Environmental effects and biophysical constraints on xylem physiology and tree growth in conifers in the Alps

Trees are impressive long-living organisms that continuously increase in size by many orders of magnitude during ontogeny by accumulating xylem biomass in stem, branches and roots. While growing taller, trees continuously adjust the xylem structure to achieve an optimal balance of carbon costs for the competing biomechanical and hydraulic requirements. One of the main function of the xylem structure is the delivery of the water from the roots up to the leaves. This must be maintained during the ontogeny, when the hydrodynamic resistance increase due to the increase in the xylem path length. However, by widening the diameter of xylem conduit (from the stem apex downwards), trees are able to minimize the negative effect of height growth. Additionally, this widening is stable during ontogeny, thus determining the radial change in conduit dimension with cambial age (from the pith outwards), implying a dependency between the variation of conduit-lumen diameter with cambial age and the rates of stem elongation. These adjustments in the xylem structure remain permanently fixed and chronologically archived in the secondary xylem, and, given the tight link between structures and functions, these provide a ‘time component’ to functional responses induced by xylem plasticity, thus allowing to reconstruct growth dynamics under different environmental conditions. However, there is a lack of detailed information and standardized procedures to explore, at the intra-specific level, the long-term modifications of xylem traits over the full life-span of trees, together with their variability along axial and radial profiles. Additionally, little is known about the relationships between the structures and functions in a view of exploring the future challenges in how a plant’s hydraulic architecture may respond to the ongoing climate change. This thesis, represent a set of studies based on dendro-anatomical and physiological approaches aimed to: - identify priorities and trade-offs among xylem functions; - determine the anatomical traits responsible for them; - retrospectively analyze how these relationships vary during ontogeny under different environmental condition; - analyze the functional response to xylem modifications occurring during ontogeny; - investigate the possibility of retrospectively analyzed height growth based on hydraulic radial profiles. Furthermore, a guidance from sample collection to xylem anatomical data and a new approach to customize cell wall thickness measurements according to the specific aims of the study, were developed. This thesis has highlighted that the xylem anatomical structure of conifer trees (Larix decidua, Picea abies, Pinus cembra) showed a high priority and biophysical determination of traits linked to hydraulic efficiency, such as conduit size, to efficiently support assimilation necessary for tree growth. Besides, other functional traits linked to mechanical support and metabolic xylem functions showed more plastic responses to intrinsic and extrinsic factors. Due to the ontogenetic stability of axial patterns of conduit size, it was possible, based on radial profiles of xylem conduit diameter of tree rings, to estimate tree growth rate, even if species-site specific, and make comparison between trees living in different epochs. In addition, despite the risk of becoming more vulnerable to air seeding cavitation, trees showed to prioritize of hydraulic efficiency vs. safety during the ontogenetic development, as the increase in xylem conductance with tree height determined a contextual decrease in the hydraulic safety margin. This study showed the importance of taking into account the three dimensional anatomical trends to better understand of the trade-offs of hydraulic safety vs. efficiency shape up the tree architecture and affect its adjustments occurring during ontogeny to cope with the arising intrinsic (i.e., size-related) and extrinsic (i.e., environmental) constraints to growth

Hydraulic traits of Juniperus communis L. along elevations and European populations

Plant hydraulics play an important role in determining plant distribution and performance, by influencing their growth and productivity. Knowledge of the hydraulic amplitude and plasticity of species is thus a prerequisite for estimating future performance under climate change. We investigated hydraulic safety and efficiency in Juniperus communis L. to estimate its intra-specific hydraulic variability. We analysed plants growing along an elevational transect (700-2000 m a.s.l., Tyrol, Austria) and plants grown in a common garden experiment from seeds collected in various European regions (France, Austria, Ireland, Germany and Sweden). Vulnerability to drought-induced embolism (i.e. hydraulic safety) was assessed via Cavitron and ultrasonic acoustic emission techniques while specific hydraulic conductivity (i.e. hydraulic efficiency) was measured with a flow meter. Hydraulic safety (water potentials inducing 12, 50 and 88% loss of conductivity) and efficiency did not differ significantly neither across elevations nor between European provenancies. Common juniper proved to a be a species with high resistance to drouht stress and showed surprisingly homogenous hydraulic traits, despite sub-species are formed at higher elevation and plant morphology differed widely across provenancies. Due to its overall high hydraulic safety, this species can be considered as less susceptible to the effects of a warmer climate

