Quercus Pyrenaica Willd Research Articles

Disentangling the effects of tree species and microclimate on heterotrophic and autotrophic soil respiration in a Mediterranean ecotone forest

Understanding how forest ecosystems influence the response of soil respiration (RS) to climate drivers is essential to accurately predict soil carbon dioxide (CO2) fluxes in a changing environment. This is particularly crucial in areas of contact between coniferous and broadleaved forests in the Mediterranean region which are already experiencing a warmer and drier climate. We present a case study in a Mediterranean ecotone forest where RS components –heterotrophic (RH) and autotrophic (RA)– were monitored beneath pure Scots pine (Pinus sylvestris L.) and Pyrenean oak (Quercus pyrenaica Willd.) stands using the root-trenching method. We used generalized linear models to predict the soil CO2 efflux based on interactions between soil water conent (SWC) and soil temperature. Regardless of the tree species, we found that the strong and inverse water availability and thermal seasonality of the Mediterranean climate intensely constrained soil microbial activity. The incorporation of the soil temperature-moisture covariation thus greatly improved the quality of the models compared to approaches that consider soil temperature alone. We also identified species-specific responses influencing both the total amount of RA and its sensitivity to environmental variables. Root respiration in the Scots pine stand showed greater vulnerability to the decline in SWC throughout the summer than in the oak stand. The RA in the Pyrenean oak stand was mainly limited by low soil temperatures in winter, indicating low maintenance rates during vegetative dormancy. Mean annual RH rates were highest in the Scots pine stand, probably driven by the larger litterfall rates and soil carbon (C) stocks; however, the apparent turnover rate of soil organic C in the oak stand was almost twice as fast as in the pine stand. While our observations are limited to a case study, our work shows that both soil moisture and forest composition can significantly control the temperature dependence of RS components under a Mediterranean climate.

Divergent phenological and leaf gas exchange strategies of two competing tree species drive contrasting responses to drought at their altitudinal boundary.

In Mediterranean mountains, Pinus sylvestris L. is expected to be displaced under a warming climate by more drought-tolerant species such as the sub-Mediterranean Quercus pyrenaica Willd. Understanding how environmental factors drive tree physiology and phenology is, therefore, essential to assess the effect of changing climatic conditions on the performance of these species and, ultimately, their distribution. We compared the cambial and leaf phenology and leaf gas exchange of Q. pyrenaica and P. sylvestris at their altitudinal boundary in Central Spain and assessed the environmental variables involved. Results indicate that P. sylvestris cambial phenology was more sensitive to weather conditions (temperature at the onset and water deficit at the end of the growing season) than Q. pyrenaica. On the other hand, Q. pyrenaica cambial and leaf phenology were synchronized and driven by photoperiod and temperatures. Pinus sylvestris showed lower photosynthetic nitrogen-use efficiency and higher intrinsic water-use efficiency than Q. pyrenaica as a result of a tighter stomatal control in response to summer dry conditions, despite its less negative midday leaf water potentials. These phenological and leaf gas exchange responses evidence a stronger sensitivity to drought of P. sylvestris than that of Q. pyrenaica, which may therefore hold a competitive advantage over P. sylvestris under the predicted increase in recurrence and intensity of drought events. On the other hand, both species could benefit from warmer springs through an advanced phenology, although this effect could be limited in Q. pyrenaica if it maintains a photoperiod control over the onset of xylogenesis.

Dry matter content during extension of twigs, buds and leaves reflects hydraulic status related to earlywood vessel development in Quercus pyrenaica Willd.

