Halepensis Plantations Research Articles

Expansion dynamics of introduced Pinus halepensis Miller plantations in an oceanic island (La Gomera, Canary Islands)

Aleppo pine (Pinus halepensis Miller) has been one of the most used Pinus species in reforestation in the Mediterranean Basin and in many other regions outside its native range. Its massive use in plantations combined with its low environmental requirements, relatively short juvenile period and adaptability to human disturbance make P. halepensis a great competitor for local species, becoming invasive in many natural and anthropized areas. As little is known about the effects of these plantations on oceanic islands ecosystems, we evaluated in this study the expansion dynamics of a series of P. halepensis plantations carried out on the island of La Gomera (Canary Islands) in the 1960s and 1970s of the past century. At short distance, we counted and measured P. halepensis individuals along transects of 140 m from the inside of the plantation to the outside, assessing also the proportion occupied by other native, endemic and ruderal species in two habitat types: thermo-sclerophyllous woodlands and laurel forests. To examine seed dispersal over longer distances, we selected one of the areas that showed a greater expansion and we analyzed factors influencing regeneration. Our results revealed that expansion is occurring mostly in open or disturbed areas instead of closed-canopy humid forests and we found a clear negative relationship between the presence of the species and the abundance of other trees or the endemic and native flora. Furthermore, we detected potential new foci of regeneration away from the plantations too. Overall, we suggest that the status of P. halepensis as a non-native species and its classification for the purposes of management in the Canary Islands needs to be re-examined.

The current situation and future perspectives of Quercus ilex and Pinus halepensis afforestation on agricultural land in Spain under climate change scenarios

Between 1994 and 2017, 137,455 ha of agricultural land were afforested in Andalusia (Spain), using a great diversity of tree species, under the Common Agricultural Policy scheme. Quercus ilex and Pinus halepensis were the most used species. In view of the need to know the spatial distribution of survival according to the potential species habitat, the current survival was assessed under current and foreseeable future climate change scenarios, using ensemble species distribution models (SDMs). Predicted potential distributions together with 2009 survival data were used to predict the distribution of the survival rate of the two species. The set modelling approach gave very accurate results for the current potential distribution of Q. ilex (AUCtrain = 0.908, TSS = 0.647, Kappa = 0.647) and P. halepensis (AUCtrain = 0.983, TSS = 0869, Kappa = 0.868) and moderately accurate estimates of the distribution of the survival rate of Q. ilex and P. halepensis (RMSE = 0.23). It was found that 25–38% of the Q. ilex and P. halepensis plantations planted between 1993 and 2000 were established in the optimal area of occurrence (probability of occurrence > 70%), but only 12.3% (Q. ilex) and 22.9% (P. halepensis) presented simultaneously an acceptable survival rate (> 50%). In addition, the volume of the environmental space defined by Q. ilex decreased, while that defined by P. halepensis remained constant in future projections under climate change. The potential of SDMs to predict the survival rate distribution of Q. ilex and P. halepensis and to assess the future stability for each of these species has been confirmed. In the worst case, ~ 5% of Q. ilex and ~ 33% of P. halepensis of planted surface would withstand climate change.

Airborne Laser Scanning Cartography of On-Site Carbon Stocks as a Basis for the Silviculture of Pinus Halepensis Plantations

Forest managers are interested in forest-monitoring strategies using low density Airborne Laser Scanning (ALS). However, little research has used ALS to estimate soil organic carbon (SOC) as a criterion for operational thinning. Our objective was to compare three different thinning intensities in terms of the on-site C stock after 13 years (2004–2017) and to develop models of biomass (Wt, Mg ha−1) and SOC (Mg ha−1) in Pinus halepensis forest, based on low density ALS in southern Spain. ALS was performed for the area and stand metrics were measured within 83 plots. Non-parametric kNN models were developed to estimate Wt and SOC. The overall C stock was significantly higher in plots subjected to heavy or moderate thinning (101.17 Mg ha−1 and 100.94 Mg ha−1, respectively) than in the control plots (91.83 Mg ha−1). The best Wt and SOC models provided R2 values of 0.82 (Wt, MSNPP) and 0.82 (SOC-S10, RAW). The study area will be able to stock 134,850 Mg of C under a non-intervention scenario and 157,958 Mg of C under the heavy thinning scenario. High-resolution cartography of the predicted C stock is useful for silvicultural planning and may be used for proper management to increase C sequestration in dry P. halepensis forests.

