Associational resistance to a pest insect fades with time

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 ofbirch 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 contributionof 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.Keywords Associational effects · Forest · ORPHEE experiment · Plant–insect interactions · Thaumetopoea pityocampa </p

Mixing oak and pine trees does not improve the functional response to severe drought in central French 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.[br/] 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.[br/] Aims We addressed how interactions between sessile oak and Scots pine in mixed forests in central France affect their functional response to drought.[br/] Methods We characterized the carbon isotope composition (δ13C) 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 (Δ13C) among contrasted years.[br/] Results The severe drought in 2003 induced a strong decrease in Δ13C 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.[br/] 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

Tree diversity effects on forest productivity: disentangling the effects of tree species addition vs. substitution

Mixture effect on stand productivity is usually apprehended through a substitutive approach, whereby productivity in mixed stands is compared to productivity in monocultures, at equivalent stand density. This approach has proved that in many cases mixed stands perform better than monospecific forests, however, we do not yet have a solid theory about species behaviour in the mixture or even guidelines for combining species. The addition of a second tree species to an existing mono-specific stand has received much less consideration. Yet, this approach has the potential to separate the facilitation effect from the complementarity effect. We compared the effect of tree species substitution vs. addition on the productivity of maritime pine and silver birch in a young tree diversity experiment implemented in 2008 in SW France. Substituting pines with birches to create two-species mixtures resulted in an increase of tree productivity at stand level beyond what was expected from monocultures (i.e., overyielding). In contrast, creating mixture through the addition of birches to pine stands had no effect on the maritime pine stand productivity (transgressive mixture effect not significant). This absence of effect is produced by two distinct density-dependence responses at an individual level. Our results allow clarifying the cases in which a mixed stand can be considered as an alternative to a monoculture of a productive species. In particular, the addition of a pioneer and soil low-demanding species during young developmental stages is a possibility to diversify the stand and potentially to increase ecosystem services without altering the productivity of the target species

Effects of tree mixture on forest productivity: tree species addition versus substitution

The effects of tree mixture on stand productivity are usually examined using a substitutive approach whereby productivity in mixed stands is compared to productivity in monocultures, at comparable tree density. This approach has proved that mixed stands usually perform better than pure stands. The addition of a second tree species to an existing mono-specific stand has received less consideration. Yet, this approach may separate the facilitation effect from the complementarity effect. We compared the effect of tree species substitution vs. addition on the productivity of maritime pine and silver birch in a 7-year-old tree diversity experiment in south-western France. Given the very young age of the stands, the 2014 tree volumes were used as a proxy for tree productivity. Substituting pines with birches resulted in a significant increase in tree productivity at the stand level, beyond expectations from the weighted mean of monocultures (i.e. overyielding). In contrast, creating a mixture by adding birches to pine stands had no significant effect on the maritime pine productivity: the transgressive mixture effect was not significant. This absence of an effect was produced by two distinct density-dependence responses at the individual tree level. Our results suggest that the addition of a pioneer species with low demands in soil water and nutrients during young developmental stages can diversify stands and increase the provision of ecosystem services without altering the productivity of the target species

Mixture effect on radial stem and shoot growth differs and varies with temperature

International audienceThe 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

Effects of tree mixture on forest productivity: tree species addition versus substitution

International audienceThe effects of tree mixture on stand productivity are usually examined using a substitutive approach whereby productivity in mixed stands is compared to productivity in monocultures, at comparable tree density. This approach has proved that mixed stands usually perform better than pure stands. The addition of a second tree species to an existing mono-specific stand has received less consideration. Yet, this approach may separate the facilitation effect from the complementarity effect. We compared the effect of tree species substitution vs. addition on the productivity of maritime pine and silver birch in a 7-year-old tree diversity experiment in south-western France. Given the very young age of the stands, the 2014 tree volumes were used as a proxy for tree productivity. Substituting pines with birches resulted in a significant increase in tree productivity at the stand level, beyond expectations from the weighted mean of monocultures (i.e. overyielding). In contrast, creating a mixture by adding birches to pine stands had no significant effect on the maritime pine productivity: the transgressive mixture effect was not significant. This absence of an effect was produced by two distinct density-dependence responses at the individual tree level. Our results suggest that the addition of a pioneer species with low demands in soil water and nutrients during young developmental stages can diversify stands and increase the provision of ecosystem services without altering the productivity of the target species

