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.
•Browsing reduces oak seedling growth rate independently of canopy openness.•Seedlings growing under open canopies are more likely to be browsed.•Fencing only increases oak seedling growth and ...survival under open canopies.•The effect of fencing on growth and survival do not correlate with cervid density.
The recruitment of forest trees is driven by both bottom-up processes (the acquisition of resources) and top-down processes (herbivory). To initiate stand regeneration, foresters commonly reduce tree density to increase light levels for seedlings and enhance primary productivity. These changes in vegetation dynamics, however, could also influence effects of ungulate browsing, resulting in unintended consequences for forest management. Here, we assessed how effects of ungulate exclusion and canopy opening interacted to affect the regeneration of two oak species: Quercus robur and Quercus petraea. We monitored the growth and survival of oak seedlings for two to three growth seasons in paired fenced and unfenced plots under contrasting conditions of canopy openness (8% to 52%) at five sites in southern Sweden and three sites in northeastern France. We scored browsing in the unfenced plots by the four cervids occurring in these areas (Alces alces, Capreolus capreolus, Cervus elaphus and Dama dama). Fencing increased the growth of (mostly taller) seedlings occurring in Sweden and the survival of (mostly smaller) seedlings in France. Both effects increased as canopies became more open. Browsing reduced oak seedling growth in both countries, independently of canopy openness. Canopy openness increased browsing levels in Sweden. Cervid densities did not appear to modify how fencing affected oak seedling growth and survival. In both contrasting forest environments, creating gaps tended to enhance ungulate damage on young forest stands as browsing frequency increased. We conclude that net forest regeneration reflects a subtle equilibrium between enhancing resource availability, boosting seedling growth, and limiting herbivory, which curtails seedling growth and survival.
The constraint caused by wild ungulates on forest regeneration is increasing worldwide. Hypotheses for plant association effects predict that species susceptible to herbivory can gain protection from ...other neighbouring plant species. In theory, such interactions could help limit the impact of browsing on the regeneration of specific tree species. However, the presence of neighbouring species can also result in increasing competition for resources between species. The resultant effects on forest regeneration of these interactions, both positive (protection against herbivores) and negative (inter-specific competition) are still unclear.
To gain insight, we coupled models of browsing by roe deer (Capreolus capreolus) and of forest dynamics to simulate trajectories of oak (Quercus petraea (Matt.) Liebl.) regeneration admixed with species of contrasted palatability and growth rate under different scenarios of browsing pressure and initial sapling density. We also investigated how releasing oak saplings from all or specific neighbours during the simulation affect regeneration.
We found that admixed species composition had a relatively weak effect on the density of oak recruits, but a strong effect on the duration of the regeneration phase. Oak regenerated faster when admixed with species of intermediate growth and low palatability (Fagus sylvatica) than with species of fast growth and high palatability (Carpinus betulus L.), except at intermediate sapling density and high browsing pressure where we found the opposite. Releasing oak from all competitors was most effective in promoting oak regeneration when admixed with both species together, although the benefit of competition release was much weaker at high browsing pressure. Lastly, we found that at low initial sapling density (i.e., 10 saplings/m2), oak regeneration was driven only by browsing and the effect of admixing species became negligible.
Our study showed that admixing oak with palatable neighbours impedes rather than improves oak regeneration due to increased competition for resources. As such, we suggest that the benefits of herbivore diversion can be off-set by increased inter-specific competition.
•In forests, saplings compete for resources and protect each other from herbivory.•We studied the balance between facilitation and competition by neighbour species.•We coupled models of oak regeneration dynamics (RReShar) and of roe deer browsing.•Our simulations suggest that herbivory diversion is generally offset by competition.
This review describes key regeneration characteristics of the genus
Fagus as represented by its four most prominent species:
F. crenata (
F.c.),
F. grandifolia (
F.g.),
F. orientalis (
F.o.) and
F. ...sylvatica (
F.s.). Similarities and differences in the relevant life phases of these species are identified. Those are related to natural disturbance regimes and synecological peculiarities of the forests where they grow, thereby establishing a basis for evaluating the likely outcome of different silvicultural measures.
Common ecological characteristics of these
Fagus species’ life cycles include the masting phenomenon, pollen dispersal with effective distances of about 100
m, seed dispersal to about 20
m, seedling sensitivity to frost, drought, and animal predation, and a very shade tolerant establishment phase. This commonality suggests its appropriateness as a “model-genus”. However, some species also have unique ecological characteristics not observed in the others.
F.g. exhibits root suckering, and beech bark disease seems to trigger vegetative regeneration by that means. Likewise, its masting behaviour deviates from
F.s.
F.o. and
F.c., occurring more frequently and more regularly. In
F.c. forests, dwarf bamboo species and their ecological characteristics are important determinants of tree regeneration establishment.
The small canopy gaps that commonly occur in
Fagus dominated natural forests fit very well with the genus’ regeneration characteristics. These conditions are best duplicated by management measures, which maintain partial overstory shading until the seedlings are large enough for release. However, such a strategy reduces chances to regenerate more light-demanding associated species. Together with differences in landowner objectives, the diversity of ecological conditions within and between the species of
Fagus requires site-specific prescriptions to insure regeneration success, e.g. cutting regimes.
