Wood anatomical traits highlight complex temperature influence on Pinus cembra L. at high elevation in the Eastern Alps

In the context of climate change, scientific community is raising attention on tree response to increasing temperature. In this sense, populations at the edge of their distributional area are crucial to understand the species climate sensitivity. Pinus cembra is of particular interest being a typical high-elevation taxon, spread with mostly scattered populations within its range. Despite its potential, this species is traditionally disregarded by dendrochronological studies because of its low tree-ring variability and climate sensitivity. In this study, we tested the potential of dendroanatomy of this species, analysing time series of xylem anatomical traits of nine trees at the species elevation limit. We measured the mean ring width (MRW) and cell number (CN) per ring. Besides, to improve the time resolution of climate/growth associations, we split each ring in ten sectors, on which we measured the mean lumen area (LA) and both radial and tangential cell-wall thickness (CWTRad and CWTTan). These parameters, assessed on 1.5 7106 tracheids, were correlated with monthly and fortnightly climatic data, obtained by the daily climate records over 89 years (1926-2014). The most important factors affecting xylem features were late-spring and summer temperatures. LA and CWT showed a stronger temperature response than MRW, starting from mid-May and early June, respectively. CWT evidenced the longest period of response to temperature, with a significant difference between CWTRad and CWTTan. Analysis of xylem anatomical traits at intra-ring level and the use of daily temperature records proved to be useful for high resolution and detailed climate/growth association inferences in Pinus cembra

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

The admixture of Quercus sp. in Pinus sylvestris stands influences wood anatomical trait responses to climatic variability and drought events

IntroductionForests are threatened by increasingly severe and more frequent drought events worldwide. Mono-specific forests, developed as a consequence of widespread management practices established early last century, seem particularly susceptible to global warming and drought compared with mixed-species forests. Although, in several contexts, mixed-species forests display higher species diversity, higher productivity, and higher resilience, previous studies highlighted contrasting findings, with not only many positive but also neutral or negative effects on tree performance that could be related to tree species diversity. Processes underlying this relationship need to be investigated. Wood anatomical traits are informative proxies of tree functioning, and they can potentially provide novel long-term insights in this regard. However, wood anatomical traits are critically understudied in such a context. Here, we assess the role of tree admixture on Pinus sylvestris L. xylem traits such as mean hydraulic diameter, cell wall thickness, and anatomical wood density, and we test the variability of these traits in response to climatic parameters such as temperature, precipitation, and drought event frequency and intensity.MethodsThree monocultural plots of P. sylvestris and three mixed-stand plots of P. sylvestris and Quercus sp. were identified in Poland and Spain, representing Continental and Mediterranean climate types, respectively. In each plot, we analyzed xylem traits from three P. sylvestris trees, for a total of nine trees in monocultures and nine in mixed stands per study location.ResultsThe results highlighted that anatomical wood density was one of the most sensitive traits to detect tree responses to climatic conditions and drought under different climate and forest types. Inter-specific facilitation mechanisms were detected in the admixture between P. sylvestris and Quercus sp., especially during the early growing season and during stressful events such as spring droughts, although they had negligible effects in the late growing season.DiscussionOur findings suggest that the admixture between P. sylvestris and Quercus sp. increases the resilience of P. sylvestris to extreme droughts. In a global warming scenario, this admixture could represent a useful adaptive management option

The admixture of Quercus sp. in Pinus sylvestris stands influences wood anatomical trait responses to climatic variability and drought events.

INTRODUCTION Forests are threatened by increasingly severe and more frequent drought events worldwide. Mono-specific forests, developed as a consequence of widespread management practices established early last century, seem particularly susceptible to global warming and drought compared with mixed-species forests. Although, in several contexts, mixed-species forests display higher species diversity, higher productivity, and higher resilience, previous studies highlighted contrasting findings, with not only many positive but also neutral or negative effects on tree performance that could be related to tree species diversity. Processes underlying this relationship need to be investigated. Wood anatomical traits are informative proxies of tree functioning, and they can potentially provide novel long-term insights in this regard. However, wood anatomical traits are critically understudied in such a context. Here, we assess the role of tree admixture on Pinus sylvestris L. xylem traits such as mean hydraulic diameter, cell wall thickness, and anatomical wood density, and we test the variability of these traits in response to climatic parameters such as temperature, precipitation, and drought event frequency and intensity. METHODS Three monocultural plots of P. sylvestris and three mixed-stand plots of P. sylvestris and Quercus sp. were identified in Poland and Spain, representing Continental and Mediterranean climate types, respectively. In each plot, we analyzed xylem traits from three P. sylvestris trees, for a total of nine trees in monocultures and nine in mixed stands per study location. RESULTS The results highlighted that anatomical wood density was one of the most sensitive traits to detect tree responses to climatic conditions and drought under different climate and forest types. Inter-specific facilitation mechanisms were detected in the admixture between P. sylvestris and Quercus sp., especially during the early growing season and during stressful events such as spring droughts, although they had negligible effects in the late growing season. DISCUSSION Our findings suggest that the admixture between P. sylvestris and Quercus sp. increases the resilience of P. sylvestris to extreme droughts. In a global warming scenario, this admixture could represent a useful adaptive management option

