Stand-structural effects on Heterobasidion abietinum-related mortality following drought events in Abies pinsapo

Climate change may affect tree–pathogen interactions. This possibility has important implications for drought-prone forests, where stand dynamics and disease pathogenicity are especially sensitive to climatic stress. In addition, stand structural attributes including density-dependent tree-to-tree competition may modulate the stands’ resistance to drought events and pathogen outbreaks. To assess the effects of stand structure on root-rot-related mortality after severe droughts, we focused on Heterobasidion abietinum mortality in relict Spanish stands of Abies pinsapo, a drought-sensitive fir. We compared stand attributes and tree spatial patterns in three plots with H. abietinum root-rot disease and three plots without root-rot. Point-pattern analyses were used to investigate the scale and extent of mortality patterns and to test hypotheses related to the spread of the disease. Dendrochronology was used to date the year of death and to assess the association between droughts and growth decline. We applied a structural equation modelling approach to test if tree mortality occurs more rapidly than predicted by a simple distance model when trees are subjected to high tree-to-tree competition and following drought events. Contrary to expectations of drought mortality, the effect of precipitation on the year of death was strong and negative, indicating that a period of high precipitation induced an earlier tree death. Competition intensity, related to the size and density of neighbour trees, also induced an earlier tree death. The effect of distance to the disease focus was negligible except in combination with intensive competition. Our results indicate that infected trees have decreased ability to withstand drought stress, and demonstrate that tree-to-tree competition and fungal infection act as predisposing factors of forest decline and mortality.     

Effects of a severe drought on Quercus ilex radial growth and xylem anatomy

We assessed the response of Quercus ilex subsp. ballota to the severe summer drought recorded in 1994 in NE Spain through the study of changes in radial growth and wood anatomy. We selected a coppice stand in the Iberian Peninsula, which is characterized by a Mediterranean climate under continental influence. We measured internode length, tree-ring width, mean and maximum vessel diameter, and vessel density for 1981–1997. The annual predicted hydraulic conductance (K_h) was calculated following Hagen-Poisseuille's law. We compared the tree-ring width, vessel diameter and K_h of Q. ilex subsp. ballota and co-existing ring-porous oaks (Q. faginea, Q. pyrenaica) for a dry summer (1994) and a wet summer (1997). To evaluate the drought-resistance of xylem for Q. ilex subsp. ballota (dominant under continental conditions) and Q. ilex subsp. ilex (dominant in mild areas) we determined vulnerability curves. Dimensionless indices of internode length, tree-ring width, and vessel density were compared with climatic data (monthly total precipitation and mean temperature) using correlation analyses. Internode length, tree-ring width, K_h, and mean and maximum vessel diameter declined in 1994. According to vulnerability curves, Q. ilex subsp. ballota showed a greater drought resistance than Q. ilex subsp. ilex. During the year of growth, we found a positive influence of January and June–August precipitation on the internode length, tree-ring width, and vessel density. The response of Q. ilex subsp. ballota radial-growth to summer drought was comparable to that of Q. faginea latewood. Overall, growth and wood anatomy of Q. ilex subsp. ballota showed a plastic response to drought.     

Motion of a DNA sliding clamp observed by single molecule fluorescence spectroscopy

DNA sliding clamps attach to polymerases and slide along DNA to allow rapid, processive replication of DNA. These clamps contain many positively charged residues that could curtail the sliding due to attractive interactions with the negatively charged DNA. By single-molecule spectroscopy we have observed a fluorescently labeled sliding clamp (polymerase III beta subunit or beta clamp) loaded onto freely diffusing, single-stranded M13 circular DNA annealed with fluorescently labeled DNA oligomers of up to 90 bases. We find that the diffusion constant for the beta clamp diffusing along DNA is on the order of 10(-14) m(2)/s, at least 3 orders of magnitude less than that for diffusion through water alone. We also find evidence that the beta clamp remains at the 3' end in the presence of Escherichia coli single-stranded-binding protein. These results may imply that the clamp not only acts to hold the polymerase on the DNA but also prevents excessive drifting along the DNA.     

