Established invasive tree species provide opportunities for climate adaptation

A rapidly changing climate weakens the resilience of forest ecosystems through loss of vitality of tree species. As a result, forests can provide fewer ecosystem services. Invasive tree species often occupy the vacant space. A global team examined how to manage five invasive tree species in the context of climate-smart forest management. They are: tree of heaven (Ailanthus altissima), blue gum (Eucalyptus globulus), monterey pine (Pinus radiata), black cherry (Prunus serotina) and black locust (Robinia pseudoacacia).

It concludes that strengthening forest resilience both reduces invasive tree species and enhances climate adaptation. The results of this study will soon be published in Current Forestry Reports. Herewith the approved pre-publication: paper-Current-Forestry-Reports.pdf.

Invasive trees, challenges and opportunities

Tree species have historically been moved across continents for various reasons. For example, for aesthetics, ecosystem restoration, biodiversity enrichment or timber production. In many cases, they have become part of regional tree species pools around the world. Some species stand out because of their rapid expansion and are therefore called ‘invasive’. Research on the effects of invasive tree species in forests often focuses on preventing new invasions. In this study, we focus on invasive tree species that are already established and widespread. We refer here to established invasive tree species (EITS).

EITS can be harmful outside forests, such as on heathlands and in grasslands. As fast-growing tree species with pioneer characteristics, EITS can change short vegetation types into forests, leading to local disappearance of light-dependent species. These effects of EITS outside forests are beyond the scope of this study. But EITS can also have significant impacts on forest-related biodiversity, ecosystem processes and related ecosystem services. They can directly or indirectly affect resource availability for other species and their rejuvenation by causing changes in the biotic or abiotic environment. For example, through shading, competition for subsurface resources, effects on biogeochemistry through rich litter, N-fixation. Moreover, in fire-prone areas, some EITS may increase fire risk. But above all, they may conflict with conservation objectives, as they are usually not part of the intended forest composition.

Nevertheless, benefits of EITS are also reported. Studies show that they can contribute to species richness, improve nutrient availability and provide ecosystem services. Forest managers and researchers also observe that EITS, under certain conditions, can contribute to forest resilience to a changing climate.

Invasive trees strengthen forest resilience

Although human-induced climate change is a relatively new phenomena, stressors are part of every forest ecosystem. The set of mechanisms that combine to enable forest ecosystems to survive and renew under these stresses determine forest resilience. Different definitions of resilience are used in forest ecology. In this study, resilience is understood as ecological resilience: ‘the ability of an ecosystem to absorb changes in environment and natural and human disturbances without loss of structure and functions as the ecosystem adapts to changing conditions’.

One of the main contributions of invasive trees to forest resilience is to fill vacant niches so that forest structure and functions recover faster after disturbance. Pioneer tree species such as EITS are at the beginning of forest succession. As nurturing tree species (‘nurse trees’), they can stimulate rejuvenation of native tree species by rapidly restoring microclimate and sometimes also by promoting litter decomposition.

Invasive trees can be integrated into forest ecosystems

Due to the pioneer nature of most EITS, spontaneous forest succession in most situations limits their dominance in time and space. EITS can be temporarily and locally dominant in pioneer situations. However, the establishment of tree species from mid- and late forest succession under the translucent canopy of EITS gradually displaces them. In forests where the number of tree species that can compete with EITS is limited, tree species diversity can be enhanced by planting. The resulting greater resilience will not only reduce dominance by EITS over time, but it also reduces the likelihood of new invasive tree species establishing. Reducing forest invasiveness is most effective for pioneers with high light requirements, such as black cherry, black locust, tree of heaven, blue gum and monterey pine. Reduction by forest succession is less effective for the exceptional EITS that combine pioneer traits with characteristics of late successional species - such as shade tolerance, strong shade-casting ability and longevity. Members of the genus Acer are typical examples of this category.

Invasive trees benefit from climate change

As forest managers seek pragmatic ways to deal with EITS, the climatic conditions under which they do so are changing. Global average temperatures have risen rapidly in recent decades, resulting in greater weather extremes, such as drought and heat. The effects of climate change have led to increased tree mortality on all continents. Sometimes forests are unable to recover, while in other situations forests respond through shifts in tree species composition. These observations suggest that persistent and increasing changes in forests will occur in response to climate change. This may reduce the provision of ecosystem services, such as climate regulation, biomass production, food, medicine, water supply and purification, pollination and habitat delivery for forest species.