Factors determining enzyme activities in soils under Pinus halepensis and Pinus sylvestris plantations in Spain: a basis for establishing sustainable forest management strategies

Water availability and soil pH seem to be major constraints for enzyme activities in calcareous soils under Pinus halepensis and acidic soils under Pinus sylvestris plantations respectively. Proposals for improving enzyme activities may include the promotion of broadleaf species to increase soil pH and the modulation of stand density or the implementation of soil preparation techniques to facilitate water infiltration. Soil enzymes play a key role in nutrient turnover in forest ecosystems, as they are responsible for the transformation of organic matter into available nutrients for plants. Enzyme activities are commonly influenced by temperature, humidity, nutrient availability, pH, and organic matter content. To assess the differences between enzyme activities in calcareous soils below Pinus halepensis and acidic soils below Pinus sylvestris plantations in Spain and to trace those differences back to edapho-climatic parameters to answer the questions: Which environmental factors drive enzyme activities in these soils? How can forest management improve them? The differences in climatic, soil physical, chemical, and biochemical parameters and the correlations between these parameters and enzyme activities in soils were assessed. Low pH and high level of phenols in acidic soils under Pinus sylvestris and water deficit in calcareous soils under Pinus halepensis plantations appeared to be the most limiting factors for enzyme activities. Options such as the promotion of native broadleaf species in the Pinus sylvestris stands and the modulation of Pinus halepensis stand density or the implementation of soil preparation techniques may improve enzyme activities and, therefore, nutrient availability.

Local basal area affects needle litterfall, nutrient concentration, and nutrient release during decomposition in Pinus halepensis Mill. plantations in Spain

Stand density has a positive effect on C, K and Mg concentration in needle litterfall and a negative one on C, N, Ca, K, Mg, P, S, Zn, and Cu release from needle litter. Consequently, forest management practices such as thinning decrease nutrient concentration in needle litterfall and accelerate nutrient release from decomposing needles in Pinus halepensis plantations in Spain. Silvicultural practices usually include stand density reduction resulting in changes in litterfall and litter decomposition rates. Little is known about the effect on nutrient concentrations in litterfall and nutrient release during decomposition even when this is the main path of nutrient return to soils. The aims of the study are to evaluate the seasonal pattern of nutrient concentration in litterfall, to study how nutrients are released from needle litterfall during decomposition, and to assess whether local basal area of the stand affects nutrient concentration of litterfall and nutrient release during litter decomposition. Eight plots were established on each of four stands covering the widest range in local basal area. A littertrap and 15 litterbags were placed on each plot. Periodically, needle litterfall and litter contained in the litterbags were analyzed for C, N, Ca, K, Mg, P, S, Fe, Cu, Mn, and Zn. Local basal area had a positive effect on C, K, and Mg concentration in needle litterfall and a negative effect on the release of all the nutrients studied but Fe and Mn during the first 2 years of litter decomposition. Density management of stands has an impact on nutrient cycling, reducing nutrient concentration in needle litterfall, and accelerating nutrient release during decomposition.

Effects of single and repeated prescribed burns on soil organic C and microbial activity in a Pinus halepensis plantation of Southern Italy

Wildfire has historically been a major disturbance in Mediterranean European ecosystems and prescribed burning is increasingly used here to mitigate possible damage and reduce hazard. This raises the issue of a possible negative impact on soil from repeated burning treatments. We investigated the effects of a single and repeated prescribed burning on total and extractable organic C, microbial biomass C, fungal mycelium, microbial activity, metabolic quotient and C mineralization rate in the fermentation layer (F-layer) and the 5-cm soil underneath of a Pinus halepensis Mill. plantation in the Cilento, Vallo di Diano e Alburni National Park, Southern Italy. Prescribed burning was sequentially applied in 2009 and 2014 and the measurements were performed during the first year following each burning treatment. The first treatment produced short-term effects on the microbial biomass and activity of the F-layer, whereas the soil layer underneath was virtually unaffected. Surprisingly, the second treatment had negligible effects on either layer, probably because of a reduction of the understory fuel load and changes in the stand structure after the first burning. Most remarkably, fire-induced changes in the soil parameters considered were generally smaller than over-time fluctuations.

