Water Sorption and Desorption of Isolated Cuticles From Three Woody Species With Focus on Ilex aquifolium.

The cuticle is a lipid-rich layer that protects aerial plant organs against multiple stress factors such as dehydration. In this study, cuticle composition and structure in relation to water loss are examined in a broad ecophysiological context, taking into consideration leaf age and side from Ilex aquifolium (holly) in comparison with Eucalyptus globulus (eucalypt) and Prunus laurocerasus (cherry laurel). Enzymatically isolated cuticular membranes from holly leaves were studied under three treatment conditions: natural (no chemical treatment), after dewaxing, and after methanolysis, and the rate of water loss was assessed. Structural and chemical changes were evaluated using different microscopy techniques and by Fourier transform infrared (FTIR) spectroscopy. The potential mechanisms of solute absorption by holly leaves were additionally evaluated, also testing if its prickly leaf margin may facilitate uptake. The results indicate that the treatment conditions led to structural changes, and that chemical composition was hardly affected because of the occurrence of cutan. Structural changes led to more hydrophilic adaxial surfaces, which retained more water and were more efficient than natural cuticles, while changes were not significant for abaxial surfaces. Across natural cuticles, age was a significant factor for eucalypt but not for holly. Young eucalypt cuticles were the group that absorbed more water and had the lowest water loss rate. When comparing older leaf cuticles of the three species, cherry laurel was found to absorb more water, which was, however, lost more slowly, compared with the other species. Evidence was gained that holly leaves can absorb foliar-applied solutes (traced after calcium chloride application) through the adaxial and abaxial surfaces, the adaxial mid veins, and to a lower extent, the spines. In conclusion, for the species examined, the results show variations in leaf cuticle composition and structure in relation to leaf ontogeny, and water sorption and desorption capacity.

Climate Change Synchronizes Growth and iWUE Across Species in a Temperate-Submediterranean Mixed Oak Forest.

Tree species have good tolerance to a range of environmental conditions, though their ability to respond and persist to environmental changes is dramatically reduced at the rear-edge distribution limits. At those edges, gene flow conferring adaptation is impaired due to lack of populations at lower latitudes. Thus, trees mainly rely on phenotypic changes to buffer against long-term environmental changes. Interspecific hybridization may offer an alternative mechanism in the generation of novel genetic recombinants that could be particularly valuable to ensure persistence in geographically isolated forests. In this paper, we take advantage of the longevity of a temperate-submediterranean mixed-oak forest to explore the long-term impact of environmental changes on two different oak species and their hybrid. Individual trees were genetically characterized and classified into three groups: pure Quercus petraea (Matt.), Liebl, pure Q. pyrenaica Willd, and hybrids. We calculated basal area increment and intrinsic water-use efficiency (iWUE) from tree-ring width and δ13C per genetic group, respectively. Tree-growth drivers were assessed using correlation analyses and generalized linear mixed models for two contrasting climatic periods: (1880-1915, colder with [CO2] < 303 ppm; and 1980-2015, warmer with [CO2] > 338 ppm). The three genetic groups have increased radial growth and iWUE during the last decades, being the least drought-tolerant QuPe the most sensitive species to water stress. However, no significant differences were found among genetic groups neither in mean growth rate nor in mean iWUE. Furthermore, little differences were found in the response to climate among groups. Genetic groups only differed in the relationship between δ13C and temperature and precipitation during the earlier period, but such a difference disappeared during the recent decades. Climate change may have promoted species-level convergence as a response to environment-induced growth limitations, which translated in synchronized growth and response to climate as well as a tighter stomatal control and increased iWUE across coexisting oak species.

Effect of irradiation and canopy position on anatomical and physiological features of Fagus sylvatica and Quercus petraea leaves.

