Abstract• It is agreed that climate (precipitation and temperature) influences the distribution of plant species. Near the margins of a species' natural range, climate becomes limiting to physiological processes. There, climate change may be expected to have a significant impact on tree growth and the species' ranges may be altered.• In order to assess what influence climate change could exert on the distribution of pine species at their margin, radial growth trends in ring-width chronologies over the last century were analysed. In the French Mediterranean area where climate change is characterized by increased temperature, forest plots were selected along an altitudinal transect on the north-facing slope of the Sainte-Baume mountain (Bouches-du-Rhône, France) where the ranges of Pinus sylvestris and Pinus halepensis overlap.• Two growth patterns were identified. For P. halepensis, radial growth has increased at all altitudes indicating that climate change has improved growth conditions of this species near the margin of its ecological range. For P. sylvestris, radial growth has increased only at low altitudes and even decreased at high altitudes.• It must be deduced that the growth changes observed cannot be generalised either at the species level or at the geographical level and must be interpreted with great caution. Mots-clés :changement global / limite bioclimatique / croissance radiale / Pinus halepensis / Pinus sylvestris Résumé -Les changements globaux ont-ils induit des tendances de croissance radiale opposées chez Pinus sylvestris et Pinus halepensis à leur limite bioclimatique ? L'exemple de la forêt de la Sainte-Baume.• Il est admis que les facteurs climatiques (températures et précipitations) sont en partie responsables de la distribution des espèces en limitant leurs processus physiologiques en limite de distribution d'aire. Dans ce cas, les conséquences des changements climatiques sur la croissance sont susceptibles d'y être le plus marqué et leurs distributions pourraient varier.• Afin d'évaluer les conséquences des changements climatiques, les tendances de croissance observées au siècle dernier dans les séries temporelles de cernes ont été analysées. En zone médi-terranéenne française, où les changements climatiques sont caractérisés par une augmentation des températures, des placettes ont été sélectionnées selon un gradient altitudinal sur le flanc nord de la montagne de la Sainte-Baume (Bouches-du-Rhône, France) où les distributions de Pinus sylvestris et Pinus halepensis se superposent.• Deux patrons de croissance ont été identifiés. La croissance radiale de P. halepensis a augmenté quelle que soit l'altitude considérée indiquant que le changement climatique a amélioré les conditions de croissance de l'espèce à sa limite de distribution. Chez P. sylvestris, des tendances de croissance opposées inattendues ont été mises en évidence : la croissance a augmenté à faible altitude et a diminué aux altitudes les plus élevées.• Il en résulte que les changements de croissance observés ne peuvent être ...

No abstract

The goal of this study was to develop management strategies favouring establishment and survival of holm oak (Quercus ilex L.) and downy oak (Quercus pubescens Willd.)-two species co-occurring in Southern France-in mature Allepo pine forests (Pinus halepensis Mill subsp. halepensis). An experimental design was assessed in a partially cut mature stand of Aleppo pine in which five soil and vegetation treatments-chopping, chopping followed by scarification in one or two directions, prescribed burning, control-and two slash treatments (presence/absence) were applied. A total of 1,600 sowing points, each composed of 3 Q. ilex or Q. pubescens acorns, were installed in the different treatments in November of two consecutive years at 6 and 18 months, after the end of treatments. Survival was monitored 3 and 2 years after sowing, soil surface at the sowing points was characterized at different dates, and predawn leaf water potentials were measured during the dry season. High mortality occurred after the first summer, but survival after 1 year was 2.3-5.2 higher in Q. ilex than in Q. pubescens, confirming that Q. ilex was better adapted to the drier parts of the Mediterranean area. Survival was significantly influenced by the treatments, but there was a variable response between the two sowing years under most of the treatments. Only intense fire proved the most beneficial treatment for seedling survival in both years. The micro-local soil cover conditions induced by the treatments played a major role in explaining oak survival. In particular, grass cover (mainly Brachypodium retusum) proved to be largely unfavourable to seedling survival and growth, and this detrimental effect was also confirmed by lower predawn leaf water potential values with increasing grass cover. Acorn introduction designed to diversify mature Aleppo pine forest after soil and vegetation treatments therefore has to be considered for treatments that most efficiently impair the pre-existing competing grass cover such as prescribed high-intensity fire treatment.

Summary1. Better knowledge of plant interactions is essential to understanding plant community dynamics and has broad applications in restoration and forest operations. Yet studies investigating positive and negative interactions, and indirect interactions, are scarce. 2. We quantified the nature and intensity of plant interactions between target species, neighbours and ground vegetation in an experimental plantation in southern France to detect competition, direct and indirect facilitation and shifts in interactions with time. 3. Mediterranean oak seedlings (Quercus pubescens and Quercus ilex) were planted as target species within the following neighbourhoods: pine trees Pinus halepensis at two densities, N-fixing shrubs (Coronilla valentina), mixed pines and shrubs, and controls. The ground vegetation was either weeded or unweeded. We computed the relative interaction index (RII) for survival, growth and elongation of the target species in different treatments over 3 years. Light and soil water content were also monitored. 4. Results showed that the outcome of plant interactions depended on the variable selected to evaluate plant response. Decrease in Quercus pubescens survival and in diameter growth of both Quercus species indicated competition, whereas enhanced Quercus elongation and higher Quercus pubescens height growth in the first year indicated facilitation. 5. Indirect facilitative interactions also occurred, resulting in the alleviation of competition by neighbours in the unweeded treatment. This result was explained by the reduction of herb competition caused by shade from neighbours. 6. With time, competitive interactions became dominant, but were strongly modulated by the type of neighbours. Competition was more severe with shrub neighbours than with pine neighbours due to much greater light interception and also higher water uptake. 7. Synthesis and applications. We show that the outcome of plant interactions is species specific and varies with time and the indicator selected. Facilitation was detected with tree neighbour treatment, but over time interactions clearly shifted to competition. Hence, forest planting operations can benefit from the interactions induced by neighbouring woody vegetation provided that neighbour species are carefully selected and active post-planting management is performed.

International audienceThe 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 (G = 32 m2/ha, irradiance 13%), a medium shelterwood (G = 19 m2/ha, irradiance 33%) and a dense shelterwood (G = 10 m2/ha, irradiance 52%). 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 three years and plant stress was assessed by measuring predawn leaf potential and photosynthetic performance through the Fv ∕Fm 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 by light availability during the three years of the study. However, as lower soil moisture and faster understorey development were also recorded in this condition, more extended observation is needed to determine whether this benefit persists in subsequent years