Integration of stakeholder choices and multi-criteria analysis to support land use planning in semiarid areas

The ecosystem services concept is increasingly considered as a policy tool to achieve the sustainable use of natural resources. However, it is still not sufficiently integrated into land use planning. We assessed five land use types (Tetraclinis articulata woodlands, Pinus halepensis plantations, Shrublands, Cereal-almond crops and Cactus groves) in a semiarid area of northern Morocco using empirical data on 17 ecosystem services whose weights were established by 67 stakeholders. The analysis included MCA and direct ranking of the five land uses. Three groups of stakeholders (scientists and managers, collaborators, and direct users of natural resources) were particularly concerned by water supply, protection against erosion and floods, soil fertility and food provision. Multi-criteria analysis showed that the three groups concurred in that Tetraclinis woodlands, crops and cactus fields were the most suitable land uses for the area, regarding ecosystem service provision. Direct ranking confirmed this tendency but showed some divergence between the three groups, as collaborators and users were more inclined towards crops and cactus. The integration of the ecosystem services concept in land use planning is needed to be more practical and easily perceived as a logical response to environmental exigencies and social aspirations.

High-intensity thinning treatments in mature Pinus halepensis plantations experiencing prolonged drought

Pinus halepensis plantations are widespread throughout semiarid–subhumid landscapes of the Mediterranean. Recently, drought-induced decline has often been reported raising concerns with regard to the future of these man-made ecosystems. The study was set out to investigate thinning as a means to alleviate water stress and improve performance of mature P. halepensis plantations experiencing prolonged drought. The study was conducted in a 40-year-old P. halepensis forest in the Jerusalem Mountains of Israel (rainfall: 550 mm year−1). Declining stands (stand basal area increment, BAIstand ≈ −3 % year−1) were treated by thinning: (1) intense thinning—tree density, BAstand, and leaf area index (LAI) reduced by 81, 68, and 59 %, respectively; (2) moderate—56, 48, and 26 %, respectively; (3) control (~560 tree ha−1). Plots of 70 × 70 m were used in four replicates. Individual tree- to stand-level variables were monitored during 4 years through stem and leaf area metrics alongside with predawn shoot water potential (ψ PD) and tree mortality. Thinning ameliorated drought stress, reduced mortality, and improved individual tree growth (ψ PD = −1.7, −1.8, and −2.0 MPa; mortality = 0.2, 2, and 5 % year−1; BAItree = 3.4, 2.0, and 1.4 % year−1 in intense, moderate, and control treatments, respectively). Thinning effects became more pronounced with time. LAI and individual tree leaf area (LAtree) fluctuated with association to annual rainfall. Higher LAtree caused by thinning reflected a “selection effect” while increased leaf area efficiency (BAI per unit LAtree) was attributed to a “release effect” of thinning.

Rootable depth controls height growth of Pinus halepensis Mill. in gypsiferous and non-gypsiferous soils

Pinus halepensis is generally considered a species adapted to soils with gypsum but there is hardly any data available to support such statement nor to assess the degree to which soil gypsum may constrain tree development. We studied fifty five 200m2-plots in a P. halepensis plantation in NE Spain, 23 on soils with gypsum and 32 on soils without gypsum. Trees were measured to estimate site index at age 40years (SI40). A soil pit was described in each plot to a depth of 1m or to a root-limiting layer, and samples of the various horizons analysed for pH, organic carbon (C), total nitrogen, Olsen phosphorus (P), exchangeable potassium (extracted with NH4OAc), calcium carbonate (calcimeter method), and gypsum concentration (Artieda method), and texture. We studied root development in the soil horizons of 15 of these plots by counting root numbers at depths of 0–30cm, 30–55cm, and 55–80cm in three 100cm2-squares per depth. Penetration resistance and bulk density were also measured in these horizons. Soils with gypsum were frequently less than 25cm deep, and had negligible concentrations of Olsen phosphorus. Values of SI40, with a maximum of 15.5m, were primarily determined, in all types of soils, by a positive effect of soil rootable depth, indicating the dominant influence of water availability, and to a lesser extent by the negative effect of the C/P ratio and rock fragment content in the upper 30cm of soil. Density of fine and very fine roots decreased in deeper soil horizons from a maximum value of 97 roots·dm−2 in the surface horizons. Mechanical impedance by increased penetration resistance was the main limitation for root development. Soil gypsum does not have a direct influence on growth but constrains the volume of soil that may be explored by roots.

