TOIGO Maude

The effect of species diversity on forest productivity and its temporal stability is known to be species-, climateand site-dependent and is mostly apprehended through stem diameter. Therefore, it remains largely unknown whether the mixture effect on the growth of tree crowns is similar to its effect on the growth of tree diameter. However, it is commonly accepted that changes in crown architecture are an important component of tree response to tree species diversity. Moreover, the mixture effect on species is often asymmetric, i.e. the effect of a species A on a species B is not equal to the effect of species B on A. It then appears that considering the effects of both species mixture and climate on shoot growth could contrast the results coming mainly from stem growth. We studied the effects of tree species mixture and temperature on the annual growth of shoots and basal area of stems in Fagus sylvatica-Quercus pubescens and Fagus sylvatica-Abies alba stands along a Mediterranean-Alpine gradient, for four years in five sites. The sample design was organized in 10 triplets: four triplets of mono-and bispecific plots of Quercus pubescens and Fagus sylvatica and six triplets of mono-and bi-specific plots of Abies alba and Fagus sylvatica along an altitudinal gradient ranging from 725 m to 1431 m. We found that the mixture effect on annual shoot volume increment (SVI) and on basal area increment (BAI) was asymmetrical in seven out of 10 cases and not significant in the three remaining cases. Mixture effect on SVI ranked from − 56% to 157% and on BAI it ranked from − 40% to 252%. Eventually we found that mixture effect was dependent on the type of limiting factor for growth, with at the driest sites a predominance of competition effects and at the coldest site a positive mixture effect on the two species studied. Branch growth appears as a variable that can be at least as informative as radial growth regarding the tree response to species interactions. This implies that considering only stem diameter in the diversity-productivity relationship can lead to biased conclusions on the global mixture effect on tree growth, which calls for a comprehensive approach of the tree response to tree species diversity. Our results are discussed in the light of the species stress tolerances and strategies to cope with competition.

The vitality of forests depends on multiple biotic and abiotic stresses that can act in an interactive way. Tree defoliation levels are therefore subject to long-term changes and shorter-term hazards that need to be monitored in the context of ongoing climate change. In this study, we analysed the combined effects of drought, insect attack and tree nutritional status on the level of defoliation in three major hardwood species in Europe: European beech (Fagus sylvatica), sessile oak (Quercus petraea) and pedunculate oak (Quercus robur). The forest condition data set was collected in the long-term intensive monitoring network in France (RENECOFOR). The average level of defoliation observed was 20% but varied considerably between plots and trees. From 1997 to 2015, beech defoliation worsened by 10% while oak defoliation stabilized over time. For the three tree species, crown defoliation was generally higher in the event of insect attack (increased by 2.6% to 7.5% depending on the tree species) and under drought (increased by 5.9%), while the effect of the tree nutritional statue was less consistent (from - 5.6% to + 10%) these factors acting synergistically. This study highlights the fact that the different stresses a tree species undergoes act in a complex and interactive way, with species-specific responses. In a context of increasing abiotic and biotic stresses, their combined analysis appears to be a necessity at a time when forest owners need to adapt their management to cope with climate change, particularly through the choice of tree species to be favoured in the future.

Background and Aims In plants, high costs of reproduction during some years can induce trade-offs in resource allocation with other functions such as growth, survival and resistance against herbivores or extreme abiotic conditions, but also with subsequent reproduction. Such trade-offs might also occur following resource shortage at particular moments of the reproductive cycle. Because plants are modular organisms, strategies for resource allocation to reproduction can also vary among hierarchical levels. Using a defoliation experiment, our aim was to test how allocation to reproduction was impacted by resource limitation. • Methods We applied three levels of defoliation (control, moderate and intense) to branches of eight Quercus ilex trees shortly after fruit initiation and measured the effects of resource limitation induced by leaf removal on fruit development (survival, growth and germination potential) and on the production of vegetative and reproductive organs the year following defoliation. • Key Results We found that defoliation had little impact on fruit development. Fruit survival was not affected by the intense defoliation treatment, but was reduced by moderate defoliation, and this result could not be explained by an upregulation of photosynthesis. Mature fruit mass was not affected by defoliation, nor was seed germination success. However, in the following spring defoliated branches produced fewer shoots and compensated for leaf loss by overproducing leaves at the expense of flowers. Therefore, resource shortage decreased resource allocation to reproduction the following season but did not affect sex ratio. • Conclusions Our results support the idea of a regulation of resource allocation to reproduction beyond the shoot scale. Defoliation had larger legacy effects than immediate effects.

