Effect of Long-Term vs. Short-Term Ambient Ozone Exposure on Radial Stem Growth, Sap Flux and Xylem Morphology of O3-Sensitive Poplar Trees

Alessio Giovannelli,Elena Paoletti,Maria Laura Traversi,Silvano Fares,Yasutomo Hoshika,Monica Anichini

doi:10.3390/f10050396

Abstract

High ozone (O3) pollution impairs the carbon and water balance of trees, which is of special interest in planted forests. However, the effect of long-term O3 exposure on tree growth and water use, little remains known. In this study, we analysed the relationships of intra-annual stem growth pattern, seasonal sap flow dynamics and xylem morphology to assess the effect of long term O3 exposure of mature O3-sensitive hybrid poplars (‘Oxford’ clone). Rooted cuttings were planted in autumn 2007 and drip irrigated with 2 liters of water as ambient O3 treatment, or 450 ppm ethylenediurea (N-[2-(2-oxo-1-imidazolidinyl)ethyl]-N0-phenylurea, abbreviated as EDU) solution as O3 protection treatment over all growing seasons. During 2013, point dendrometers and heat pulses were installed to monitor radial growth, stem water relations and sap flow. Ambient O3 did not affect growth rates, even if the seasonal culmination point was 20 days earlier on average than that recorded in the O3 protected trees. Under ambient O3, trees showed reduced seasonal sap flow, however, the lower water use was due to a decrease of Huber value (decrease of leaf area for sapwood unit) rather than to a change in xylem morphology or due to a direct effect of sluggish stomatal responses on transpiration. Under high evaporative demand and ambient O3 concentrations, trees showed a high use of internal stem water resources modulated by stomatal sluggishness, thus predisposing them to be more sensitive water deficit during summer. The results of this study help untangle the compensatory mechanisms involved in the acclimation processes of forest species to long-term O3 exposure in a context of global change.

Highlights

Tropospheric ozone (O3 ) is considered a serious health issue for forests [1] in many areas of the world [2]
The net primary productivity of three forest communities composed of trembling aspen, paper birch and sugar maple was unaffected by long-term O3 fumigation, because the growth of O3 -tolerant genotypes compensated the reduction of Net primary productivity (NPP) of O3 -sensitive genotypes [12]
Our results showed that long-term O3 exposure of O3 -sensitive poplar trees induced significant changes in stem radial growth and tree water use

Summary

Introduction

Tropospheric ozone (O3 ) is considered a serious health issue for forests [1] in many areas of the world [2]. One of the major unanswered questions about O3 impacts on mature trees, is still about growth under real-world conditions [8]. The assessment of the long-term O3 effect on forest stand productivity still remains insufficiently detailed [9] and contrasting results were often reported in the literature. Net primary productivity (NPP) of aspen trees was insensitive to long-term O3 fumigation treatments [10] and similar results were reported for Picea abies (L.) H. The net primary productivity of three forest communities composed of trembling aspen, paper birch and sugar maple was unaffected by long-term O3 fumigation, because the growth of O3 -tolerant genotypes compensated the reduction of NPP of O3 -sensitive genotypes [12]

Methods

Results

Discussion

Conclusion