Mediterranean Tree Species Research Articles

Dynamics of competition over water in a mixed oak-pine Mediterranean forest: Spatio-temporal and physiological components

Understanding inter- and intra-specific plant interactions and competition over water is challenging because of the lack of effective approaches for accessing and monitoring root distribution and activity. In this context, stable isotopes are excellent eco-hydrological tracers that allow characterizing the dynamics of water uptake patterns in trees and shrubs. Here, we studied biotic interactions for water uptake between two typical Mediterranean tree species, Aleppo pine (Pinus halepensis) and holm oak (Quercus ilex), coexisting in a mixed forest. We measured stable isotope composition (δ18O and δ2H) of xylem water in all trees found in the studied stand during one growing season, covering an exceptionally long summer drought and subsequent recovery. We applied point-process statistics together with stand density information to evaluate tree-to-tree interactions for water use. In pines, we observed a clear uncoupling between soil and xylem water isotope composition after two months of persistent drought. Conversely, the isotope composition of xylem water in oaks tracked observed changes in the soil during the first two months of drought, but began to depart from soil values after three months. These results suggest that during drought the oaks were able to keep active for longer using alternative soil water sources, not available for the pines. Point-process statistics revealed more positive isotope compositions at distances below 4–6m, but only between con-specific individuals (i.e. pine-pine, oak-oak). These intra-specific responses were first seen in the pines (after two months of drought) and subsequently in oaks (after three months), coinciding with the onset of soil-xylem uncoupling for each species. On the other hand, the isotope composition of individual oaks decreased with increasing neighbor pine density, but increased in response to oak density. Conversely, the pines showed more positive values with increasing oak density. Our results suggest that the use of shallow water in oaks is limited by the presence of pines, which force them to shift to deep-soil water use, whereas pines have more restricted access to deep water in the presence of oaks, leading to more positive isotope values. According to the dynamics of interaction patterns, we conclude that inter-specific differences in pine-oak mixed forests hold two components: a static, spatial component determined by root distribution, and a dynamic, physiological component related to water uptake capacity within the soil profile.

Linking nursery nutritional status and water availability post-planting under intense summer drought: the case of a South American Mediterranean tree species

Drought-avoidance traits of South American Mediterranean tree species are crucial attributes to be considered in nursery practices aimed at improving the performance of seedlings exposed to intense summer drought in dryland reforestation projects. In this study, we determined the relation between nursery fertilization doses and the development of drought-avoidance traits of the soapbark tree Quillaja saponaria (Mol.) under contrasting watering regimes following post-planting. Seedlings were grown for 6 months using four increasing doses of controlled-release fertilizer (0, 3, 6, and 12 g L-1 of Basacote® Plus 15:8:12). After outplanting, half of the seedlings were watered weekly and the other half were left unwatered for one growing season from September 2011 to May 2012. Seedlings were periodically measured for morphological and ecophysiological parameters, and carefully harvested for root measurements at the end of the study. Our results showed that high fertilization doses produced significantly larger seedlings in the nursery with high nitrogen and phosphorous foliar concentrations, which resulted in a significantly higher shoot dry mass after outplanting. Unfertilized seedlings grown with water application had a significantly higher stem diameter, root dry mass and lower shoot/root compared with seedlings with high fertilizer dose. These results highlight the ability of this species to maintain drought-avoidance traits, such as high xylem water potential and chlorophyll fluorescence, during the first 3 months of the 7-month drought period. High nutrient loading, although resulting in improved shoot productivity after outplanting, did not make a significant contribution to the early development of drought-avoidance traits in Q. saponaria.

Ecological implications of different water use strategies in three coexisting mediterranean tree species

The ability of trees to grow and survive during periods of water scarcity is determined by the water use strategy of each individual. In this study we evaluated how water use strategies can influence the establishment phase of three coexisting species in forests where harvesting intensity and silvicultural interventions have been reduced over recent decades. We conducted a greenhouse experiment between February and July, applying two watering regimes (moist and dry) to current year seedlings in 1.5m high pots to avoid root growth constraint. At the same time, the physiological performance of seedlings of the same species was monitored under natural conditions in the field over a period of decreasing soil moisture. Our results indicated that resprouter species allocated more than 40% of total biomass to roots whereas the non-resprouter P. pinea allocated only 26%. The plasticity index for the root traits was highest in Q. ilex, in which the highest stomatal conductance was measured in early summer. The slowest growth and lowest root plasticity, root hydraulic conductance and leaf water potential were measured in J. oxycedrus. The three species exhibited a positive carbon balance under low water availability. Coexisting species displayed a continuum in the water use strategies adopted. P. pinea would be the least competitive species under the forecasted increasing drought conditions in contrast to J. oxycedrus, which presents traits more suited to coping with droughts.