A quantitative method was tested to describe crown phenophases in relation to water content and to secondary growth in ring-porous species, based on the hypothesis that new shoots require hydrated tissues to maintain the necessary turgor for extension, leading to a reduction in dry matter content (DMC). We collected a three-year-old branch from 11 Quercus pyrenaica Willd. trees at 10-day intervals to estimate DMC of newly developing buds, leaves, and twigs, and processed two opposite stem microcores for xylogenesis. Branch phenophases and shoot length were recorded in the field. The DMC of all organs decreased during crown development, with a minimum in early June, followed by a gradual increase up to initial values in late September. The shoot extension period concurred with the lowest DMC, but also with the beginning of earlywood maturation in the main stem, suggesting a high tissue hydration only when earlywood vessels become functional to fulfill enough water requirements for shoot and leaf extension. These results confirm the usefulness of DMC to accurately quantify the phenology of primary growth from bud swelling up to full leaf extension, as a complement to qualitative methods. This variation in DMC appears to be linked to secondary growth as a result of earlywood vessel development.

Total and Active Core Microbiomes of Melojo Oak Are Composed by a Minority of Bacterial Taxa Highly Abundant

Melojo oak (Quercus pyrenaica Willd.) is a key tree species of Mediterranean forests with high landscape value in the Iberian Peninsula. However, these forests exhibit an advanced stage of deterioration and their distribution area has been seriously reduced over the last decades. Plant-associated microorganisms play an essential role improving their host’s fitness, hence, a better understanding of oak rhizospheric microbiome, especially of those active members, could be the first step towards microbiome-based approaches for the improvement of the survival and growth of oak trees. Here we reported, for the first time, the diversity of total (DNA-based) and potentially active (RNA-based) bacterial communities of three types of Quercus pyrenaica Willd. forest formations through 454 pyrosequencing of 16S rDNA amplicons, and defined the rhizospheric core microbiome of Q. pyrenaica at Sierra Nevada National and Natural Park during spring season. In addition rRNA:rDNA ratio was calculated as a proxy of the contribution of each core taxon to the rhizosphere environment. We found that potentially active bacterial communities were as rich and diverse as DNA-based populations, but different in terms of relative abundance patterns in some of the studied areas. Both microbiomes were dominated by a relatively small percentage of OTUs, most of which showed positive correlation between both libraries. However, the uncoupling between abundance (rDNA) and potential activity (rRNA) for some taxa suggests that the most abundant taxa are not always the most active, and that low-abundance OTUs may have a strong influence on oak’s rhizospheric ecology. Thus, measurement of rRNA:rDNA ratio could be helpful in characterizing rhizospheric bacterial communities, their potential involvement in the functioning of the soil ecosystem and, therefore, in identifying major bacterial players for the development of microbiome-based strategies for future afforestation tasks.

Coexisting oak species, including rear-edge populations, buffer climate stress through xylem adjustments.

The ability of trees to cope with climate change is a pivotal feature of forest ecosystems, especially for rear-edge populations facing warm and dry conditions. To evaluate current and future forests threats, a multi-proxy focus on the growth, anatomical and physiological responses to climate change is needed. We examined the long-term xylem adjustments to climate variability of the temperate Quercus robur L. at its rear edge and the sub-Mediterranean Quercus pyrenaica Willd. Both species coexist at a mesic (ME, humid and warmer) and a xeric (XE, dry and cooler) site in northern Spain, the latter experiencing increasing temperatures in recent decades. We compared xylem traits at each site and assessed their trends, relationships and responses to climate (1960-2008). Traits included basal area increment, earlywood vessel hydraulic diameter, density and theoretical-specific hydraulic conductivity together with latewood oxygen (δ18O) stable isotopes and δ13C-derived water-use efficiency (iWUE). Quercus robur showed the highest growth at ME, likely through enhanced cambial activity. Quercus pyrenaica had higher iWUE at XE compared with ME, but limited plasticity of anatomical xylem traits was found for the two oak species. Similar physiological performance was found for both species. The iWUE augmented in recent years especially at XE, likely explained by stomatal closure given the increasing δ18O signal in response to drier and sunnier growing seasons. Overall, traits were more correlated at XE than at ME. The iWUE improvements were linked to higher growth up to a threshold (~85 μmol mol-1) after which reduced growth was found at XE. Our results are consistent with Q. pyrenaica and Q. robur coexisting at the central and dry edge of the climatic species distribution, respectively, showing similar responses to buffer warmer conditions. In fact, the observed adjustments found for Q. robur point towards growth stability of similar rear-edge oak populations under warmer climate conditions.

