Xylogenesis under climatic stress: wood anatomical evidence of harsh conditions

Pinus halepensis Mill. and Pinus sylvestris L. have been widely used for reforestation of poor and degraded soils in Spain. Pinus halepensis is a Mediterranean species adapted to drought and to a wide range of substrates and able to colonize very poor soils and improve them, promoting the growth of native broadleaved species such as Quercus ilex L. or Quercus faginea Lam. Notwithstanding the above, it has been poorly studied because of the limited economic interest of its wood. Pinus sylvestris presents great ecological amplitude responsible for its great distribution. Pinus sylvestris plantations were established in former Quercus pyrenaica Willd. stands that were converted into crops or overexploited for firewood. The aim of this thesis is to widen the knowledge about the existing relationships between the environmental parameters and the dynamics of these plantations to serve as a guide for sustainable forest management of stands. Forest productivity determination is crucial to accomplish the established managerial objectives through sustainable forest management. It can be estimated through the site index of the stand and, when dominant height is not available, can also be estimated by means of environmental parameters (soil, climatic and physiographic). In this thesis, a discriminant model was developed to predict the site index for Pinus sylvestris by using latitude, soil porosity, inorganic Al and microbial biomass C as predictors. Besides, another discriminant model was developed to predict the site index for Pinus halepensis plantations with soil porosity, the Annual Hydric Index, the slope and the soil microbial biomass N as predictors. As seen in the previous models, soil biochemical parameters are determining factors for forest productivity. Organic matter decomposition and nutrient cycling are mainly driven by the activities of the enzymes produced by bacteria and fungi. Therefore, it is essential to know how environmental parameters affect microbial performance in forest soils to understand how ecosystems function. The activities of several enzymes (FDA, DHA, catalase, urease and phosphatases) were studied in soils under Pinus halepensis and Pinus sylvestris plantations to determine those environmental parameters that influence microbial performance in these ecosystems. Hydric deficit seems to be the most limiting factor for enzyme activities in the calcareous soils under Pinus halepensis plantations. However, in soils under Pinus sylvestris plantations, the low pH and the high amount of soluble phenols seem to limit both the activity and the composition of the microbial communities. Litterfall is the main source of nutrients for forest soils. The amount of litterfall shed by the stand and the dynamics of litter decomposition are driving factors for soil fertility. Decomposition processes are driven by the abundance and diversity of microorganisms, the quality of the substrate and the climate, since temperature, humidity and nutrient availability drive, in turn, the decomposers performance. Usual silviculture often includes stand density management. These practices may alter the amount and chemical composition of litterfall and can also modify the microclimate altering the microbial performance and therefore, the litter decomposition processes. To shed light on these relationships, the effect of local basal area on litterfall, litter decomposition and soil temperature and humidity was studied in four Pinus sylvestris and four Pinus halepensis stands. The local basal area of the plot significantly affected the amount of litterfall in the stands of both Pinus species. The needle litter decomposition rate was significantly affected by the local basal area in Pinus halepensis stands, but not in Pinus sylvestris stands. Besides, a significant and negative correlation between the local basal area of the stand and the topsoil humidity was found in Pinus halepensis and Pinus sylvestris plantations. Therefore, the amount of litterfall is lower in plots with lower local basal area due to the lower aboveground tree biomass. The lower amount of tree biomass also intercept less amount of water from precipitations, and then, a higher amount of water reach the soil increasing soil humidity. The activity of decomposers is higher in plots with lower local basal area in Pinus halepensis plots (which are the ones limited by hydric deficit) because of the higher soil humidity and finally, the needle litter decomposition rate is also higher. The chemical composition of the litterfall in Pinus halepensis and the nutrient release from decomposing needle litter of both Pinus species is also affected by the local basal area of the stand. Therefore, silvicultural practices involving density management also have an impact on the nutrient cycling of the Pinus sylvestris and Pinus halepensis plantations studied.

