Catherine Collet

ABSTRACT In recent years, there has been increasing interest in modelling of species abundance data in addition to presence data. In this study, we assessed the similarities and differences between presence-absence distributions and abundance distributions along similar environmental gradients, derived, respectively, from presence-absence and abundance data. Moreover, we examined the possibility of using presence-absence distribution models to derive abundance distributions. For this purpose, we used Braun-Blanquet abundance scores for 243 vascular species at 10 996 French forest sites. Species distribution models were used to analyse the link between the patterns of occurrence, low abundance and high abundance for each species with regard to mean annual temperature, June water balance, and soil pH. For each species, differences in the modelled distributions were characterised by the ecological optimum and ecological amplitude. A comparison of the presence-absence and abundance distributions for all species revealed similar optima and different amplitudes along the three ecological factors. An abundant-centre distribution was observed in environmental space, with species abundance being greatest at the optimal conditions and lower at less favourable conditions of the species occurrence response. Geographical habitat mapping also shows centred, high-abundance suitability within the presence habitat of each species. We conclude that species distribution models derived from presence-absence data provide useful information about the ecological optima of abundance distributions but overestimate the range of habitats suitable for high species abundance. This study demonstrates the utility of presence-absence data for ecologist and conservation biologist when they are interested in the optimal conditions of high species abundance.

The objective of this research was to evaluate the influence of vegetation control on the microclimate (light, air temperature, vapor-pressure deficit (VPD)) and on the growth of young planted, sessile oak seedlings. Three types of vegetation control, creating a range of above-ground interaction intensity, were compared. In the open treatment, vegetation was chemically controlled and was maintained at a low height; in the closed treatment, vegetation was manually controlled and a few dominant individuals that overtopped the oak seedlings were cut; and in the sheath treatment, the vegetation in a 0.5-m diameter circle around each seedling was manually cut to a height equal to three quarters of the total seedling height. In all treatments, a strong reduction of incoming radiation was observed, as well as an increase in air temperature and VPD during the day, and a decrease in daily PET, compared to the reference located 4 m above the vegetation. However, the neighboring vegetation did not modify the microclimate around the seedling to a level that could have induced a significant reduction in seedling photosynthesis and, thus, in seedling growth. Seven years after planting, seedlings were shortest in the open treatment and tallest in the closed treatment (334, 372, 378 cm in the open, sheath and closed treatments, respectively). Seedlings in the open treatment allocated proportionally more biomass to diameter increment than to height growth, compared to seedlings in the sheath and closed treatments. The seedlings averaged 1.77, 1.10 and 1.00 forks in the open, sheath and closed treatments, respectively, and the average height of the lowest fork was 171, 206 and 226 cm in the same treatments. The reduction of the number of forks per seedling was not related to a reduction of the appearance of new forks, but rather to a reduction in the persistence of the existing forks. These changes in stem allometry and in fork development seemed to be related to the lateral shade afforded by the vegetation.

By increasing resource availability, canopy opening enhances tree recruitment as well as the development of neighbouring vegetation. The proliferation of early successional and highly competitive vegetation may have dramatic consequences on seedling establishment. However, differences in competitive abilities have been shown among the plant growth forms commonly encountered in forests. We may thus expect that vegetation management leading to control of different plant growth forms would have different consequences on tree seedling growth and development. To test this hypothesis, we analysed the effects of an intensity gradient of four vegetation control treatments (untreated, coppice control, coppice and non-tree plant control, and coppice, non-tree and pioneer (tree) plant control) on plant species richness and natural tree regeneration in three post-storm sites. Higher plant species richness and a better natural tree regeneration were observed in the more intensive treatments that significantly improved the balance of the relative abundance of tree species. Suppressing the more competitive vegetation, mainly Rubus fruticosus and graminoids, led to the recruitment and growth of tree species sensitive to competition and good tree species diversity. Practical recommendations in terms of vegetation control relative to tree regeneration and plant species richness are given.