Transient Effects of Snow Cover Duration on Primary Growth and Leaf Traits in a Tundra Shrub

With the recent climate warming, tundra ecotones are facing a progressive acceleration of spring snowpack melting and extension of the growing season, with evident consequences to vegetation. Along with summer temperature, winter precipitation has been recently recognised as a crucial factor for tundra shrub growth and physiology. However, gaps of knowledge still exist on long-living plant responses to different snowpack duration, especially on how intra-specific and year-to-year variability together with multiple functional trait adjustments could influence the long-term responses. To fill this gap, we conducted a 3 years snow manipulation experiment above the Alpine treeline on the typical tundra species Juniperus communis, the conifer with the widest distributional range in the north emisphere. We tested shoot elongation, leaf area, stomatal density, leaf dry weight and leaf non-structural carbohydrate content of plants subjected to anticipated, natural and postponed snowpack duration. Anticipated snowpack melting enhanced new shoot elongation and increased stomatal density. However, plants under prolonged snow cover seemed to compensate for the shorter growing period, likely increasing carbon allocation to growth. In fact, these latter showed larger needles and low starch content at the beginning of the growing season. Variability between treatments slightly decreased over time, suggesting a progressive acclimation of juniper to new conditions. In the context of future warming scenarios, our results support the hypothesis of shrub biomass increase within the tundra biome. Yet, the picture is still far from being complete and further research should focus on transient and fading effects of changing conditions in the long term

Osmolality and non-structural carbohydrate composition in the secondary phloem of trees across a latitudinal gradient in Europe

Phloem osmolality and its components are involved in basic cell metabolism, cell growth, and in various physiological processes including the ability of living cells to withstand drought and frost. Osmolality and sugar composition responses to environmental stresses have been extensively studied for leaves, but less for the secondary phloem of plant stems and branches. Leaf osmotic concentration and the share of pinitol and raffinose among soluble sugars increase with increasing drought or cold stress, and osmotic concentration is adjusted with osmoregulation. We hypothesize that similar responses occur in the secondary phloem of branches. We collected living bark samples from branches of adult Pinus sylvestris, Picea abies, Betula pendula and Populus tremula trees across Europe, from boreal Northern Finland to Mediterranean Portugal. In all studied species, the observed variation in phloem osmolality was mainly driven by variation in phloem water content, while tissue solute content was rather constant across regions. Osmoregulation, in which osmolality is controlled by variable tissue solute content, was stronger for Betula and Populus in comparison to the evergreen conifers. Osmolality was lowest in mid-latitude region, and from there increased by 37% toward northern Europe and 38% toward southern Europe due to low phloem water content in these regions. The ratio of raffinose to all soluble sugars was negligible at mid-latitudes and increased toward north and south, reflecting its role in cold and drought tolerance. For pinitol, another sugar known for contributing to stress tolerance, no such latitudinal pattern was observed. The proportion of sucrose was remarkably low and that of hexoses (i.e., glucose and fructose) high at mid-latitudes. The ratio of starch to all non-structural carbohydrates increased toward the northern latitudes in agreement with the build-up of osmotically inactive C reservoir that can be converted into soluble sugars during winter acclimation in these cold regions. Present results for the secondary phloem of trees suggest that adjustment with tissue water content plays an important role in osmolality dynamics. Furthermore, trees acclimated to dry and cold climate showed high phloem osmolality and raffinose proportion.Peer reviewe

Environmental effects and biophysical constraints on xylem physiology and tree growth in conifers in the Alps