Wildfire effects on Pinus halepensis Mill. plantations in a semi-arid region of north-eastern Algeria. A case study of Zenadia forest, Sétif

Fire is considered as the most important factor responsible for the diminution of Algerian forested area. Every year, thousands of hectares of vegetation burn affecting different species. The natural Aleppo pine forests have been the most affected, but recently, fire tends to affect also the planted forests. The effect of fire on Aleppo pine natural stands is well documented ; however, there is a lack of knowledge on planted ones. The objectives of this paper were to assess the fire effects on Pinus halepensis plantations in northern Algeria in terms of floristic diversity and pine regeneration. Floristic diversity surveys were performed three years after fire in burned and unburned plots, using the Braun-Blanquet method. Additionally, the number and height of P. halepensis seedlings were assessed and continued to be monitored in the best regenerated area four, six, eight, and ten years after fire occurrence. The floristic surveys performed showed large differences between the unburned and the burned plots, both in terms of vegetation structure and plant diversity. Three years after fire, the burned plots were dominated by an herbaceous layer and had a much higher floristic richness when compared with the unburned plots. Pine regeneration was absent in the unburned area, while in the burned area it reached up to nearly three seedlings per m2 after three years. Seedling density decreased over time while maximum height increased up to two meters ten years after fire. Our results suggest that prescribed burning, associated to selective logging, could be an interesting management tool to explore in these artificial stands. This could create forest gaps that would permit the natural regeneration of Aleppo pine and others species, favouring multi-strata forest and biodiversity.

Warming reduces the cover and diversity of biocrust-forming mosses and lichens, and increases the physiological stress of soil microbial communities in a semi-arid Pinus halepensis plantation.

Soil communities dominated by lichens and mosses (biocrusts) play key roles in maintaining ecosystem structure and functioning in drylands worldwide. However, few studies have explicitly evaluated how climate change-induced impacts on biocrusts affect associated soil microbial communities. We report results from a field experiment conducted in a semiarid Pinus halepensis plantation, where we setup an experiment with two factors: cover of biocrusts (low [<15%] versus high [>50%]), and warming (control versus a ∼2°C temperature increase). Warming reduced the richness and cover (∼45%) of high biocrust cover areas 53 months after the onset of the experiment. This treatment did not change the ratios between the major microbial groups, as measured by phospholipid fatty acid analysis. Warming increased the physiological stress of the Gram negative bacterial community, as indicated by the cy17:0/16:1ω7 ratio. This response was modulated by the initial biocrust cover, as the increase in this ratio with warming was higher in areas with low cover. Our findings suggest that biocrusts can slow down the negative effects of warming on the physiological status of the Gram negative bacterial community. However, as warming will likely reduce the cover and diversity of biocrusts, these positive effects will be reduced under climate change.

Monthly foliar-nutrient pattern in a semiarid Aleppo pine plantation five years after thinning

Foliar-nutrient concentrations in tree species have traditionally been used to assess the nutrient status of forests. Monitoring these parameters in artificial forest plantations could be crucial to planning the best silvicultural practices as a basic strategy for improving stand resilience. This work examines the monthly patterns of foliar-nutrient concentrations over a 13-month period in a Pinus halepensis plantation five years after four different thinning regimes were applied (T0 treatment: control plots; T75, T60, and T48 treatments: 75%, 60%, and 48% of mean basal area was removed, respectively). Mean leaf-nutrient concentrations reported were among the normal and adequate ranges shown in the literature for P. halepensis, with the exception of Ca (higher), as well as K and Zn (lower). Thinning did not affect the leaf P or K concentrations in any case or sampling period. Moreover, we found no clear differences between thinned and control stands for other nutrients. Only B was higher in control stands and Zn in thinned stands. Exceptionally, N, Mg, Fe, Zn, and Ca concentrations proved higher in the most intense thinning (T75), for specific sampling periods. Patterns of foliar-nutrient concentrations were similar among thinned treatments and control. All nutrient concentrations decreased over the study period except the Ca concentration (and Mg in T75), which increased. Foliar-nutrient concentrations showed a positive linear correlation between nutrients (especially N–P, P–B and N–B), except K–Ca, K–Mg, and Fe–Ca, which proved negative. Partial correlations (month by month) among nutrient concentrations differed from the general patterns. Foliar-nutrient concentrations and paired relationships were influenced mainly by the sampling period, suggesting the importance of standardized leaf-sampling designs in studies analyzing and comparing foliar-nutrient concentrations.