Taxonomic and Functional Diversity of a Quercus pyrenaica Willd. Rhizospheric Microbiome in the Mediterranean Mountains

Altitude significantly affects vegetation growth and distribution, including the developmental stages of a forest. We used shotgun Illumina sequencing to analyze microbial community composition and functional potential in melojo-oak (Quercus pyrenaica Willd.) rhizospheric soil for three different development stages along an altitudinal gradient: (a) a low altitude, non-optimal site for forest maintenance; (b) an intermediate altitude, optimal site for a forest; and (c) a high altitude, expansion site with isolated trees but without a real forest canopy. We observed that, at each altitude, the same microbial taxa appear both in the taxonomic analysis of the whole metagenome and in the functional analysis of the methane, sulfur and nitrogen metabolisms. Although there were no major differences at the functional level, there were significant differences in the abundance of each taxon at the phylogenetic level between the rhizospheres of the forest (low and intermediate altitudes) and the expansion site. Proteobacteria and Actinobacteria were the most differentially abundant phyla in forest soils compared to the expansion site rhizosphere. Moreover, Verrucomicrobia, Bacteroidetes and Nitrospirae phyla were more highly represented in the non-forest rhizosphere. Our study suggests that rhizospheric microbial communities of the same tree species may be affected by development stage and forest canopy cover via changes in soil pH and the C/N ratio.

Xylem and Leaf Functional Adjustments to Drought in Pinus sylvestris and Quercus pyrenaica at Their Elevational Boundary.

Climatic scenarios for the Mediterranean region forecast increasing frequency and intensity of drought events. Consequently, a reduction in Pinus sylvestris L. distribution range is projected within the region, with this species being outcompeted at lower elevations by more drought-tolerant taxa such as Quercus pyrenaica Willd. The functional response of these species to the projected shifts in water availability will partially determine their performance and, thus, their competitive success under these changing climatic conditions. We studied how the cambial and leaf phenology and xylem anatomy of these two species responded to a 3-year rainfall exclusion experiment set at their elevational boundary in Central Spain. Additionally, P. sylvestris leaf gas exchange, water potential and carbon isotope content response to the treatment were measured. Likewise, we assessed inter-annual variability in the studied functional traits under control and rainfall exclusion conditions. Prolonged exposure to drier conditions did not affect the onset of xylogenesis in either of the studied species, whereas xylem formation ceased 1–3 weeks earlier in P. sylvestris. The rainfall exclusion had, however, no effect on leaf phenology on either species, which suggests that cambial phenology is more sensitive to drought than leaf phenology. P. sylvestris formed fewer, but larger tracheids under dry conditions and reduced the proportion of latewood in the tree ring. On the other hand, Q. pyrenaica did not suffer earlywood hydraulic diameter changes under rainfall exclusion, but experienced a cumulative reduction in latewood width, which could ultimately challenge its hydraulic performance. The phenological and anatomical response of the studied species to drought is consistent with a shift in resource allocation under drought stress from xylem to other sinks. Additionally, the tighter stomatal control and higher intrinsic water use efficiency observed in drought-stressed P. sylvestris may eventually limit carbon uptake in this species. Our results suggest that both species are potentially vulnerable to the forecasted increase in drought stress, although P. sylvestris might experience a higher risk of drought-induced decline at its low elevational limit.