Trees are impressive long-living organisms that continuously increase in size by many orders of magnitude during ontogeny by accumulating xylem biomass in stem, branches and roots. While growing taller, trees continuously adjust the xylem structure to achieve an optimal balance of carbon costs for the competing biomechanical and hydraulic requirements. One of the main function of the xylem structure is the delivery of the water from the roots up to the leaves. This must be maintained during the ontogeny, when the hydrodynamic resistance increase due to the increase in the xylem path length. However, by widening the diameter of xylem conduit (from the stem apex downwards), trees are able to minimize the negative effect of height growth. Additionally, this widening is stable during ontogeny, thus determining the radial change in conduit dimension with cambial age (from the pith outwards), implying a dependency between the variation of conduit-lumen diameter with cambial age and the rates of stem elongation. These adjustments in the xylem structure remain permanently fixed and chronologically archived in the secondary xylem, and, given the tight link between structures and functions, these provide a ‘time component’ to functional responses induced by xylem plasticity, thus allowing to reconstruct growth dynamics under different environmental conditions. However, there is a lack of detailed information and standardized procedures to explore, at the intra-specific level, the long-term modifications of xylem traits over the full life-span of trees, together with their variability along axial and radial profiles. Additionally, little is known about the relationships between the structures and functions in a view of exploring the future challenges in how a plant’s hydraulic architecture may respond to the ongoing climate change. This thesis, represent a set of studies based on dendro-anatomical and physiological approaches aimed to: - identify priorities and trade-offs among xylem functions; - determine the anatomical traits responsible for them; - retrospectively analyze how these relationships vary during ontogeny under different environmental condition; - analyze the functional response to xylem modifications occurring during ontogeny; - investigate the possibility of retrospectively analyzed height growth based on hydraulic radial profiles. Furthermore, a guidance from sample collection to xylem anatomical data and a new approach to customize cell wall thickness measurements according to the specific aims of the study, were developed. This thesis has highlighted that the xylem anatomical structure of conifer trees (Larix decidua, Picea abies, Pinus cembra) showed a high priority and biophysical determination of traits linked to hydraulic efficiency, such as conduit size, to efficiently support assimilation necessary for tree growth. Besides, other functional traits linked to mechanical support and metabolic xylem functions showed more plastic responses to intrinsic and extrinsic factors. Due to the ontogenetic stability of axial patterns of conduit size, it was possible, based on radial profiles of xylem conduit diameter of tree rings, to estimate tree growth rate, even if species-site specific, and make comparison between trees living in different epochs. In addition, despite the risk of becoming more vulnerable to air seeding cavitation, trees showed to prioritize of hydraulic efficiency vs. safety during the ontogenetic development, as the increase in xylem conductance with tree height determined a contextual decrease in the hydraulic safety margin. This study showed the importance of taking into account the three dimensional anatomical trends to better understand of the trade-offs of hydraulic safety vs. efficiency shape up the tree architecture and affect its adjustments occurring during ontogeny to cope with the arising intrinsic (i.e., size-related) and extrinsic (i.e., environmental) constraints to growth.Gli alberi sono organismi viventi che aumentano continuamente di dimensione (anche diversi ordini di grandezza) durante l'ontogenesi, accumulando biomassa