Aleppo pine forests from across Spain show drought-induced growth decline and partial recovery

Drought-induced growth decline is a phenomenon widely described in forests growing in regions subjected to seasonal water shortage such as the Mediterranean Basin. However, how tree species react to drought by modifying their growth pattern across the species distribution range is a question that remains to be answered.We investigate this question using a network of 27 Aleppo pine forests distributed across the eastern Spain and encompassing diverse site characteristics and climatic conditions. Using dendrochronology we quantify the radial growth responses of this Mediterranean tree species in 1994–1995, when the most severe drought after 1950 affected the study area. We quantify the variation between trees and sites in terms of growth resilience to drought, i.e. the resistance to the drought event and the recovery after it. Then, we quantify the relationship of these responses to tree traits (tree diameter and height, tree age, sapwood area) at the individual tree level, and to site characteristics (climate conditions, stand structure–tree density, basal area and the presence of co-dominant tree species) at the site level.We found that 1994–95 dry spell strongly impacted tree growth since 25 out of 27 sites and about 88% of trees showed a marked growth reduction of around 60% in radial growth. Climatic conditions were by far more important than tree traits on explaining resilience indices. In particular, the resistance to drought decreased with the increase in drought intensity, whereas the post-drought recovery was linked to site precipitation. Trees from dry sites were less resistant but recovered faster than trees from wet sites.We demonstrate that the reaction of different tree populations to drought is structured according to local climate conditions. If droughts become more severe and frequent as predicted in the Mediterranean region where climate warming is forecasted, Aleppo pine stands growing in the driest regions will be increasingly more sensitive to intense dry spells.

Projecting the distribution and abundance of Mediterranean tree species under climate change: a demographic approach

Projecting the distribution and abundance of Mediterranean tree species under climate change: a demographic approach

Climate response of cork growth in the Mediterranean oak (Quercus suber L.) woodlands of southwestern Portugal

In the Mediterranean climate regions, drought events are expected to affect the growth of forests ecosystems by changing trees growth rates and eventually inducing shifts in their growth patterns. Cork oak (Quercus suber L.) is a strictly western Mediterranean tree species periodically harvested for its bark, the cork. So far, cork oak has received limited attention for dendroclimatological studies due to its typical faint and erratic tree wood rings. Moreover, its distinct cork rings chronologies have been completely neglected. In this study we introduce an approach using cork ring chronologies dated back 9–10 years for climate response. Despite enhancing interannual variability and increasing statistical response to short-term climatic variability, still poorly understood, this study will possibly allow infer long-term climate response. We analyzed the cork ring chronologies of 55 cork samples collected in mature (under exploitation) trees in three distinct locations in southwestern Portugal. Cork growth recorded a high climate signal, with highly significant and coherent responses to the yearly climate-related sources of variation. We successfully assessed trends of cork growth via correlation analysis including selected climate variables among mean monthly temperature, monthly precipitation and, on an annual basis, eight precipitation indices. The high mean sensitivities and inter-series correlations found for cork ring chronologies combined with the significant variance explained by climate variables suggest that climate is likely one dominant signal that affects cork growth, but local environmental stresses can decisively affect this (climate) signal. Assuming cork growth as a proxy for cork oak growth, it seems conceivable that despite the trees being highly resistant to drought stress, cork oak woodlands in southwestern Portugal would have to face lesser growth in a global warming scenario.

Diversifying sub‐Mediterranean pinewoods with oak species in a context of assisted migration: responses to local climate and light environment

