Can we use shelterwoods in Mediterranean pine forests to promote oak seedling development?

The use of shelterwoods to favour the development of natural or underplanted seedlings is common in temperate forests but rare in the pine forests of the Mediterranean area. Our aim was to assess the use of shelterwoods in Aleppo pine (Pinus halepensis) woodlands in southern France to promote the survival and growth of two co-occurring oak species: the deciduous Quercus pubescens and the evergreen Quercus ilex.Twelve Aleppo pine stands were selected and differentially thinned to create a light shelterwood (basal area = 10 m²/ha, irradiance 52%), a medium shelterwood (basal area = 19 m²/ha, irradiance 33%) and a dense shelterwood (basal area = 32 m²/ha, irradiance 13%). A total of 1248 sowing points, half composed of Q. pubescens and half of Q. ilex, were then set up in these three conditions. Seedling survival and growth were monitored for 3 years. Plant stress was assessed by measuring predawn leaf potential and photosynthetic performance through the F_v/F_m ratio. Soil moisture was also recorded at two depths during two growing seasons.Survival was high for both species in all three conditions due to three consecutive wet years. The lowest survival was recorded for Q. pubescens in the dense shelterwoods. Growth in diameter and height increased from the dense to the light shelterwoods. Shrubs developed more strongly in the light shelterwood, and increasing shrub cover enhanced height growth. Photosynthetic performance was lowest for Q. pubescens in dense shelterwoods and highest in light shelterwoods, whereas the reverse was true for Q. ilex. The lowest predawn potentials were recorded in the dense shelterwoods even though higher soil water content values were measured in this treatment during the summer drought.We show that light shelterwoods were more beneficial to growth than denser ones, indicating control mainly by light availability during the 3 years of the study. However, as lower soil moisture at 30-50 cm depth and faster understorey development were also recorded in this condition, more extended observation is needed to determine whether this benefit persists in subsequent years.     

Response of Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) to soil and atmospheric water deficits under Mediterranean mountain climate

The physiological responses to water deficits of Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) were studied under Mediterranean mountain climate. Minimum leaf water potentials were ?3.2 MPa for oak and ?2.1 MPa for pine, with higher predawn values for pubescent oak. Relative sap flow declined in both species when vapour pressure deficit (D) went above ca. 1.2 kPa, but stomatal control was stronger for pine during the 2003 summer drought. P. sylvestris plant hydraulic conductance on a half-total leaf area basis (k _L,s?1) was 1.2–2.6 times higher than the values shown by Q. pubescens, and it showed a considerably steeper decrease during summer. Leaf-level gas exchange was positively related to k _L,s?1 in both species. Scots pine was more vulnerable to xylem embolism and closed stomata to prevent substantial conductivity losses. The results of this study confirm that pubescent oak is more resistant to extreme drought events.     

Why do large,nitrogen rich seedlings better resist stressful transplanting conditions? A physiological analysis in two functionally contrasting Mediterranean forest species

We analysed the physiological bases that explain why large and high nitrogen (N) concentration seedlings frequently have improved survival and growth relative to small seedlings in Mediterranean woodland plantations. Large seedlings of Aleppo pine (Pinus halepensis Mill.) and holm oak (Quercus ilex L.) with high N concentration (L+), and small seedlings with either high (S+) or low (S−) N concentration, were planted on two sites of different weed competition intensity that created contrasting stress conditions. Seedling survival, growth, gas exchange, N remobilization (N_R) and uptake (N_U), and water potential were assessed through the first growing season. Weeds reduced survival and growth, but seedling response to weed competition varied among phenotypes and between species. At the end of the first growing season, L+ Aleppo pine seedlings had higher survival than both small seedling types in presence of weeds but no differences were observed in absence of weeds. Mortality differences among phenotypes occurred in spring but not in summer. L+ Aleppo pines grew more than small Aleppo pines independently of weed competition. No holm oak seedling type survived in presence of weeds and no mortality differences among phenotypes where observed in absence of weeds, although L+ holm oak seedlings grew more than small seedlings. Mortality and growth differences in Aleppo pine were linked to marked physiological differences among phenotypes while physiological differences were small among holm oak phenotypes. L+ Aleppo pines had greater root growth, gas exchange, N_R, and N_U than small seedlings, irrespective of their N concentration. Seedling size in Aleppo pine had a greater role in the performance of transplanted seedlings than N concentration. The functional differences among oak phenotypes were small whereas they were large in pine seedlings, which led to smaller differences in transplanting performance in holm oak than in pine. This suggests that the nursery seedling quality improvement for planting in dry sites could depend on the species-specific phenotypic plasticity and functional strategy. Improved transplanting performance in large Aleppo pine seedlings relative to small seedlings was linked to greater gas exchange, root growth and N cycling.     