Pine afforestation decreases the long‐term performance of understorey shrubs in a semi‐arid Mediterranean ecosystem: a stable isotope approach

Summary Plant–plant interactions shape the structure and composition of plant communities, but shifts in interaction outcomes might occur in the face of ongoing climate change. We assessed the influence of Pinus halepensis plantations on the long‐term ecophysiological performance of understorey vegetation, by conducting a retrospective comparison (1989–2007) of growth‐ring widths, δ13C and δ18O between Rhamnus lycioides shrubs from two contrasting vegetation types: P. halepensis plantations vs. open woodlands. We also measured the leaf δ13C, δ18O, δ15N, and nutrient concentrations of understorey R. lycioides shrubs growing at varying distances from planted trees within pine plantations. Dendroecological and stable isotope data revealed strong competitive effects of planted P. halepensis trees on R. lyciodies. Shrubs in pine plantations showed lower radial growth and higher growth ring δ13C and δ18O than those in open woodlands, indicating lower stomatal conductance and photosynthesis in the former. The strong competitive effects of P. halepensis on understorey R. lycioides were most evident in wet, productive years. Conversely, in very dry years, there were indications of a facilitative effect of planted P. halepensis canopies on understorey shrubs. Within pine plantations, understorey R. lycioides shrubs growing at shorter distances from planted trees were forced to rely on more superficial and ephemeral soil water sources, which reduced their stomatal conductance (higher leaf δ18O) and interfered with nutrient uptake (lower leaf N and P concentrations and more negative δ15N). The intrinsic water use efficiency of R. lycioides shrubs growing in open woodlands has increased during recent decades as a result of their ability to adjust their stomatal conductance in response to increasing temperature and atmospheric CO2. However, this adaptive response was much weaker or absent in the severely drought‐stressed shrubs from pine plantations. Pine afforestation strongly reduces water and nutrient availability for understorey shrubs in drylands, with potential long‐term consequences for ecosystem biodiversity, structure and functioning. Competition by P. halepensis on R. lycioides clearly outweighed facilitation in the long‐term, thus compromising the ability of understorey shrubs in semi‐arid pine plantations to cope with climate change.

Multi-criteria participative evaluation of Pinus halepensis plantations in a semiarid area of southeast Spain

Semiarid areas host several hotspots of biodiversity, and provide vital ecosystem services. Afforestation has been extensively used to restore these areas. However, studies on the impact of afforestation on the provision of ecosystem goods and services are scarce. We have evaluated changes in the provision of ecosystem services after afforestation in a semiarid area in southeastern Spanish. Through a multi-criteria analysis, Pinus halepensis plantations were compared to four unrestored landscape units (grasslands, Stipa tenacissima steppes, shrublands and abandoned agricultural fields), using six ecological and socioeconomic criteria and fourteen indicators. The weights of criteria and indicators were obtained from the opinion of 38 stakeholders. The values of each indicator per landscape unit were obtained from empirical data and bibliographic sources. From the stakeholders’ perspective, regulating and supporting services and biodiversity were more important than provisioning, cultural and economical services. Pine plantations enhanced ecosystem services as hydrological and climatic regulation, and aesthetic value, but decreased the provision of other services as forage productivity, water availability, and small and big game habitat. Pine plantations provided the highest levels of ecosystem services. The multi-criteria and participatory assessment allowed managers to check the overall impact of their decisions, and it represents an efficient tool for reciprocal learning and negotiation.

The impact of Pinus halepensis afforestation on Mediterranean spontaneous vegetation: do soil treatment and canopy cover matter?

