Stem hydraulic capacitance decreases with drought stress : implications for modelling tree hydraulics in the Mediterranean oak Quercus ilex

Hydraulic modelling is a primary tool to predict plant performance in future drier scenarios. However, as most tree models are validated under non-stress conditions they may fail when water becomes limiting. To simulate tree hydraulic functioning under moist and dry conditions, the current version of a water flow and storage mechanistic model was further developed by implementing equations that describe variation in xylem hydraulic resistance (RX) and stem hydraulic capacitance (CS) with predawn water potential (ΨPD). The model was applied in a Mediterranean forest experiencing intense summer drought, where six Quercus ilex trees were instrumented to monitor stem diameter variations and sap flow, concurrently with measurements of predawn and midday leaf water potential. Best model performance was observed when CS was allowed to decrease with decreasing ΨPD. Hydraulic capacitance decreased from 62 to 25 kg m-3 MPa-1 across the growing season. In parallel, tree transpiration decreased to a greater extent than the capacitive water release and the contribution of stored water to transpiration increased from 2.0% to 5.1%. Our results demonstrate the importance of stored water and seasonality in CS for tree hydraulic functioning, and they suggest that CS should be considered to predict the drought-response of trees with models

Influencia de la luz y la sequía estival en la respuesta funcional de brinzales de Quercus petraea (Matt.) Liebl. y Quercus pyrenaica Willd. : implicaciones para la regeneración

El estudio de las respuestas funcionales de brinzales de roble albar (Q. petraea) y melojo (Q. pyrenaica) a la disponibilidad de luz y agua constituye la base para un buen conocimiento de su capacidad de regeneración. En el desarrollo de esta tesis se ha evaluado la capacidad de aclimatación a la luz y la influencia en la respuesta a la sequía estival, de brinzales de las dos especies. Para ello se ha realizado un experimento en invernadero en ausencia de estrés hídrico en el que se ha estudiado la capacidad de aclimatación a la luz a corto y a largo plazo, y se han realizado varias plantaciones en sitios próximos al “Hayedo de Montejo” (NE de Madrid) con diferente cobertura arbórea, examinando la evolución de diversos parámetros ambientales y funcionales a lo largo del verano y la supervivencia durante los 3-4 primeros años. En invernadero, las hojas de los brinzales de roble albar tuvieron mayor superficie específica e individual, mayor concentración de clorofila, y mayor proporción de nitrógeno invertido en componentes del aparato fotosintético que las de melojo; en condiciones de elevada iluminación crecieron más rápidamente que los brinzales de melojo, invirtiendo menos biomasa en raíces y mostrando un patrón arquitectural más eficiente en la captación de luz. Sin embargo, en el campo la supervivencia de melojos fue igual o mayor que la de robles albares en un amplio rango de ambientes lumínicos, probablemente porque la variación de la luz disponible para las plantas estuvo asociada a la variación de su estado hídrico. La humedad del suelo en verano fue mayor en zonas aclaradas de pinares de silvestre que en claros, o zonas sin aclarar con una elevada densidad de pinos, donde además, el desarrollo de las plantas fue escaso. Ambas circunstancias contribuyeron a que el mayor grado de estrés hídrico de los brinzales se produjese en las zonas no aclaradas, y a que la supervivencia fuese mayor en las parcelas establecidas bajo los pinares aclarados. No se detectaron diferencias entre las dos especies en el estado hídrico, pero los brinzales de melojo siempre mantuvieron tasas de intercambio gaseoso más altas en todos los ambientes; sus hojas mostraron una eficiencia en el uso del agua menor que las de roble albar al final del verano. Los resultados demuestran que las dos especies tienen requerimientos ecológicos diferentes en la etapa de plántula, y apuntan a una mayor capacidad del melojo para regenerarse en ambientes submediterráneos. ABSTRACT The ecophysiological responses of seedlings to light and water resources can be a useful tool to approaching their regeneration performance. The present PhD Thesis addresses the light acclimatory responses of seedlings of sessile oak (Q. petraea) and pyrenean oak (Q. pyrenaica) to light in a glasshouse, and the role of light in modifying waterstress responses and initial survival of seedlings planted in sites of contrasting canopy cover in a sub-Mediterranean mid-mountain field site. Results at both leaf and plant level under glasshouse conditions pointed to a more competitive ability to light in seedlings of sessile oak. They had larger leaves with more specific area and higher nitrogen proportion in photosynthetic components than pyrenean oak’ leaves, which all support its higher growth in low and high light (5.3 vs 70 % of full sunlight). In high light, seedlings of sessile oak showed a more efficient pattern of growth in terms of light capturing, with longer internodes and lesser number of basal sprouts resulting in reduced self-shading compared to pyrenean oak. Survival was though similar or lower in sessile oak than in pyrenean oak in several plantations carried out in gaps, clearings and Scots pine stands close to a beech-oak forest in Montejo de la Sierra (NE Madrid, Spain). This could be so because variation of understory light availability generated by the distinct canopy-tree densities was related to the variation of seedlings’ water status. Summer soil moisture was higher in thinned areas of Scots pine forests than in un-thinned areas, gaps or clearings. Both lower soil moisture and plant development likely translated into the highest degree of water stress found in understory seedlings. Survival was highest in thinned areas in all plantations compared to un-thinned areas, gaps or clearings. There was barely any difference between the two species in leaf water potentials across plantations, either at dawn or midday, supporting a similar water-avoidance capacity. However, seedlings of pyrenean oak had higher foliar gas exchange rates along the summer dates, its leaves being less efficient in using water at the end of summer, the period of maximum water stress. The results demonstrate that seedlings’ ecological requirements differ between the two oaks, pointing to a better ability of Q. pyrenaica to regenerate in sub-Mediterranean environments

