In past blog posts we have been discussing how forest landscapes can be seen as interconnected and functional complex networks – and shown how network analysis can be combined with modelling and forest management. But is the so-called functional network approach really an efficient way to optimize forest landscape management and to promote ecological resilience in the face of unexpected global change stresses?
When we go hiking in the mountains, we know that before reaching an appealing and gratifying view we often need to walk up a few hundred meters inside a forest. Sounds natural, it has always been this way. We have cities, crop fields, grasslands, forests, rocky mountain peaks, etc. Forests are intrinsically part of our cultural landscape, and it is normal to think they will always be. Although such landscapes look simple, when we disentangle each single element, we realise that it is a very complex socio-ecological network, with both human and biophysical processes linked across different spatial and temporal scales.
To make sure all its elements provide us a series of important goods and services, our easiest option is to manage all its decoupled entities separately. In the case of forests, this means applying a classic stand-scale approach, or, in the best scenario, planning interventions on groups of stands that share similar characteristics. However, the rapid increase of global change stressors (e.g., climate change, sudden drought events, severe fires, insect outbreaks) requires us to go beyond that. Planning must be carried out with a larger vision, by looking strategically at the entirety of interconnected forest patches across large landscapes. However, planning interventions in the long-term and at the regional scale makes everything more complicated, and it requires applying new methods of integrated landscape management.
The highest uncertainty in such process is not knowing what is coming to disrupt forest´s natural development. Researchers made great progresses to develop sets of climate scenarios representing a range of possible climate futures. Clearly, we are still unsure what the climate in the next decade will be but at least we have a rough idea thanks to climate projections. However, no divergent projections exist globally for the possible occurrence of extreme events and disturbances. A highly cited review article published a few years ago concludes that natural disturbances affecting forest ecosystems will likely increase in the future. Disturbances such as fire, insect, wind, and pathogens will be fueled by direct climate effects, but interestingly also indirect effects (e.g., changes in vegetation composition and structure) would play an important role sometimes dampening disturbances increase and thus making the situation very variable across regions.
Several studies highlight the role of forest management for modulating the impact of disturbance events, and some large collaborative projects are targeting these challenges. Management can modify forest structure and composition, and – in conjunction with reducing carbon emission – it is our best and strategic weapon to make forests more resilient to climate change and disturbance. But what does resilience mean, in this case? There are multiple definitions of resilience, from its classic notion of engineering resilience to a more holistic concept of ecological resilience. These have been widely discussed in a comprehensive review article by EFI colleagues, who summarized it in a previous post. If we embrace a wider concept of resilience (i.e., ecological resilience), there are three of its core components that we can target with management: persistence, recovery, and reorganization (see this interesting article discussing these aspects). Increasing persistence can be achieved by boosting the capacity of trees to tolerate disturbances without major changes. For example, by increasing structural diversity we can make sure that stands will better resist to wind guts. Or by reducing stand density we can decrease overall respiration and therefore lessen the cases of individual tree mortality against drought. Managing for persistence, however, can be done if we somewhat know the disturbance type that will likely affect our ecosystem, which is not always the case. But when a forested area is affected by multiple and diverse disturbances, the only option for management is to target recovery and reorganization of the system. This can be done by increasing genetic, compositional, and functional trait diversity of tree communities because diverse communities are likely to respond quicker to perturbations. Furthermore, we can foster landscape-level functional connectivity, as high potential trait dispersal ensures a rapid tree recolonization of disturbed stands by seeds coming from the surrounding intact stands, contributing to a swift and efficient reorganization of the system.
Simulating forest resilience with models
Unless we are willing to wait for decades or centuries, assessing future changes in slow-responding systems like forests require simulations models. Process-based forest landscape models, thanks to their ability to capture ecological processes and scaling them up to large regions in a spatially explicit manner, are among the most powerful tools in forest resilience research. The use of simulation models to simulate and evaluate forest resilience have been recently synthesized in a review article, and their advantages in the context of resilience studies have been discussed in our previous post. On that occasion, we also described how forest landscape models can be coupled with the principles of functional diversity (i.e., seeing forest stands as communities of functional traits) and network analysis (i.e., representing forest landscapes as functional complex network of stands linked by levels of functional connectivity) to assess how large, fragmented and complex forest regions will be affected by changing climate and disturbances – and if current management practices are suitable to maintain their ecological resilience. However, we left ourselves with a key point in our to-do list: to design and assess forest management scenarios based on the principles of this so-called functional network approach. This way we can verify whether these innovative methods can really make a difference in the long-term to boost ecosystem resilience, even under unexpected disturbances.
Optimizing management by seeing forest landscapes as functional networks
We all know that resources to manage forests are very often limited. Governments may allocate extraordinary funds to forestry to cope with disasters, for example for salvaging interventions after major disturbances (e.g., big fires or windthrow events), But as in many other sectors, resources to do prevention are often scarce. For this reason, it is key to maximize the effect of stand-level silvicultural interventions across the landscape if we aim at increasing forest resilience in the long-term. But how?
To answer these questions, we selected a large forested region in southern Quebec, which we had already implemented in the LANDIS-II simulation model in a previous study. Across such large landscape (about 7000 km2), management interventions are typically executed at the stand scale, following principles of sustainable forest management and even by attempting to adapt management to climate change (e.g., managers are starting to promote and plant species that are known to be more suitable in the future). To better understand how to maximize management interventions, we pre-analysed the landscape following the principle of the functional network, looking at the distribution of stand- and neighbouring-stands functional diversity, as well as functional connectivity, to create a map of the landscape subdivided into strategic zones. In this way, priority can be given to zones with low level of functional diversity, but also to those that have higher level of functional connectivity (i.e., the potential of dispersing functional traits by seeds from the surrounding stands), because they can ensure a rapid recolonization of disturbed stands in case of unfortunate events such as fire, pest outbreaks or windthrow.