Abstract Background and Aims In plants, high costs of reproduction during some years can induce trade-offs in resource allocation with other functions such as growth, survival and resistance against herbivores or extreme abiotic conditions, but also with subsequent reproduction. Such trade-offs might also occur following resource shortage at particular moments of the reproductive cycle. Because plants are modular organisms, strategies for resource allocation to reproduction can also vary among hierarchical levels. Using a defoliation experiment, our aim was to test how allocation to reproduction was impacted by resource limitation. Methods We applied three levels of defoliation (control, moderate and intense) to branches of eight Quercus ilex trees shortly after fruit initiation and measured the effects of resource limitation induced by leaf removal on fruit development (survival, growth and germination potential) and on the production of vegetative and reproductive organs the year following defoliation. Key Results We found that defoliation had little impact on fruit development. Fruit survival was not affected by the intense defoliation treatment, but was reduced by moderate defoliation, and this result could not be explained by an upregulation of photosynthesis. Mature fruit mass was not affected by defoliation, nor was seed germination success. However, in the following spring defoliated branches produced fewer shoots and compensated for leaf loss by overproducing leaves at the expense of flowers. Therefore, resource shortage decreased resource allocation to reproduction the following season but did not affect sex ratio. Conclusions Our results support the idea of a regulation of resource allocation to reproduction beyond the shoot scale. Defoliation had larger legacy effects than immediate effects.

Key message: In this exploratory study, we show how combining the strength of tree diversity experiment with the long-term perspective offered by forest gap models allows testing the mixture yielding behavior across a full rotation period. Our results on a SW France example illustrate how mixing maritime pine with birch may produce an overyielding (i.e., a positive net biodiversity effect). Context: Understanding the link between tree diversity and stand productivity is a key issue at a time when new forest management methods are investigated to improve carbon sequestration and climate change mitigation. Well-controlled tree diversity experiments have been set up over the last decades, but they are still too young to yield relevant results from a long-term perspective. Alternatively, forest gap models appear as appropriate tools to study the link between diversity and productivity as they can simulate mixed forest growth over an entire forestry cycle. Aims: We aimed at testing whether a forest gap model could first reproduce the results from a tree diversity experiment, using its plantation design as input, and then predict the species mixing effect on productivity and biomass in the long term. Methods: Here, we used data from different forest experimental networks to calibrate the gap model ForCEEPS for young pine (Pinus pinaster) and birch (Betula pendula) stands. Then, we used the refined model to compare the productivity of pure and mixed pine and birch stands over a 50-year cycle. The mixing effect was tested for two plantation designs, i.e., species substitution and species addition, and at two tree densities. Results: Regarding the comparison with the experiment ORPHEE (thus on the short term), the model well reproduced the species interactions observed in the mixed stands. Simulations showed an overyielding (i.e., a positive net biodiversity effect) in pine-birch mixtures in all cases and during the full rotation period. A transgressive overyielding was detected in mixtures resulting from birch addition to pine stands at low density. These results were mainly due to a positive mixing effect on pine growth being larger than the negative effect on birch growth. Conclusion: Although this study remains explorative, calibrating gap models with data from monospecific stands and validating with data from the manipulative tree diversity experiment (ORPHEE) offers a powerful tool for further investigation of the productivity of forest mixtures. Improving our understanding of how abiotic and biotic factors, including diversity, influence the functioning of forest ecosystems should help to reconsider new forest managements optimizing ecosystem services.

More diverse forests are generally more resistant to insect herbivores. This might be due to positive effects of tree diversity on predation. Although the enemies hypothesis has received conflicting evidence in forest ecosystems. Carabids were sampled by pitfall trapping in a tree diversity experiment, at the centre of plots ranging from one to five tree species mixtures. The composition and vertical structure of the vegetation was assessed at three scales, in the understorey, in the canopy of the experimental plots, and in the surrounding area of each plot. None of the tested vegetation variables had an effect on the species richness of carabids. In contrast, the vegetation compositional diversity at the understorey, canopy and surrounding scales had additive and positive effects on the activity-density of the carabids. Our findings indicate that more diverse forests can host a higher activity-density of predatory carabids, as a result of the combined effect of horizontal and vertical vegetation diversity, which might increase both habitat quality and the amount of feeding resources. This highlights the relevance of manipulative tree diversity experiments to identify the ecological filters shaping local carabid communities.