The relevance of seed size in modulating leaf physiology and early plant performance in two tree species

he size of seeds and the microsite of seed dispersal may affect the early establishment of seedlings through different physiological processes. Here, we examined the effects of seed size and light availability on seedling growth and survival, and whether such effects were mediated by water use efficiency. Acorns of Quercus petraea and the more drought-tolerant Quercus pyrenaica were sowed within and around a tree canopy gap in a sub-Mediterranean forest stand. We monitored seedling emergence and measured predawn leaf water potential (Ψpd), leaf nitrogen per unit area (Na), leaf mass per area, leaf carbon isotope composition (δ13C) and plant growth at the end of the first summer. Survival was measured on the next year. Path analysis revealed a consistent pattern in both species of higher δ13C as Ψpd decreased and higher δ13C as seedlings emerged later in the season, indicating an increase in 13C as the growing season is shorter and drier. There was a direct positive effect of seed size on δ13C in Q. petraea that was absent in Q. pyrenaica. Leaf δ13C had no effect on growth but the probability of surviving until the second year was higher for those seedlings of Q. pyrenaica that had lower δ13C on the first year. In conclusion, leaf δ13C is affected by seed size, seedling emergence time and the availability of light and water, however, leaf δ13C is irrelevant for first year growth, which is directly dependent on the amount of seed reserves

GC-TOF-MS metabolite profiling of drought tolerant Quercus ilex

Perfil metabólico primario de hojas de encina en respuesta al estrés hídrico por sequía

Influencia de la espesura de un pinar albar (Sistema Central, España) en la disponibilidad de luz y agua en el sotobosque, y la supervivencia de robles plantados en su interior

Information of tree-nurse shelterwood effects on survival of underplanted seedlings is particularly scant in Mediterranean forest ecosystems. To study light and water resources availability and survival associated to overstory density, two-year-old seedlings of Quercus petraea (Matt.) Liebl. (sessile oak) and Quercus pyrenaica Willd. (pyrenean oak) were planted in the understory of an even-aged Scots pine (Pinus sylvestris L.) plantation located in central Spain, which had been previously assigned to three density treatments: uncut, 33% thinned and 50% thinned of the original density, each replicated four times. Soil moisture was measured with a TDR during the first growing season after planting. Light conditions were estimated by hemispherical photography. Survival was measured at the end of the first growing season in the field and at the beginning of the next two growing seasons. The reduction in density after thinning had a positive effect on light availability and on near-surface soil moisture. Pyrenean oak had higher survival rates than sessile oak, which showed similarly high mortality rates in all three overstory treatments. Thinning had a positive effect on the survival of pyrenean oak, though irrespective of the intensity. Overall, these results point to the necessity to reduce canopy tree density in Mediterranean mountain pinewoods before carrying out enrichment plantations beneath.Con el fin de estudiar el efecto de la espesura de la cubierta forestal en la disponibilidad de luz y agua para las plantas y su supervivencia, se plantaron brinzales de dos savias de Quercus petraea (Matt.) Liebl. (roble albar) y Quercus pyrenaica Willd. (melojo) en el interior de un pinar de Pinus sylvestris L. (pino albar) situado en el NE de la provincia de Madrid sometido previamente a tres tratamientos: clara del 33% de la densidad original, clara del 50% y ausencia de clara. Se midió la humedad volumétrica del suelo con un TDR y la disponibilidad de luz en el sotobosque por medio de fotografias hemisféricas. La supervivencia se midió al final del primer año y al comienzo del verano de los dos años siguientes. La reducción de la densidad del dosel incrementó la luz disponible en el sotobosque, la humedad de los primeros 10 cm de suelo y la supervivencia de las plantas de melojo, aunque no se apreció un efecto diferente segun la intensidad de la clara aplicada. La mortalidad de las plantas de roble albar fue elevada en todos los tratamientos, y superior a la del melojo. Estos resultados apuntan a la necesidad de reducir la espesura del pinar albar en la zona de estudio antes de llevar a cabo plantaciones con melojo en su interior