AbstractQuestionsHow do thermal migration distance and extreme cold events affect seedling emergence and survival in assisted migration schemes in the sub‐Mediterranean context? What role does plant provenance play? Can biotic interactions such as nurse effect of the overstorey and shrub layer buffer the negative responses to plant translocation? Are any of these effects species‐specific?LocationThree pinewoods in the Catalan Pre‐Pyrenees, northeast Iberian Peninsula.MethodsWe used a replicated field trial to test the early years establishment of two contrasted provenances of four Quercus species (Q. coccifera, Q. ilex, Q. faginea and Q. pubescens) that were sown and planted along gradients of elevation and understorey microsite conditions in sub‐Mediterranean pinewoods. Seedling responses to translocation were evaluated through seedling emergence, seedling survival and re‐sprouting after dieback events according to seedling provenance, thermal migration distance, extreme cold events and microenvironment.ResultsThe study reports high success of both the planting (with an overall 76.3% of initial 3‐yr survival) and sowing (with an overall 50% of seedling emergence) experiments. The results show that: (1) the thermal migration distance and the occurrence of extreme cold events have strong effects on the responses of the translocated species (particularly the evergreen oaks); (2) the forest overstorey plays an important role in attenuating the negative effects of thermal migration distance on seedling survival; and (3) these responses are species‐specific. The evergreen Quercus species showed more evidence of high ecotypic differentiation in terms of cold tolerance, enabling local provenances to respond better to translocation. In contrast, marcescent species, showed high phenotypic plasticity that led to a better overall establishment success.ConclusionThe implementation of assisted migration is a feasible option to increase the diversity and resilience of the sub‐Mediterranean pinewoods. Assisted migration programmes should manage risks by thoroughly considering thermal migration distances and the occurrence of extreme cold events when selecting species and seed sources, since Mediterranean tree species show different strategies regarding adaptation to cold. Programme managers should also consider the advantage of planting/sowing under relatively closed canopy to buffer some of the negative responses associated with translocation.

Photosynthesis, stomatal conductance and terpene emission response to water availability in dry and mesic Mediterranean forests

Warmer summer conditions result in increased terpene emissions except under severe drought, in which case they strongly decrease. Water stress results in a reduction of the metabolism of plants and in a reorganization of their use of resources geared to survival. In the Mediterranean region, periods of drought accompanied by high temperatures and high irradiance occur in summer. Plants have developed various mechanisms to survive in these conditions by resisting, tolerating or preventing stress. We used three typical Mediterranean tree species in Israel, Pinus halepensis L., Quercus calliprinos and Quercus ithaburensis Webb, as models for studying some of these adaptive mechanisms. We measured their photosynthetic rates (A), stomatal conductance (g s), and terpene emission rates during spring and summer in a geophysical gradient from extremely dry to mesic from Yatir (south, arid) to Birya (north, moist) with intermediate conditions in Solelim. A and g s of P. halepensis were threefold higher in Birya than in Yatir where they remained very low both seasons. Quercus species presented 2–3-fold higher A and g s but with much more variability between seasons, especially for Q. ithaburensis with A and g s that decreased 10–30-fold from spring to summer. Terpene emission rates for pine were not different regionally in spring but they were 5–8-fold higher in Birya than in Yatir in summer (P < 0.05). Higher emissions were also observed in Solelim for the drought resistant Q. ithaburensis (P < 0.001) but not for Q. calliprinos. α-Pinene followed by limonene and 3-carene were the dominant terpenes. Warmer summer conditions result in increased Terpene emission rates except under severe drought, in which case they strongly decrease.

Cold temperature extremes during spring do not limit the range shift of Mediterranean pines into regions with intermittent frost

Summary Bioclimatic envelope models have predicted latitudinal range shifts of tree species in Europe following climate change. Accordingly, Mediterranean species will be able to migrate northwards if climatic conditions become warmer and dryer. Frost, on the other hand, is an important and recurring factor in temperate and boreal regions causing damage to buds and leaves, and potentially limiting the survival of Mediterranean tree species or populations at higher latitudes. Since species distribution models rely on average climatic parameters, they may underestimate the risk of frost damage from low temperature extremes. We measured the cold hardiness of Pinus sylvestris, Pinus nigra and Pinus halepensis seedlings from a total of 11 European populations growing in a common garden in a cold Central Alpine valley on seven dates between February and July 2013. On each date, needles were artificially frozen at several temperatures and the temperature estimated at which 50% of the needle tissue is damaged (LT50; relative electrolyte leakage). Cold hardiness did not differ between populations of the same species and was not related to the minimum temperatures at the seed origin. In comparison with deciduous trees, Mediterranean P. sylvestris and P. nigra maintained extremely wide safety margins against frost throughout late winter and spring. By contrast, safety margins of P. halepensis were much narrower until March and winter cold hardiness was in the range of regularly recurring low temperature events in Central Europe. According to the measured LT50 values, the migration of a wide range of drought‐tolerant populations of P. sylvestris and P. nigra from the Mediterranean to Central and Western Europe is not limited by intermittent cold temperature extremes in spring. They are notably as well adapted to frost as populations from Central Alpine origin. Differences in dehardening patterns between species demonstrate the importance of analysing cold hardiness repeatedly during potentially sensitive periods in order to predict species range shifts in the context of climatic change.