Ozone (O3) is a toxic oxidative air pollutant with significant detrimental effects on natural vegetation and crop species. Free-air controlled exposure (FACE) facilities provide an ideal tool for O3 effect studies, producing realistic results. The O3 Free-Air Controlled Exposure (FO3X - FACE) facility, located in Sesto Fiorentino, Italy, and established in 2015, is an AnaEE (Analysis and Experimentation on Ecosystems) European research platform. The facility permits the exposure of plants to three levels of O3 concentrations (1.0, 1.5, and 2.0 times the ambient concentration, denoted as AA, x1.5AA, x2.0AA, respectively), with main environmental variables continuously monitored. Over the years, the accumulated exposure over 40 ppb hourly concentrations (AOT40) was calculated and used as an exposure-based O3 index. The stomatal conductance (gsto) model, based on the multiplicative algorithm, was used to parameterize the gsto of 12 species (7 deciduous: Oxford poplar, Quercus robur, Quercus pubescens, Sorbus aucuparia, Alnus glutinosa, Vaccinium myrtillus, I-214 poplar, and 5 evergreens: Quercus ilex, Phillyrea angustifolia, Arbutus unedo, Pinus halepensis, Pinus pinea), in order to calculate the hourly stomatal O3 flux (Fst), and thus the phytotoxic O3 dose above an hourly threshold y of uptake (y = 1, POD1) used as a flux-based O3 index. Our studies have evaluated the effect of O3 in gas exchange parameters as light-saturated photosynthesis (Asat / R2 POD1 = 0.46 vs. AOT40 = 0.20) and stomatal conductance (gsto /R2 POD1 = 0.18 vs. AOT40 = 0.18), as well as for the induction of specific visible foliar injury (VFI / R2 POD1 =0.40 vs. AOT40 = 0.32) and biomass loss (Bloss / R2 POD1 = 0.50 vs. AOT40 = 0.22). We demonstrate that species-specific flux-based O3 index POD1 is more relevant compared to the exposure-based O3 index AOT40, showing the importance of comprehending the mechanism of O3 damage to plants after the uptake through stomata. Our research has also been improving the derivation of experimentally based critical levels (CLs) for the protection of forests and crops vegetation from O3 damage.

Moroccan vegetation faces significant pressure particularly from human activities and climate change, while most ecosystems lack detailed assessments. Phytoecological studies and species assessments are implemented using vegetation sampling, analysis of climate data, geological substrate maps, and the Digital Elevation Model (DEM). The study area hosts 565 plant species distributed into 74 families, with Asteraceae being the most abundant family, representing 17.7%. In addition, the correspondence analysis test demonstrates that species are grouped into six distinct blocks. Block 1 comprises a set of Quercus ilex forests. Block 2 encompasses Juniperus phoenicea lands and transition zones between Quercus ilex and Juniperus phoenicea. Block 3 represents Pinus halepensis forests and pine occurrences within Quercus ilex and Juniperus phoenicea stands. Block 4 indicates the emergence of xerophytic species alongside the aforementioned species; it forms the upper limits of Blocks 1, 2, and 3. Block 5 corresponds to formations dominated by Juniperus thurifera in association with xerophytes. Block 6 groups together a set of xerophytic species characteristic of high mountain environments. Additionally, Quercus ilex colonizes the subhumid zones and prefers limestone substrates, Juniperus phoenicea and Tetraclinis articulata, and Pinus halepensis occupies the hot part of the semi-arid in limestone, clays, and conglomerates, while the Juniperus thurifera and xerophytes inhabit the cold parts and limestone substrates. The thermo-Mediterranean vegetation level occupies low altitudes, dominated by Tetraclinis articulata, Juniperus phoenicea, and Olea europaea. The meso-Mediterranean level extends to intermediate altitudes, dominated by Quercus ilex and Juniperus phoenicea. While the supra-Mediterranean level is dominated by Quercus ilex, Arbutus unedo, and Cistus creticus. The mountain Mediterranean level, located in the high mountains, is dominated by Juniperus thurifera associated with xerophytes. Finally, the oro-Mediterranean level, found at extreme altitudes, is dominated by xerophytes. Some species within this region are endemic, rare, and threatened. Consequently, the implementation of effective conservation and protection policies is recommended.

