title: Unravelling spatiotemporal tree-ring signals in Mediterranean oaks: a variance-covariance modelling approach of carbon and oxygen isotope ratios.
abstract: Identifying how physiological responses are structured across environmental gradients is critical to understanding in what manner ecological factors determine tree performance. Here, we investigated the spatiotemporal patterns of signal strength of carbon isotope discrimination (Δ(13)C) and oxygen isotope composition (δ(18)O) for three deciduous oaks (Quercus faginea (Lam.), Q. humilis Mill. and Q. petraea (Matt.) Liebl.) and one evergreen oak (Q. ilex L.) co-occurring in Mediterranean forests along an aridity gradient. We hypothesized that contrasting strategies in response to drought would lead to differential climate sensitivities between functional groups. Such differential sensitivities could result in a contrasting imprint on stable isotopes, depending on whether the spatial or temporal organization of tree-ring signals was analysed. To test these hypotheses, we proposed a mixed modelling framework to group isotopic records into potentially homogeneous subsets according to taxonomic or geographical criteria. To this end, carbon and oxygen isotopes were modelled through different variance-covariance structures for the variability among years (at the temporal level) or sites (at the spatial level). Signal-strength parameters were estimated from the outcome of selected models. We found striking differences between deciduous and evergreen oaks in the organization of their temporal and spatial signals. Therefore, the relationships with climate were examined independently for each functional group. While Q. ilex exhibited a large spatial dependence of isotopic signals on the temperature regime, deciduous oaks showed a greater dependence on precipitation, confirming their higher susceptibility to drought. Such contrasting responses to drought among oak types were also observed at the temporal level (interannual variability), with stronger associations with growing-season water availability in deciduous oaks. Thus, our results indicate that Mediterranean deciduous and evergreen oaks constitute two clearly differentiated functional groups in terms of their carbon and water economies, despite co-existing in a wide range of environments. In contrast, deciduous oaks form a rather homogeneous group in terms of climate sensitivity.
authors.affiliation: Department of Ecology, University of Barcelona, Avda Diagonal 645, E-08028 Barcelona, Spain.
authors.forename: Tatiana A
authors.initials: TA
authors.lastname: Shestakova
authors.affiliation: Department of Crop and Forest Sciences-AGROTECNIO Center, University of Lleida, Avda Rovira Roure 191, E-25198 Lleida, Spain.
authors.forename: Mònica
authors.initials: M
authors.lastname: Aguilera
authors.affiliation: Department of Crop and Forest Sciences-AGROTECNIO Center, University of Lleida, Avda Rovira Roure 191, E-25198 Lleida, Spain.
authors.forename: Juan Pedro
authors.initials: JP
authors.lastname: Ferrio
authors.affiliation: Department of Ecology, University of Barcelona, Avda Diagonal 645, E-08028 Barcelona, Spain.
authors.forename: Emilia
authors.initials: E
authors.lastname: Gutiérrez
authors.affiliation: Department of Crop and Forest Sciences-AGROTECNIO Center, University of Lleida, Avda Rovira Roure 191, E-25198 Lleida, Spain jvoltas@pvcf.udl.cat.
authors.forename: Jordi
authors.initials: J
authors.lastname: Voltas
mesh_terms: D000222:Adaptation, Physiological; D002244:Carbon / Q000378:metabolism*; D002247:Carbon Isotopes / Q000378:metabolism*; D002980:Climate; D055864:Droughts; D065928:Forests; D019083:Mediterranean Region; D008954:Models, Biological; D010100:Oxygen / Q000378:metabolism*; D010103:Oxygen Isotopes / Q000378:metabolism*; D029963:Quercus / Q000254:growth & development / Q000378:metabolism / Q000502:physiology*; D015398:Signal Transduction; D013045:Species Specificity; D014197:Trees / Q000502:physiology; D014867:Water; D014934:Wood / Q000254:growth & development* / Q000378:metabolism
chemical_list: D002247:Carbon Isotopes; D010103:Oxygen Isotopes; D014867:Water; D002244:Carbon; D010100:Oxygen
keywords: Iberian Peninsula; Quercus; dendroecology; mixed models; summer drought; tree-ring networks; water availability