An inspection tells us about a tree at a point in time. But the conditions affecting that tree continue to change.
After an exceptionally hot, dry growing season, recent warnings about drought-stressed trees have highlighted why that matters. In a recent Guardian article, Kevin Martin, Head of Tree Collections at Royal Botanic Gardens, Kew, warned about the potential effects of prolonged heat and drought as trees move into autumn, including increased brittleness, impacts below ground and consequences that may not become fully apparent for several years.
For those responsible for managing large tree populations across highways, communities and public spaces, that raises an important question: when the conditions affecting our trees change, how should our understanding of risk change with them?
Tree risk isn’t static
Routine inspections are an essential part of understanding tree condition and managing risk, but every inspection represents what was observed at a particular point in time. Periods of drought, extreme heat, flooding, storms and changing ground conditions can all influence trees between inspection cycles.
That does not make existing inspection information less valuable. It means the wider context around that information matters too.
Kevin’s comments are particularly relevant because some of the effects of this growing season may take time to emerge. Understanding risk therefore means more than asking when a tree was last inspected. It also means considering what may have changed since.
Stress doesn’t equal risk in isolation
A drought-stressed tree should not automatically be considered a high-risk tree.
Condition or potential for failure is only part of the picture. The consequence of failure and the level of exposure matter too. For authorities managing thousands of trees, responding to changing climatic conditions cannot simply mean treating every tree in the same way.
The more useful question is where changing conditions intersect with existing vulnerability and greater potential consequence.
Location changes the consequence
Two trees may show similar signs of stress, but present very different levels of concern depending on where they are located.
A tree beside a heavily trafficked road, school route, footway, building or busy public space presents a very different consideration from an equivalent tree in a low-use location. Location, falling distance and what could be affected if failure occurred all provide important context when deciding where professional attention should be focused.
For highway authorities in particular, this matters because trees sit alongside road users, pedestrians, properties and infrastructure across a very large network.
Bringing more information together
Authorities already hold significant amounts of information about their tree populations. The opportunity is to make better use of it.
Inspection history, species, age and size, previous defects or failures, location and maintenance history can be considered alongside wider information such as falling distance, road and footway exposure, nearby buildings, community use, previous incidents, rainfall, flooding, drought and other environmental factors.
Bringing those sources together gives authorities a stronger basis for understanding where attention may be most valuable.
Using intelligence to focus attention
The practical response to a changing climate cannot simply be to inspect every tree more frequently. For large tree populations, that would create significant pressure on budgets, arboricultural resources and operational teams.
A more targeted approach starts by asking where changing conditions matter most. Where are trees already vulnerable? Where have conditions changed significantly? Where would failure have the greatest consequence? Where does existing inspection information suggest further attention may now be appropriate?
Using data to identify those intersections can help authorities focus specialist resources where they can provide the greatest value.
Preparing for what comes next
The concern is not only what drought has already done, but what happens next. Trees that have experienced a prolonged period of heat and drought are now moving towards autumn and the possibility of wetter, windier conditions. Kevin has also highlighted the potential interaction between very dry ground, subsequent heavy rainfall and tree-root stability, while some effects may take years rather than weeks to become apparent.
That makes this more than a response to one unusually dry summer. It is another example of why tree management needs to account for an environment that is changing around the assets being managed.
Tree risk isn’t static. Neither is the information we should use to understand it.