Ana I. García-Cervigón

Quantitative Tracheid Anatomy Reveals a Complex Environmental Control of Wood Structure in Continental Mediterranean Climate

A detailed understanding of how the climate models the hydraulic system of trees is still lacking, in spite of the need to understand tree response to the ongoing process of climatic change. A systematic correlation of tracheid anatomical features with climate has not been implemented in Mediterranean ecosystems, where climatic change is expected to be particularly intense. We measured ring width and cell number—in addition to intraring position, lumen size, and wall thickness of tracheids—in 10 Juniperus thurifera individuals from north-central Spain. We used this information to perform an exploratory analysis of how these parameters correlated with climatic variables in 1965–2004. Cell number and ring width shared a relatively similar climatic signal, whereas the anatomical variables provided differentiated and diverse signals about climatic conditions during their formation. Earlywood and latewood tracheids differed in controlling factors, with earlywood tracheid size and wall thickness being positively related to rainfall during early summer and latewood tracheid size being positively related to August temperature. Tracheid anatomical variables improved our understanding of climate effects on tree growth and wood formation under harsh environmental conditions, as those experienced in continental Mediterranean climates, where limiting factors show multiple shifts across the year.

New star on the stage: amount of ray parenchyma in tree rings shows a link to climate

Tree-ring anatomy reflects the year-by-year impact of environmental factors on tree growth. Up to now, research in this field has mainly focused on the hydraulic architecture, with ray parenchyma neglected despite the growing recognition of its relevance for xylem function. Our aim was to address this gap by exploring the potential of the annual patterns of xylem parenchyma as a climate proxy. We constructed ring-width and ray-parenchyma chronologies from 1965 to 2004 for 20 Juniperus thurifera trees growing in a Mediterranean continental climate. Chronologies were related to climate records by means of correlation, multiple regression and partial correlation analyses. Ray parenchyma responded to climatic conditions at critical stages during the xylogenetic process; namely, at the end of the previous years xylogenesis (October) and at the onset of earlywood (May) and latewood formation (August). Ray parenchyma-based chronologies have potential to complement ring-width chronologies as a tool for climate reconstructions. Furthermore, medium- and low-frequency signals in the variation of ray parenchyma may improve our understanding of how trees respond to environmental fluctuations and to global change.

Drought-induced increase in water-use efficiency reduces secondary tree growth and tracheid wall thickness in a Mediterranean conifer

In order to understand the impact of drought and intrinsic water-use efficiency (iWUE) on tree growth, we evaluated the relative importance of direct and indirect effects of water availability on secondary growth and xylem anatomy of Juniperus thurifera, a Mediterranean anisohydric conifer. Dendrochronological techniques, quantitative xylem anatomy, and 13C/12C isotopic ratio were combined to develop standardized chronologies for iWUE, BAI (basal area increment), and anatomical variables on a 40-year-long annually resolved series for 20 trees. We tested the relationship between iWUE and secondary growth at short-term (annual) and long-term (decadal) temporal scales to evaluate whether gains in iWUE may lead to increases in secondary growth. We obtained a positive long-term correlation between iWUE and BAI, simultaneously with a negative short-term correlation between them. Furthermore, BAI and iWUE were correlated with anatomical traits related to carbon sink or storage (tracheid wall thickness and ray parenchyma amount), but no significant correlation with conductive traits (tracheid lumen) was found. Water availability during the growing season significantly modulated tree growth at the xylem level, where growth rates and wood anatomical traits were affected by June precipitation. Our results are consistent with a drought-induced limitation of tree growth response to rising CO2, despite the trend of rising iWUE being maintained. We also remark the usefulness of exploring this relationship at different temporal scales to fully understand the actual links between iWUE and secondary growth dynamics.

Back to the Future: The Responses of Alpine Treelines to Climate Warming are Constrained by the Current Ecotone Structure

