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Water availability drives gas exchange and growth of trees in northeastern US, not elevated CO 2 and reduced acid deposition

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ABSTRACT

Dynamic global vegetation models (DGVM) exhibit high uncertainty about how climate change, elevated atmospheric CO2 (atm. CO2) concentration, and atmospheric pollutants will impact carbon sequestration in forested ecosystems. Although the individual roles of these environmental factors on tree growth are understood, analyses examining their simultaneous effects are lacking. We used tree-ring isotopic data and structural equation modeling to examine the concurrent and interacting effects of water availability, atm. CO2 concentration, and SO4 and nitrogen deposition on two broadleaf tree species in a temperate mesic forest in the northeastern US. Water availability was the strongest driver of gas exchange and tree growth. Wetter conditions since the 1980s have enhanced stomatal conductance, photosynthetic assimilation rates and, to a lesser extent, tree radial growth. Increased water availability seemingly overrides responses to reduced acid deposition, CO2 fertilization, and nitrogen deposition. Our results indicate that water availability as a driver of ecosystem productivity in mesic temperate forests is not adequately represented in DGVMs, while CO2 fertilization is likely overrepresented. This study emphasizes the importance to simultaneously consider interacting climatic and biogeochemical drivers when assessing forest responses to global environmental changes.

No MeSH data available.


Hypothetical structural equation model used for assessing the influence of the summer climatic water balance, atmospheric CO2 concentration, SO4 and N wet deposition on tree growth (basal area increment, BAI), and tree gas exchange inferred from Δ13C and δ18O measured in tree rings.Single-headed arrows indicate causal relationships and double-headed arrows denote covariation between response variables. Grey paths indicate covariation between explanatory variables.
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f2: Hypothetical structural equation model used for assessing the influence of the summer climatic water balance, atmospheric CO2 concentration, SO4 and N wet deposition on tree growth (basal area increment, BAI), and tree gas exchange inferred from Δ13C and δ18O measured in tree rings.Single-headed arrows indicate causal relationships and double-headed arrows denote covariation between response variables. Grey paths indicate covariation between explanatory variables.

Mentions: Here, we assess the simultaneous effects of changes in key environmental factors on gas exchange and tree growth in a temperate mesic forest of northeastern US using isotopic records from tree rings of two dominant and widely distributed tree species in eastern North America, Liriodendron tulipifera L. and Quercus rubra L. (see Methods, Fig. 1, Supplementary Table S1). Under dry conditions, L. tulipifera has an isohydric behavior and constrains its stomatal conductance so that mid-day water potential minima is kept below a critical threshold21. In contrast, Q. rubra shows an anisohydric behavior and maintains constant levels of stomatal conductance during drought at the risk of incurring xylem cavitation21. Contrasting physiological behavior and habitats of our study trees make them ideal for isolating growth and physiological responses to concurrent but divergent changes in key environmental factors. We first assess the simultaneous influences of changes in atmospheric CO2 concentration, climatic water balance, and SO4 and N deposition on tree growth and physiological mechanisms with structural equation models (SEM, Fig. 2). Second, we analyze the growth, carbon isotope discrimination (Δ13C), intrinsic water-use efficiency (iWUE), and oxygen isotopic ratio (δ18O) responses of trees to shifting moisture conditions, from the extreme 1960s drought to repeated pluvial periods since the 1980s (Fig. 1c).


Water availability drives gas exchange and growth of trees in northeastern US, not elevated CO 2 and reduced acid deposition
Hypothetical structural equation model used for assessing the influence of the summer climatic water balance, atmospheric CO2 concentration, SO4 and N wet deposition on tree growth (basal area increment, BAI), and tree gas exchange inferred from Δ13C and δ18O measured in tree rings.Single-headed arrows indicate causal relationships and double-headed arrows denote covariation between response variables. Grey paths indicate covariation between explanatory variables.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5385545&req=5

f2: Hypothetical structural equation model used for assessing the influence of the summer climatic water balance, atmospheric CO2 concentration, SO4 and N wet deposition on tree growth (basal area increment, BAI), and tree gas exchange inferred from Δ13C and δ18O measured in tree rings.Single-headed arrows indicate causal relationships and double-headed arrows denote covariation between response variables. Grey paths indicate covariation between explanatory variables.
Mentions: Here, we assess the simultaneous effects of changes in key environmental factors on gas exchange and tree growth in a temperate mesic forest of northeastern US using isotopic records from tree rings of two dominant and widely distributed tree species in eastern North America, Liriodendron tulipifera L. and Quercus rubra L. (see Methods, Fig. 1, Supplementary Table S1). Under dry conditions, L. tulipifera has an isohydric behavior and constrains its stomatal conductance so that mid-day water potential minima is kept below a critical threshold21. In contrast, Q. rubra shows an anisohydric behavior and maintains constant levels of stomatal conductance during drought at the risk of incurring xylem cavitation21. Contrasting physiological behavior and habitats of our study trees make them ideal for isolating growth and physiological responses to concurrent but divergent changes in key environmental factors. We first assess the simultaneous influences of changes in atmospheric CO2 concentration, climatic water balance, and SO4 and N deposition on tree growth and physiological mechanisms with structural equation models (SEM, Fig. 2). Second, we analyze the growth, carbon isotope discrimination (Δ13C), intrinsic water-use efficiency (iWUE), and oxygen isotopic ratio (δ18O) responses of trees to shifting moisture conditions, from the extreme 1960s drought to repeated pluvial periods since the 1980s (Fig. 1c).

View Article: PubMed Central - PubMed

ABSTRACT

Dynamic global vegetation models (DGVM) exhibit high uncertainty about how climate change, elevated atmospheric CO2 (atm. CO2) concentration, and atmospheric pollutants will impact carbon sequestration in forested ecosystems. Although the individual roles of these environmental factors on tree growth are understood, analyses examining their simultaneous effects are lacking. We used tree-ring isotopic data and structural equation modeling to examine the concurrent and interacting effects of water availability, atm. CO2 concentration, and SO4 and nitrogen deposition on two broadleaf tree species in a temperate mesic forest in the northeastern US. Water availability was the strongest driver of gas exchange and tree growth. Wetter conditions since the 1980s have enhanced stomatal conductance, photosynthetic assimilation rates and, to a lesser extent, tree radial growth. Increased water availability seemingly overrides responses to reduced acid deposition, CO2 fertilization, and nitrogen deposition. Our results indicate that water availability as a driver of ecosystem productivity in mesic temperate forests is not adequately represented in DGVMs, while CO2 fertilization is likely overrepresented. This study emphasizes the importance to simultaneously consider interacting climatic and biogeochemical drivers when assessing forest responses to global environmental changes.

No MeSH data available.