conifer (Tsuga canadensis) reveals a complex physiological response to climatic and anthropogenic stressors

Abstract

Rising atmospheric CO 2 (c a) is expected to promote tree growth and lower water loss via changes in leaf gas exchange. However, uncertainties remain if gas-exchange regulation strategies are homeostatic or dynamical in response to increasing c a , as well as evolving climate and pollution inputs. Using a suite of tree ring-based δ 13 C-derived physiological parameters (Δ 13 C, c i , iWUE) and tree growth from a mesic, low elevation stand of canopy-dominant Tsuga canadensis in northeastern USA, we investigated the influence of rising c a , climate and pollution on, and characterised the dynamical regulation strategy of, leaf gas exchange at multidecadal scales. Isotopic and growth time series revealed an evolving physiological response in which the species shifted its leaf gas-exchange strategy dynamically (constant c i ; constant c i /c a ; constant c a − c i) in response to rising c a , moisture availability and site conditions over 111 yr. Tree iWUE plateaued after 1975, driven by greater moisture availability and a changing soil biogeochemistry that may have impaired a stomatal response. Results suggested that trees may exhibit more complex physiological responses to the changing environmental conditions over multidecadal periods, and complicating the parameterisation of Earth system models and the estimation of future carbon sink capacity and water balance in midlatitude forests and elsewhere.

Precipitation alters the CO2 effect on water‐use efficiency of temperate forests

“…The increasing Δ 13 C trend was attributed to increases in stomatal conductance and A , and was interpreted as a recovery of tree physiology and growth from peak‐pollution levels, but, also in response to N status and warming growing season temperature. In one of the investigated sites in this study (ABP), acid deposition and soil biogeochemistry (Ca 2+ leaching) were shown to drive the increasing Δ 13 C trends after 1970 CE, albeit, to a lesser extent than soil moisture and VPD (Rayback et al, 2020). For some broadleaf species located in NE‐US, water availability was shown to override the effects of CO 2 , nitrogen, and acid deposition on leaf‐gas exchange and tree growth after 1980 CE (Levesque et al, 2017).…”

“…Individual whole rings were separated using a razor blade under a microscope, and α‐cellulose was extracted from the wood samples following the protocol described by Leavitt and Danzer (1993). We measured δ 13 C from 1901 to 2012 at three sites including US‐Ha1 and US‐Ho1 (two flux tower sites at Harvard and Howland Forests, respectively) and ABP (Abbey Pond Forest, Rayback et al, 2020). We measured δ 13 C from 1992 to 2012 at eight additional sites to overlap with the common period of the longest flux‐tower record at Harvard Forest (US‐Ha1; Figure 1a; Table S1).…”

On tree longevity

“…It is worth mentioning that large uncertainties still exist with regard to sinks and fluxes in soils, vegetation and the associated boundary layer (e.g. Rayback et al ., 2020).…”

Adding Tree Rings to North America's National Forest Inventories: An Essential Tool to Guide Drawdown of Atmospheric CO2

“…CO2 enrichment experiments, eddy flux measurements, and remote sensing observations all show that increased atmospheric CO2 leads to increased water-use efficiency and net primary productivity, although the magnitude and duration of this fertilization effect varies (Walker et al 2020 ). Evidence from tree rings also finds increased water-use efficiency (via stable isotope studies), although more recently these estimates have been shown to be influenced by tree size and environmental factors (Marchand et al 2020 , Rayback et al 2020a ). In contrast, tree-ring evidence for a fertilization effect on radial tree growth is limited (Hickler et al 2008 , Girardin et al 2011 , 2016 , Frank et al 2015 , Giguère-Croteau et al 2019 , Hararuk et al 2019 ).…”