Alpine treeline ecotones are considered early-warning monitors of the effects of climate change on terrestrial ecosystems, but it is still unclear how accurately treeline dynamics may track the expected temperature rises. Site-specific abiotic constraints, such as topography and demographic trends may make treelines less responsive to environmental fluctuations. A better understanding on how local processes modulate treelines’ response to warming is thus required. We developed a model of treeline dynamics based on individual data of growth, mortality and reproduction. Specifically, we modeled growth patterns, mortality rates and reproductive size thresholds as a function of temperature and stand structure to evaluate the influence of climate- and stand-related processes on treeline dynamics. In this study, we analyze the dynamics of four Pyrenean mountain pine treeline sites with contrasting stand structures, and subjected to differing rates of climate warming. Our models indicate that Pyrenean treelines could reach basal areas and reproductive potentials similar to those currently observed in high-elevation subalpine forest by the mid twenty-first century. The fastest paces of treeline densification are forecasted by the late twenty-first century and are associated with higher warming rates. We found a common densification response of Pyrenean treelines to climate warming, but contrasting paces arise due to current size structures. Treelines characterized by a multistratified stand structure and subjected to lower mean annual temperatures were the most responsive to climate warming. In monostratified stands, tree growth was less sensitive to temperature than in multistratified stands and trees reached their reproductive size threshold later. Therefore, our simulations highlight that stand structure is paramount in modulating treeline responsiveness to ongoing climate warming. Synthesis. Treeline densification over the twenty-first century is likely to occur at different rates contingent on current stand structure and its effects on individual-level tree growth responses to warming. Accurate projections of future treeline dynamics must thus incorporate site-specific factors other than climate, specifically those related to stand structure and its influence on tree growth.

Atlantic and Mediterranean synoptic drivers of central Spanish juniper growth

Atlantic and Mediterranean air masses influence the climate over the Iberian System mountain range. The relatively short instrumental records in central Spain though limit any long-term assessment of these synoptic systems. We here evaluate the potential to analyze such changes using ring width data from Juniperus thurifera trees growing in the northwestern and southeastern regions of the Iberian System, exposed to Atlantic and Mediterranean cyclonic activity, respectively. Comparison of tree rings with regional precipitation, temperature, and Palmer Drought Severity Index (PDSI) data indicates that juniper trees contain information on late spring and early summer drought conditions. Calibration trials using spatially resolved, gridded climate data reveal that the northwestern sampling site is predominantly controlled by Atlantic weather, while the southeastern site mainly reflects Mediterranean climate patterns. The strength and position of the blocking Azores high during spring to early summer is of particular importance for the distinct growth reactions in the Iberian System. The climate signal is remarkably strong in the southeastern site, where we developed the longest and best-replicated juniper tree ring record of the Iberian Peninsula. Data from this site allowed the reconstruction of May-June PDSI variability back to the early eighteenth century, indicating severe drought (PDSI < −9) in southeastern Spain in 1782, 1828, 1869, 1981, and 2005. The new PDSI record coheres well with historical rogation ceremony data from eastern Spain, indicating that common information on past drought events is inherent in both proxy archives.

Facilitation promotes changes in leaf economics traits of a perennial forb.

Optimal allocation of resources is crucial to maximize plant success. Plants modify their economic strategies by adjusting functional traits in response to shifts in environmental conditions. Facilitation has been recognized as a major biotic filter of trait distribution in communities, although the effect of facilitation on intraspecific variability has been scarcely explored. We evaluated intraspecific shifts in leaf functional traits of a perennial forb (Helleborus foetidus) in relation to the presence of a nurse plant (Juniperus sabina) in two sites with contrasting abiotic stress levels. The effects of abiotic (site) and biotic (juniper presence, microsite) environments on specific leaf area (SLA), leaf area (LA), lamina/petiole length ratio (LPR), intrinsic water use efficiency (iWUE) and leaf nutrient content (N, P and N:P) per mass were evaluated. Alleviation of drought stress associated with nurse plant presence was reflected in SLA, LA, LPR and iWUE at the high-stress site. Individuals growing in open areas showed more resource-conservative traits, supporting the argument that this strategy is advantageous in environments that limit opportunities for rapid carbon gain. Leaf nutrients were unrelated to other traits. The large amount of intraspecific variation in leaf functional traits related to facilitative processes highlights the importance of facilitation as a major source of plant trait variation. Both positive and negative biotic interactions, as well as intraspecific trait variability, should be considered in mechanistic models of plant communities’ functional responses to environmental changes.

Intra-annual wood density fluctuations and tree-ring width patterns are sex- and site-dependent in the dioecious conifer Juniperus thurifera L.