Breeding and scientific advances in the fight against Dutch elm disease - will they allow the use of elms in forest restoration?

Revisión de los avances científicos y de producción de planta con fin de evaluar el potencial en la recuperación del olmo común

Impact of successive spring frosts on leaf phenology and radial growth in three deciduous tree species with contrasting climate requirements in central Spain

Rear-edge tree populations forming the equatorward limit of distribution of temperate species are assumed to be more adapted to climate variability than central (core) populations. However, climate is expected to become more variable and the frequency of climate extremes is forecasted to increase. Climatic extreme events such as heat waves, dry spells and spring frosts could become more frequent, and negatively impact and jeopardize rear-edge stands. To evaluate these ideas, we analyzed the growth response of trees to successive spring frosts in a mixed forest, where two temperate deciduous species, Fagus sylvatica L. (European beech) and Quercus petraea (Matt.) Liebl. (sessile oak), both at their southernmost edge, coexist with the Mediterranean Quercus pyrenaica Willd. (Pyrenean oak). Growth reductions in spring-frost years ranked across species as F. sylvatica > Q. petraea > Q. pyrenaica. Leaf flushing occurred earlier in F. sylvatica and later in Q. pyrenaica, suggesting that leaf phenology was a strong determinant of spring frost damage and stem growth reduction. The frost impact depended on prior climate conditions, since warmer days prior to frost occurrence predisposed to frost damage. Autumn Normalized Difference Vegetation Index data showed delayed leaf senescence in spring-frost years and subsequent years as compared with pre-frost years. In the studied forest, the negative impact of spring frosts on Q. petraea and especially on F. sylvatica growth, was considerably higher than the impacts due to drought. The succession of four spring frosts in the last two decades determined a trend of decreasing resistance of radial growth to frosts in F. sylvatica. The increased frequency of spring frosts might prevent the expansion and persistence of F. sylvatica in this rear-edge Mediterranean population

Linking functional composition moments of the sub-Mediterranean ecotone with environmental drivers