Thinning enhances the species-specific radial increment response to drought in Mediterranean pine-oak stands

Radial increment analyses allow us to determine tree responses to weather and tree competition, thus enabling the development of management strategies for adapting forest stands to forecasted climate change scenarios. In this study, the responses of pine-oak mixed stands (Pinus pinaster Ait. and Quercus pyrenaica Willd.) to thinning treatments were assessed at two sites in central Spain with contrasting drought conditions. Inter- and intra-annual radial increments were recorded every two weeks from dendrometer band measurements, using a Latin square design. Each site consisted of a control (unthinned) and two thinning intensities: moderate (25% of pine basal area removed) and heavy (40% of pine basal area removed). Thinning effects were monitored over three years of different weather conditions (2010–2012), with an extreme drought event in 2012. Linear mixed models were fitted to analyze weather and thinning effects on the inter- and intra-annual radial increment rates. Additionally, the sum of two logistic functions was used to estimate cumulative radial increment patterns. Heavy thinning resulted in the greatest annual radial increment for pine, regardless of site and year, even during the 2012 drought, when compared to densely stocked forest stands. The model results indicated that heavy thinning positively affected the intra-annual pattern, increasing inflection point and spring and autumn asymptotes. Thinning also had a slight positive effect on inter- and intra-annual radial increment in oak, although this was not clear during extreme drought in 2012. The radial increment rate was related to weather variables, which reflected light (positive for solar radiation) and water availability dependence (increased with precipitation, but decreased with vapor pressure deficit) for both species. Besides water status, air humidity (positive relation with air temperature and relative humidity) also affected oak. Reducing competition by thinning stimulated radial increment in oak-pine mixed stands, which may provide an adaptive forest management tool for reducing tree vulnerability to extreme droughts, which are linked to climate change.

Earlywood vessel area of Quercus pyrenaica Willd. is a powerful indicator of soil water excess at growth resumption

We selected two sites dominated by the sub-Mediterranean oak Quercus pyrenaica Willd. close to its distribution boundary in northwestern Iberia, within a mountain region with a high winter precipitation. The sites differed in their soil water regime, corresponding to the edge of a peat bog, and to a moderate slope. We obtained tree-ring chronologies of total ring width (RW), and mean earlywood vessel area (MVA); their responses to climatic factors were compared for the period 1945–2002. RW presented a higher chronology quality than MVA, but was rather independent of climate, probably because of the presence of recurrent growth reductions. In contrast, MVA was closely related to precipitation during April and May, whereby a high water availability was coupled to smaller vessels. We found remarkable differences between the climatic signal of both stands, as trees growing on the peat soil responded later and with considerably lower intensity. We hypothesize that spring waterlogging causes that the response at the wettest site occurs only when soil desiccation begins, which results in a delayed climatic signal, and also lower intra- and inter-annual variation due to more homogeneous conditions. Climate–growth relationships at the driest site were mainly associated with the first row, whereas it is vessels expanding later in the season that show this relation for the moist site. Our results confirm that MVA chronologies are reliable proxies of both regional and local climatic conditions, but only a careful optimization by selecting vessel subsets does provide a complete view of their potential.

Variation of the chemical composition of Pyrenean oak (Quercus pyrenaica Willd.) heartwood among different sites and its relationship with the soil chemical characteristics

This work presents the results of a study on the variation of the chemical composition of Pyrenean oak (Quercus pyrenaica Willd.) heartwood among different sites and its relationship with the soil chemical characteristics. The chemical characteristics of the heartwood are quantified in terms of proximate and ultimate analysis, calorific value and ash composition. Subsequently, the relationship between the wood chemical characteristics and the soil characteristics was also examined. The results showed no significant differences between the wood chemical characteristics from different sites. There are, however, significant differences between trees from the same site regarding the wood ash content, the calorific value and the contents of nitrogen and hydrogen. Strong correlations were observed between the heartwood ash contents of BaO, MnO, CuO and TiO2 and the soil nitrogen, phosphorous and potassium and the soil exchangeable base contents.