Tree diversity is one of the drivers of forest resistance to herbivores. Most of the current understanding of the diversity resistance relationship comes primarily from short-term studies. Knowing whether tree diversity effects on herbivores are maintained over time is important for perennial ecosystems like forests. We addressed the temporal dynamics of the diversity resistance relationship by conducting a 6-year survey of pine attacks by the pine processionary moth Thaumetopoea pityocampa (PPM) in a tree diversity experiment where we could disentangle tree composition from host density effects. During the first years after planting the trees, PPM attacks on maritime pine Pinus pinaster were reduced in the presence of birch Betula pendula, a fast-growing non-host tree (i.e. associational resistance). This effect was maintained but faded with time as the pines eventually grew taller than neighbouring birches. The number of repeated attacks on individual pine trees also decreased in mixed pine-birch stands. Pine density had a positive effect on stand colonisation by PPM and a negative effect on the proportion of trees that were attacked. Pines were less likely to be repeatedly attacked as pine density increased, with attacks being spread over a larger number of host trees. Collectively, these results unravel the independent contribution of tree species composition and host density to tree resistance to herbivores. Both processes had directional changes over time. These results indicate that the resistance of planted forests to herbivores can be improved by carefully choosing the composition of mixed forests and the timing of species planting.

No summary available.

The spatial distribution and niche differentiation of three closely related species (Erysiphe alphitoides, Erysiphe quercicola and Erysiphe hypophylla) causing oak powdery mildew was studied at scales ranging from the European continent, where they are invasive, to a single leaf. While E. alphitoides was dominant at all scales, E. quercicola and E. hypophylla had restricted geographic, stand and leaf distributions. The large-scale distributions were likely explained by climatic factors and species environmental tolerances, with E. quercicola being more frequent in warmer climates and E. hypophylla in colder climates. The extensive sampling and molecular analyses revealed the cryptic invasion of E. quercicola in nine countries from which it had not previously been recorded. The presence of the three species was also strongly affected by host factors, such as oak species and developmental stage. Segregation patterns between Erysiphe species were observed at the leaf scale, between and within leaf surfaces, suggesting competitive effects.

Presentation during the signature of the framework agreement of works carried out within the framework of the particular agreement "Growth and production of mixed forests".

Key message: Mixing sessile oak and Scots pine in central France to reduce intraspecific competition for water resources did not improve the ability of these two species to withstand severe drought during the summer. Context: In order to reduce the impact of increasingly extreme droughts on forests, managers must adapt their practices to future climate conditions. Maintaining a greater diversity of tree species in temperate forest ecosystems is one of the recommended options. Aims: We addressed how interactions between sessile oak and Scots pine in mixed forests in central France affect their functional response to drought. Methods: We characterized the carbon isotope composition (δ 13 C) in the tree growth rings formed during wet (2001, 2007) or dry (2003, 2004) summers for each of the two species growing both in pure and in mixed stands in order to compare the effect of stand composition on variations in carbon isotope discrimination (Δ 13 C) among contrasted years. Results: The severe drought in 2003 induced a strong decrease in Δ 13 C for all trees and in all stands as compared to 2001. This decrease was greater in pine than in oak. There was no significant difference between pure and mixed stands in the response of either species to drought. Conclusion: Mixing sessile oak and Scots pine in stands in central France does not improve the ability of either species to withstand severe drought during the summer.

1. Assessing how species productivity in mixtures is influenced by species shade tolerance (ST) and phylogeny would be helpful to improve our general understanding of the relationship between tree species diversity and productivity in forests. 2. We investigated the effects of differences in ST and phylogenetic distances (PDs) between pairs of species on the productivity of Quercus petraea growing in 18 mixtures in lowland temperate forests. We calculated the mixture effect as the difference in productivity of Q. petraea in mixed vs. pure stands. Our analyses were based on data from seven annual campaigns of the French National Forest Inventory covering 1,573 plots. 3. The mixture effect on Q. petraea productivity increased when the ST of the companion species decreased. Compared to its productivity in pure stands, Q. petraea productivity in mixed coniferous stands varied from from.