This article is part of the Research Topic Plant Diversity: The Key to Ecosystem Resilience in a Changing World[EN] Introduction: Functional trait-based approaches are extensively applied to the study of mechanisms governing community assembly along environmental gradients. These approaches have been classically based on studying differences in mean values among species, but there is increasing recognition that alternative metrics of trait distributions should be considered to decipher the mechanisms determining community assembly and species coexistence. Under this framework, the main aim of this study is to unravel the effects of environmental conditions as drivers of plant community assembly in sub- Mediterranean ecotones. Methods: We set 60 plots in six plant communities of a sub-Mediterranean forest in Central Spain, and measured key above- and belowground functional traits in 411 individuals belonging to 19 species, along with abiotic variables. We calculated community-weighted mean (CWM), skewness (CWS) and kurtosis (CWK) of three plant dimensions, and used maximum likelihood techniques to analyze how variation in these functional community traits was driven by abiotic factors. Additionally, we estimated the relative contribution of intraspecific trait variability and species turnover to variation in CWM. Results and discussion: The first three axes of variation of the principal component analyses were related to three main plant ecological dimensions: Leaf Economics Spectrum, Root Economics Spectrum and plant hydraulic architecture, respectively. Type of community was the most important factor determining differences in the functional structure among communities, as compared to the role of abiotic variables. We found strong differences among communities in their CWMs in line with their biogeographic origin (Eurosiberian vs Mediterranean), while differences in CWS and CWK indicate different trends in the functional structure among communities and the coexistence of different functional strategies, respectively. Moreover, changes in functional composition were primarily due to intraspecific variability. Conclusion: We observed a high number of strategies in the forest with the different communities spreading along the acquisitive-conservative axis of resource-use, partly matching their Eurosiberian-Mediterranean nature, respectively. Intraspecific trait variability, rather than species turnover, stood as the most relevant factor when analyzing functional changes and assembly patterns among communities. Altogether, our data support the notion that ecotones are ecosystems where relatively minor environmental shifts may result in changes in plant and functional compositionSIThis work was financially supported by the German Research Foundation (Deutsche Forschungsgemeinschaft), being part of the project “the Functional Frontier among Mediterranean and Eurosiberian Plant Communities” (ECOFUMER, 441909701). ER and JG are respectively supported by a Marı́a Zambrano and a Margarita Salas fellowships funded by the Spanish Ministry of Universities and European Union-Next Generation Plan. IP acknowledges funding from a Ramón y Cajal contract (RYC2021-033081-I) funded by the Ministry of Science and Innovation and co-funded by the European Union-Next Generation Plan funded by European Union-NextGenerationE

Tree-ring density and carbon isotope composition are early-warning signals of drought-induced mortality in the drought tolerant Canary Island pine

Tree death is not always preceded by a visible decline in vigor (canopy dieback) or a progressive loss in crown volume. Identifying early-warning signals of incipient decline can help to implement the necessary measures to prevent tree death. The aim of this work is to understand what functional alterations preceded the massive drought-induced death of adult Pinus canariensis trees in an arid stand, located in the Canary Islands. To this aim, we analyzed interannual variations in earlywood and latewood width, wood density and anatomy, and carbon isotope composition, and the relationships among these variables, in dead and living trees from 1980 to 2013. Dead trees grew less since the 1990's, produced fewer parenchyma rays and resin canals, and exhibited a trend of decreasing latewood density and a marked shift in carbon isotope discrimination over the last 34 years that were reversed in living trees. Higher wood density in living trees resulted from thicker tracheid cell walls rather than narrower lumens. The intrinsic water use efficiency shifted from higher to lower values in dead trees after the 2000s. These results suggest a carbon limitation to maintain hydraulic safety under xylem tension, as well as to maintain storage and defense capacity, which can render trees more vulnerable to severe drought episodes. A long-term trend of decreasing tree-ring density and a reduced intrinsic water use efficiency in the short-term can be early-warning signals of carbon limitation and tree decline in drought-stressed P. canariensis. The analysis of these variables can be used to assess tree decline risks in similarly vulnerable conifer populations inhabiting drought-prone regions

Small-scale variation of vegetation in a mixed forest understorey is partly controlled by the effect of overstory composition on litter accumulation

We investigated how richness and composition of vascular plant species in the understory of a mixed hardwood forest stand varied with respect to the abundance and composition of the overstory. The stand is in central Spain and represents the southernmost range of distribution of several tree and herbaceous species in Europe. Understory species were identified in 46 quadrats (0.25 m2) where variables litter depth and light availability were measured. In addition, we estimated tree density, basal area, and percent basal area by tree species within 6-m-radius areas around each plot. Species richness and composition were studied using path analysis and scale-dependent geostatistical methods, respectively. We found that the relative abundance of certain trees species in the overstory was more important than total overstory abundance in explaining understory species richness. Richness decreased as soil litter depth increased, and soil litter increased as the relative proportion of Fagus sylvatica in the overstory increased, which accounted for a negative, indirect effect of Fagus sylvatica on richness. Regarding understory species composition, we found that some species distributed preferentially below certain tree species. For example, Melica uniflora was most frequent below Fagus sylvatica and Quercus petraea while the increasing proportion of Q. pyrenaica in the overstory favored the presence of Cruciata glabra, Arenaria montana, Prunus avium, Conopodium bourgaei, Holcus mollis, Stellaria media and Galium aparine in the understory. Overall, these results emphasize the importance of individual tree species in controlling the assemblage and richness of understory species in mixed stands. We conclude that soil litter accumulation is one way through which overstory composition shapes the understory community