Which management practices for the adaptation of forests to climate change?

• INT89 • Beyond mean fitness: demographic stochasticity, species interactions and resilience to disturbance matter at tree species climatic edges > G. Georges KUNSTLER
Content : Georges KUNSTLER, Arnaud GUYENNON, Bjorn REINEKING
Univ. Grenoble Alpes, INRAE, LESSEM, 2 rue de la Papeterie - BP 76 F-38402 St-Martin-d’Hères, France – georges.kunstler@inrae.fr
In the face of climate change, there are increasing concerns about the future redistribution of tree species ranges. To anticipate how these redistributions will take place, linking local population dynamics and species distribution becomes essential. While many studies focus on the mean fitness of populations, theory shows that species distributions can be shaped by demographic stochasticity or population resilience. Here we examine how mean fitness (measured by invasion rate), demographic stochasticity, and resilience (measured by the ability to recover from disturbance) constrain populations at the edges compared to the climatic center.
We developed dynamic population models covering the entire life cycle of 25 European tree species with climatically dependent recruitment models fitted to forest inventory 42 data. We then ran simulations using integral projection and individual-based models to test how invasion rates, risk of stochastic extinction, and ability to recover from stochastic disturbances differ between the center and edges of species’ climatic niches.
Results varied among species, but in general, demographic constraints were stronger at warm edges and for species in harsher climates. Conversely, recovery was more limiting at cold edges. In addition, we found that for several species, constraints at the edges were due to demographic stochasticity and recovery capacity rather than mean fitness.
Our results highlight that mean fitness is not the only mechanism at play at the edges; demographic stochasticity and population capacity to recover also matter for European tree species. To understand how climate change will drive species range shifts, future studies will need to analyse the interplay between population mean growth rate and stochastic demographic processes as well as disturbances. • INT90 • Potential and limitations of stand density management to mitigate drought stress in trees > J. Jürgen BAUHUS
Content : Jürgen BAUHUS and Julius WILLIG
Univ. Freiburg - juergen.bauhus@waldbau.uni-freiburg.de
The adaptation of forests to extremely dry and hot climatic conditions has become a major challenge in forestry. For already existing forest stands, the reduction in stand density through thinning has been proposed as a suitable approach to increase the resistance and resilience of trees to drought. Many studies show that thinning can reduce drought-related reductions and promote post-drought recovery of radial growth. There are fewer studies that have investigated the potential of thinning to reduce drought related mortality. There are observations from forestry practice that indicate that the vulnerability to drought in older and taller trees actually increases with declining stand density. In this presentation we review the evidence for thinning effects on growth and mortality, try to resolve obviously contradictory observations, and identify research gaps that need to be addressed to use this practice with more predictable outcomes in forest adaptation approaches. • INT91 • Introducing exotic tree species to adapt forests to climate change: a risky business. > G. Guillaume DECOCQ
Content : Guillaume DECOCQ
Université de Picardie Jules Verne, UMR CNRS 7058 « Ecologie et Dynamique des Systèmes Anthropisés » (EDYSAN), 1 rue des Louvels, 80037 Amiens Cedex 1- guillaume.decocq@u-picardie.fr
The increasing frequency of hotter drought summers and extreme meteorological events like storms challenges the resilience of many temperate forests worldwide. In western Europe, this is associated with widespread forest die-off events and windthrow-induced destruction. For this reason forest adaptation to climate change has become a priority target for European and national policies. The main goals are to maintain timber production in extent production forests, and to mitigate climate change by increasing carbon sequestration and storage through massive tree planting. To achieve these goals, ongoing programs aim to help adapt forest to climate change by planting tree species adapted to projected future climate, including many non-natives. As a result, exotic tree, low-diversity plantations often replace previous native forests or treeless ecosystems. However, these exotic tree planting could produce counterproductive side effects, as they are selected on the basis of their drought tolerance, growth performance and timber quality, but not according to their response to a new environment or ecosystem effects. Here we review the actual and potential risks associated with the introduction of non-native tree species in forest ecosystems, with a special focus on (1) the invasion by alien tree species, when introduced exotic tree species proliferate within or beyond forests at the expense of native biodiversity, thereby impacting ecosystem functioning; (2) the introduction of non-native pests, including arthropods, parasites and pathogens, that can sometimes cause dramatic epidemics; (3) the