nel fusto, nei rami e nelle radici. Durante la crescita, la struttura xilematica degli alberi continua ad adattarsi mantenendo un equilibrio nell’ottimizzazione del carbonio, garantendo contemporaneamente un’adeguata stabilità meccanica ed efficienza idrica della pianta. Il trasporto dell'acqua dalle radici fino alle foglie è una funzione fondamentale dello xilema e deve essere mantenuto efficiente durante tutte le fasi ontogenetiche. La resistenza idraulica del sistema infatti è fortemente influenzata dall’incremento della lunghezza del percorso idrico. Tuttavia, allargando la dimensione degli elementi di conduzione dello xilema (dall'apice alla la base del fusto), le piante sono in grado di minimizzare l'effetto negativo della crescita in altezza. Inoltre, data la stabilità di questo trend assiale durante l’ontogenesi, le dimensioni dei condotti xilematici aumentano anche in direzione radiale con l'età cambiale (dal midollo verso l'esterno), determinando una forte relazione tra la variazione del diametro dell’elemento conduttivo con l'età cambiale ed il tasso di allungamento del fusto. Le modifiche nella struttura xilematica, rimanendo impresse e cronologicamente archiviate nel legno, rappresentano un’importante fonte di informazioni che permette di aggiungere una componente temporale legata a meccanismi funzionali e di plasticità xilematica e, quindi, permetterebbe di ricostruire le dinamiche di crescita in diverse condizioni ambientali. Esiste tuttavia, una carenza di conoscenza e di procedure standard atte ad esplorare, a livello intra-specifico, le modificazioni a lungo termine dello xilema e la variabilità della sua struttura lungo profili assiali e radiali. Rimangono inoltre poco chiari i rapporti tra la struttura e la funzionalità, utili a prevedere in futuro eventuali adattamenti del sistema idraulico e metabolico al cambiamento climatico. Questa tesi riporta una serie di studi che si basano su un approccio dendro-anatomico e fisiologico, allo scopo di: - individuare priorità e compromessi tra le varie funzioni xilematiche; - determinarne i tratti anatomici responsabili; - analizzare in maniera retroattiva la loro variazione durante l'ontogenesi e in diverse condizioni ambientali; - analizzare risposte funzionali alle modifiche anatomiche che occorrono durante l’ontogenesi; - esaminare la possibilità di ricostruire i trend di accrescimento in altezza basandosi su profili idraulici radiali. E’ stata definita una guida alla standardizzazione della procedura, dalla raccolta del campione al dato anatomico dei tratti xilematici. Inoltre è stato sviluppato un nuovo approccio di quantificazione dello spessore della parete cellulare al fine di soddisfare gli obiettivi specifici dello studio. La struttura xilematica delle conifere (Larix decidua, Picea abies, Pinus cembra) evidenzia priorità e determinazione biofisica di tratti legati all’efficienza idraulica, come le dimensioni delle tracheidi, al fine di sostenere l'assimilazione necessaria per la crescita degli alberi. Altri caratteri funzionali invece, legati al supporto meccanico ed all’attività metabolica, mostrano più plasticità a fattori intrinseci ed estrinseci. Grazie alla stabilità del trend assiale dei condotti idraulici durante l’ontogenesi è stato possibile, basandosi sul conseguente pattern radiale, stimare il tasso di accrescimento delle piante, anche se specie-sito specifico, e confrontare quindi i trend con le piante che sono vissute in epoche diverse. Nonostante il rischio di aumentare la vulnerabilità alla cavitazione, gli alberi tendono a priorizzare l’efficienza a discapito della sicurezza idraulica durante lo sviluppo ontogenetico, a causa dell’aumento della conduttanza e conseguente riduzione del margine di sicurezza idraulica. Questo studio dimostra l'importanza di considerare la tridimensionalità dei trend anatomici al fine di comprendere meglio i rapporti tra la sicurezza idraulica e l’efficienza che modella l’architettura della pianta, influenzandone le modifiche ontogenetiche e compensandone i vincoli di crescita intrinsechi (dimensione-dipendenti) ed estrinseci (ambiente-dipendenti)