Mixture effect on radial stem and shoot growth differs and varies with temperature

International audienceThe 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

Resource manipulation through experimental defoliation has legacy effects on allocation to reproductive and vegetative organs in Quercus ilex

International audienceBackground and Aims In plants, high costs of reproduction during some years can induce trade-offs in resourceallocation with other functions such as growth, survival and resistance against herbivores or extreme abioticconditions, but also with subsequent reproduction. Such trade-offs might also occur following resource shortageat particular moments of the reproductive cycle. Because plants are modular organisms, strategies for resource allocationto reproduction can also vary among hierarchical levels. Using a defoliation experiment, our aim was totest how allocation to reproduction was impacted by resource limitation.• Methods We applied three levels of defoliation (control, moderate and intense) to branches of eight Quercusilex trees shortly after fruit initiation and measured the effects of resource limitation induced by leaf removal onfruit development (survival, growth and germination potential) and on the production of vegetative and reproductiveorgans the year following defoliation.• Key Results We found that defoliation had little impact on fruit development. Fruit survival was not affected bythe intense defoliation treatment, but was reduced by moderate defoliation, and this result could not be explainedby an upregulation of photosynthesis. Mature fruit mass was not affected by defoliation, nor was seed germinationsuccess. However, in the following spring defoliated branches produced fewer shoots and compensated for leafloss by overproducing leaves at the expense of flowers. Therefore, resource shortage decreased resource allocationto 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 theshoot scale. Defoliation had larger legacy effects than immediate effects

Resource manipulation through experimental defoliation has legacy effects on allocation to reproductive and vegetative organs in Quercus ilex

International audienceAbstract 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

Can mixing Quercus robur and Quercus petraea with Pinus sylvestris compensate for productivity losses due to climate change?

The climate change scenarios RCP 4.5 and RCP 8.5, with a representative concentration pathway for stabilization of radiative forcing of 4.5 W m−2 and 8.5 W m−2 by 2100, respectively, predict an increase in temperature of 1–4.5° Celsius for Europe and a simultaneous shift in precipitation patterns leading to increased drought frequency and severity. The negative consequences of such changes on tree growth on dry sites or at the dry end of a tree species distribution are well-known, but rarely quantified across large gradients. In this study, the growth of Quercus robur and Quercus petraea (Q. spp.) and Pinus sylvestris in pure and mixed stands was predicted for a historical scenario and the two climate change scenarios RCP 4.5 and RCP 8.5 using the individual tree growth model PrognAus. Predictions were made along an ecological gradient ranging from current mean annual temperatures of 5.5–11.4 °C and with mean annual precipitation sums of 586–929 mm. Initial data for the simulation consisted of 23 triplets established in pure and mixed stands of Q. spp. and P. sylvestris. After doing the simulations until 2100, we fitted a linear mixed model using the predicted volume in the year 2100 as response variable to describe the general trends in the simulation results. Productivity decreased for both Q. spp. and P. sylvestris with increasing temperature, and more so, for the warmer sites of the gradient. P. sylvestris is the more productive tree species in the current climate scenario, but the competitive advantage shifts to Q. spp., which is capable to endure very high negative water potentials, for the more severe climate change scenario. The Q. spp.-P. sylvestris mixture presents an intermediate resilience to increased scenario severity. Enrichment of P. sylvestris stands by creating mixtures with Q. spp., but not the opposite, might be a right silvicultural adaptive strategy, especially at lower latitudes. Tree species mixing can only partly compensate productivity losses due to climate change. This may, however, be possible in combination with other silvicultural adaptation strategies, such as thinning and uneven-aged management