<p>'Blue Rings' (BRs) are distinct wood anatomical anomalies recently discovered in several tree species. Previous studies connect their occurrence to lower than normal temperatures during the cell wall lignification phase of xylogenesis. Cell wall lignification usually continues after radial growth is completed, after the growth season. Therefore, systematic analysis of blue rings can add another level of time resolution to dendroclimatic studies. Additionally, BRs are more sensitive temperature recorders than frost rings which require freezing temperatures to form. We  present a continuous chronology of blue rings in North American bristlecone pine covering the last millennium and their connections to volcanic eruptions known both from historic and ice core records. Most recorded BR years coincide with cooling following large volcanic eruptions. The three most prominent events during the last 1000 years, with the highest share of blue rings in bristlecone pine from the White Mountains of California are at: 1453, 1601 and 1884CE (83%, 91%, 69% of blue rings respectively), attributed to known eruptions of Kuawe (attribution still debated) 1452CE -Vanuatu, Huaynaputina 1600CE – Peru, and Krakatoa 1883CE - Indonesia. Fourth most prominent event with 58% of blue rings is noted in 1200CE. Acidity peak in 1200CE is so far recorded only in Greenland ice-cores suggesting northern hemisphere high latitude eruption, but strong BR signal would suggest a broader climatic significance of this event. It is interesting to note that BRs were indicated in 69% of samples in 1884, relating to the known eruption and associated climate impact of Krakatoa (1883), yet no BRs were observed in 1816, the so-called year without a summer which followed the largest historically noted and well described eruption of Tambora, Indonesia (1815). We did find a strong BR signal in 1809 (with BRs continuing in 1810 and 1811) following an unidentified but prominent eruption seen in ice core records. The 1809 and 1815 eruptions are thought to be responsible for the cold decade from 1810 to 1819 thought by some to be the coldest decade of the last 500ys. The source of 1809 eruption remains unknown and scientific debate over the scale of the eruption continues, but bipolar acidity peaks in ice cores point to a tropical eruption with widespread sulfate distribution to both hemispheres and tephra in ice cores points to two coinciding high latitude eruptions of only regional prominence. The BR record supports 1809 CE as an event of global climatic significance illustrating the capacity for BRs  to capture cooling events with even higher time resolution (after the radial growth is completed) and of smaller magnitude than frost rings, TRW or MXD studies to help better investigate and understand the impacts of volcanism on climate and society.</p>

Formation, structure and climatic significance of blue rings and frost rings in high elevation bristlecone pine (Pinus longaeva D.K. Bailey)

Terpene concentration and emission were studied in potted plants of some of the most common Mediterranean woody species (Pinus halepensis L., Pistacia lentiscus L., Cistus albidus L., Cistus monspeliensis L., Quercus ilex L., Quercus coccifera L., Phillyrea latifolia L., Phillyrea angustifolia L., and Arbutus unedo L.) under irrigation and under severe drought conditions that dropped relative water content to a range between 40% in Q. ilex and 85% in Phillyrea latifolia after withholding watering for one dry summer week. Terpene concentrations were detected in Pinus halepensis, Pistacia lentiscus, C. albidus, and C. monspeliensis, and they increased after withholding watering except in C. albidus. Terpene emission was detected in all species except Phillyrea angustifolia and A. unedo. Pinus halepensis showed the highest diurnal terpene emission rates of 86 µg·g-1 dry wt.·h-1 followed by C. albidus, Pistacia lentiscus, Q. ilex, Q. coccifera, Phillyrea latifolia, and C. monspeliensis (4 µg·g-1 dry wt.·h-1). Emitted terpenes represented from 0.33% of C fixed in C. monspeliensis to 10% in C. albidus. All species severely decreased their terpene emission rates under severe drought conditions. Emission by terpene-storing species (e.g., Pinus halepensis) was more related to temperature than in nonstoring species (e.g., Q. ilex), which showed emission relationships with photosynthetic rates. The monoterpenes alpha-pinene, beta-pinene, beta-phellandrene, and limonene and the sesquiterpene caryophyllene were the most abundant terpenes stored and emitted by these Mediterranean plant species.Key words: drought, Mediterranean conditions, terpene concentration, terpene emission, woody plants.