We investigated central Mediterranean Pinus halepensis plantations under semi-arid climate in order to evaluate the combined effect of soil treatment and afforestation practices on spontaneous plant species composition, richness and evenness, and on the trend and speed of vegetation dynamics. Phytosociological relevés of three different plot typologies, i.e. (1) soil-treatment and plantation, (2) only soil-treatment, (3) no soil-treatment and no plantation, were compared by (a) multivariate analysis and (b) with reference to species richness and evenness. Moreover, in order to compare vegetation dynamics within the plantations with those ones ongoing in semi-natural garrigue communities, we compared life form and syntaxonomic spectra between phytosociological relevés taken at 8 years of distance. DCA showed that floristic species composition and similarity are influenced by the canopy cover of Pine trees as well as by soil-treatment practices. Although species richness and evenness are not clearly related to neither soil treatment nor Pine afforestation, canopy cover clearly plays a major role: in fact, the highest Ph cover rates correspond to the lowest values of understory species richness. This is true also if only species of biogeographical/conservation interest are considered. Regarding vegetation dynamics, sites with dense Pine canopy cover evolve much slower than the adjacent garrigue communities. The same factors invoqued to explain the patterns of floristic composition and similarity (i.e. allelopathy and competition for light, water and nutrients) may also explain the lowering of diversity of therophytes and the strong decline of the cover perfomed by both therophytes and hemicryptophytes underneath the canopy of dense Ph plantations. Thus, in sites where Ph cover exceeds about 80%, thinning is recommended not only in order to accelerate succession, but also to give a natural ‘shape’ to afforestations.

Improving substrate fertility to enhance growth and reproductive ability of a Pinus halepensis Mill. afforestation in a restored limestone quarry

We have evaluated the effects of improving substrate fertility on the growth and reproduction of a P. halepensis plantation in a restored limestone quarry on a stony calcareous regolith (R plots). The natural substrate was supplemented by adding a 0.2 m layer of a fine textured soil (RS plots) or a sewage sludge amended soil (RSS plots). The treatments were performed when the pines were 7 years old, and tree growth (height and trunk and canopy diameter) was monitored over the subsequent 12 years. The reproductive status of the trees was also measured when the pines were 20 years old. Tree growth was proportional to the amount of soil nutrients: 12 years after treatment the mean height of the R, RS and RSS trees was 1.5, 3.1 and 6.2 m respectively and growth increases over the baseline were 76, 264 and 632%. The treatment also affected the age of onset of reproduction (15, 11 and 9 years, respectively), the average number of cones per tree (12, 43 and 61), and the amount of seeds per cone (37, 52 and 72), but did not modify the germination percentage of pine seeds (ca 71.5%). Soil organic carbon increased proportionally to the vegetation development, contributing to carbon sequestration. These results suggest that improving the nutritional status of the soil not only improves the growth of trees, but it also ameliorates their reproductive ability (earlier reproduction onset and larger seed crop size). Implications for soil restoration through afforestation are also discussed.

Understory woody vegetation in manmade Mediterranean pine forests: variation in community structure along a rainfall gradient

Natural regeneration that occurs in the understory of Mediterranean pine monocultures provides the basis for the transition of these simply structured systems toward a more complex and sustainable state. However, the course and consequences of this process, and its relationships with environmental and silvicultural variables, are still inadequately understood. We investigated the relationship between rainfall amount and understory woody vegetation (UWV) structure in mature (40–50 year) Pinus halepensis plantations in the Mediterranean zone of Israel, where rainfall ranges from 280 mm/year in the south to 900 mm/year in the north. We measured abundance, diversity and species composition on south- and north-facing slopes, in forest sites distributed along the rainfall gradient. UWV abundance, as measured by cover percentage and height, increased with rainfall amount along the entire gradient (2–113% and 0.1–3.4 m, respectively), more rapidly on north-facing slopes. Species composition varied along the rainfall gradient, with ranges of species occurrences corresponding to those in unforested habitats. The relationship between rainfall and UWV species richness was positive throughout most of the rainfall gradient, possibly with a shift in pattern at the highest rainfall levels. UWV richness increased sharply with increasing abundance, up to a certain point with no further increase in richness as abundance increased further. We concluded that UWV structure in the studied forest environment and climatic range is strongly determined by rainfall and suggested that the design and management of Mediterranean forests should focus more on optimizing water availability for the various components.