The Deuterator: software for the determination of backbone amide deuterium levels from H/D exchange MS data

Background  

The combination of mass spectrometry and solution phase amide hydrogen/deuterium exchange (H/D exchange) experiments is an effective method for characterizing protein dynamics, and protein-protein or protein-ligand interactions. Despite methodological advancements and improvements in instrumentation and automation, data analysis and display remains a tedious process. The factors that contribute to this bottleneck are the large number of data points produced in a typical experiment, each requiring manual curation and validation, and then calculation of the level of backbone amide exchange. Tools have become available that address some of these issues, but lack sufficient integration, functionality, and accessibility required to address the needs of the H/D exchange community. To date there is no software for the analysis of H/D exchange data that comprehensively addresses these issues.     

Geographically Structured Growth decline of Rear-Edge Iberian Fagus sylvatica Forests After the 1980s Shift Toward a Warmer Climate

Ecosystems - Warming-related growth decrease on southern Fagus sylvatica forests has been observed in different regions; however, whether it is a generalized fact or not remains unclear. Here we...     

Testing the shared-pathway hypothesis in the carotenoid-based coloration of red crossbills

The mechanisms involved in the production of red carotenoid-based ornaments of vertebrates are still poorly understood. These colorations often depend on enzymatic transformations (ketolation) of dietary yellow carotenoids, which could occur in the inner mitochondrial membrane (IMM). Thus, carotenoid ketolation and cell respiration could share biochemical pathways, favoring the evolution of ketocarotenoid-based ornaments as reliable indices of individual quality under sexual selection. Captive male red crossbills (Loxia curvirostra Linnaeus) were exposed to redox-active compounds designed to penetrate and act in the IMM: an ubiquinone (mitoQ) or a superoxide dismutase mimetic (mitoTEMPO). MitoQ can act as an antioxidant but also distort the IMM structure, increasing mitochondrial free radical production. MitoQ decreased yellow carotenoids and tocopherol levels in blood, perhaps by being consumed as antioxidants. Contrarily, mitoTEMPO-treated birds rose circulating levels of the second most abundant ketocarotenoid in crossbills (i.e., canthaxanthin). It also increased feather total red ketocarotenoid concentration and redness, but only among those birds exhibiting a redder plumage at the start of the study, that is, supposedly high-quality individuals. The fact that mitoTEMPO effects depended on original plumage color suggests that the red-ketocarotenoid-based ornaments indicate individual quality as mitochondrial function efficiency. The findings would thus support the shared pathway hypothesis.     

Climate–human interactions contributed to historical forest recruitment dynamics in Mediterranean subalpine ecosystems

Long‐term tree recruitment dynamics of subalpine forests mainly depend on temperature changes, but little is known about the feedbacks between historical land use and climate. Here, we analyze a southern European, millennium‐long dataset of tree recruitment from three high‐elevation pine forests located in Mediterranean mountains (Pyrenees, northeastern Spain; Pollino, southern Italy; and Mt. Smolikas, northern Greece). We identify synchronized recruitment peaks in the late 15th and early 16th centuries, following prolonged periods of societal and climate instability. Major European population crises in the 14th and 15th centuries associated with recurrent famines, the Black Death pandemic, and political turmoil are likely to have reduced the deforestation of subalpine environments and caused widespread rewilding. We suggest that a distinct cold phase in the Little Ice Age around 1450 ce could also have accelerated the cessation of grazing pressure, particularly in the Pyrenees, where the demographic crisis was less severe. Most pronounced in the Pyrenees, the enhanced pine recruitment from around 1500–1550 ce coincides with temporarily warmer temperatures associated with a positive phase of the North Atlantic Oscillation. We diagnose that a mixture of human and climate factors has influenced past forest recruitment dynamics in Mediterranean subalpine ecosystems. Our results highlight how complex human–climate interactions shaped forest dynamics during pre‐industrial times and provide historical analogies to recent rewilding.     