We lack information regarding the main factors driving growth responses to drought in tree species with different vulnerability against this stressor and considering sites with contrasting climatic conditions. In this paper, we identify the main drivers controlling growth response to a multi-scalar drought index (Standardized Precipitation Index, SPI) in eight tree species (Abies alba, Pinus halepensis, Quercus faginea, Pinus sylvestris, Quercus ilex, Pinus pinea, Pinus nigra, Juniperus thurifera). We sampled forests growing across a pronounced climatic gradient under Mediterranean conditions in north-eastern Spain. To summarize the patterns of growth responses to drought, we used principal component analysis (PCA). To determine the main factors affecting growth responses to drought, correlation and regression analyses were carried out using a set of abiotic (climate, topography, soil type) and biotic (Normalized Difference Vegetation Index, Enhanced Vegetation Index, tree-ring width, diameter at breast height) predictors and the PCs loadings as response variables. The PCA analysis detected two patterns of growth responses to drought corresponding to xeric and mesic sites, respectively. The regression analyses indicated that growth responses to drought in xeric forests were mainly driven by the annual precipitation, while in mesic sites the annual water balance was the most important driver. The management of Mediterranean forests under the forecasted warmer and drier conditions should focus on the main local factors modulating the negative impacts of drought on tree growth in xeric and mesic sites.

AimLarge‐scale patterns of limitations in tree recruitment remain poorly described in the Mediterranean Basin, and this information is required to assess the impacts of global warming on forests. Here, we unveil the existence of opposite trends of recruitment limitation between the dominant genera Quercus and Pinus on a large scale and identify the key ecological drivers of these diverging trends.LocationSpainMethodsWe gathered data from the Spanish National Forest inventory to assess recruitment trends for the dominant species (Pinus halepensis, Pinus pinea, Pinus pinaster, Pinus nigra, Pinus sylvestris, Pinus uncinata, Quercus suber, Quercus ilex, Quercus petraea, Quercus robur, Quercus faginea and Quercus pyrenaica). We assessed the direct and indirect drivers of recruitment by applying Bayesian structural equation modelling techniques.ResultsSevere limitations in recruitment were observed across extensive areas for all Pinus species studied, with recruitment failure affecting 54–71% of the surveyed plots. In striking contrast, Quercus species expanded into 41% of the plots surveyed compared to only 10% for Pinus and had a lower local recruitment failure (29% of Quercus localities compared to 63% for Pinus species). Bayesian structural equation models highlighted the key role of the presence of Q. ilex saplings and the increase in the basal area of Q. ilex in limiting recruitment in five Pinus species. The recruitment of P. sylvestris and P. nigra showed the most negative trends and was negatively associated with the impacts of fire.Main conclusionsThis study identified Q. ilex, the most widespread species in this area, as a key driver of recruitment shifts on a large scale, negatively affecting most pine species with the advance of forest succession. These results highlight that the future expansion/contraction of Q. ilex stands with ongoing climate change will be a key process indirectly controlling the demographic responses of Pinus species in the Mediterranean Basin.

This article describes a data-driven framework based on spatiotemporal machine learning to produce distribution maps for 16 tree species (Abies alba Mill., Castanea sativa Mill., Corylus avellana L., Fagus sylvatica L., Olea europaea L., Picea abies L. H. Karst., Pinus halepensis Mill., Pinus nigra J. F. Arnold, Pinus pinea L., Pinus sylvestris L., Prunus avium L., Quercus cerris L., Quercus ilex L., Quercus robur L., Quercus suber L. and Salix caprea L.) at high spatial resolution (30 m). Tree occurrence data for a total of three million of points was used to train different algorithms: random forest, gradient-boosted trees, generalized linear models, k-nearest neighbors, CART and an artificial neural network. A stack of 305 coarse and high resolution covariates representing spectral reflectance, different biophysical conditions and biotic competition was used as predictors for realized distributions, while potential distribution was modelled with environmental predictors only. Logloss and computing time were used to select the three best algorithms to tune and train an ensemble model based on stacking with a logistic regressor as a meta-learner. An ensemble model was trained for each species: probability and model uncertainty maps of realized distribution were produced for each species using a time window of 4 years for a total of six distribution maps per species, while for potential distributions only one map per species was produced. Results of spatial cross validation show that the ensemble model consistently outperformed or performed as good as the best individual model in both potential and realized distribution tasks, with potential distribution models achieving higher predictive performances (TSS = 0.898, R2logloss = 0.857) than realized distribution ones on average (TSS = 0.874, R2logloss = 0.839). Ensemble models for Q. suber achieved the best performances in both potential (TSS = 0.968, R2logloss = 0.952) and realized (TSS = 0.959, R2logloss = 0.949) distribution, while P. sylvestris (TSS = 0.731, 0.785, R2logloss = 0.585, 0.670, respectively, for potential and realized distribution) and P. nigra (TSS = 0.658, 0.686, R2logloss = 0.623, 0.664) achieved the worst. Importance of predictor variables differed across species and models, with the green band for summer and the Normalized Difference Vegetation Index (NDVI) for fall for realized distribution and the diffuse irradiation and precipitation of the driest quarter (BIO17) being the most frequent and important for potential distribution. On average, fine-resolution models outperformed coarse resolution models (250 m) for realized distribution (TSS = +6.5%, R2logloss = +7.5%). The framework shows how combining continuous and consistent Earth Observation time series data with state of the art machine learning can be used to derive dynamic distribution maps. The produced predictions can be used to quantify temporal trends of potential forest degradation and species composition change.