Vivere al limite: risposte anatomiche e fisiologiche di specie legnose sottoposte a condizioni estreme

Gli ambienti limitati dalle basse temperature si stanno scaldando ad un tasso superiore alla media globale con effetti già evidenti benché eterogenei sulla vegetazione. Poiché ad alta quota le specie legnose operano spesso al loro limite fisiologico, comprendere le loro risposte climatiche è cruciale per prevedere scenari futuri o indirizzare misure di gestione forestale. Le piante possono adeguare i tratti morfo-fisiologici entro i limiti della loro plasticità ma, se le prestazioni richieste dalle condizioni esterne superano questi limiti, possono indebolirsi o morire. Pertanto, la presente tesi mira a quantificare la variabilità spaziale e temporale delle risposte climatiche delle conifere (i taxa dominanti della fascia di alta quota nell'Europa meridionale), sia erette che prostrate, che crescono al limite della distribuzione della specie, attraverso un approccio dendroanatomico e ecofisiologico. Juniperus communis è un arbusto tipico dell’ambiente alpino in quota, oltre a essere la conifera con la più ampia distribuzione nell'emisfero settentrionale. E’ altamente resistente alla siccità e con prestazioni idrauliche stabili, indipendenti da differenze genetiche o ambientali. Tuttavia, nonostante sembri in grado di ben tollerare future condizioni più xeriche, le popolazioni al limite più settentrionale potrebbero essere le prime ad essere idraulicamente compromesse. Inoltre, J. communis sembra beneficiare dello scioglimento nivale anticipato in termini di crescita, anche se, in caso di copertura prolungata, pare compensi la stagione vegetativa più breve con aggiustamenti fisiologici volti a massimizzare la crescita. Eppure, tali effetti si riducono negli anni grazie alla velocità di acclimatazione della specie, evidenziando l'importanza di considerare la dimensione temporale nelle previsioni sulle dinamiche vegetazionali della tundra. Bassa variabilità intraspecifica è emersa anche analizzando la sensitività climatica dei tratti anatomici in popolazioni di Pinus cembra tra Alpi e Carpazi, nonostante le differenze ambientali. In generale, condizioni fresche e umide inducono cellule più larghe con pareti sottili, al contrario, con condizioni più miti e asciutte, che si prevede possano verificarsi in futuro, la tendenza è inversa. Infine, temperature più elevate sembrano promuovere la crescita radiale. Le associazioni tra condizioni climatiche e tratti xilematici non sembrano però stabili: dagli anni 80 questa specie è sempre meno limitata, specie nei Carpazi, sia dalla temperatura che dalle precipitazioni. Tuttavia, nonostante il potenziale beneficio del riscaldamento, P. cembra potrebbe venire confinato ancor più in aree disgiunte di alta quota da altre specie più competitive che stanno avanzando in altitudine. Inoltre, per l’ecologia degli alberi al limite superiore del bosco sono cruciali anche le condizioni invernali. Infatti, le conifere al limite della foresta soffrono comunemente di una drastica perdita di conduttività idraulica, principalmente a causa di embolie da gelo. Si è visto però che molte specie recuperano tali danni a fine inverno, quando suolo e porzioni di xilema sono ancora gelati. Si è dimostrato che Larix decidua e Picea abies riescono ad assorbire acqua attraverso i rami, probabilmente derivata dallo scioglimento nivale in chioma, per poi redistribuirla all'interno della chioma stessa. In questo caso, il cambiamento climatico potrebbe avere un duplice effetto: positivo, favorendo il recupero idraulico per via dell’innalzamento termico, ma anche negativo diminuendo la quantità di neve sui rami. Questi risultati contribuiscono a capire come le conifere affrontino condizioni di crescita estreme in aree termicamente limitate e di quale sia la variabilità spazio-temporale delle loro risposte climatiche, supportando previsioni future sui dinamismi delle comunità vegetali di alta quota e decisioni gestionali volte a preservare questi sistemi così vulnerabili.Heat-limited environments are facing significant climatic changes since they are warming at higher rate than the global average. This has already evident but heterogeneous effects on vegetation communities. At high-elevation long-lived woody species often operate at their physiological limit, and for this reason a deep understanding of their climate responses to changing conditions is of key importance when predicting future scenarios or taking forest management decisions. In fact, plants may adjust their traits spanning within the range of their plasticity and, when required performance to cope with external conditions exceed this threshold, plant is likely to lose fitness or die. In this context, this thesis aims to give new insight on spatial and temporal variability of the climatic responses of conifers, the dominant taxa of all the high-elevation belt of southern Europe, with both erect and prostrate growth form, growing at the limit of their distribution. To this end I applied both dendroanatomical and physiological approaches. Juniperus communis is a typical shrub species at high elevation on Alps and is the conifer with the widest distribution range in the Northern Hemisphere. It was found to have a strong drought-resistant and conservative hydraulic performance independently from phenotypical and genotypical differences or from environmental setting. However, despite this species is likely to well cope to future more xeric conditions, populations at the northernmost limit might be the first to be hydraulically impaired. In addition, common juniper seemed to benefit from anticipated snowpack melt in term of growth, though, in case of prolonged snow-cover, it seems able to offset shorter vegetative season through physiological adjustments likely aimed to maximize growth rate. Nonetheless, considering the remarkable acclimation speed, the effects of changed snow-melt timing weaken over years and highlighted the importance of considering temporal variability when making future predictions on tundra vegetation. A low intra-specific variability emerged also looking at the climate sensitivity of Pinus cembra’s anatomical traits in the Alps and Carpathians, despite distance and environmental differences of these two regions. In general, cool and wet conditions induce larger cells with thin cell wall and, on the opposite, warm and dry conditions, that are predicted to occur in the next future, tent to produce smaller and thicker walled cells. Lastly, high temperature seems to enhance radial growth. All these associations seemed not absolutely stable and from the 80s this species is less and less limited by both temperature and precipitation, especially in the Carpathians. Yet, despite the potential benefit of warming, Pinus cembra might be further restrained to high-elevation patches due to its low competitive advantage with other species that are advancing upward. For tree life at the upper forest limit not just the growing season conditions are crucial, but also winter ones. In fact, conifers at treeline commonly suffer of drastic loss of conductivity, mainly due to frost-drought induced embolisms, but they recover in late winter when soil and xylem portions are still frozen. Larix decidua and Picea abies are demonstrated to absorb water, likely from melting snow, through branches and redistribute it within the crown. In this case climate change may have twofold effects since it is likely to favour hydraulic recovery but also to reduce embolism recovery decreasing snow amount on branches. These results contributed to understand how conifers deal with extreme growing conditions in heat-limited areas and which is the spatial and temporal variability of their climatic responses, likely supporting future predictions and management decisions on such vulnerable systems