Trade-Offs between Drought Survival and Rooting Strategy of Two South American Mediterranean Tree Species: Implications for Dryland Forests Restoration

Differences in water-acquisition strategies of tree root systems can determine the capacity to survive under severe drought. We evaluate the effects of field water shortage on early survival, growth and root morphological variables of two South American Mediterranean tree species with different rooting strategies during two growing seasons. One year-old Quillaja saponaria (deep-rooted) and Cryptocarya alba (shallow-rooted) seedlings were established under two watering treatments (2 L·week−1·plant−1 and no water) in a complete randomized design. Watering improved the final survival of both species, but the increase was only significantly higher for the shallow-rooted species. The survival rates of deep- and shallow-rooted species was 100% and 71% with watering treatment, and 96% and 10% for the unwatered treatment, respectively. Root morphological variables of deep-rooted species such as surface area, volume, and diameter were higher under unwatered treatment. On the other hand, shallow-rooted species had a higher total root dry mass, length, surface area with watering treatments. Our findings suggest that deep-rooted species are highly recommended for reforestation in dry conditions, even under low soil water availability. Water supplements during the summer season can attenuate the differences between deep- and shallow-rooted species in their ability to survive drought during the early stage.

Explanatory ecological factors for the persistence of desiccation-sensitive seeds in transient soil seed banks:Quercus ilexas a case study

Dominant tree species in northern temperate forests, for example oak and beech, produce desiccation-sensitive seeds. Despite the potentially major influence of this functional trait on the regeneration and distribution of species under climate change, little is currently known about the ecological determinants of the persistence of desiccation-sensitive seeds in transient soil seed banks. Knowing which key climatic and microsite factors favour seed survival will help define the regeneration niche for species whose seeds display extreme sensitivity to environmental stress Using the Mediterranean Holm oak (Quercus ilex) forest as a model system, an in situ time-course monitoring of seed water status and viability was performed during the unfavourable winter season in two years with contrasting rainfall, at an instrumented site with detailed climate records. In parallel, the characteristics of the microhabitat and their influence on the post-winter water status and viability of seeds were investigated in a regional survey of 33 woodlands representative of the French distribution of the species. Time-course monitoring of seed water status in natural conditions confirmed that in situ desiccation is the main abiotic cause of mortality in winter. Critical water contents could be reached in a few days during drought spells. Seed dehydration rates were satisfactorily estimated using integrative climate proxies including vapour pressure deficit and potential evapotranspiration. Seed water status was therefore determined by the balance between water uptake after a rainfall event and water loss during dry periods. Structural equation modelling of microhabitat factors highlighted the major influence of canopy openness and resulting incident radiation on the ground. This study provides part of the knowledge required to implement species distribution models which incorporate their regeneration niche. It is an important step forward in evaluating the ecological consequences of increasing winter drought and environmental filtering due to climate change on the regeneration of the most dominant Mediterranean tree species.

Cambial activity and xylogenesis in stems of Cedrus libani A. Rich at different altitudes.

BackgroundThe dynamics of cambial activity and xylogenesis provide information on how and to what extent wood formation respond to climatic variability. The Lebanon Cedar (Cedrus libani A.Rich) is a montane tree species which is distributed along a wide altitudinal range in the northeastern Mediterranean region, currently considered as a potential forest species for Central Europe with respect to climate change. This study provides first data on intra-annual growth dynamics at cellular level using the microcore technique for a montane Mediterranean tree species at different altitudes within and outside its natural range.ResultsMicrocores were collected fortnightly in the growing season of 2013 in order to study temporal dynamics of cambial activity and xylogenesis in stems of C. libani at different altitudes in the Taurus Mountains (1000 – 2000 m a.s.l.) and at a plantation at Bayreuth (330 m a.s.l.; Germany). The dormant cambium consisted of about 5 cells at the Turkish sites and 7 cells at Bayreuth. Cambial activity set in, when daily minimum temperatures exceeded 0 °C and daily means of air and stem temperature exceeded 5 °C. Xylogenesis started between April and May, ended approximately the end of September to the beginning of October and lasted 134 (at tree line) to 174 days (at the lowest Turkish site). Mean ring widths varied from 0.55 to 3.35 mm, with highest values observed at Bayreuth very likely resulting from a steady water supply during growing season. Means of daily cell production rates varied from 0.73 to 0.12. Samples containing traumatic resin ducts occurred only rarely and where not used for analysis.ConclusionsIn C. libani, onset and dynamics of cambial activity and xylogenesis are triggered by daily means of stem and air temperatures whereas water availability has a higher influence on growth rates and cessation of wood formation. Within sites, duration of xylogenesis does not significantly differ with respect to age and tree size. C. libani grows well outside its natural range and thus may be a promising species for forestation in Central Europe with respect to climate change. We suggest further studies on if/how traumatic resin ducts influence tree ring width.Electronic supplementary materialThe online version of this article (doi:10.1186/s40529-015-0100-z) contains supplementary material, which is available to authorized users.