The effects of mixed stands on biodiversity are increasingly being studied since they are supposed to offer higher habitat heterogeneity. Nevertheless, for tree-associated diversity, including epiphytes and terricolous species near tree trunks, few studies exist, and still fewer compare mixed stands with each corresponding pure tree species stand. We evaluated and quantified the influence of forest composition on tree-associated bryophyte diversity (species richness, abundance, composition) in mixed and pure oak-pine stands in a French lowland forest. The main explanatory variables for bryophyte diversity at tree-level were the identity of the phorophyte tree species and the mixture type (pure versus mixed). At the plot level, the main explanatory variable was the stand type (pure oak, pure pine and mixed). We also explored the role of other variables including the chemistry of the bryophyte substrates (soil, bark) and water supply (stemflow, throughfall), as well as stand abundance variables (basal area, interfering plant cover). We analyzed data with Generalized Linear Models under Bayesian statistics, to take into account the spatial autocorrelation between plots and any under- or over-dispersion of our data. At the tree-level, bryophyte richness and abundance were higher on oak than on pine. Pine bryophyte richness was higher in mixed compared to pure stands, whereas for oak, mixed stands did not enhance bryophyte richness. At the plot level, mixed stands hosted bryophyte communities of similar richness to those in pure oak stands, whereas pure pine stands were clearly poorer. Our exploratory models suggested strong effects of water supply chemistry (stemflow and throughfall pH or conductivity) and basal area. the latter had a strong quadratic effect on epiphytic richness at the plot level. In terms of composition, three species were more likely to be found on pine phorophytes, seven species clearly occurred more frequently on oaks. Some species were more likely to be associated to pine in mixed than in pure stands, and one species was found more often under pines in pure stands. Therefore, bryophyte diversity at the landscape level should benefit from the simultaneous presence of the three stand composition types - pure oak, pure pine and mixed stands.

Geographical variation in forest insect sensitivity to environmental conditions is often overlooked. Yet considering regional variation in response to climate is necessary to better predict the consequences of climate change on pest population dynamics and associated damage. We investigated the climatic drivers of demographic fluctuations in a forest tree defoliator, the pine pro cessionary moth (PPM), in 8 biogeographical regions in France. We studied the effects of precipitation and temperature for both cold and warm periods, in accordance with PPM biology. We trained second-order log linear models of infestation dynamics based on records spanning over 32 years (1981-2014) and 1239 forest plots, and tested their forecast accuracy on two additional years (2013-2014). PPM population growth rate decreased with higher precipitations in 5 regions and increased with increasing temperatures in the cold period in 4 regions. The magnitude and direction of temperature effects in the warm period varied among biogeographical regions. Our results also revealed that PPM population dynamics were largely triggered by density-dependent factors independent from climatic drivers. Short-term forecasts were generally accurate irrespective of the inclusion of climatic variables into the models. To better predict the consequences of climate change on insect population dynamics, it is advisable to take geographical variability of species response into account and develop regional forecasting models.

The effects of mixed stands on biodiversity are increasingly being studied. Nevertheless, among the few studies dealing with epiphyte taxa, even fewer compare mixed stands with the pure stands of each species. We evaluated the diversity of tree-dwelling bryophyte communities in mixed and pure oak-pine stands (Quercus petraea (Matt.) Liebl., Pinus sylvestris L.) in French lowland forests. The main explanatory variables were the phorophyte species and the stand type (pure versus mixed) at tree level, the stand composition (pure oak, pure pine and mixed) at plot level. In addition, we investigated the role of the chemistry of the bryophyte substrates (soil, bark) and water supplies (stemflows, throughfalls), the basal area and interfering plant cover. Data were analyzed using Generalized Linear Models under Bayesian statistics. Richness and abundance of bryophytes were significantly and strongly higher on oak. Pine bryophyte richness at tree level was higher in mixed compared to pure stands, whereas mixed stands did not enhance oak bryophyte richness. Three species had a clearly higher probability to be found on pine, whereas seven species occurred clearly more frequently on oaks. Some species had a higher probability to be associated to pine in mixed stand than in pure stands, and one species was more prone to be found on pine within pure stands. As a conclusion, bryophyte biodiversity at the landscape level should benefit from the simultaneously presence of the three stand types: pure oak, pure pine and mixed. Exploratory models suggested strong effects of water supply chemistry which could explain the phorophyte species effect on bryophyte diversity. Basal area had a strong quadratic effect on plot epiphytic richness, and the best models for bryophyte diversity per tree always promoted the basal area effect in addition to (or interaction with) the main species and composition effects.