erosion of native biodiversity, especially when non-native evergreen monocultures replace native broadleaved forests, causing dramatic local and proximal alterations of the environment, either directly or via associated silvicultural practices; (4) the exacerbating effects of catastrophic events, when, for instance, planting flammable tree species increases the amount of fuels in a warming world. I argue that introducing exotic tree species in forests is a risky business and that public policies should aim at reducing these risks at a lower level than the expected benefit for society. Forest adaptation that does not take into account these risks may accentuate the biodiversity crisis and decrease timber production and carbon sequestration, and ultimately increases the risk of ecosystem collapse. • INT92 • Managing mixed forests: the role of tree species diversity for productivity in a changing climate > C. Christian AMMER
Content : Christian AMMER
Univ. Göttingen - Christian.Ammer@forst.uni-goettingen.de
The diversity–productivity relationship describes the effects that differing numbers of tree species may have on stand-level productivity under ceteris paribus conditions, resulting in neutral growth, over- or underyielding. It was shown, that the productivity of mixed stands is mainly driven by two determinants: actual environmental conditions and species composition, i.e. species identity and complementarity of their functional traits, all of which are affected by climate change. Here, three scenarios for changes of the diversity-productivity relationship under a changing climate are presented. First, interactions between coexisting complementary tree species may change such that one or more species become dominant. Over the long term, such a selection effect sensu Loreau may lead to competitive exclusion of species. Second, a changing climate may lead to higher complementarity between species because one or more species become less dominant. In this case, overyielding may not change or may even increase. The third scenario may occur if one or more species migrate to an area with comparably fewer tree species. In all three cases, changed environmental conditions will ultimately lead to a reorganization of the interactions between species’ resource demands, their competitive interactions, and facilitation effects. Consequences for adaptation of managed forests to climate change are discussed. • INT93 • Forest biodiversity and of ecosystem functioning: new insights from modelling studies. > X. Xavier MORIN
Content : Xavier MORIN
CNRS, UMR 5175 « Centre d’Ecologie Fonctionnelle et Evolutive » (CEFE), 1919 route de Mende, 34293 Montpellier cedex 5 - xavier.morin@cefe.cnrs.fr
Ongoing and future climate change puts forests at risk, as harsher environmental conditions strongly impact forests and their functioning. Yet, large uncertainties remain regarding how climate change will affect forest dynamics, notably because forest ecosystems have slower dynamics than other terrestrial ecosystems. Furthermore, these climate-change impacts cannot be properly understood and anticipated without considering the role of species diversity on forest functioning, which has received an increasing attention in the last decades.
Yet, our understanding of the underlying processes linking species diversity and forest functioning, especially species richness and ecosystem productivity, remains weak. In fact, the slow dynamics of forest ecosystems makes experimental tests difficult, although long-term experiment start bringing interesting results. Yet, the study of forest ecosystems beneficiates of a long history of modelling efforts, and such forest models represent key tools to explore the links among species composition, forest functioning and climate, complementing former experimental and empirical approaches.
In this talk, we briefly review how forest models (empirical and process-based ones) have been used to explore the relationship between species richness and forest productivity, and its response to climate drivers. Then we present new results showing how forest models can bring new insights on this complex topic by linking species coexistence and ecosystem functioning. These results notably highlight that the role of forest composition on functioning may greatly vary depending on forest type and site location, but also be strongly dependent on stand density. We further discuss how forest models can be used to explore the implications for forest management, especially in new climatic conditions. • INT94 • Mixing tree species to improve forest drought resistance? A perspective based on plant hydraulics > N. Nicolas MARTIN
Content : Nicolas MARTIN-ST PAUL1 & Joannès GUILLEMOT
1 URFM, INRAE, 84000, Avignon, France, nicolas.martin@inrae.fr
2 UMR Eco&Sols, CIRAD, 34000, Montpellier, France, joannes.guillemot@cirad.fr
Increasing atmospheric and soil drought is responsible for massive waves of tree mortality, growth reduction and plantation failure. Mixing species with different water use or drought response strategies is viewed as a mean to alleviate water stress and adapt forests to climate change. However, empirical data show conflicting results regarding the effects of tree diversity on drought resistance, and the underlying mechanisms are still not well understood, which limits our ability to design successful species combinations.