Growing conditions at different tree canopy positions may significantly vary and lead to foliar changes even within the same tree. An assessment of foliar anatomy, including also epidermal features, can help us understand how plants respond to environmental factors. Working with two model tree species (i.e., Quercus petraea and Fagus sylvatica) grown at their southernmost European distribution area in Central Spain, the influence of irradiation and canopy height was examined by sampling lower canopy leaves and comparing them with fully irradiated, top canopy leaves and shaded top canopy leaves grown for months within a bag made of shade netting fabric before they sprouted. At the end of the summer, samples were collected, and several parameters were analysed. The results indicate that SLA (specific leaf area) differences are significant both between species and groups. Leaf and cuticle thickness differed significantly between groups while stomatal densities only between species. Regarding mineral concentrations, differences between species were significant for K, Mn, N and N: P ratios. It is concluded that leaf responses to environmental conditions may be variable both within the same tree and between species.

Unprecedented herbivory threatens rear-edge populations of Betula in southwestern Eurasia.

Mediterranean rear-edge populations of Betula, located at the southwestern Eurasian margin of the distribution range, represent unique reservoirs of genetic diversity. However, increasing densities of wild ungulates, enhanced dryness, and wildfires threaten their future persistence. A historical perspective on the past responses of these relict populations to changing herbivory, fire occurrence and climatic conditions may contribute to assessing their future responses under comparable scenarios. We have reconstructed vegetation and disturbance (grazing, fire) history in the Cabañeros National Park (central-southern Spain) using the paleoecological records of two small mires. We particularly focused on the historical range of variation in disturbance regimes, and the dynamics of rear-edge Betula populations and herbivore densities. Changes in water availability, probably related to the North Atlantic Oscillation (NAO) index, and land-use history have played a crucial role in vegetation shifts. Our data suggest that heathlands (mainly Erica arborea and E. scoparia) and Quercus woodlands dominated during dry phases while Sphagnum bogs and Betula stands expanded during wet periods. Betula populations survived past moderately dry periods but were unable to cope with enhanced land use, particularly increasing livestock raising since ~1,100-900 cal. yr BP (850-1,050 CE), and eventually underwent local extinction. High herbivore densities not only contributed to the Betula demise but also caused the retreat of Sphagnum bogs. Ungulate densities further rose at ~200-100 cal. yr BP (1750-1850 CE) associated with the historically documented intensification of land use around the Ecclesiastical Confiscation. However, herbivory reached truly unprecedented values only during the last decades, following rural depopulation and subsequent promotion of big game hunting. For the first time in temperate and Mediterranean Europe, we have used the abundances of fossil dung fungal spores to assess quantitatively that current high herbivore densities exceed the historical range of variation. In contrast, present fire activity lies within the range of variation of the last millennia, with fires (mainly human-set) mostly occurring during dry periods. Our paleodata highlight the need of controlling the densities of wild ungulates to preserve ecosystem composition and functioning. We also urge to restore Betula populations in suitable habitats where they mostly disappeared because of excessive human activities.

Daytime depression in temperature-normalised stem CO2 efflux in young poplar trees is dominated by low turgor pressure rather than by internal transport of respired CO2.

Daytime decreases in temperature-normalised stem CO2 efflux (EA_D ) are commonly ascribed to internal transport of respired CO2 (FT ) or to an attenuated respiratory activity due to lowered turgor pressure. The two are difficult to separate as they are simultaneously driven by sap flow dynamics. To achieve combined gradients in turgor pressure and FT , sap flow rates in poplar trees were manipulated through severe defoliation, severe drought, moderate defoliation and moderate drought. Turgor pressure was mechanistically modelled using measurements of sap flow, stem diameter variation, and soil and stem water potential. A mass balance approach considering internal and external CO2 fluxes was applied to estimate FT . Under well-watered control conditions, both turgor pressure and sap flow, as a proxy of FT , were reliable predictors of EA_D . After tree manipulation, only turgor pressure was a robust predictor of EA_D . Moreover, FT accounted for < 15% of EA_D . Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause of EA_D in young poplar trees. Turgor pressure is determined by both carbohydrate supply and water availability, and should be considered to improve our widely used but inaccurate temperature-based predictions of woody tissue respiration in global models.

Temporal and spatial patterns of internal and external stem CO2 fluxes in a sub-Mediterranean oak.

