Timing of False Ring Formation in Pinus halepensis and Arbutus unedo in Southern Italy: Outlook from an Analysis of Xylogenesis and Tree-Ring Chronologies.

Veronica De Micco,Giovanna Battipaglia,Giovanna Aronne,Katarina Čufar,Maks Merela,Jožica Gričar,Angela Balzano

doi:10.3389/fpls.2016.00705

Abstract

Mediterranean tree rings are characterized by intra-annual density fluctuations (IADFs) due to partly climate-driven cambial activity. IADFs are used as structural signals to gain information on relations between environmental conditions and eco-physiological processes during xylogenesis, with intra-annual resolution. To reach an unbiased synchronization of the IADF position within tree rings and seasonal fluctuations in environmental conditions, it is necessary to know the timing of cambial activity and wood formation, which are species- and site-specific processes. We applied the microcoring technique to analyze xylogenesis in Pinus halepensis and Arbutus unedo. To the best of our knowledge, this is the first attempt to study xylogenesis in a hardwood species forming frequent IADFs. Both species co-occur at a site in southern Italy characterized by a Mediterranean climate. To facilitate tree-ring dating and identification of IADFs, we performed traditional dendroecological analysis. We analyzed xylogenesis during summer, which is considered a constraint for xylogenesis and a trigger for IADF formation. We followed the different phases of cell development in the current wood increment with the aim of evaluating whether and which type of IADFs were formed. We additionally analyzed the same phases again in September and in winter to verify the possible formation of IADFs in fall and whether cell production and differentiation was completed by the end of the calendar year. Both species formed the same type of IADFs (earlywood-like cells within latewood), due to temporary growth restoration triggered by rain events during the period of summer drought. At the end of the calendar year, no cells in the phases of enlargement and secondary cell wall deposition occurred. A. unedo was more sensitive than P. halepensis because IADFs were formed earlier in the season and were more frequent in the tree-ring series. The dendro-anatomical approach, combining analysis of tree-ring series and of xylogenesis, helped to detect the period of IADF formation in the two species. Results are discussed in functional terms, highlighting the environmental conditions triggering IADFs, and also in methodological terms, evaluating the applicability of xylogenesis analysis in Mediterranean woods, especially when the formation of IADFs is not uniform around the stem.

Highlights

In P. halepensis, we focused on the development of tracheids, using visual criteria based on lumen size and wall thickness measured by means of the eyepiece micrometer while looking through the microscope
In A. unedo we focused on the development of vessels and imperforate tracheary elements, using visual criteria based on lumen size and wall thickness and on vessel frequency; the transition between earlywood and latewood was often diffuse but latewood could be distinguished by the presence of narrower and less frequent vessels than earlywood (De Micco et al, 2016b)
High expressed population signal (EPS) values (>0.85) for the period of 1921–2014 indicated that the mean chronology was representative of radial growth variations of the whole population of trees (Wigley et al, 1984)

Summary

Introduction

Tree rings are well-established climate proxies: environmental information can be extracted from dated tree-ring series by analyzing the variability in tree-ring width, earlywood and latewood widths, wood density and functional anatomical traits (e.g., Eckstein and Schmidt, 1974; Schweingruber, 1978; Eckstein et al, 1979; Tardif, 1996; Cherubini et al, 2003; Grudd, 2008; Fonti et al, 2010; Esper et al, 2012; Beeckman, 2016). The analysis of tree-ring series is more and more applied in various climatic regions worldwide and has a great potential to reconstruct environmental information with seasonal or intraseasonal resolution, especially under conditions promoting an alternation of growth flushes and dormancy during the year (De Micco et al, 2016a) Within this context, wood of Mediterranean species is interesting because frequent fluctuations in climatic factors exert a control on cambial activity, triggering the formation of intra-annual density fluctuations (IADFs) in tree rings (Cherubini et al, 2003; De Micco and Aronne, 2009; De Micco et al, 2016a). Since different species can show different sensitivities to fluctuating environmental conditions and can be differently prone to form IADFs in various environments, understanding the patterns and processes of xylem formation in response to variable environmental conditions is valuable for forecasting species growth fitness and adaptation capability (Camarero et al, 2010), which are linked to forest dynamics, biomass production and biogeochemical cycles (Cuny et al, 2015; Xia et al, 2015; Pacheco et al, 2016)

Objectives

Methods

Results

Conclusion