Often EITS are better adapted to these changes than native tree species, due to higher ecological plasticity and wider ecological amplitude. Moreover, climate change perturbations favour pioneer trees with intensive rejuvenation dynamics, rapid juvenile growth and high light requirements, traits characteristic of most EITS. Forests sensitive to climate change often appear to be dominated by drought- and heat-sensitive tree species. Or they are highly disturbed forest ecosystems with altered abiotic conditions and species composition, such as (former) plantations. One of the expected consequences of climate change is that tree species failure will increase the proportion of vulnerable forest ecosystems, resulting in an increasing proportion of EITS.

Invasive trees contribute to climate adaptation

The integration of EITS into the forest ecosystem can be enhanced by measures aimed at strengthening resilience to climate change. These include optimising abiotic conditions, strengthening forest stratification, increasing tree species diversity and small-scale forest management. Consistent implementation of climate adaptation will reduce the (dominant) occurrence of EITS in forests, eventually making EITS common trees among other tree species.

In resilient forest ecosystems, EITS can contribute to climate adaptation because of their often higher drought and heat tolerance and their pioneer nature. This translates into rapid recovery of a forest microclimate after large-scale disturbances and the ability, as nurse trees, to promote the establishment of native trees. EITS can enhance several elements of forest structure: abiotic conditions, vegetation structure and species composition.

Integrated EITS contribute to the diversity of tree species in forests, which helps ensure the continuous delivery of ecosystem services. For example, monterey pine can fulfil functions of native conifer species affected by severe pests or disturbances. Similarly, black locust, black cherry, tree of heaven and blue gum can act as alternative fast-growing pioneer deciduous trees when native pioneer species are losing the battle with climate change.

However, combining climate adaptation with increased resilience against dominance by EITS is only possible in forests where forest adaptation to climate change is served by a structure-rich forest with high functional diversity of tree species. For example, in (semi-)arid areas, where forest resilience to climate change is enhanced by allowing trees a large growth space in an open forest, the proportion of pioneer tree species, and thus EITS, is likely to increase.

Integration of invasive trees promotes species diversity

Adding a tree species - with its species-specific characteristics - to an existing forest ecosystem changes biological diversity. This applies to both native tree species and EITS. These effects on species diversity depend heavily on the density at which EITS occur. When the density is high, these effects, negative or positive, are greater than when it is lower.

In monoculture plantations, EITS are often intensively managed with short rotation periods. This short forest development period usually prevents natural rejuvenation of native tree species. This is similar to the effect of dense, young stands of native trees on rejuvenation. However, less intensively managed EITS-dominated forests and abandoned EITS plantations harbour a high diversity of plants and animals. This phenomenon is well known for plantations of blue gum, monterey pine and blac locust. Tree of heaven and black cherry are mostly not managed as plantations outside their areas of origin.

In forest ecosystems that are (made) resilient against dominance by EITS, their local dominance is a transitional phase viewed at the time scale of forest development. As a result, both large-scale negative and positive effects of EITS on the species composition of forest ecosystems resolve during forest succession. The negative effects of the presence of dominant EITS can turn into positive diversity effects in resilient forests with a low proportion of EITS due to increased habitat and food supply variation. Moreover, during the presence of EITS, a gradual adaptation takes place between EITS and long-established species.

Integrated invasive trees provide opportunities for timber production

For some EITS, such as blue gum, monterey pine and black locust, timber production in plantations is an important reason for plantation. The question is therefore relevant whether these EITS and EITS whose invasive nature prevents forest managers from using them for timber production (e.g. tree of heaven and black cherry) can contribute to timber supply in resilient forests. Although timber production with EITS in plantation forests usually focuses on low-value uses such as firewood, pulp and paper, EITS can also be used to provide timber for high-value uses such as platform framing, furniture and veneer. This is valid for all five tree species studied. The management of resilient forests is characterised by small-scale interventions that limit the amount of light on the forest floor and concentrate secondary growth on a limited number of high-quality trees. So here, too, the density of pioneer tree species - and thus of EITS - decreases in favour of late successional species.

Integration of invasive trees is promising for forest managers

Forest managers usually see preventing dominance by invasive trees and climate adaptation as separate challenges. However, this study shows that the two ambitions overlap considerably in practice. Forest management measures aimed at climate adaptation simultaneously reduce the risk of invasive trees becoming dominant in the forest. This is especially true if these measures enhance the complexity of the forest structure and the diversity of tree species.

The contribution that invasive trees can make to climate adaptation in forests is promising for forest managers. The large budgets previously earmarked for control can now be used to further develop forests and strengthen their resilience. Moreover, by incorporating invasive trees, forest managers have additional tree species at their disposal to achieve their management objectives. Despite all the differences in how the presence of different invasive trees impact various forest ecosystems, society has much to gain from a non-biased approach to these ‘new’ tree species. The sustainable delivery of forest-related ecosystem services under rapidly changing climatic conditions may partly depend on them.