ABSTRACT This study aimed at evaluating alternative methods to ensure regeneration success in temperate low-mountain forest stands by (1) estimating the effects of seed availability, competition from the adult stand and from neighbouring vegetation and interaction with the litter layer on seedling density, and by (2) comparing the effects of various silvicultural methods on regeneration success. The experiment was conducted in a monospecific beech (Fagus sylvatica) stand and a mixed silver fir-beech (Abies alba, Fagus sylvatica) stand with contrasted vegetation communities, in north-east France. Different methods of soil preparation, i.e. chemical (herbicide), mechanical (surface hoeing and deep scarification using light-weight machines) and biological (cover crops after surface hoeing) methods, were applied along a canopy opening gradient. After soil preparation (in 2009), vegetation colonisation and tree seedling density were monitored once a year from 2010 to 2013. Results were similar for the two sites. Seedling density the first year indicated a predominant effect of seed availability and soil scarification over potential competitive effects of adult stand and neighbouring vegetation. Despite continuous vegetation colonisation after soil preparation, seedling density remained stable over the 4 years of the experiment. For each of the 4 years, seedling density increased with canopy cover. Seedling density was higher after mechanical soil preparation than after herbicide application. Cover crops (following surface hoeing) appeared as the best method, ensuring both the lowest vegetation colonisation and the highest seedling density.

• In mixed-species forest stands, large losses in tree species diversity often occur during the regeneration phase. In a former coppice-with-standards, we investigated whether the limiting stage in the recruitment process of advance regeneration is the immediate seedling response to canopy release. Experimental canopy gaps were opened and the survival and growth of advance seedlings (Fagus sylvatica, Acer pseudoplatanus, Acer campestre, Acer platanoides) growing in the gaps or under closed canopy were monitored for three years. • All species responded positively and rapidly to canopy release. Survival was not affected by gap opening. Diameter increment after gap opening was similar across species, and height increment was greater for Acer platanoides and for Acer pseudoplatanus. Post-release diameter and height growth were mainly determined by pre-release seedling size. Competition from neighbouring seedlings did not affect growth in the three years following canopy opening. • In the recruitment process of F. sylvatica and Acer sp. advance regeneration, the recovery from canopy release did not appear as a limiting step that would filter against some species. Pre-release size was the main factor accounting for post-release growth and is probably a major determinant of long-term seedling dominance. • Dans les peuplements mélangés, une forte réduction de la diversité spécifique a lieu au cours de la phase de régénération. Nous avons examiné dans un ancien taillis-sous-futaie dans quelle mesure la réponse immédiate des semis à l’ouverture du couvert adulte pouvait limiter le recrutement de semis préexistants. Des trouées ont été ouvertes dans le couvert, et la survie et la croissance de semis préexistants (Fagus sylvatica, Acer pseudoplatanus, Acer campestre, Acer platanoides), situés dans les trouées ou bien sous couvert fermé, ont été suivies pendant trois années. • Les quatre espèces ont répondu positivement et rapidement à l’ouverture du couvert. La survie n’a pas été affectée par l’ouverture. Pour les semis dans les trouées, l’accroissement en diamètre était similaire pour les quatre espèces et l’accroissement en hauteur était plus important pour Acer platanoides et Acer pseudoplatanus. La croissance en hauteur et en diamètre après ouverture étaient principalement déterminées par les dimensions des semis avant ouverture. La compétition exercée par les semis voisins n’a pas affecté significativement la croissance dans les trois années qui ont suivi l’ouverture. • La période d’acclimatation des semis aux nouvelles conditions après ouverture de la canopée n’est pas apparue comme un stade limitant le recrutement des semis préexistants de Fagus sylvatica, Acer sp., ni comme un filtre vis-à-vis de l’une des quatre espèces. Les dimensions initiales des semis constituaient le facteur principal expliquant la croissance après ouverture, et probablement un facteur explicatif important pour la dominance future des semis vis-à-vis de leurs voisins.