Changes in thylakoid membrane thickness associated with the reorganization of photosystem II light harvesting complexes during photoprotective energy dissipation

Using freeze-fracture electron microscopy we have recently shown that non-photochemical quenching (NPQ), a mechanism of photoprotective energy dissipation in higher plant chloroplasts, involves a reorganization of the pigment-protein complexes within the stacked grana thylakoids.¹ Photosystem II light harvesting complexes (LHCII) are reorganized in response to the amplitude of the light driven transmembrane proton gradient (ΔpH) leading to their dissociation from photosystem II reaction centers and their aggregation within the membrane.¹ This reorganization of the PSII-LHCII macrostructure was found to be enhanced by the formation of zeaxanthin and was associated with changes in the mobility of the pigment-protein complexes therein.¹ We suspected that the structural changes we observed were linked to the ΔpH-induced changes in thylakoid membrane thickness that were first observed by Murikami and Packer.^2,3 Here using thin-section electron microscopy we show that the changes in thylakoid membrane thickness do not correlate with ΔpH per se but rather the amplitude of NPQ and is thus affected by the de-epoxidation of the LHCII bound xanthophyll violaxanthin to zeaxanthin. We thus suggest that the change in thylakoid membrane thickness occurring during NPQ reflects the conformational change within LHCII proteins brought about by their protonation and aggregation within the membrane.Key words: nonphotochemical quenching, photoprotection, LHCII, photosystem II, thylakoid membrane     

Challenges to understand plant responses to wind

Understanding plant response to wind is complicated as this factor entails not only mechanical stress, but also affects leaf microclimate. In a recent study, we found that plant responses to mechanical stress (MS) may be different and even in the opposite direction to those of wind. MS-treated Plantago major plants produced thinner more elongated leaves while those in wind did the opposite. The latter can be associated with the drying effect of wind as is further supported by data on petiole anatomy presented here. These results indicate that plant responses to wind will depend on the extent of water stress. It should also be recognized that the responses to wind may differ between different parts of a plant and between plant species. Physiological research on wind responses should thus focus on the signal sensing and transduction of both the mechanical and drought signals associated with wind, and consider both plant size and architecture.Key words: biomechanics, leaf anatomy, phenotypic plasticity, plant architecture, signal transduction thigmomorphogenesis, windWind is one of the most ubiquitous environmental stresses, and can strongly affect development, growth and reproductive yield in terrestrial plants.^1–3 In spite of more than two centuries of research,⁴ plant responses to wind and their underlying mechanisms remain poorly understood. This is because plant responses to mechanical movement themselves are complicated and also because wind entails not only mechanical effects, but also changes in leaf gas and heat exchange.^5–7 Much research on wind has focused primarily on its mechanical effect. Notably, several studies that determine plant responses to mechanical treatments such as flexing, implicitly extrapolate their results to wind effects.^8–10 Our recent study¹¹ showed that this may lead to errors as responses to wind and mechanical stimuli (in our case brushing) can be different and even in the opposite direction. In this paper, we first separately discuss plant responses to mechanical stimuli, and other wind-associated effects, and then discuss future challenges for the understanding of plant responses to wind.It is often believed that responses to mechanical stress (thigmomorphogenesis) entail the production of thicker and stronger plant structures that resist larger forces. This may be true for continuous unidirectional forces such as gravity, however for variable external forces (such as wind loading or periodic flooding) avoiding such mechanical stress by flexible and easily reconfigurable structures can be an alternative strategy.^12–14 How plants adapt or acclimate to such variable external forces depends on the intensity and frequency of stress and also on plant structures. Reduced height growth is the most common response to mechanical stimuli.^15,16 This is partly because such short stature increases the ability of plants to both resist forces (e.g., real-locating biomass for radial growth rather than elongation growth), and because small plants experience smaller drag forces (Fig. 1). Some plant species show a resistance strategy in response to mechanical stress by increasing stem thickness^1,10 and tissue strength.⁷ But other species show an avoidance strategy by a reduction in stem or petiole thickness and flexural rigidity in response to MS.^11,15–18 These different strategies might be associated with plant size and structure. Stems of larger plants such as trees and tall herbs are restricted in the ability to bend as they carry heavy loads^7,10,19 (Fig. 1). Conversely short plants are less restricted in this respect and may also be prone to trampling for which stress-avoidance would be the only viable strategy.^18,20 Systematic understanding of these various responses to mechanical stress remains to be achieved.Open in a separate windowFigure 1A graphical representation of how wind effects can be considered to entail both a drying and a mechanical effect. Adaptation or acclimation to the latter can be through a force resistance strategy or a force avoidance strategy, the benefit of which may depend on the size and architecture of plants as well as the location of a given structure within a plant.Wind often enhances water stress by reducing leaf boundary layers and reduces plant temperature by transpiration cooling. The latter effect may be minor,¹¹ but the former could significantly affect plant development. Anten et al. (2010) compared phenotypic traits and growth of Plantago major that was grown under mechanical stimuli by brushing (MS) and wind in the factorial design. Both MS and wind treatments reduced growth and influenced allocation in a similar manner. MS plants, however, had more slender petioles and narrower leaf blades while wind exposed plants exhibited the opposite response having shorter and relatively thicker petioles and more round-shaped leaf blades. MS plants appeared to exhibit stress avoidance strategy while such responses could be compensated or overridden by water stress in wind exposure.¹¹ A further analysis of leaf petiole anatomy (Fig. 2) supports this view. The vascular fraction in the petiole cross-section was increased by wind but not by MS, suggesting that higher water transport was required under wind. Our results suggest that drying effect of wind can at least to some extent override its mechanical effect.Open in a separate windowFigure 2Representative images of petiole cross-sections of Plantago major grown in 45 days in continuous wind and/or mechanical stimuli (A–D). Petiole cross-section area (E) and vascular bundle fraction in the cross-section of petiole (F). mean + SD (n = 12) are shown. Significance levels of ANOVA; ***p < 0.001, **p < 0.01, *p < 0.05, ns p > 0.05.Physiological knowledge on plant mechanoreception and signal transduction has been greatly increased during the last decades. Plants sense mechanical stimuli through membrane strain with stretch activated channels²¹ and/or through some linker molecules connecting the cell wall, plasma membrane and cytoskeleton.^4,22,23 This leads to a ubiquitous increase in intracellular Ca²⁺ concentration. The increased Ca²⁺ concentration is sensed by touch induced genes (TCHs),^24,25 which activates downstream transduction machineries including a range of signaling molecules and phytohormones, consequently altering physiological and developmental processes.²⁶ Extending this knowledge to understand plant phenotypic responses to wind however remains a challenge. As responses to wind have been found to differ among parts of a plant (e.g., terminal vs. basal stem) and also across species, physiological studies should be extended to the whole-plant as integrated system rather than focusing on specific tissue level. Furthermore to understand the general mechanism across species, it is required to study different species from different environmental conditions. Advances in bioinformatics, molecular and physiological research will facilitate cross-disciplinary studies to disentangle the complicated responses of plants to wind.     

