Abstract
The uptake of carbon dioxide (CO2) from the atmosphere through photosynthesis is accompanied by an inevitable loss of water vapor through the stomata of leaves. The rate of leaf-level CO2 assimilation per unit stomatal conductance, i.e. intrinsic water-use efficiency (WUEi), is thus a key characteristic of terrestrial ecosystem functioning that is central to the global hydroclimate system. Empirical evidence and theory suggest a positive response of forest WUE to increased CO2 levels globally. Although evidence exists for a positive effect of ecosystem nitrogen (N) inputs on WUEi, it is not clear how trends in atmospheric N deposition have affected WUEi in the past. Here we combine twentieth-century climate and nitrogen deposition with stable isotope signature in tree rings and document a WUEi trend reversal at two sites in Switzerland, that matches the timing of a trend reversal in atmospheric N deposition. Using generalized additive models (GAMs), we fitted observed WUEi time series to multiple environmental covariates. This suggested N deposition to have a significant effect on long-term WUEi at the site that was exposed to higher N deposition levels. The ratio of the increase in WUEi in response to increase in CO2 (dWUEi/dCO2) declined by 96% after 1980 (from 0.53 to 0.02) in the beech forest and declined by 72% in the spruce forest (from 0.46 to 0.13) concurrent with a sharp decline in N deposition. Using the GAM model for two scenarios, we show that had N deposition levels not declined after 1980s, WUEi would have increased more strongly in response to increasing CO2. Although the increase in N deposition was limited to the 1950–1980 decades and the signals have declined with improvements in air quality across Europe, the role of atmospheric pollution must be reconsidered in interpretation of tree ring studies and for building environmental proxies that are pivotal to understanding future sink capacity of terrestrial ecosystems in response to climate change.
Highlights
Intrinsic water-use efficiency (WUEi), or the ratio of photosynthesis to stomatal conductance determines the balance between carbon gain and water loss through stomatal pores and is a key characteristic of terrestrial ecosystem functioning (Beer et al 2009)
At the leaf level, N fertilization can increase investment in enzymes resulting in increased assimilation (A) for a given stomatal conductance, at the plant and ecosystem level effect of N deposition on WUEi can be negative via enhancing leaf area and concurrently transpiration and compensating leaf-level water saving effects (Zhu et al 2016, Lu et al 2018, Liang et al 2020)
The response of trees and their WUEi to N deposition are further complicated by the interaction with concomitantly increasing air temperatures, evaporative demand, and atmospheric CO2 concentrations (Fernandez-Martínez et al 2014, Grossiord et al 2020), which all can lead to stomatal closure and increasing WUEi (Hatfield and Dold 2019)
Summary
Intrinsic water-use efficiency (WUEi), or the ratio of photosynthesis to stomatal conductance determines the balance between carbon gain and water loss through stomatal pores and is a key characteristic of terrestrial ecosystem functioning (Beer et al 2009). A large body of evidence shows a steadily increasing WUEi trend over the past decades inferred from multiple types of observations including atmospheric and ecosystem flux measurements, and tree-ring studies (Keenan et al 2013, Frank et al 2015, Keeling et al 2017, Guerrieri et al 2019, Belmecheri et al 2020, Mathias and Thomas 2021). To date, understanding how centennial WUEi is affected by long-term climate and atmospheric changes has focused predominantly on climatic factors such as precipitation patterns, temperature, and atmospheric CO2 concentrations (e.g. Keenan et al 2013, Saurer et al 2014, Frank et al 2015, Belmecheri et al 2021) and considerably fewer studies have explored the role of N deposition (but see Leonardi et al 2012, Levesque et al 2017, Adam et al 2021) despite the importance of N supply for assimilation and plant response to atmospheric CO2 (Oren et al 2001). Plant water loss is affected by increase in air temperature and evaporative demand on two opposing directions: in one direction increase in evaporative demand directly increases transpiration, and on the other hand increase in evaporative demand indirectly reduces transpiration due to stomatal closure (Massmann et al 2018)
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