Effects of nursery shading on seedling quality and post-planting performance in two Mediterranean species with contrasting shade tolerance

In Mediterranean climates, seedlings are frequently shaded in the nursery to avoid heat damage and save water. However, the impact of this shading on the seedling quality and transplanting performance of Mediterranean species is not well known. We studied the effect of nursery shading on pre-planting features and post-planting performance of two Mediterranean tree species: the shade-intolerant pioneer Pinus halepensis and the shade-tolerant late-successional Quercus ilex. We grew one-year-old seedlings of both species under 100, 40 and 5% full sunlight. Shade had a low impact on the morphology and physiology of Q. ilex seedlings. In pines, only the deep shade treatment produced low quality seedlings with poor root development. In both species, transference to high light at planting in autumn did not impose any additional stress than that caused by frosts, but initial root growth was impaired in the two shaded treatments in pine. Post-planting growth and survival of oak seedlings showed no difference between treatments. Pine seedlings grown in deep shade showed higher mortality and lower growth after planting than those grown in full sun and intermediate light treatments, while intermediate light only reduced growth. For the nursery culture of Q. ilex seedlings, we advise using low light levels during summer to save water without impairing field performance. In P. halepensis, seedlings should be cultured under full sunlight conditions to maximize post-planting growth, but they can be cultured under intermediate light without impairing survival.     

How nutrient availability influences acclimation to shade of two (pioneer and late-successional) Mediterranean tree species?

Examining ecological limits to shade acclimation at whole-plant level is determinant for evaluating the success of sapling establishment in low-light environments. We studied nutritional effects on whole-plant development in response to shade in two Mediterranean forest tree species with different successional status: the early-successional Pinus halepensis Mill. and the late-successional Quercus pubescens Wild. Through a nursery-based factorial experimental design approach, we measured height increment along 2 years and final leaf, stem and root biomass in both species saplings subjected to two lights and two soil nutrient availability treatments. The shade avoidance response was exclusive to P. halepensis, appeared as timely dependent, and persisted longer in saplings exposed to higher nutrient availability. Q. pubescens benefited from the higher nutrient availability by lowering the light-driven plastic response in aerial support investment and belowground carbon allocation, whereas P. halepensis heightened its light-driven plastic response. These contrasted responses are thus clearly related to the shade acclimation strategy of each species: the shade-intolerant P. halepensis enhances shade avoidance when non-nutrient-limited, whereas the shade-tolerant Q. pubescens assumes a conservative strategy by limiting phenotypic plasticity-induced costs. Maintaining greater shade avoidance in non-nutrient-limited soil conditions might be an adaptive advantage for P. halepensis seedlings growing in the understory, in response to gap formation in the overstory. In contrast, the more conservative and less costly shade responsiveness of Q. pubescens may confer it a better adaptive advantage in long-term light-limited environments.     