No summary available.

Global environmental changes can result in a modification of abiotic conditions and biological diversity. The production function of forest ecosystems has the particularity of being both subject to and regulating these environmental changes. It is therefore essential to understand how species diversity and abiotic factors control this function. In my work, I studied the variation in the effect of species mixing on their productivity in a changing climatic and edaphic context. Using a double approach, based on local data and data from the National Forest Inventory, I focused on five major European forest species in pure and bispecific stands: Quercus petraea, Pinus sylvestris, Fagus sylvatica, Abies alba and Picea abies. In lowland areas, I show that the effect of species mixing varies little with abiotic factors. Moreover, this effect is determined by the shade tolerance of the accompanying species. In the mountains, the effect of species mixing on productivity is stronger and more positive when abiotic factors are the most limiting for growth. These results illustrate the interest of integrating abiotic factors and functional characteristics of species as determinants of the effect of biological diversity on ecosystem functions.

There is a rising interest in the role of species diversity in ecosystem functioning and services, including productivity. Yet, how the diversity-productivity relationship depends on species identity and abiotic conditions remains a challenging issue. We analysed mixture effects on species productivity along site productivity gradients, calculated from a set of abiotic factors, in two biogeographic contexts (highlands and lowlands). We compared the productivity of 5 two-species mixtures (i.e. 10 cases of mixed species) with that of monocultures of the same species. Five main European tree species were considered: sessile oak (Quercus petraea Liebl.), Scots pine (Pinus sylvestris L.), European beech (Fagus sylvatica L.), silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) H. Karst). Our data set was compiled from the 2006 to 2010 French National Forest Inventory data base and covers 2361 plots including pure and mixed stands. Overall productivity of mixtures in highlands, that is European beech-Norway spruce, European beech-silver fir and to a lesser extent, silver fir-Norway spruce, was found to be higher than expected from the productivity of corresponding monospecific stands. Overyielding was mainly due to European beech for the first two mixtures and to silver fir for the third one. No effect of mixture was found for sessile oak-Scots pine and sessile oak-European beech stands in lowlands. Overyielding of sessile oak mixed with Scots pine was not strong enough to significantly increase overall stand productivity. Overyielding of European beech was balanced by an underyielding of sessile oak. The mixture effect changed along site productivity gradients for six cases out of the 10 studied, with a stronger and positive effect on sites with low productivity. The magnitude of this change along site productivity gradients varied up to 89% depending on the tree species.Synthesis. The nature of species interaction in mixtures with regard to productivity changes with species assemblage and abiotic conditions. Overyielding is strongest when species grow in highlands on less productive sites. A negative link between mixture effect and site productivity was found, in line with the stress-gradient hypothesis.

Mixing species has been proposed as a forest management option to mitigate the effects of the increase in summer drought episodes predicted by climate change models in temperate zones. Studying how the relationship between tree rings and summer drought episodes is modified by species mixture is of primary importance in this context. Based on a dendroclimatology study, we (i) qualified and quantified the relationship between ring width (RW) or mean ring density (MRD) and soil water deficit (SWD) in a ring-porous species, Quercus petraea, and a coniferous species, Pinus sylvestris, and (ii) determined if the relationships between tree ring characteristics and SWD are modified in mixtures. We found interspecific differences in RW patterns depending on the seasonality of the drought periods: RW in Q. petraea responded mainly to SWD from May to July, and RW in P. sylvestris to SWD from May to September. The ring-width of both species was strongly reduced by SWD. however, inter-specific differences in response to SWD seasonality triggered RW variations of opposite directions between the two species, for several years. In contrast, MRD for both species was dependent on the SWD from June to July. MRD in Q petraea increased slightly with SWD whereas MRD in P. sylvestris decreased strongly with SWD. The relationship between RW and SWD was not modified in mixtures compared to pure stands. We found that the effect of SWD on MRD was stronger in mixtures. The impact of SWD on species may be worsened in mixtures, but mixture of species with different growth sensitivities to the seasonality of the drought periods and to the drought intensity might help to buffer the effects of climatic fluctuations on the overall mixed stands productivity compared to pure stands. (C) 2015 Elsevier B.V. All rights reserved.

