Water stress is already affecting forest growth in southern Finland. Norway spruce and Scots pine are responding differently: our study found a sharper growth decline in spruce, while pine showed a stronger adjustment in the balance between carbon uptake and water loss.
As temperatures rise, the atmosphere can draw more water from trees. This growing demand can constrain growth when water supply cannot keep pace, with consequences for timber production and carbon storage.
In our study published in Biogeosciences, we combined tree-ring growth and carbon isotopes with climate data and National Forest Inventory records. This allowed us to connect responses at individual forest sites with regional growth patterns.
Tree rings record annual stem growth. Their carbon isotopes provide information about intrinsic water-use efficiency, which relates carbon uptake to how readily water vapour passes through leaf pores. Together, these records reveal how growth and physiology have responded over time.
Norway spruce growth declined markedly after 2016. During 2016–2022, higher atmospheric water demand was associated with lower spruce growth. Scots pine showed smaller growth reductions.
The physiological records add an important part of the story. Pine showed a stronger increase in intrinsic water-use efficiency, consistent with tighter regulation of water loss. Spruce showed more modest changes despite its substantial growth decline. The two species therefore differed in both growth and the physiological responses accompanying it.
Local moisture conditions shaped these patterns. Pine growth was relatively stable on wetter plots but declined on drier plots. Spruce growth declined under both conditions, although average growth was higher on the drier plots. Pine was studied on mineral soils and spruce on drained organic soils, so the species comparison also reflects their different site contexts.
For me, these results make water stress a practical concern for forest planning. The decline in spruce growth matters for timber production and for expectations of how much carbon forests can store.
A next step is to test whether growth and isotope records, combined with satellite observations, can identify forests likely to recover poorly after drought. Testing this across other sites and regions could extend the value of these records to broader forest monitoring.
The practical priority is to give water availability and species responses greater attention in forest planning. Our findings provide a basis for that discussion by showing how southern Finland's forests are already responding.
The research was funded by the Natural Resources Institute Finland (Luke), the Ministry of Agriculture and Forestry of Finland, the European Union’s Horizon Europe research and innovation programme, and NordForsk.