Effects of resource availability on condensed tannins and nitrogen in two Quercus species differing in leaf life span

Seedlings of Quercus ilex and Q. cerrioides, an evergreen and a winter-deciduous oak co-occurring in western-Mediterranean forests, were grown at two light regimes (8 and 36% of photosynthetically active radiation), at two water regimes (500 and 800 mm) and with two nutrient availabilities (standard substrate and 7% increase in soil N). The concentrations of soluble condensed tannins (CT) and nitrogen in the leaves were analyzed to test the phenotypic plasticity of these commonly related parameters in two con-generic species with contrasting leaf habit. Q. ilex contains seven times more CT and a few less N than Q. cerrioides. Light increased CT, whereas neither fertilization nor water had an effect on CT. N concentration was decreased by light, increased by fertilization and not affected by water treatment. Plant growth was increased by light but not affected by fertilization or water treatment. CT were negatively correlated with N concentration. CT of the evergreen species exhibited greater plasticity than the deciduous one as reflected by a steeper negative correlation among nitrogen and CT concentrations in Q. ilex. Given the antiherbivory activity of CT, this implies that in less shaded environments, e.g. canopy aperture by disturbances, leaf tissue quality for herbivores will be much more reduced in Q. ilex than in Q. cerrioides. Higher leaf CT in Q. ilex and its higher plasticity to light availability may explain the higher browsing by sheep in Q. cerrioides than in Q. ilex resprouts, as well as the low recruitment rates of seedlings of the former species, reported in other studies.     

Is light the key factor for success of tube shelters in forest restoration plantings under Mediterranean climates?

Tube shelters were designed to protect against browsing, but they improve seedling survival in Mediterranean dry climates. Mechanisms for this response, however, are not fully understood and this knowledge can be useful to help design optimal tube shelters for Mediterranean species and climates. Our objective in this study was to determine if the positive effect of tube shelters is due to enhanced growth during the wet season or to reduced light stress during the dry season. We performed two independent experiments. In the first, we assessed root growth during the wet season in two Mediterranean species with contrasting light tolerance (Quercus ilex L. and Pinus halepensis Mill.) growing in tubes with varying light transmissivity. In the second experiment, we studied the response of a Quercus ilex plantation to different shelter treatments. Root growth during the wet season was reduced with decreasing light transmissivity in the shade intolerant P. halepensis, but not in the shade tolerant Quercus ilex. Survival of Q. ilex shaded by a mesh shelter only during summer was higher than in unsheltered seedlings and similar to the survival in tube and mesh shelters during the whole season. This suggests that shade during the dry period was the main factor explaining survival in this species. This effect could be related to the lower leaf temperature recorded in sheltered seedlings. We conclude that Q. ilex (and perhaps other late successional, shade tolerant Mediterranean species) should be planted in tubes with the currently used light transmissivity because these shelters reduce light stress in summer without impairing root growth in the wet season. However, current tubes impair root growth in P. halepensis (and likely other pioneer, shade intolerant Mediterranean species), so higher transmissivity tubes may be necessary. Optimal transmissivity for tube shelter in Mediterranean climates is species-specific and identifiable as the point that minimizes light stress during summer without impairing root growth in the wet season.     

A GIS-based integrated approach predicts accurately post-fire Aleppo pine regeneration at regional scale

Context

This study investigates post-fire natural regeneration of Aleppo pine (Pinus halepensis) forests at Ilia region (Peloponnesus, Greece) following the catastrophic fire of 2007.

Aims

The objective of this study is the prediction of P. halepensis post-fire regeneration at a regional scale through an integrated geographic information systems (GIS) model as a basis for post-fire management plans.

Methods

The model was developed in three interconnected stages: (1) field data collection, (2) development of two prediction models (based on interpolation of field data and multi-criteria evaluation (MCE) that combined factors known to affect regeneration), and (3) combination of applied models using Bayesian statistics.

Results

Post-fire pine regeneration presented high variation among the studied plots. Redundancy analysis revealed the positive effect of fallen branches and a negative correlation with altitude. Both modeling approaches (geostatistical and MCE) predicted the post-fire pine regeneration with high accuracy. A very significant correlation (r?=?0.834, p?<?0.01) was found between the combined final model and the actual number of counted seedlings, illustrating that less than 10?% of the studied area corresponds to sites of very low post-fire pine regeneration.