Mapping forest site quality at national level

ABSTRACTMediterranean trees and shrubs form intra-annual density fluctuations (IADFs) in tree rings as a sign of their plasticity in wood formation in response to intraseasonal variations of environmental conditions. Different species show a different aptitude to form IADFs, due to their diverse ability to cope with climate stressors, since the occurrence of IADFs may affect plant hydraulics. Dendroecology and quantitative wood anatomy were used to characterise IADFs in Pinus pinea and Arbutus unedo co-occurring at a Mediterranean site in Italy. The relations between climate parameters (i.e. temperature and precipitation) and intra-annual tree-ring traits (i. e. IADF frequency and conduit size) were analysed to highlight the main triggers for IADF formation and their functional role.Data showed that both species are characterised by a high plastic response to climate and formed a high frequency of L-IADFs (occurrence of earlywoodlike conduits in latewood). The two species, although forming the same type of IADFs, showed different sensitivity to environmental factors. Pinus pinea showed a high dependence of tracheid size on temperature, while Arbutus unedo was more sensitive to precipitation in spring and autumn. Arbutus unedo promptly developed more than one IADF per year in response to rainfall events following drought periods.The overall results were useful to compare the aptitude of the two species in forming IADFs and to highlight the factors priming their formation. This is useful to understand wood growth reactions to environmental drivers and to evaluate the adaptive capabilities in these two species, and thus to predict forest reactions after climate changes.

We conducted a 6-year field manipulation drought experiment in an evergreen Quercus ilex forest where we simulated the drought predicted by GCM and ecophysiological models for the coming decades (an average of 15% soil moisture reduction). We thereby tested the hypothesis that enhanced drought will change Ca, Fe, Mg, Mo and S availability, concentrations and accumulation patterns in Mediterranean ecosystems. The strongest effects of drought occurred in the soil. Drought increased the total soil concentrations of S, the soil extract concentrations of Fe, Mg and S, the Mg saturation in the soil exchangeable complex and tended to increase the percentage base saturation of the soil exchangeable complex. These increased soil concentrations were related to a decrease of plant uptake capacity and not to an increase of soil enzyme activity, which in fact decreased under drier conditions. Drought increased leaf Mg concentrations in the three dominant species although only significantly in Quercus ilex and Arbutus unedo (20 and 14%, respectively). In contrast, drought tended to decrease Ca in Phillyrea latifolia (18%) and Ca and Fe concentrations in the wood of all three species. Drought increased Ca and Fe concentrations in the roots of Quercus ilex (26 and 127%). There was a slight general trend to decrease total biomass accumulation of nutrients that depend on water flux such as Mg, Fe and S. This effect was related to a decrease of soil moisture that reduced soil flow, and to a decrease in photosynthetic capacity, sap flow, transpiration and growth, and therefore plant uptake capacity under drought observed in Quercus ilex and Arbutus unedo. On the contrary, drought increased Mo accumulation in aboveground biomass in Phillyrea latifolia and reduced Mo accumulation in Arbutus unedo by reducing growth and wood Mo concentrations (51%). Phillyrea latifolia showed a great capacity to adapt to drier conditions, with no decrease in growth, an increase of Mo uptake capacity and a decrease in leaf Ca concentration, which was related to a decrease in transpiration under drought. The results indicate asymmetrical changes in species capacity to accumulate these elements, which are likely to produce changes in inter-specific competitive relations among dominant plant species and in their nutritional quality as food sources. The results also indicate that drought tended to decrease nutrient content in aboveground biomass, mainly through the decrease in growth and transpiration of the most sensitive species and caused an increase in the availability of these nutrients in soil. Thus, drought decreased the ecosystem’s capacity to retain Mg, Fe and S, facilitating their loss in torrential rainfalls.