The objective of this study was to determine if various morphological components of height growth were sensitive to environmental factors (i.e., grass root competition, frost and shading) and to see if these components could be used to quantify the effect of environmental stress on the height growth of sessile oak (Quercus petraea Liebl.) seedlings. The following components were measured: number of flushes produced on the main stem, length of the growth unit (GU) developed during each flush, annual GU length, number of nodes per GU, and state of the terminal bud of each GU. The number of flushes produced was reduced by all environmental treatments. The annual GU length was positively related to the number of flushes produced. Grass competition reduced GU length in each flush and therefore reduced annual GU length. On the other hand, partial shading increased GU length in each flush and annual GU length. Number of nodes was not apparently affected by environmental factors and was positively related to GU length. Where factors that affect terminal bud mortality were partially controlled (i.e., in grass competition and shading experiments), mortality rate was low (5% for GUs of the first flush in grass competition experiment), but in the frost experiment terminal bud mortality rate was higher (64% for GUs of the first flush). The sensitivity of the different morphological components to environmental factors and the quantification of the effects of the factors using generalized linear models were discussed.

ABSTRACT The objective of the study was to analyse to which extent horizontal crown plasticity reduces inter-tree competition at stand scale, and how it relates to species growth strategy. Two components of crown plasticity defined at the individual tree level (crown shape distortion, CSD and crown displacement relative to stem, CRD) were analysed and their relative importance in the reduction of competition was quantified. Inter-tree competition at stand scale was estimated using spatial pattern analysis and crown overlap estimation. Measurements were performed in a mixed broadleaved stand of Western Europe. Crown plasticity was shown to regularise the spatial distribution of crowns in comparison with the corresponding stems and to reduce inter-tree competition by optimising space occupation. A significant reduction in crown overlap was observed, mainly due to CRD and secondarily to CSD. At the species level, CSD and CRD were positively correlated. In addition, both were negatively correlated with species shade-tolerance scores. In particular, three European temperate tree species showed contrasting responses, which were related to their known specific ecological strategies. Fagus sylvatica, known to be a highly shade-tolerant species, showed large crowns, low CSD and CRD, indicating a low plasticity and suggesting a strong competitive ability. At the opposite, Quercus sp., known to be clearly less shade-tolerant, exhibited a reduced growth, associated with high CSD and CRD, indicating high crown plasticity and probably a lower competitive ability. For this species, plasticity could be described as passive. Last, Carpinus betulus, known to have a relatively good shade-tolerance, showed a contrasting behaviour with high CSD and CRD and a strong presence in the stand, suggesting high crown plasticity and a good competitive ability. In that case, plasticity was described as rather adaptive.

ABSTRACT Although light is a key factor in forestry, it is surprisingly seldom measured in day-to-day management of European forests. The spherical convex densiometer is a simple instrument that allows to evaluate the canopy openness (CO) by counting the number of ‘canopy’ dots on a grid lying on a convex mirror reflecting the canopy. In this contribution, we compared the performances of this instrument in mixed oak–beech hardwood forests spanning the lower end of the light gradient [1–17 % above canopy photosynthetically active radiation (PAR)] to two reference techniques: PAR measurements and fish-eye photography, based on a detailed analysis of its functioning. Discrepancies between the densiometer and the fish-eye estimates of CO were due to a combination of differences in dot resolution, dot counting and portion of the hemisphere considered. By contrast, the various effects of operator on densiometer estimates, including the influence of conformation on the angle of view, were found to be relatively minor. Densiometer readings were closely related to the relative light intensity assessed by PAR sensors in overcast conditions, which suggests that the use of this inexpensive tool should be expanded.

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ABSTRACT The area of forests planted with exotic tree species is increasing worldwide in order to fulfill various economic and environmental demands. Numerous species currently used in forest plantations are considered to be invasive in many parts of the world. Exotic plantation tree species are endowed with a series of life-history traits that are characteristic of invasive species: easy establishment, fast growth, high propagule pressure, and low or intermediate shade tolerance. In addition, plantation forestry presents many features that increase ecosystem invasibility, including a regime of frequent and intensive canopy disturbances, a dense network of roads and trails with continuous traffic, and wide plantation areas. Exotic trees planted for production purposes have strong direct positive economic impacts on the local and national economies of many countries, often leading to notable conflicts of interest when the species becomes invasive, as well as to negative impacts on the ecosystem. Studies and management programs mainly focus on Pinus and Acacia, which are used as model species. Complementary management strategies have been established to control these species at different stages of the invasion process. Knowledge gained is useful to predict problems in other regions of the world with the same species and to guide research or management actions on other problematic but less studied tree species.