Point processes statistics of stable isotopes: analysing water uptake patterns in a mixed stand of Aleppo pine and Holm oak

Aim of study: Understanding inter- and intra-specific competition for water is crucial in drought-prone environments. However, little is known about the spatial interdependencies for water uptake among individuals in mixed stands. The aim of this work was to compare water uptake patterns during a drought episode in two common Mediterranean tree species, Quercus ilex L. and Pinus halepensis Mill., using the isotope composition of xylem water (δ18O, δ2H) as hydrological marker.Area of study: The study was performed in a mixed stand, sampling a total of 33 oaks and 78 pines (plot area= 888 m2). We tested the hypothesis that both species uptake water differentially along the soil profile, thus showing different levels of tree-to-tree interdependency, depending on whether neighbouring trees belong to one species or the other.Material and Methods: We used pair-correlation functions to study intra-specific point-tree configurations and the bivariate pair correlation function to analyse the inter-specific spatial configuration. Moreover, the isotopic composition of xylem water was analysed as a mark point pattern.Main results: Values for Q. ilex (δ18O= -5.3 ±0.2‰, δ2H=-54.3±0.7‰) were significantly lower than for P. halepensis (δ18O= -1.2±0.2‰, δ2H = -25.1±0.8‰), pointing to a greater contribution of deeper soil layers for water uptake by Q. ilex. Research highlights: Point-process analyses revealed spatial intra-specific dependencies among neighbouring pines, showing neither oak-oak nor oak-pine interactions. This supports niche segregation for water uptake between the two species.Key words: Cross-pair correlation function; Deuterium; Mark correlation function; Oxygen-18; Point patterns; Xylem.Abbreviations: d18O, oxygen isotope composition; d2H, hydrogen isotope composition; BA, basal area.

Contrasting growth and water use strategies in four co-occurring Mediterranean tree species revealed by concurrent measurements of sap flow and stem diameter variations

Drought limits tree water use and growth of Mediterranean trees. However, growth and water use strategies are rarely addressed simultaneously across species and drought conditions. Here, we investigate the link between stem diameter variations and sap flow in four co-existing Mediterranean trees (Pinus halepensis Mill., Quercus pubescens Willd., Quercus ilex L. and Arbutus unedo L.), under relatively wet (2011) and dry (2012) conditions. Continuous stem diameter variations were converted to basal area increment (BAI) and de-trended to estimate tree water deficit (ΔW), an indicator of stem hydration. P. halepensis and Q. pubescens showed the most and the least conservative sap flow density (JS) regulation under drought, respectively, with Q. ilex and A.unedo showing intermediate drought responses. All species, except A. unedo, showed some between-year variability in the environmental control of JS. Seasonal stem shrinkage in response to drought (i.e., increasing ΔW) and subsequent trunk rehydration after rainfall (i.e., decreasing ΔW) occurred in all species. Vapor pressure deficit (VPD) and soil moisture (θ) interacted to determine seasonal variation in ΔW. Interestingly, in the dry year, 2012, more species-specific differences were found in the responses of ΔW to θ and VPD. Across species, JS and ΔW began to decline at similar soil moisture thresholds, underpinning the tight link between JS and ΔW under varying drought conditions. Annual BAI decreased proportionally more than tree-level transpiration (JT) between the wet (2011) and the dry (2012) year, hence growth-based WUE (WUEBAI=BAI/JT) decreased for all species, albeit less acutely for P. halepensis. Overall, despite their contrasting leaf habit and wood type, the studied Mediterranean tree species show coordinated responses of transpiration, water storage dynamics and growth-based WUE which allow them to cope with seasonal and interannual drought.

