Douglas-fir plantations face limited pest risk: New study by Aditya Ganesh in Forest Ecology and Management

A new perspective on pest risk in non-native forests

A new study published in Forest Ecology and Management, led by Aditya Ganesh (FISC) and co-authored by Deepa S. Pureswaran, leader of the FISC working group “Transient impacts of invasive species”, and Andrew M. Liebhold, Chief Scientist of FISC, brings new insights into the risks associated with planting non-native tree species.

Douglas-fir (Pseudotsuga menziesii), one of the most widely planted tree species globally, benefits from what ecologists call “enemy release” – the absence of its natural pests outside its native range. However, this advantage may decline if co-evolved pests are introduced.

The study addresses a key question: Where could these pests not only establish, but actually cause damage?

From pest presence to pest damage

Unlike most previous studies, the authors distinguish between:

• climatic suitability for pest establishment, and
• climatic suitability for damaging outbreaks

Using data from North America, the team modelled the potential host ranges and pest pest damage of three major Douglas-fir pests:

• Douglas-fir beetle (Dendroctonus pseudotsugae)
• Douglas-fir tussock moth (Orgyia pseudotsugata)
• Western spruce budworm (Choristoneura freemani)

Their approach combines species distribution modelling with historical outbreak data to better capture real ecological risk.

Where Douglas-fir grows – and where pests can damage

The study confirms that large parts of Europe and New Zealand are climatically suitable for Douglas-fir plantations.

However, the conditions required for pest damage are far more restrictive.

Key finding: Low risk of damaging outbreaks

The main result is clear:

Even if these pests were introduced, most Douglas-fir plantations in Europe and New Zealand are unlikely to experience damaging outbreaks under current climate conditions.

• In Europe, only a small fraction (< 2%) of Douglas-fir plantations overlap with conditions suitable for pest damage
• In New Zealand, the risk is close to zero for two of the three pests

This means that while pests could establish over larger areas, outbreaks would be far more constrained.

Why this matters for biosecurity

The study highlights an important shift in how invasion risks should be assessed:

• Traditional models ask: Where can a species occur?
• This study asks: Where can it cause damage?

This distinction is crucial for:

• forest management
• biosecurity planning
• risk prioritisation

A more nuanced view of invasion risk

The findings suggest that non-native plantations are not automatically at high risk, even if pests arrive.

At the same time, the authors stress that:

• climate change may alter these risks
• local disturbances (e.g. drought, windthrow) can still trigger outbreaks
• interactions with other species play an important role