Conclusions

The combination of GIS models increased the prediction success of different levels of pine regeneration. Low-altitude areas with low grass cover overlying tertiary deposits were proved the most suitable for pine regeneration, while stands developing on limestone proved least suitable. The proposed methodology provides management authorities with a sound tool to quickly assess Aleppo pine post-fire regeneration potential.     

Prediction of planted seedling survival of five Mediterranean species based on initial seedling morphology

Field performance can be predicted by evaluating nursery stock quality, but optimal morphological variables for use in these assessments may vary by species especially under dry Mediterranean conditions. Our objective was to identify initial seedling morphological characteristics that successfully predict field performance of five Mediterranean species (Pinus halepensis, Quercus ilex, Quercus coccifera, Ceratonia silqua and Pistacia lentiscus). Container seedling morphology was evaluated following the nursery phase, and then seedlings were outplanted in the field where field survival was monitored for two successive years. Results indicate that survival can successfully be predicted from seedling initial morphological characteristics for all these species, yet not all the initial characteristics were good predictors. Survival of P. halepensis and Pist. lentiscus seedlings was positively correlated to initial seedling root-collar diameter, total dry weight and Dickson’s quality index, and can be reliably predicted by these variables. In contrast, seedling field survival of the two Mediterranean evergreen oak species was correlated with few initial morphological attributes; initial diameter provided an accurate index to predict second-year outplanting survival for both species while height/diameter was a good survival predictor for Q. coccifera seedlings. For C. siliqua seedlings, seedlings with larger initial diameter and total biomass survive better in the field. Thus, diameter was the common variable that accurately predicted survival for all species, which should be >5 mm for P. halepensis seedlings and >7 mm for the remaining species.     

Reforestation of degraded Kermes oak shrublands with planted pines: effects on vegetation cover, species diversity and community structure

This paper examines the results of plantings of the Mediterranean pine species, Pinus halepensis and Pinus pinea, in a degraded Mediterranean kermes oak (Quercus coccifera) shrubland in Northern Greece, which were accomplished in order to mitigate ecosystem degradation. Plant establishment and the vegetation differences between the degraded ecosystem’s previous state and the new state following reforestation were measured in order to evaluate the effect of reforestation. Monitoring of the seedling survival and growth of the planted species was carried out during the next five years. In the fifth year we conducted botanical inventories in 18 and 15 plots (50 m² in size) from the reforested and control area, respectively. Plant community parameters estimated were: vegetation composition, total plant cover, planted species cover, native woody, herb and grass species cover, plant species richness, Shannon-Weiner index, community structure and dominant plant height. P. halepensis exhibited higher survival and growth than P. pinea. The reforested area exhibited higher plant diversity, higher vegetation cover, taller plants and more complex community structures than the control area, which concludes that plantings of pines can be successfully used in degraded ecosystem reforestation projects, in areas with similar site conditions.     

Phenology and growth dynamics in Mediterranean evergreen oaks: Effects of environmental conditions and water relations

Budburst date and shoot elongation were measured in two mature Mediterranean evergreen oaks (Quercus suber and Quercus ilex) and their relationships with meteorological and tree water status (predawn leaf water potential) data were analysed. Experimental work took place at two sites: Mitra 2 - Southern Portugal (2002-2003) and Lezirias - Central Portugal (2007-2010). Quercus suber phenology was studied at both sites whereas Q. ilex was only studied at Mitra 2. Quercus suber budburst date occurred at a photoperiod around 13.8 h (± 0.26) - late April/early May - and was highly related to the average daily temperature in the period 25 March - budburst date (ca. 1.5 months prior to budburst), irrespective of site location. In that period, budburst date was much more dependent on average maximum than average minimum daily temperature. Base temperature and thermal time for Q. suber were estimated as 6.2 °C (within the reported literature values) and 323 degree-days, respectively. Q. ilex budburst occurred about 6 weeks earlier than in Q. suber (photoperiod: 12.3 h (±0.3)). Relationships of Q. ilex budburst date and temperature were not studied since only 2 years of data were available for this species. Q. suber shoot elongation underlying mechanisms were quite different in the two sites. At Mitra 2 (Q. suber and Q. ilex), there was a considerable tree water stress during the dry season which restricted shoot elongation. Shoot growth was resumed later in the wet autumn when tree water status recovered again. At the Lezirias site Q. suber water status was not restrictive. Therefore, shoot elongation was mainly dependent on nutrient availability in top soil, as suggested by the strong and positive relationships between annual shoot growth and long-term cumulative rainfall (2-4 months) and short-term average temperature (1 month) prior to budburst. Annual shoot elongation at this well-watered site was higher than in Mitra 2, and variability of growth between trees was enhanced after warm, wet springs when shoot elongation was higher. Results obtained are relevant to the carbon balance, productivity and management of evergreen Mediterranean oak woodlands, particularly under the foreseen climate change scenarios.     