Volatile organic compounds emitted by Quercus pyrenaica Willd. and its relationship with saproxylic beetle assemblages

Volatile organic compounds (VOCs) emitted by plants play a critical role in the structure of the faunal communities that are associated with them. The aim of this work was to provide a detailed list of VOCs emitted by the oak species Quercus pyrenaica Willd. and to analyse the spatio-temporal variation in the emission rates of these compounds and in the diversity of saproxylic beetle assemblages associated with this tree species. VOCs sample collection was carried out by a dynamic flow-through enclosure technique with subsequent analysis by gas chromatography–mass spectrometry. Statistical differences in the emission rates of VOCs and in the diversity of saproxylic beetles were found between seasons. Temperature and relative humidity seemed to be related to these variations. Spatially, neither qualitative nor quantitative statistical differences in the emission of VOCs were found. Nevertheless, the results demonstrated a trend such that larger trees emitted VOCs with higher intensity and hosted a greater diversity of saproxylic beetles. Our hypothesis that beetles responded to certain blends of VOCs emitted at different rates by potential host tress and not so much to the absence or presence of a particular compound was reinforced by the absence of qualitative differences in the VOCs emitted by trees of different sizes. These results open a new field of study, and it will be necessary to become more involved with the subject to evaluate the real influence of these VOCs released by trees in saproxylic beetle assemblages.

Evolution of forest soil properties after liming with a by-product derived from a sugar beet mill

Deciduous oak coppices (Quercus pyrenaica Willd.) of the “Sierra de Gata” mountains (central western Spain) are located on acid rocks and subjected to annual rainfall higher than 700 mm year−1. Accordingly, the soils are acidic (soil pH is often below 5.0) and feature the presence of Mn2+ and Al3+ in the exchangeable complex. The aim of the present study was to assess the response of these soils to the addition of a lime residue from a sugar beet mill to reach pH values close to 7.0 and 6.0 with in situ and in vitro experiments, respectively. Initially, the quality of the lime by-product was evaluated, which showed content close to 75 % CaCO3. Based on that analysis, calculations were accomplished to determine the dose of lime by-product necessary for achieving the targeted soil pH. We first performed an in vitro liming experiment, incubating 0.50 kg soil (0–20 cm layer) with 23.92 g CaCO3 kg−1 soil (20 °C and soil moisture equivalent to that of field capacity). Subsequently, another experiment was done adding an equivalent quantity of sub-product in lieu of the pure CaCO3; the factors monitored were: soil pH, exchange acidity, CEC, and exchangeable bases, every 24 h. The in situ experiment was performed in a forest plot located in Western Spain (Navasfrias), liming with 16.0 Mg DM by-product ha−1 (September 1999). Soil samples (two depths: 0–10 and 10–20 cm) were taken from both the control and the experimental forest plots for monitoring the liming effect. Sampling dates were September and December 1999; June 2000, January, June and September 2001; October 2002 and 2003, and March 2005). The dissolution of the lime by-product at the laboratory was almost immediate, but the effects in the field occurred approximately 2 years after initial liming, since lime dissolution depends on both the amount and distribution of rainfall and, also, on the application procedure.

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.

Space–time modeling of changes in the abundance and distribution of tree species

Land use change and global warming are important drivers in the distribution of tree species. The Mediterranean mountain forest ecosystems can be severely affected by climate change since an increment in temperature has been observed. The main aim of this work was to analyze shifts in the distribution and changes in the abundance of Pyrenean oak (Quercus pyrenaica Willd.) and Scots pine (Pinus sylvestris L.) in Mediterranean mountains over a period of 47years (1965–2012) by using data from the National Forest Inventories, extending the Universal Kriging/Cokriging models to the space–time case for long-term forest inventory data analysis including climatic variables. Our results indicated that both the distribution and the abundance of Scots pine remained quite constant during the study period. Pyrenean oak increased its presence by 42% whereas its abundance doubled between 1965 and 2012. This movement took place towards both higher and lower altitudes. With regard to climatic factors, the kriging models showed a negative association between the presence of Scots pine and the temperature. However, we found a quadratic relationship between the Pyrenean oak and temperature, pointing to the occurrence of at intermediate altitudes. Additionally, we found significant relationships between the rainfall and the abundance of both species. Rainfall was positively related to the abundance of Scots pine and negatively to that of Pyrenean oak. Our results indicate that there has been a threefold increase in the area covered by mixed stands. However, successive land use changes as well as forest policies related to conservation have played an important role in the distribution of both species and therefore are taken into account in the discussion of these results.

