A conifer study reveals how a changing climate could make commercial forestry more complicated

Conifers dominate the taiga in the Arctic and are a common feature of temperate latitudes. Their growth is a factor in their popularity in commercial forestry. What isn’t understood properly is how their stem growth will respond to a changing climate. The trees that grow well now might suffer in droughts of the future. Yanjun Song and colleagues at Wageningen University set out to study nineteen conifer species to see how they reacted to climate.

While the weather can change from day, climate change is comparatively slow. Fortunately, the team were able to use a common garden set up at the Schovenhorst Estate that was first planted in 1916. These trees gave the team the advantage of a long-term project, ready for use, with over forty years of climate tracking. The downside, they note, is they were stuck with the experimental design that other botanists set up decades earlier.

The botanists used tree rings to track how the stems of the trees grew and compared what they saw with weather records for the area. While you can count rings from sawing down the trees, this is an expensive way to research. Instead, the team used corers and bored into the trees from opposite sides around 1.3 metres above the ground.

Not surprisingly, almost ninety per cent of species showed a reduction in growth during summers with drought. However, looking closely at the trees, the scientists could pull more information out about other seasonal responses. They found that over a third of trees had trouble with spring frosts, but also that a third benefited from warmer winters.

So can you identify the species that will respond best to climate change to maximise growth? Surprisingly not, say Song and colleagues.

The team had expected that the species with high growth potential would be more sensitive to climate variation. They thought that large cells with thin cell walls would be more susceptible to changing temperatures.

“However, we only observed a weak pattern that contrasted with the expected positive relationship, since stem diameter growth and growth sensitivity to summer drought were negatively – and not positively – associated (r=- 0.40, P=0.08)… Particularly Abies grandis combined a large growth potential with high tolerance to climate extremes, as shown by the low impact of spring frost and summer drought on stem growth of this species,” they write.

“Overall, our study implies that for a given site, forest managers have the option to select and plant conifer species that combine a high growth rate with a high tolerance to summer droughts. This is an important prerequisite for designing climate-smart forests for the future (Nabuurs et al., 2018) that combine a high productivity with a large carbon storage potential and strong drought resilience.”