Of particular interest to research are the challenges of managing mixed-species stands for high quality timber production in Central European and Caspian beech forests, the decline of
F.g. and how to deal with the aftermath forest, and effective ways to manage
F.c. in coexistence with dwarf bamboo. Further, the historic dispersal of heavy seeded
Fagus species over long distances is still poorly understood. In addition, since their drought sensitive seedlings may be damaged or killed during extreme weather, research must address the possible effects of global climate change on the regeneration potential of beech forests. Species-bridging research may be needed to address these questions.
Key message
Fifteen species are most susceptible to require vegetation control during tree regeneration in the range of our study. Among these 15 species,
Rubus fruticosus
,
Pteridium aquilinum
, and
...Molinia caerulea
cover each more than 300,000 ha of open-canopy forests.
Context
Vegetation control, i.e., the reduction of competitive species cover, is often required to promote tree seedling establishment during the forest regeneration stage. The necessity to control understory vegetation largely depends on the species to be controlled. In order to plan forest renewal operations, it is critical to identify which species require vegetation control during the regeneration stage and to quantify the forest area affected by these species.
Aims
We aimed at identifying the main species requiring vegetation control and at estimating the forest area they cover at the national level.
Methods
Using National Forest Inventory data, we created four indicators based on two levels of plant cover, cross-referenced with two levels of canopy opening, and compared them to the outcome of a survey of forest manager practices.
Results
The best indicator was the one that represented the proportion of forests with open canopy where the species was present with a large cover in the understory. In non-Mediterranean France, according to the indicator, a total of 15 species were found to frequently require vegetation control during the tree regeneration stage.
Pteridium aquilinum
,
Molinia caerulea
, and
Rubus fruticosus
were the main species, and each covered more than 300,000 ha of forest with open canopies, representing about 13% of the total forest area with open canopies outside of the Mediterranean area.
Conclusions
Forests covered by species requiring vegetation control according to forest managers represent a large share of the forest area undergoing regeneration. This study provides the first list of species that require vegetation control based on a methodological protocol that makes it possible to calculate the area associated with each species.
•We investigated SOC storage in two-species forests and their corresponding monocultures in Europe.•Tree species identity was the main driver of SOC storage, compared with species mixing.•Tree ...species identity influenced SOC storage predominantly in the topsoil layers.•Species mixing effect on deep SOC storage was limited to pine-beech triplets.
Mixed forests are usually associated with higher aboveground carbon storage compared to the corresponding monocultures but information on the impact of tree species mixing on soil organic carbon (SOC) is still limited. Yet, maximizing SOC storage is crucial for ecosystem C sequestration and many other ecosystem services. This study used a triplet approach (ie. two-species mixed stand and respective pure stands at the same site) to assess the impact of tree species identity and mixing on SOC storage in eight pine-oak, eight pine-beech and five beech-oak triplets in Europe. We sampled the forest floor (FF) and 0–40 cm in the mineral soil per 10 cm interval. For each triplet type, we fitted basal area (BA) proportion of one component species (for species identity) and a BA-based plot-level True Shannon Diversity index (for species mixing) as explanatory variables for SOC stocks in linear mixed effects models, which included stone content and plot BA as covariates, and site as a random intercept. Considering the total soil depth (FF + 0–40 cm), species identity effect on SOC stocks was only significant for pine-beech and pine-oak triplets but explained more variability in SOC stocks than species mixing across triplet types. Species mixing effect was not significant for any triplet type in the total soil depth. While species identity consistently drove SOC storage in the topsoil layers across triplet types, species mixing explained more variability in SOC stocks in the deeper soil layers except for pine-oak triplets. The results showed that species identity is a stronger driver of SOC storage than species mixing. While tree species identity effect was strongly related to a conifers vs broadleaves signature, the drivers behind mixing effects remained elusive. The results suggest that targeted selection of tree species could better enhance SOC storage in European forests than a mere increase in species richness.
•Belowground biomass equations for 14 juvenile tree species.•Estimators of root biomass based on tree sapling height, diameter, aboveground biomass.•Broadleaf species with higher root biomass for a ...given dimension.•Trend of increasing relative belowground biomass with increasing light availability.•Height to diameter ratio (HD ratio) negatively correlated to relative belowground biomass.
Just as the aboveground tree organs represent the interface between trees and the atmosphere, roots act as the interface between trees and the soil. In this function, roots take-up water and nutrients, facilitate interactions with soil microflora, anchor trees, and also contribute to the gross primary production of forests. However, in comparison to aboveground plant organs, the biomass of roots is much more difficult to study. In this study, we analyzed 19 European datasets on above- and belowground biomass of juvenile trees of 14 species to identify generalizable estimators of root biomass based on tree sapling dimensions (e.g. height, diameter, aboveground biomass). Such estimations are essential growth and sequestration modelling. In addition, the intention was to study the effect of sapling dimension and light availability on biomass allocation to roots. All aboveground variables were significant predictors for root biomass. But, among aboveground predictors of root biomass plant height performed poorest. When comparing conifer and broadleaf species, the latter tended to have a higher root biomass at a given dimension. Also, with increasing size, the share of belowground biomass tended to increase for the sapling dimensions considered. In most species, there was a trend of increasing relative belowground biomass with increasing light availability. Finally, the height to diameter ratio (H/D) was negatively correlated to relative belowground biomass. This indicates that trees with a high H/D are not only more unstable owing to the unfavorable bending stress resistance, but also because they are comparatively less well anchored in the ground. Thus, single tree stability may be improved through increasing light availability to increase the share of belowground biomass.