Climate response of a glacial relict conifer across its distribution range is invariant in space but not in time

: Climate change impacts on forest trees will be particularly severe for relict species endemic to the subalpine forest, such as Pinus cembra in the Alps and Carpathians. Most current knowledge about the response of this species to climate comes from tree-ring width analysis. However, this approach cannot perform in-depth and highly time-resolved analysis on the climate influence on specific growth processes and xylem functions. We analyzed xylem anatomical traits from six sites covering most of the longitudinal range of this species. Associations between climate and cell number, lumen area and cell wall thickness were computed for the 1920-2010 period using climate records aligned to degree-day temperature sum thresholds. The anatomical chronologies were clearly distinct between the Alps and Carpathians. However, climate responses were similar for all sites, suggesting common species-specific response mechanisms. Temperature showed a positive correlation with both cell number and cell wall thickness. Cell lumen size exhibited an early positive association, followed by strong negative association with temperature and a positive one with precipitation. This highlights that the cell enlargement process was negatively related to high temperature at high elevation, where meristematic processes are rather supposed to be constrained by low temperatures. Therefore, long-term climate warming can have negative consequences on the xylem potential to transport water at all investigated sites. Moreover, in the last 30 years, we observed a slight anticipation of some responses and a decrease in climate sensitivity of some xylem parameters. Our findings provide evidence of temporally unstable but spatially consistent climate response of Pinus cembra from the Alps to the Carpathians. The low diversity in xylem phenotypic responses to climate suggests that future warming could extensively and evenly affect the species throughout its entire distribution