Competition and drought limit the response of water-use efficiency to rising atmospheric carbon dioxide in the Mediterranean fir <Emphasis Type="Italic">Abies pinsapo</Emphasis>

The gas-exchange and radial growth responses of conifer forests to climatic warming and increasing atmospheric CO₂ have been widely studied. However, the modulating effects of variables related to stand structure (e.g., tree-to-tree competition) on those responses are poorly explored. The basal-area increment (BAI) and C isotope discrimination (C stable isotope ratio; δ¹³C) in the Mediterranean fir Abies pinsapo were investigated to elucidate the influences of stand competition, atmospheric CO₂ concentrations and climate on intrinsic water-use efficiency (WUEi). We assessed the variation in δ¹³C of tree-rings from dominant or co-dominant trees subjected to different degrees of competition. A high- (H) and a low-elevation (L) population with contrasting climatic constraints were studied in southern Spain. Both populations showed an increase in long-term WUEi. However, this increase occurred more slowly at the L site, where a decline of BAI was also observed. Local warming and severe droughts have occurred in the study area over the past 30 years, which have reduced water availability more at lower elevations. Contrastingly, trees from the H site were able to maintain high BAI values at a lower cost in terms of water consumption. In each population, trees subjected to a higher degree of competition by neighboring trees showed lower BAI and WUEi than those subjected to less competition, although the slopes of the temporal trends in WUEi were independent of the competitive micro-environment experienced by the trees. The results are consistent with an increasing drought-induced limitation of BAI and a decreasing rate of WUEi improvement in low-elevation A. pinsapo forests. This relict species might not be able to mitigate the negative effects of a decrease in water availability through a reduction in stomatal conductance, thus leading to a growth decline in the more xeric sites. An intense and poorly asymmetric competitive environment at the stand level may also act as an important constraint on the adaptive capacity of these drought-sensitive forests to climatic warming. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.