Key messageSex and site conditions modulate intra- and inter-annual secondary growth and its climatic sensitivity in a dioecious Mediterranean conifer.AbstractDivergent evolutionary pressures associated with differential reproductive costs in dioecious trees may lead to sex-related variation in non-reproductive functions. Sex-related differences may be site-dependent, with different outcomes depending on environmental conditions. We explored the effects of sex and environmental conditions on the climatic control of annual secondary growth and intra-annual wood density fluctuations (IADF) of a dioecious conifer (Juniperus thurifera L.) growing in two sites with contrasting hydrological conditions under a continental Mediterranean climate. Different sex-related strategies had variable effects on relative secondary growth, with females outperforming males under more favorable hydrological conditions, and males outperforming females under water-limited conditions. Ring width and IADF formation were driven by climatic factors occurring at different temporal scales. Tree-ring growth depended on factors acting prior to the initiation of the xylogenesis and to conditions directly affecting the duration and pace of cambial activity, and ring width, therefore, integrated a complex signal of factors occurring over a relatively long period, and on an annual cycle. In contrast, IADFs responded to singular short-term events that alleviated drought and promoted cambial reactivation during the summer arrest. Female trees showed a more opportunistic water use, displayed in the stronger ring-width response to June–July conditions. Enhanced cambial sensitivity in females set a lower threshold for IADF occurrence, leading to a higher frequency of IADFs irrespective of site. Intra-annual and inter-annual female growth patterns reflect an opportunistic strategy to benefit from favorable climatic windows.

Alpine Ecology in the Iberian Peninsula: What Do We Know, and What Do We Need to Learn?

Abstract The 11th Conference of the Spanish Association of Terrestrial Ecology, held in Pamplona, Spain, on 6–10 May 2013, included a symposium on alpine ecological research in the Iberian Peninsula. This session offered an excellent opportunity to assess the state and progress of alpine ecology in this region, identify knowledge gaps, and discuss further directions for research. Iberian alpine ecosystems are biodiversity hotspots and have traditionally contributed to sustaining rural livelihoods. Today, these ecosystems are subjected to large changes in land uses, including land abandonment, and are affected by climate change. This article reviews the current state of Iberian alpine ecology and proposes a research agenda. Alpine ecology in the Iberian Peninsula is a growing field of research. The need for larger spatial and temporal scales in research and monitoring, along with the integration of socioecological aspects, is a critical issue for understanding the major drivers of change in the alpine ecosystems of the Iberian Peninsula. The implementation of effective mitigation strategies aimed at reducing the impact of the pressing environmental and socioeconomic problems of Iberian mountain areas can only be accomplished through a multidisciplinary and integrative approach.

Disentangling Facilitation Along the Life Cycle: Impacts of Plant–Plant Interactions at Vegetative and Reproductive Stages in a Mediterranean Forb

Facilitation enables plants to improve their fitness in stressful environments. The overall impact of plant–plant interactions on the population dynamics of protégées is the net result of both positive and negative effects that may act simultaneously along the plant life cycle, and depends on the environmental context. This study evaluates the impact of the nurse plant Juniperus sabina on different stages of the life cycle of the forb Helleborus foetidus. Growth, number of leaves, flowers, carpels, and seeds per flower were compared for 240 individuals collected under nurse canopies and in open areas at two sites with contrasting stress levels. Spatial associations with nurse plants and age structures were also checked. A structural equation model was built to test the effect of facilitation on fecundity, accounting for sequential steps from flowering to seed production. The net impact of nurse plants depended on a combination of positive and negative effects on vegetative and reproductive variables. Although nurse plants caused a decrease in flower production at the low-stress site, their net impact there was neutral. In contrast, at the high-stress site the net outcome of plant–plant interactions was positive due to an increase in effective recruitment, plant density, number of viable carpels per flower, and fruit set under nurse canopies. The naturally lower rates of secondary growth and flower production at the high-stress site were compensated by the net positive impact of nurse plants here. Our results emphasize the need to evaluate entire processes and not only final outcomes when studying plant–plant interactions.

Assessing seed and microsite limitation on population dynamics of a gypsophyte through experimental soil crust disturbance and seed addition

Understanding the factors limiting population growth is crucial for species management and conservation. We assessed the effects of seed and microsite limitation, along with climate variables, on Helianthemum squamatum, a gypsum soil specialist, in two sites in central Spain. We evaluated the effects of experimental seed addition and soil crust disturbance on H. squamatum vital rates (survival, growth and reproduction) across four years. We used this information to build integral projection models (IPMs) for each combination of management (seed addition or soil disturbance), site and year. We examined differences in population growth rate (λ) due to management using life table response experiments. Soil crust disturbance increased survival of mid to large size individuals and germination. Contributions to λ of positive individual growth (progression) and negative individual growth (retrogression) due to managements varied among years and sites. Soil crust disturbance increased λ in the site with the highest plant density, and seed addition had a moderate positive effect on λ in the site with lowest plant density. Population growth rate (λ) decreased by half in the driest year. Differences in management effects between sites may represent a shift from seed to microsite limitation at increasing densities. This shift underscores the importance of considering what factors limit population growth when selecting a management strategy.