title: Assessing the effects of nitrogen deposition and climate on carbon isotope discrimination and intrinsic water-use efficiency of angiosperm and conifer trees under rising CO2 conditions.
abstract: The objective of this study is to globally assess the effects of atmospheric nitrogen deposition and climate, associated with rising levels of atmospheric CO2 , on the variability of carbon isotope discrimination (Δ(13) C), and intrinsic water-use efficiency (iWUE) of angiosperm and conifer tree species. Eighty-nine long-term isotope tree-ring chronologies, representing 23 conifer and 13 angiosperm species for 53 sites worldwide, were extracted from the literature, and used to obtain long-term time series of Δ(13) C and iWUE. Δ(13) C and iWUE were related to the increasing concentration of atmospheric CO2 over the industrial period (1850-2000) and to the variation of simulated atmospheric nitrogen deposition and climatic variables over the period 1950-2000. We applied generalized additive models and linear mixed-effects models to predict the effects of climatic variables and nitrogen deposition on Δ(13) C and iWUE. Results showed a declining Δ(13) C trend in the angiosperm and conifer species over the industrial period and a 16.1% increase of iWUE between 1850 and 2000, with no evidence that the increased rate was reduced at higher ambient CO2 values. The temporal variation in Δ(13) C supported the hypothesis of an active plant mechanism that maintains a constant ratio between intercellular and ambient CO2 concentrations. We defined linear mixed-effects models that were effective to describe the variation of Δ(13) C and iWUE as a function of a set of environmental predictors, alternatively including annual rate (Nrate ) and long-term cumulative (Ncum ) nitrogen deposition. No single climatic or atmospheric variable had a clearly predominant effect, however, Δ(13) C and iWUE showed complex dependent interactions between different covariates. A significant association of Nrate with iWUE and Δ(13) C was observed in conifers and in the angiosperms, and Ncum was the only independent term with a significant positive association with iWUE, although a multi-factorial control was evident in conifers.
authors: Dipartimento di Scienze Ambientali, Università di Parma, via G. P. Usberti 11, 43100, Parma, Italy.
authors: Stefano
authors: S
authors: Leonardi
authors: Tiziana
authors: T
authors: Gentilesca
authors: Rossella
authors: R
authors: Guerrieri
authors: Francesco
authors: F
authors: Ripullone
authors: Federico
authors: F
authors: Magnani
authors: Maurizio
authors: M
authors: Mencuccini
authors: Twan V
authors: TV
authors: Noije
authors: Marco
authors: M
authors: Borghetti
keywords: Carbon dioxide; Carbon isotope discrimination; Climate; Forest; Linear Mixed-Effect Model; Nitrogen deposition; Tree ring; Water-use efficiency