Disentangling the effect of competition, CO2 and climate on intrinsic water‐use efficiency and tree growth

Summary Climate change scenarios forecast rising temperatures for the Mediterranean region, which could enhance the vulnerability to drought stress in forest ecosystems. The long‐term effects of climate forcing on tree performance can be, however, modulated by other environmental factors, such as competition and rising atmospheric CO2 concentrations. We assessed the concomitant effect of competition, climate and CO2 concentrations on the tree‐ring δ13C‐derived intrinsic water‐use efficiency (iWUE) and basal area increments (BAI) of species with different drought tolerance: two Mediterranean deciduous species (Quercus faginea Lam. and Quercus pyrenaica Willd.) and one conifer (Pinus sylvestris L.). Additionally, given that competition may be managed to mitigate the effect of increasing drought stress, we further examined the influence of this variable on iWUE and growth using data compiled from the literature, providing the first review on the response of iWUE to competition. Competition had no significant effect on iWUE in any of the three species studied, whereas, as expected, growth rates were significantly higher under low‐competition levels. This was consistent with the literature review, which showed that shifts in iWUE with competition changes are rare; supporting the hypothesis that leaf‐level gas exchange tends to be a homeostatic trait. In the long term, the three species exhibited a significant increasing trend in iWUE due to the combined effect of increased CO2 concentration, climate and age. Growth, however, was mostly affected by competition and climate and in most cases was not enhanced as a result of the increase in iWUE. Synthesis. Regardless of their functional response to drought, trees respond to reduced competition through the structural shifts such as increased radial growth rather than leaf‐level gas exchange adjustments. CO2 and climate are, therefore, the main drivers of iWUE variability, rather than competition. Thus, if temperature‐induced drought becomes limiting, reducing competition for resources may not offset the detrimental effect of increasing drought stress on tree physiology and growth decline may occur without a CO2 fertilization effect.

Comparison of ALS based models for estimating aboveground biomass in three types of Mediterranean forest

This study aimed to develop ALS-based models for estimating stem, crown and aboveground biomass in three types of Mediterranean forest, based on low density ALS data. Two different modelling approaches were used: (i) linear models with different variable selection methods (Stepwise Selection [SS], Clustering/Exhaustive search [CE] and Genetic Algorithm [GA]), and (ii) previously Published Models (PM) applicable to diverse types of forest. Results indicated more accurate estimations of biomass components for pure Pinus pinea L. (rRMSE = 25.90-26.16%) than for the mixed (30.86-36.34%) and Quercus pyrenaica Willd. forests (32.78-34.84%). All the tested approaches were valuable, but SS and GA performed better than CE and PM in most cases.

Dataset of MIGRAME Project (Global Change, Altitudinal Range Shift and Colonization of Degraded Habitats in Mediterranean Mountains).

In this data paper, we describe the dataset of the Global Change, Altitudinal Range Shift and Colonization of Degraded Habitats in Mediterranean Mountains (MIGRAME) project, which aims to assess the capacity of altitudinal migration and colonization of marginal habitats by Quercus pyrenaica Willd. forests in Sierra Nevada (southern Spain) considering two global-change drivers: temperature increase and land-use changes. The dataset includes information of the forest structure (diameter size, tree height, and abundance) of the Quercus pyrenaica ecosystem in Sierra Nevada obtained from 199 transects sampled at the treeline ecotone, mature forest, and marginal habitats (abandoned cropland and pine plantations). A total of 3839 occurrence records were collected and 5751 measurements recorded. The dataset is included in the Sierra Nevada Global-Change Observatory (OBSNEV), a long-term research project designed to compile socio-ecological information on the major ecosystem types in order to identify the impacts of global change in this mountain range.