In addition to Business-as-usual, we simulated two management scenarios. The first, climate change adaptation, represents our best guess on how managers will change practices by adapting forests to a warming climate. This includes more vigorous stand-scale harvesting complemented by enrichment planting with native tree species, for instance those not very abundant but believed to be more suitable in a warmer world (e.g., oaks, pines). The second, as mentioned above, is the functional diversification network management; this includes zoning the management units into priority area and performing assisted migration with several tree species, with the aim of enriching the pool of functional traits in tree communities (i.e., not just enhancing species diversity). We made several assumptions in designing such management scenario, but yet we tried to make it as feasible as possible. For example, we only simulated assisted migration with species that are believed will be highly accepted by stakeholders and forest managers, excluding the introduction of exotic species or of those that are already widely affected by pathogens. We also increased stand- and landscape-level harvesting efforts but made sure not to undermine the capacity of forest to keep sequestering carbon and mitigate climate change (following the principles of Climate-smart forestry).
Expecting extremes and unexpected disturbance events
To test the effectiveness of the different management strategies, we hypothesised the occurrence of multiple forest disturbances. In the case of Eastern north American forests, biotic disturbances are the most worrying ecological threats. Northeastern forests currently have the largest concentration of non-native insects in the continent, which are increasingly expanding their range thanks to rapid climate change. Because of this, we simulated the occurrence of insect outbreaks, designing two disturbance scenarios in LANDIS-II, also in combination with unsystematic and unexpected reduction of annual precipitation. We also hypothesized the upcoming arrival of insects that have not being detected yet, but that can likely be introduced in the coming years. One of them is the Asian long-horned beetle (Anoplophora glabripennis), an exotic wood-boring pest attacking mostly maples and other hardwoods trees, a pest of primary concern in eastern Canada given its generalist nature.
Without much surprise, we found that southern Quebec´s forests are highly susceptible to the Asian long-horned beetle. Other pests, such as the Spongy moth (Limantria dispar) and the native spruce budworm (Choristoneura fumiferana) also have high damaging potential, while other insects – such as the Hemlock woolly adelgid (Adelges tsugae) and the mountain pine beetle (Dendroctonus ponderosae) – have the potential to drastically affect single species but with an overall low damaging potential at the landscape scale. These results give us a better idea on what insect pest should be given priority for monitoring, detection and rapid eradication before they can cause widespread damages across large forest regions. Our simulation results were also useful to show that some insect outbreaks might be more intense if through management interventions we promote tree species that we believe are more suitable to grow under climate change. The typical example is the promotion of pines and oaks, known to be more drought tolerant than other species, but suitable host species by the pine beetle and the Spongy moth, respectively. Thus, modifying forest composition and structure via management to adapt forest to climate change must be done with caution – and we need to be aware that this might interact with potentially new biotic disturbances.
Why changes in forest practices need to happen soon
To evaluate ecological resilience and compare management strategies under disturbances, we analysed multiple indicators at landscape scale. All indicators were visualized together using bubble or 3D plots, that can give us an overview of what management is able to maximize all the different drivers of ecological resilience. But we also gave special attention to net primary productivity (NPP), a direct indicator of the rate at which an ecosystem accumulates aboveground carbon. Results are clear: managing forest landscapes as functional complex network provide higher ecological resilience than business-as-usual forest management. While business-as-usual would still maintain productivity, key resilience indicators such as functional connectivity can be only be boosted by executing interventions (such as enrichment planting and assisted migration) strategically following network analysis. This way, more diverse forest communities would be able to better spread functional diversity to neighboring stands. However, higher levels of functional connectivity become apparent only after several decades in the future. This means that long time lags are required to boost such property and that changes in forest practices should take place as soon as possible to build more functionally connected, and thus naturally resilient, future forest landscapes.
Increasing resilience in complex forest landscapes requires complex solutions
Although modelling studies always require many assumptions and have methodological limitations, these results clearly demonstrate that building forest landscapes as functionally rich, well-structured complex networks can increase ecological resilience to climate change and unexpected disturbances. This is, of course, a great and complex challenge. Given that resources for ecosystem management interventions are often limited, adopting a landscape perspective by planning interventions strategically in space – thus maximizing their impact – and adopting a functional trait approach to diversify forests by maximizing the response range to unknown threats is a promising approach for enhancing forest ecological resilience under global change. We believe that these approaches should also be merged with regional-scale risk assessment for key ecosystem services provided by forest ecosystems, which should also consider a socioeconomic perspective, and by feasibility analysis by engaging stakeholders and management to assess the applicability of such adaptations in real forest landscapes.
And now, what are the next steps? It is now the time to assess these approaches in multiple forest regions, spanning a large biogeographical and climatic gradient, and verify if these results can be generalised for multiple forest types with diverse governance regimes – for example in the Alps as in the MSCA-H2020 REINFORCE project – but also affected by different natural disturbance types (e.g., fire, windthrow, pathogens, other insects). It is also the time to complement modelling studies with field trials and real-world experiments, start involving local managers and administrators (which is e.g. done in the H2020 project RESONATE that aims to generate the needed knowledge and practical guidance for making European forests, the services they provide, and related economic activities more resilient to future climate change and disturbances). This way we can show that these are not ideas ending up in academic journals only, but that can be converted in practical actions to build more resilient forests, together.
Read the article featured in this post:
Mina, M., Messier, C., Duveneck, M., Fortin, M., & Aquilué, N. 2022. Managing for the unexpected: building resilient forest landscapes to cope with global change. Global Change Biology, 1–19. https://doi.org/10.1111/gcb.16197 (Open Access)