Seasonal and local differences in leaf litter flammability of six Mediterranean tree species.

One of the suggested management options for reducing fire danger is the selection of less flammable plant species. Nevertheless, vegetation flammability is both complex and dynamic, making identification of such species challenging. While large efforts have been made to connect plant traits to fire behavior, seasonal changes and within species variability of traits are often neglected. Currently, even the most sophisticated fire danger systems presume that intrinsic characteristics of leaf litter stay unchanged, and plant species flammability lists are often transferred from one area to another. In order to assess if these practices can be improved, we performed a study examining the relationship between morphological characteristics and flammability parameters of leaf litter, thereby taking into account seasonal and local variability. Litter from six Mediterranean tree species was sampled throughout the fire season from three different locations along a climate gradient. Samples were subjected to flammability testing involving an epiradiator operated at 400 °C surface temperature with 3 g sample weight. Specific leaf area, fuel moisture content, average area, and average mass of a single particle had significant influences on flammability parameters. Effects of sampling time and location were significant as well. Due to the standardized testing conditions, these effects could be attributed to changes in intrinsic characteristics of the material. As the aforementioned effects were inconsistent and species specific, these results may potentially limit the generalization of species flammability rankings. Further research is necessary in order to evaluate the importance of our findings for fire danger modeling.

Influence of species interactions on transpiration of Mediterranean tree species during a summer drought

Recent research has shown that interactions between species with different functional traits can promote forest ecosystem processes. In the context of climate change, understanding whether species interactions in mixed-species ecosystems can improve the adaptation of these ecosystems to extreme climatic events is crucial to developing new management strategies. In this study, we investigated the impact of species interactions on the sap flux density of three Mediterranean tree species (Quercus faginea, Pinus nigra and Pinus sylvestris) during a summer drought. Measurements of foliar carbon isotopic composition (δ 13C) were also conducted on the same trees. The decline in transpiration during drought was the greatest for P. sylvestris and the least pronounced for Q. faginea. For P. nigra and Q. faginea, the decrease in transpiration as the drought progressed was lower when these species where interacting with another tree species, particularly with P. sylvestris. In contrast, the decrease for P. sylvestris was higher when this species was interacting with another species. Differing drought effects were consistent with the δ 13C values. We showed that the identity of the species present in the direct neighbourhood of a given tree can differentially influence water availability and water-use of these three co-existing Mediterranean tree species during a summer drought. Our findings suggest that species interactions play an important role in modulating the response of tree species to drought. Favouring tree species diversity in this region does not seem to be systematically beneficial in terms of soil water availability and water-use for all the interacting species.

Mapping Tree Species in Coastal Portugal Using Statistically Segmented Principal Component Analysis and Other Methods

Hyperspectral sensors record radiances in a large number of wavelengths of the electromagnetic spectrum and can be used to distinguish different tree species based on their characteristic reflectance signatures. Reflectance spectra were measured from airborne hyperspectral AISA Eagle/Hawk imagery in order to identify different Mediterranean tree species at a coastal test site in Portugal. A spectral range from 400 to 2450 nm was recorded at 2-m spatial resolution. The hyperspectral data are divided into five spectral data ranges. The chosen ranges for segmentation are based on statistical properties as well as on their wavelengths, as radiances of a particular wavelength may overlap with neighboring wavelengths. Principal component analysis (PCA) is applied individually to each spectral range. The first three principal components (PCs) of each range are chosen and are fused into a new data segment of reduced dimensionality. The resulting 15 PCs contain 99.42% of the information content of the original hyperspectral image. These PCs were used for a maximum likelihood classification (MLC). Spectral signatures were also analyzed for the hyperspectral data, and were validated with ground data collected in the field by a handheld spectro-radiometer. Different RGB combinations of PC bands of segmented PC image provide distinct feature identification. A comparison with other classification approaches (spectral angle mapper and MLC of the original hyperspectral imagery) shows that the MLC of the segmented PCA achieves the highest accuracy, due to its ability to reduce the Hughes phenomenon.