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. is 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 study based on 90 mature Scots pine stands along a productivity gradient across Europe.•Growth partitioning became more asymmetric and structuring with increasing site quality.•Mortality ...eliminated predominantly small trees with increasing site quality.•We found the highest size variation on poor sites and the lowest on rich sites.•As a result stand structure became more homogeneous with increasing site quality.
Heterogeneity of structure can increase mechanical stability, stress resistance and resilience, biodiversity and many other functions and services of forest stands. That is why many silvicultural measures aim at enhancing structural diversity. However, the effectiveness and potential of structuring may depend on the site conditions. Here, we revealed how the stand structure is determined by site quality and results from site-dependent partitioning of growth and mortality among the trees. We based our study on 90 mature, even-aged, fully stocked monocultures of Scots pine (Pinus sylvestris L.) sampled in 21 countries along a productivity gradient across Europe. A mini-simulation study further analyzed the site-dependency of the interplay between growth and mortality and the resulting stand structure. The overarching hypothesis was that the stand structure changes with site quality and results from the site-dependent asymmetry of competition and mortality.
First, we show that Scots pine stands structure across Europe become more homogeneous with increasing site quality. The coefficient of variation and Gini coefficient of stem diameter and tree height continuously decreased, whereas Stand Density Index and stand basal area increased with site index.
Second, we reveal a site-dependency of the growth distribution among the trees and the mortality. With increasing site index, the asymmetry of both competition and growth distribution increased and suggested, at first glance, an increase in stand heterogeneity. However, with increasing site index, mortality eliminates mainly small instead of all-sized trees, cancels the size variation and reduces the structural heterogeneity.
Third, we modelled the site-dependent interplay between growth partitioning and mortality. By scenario runs for different site conditions, we can show how the site-dependent structure at the stand level emerges from the asymmetric competition and mortality at the tree level and how the interplay changes with increasing site quality across Europe.
Our most interesting finding was that the growth partitioning became more asymmetric and structuring with increasing site quality, but that the mortality eliminated predominantly small trees, reduced their size variation and thus reversed the impact of site quality on the structure. Finally, the reverse effects of mode of growth partitioning and mortality on the stand structure resulted in the highest size variation on poor sites and decreased structural heterogeneity with increasing site quality. Since our results indicate where heterogeneous structures need silviculture interventions and where they emerge naturally, we conclude that these findings may improve system understanding and modelling and guide forest management aiming at structurally rich forests.
•Procedures, standards and attributes used to control seedling quality vary greatly across Europe.•All countries monitor the origin of FRM as a potential genetic quality indicator.•Morphological ...attribute standards differ among countries.•The use of physiological attributes is very limited to a few countries of northern Europe.•European countries in general do not apply the “target plant concept“, except for the origin of FRM.•This indicates the need to review seedling quality attributes and standards based on scientific knowledge and the need to harmonize some of them between EU countries.
The relationship between the quality of forest seedlings and their outplanting survival and growth has long been recognized. Various attributes have been proposed to measure the quality of planted seedlings in forest regeneration projects, ranging from simple morphological traits to more complex physiological and performance attributes, or a combination thereof. However, the utility and meaning of seedling quality attributes can differ significantly among regions, nursery practices, site planting conditions, species and the establishment purpose. Here, forest scientists compiled information using a common agreed questionnaire to provide a review of current practices, experiences, legislation and standards for seedling quality across 23 European countries.
Large differences exist in measuring seedling quality across countries. The control of the origin of seed and vegetative material (genetic component of plant quality), and control of pests and diseases are common practices in all countries. Morphological attributes are widely used and mandatory in most cases. However, physiological attributes are hardly used at the operative level and mainly concentrated to Fennoscandia. Quality control legislation and seedling quality standards are less strict in northern European countries where seedling production is high, and quality control relies more on the agreements between producers and local plant material users. In contrast, quality standards are stricter in Southern Europe, especially in the Mediterranean countries.
The control of seedling quality based on plantation and reforestation success is uncommon and depends on the conditions of the planting site, the traditional practices and the financial support provided by each country. Overall, European countries do not apply the “target seedling concept” for seedling production except for seed origin. Seedling production in many countries is still driven by traditional “know-how” and much less by scientific knowledge progress, which is not adequately disseminated and transferred to the end-users.
Our review highlights the need for greater harmonization of seedling quality practices across Europe and the increased dissemination of scientific knowledge to improve seedling quality in forest regeneration activities.
PDF