Litter flammability in oak woodlands and shrublands of southeastern France

Characterizing the flammability of litter fuels is of major importance for assessing wildland fire ignition hazard. Here we compared the flammability of litter within a mosaic of Quercus suber (cork oak) woodlands and shrublands in a Mediterranean fire-prone area (Maures massif, southeastern France) to test whether the characteristics and the flammability of litter vary with the vegetation types. We tested experimentally the ignitability, the sustainability, the combustibility and the consumability of undisturbed (=non-reconstructed) litter samples with a point-source mode of ignition. Although the frequency of ignition was similar between all the vegetation types, we distinguished four groups having litter of specific composition and flammability: low and sparse shrublands dominated by Cistus species, medium shrublands with cork oak, high Erica shrublands with sparse cork oak woodlands, and mixed mature oak woodlands with Q. suber, Q. ilex and Q. pubescens. As these vegetation types corresponded to a specific range of past fire recurrence, we also tested the effect of the number of fires and the time since the last fire on litter flammability. Litters of plots recurrently burned had low ability to propagate flames and low flame sustainability. We discuss how the recent fire history can modify vegetation and litter flammability, and thus the fire ignition hazard.     

Afforestation method affects the isotopic composition of planted Pinus halepensis in a semiarid region of Spain

We used an isotopic approach to evaluate the effects of three afforestation methods on the ecophysiology of an Aleppo pine plantation in semiarid Spain. The site preparation methods tested were excavation of planting holes (H), subsoiling (S), and subsoiling with addition of urban solid refuse to soil (S + USR). Five years after plantation establishment, trees in the S + USR treatment were over three times larger than those in the S treatment, and nearly five-fold larger than those planted in holes. Differences in tree biomass per hectare were even greater due to disparities in initial planting density and pine tree mortality among treatments. Pine trees in the S + USR treatment showed higher foliar P concentration, δ¹³C and δ¹⁵N than those in the S or H treatments. Foliar δ¹⁵N data proved that trees in the S + USR treatment utilized USR as a source of nitrogen. Foliar δ¹³C and δ¹⁸O data suggest that improved nutrient status differentially stimulated photosynthesis over stomatal conductance in the pine trees of the S + USR treatment, thus enhancing water use efficiency and growth. In the spring of 2002, trees in the S + USR treatment exhibited the most negative predawn water potentials of all the treatments, indicating that the rapid early growth induced by USR accelerated the onset of intense intra-specific competition for water. The results of this study have implications for the establishment and management of Aleppo pine plantations on semiarid soils. Planting seedlings at low density and/or early thinning of pine stands are strongly recommended if fast tree growth is to be maintained beyond the first few years after USR addition to soil. Foliar C, O and N isotope measurements can provide much insight into how resource acquisition by trees is affected by afforestation techniques in pine plantations under dry climatic conditions.     