The light, temperature and stomatal conductance dependencies of volatile organic compound (VOC) emissions from ten plant species commonly found in the Mediterranean region were studied using a fully controlled leaf cuvette in the laboratory. At standard conditions of temperature and light (30°C and 1000 μmol m −2 s −1 PAR), low emitting species ( Arbutus unedo, Pinus halepensis, Cistus incanus, Cistus salvifolius, Rosmarinus officinalis and Thymus vulgaris) emitted between 0.1 and 5.0 μg (C) (total VOCs) g −1 dw h −1, a medium emitter ( Pinus pinea) emitted between 5 and 10 μg (C) g −1 dw h −1 and high emitters ( Cistus monspeliensis, Lavendula stoechas and Quercus sp.) emitted more than 10 μg (C) g −1 dw h −1. VOC emissions from all of the plant species investigated showed some degree of light dependency, which was distinguishable from temperature dependency. Emissions of all compounds from Quercus sp. were light dependent. Ocimene was one of several monoterpene compounds emitted by P. pinea and was strongly correlated to light. Only a fraction of monoterpene emissions from C. incanus exhibited apparent weak light dependency but emissions from this plant species were strongly correlated to temperature. Data presented here are consistent with past studies, which show that emissions are independent of stomatal conductance. These results may allow more accurate predictions of monoterpene emission fluxes from the Mediterranean region to be made.

Aleppo pine (Pinus halepensis) and the evergreen holm oak (Quercus ilex) dominate forest areas of the Mediterranean Basin. Both species regenerate abundantly after fires: pine through seedlings and holm oak through resprouts. Cumulative nutrient losses caused by frequent fires may have decreased soil nutrient availability in such areas. To assess the role of nitrogen and phosphorus as limiting factors for growth of these species during post-fire recovery, a field fertilisation and competition experiment was conducted in a 5-year post-fire shrubland on calcareous soil, where naturally-regenerated saplings of Aleppo pine and resprouts of interior holm oak (Quercus ilex subsp. rotundifolia) coexist. Three years after fertilisation, relative basal area increment was 56% greater in pines fertilised with 250 kg P ha–1 than in non fertilised ones. N fertilisation had small or no effects. Interactions between N and P fertilisation were not observed. Growth of Aleppo pine only increased with P fertilisation when neighbours were removed. Hence, the negative effect of neighbours on growth was greater when P availability was enhanced by fertilisation. In contrast, holm oak was able to grow more (110%) in response to increased P supply even without neighbour removal. A common garden experiment was then conducted with potted seedlings to investigate whether the suggested higher competitive capacity of holm oak for P held under a range of P amendments on different soils and competitive situations. P fertilisation increased seedling biomass yield of both species. When P availability increased, a negative effect of neighbours on growth was observed for holm oak and in 70 a lesser extent for Aleppo pine. In conclusion, in the field, holm oak resprouts showed higher competitive ability for P uptake compared to Aleppo pine saplings, but in potted seedlings in common garden conditions this trend was not observed. Therefore holm oak is not always competitively superior to Aleppo pine for P. Potted seedlings of both species had a notable plasticity in shoot/root biomass allocation, but only holm oak increased its proportional allocation to roots when neighbours were present. P availability can be a key factor in growth and competitive relations of these two species, but effects differ depending on soil type, individual age, regeneration type (i.e., seedling versus resprouts), and competitive situation.

Botanic analysis of Livia’s villa painted flora (Prima Porta, Roma)