Relying on experimental data and model analyses, we propose to leverage the explanatory framework of plant hydraulics (i.e. the control and constraints of water transport in plants) to identify a list of testable hypotheses regarding the key mechanisms and functional traits determining mixtures performances under drought. This approach allows better predict which species or traits combination could improve forest and tree plantation resistance to the increased drought associated with climate change. • INT95 • The role of forest species diversity in resisting climate change-induced insect outbreaks > H. Hervé JACTEL
Content : Hervé JACTEL
INRAE, Université de Bordeaux, UMR Biogeco, 33612 Cestas, France - herve.jactel@inrae.fr
Climate change is a multifaceted phenomenon, including higher temperatures, more severe droughts, and more frequent storms. Most responses of forest insect herbivores to these multiple, interacting hazards are expected to be positive, with shorter generation times, higher fecundity and survival rates, leading to an increased likelihood of outbreaks. A striking example illustrating this phenomenon is the increase in bark beetle outbreaks in conifer plantations in Europe.
It is now well established that curative control solutions against these pests, based on the use of insecticides, should be banned due to their low efficiency and their economic and environmental cost. To reduce the impact of these emerging risks, it is better to adopt a preventive and generic method capable of affecting the multiple species of insect pests that can affect forest health. A growing body of empirical and experimental evidence shows that multi-species forests are, on average, more resistant to insect pest attacks than tree monocultures.
In this presentation, we will examine how forest insects favored by climate change, primarily bark beetles, respond to increased forest diversity. We will propose ecological mechanisms to explain the greater resistance of mixed forests to these herbivorous insects. We will consider how these findings can be used to propose changes in forest design and management to maintain forest integrity and vitality under climate change. • INT96 • Avoided emissions from wood use are not systematic : a meta-analysis of the substitution potential of wood > A. Aude VALADE
Content : Aude VALADE
UMR Eco&Sols, CIRAD, 34000, Montpellier, France - aude.valade@cirad.fr
Forest management can reduce net greenhouse gas (GHG) emissions to the atmosphere by three processes: (i) carbon sequestration in the forest soil and biomass (i) carbon storage in harvested wood products (HWP) and (iii) using wood harvest to avoid fossil fuel emissions, often referred to as substitution or displacement. Of these three levers, European forest climate policies primarily incentivize substitution at the expense of in-situ forest sequestration. Contrary to carbon sequestration in forests and products, the substitution potential depends –by definition–on the reference chosen and on the system’s boundaries making this approach highly subjective.
Here we define the concept of substitution as all change in carbon balance resulting from an increase in wood use. With this definition in mind, literature on forest science, life cycle analysis (LCA), energy and construction engineering and applied economics was systematically reviewed in the form of a meta-analysis to assess: (1) available methodologies and current practices for quantifying the carbon impact of wood use as well as (2) the effect of biological, physical and socio-economical components of the wood sector and system’s boundaries on substitution estimates.
Current approaches for quantifying the substitution potential which are already used to inform policy making, are incomplete and may cause adverse outcomes of well-intended but poorly quantified measures to avoid GHG emissions. Our review shows an important variability in carbon impact of increased wood use. A large portion of the reviewed data even show an increase in carbon emissions as a result of increased wood use. The time horizon of the studies, the inclusion or not of biogenic processes as well as the wood origin and mobilization lever appear as key determinants of the carbon impact of increased wood use.
Our results question the values usually assumed for carbon balance of wood use, calling for caution and rigor in biomass-based climate policies. These results also call for turning substitution into full carbon impact estimates by extending the system’s space and time boundaries by accounting for both biogenic carbon including in-situ sequestration, carbon storage in HWP and end-of life release, and fossil emissions from life cycle stages. Moreover, we find that methodologies are mature in climate science, ecophysiology, LCA science and economics to account for the effect of environmental and sociotechnical dynamic drivers necessary for prospective estimates of the climate effects of wood use.