Effect of geographic range discontinuity on species differentiation—East-Mediterranean Abies cilicica: a case study

In the present study, a genetic analysis of nuclear microsatellites was conducted in Abies cilicica populations growing in West Taurus, East Taurus, and Lebanon mountains to obtain evidence for significant genetic differentiation. Despite range fragmentation, all examined populations tended to maintain a relatively high level of genetic diversity (H E = 0.724) and showed moderate genetic differentiation (F ST = 0.134) that is frequently noted in Mediterranean tree species. However, significant inbreeding, bottleneck effect, and low effective population sizes were detected, that may negatively affect species genetic resources. Bayesian clustering revealed three distinct groups, “West Taurus”, “East Taurus,” and “Lebanon”, which reflect geographic structure of the species natural range. Minor genetic admixture between Lebanese and East Taurus populations of A. cilicica was found, but no genetic admixture between Lebanese populations and West Taurus. We argue that significant genetic differentiation discovered among areas of species occurrence is probably due to the effect of long-lasting genetic isolation that started in that region along with the Pleistocene climatic alternation and was further enhanced with human-induced deforestations.

More than just drought: complexity of recruitment patterns in Mediterranean forests.

Understanding community dynamics during early life stages of trees is critical for the prediction of future species composition. In Mediterranean forests drought is a major constraint for regeneration, but likely not the only factor determining the observed spatial patterns. We carried out a sowing experiment aimed at identifying main filters during seed-seedling transition. Specifically, we studied seed fate (predation, fungi infection, emergence) and subsequent seedling performance (mortality during the first summer and overall recruitment after 2 years) of four co-occurring Mediterranean tree species (Quercus ilex, Quercus faginea, Juniperus thurifera, Pinus nigra). We related these processes to the dominant species composition, microhabitat heterogeneity, herb cover and seed mass. The identity of the dominant species in the forest canopy was more important for recruitment than the forest canopy being dominated by conspecific vs. heterospecific species. The patterns we found suggest that biotic interactions such as facilitation (lower mortality under the canopies) and herb competition (during emergence of J. thurifera) are relevant during recruitment. Moreover, our results pointed to ontogenetic conflicts regarding the seed mass of Q. faginea and to density-dependent seed mortality for Q. ilex, rarely described in Mediterranean ecosystems. We propose that our study species experience population growth in forests dominated by heterospecifics where the recruitment success depends on habitat heterogeneity and on moderated biotic and abiotic stresses created by each species. Our results reveal patterns and mechanisms involved in recruitment constraints that add complexity to the well-known drought-related processes in Mediterranean ecosystems.

Long‐term expansion of juniper populations in managed landscapes: patterns in space and time

Summary Forest cover has increased world‐wide over the last decade despite continuous forest fragmentation. However, a lack of long‐term demographic data hinders our understanding of the spatial dynamics of colonization in remnant populations inhabiting recently protected areas or set‐aside rural lands. We investigated the population expansion of the Phoenician juniper (Juniperus phoenicea subsp. turbinata), which is an endozoochorous Mediterranean tree species inhabiting landscapes that have been managed for many centuries. By combining the photointerpretation of aerial photos that have been taken over the last 50 years with in situ sampling and spatial analyses of replicated plots, we estimated the population growth over the chronosequence; identified hotspots, coldspots and outliers of regeneration; and assessed the roles of key environmental factors in driving demographic expansion patterns, including elevation, initial density and distance to remnant forests. Ecological factors leading to seed limitation, such as initial plant density, are expected to drive colonization patterns at the early stages. Factors mediating the competition for limiting resources, such as water availability, would prevail at later stages of expansion. We further expect that nucleated colonization patterns emerge driven by vertebrate seed dispersal. The photointerpretation of aerial images in combination with in situ measurements has yielded reliable density data. Overall, our results show a marked demographic expansion during the first decade followed by a period of steady and heterogeneous population growth with signs of local population decline. We found evidence of nucleated establishment patterns as expected for an endozoochorous species. Hotspots and outliers of regeneration emerged throughout the study chronosequence, whereas coldspots of regeneration only appeared at advanced colonization stages. Factors influencing dispersal limitation had contrasting effects at different colonization stages, and the initial density influenced population growth at various spatial scales. Synthesis. The photointerpretation of aerial images shows that the influence of dispersal limitation versus factors mediating competitive responses changes throughout colonization stages. Whereas dispersal limitation is the main factor influencing colonization at early stages, competition for local resources controls population growth at later stages. Therefore, long‐term studies are required to capture the overall combined influence of key ecological factors in shaping long‐term spatial demographic trends.