Canopy cover effects on cellulose decomposition in oak and pine stands

Cellulose mass loss was measured for four levels of canopy cover,i.e., clearcut, 25%, 75%, and uncut, in northern red oak (Quercus rubra) and red pine (Pinus resinosa) stands in northern Lower Michigan, USA. Cellulose mass loss was more rapid in the clearcut and 25% canopy cover treatments than in the 75% canopy cover and uncut treatments. After 4 month incubation of cellulose filter papers, mass loss rates averaged 75.2% in the clearcut, 56.3% in the 25% canopy cover, 46.9% in the 75% canopy cover, and 45.7% in the uncut stands. For the clearcut and the 25% canopy cover treatments, cellulose mass loss in the mineral soil layer was significantly higher than in the forest floor after 2 and 4 months of incubation, while cellulose mass loss of the uncut treatment was significantly lower in the soil layer than in the forest floor after 4 months of incubation. Cellulose mass loss was not significantly different between the oak and the pine stands (p > 0.05), but cellulose mass loss rates in other canopy cover treatments except for the clearcut were generally higher in red oak stands than in red pine stands. These results suggest that canopy manipulation increases cellulose decomposition and may stimulate nutrient cycling process in canopy removal stands. This study was supported in part by USDA Forest Service and Michigan Technological University.     

Weed control modifies Tuber melanosporum mycelial expansion in young oak plantations

Context

Black truffle (Tuber melanosporum) cultivation is a promising agro-forestry alternative for Mediterranean rural areas, but adequate weed control at seedling establishment still remains a challenge in black truffle plantations.

Aims

The aim of this study is to evaluate the effects of several weed control strategies on early development of Quercus ilex seedlings and the symbiotic T. melanosporum.

Materials and methods

In a young black truffle-inoculated holm oak plantation, we assessed for 3 years the effects of two types of mechanical weed control and five mulches in a young Q. ilex plantation inoculated with T. melanosporum. Herbaceous cover, seedling growth and abundance of T. melanosporum mycelium, based on PCR analysis of soil DNA extracts using T. melanosporum-specific primers, were estimated to determine the effectiveness of these treatments in controlling weeds and supporting the growth of both the host tree and the target fungus.

Results

The amount of T. melanosporum mycelium in the soil 30 cm around the seedlings was larger under double-layer white mulch than in the rest of treatments tested. Under the white colour mulches, which had the largest light reflection, we registered the cooler soil temperature, and the best weed control was observed on the single- and double-layer black truffles and double-layer white mulch.

Conclusion

The effects of double-layer white mulch on herbaceous cover, soil temperature, reflected light, and the expansion of T. melanosporum bring us closer to being able to substitute traditional tilling of truffle orchards for the less expensive mulching treatments.     

Spatial pattern of beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) and oak (<Emphasis Type="Italic">Quercus pubescens</Emphasis> Mill.) seedlings in natural pine (<Emphasis Type="Italic">Pinus sylvestris</Emphasis> L.) woodlands

Spatial pattern of recruitment is an important factor influencing population dynamics of plant communities. The spatial pattern is determined by seed dispersal and by the spatial variability of germination and initial survival. In the process of forest expansion following farmland abandonment, mid- and late-successional species are often dispersed in pioneer forests by birds. This could result in an aggregated spatial pattern of seeds that could influence the dynamics of these species in mixed pioneer forests. In the sub-Mediterranean area, mid- and late-successional species such as Quercus pubescens (downy oak) and Fagus sylvatica (European beech) are expected to replace pioneer Pinus species. Using a point sampling method we demonstrated that beech and oak seedlings (height <2 m) have a clumped distribution in the understorey of pine. This could result from an aggregated dispersal by jays (dispersal effect) or from preferential recruitment in particular habitats (habitat effect). To test these hypotheses we proposed a statistical analysis of spatial patterns of regeneration of beech and oak. Ground cover variables (i.e. cover by rock outcrops, herbs, box shrubs, mosses or pine) did not differ significantly around seedlings as compared with random sample plots. Likewise, clumped seedlings had growth similar to isolated seedlings, thus refuting the hypothesis of preferential location in the most favourable microsites. Aggregated dispersal seems to be involved in the process of regeneration. Since beech and oak seedlings have contrasting ecological demands, we discuss the implication of this pattern for the replacement dynamics of pine by these species.     