To accurately estimate stem respiration (RS), measurements of both carbon dioxide (CO2) efflux to the atmosphere (EA) and internal CO2 flux through xylem (FT) are needed because xylem sap transports respired CO2 upward. However, reports of seasonal dynamics of FT and EA are scarce and no studies exist in Mediterranean species under drought stress conditions. Internal and external CO2 fluxes at three stem heights, together with radial stem growth, temperature, sap flow and shoot water potential, were measured in Quercus pyrenaica Willd. in four measurement campaigns during one growing season. Substantial daytime depressions in temperature-normalized EA were observed throughout the experiment, including prior to budburst, indicating that diel hysteresis between stem temperature and EA cannot be uniquely ascribed to diversion of CO2 in the transpiration stream. Low internal [CO2] (<0.5%) resulted in low contributions of FT to RS throughout the growing season, and RS was mainly explained by EA (>90%). Internal [CO2] was found to vary vertically along the stems. Seasonality in resistance to radial CO2 diffusion was related to shoot water potential. The low internal [CO2] and FT observed in our study may result from the downregulation of xylem respiration in response to a legacy of coppicing as well as high radial diffusion of CO2 through cambium, phloem and bark tissues, which was related to low water content of stems. Long-term studies analyzing temporal and spatial variation in internal and external CO2 fluxes and their interactions are needed to mechanistically understand and model respiration of woody tissues.

Seasonal and diel variation in xylem CO2 concentration and sap pH in sub-Mediterranean oak stems.

Since a substantial portion of respired CO2 remains within the stem, diel and seasonal trends in stem CO2 concentration ([CO2]) are of major interest in plant respiration and carbon budget research. However, continuous long-term stem [CO2] studies are scarce, and generally absent in Mediterranean climates. In this study, stem [CO2] was monitored every 15min together with stem and air temperature, sap flow, and soil water storage during a growing season in 16 stems of Quercus pyrenaica to elucidate the main drivers of stem [CO2] at different temporal scales. Fluctuations in sap pH were also assessed during two growing seasons to evaluate potential errors in estimates of the concentration of CO2 dissolved in xylem sap ([CO2*]) calculated using Henry's law. Stem temperature was the best predictor of stem [CO2] and explained more than 90% and 50% of the variability in stem [CO2] at diel and seasonal scales, respectively. Under dry conditions, soil water storage was the main driver of stem [CO2]. Likewise, the first rains after summer drought caused intense stem [CO2] pulses, suggesting enhanced stem and root respiration and increased resistance to radial CO2 diffusion. Sap flow played a secondary role in controlling stem [CO2] variations. We observed night-time sap pH acidification and progressive seasonal alkalinization. Thus, if the annual mean value of sap pH (measured at midday) was assumed to be constant, night-time sap [CO2*] was substantially overestimated (40%), and spring and autumn sap [CO2*] were misestimated by 25%. This work highlights that diel and seasonal variations in temperature, tree water availability, and sap pH substantially affect xylem [CO2] and sap [CO2*].

Stem CO2 efflux in six co-occurring tree species: underlying factors and ecological implications.

Stem respiration plays a role in species coexistence and forest dynamics. Here we examined the intra- and inter-specific variability of stem CO2 efflux (E) in dominant and suppressed trees of six deciduous species in a mixed forest stand: Fagus sylvatica L., Quercus petraea [Matt.] Liebl, Quercus pyrenaica Willd., Prunus avium L., Sorbus aucuparia L. and Crataegus monogyna Jacq. We conducted measurements in late autumn. Within species, dominants had higher E per unit stem surface area (Es ) mainly because sapwood depth was higher than in suppressed trees. Across species, however, differences in Es corresponded with differences in the proportion of living parenchyma in sapwood and concentration of non-structural carbohydrates (NSC). Across species, Es was strongly and NSC marginally positively related with an index of drought tolerance, suggesting that slow growth of drought-tolerant trees is related to higher NSC concentration and Es . We conclude that, during the leafless period, E is indicative of maintenance respiration and is related with some ecological characteristics of the species, such as drought resistance; that sapwood depth is the main factor explaining variability in Es within species; and that the proportion of NSC in the sapwood is the main factor behind variability in Es among species.