Long term and seasonal courses of leaf area index in a semi-arid forest plantation

Effective leaf area index (LAI e) in the semi-arid Pinus halepensis plantation, located between arid and semi-arid climatic zones at the edge of the Negev and Judean deserts, was measured bi-annually during four years (2001–2004) and more intensively (monthly) during the following two years (2004–2006) by a number of non-contact optical devices. The measurements showed a gradual increase in LAI e from ∼1 (±0.25) to ∼1.8 (±0.11) during these years. All instruments, when used properly, gave similar results that were also comparable with actual leaf area index measured by litter collection and destructive sampling and allometric estimates. Because of the constraint of clear sky conditions, which limited the use of the fisheye type sensors to times of twilight, the fisheye techniques were less useful. The tracing radiation and architecture of canopies system, which includes specific treatment of two levels of clumpiness of the sparse forest stand, was used successfully for the intensive monitoring. The mean clumpiness index, 0.61, is considered representative for the specific environment. Finally, the LAI e measurements at the start of each season were used to constrain phenology-based estimates of annual LAI e development, resulting in a continuous course of LAI e in the forest over the five-year period. Intra-seasonal LAI e variation in the order of 10% of total LAI e predicted by the model was also observed in the intensive TRAC measurements, giving confidence in the TRAC system and indicating its sensitivity and applicability in woodlands even with low LAI e values. The results can be important for forest management decision support as well as for use in evaluation of remote sensing techniques for forests at the lowest range of LAI e values.

Drought-induced growth decline of Aleppo and maritime pine forests in south-eastern Spain.

Climate warming may enhance the negative effects of droughts on radial growth in areas with severe water deficit, such as Mediterranean mountains under semi-arid conditions. The impacts of drought on growth decline of Mediterranean pines have not been evaluated considering species with different vulnerability and areas with contrasting climates. Dendrochronological methods were used to assess the responses of basal area increment to drought in Pinus pinaster and P. halepensis plantations. We compared growth trends of trees with different defoliation degree in two sites in south-eastern Spain (Sierra de los Filabres and Sierra de Baza) with contrasting climatic conditions. In the more xeric area (Filabres) both pine species showed a sharp growth reduction in response to extreme droughts such as those observed in 1994-1995, 1999 and 2005. The radial growth of both species was enhanced by May and June precipitation of the year of tree-ring formation. P. pinaster showed higher defoliation in the xeric area (Filabres) than in the more mesic area (Baza) but needle loss was not linked to an abrupt growth reduction. Contrastingly, divergent radial growth patterns between trees showing high and low defoliation degrees were found for P. halepensis in the more xeric area, where a negative relationship between recent basal area increment and defoliation was found. Pine plantations in Mediterranean mountains under semi-arid conditions are highly vulnerable to warming-induced droughts. Such marginal stands constitute valuable monitoring systems to assess the negative impacts of drought on tree growth, and to test if management strategies as thinning can mitigate the negative impacts of climate warming on similar drought-stressed forests.

Storage of organic carbon and Black carbon in density fractions of calcareous soils under different land uses

The association between soil particles and their spatial arrangement plays a key role in soil organic matter (OM) dynamics. Density fractionation combined with ultrasonic dispersion enables separation and study of soil OM fractions, considered on the basis of the mechanisms of physical protection: non-physically protected OM (FF), OM occluded into aggregates (OF), and OM stabilized in organo-mineral complexes (DF). In the present study, whole soils and density fractions of calcareous soils under three different management systems — native Quercus ilex forest, a Pinus halepensis plantation and cropped land — were analyzed for organic C (OC), total N, and Black carbon (BC) content. Black carbon is often considered as a very recalcitrant pool in the soil. However, as well as BC content of soils has seldom been quantified, long-term studies on BC stability are scarce and conclusions about BC stability are not widespread. About 67% of the total C in the topsoil was lost as a result of converting the natural Q. ilex forest to cropped land, 100 years ago. After crop abandon, the stock of OC in the topsoil greatly increased upon reforestation of the studied plot with P. halepensis. An average recovery of 71% of the previously lost OC had been recovered, after 40 years of pine plantation. The changes in OC stocks affected mainly the free fraction (FF). Black carbon represented between 1.2 and 2.3% of the TOC of soil with the highest concentrations in OF. The maintenance of BC proportion through land uses suggests an equilibrium between inputs and outputs, and leads to the suspicion that BC could be less stable and less resistant to biodegradation than is often taken for granted.