Progress report for year 3 Description and results of the project - year 3.

Global environmental changes can result in a modification of abiotic conditions and biological diversity. The production function of forest ecosystems has the particularity of being both subject to and regulating these environmental changes. It is therefore essential to understand how species diversity and abiotic factors control this function. In my work, I studied the variation in the effect of species mixing on their productivity in a changing climatic and edaphic context. Using a double approach, based on local data and data from the National Forest Inventory, I focused on five major European forest species in pure and bispecific stands: Quercus petraea, Pinus sylvestris, Fagus sylvatica, Abies alba and Picea abies. In lowland areas, I show that the effect of species mixing varies little with abiotic factors. Moreover, this effect is determined by the shade tolerance of the accompanying species. In the mountains, the effect of species mixing on productivity is stronger and more positive when abiotic factors are the most limiting for growth. These results illustrate the interest of integrating abiotic factors and functional characteristics of species as determinants of the effect of biological diversity on ecosystem functions.

Presentation of a part of Maude Toigo's thesis on the study of density and ring width of 108 sessile oak and Scots pine trees in the Orléans forest.

Analysis of the effect of mixing on growth for five species pairs from forest inventory data. Analysis of the influence of abiotic factors on this relationship. The effect decreases with the productivity gradient.

The diversity of bryophyte communities in mixed stands of sessile oak (Quercus petraea) and Scots pine (Pinus sylvestris) was compared to pure stands of these species, for trunk epiphytes and humo-terric bryophytes around the trunk. At the tree scale, pines harbor lower diversity than oaks. Mixing has a positive effect on bryophyte community richness on/under pine, while it has no significant effect on oak. At the stand scale, pure pine stands are less species rich, and there is no difference in richness between mixture and pure oak stands. However, the composition of the species assemblages is influenced by the interaction between mixture and species, so diversity at the scale of the massif is enhanced if all three stand types remain represented.

Presentation of part of Maude Toïgo's thesis work using data from the IGN forest inventory to evaluate the effect of mixing for 5 species pairs, as well as the variation of this effect along forest productivity gradients.

This report of the last part of the ONF/Irstea Nogent convention includes the following parts: - Modeling of mixed stands Oak - other, including the progress of the thesis of Maude Toïgo - An article RdVT on the modeling of Fir-Epicea-Beech mixtures from IFN data, and the evolution of the module Capsis Melies to Deesses - Local studies on the mixture Oak-Pine in the forest of Orleans, including a report on the balance of soil water monitoring in connection with intra-annual growth.

Progress report on the project.

Report on the lowland forest research convention. The report includes progress on : - The modeling of mixed stands Oak + others - The modeling of fir - spruce - beech mixtures, and their implementation in the Melies module - The local study of the Sessile Oak - Scots Pine mixture.

Unmanaged forest reserves are designed to preserve or restore typical forest biodiversity, such as forest specialist or dispersal-limited forest species. Yet some species groups might be more dependent on specific habitat features than on forest management per se. We therefore investigated the respective influence of forest management abandonment and habitat characteristics on ground beetles in six French forests comprising both managed and unmanaged areas (85 plots). We hypothesised contrasted responses of carabid species richness depending on ecological and life-history traits (habitat affinity, dispersal ability, diet and moisture affinity). Management abandonment favoured only two ecological groups: forest specialists and openland species. For the other groups, management abandonment was not the main driver. Basal area and humus activity – respectively proxies for canopy closure and food supply – increased total species richness and richness of four ecological groups (forest, wingless, moisture indifferent and carnivorous species). Small scale variables, such as ground vegetation structure, most influenced habitat generalists, winged, hygrophilous and xerophilous species. The effect of forest management abandonment may have been limited either because the reserves we studied have been set aside too recently (15–45 years ago), or because harvesting in the managed forests was relatively extensive (no clearcutting or slash harvesting). We emphasise the importance of taking stand structure into account to plan for biodiversity conservation in managed forests.

Progress report for the first year of the SYREMI project.