Environmental effects and biophysical constraints on xylem physiology and tree growth in conifers in the Alps

Gli alberi sono organismi viventi che aumentano continuamente di dimensione (anche diversi ordini di grandezza) durante l'ontogenesi, accumulando biomassa nel fusto, nei rami e nelle radici. Durante la crescita, la struttura xilematica degli alberi continua ad adattarsi mantenendo un equilibrio nell\u2019ottimizzazione del carbonio, garantendo contemporaneamente un\u2019adeguata stabilit\ue0 meccanica ed efficienza idrica della pianta. Il trasporto dell'acqua dalle radici fino alle foglie \ue8 una funzione fondamentale dello xilema e deve essere mantenuto efficiente durante tutte le fasi ontogenetiche. La resistenza idraulica del sistema infatti \ue8 fortemente influenzata dall\u2019incremento della lunghezza del percorso idrico. Tuttavia, allargando la dimensione degli elementi di conduzione dello xilema (dall'apice alla la base del fusto), le piante sono in grado di minimizzare l'effetto negativo della crescita in altezza. Inoltre, data la stabilit\ue0 di questo trend assiale durante l\u2019ontogenesi, le dimensioni dei condotti xilematici aumentano anche in direzione radiale con l'et\ue0 cambiale (dal midollo verso l'esterno), determinando una forte relazione tra la variazione del diametro dell\u2019elemento conduttivo con l'et\ue0 cambiale ed il tasso di allungamento del fusto. Le modifiche nella struttura xilematica, rimanendo impresse e cronologicamente archiviate nel legno, rappresentano un\u2019importante fonte di informazioni che permette di aggiungere una componente temporale legata a meccanismi funzionali e di plasticit\ue0 xilematica e, quindi, permetterebbe di ricostruire le dinamiche di crescita in diverse condizioni ambientali. Esiste tuttavia, una carenza di conoscenza e di procedure standard atte ad esplorare, a livello intra-specifico, le modificazioni a lungo termine dello xilema e la variabilit\ue0 della sua struttura lungo profili assiali e radiali. Rimangono inoltre poco chiari i rapporti tra la struttura e la funzionalit\ue0, utili a prevedere in futuro eventuali adattamenti del sistema idraulico e metabolico al cambiamento climatico. Questa tesi riporta una serie di studi che si basano su un approccio dendro-anatomico e fisiologico, allo scopo di: - individuare priorit\ue0 e compromessi tra le varie funzioni xilematiche; - determinarne i tratti anatomici responsabili; - analizzare in maniera retroattiva la loro variazione durante l'ontogenesi e in diverse condizioni ambientali; - analizzare risposte funzionali alle modifiche anatomiche che occorrono durante l\u2019ontogenesi; - esaminare la possibilit\ue0 di ricostruire i trend di accrescimento in altezza basandosi su profili idraulici radiali. E\u2019 stata definita una guida alla standardizzazione della procedura, dalla raccolta del campione al dato anatomico dei tratti xilematici. Inoltre \ue8 stato sviluppato un nuovo approccio di quantificazione dello spessore della parete cellulare al fine di soddisfare gli obiettivi specifici dello studio. La struttura xilematica delle conifere (Larix decidua, Picea abies, Pinus cembra) evidenzia priorit\ue0 e determinazione biofisica di tratti legati all\u2019efficienza idraulica, come le dimensioni delle tracheidi, al fine di sostenere l'assimilazione necessaria per la crescita degli alberi. Altri caratteri funzionali invece, legati al supporto meccanico ed all\u2019attivit\ue0 metabolica, mostrano pi\uf9 plasticit\ue0 a fattori intrinseci ed estrinseci. Grazie alla stabilit\ue0 del trend assiale dei condotti idraulici durante l\u2019ontogenesi \ue8 stato possibile, basandosi sul conseguente pattern radiale, stimare il tasso di accrescimento delle piante, anche se specie-sito specifico, e confrontare quindi i trend con le piante che sono vissute in epoche diverse. Nonostante il rischio di aumentare la vulnerabilit\ue0 alla cavitazione, gli alberi tendono a priorizzare l\u2019efficienza a discapito della sicurezza idraulica durante lo sviluppo ontogenetico, a causa dell\u2019aumento della conduttanza e conseguente riduzione del margine di sicurezza idraulica. Questo studio dimostra l'importanza di considerare la tridimensionalit\ue0 dei trend anatomici al fine di comprendere meglio i rapporti tra la sicurezza idraulica e l\u2019efficienza che modella l\u2019architettura della pianta, influenzandone le modifiche ontogenetiche e compensandone i vincoli di crescita intrinsechi (dimensione-dipendenti) ed estrinseci (ambiente-dipendenti)