Xylem and soil CO2 fluxes in a Quercus pyrenaica Willd. coppice: root respiration increases with clonal size

Xylem and soil CO 2 fluxes in coppiced oak forests increase with clonal size, suggesting larger expenditures of energy for root respiration. An imbalance between root demand and shoot production of carbohydrates may contribute to the degradation of abandoned coppices. Our understanding of root respiration is limited, particularly in root-resprouting species with many stems and a large system of interconnected roots resulting from long-term coppicing. We tested the hypothesis that clone size influences the internal flux of CO2 dissolved in xylem sap (F T) from roots into the stem and soil CO2 efflux (F S) as indicators of root respiration. We predicted that large clones would exhibit higher F T per stem and F S than small clones due to larger root system per stem in large clones. Genetic analyses were performed to elucidate clonal grouping. F T was measured continuously for 100 days in 16 similar-sized stems of Quercus pyrenaica belonging to two large and two small clones. F S was measured in 20 clones of varying size. F T per stem and F S were higher in large clones. F T was 2 % of the root-respired CO2 that diffused through soil to the atmosphere. Relative to other studies, the contribution of F T to root respiration was very low, pointing to large differences depending on species or site. Higher stem F T and F S in large clones compared with small clones suggest greater carbon consumption by roots in large clones, pointing to a root/shoot biomass and physiological imbalance resulting from long-term coppicing that would partially explain the degradation of currently abandoned stands of Q. pyrenaica.

Epiphytic bryophytes of Quercus forests in Central and North inland Iberian Peninsula

Diversity patterns are governed by a complex network of interacting factors. Studies directed to disentangle the most important factors affecting diversity have frequently shown divergent results, which has encouraged a rewarding debate about the relative importance of each factor. Scale dependency has been identified as a direct cause of at least part of such divergences. However, studies with spatially-explicit measurements at different scales are costly and therefore they are relatively scarce despite their importance. Here, we present a database to disentangle the cross-scale variation in the importance of factors affecting the diversity of epiphytic bryophyte communities in Quercus dominated forests (Quercus ilex L., Quercus pyrenaica Willd. and Quercus faginea Lam.) in the North-western region of the Iberian Peninsula. We provide species-per-site abundance information with more than 9000 entries and an environmental table containing 20 in situ measured variables at three different scales (forest, stand, and sample). The database will help to advance the research of cross-scale effects of diversity patterns while at the same time providing valuable information on the distribution of a poorly known group of organisms.&nbsp

Regeneration dynamics of Quercus pyrenaica Willd. in the Central System (Spain)

In the Mediterranean region, over the past few centuries human activity has modeled the landscape, leading to forest degradation. However, as in many parts of the European continent since the second half of the twentieth century, the Iberian Peninsula has been subject to a substantial degree of land abandonment that has led to the expansion of important secondary forests. One of the dominant tree species in the transitional forests of mountain areas between the Mediterranean and Euro-Siberian regions of the Iberian Peninsula is the deciduous oak, Quercus pyrenaica Willd. The main objectives of the present work were to study the mode of regeneration and forest dynamics of Q. pyrenaica in several types of stands: young forests, mature and previously perturbed forests, and abandoned Pinus reforestations. With this in mind we established eight plots in the mountain range of the Sierra de Francia–Quilamas, in the west subdivision of the Spanish Central System. We analyzed the age and dbh distributions, regeneration density, and the spatial structure of trees and saplings. Young Q. pyrenaica stands were seen to show episodic recruitment after land abandonment consistent with a ‘catastrophic’ mode of regeneration. These stands were characterized by unimodal age-cohorts, with tree recruitment dropping drastically as the canopy developed and closed in, except at some study sites where the more shade-tolerant Castanea sativa was found. Older and previously disturbed Q. pyrenaica stands showed bimodal age cohorts, the young ones exhibiting a clumped pattern associated with canopy gaps and/or a lower tree density of the older cohort. In abandoned Pinus reforestations, the recruitment of Q. pyrenaica was also associated with canopy openings. These findings show that Q. pyrenica also undergoes a gap-phase mode of regeneration. An abundant regeneration of Q. pyrenaica could be found at all the stands, guaranteeing the persistence of seedling banks (but a scarcity or lack of saplings) under closed forests, until canopy gaps may allow some Q. pyrenaica individuals to grow and reach the main canopy.