Viability of holm and cork oak seedlings from acorns sown in soils naturally infected with Phytophthora cinnamomi

The emergence and survival of pregerminated holm oak (Quercus ilex) and cork oak (Quercus suber) acorns from two ecologically different dehesas (Mediterranean open woodlands) were studied in two soils from these stands naturally infected with Phytophthora cinnamomi, and in the same soils previously sterilized in the autoclave. Phytophthora cinnamomi was consistently isolated from the radicles of all unemerged and all emerged but dead seedlings from the unsterilized substrates. Seedlings of holm oak were more susceptible to P. cinnamomi than those of cork oak. Mortality of holm oak seedlings was significantly different depending only on soil treatment (sterilized or unsterilized), and it was 100% in unsterilized soils, independent of acorn provenance and soil origin. Mortality of cork oak seedlings was significantly different depending on the acorn origin and soil treatment, and on the interactions acorn origin × soil origin and soil origin × soil treatment. The demonstrated high susceptibility of holm and cork oak young seedlings to P. cinnamomi could be a limiting factor in Mediterranean open woodlands (dehesas) not only in natural regeneration processes but also when reforestation by direct sowing is implemented.     

Thinning has a positive effect on growth dynamics and growth–climate relationships in Aleppo pine (Pinus halepensis) trees of different crown classes

? Context

Modification of stand density by thinning may buffer the response of tree growth and vigor to changes in climate by enhancing soil water availability.

? Aims

We tested the impact of thinning intensity on cambial growth of Aleppo pine (Pinus halepensis L.) under semi-arid, Mediterranean conditions.

? Methods

A multiple thinning experiment was established on an Aleppo pine plantation in Spain. We analysed the stem growth dynamics of two different crown classes under four different thinning intensities (15 %, 30 %, and 45 % removal of the basal area) for 2 years, based on biweekly band dendrometer recordings. Local relative extractable soil water was derived from the use of a water balance model Biljou© (available at https://appgeodb.nancy.inra.fr/biljou/) and used as an explanatory variable.

? Results

Radial growth was mainly controlled by soil water availability during the growing season, and differed by crown class. The growth rates of dominant trees were significantly higher than the growth rates of suppressed trees. Removal of 30 % and 45 % of the initial basal area produced a growth release in both dominant and suppressed trees that did not occur under less intense thinning treatments.

? Conclusions

Soil water availability was the main driver of radial growth during the growing season. Forest management confirmed its value for ameliorating the effects of water limitations on individual tree growth. These results may help managers understand how altering stand density will differentially affect diameter growth responses of Aleppo pine to short-term climatic fluctuations, promoting forests that are resilient to future climatic conditions.     

Soil water content and water relations in planted and naturally regenerated Pinus halepensis Mill. seedlings during the first year in semiarid conditions

The capacity of Aleppo pine (Pinus halpensis Mill.) seedlings to overcome the planting shock in dry conditions was assessed by firstly studying the survival and water status during the first year after planting in relation to soil water content. In spite of receiving only 177 mm rainfall during the year studied, survival of planted Aleppo pine was very high (88.5%). Soil water during summer months (after receiving 67 mm rainfall in winter and spring) was only available at 30–60 cm depth, with tension values of –1.1 and –1.3 MPa in July and August respectively; in these conditions, July predawn xylem water potential measured was –2.5 MPa, and midday potential was –3.6 MPa. According to different authors, these values don't jeopardize the survival of Aleppo pine. In addition, acclimation of outplanted seedlings to environmental conditions was followed by comparing their water relations with those of naturally regenerated seedlings on the site. Predawn and midday xylem water potential showed differences in favour of outplanted seedlings since June, indicating an adjustment to this dry site. Compared to naturally regenerated trees, nursery grown stock of the same age before field planting had much more biomass and higher N and P concentrations and contents; although shoot:root balance and Dickson quality index were not significantly different. Finally, planted seedlings acclimation level during first year was also evaluated by Transplant Stress Index, which value (–0.1278) indicated a slight planting impact.