Response of mature forests to nitrogen deposition: insights from a dendroecological approach in a nitrogen manipulation experiment in Italy 

Abstract

<p>The ability of forests to continue absorbing atmospheric carbon dioxide (CO<sub>2</sub>), and hence mitigating climate change, depends on the extent to which their productivity is limited by nutrients, with nitrogen hypothesised to be particularly important in temperate regions. Fertilization experiments offer an opportunity to directly determine levels of nitrogen limitation in forests, and whether nitrogen input from atmospheric deposition can alleviate it. Yet, very few such studies have been carried out in mature temperate broadleaf forests. These mature forests are considered to play a critical role in the current carbon (C) sink in temperate regions, and thus our incomplete understanding of their level of nitrogen limitation, under a warmer and CO<sub>2</sub> richer conditions, represents a major knowledge gap. In this context, the nitrogen fertilization experiment established in a mature beech (<em>Fagus sylvatica</em> L.) forest (average age 140 years) in Northern Italy represents a unique resource for advancing understanding on forest responses to global change. At this site, four different treatments (n=3 plots per treatment) have been carried out since 2015: control, canopy (30 kg ha<sup>-1</sup> yr<sup>-1</sup>) and soil (30 and 60 kg ha<sup>-1</sup> yr<sup>-1</sup>) N additions in the form of ammonium nitrate (NH<sub>4</sub>NO<sub>3</sub>). This study aimed at: <em>i)</em> exploring long-term changes in tree growth (i.e., 1940-2020) and elucidate main drivers (climate vs. anthropogenic factors), and <em>ii)</em> investigating whether tree growth and intrinsic water-use efficiency (i.e., iWUE, the ratio between photosynthesis and stomatal conductance) are enhanced by the simulated increase in atmospheric nitrogen deposition. To achieve these goals, wood cores were sampled from n=15 trees per treatment (5 trees per plot) and used for dendrochronological analyses to obtain tree-ring width series, which were then converted to basal area increments (BAI). Preliminary analyses on growth data indicated that BAI has increased over the last 80 years, and that variations in BAI were more explained by climate than anthropogenic factors. Indeed, changes in BAI were neither related to atmospheric CO<sub>2</sub> nor NO<sub>y</sub> deposition, while a positive relationship between BAI and NH<sub>x</sub> deposition was observed, likely reflecting different dynamics of the two N forms when moving within the ecosystem (e.g., preferential uptake of NH<sub>4</sub><sup>+</sup> by trees and nitrogen loss from the ecosystem through NO<sub>3</sub><sup>- </sup>leaching). When focusing on the 6 years of the manipulation experiment (2015-2020), trees in the soil nitrogen treatments showed greater BAI (expressed as difference from the control) compared to those that received nitrogen misting over tree canopies. Further analyses, including the measure of stable carbon, oxygen and nitrogen isotope ratios in tree rings, will contribute to elucidating physiological mechanisms underpinning growth changes in relation to differences in ecosystem nitrogen dynamics.</p>