The research team combined two complementary lines of evidence spanning 1990 to 2022. At the regional scale, they drew on standardized growth indices derived from the Finnish National Forest Inventory (NFI), which removes size- and age-related growth trends so that remaining fluctuations reflect environmental variability such as climate. At the site level, they established three study sites per species, with Scots pine on mineral soils and Norway spruce on drained organic soils, and collected tree-ring cores from dominant and codominant trees. From these cores they measured basal area increment (BAI), a direct measure of wood production per unit area, and analyzed carbon isotope discrimination (Δ13C) in α-cellulose extracted from annual rings covering 1999 to 2022.

The isotope analysis is the technical heart of the study. Δ13C primarily reflects the ratio of intercellular to ambient CO2 concentrations (ci/ca) in the leaf, which is governed by the balance between photosynthetic demand for CO2 and its supply through stomata. Because intrinsic water-use efficiency (iWUE), the ratio of net photosynthetic rate to stomatal conductance, increases as ci/ca decreases, declining Δ13C values signal tighter stomatal regulation. In other words, the isotopic signature locked into each annual ring is a fossil record of how carefully the tree rationed water loss against carbon gain during that growing season.

The results reveal a sharp species contrast. In Norway spruce, segmented regression detected significant breakpoints in all four analyzed variables, with most change points clustering between 2015 and 2017. The NFI-based growth index declined sharply after 2016 on both mineral and organic soils, with slopes of -9.71 and -5.87 index units per year respectively, and the decline on mineral soils was resolved in every one of 1000 statistical realizations that propagated NFI sampling uncertainty. Site-level BAI fell by roughly -0.069 and -0.061 square meters per hectare per year under dry and wet conditions alike. Yet spruce’s iWUE rose only modestly, and mean iWUE was actually slightly lower on dry plots (69.6 versus 72.6 micromoles per mole), a difference that did not reach statistical significance.

Scots pine told a different story. Its growth index showed no statistically supported regional breakpoint, and site-level BAI declined only modestly under dry conditions (-0.0082 per year) while remaining essentially stable on wet plots. Meanwhile, pine’s iWUE climbed steeply, increasing more under dry conditions (2.160 per year) than wet ones (1.400 per year), with significantly higher mean iWUE on dry plots. The carbon isotope data reinforced this pattern: Scots pine’s Δ13C responded strongly to vapor pressure deficit (VPD), with slopes of approximately -4.5 to -4.9 per mille per unit VPD, compared with only -1.92 and -0.57 per mille in Norway spruce before and after 2016. Pine, in short, was physiologically adjusting while spruce was not.

Linear mixed-effects models examining the May-July climate window clarified the drivers. When slopes were rescaled to represent a one-standard-deviation increase in each predictor, VPD emerged as the dominant influence on Scots pine across both growth and isotope responses, whereas Norway spruce responded to atmospheric demand and the summer water balance (captured by the Standardized Precipitation-Evapotranspiration Index, SPEI) to a similar degree. Critically, after 2016, VPD became a significant constraint on Norway spruce growth, with a negative, moderate-effect-size slope, while Scots pine’s growth remained comparatively buffered. The authors note that the evidence supports a significant VPD constraint on spruce growth after 2016 rather than a measured increase in the strength of that constraint, since the between-period slope difference itself was not statistically supported.

The mismatch between spruce’s steep growth decline and its flat iWUE trajectory carries a troubling implication. The authors draw a parallel with earlier work on drought-induced mortality in ponderosa pine, where trees that died showed large growth declines but low sensitivity in water-use efficiency, indicating weak stomatal regulation during intensifying water stress. Norway spruce’s pattern fits this dual signature: growth reductions not tightly coupled to increased stomatal closure, pointing instead to non-stomatal limitations such as hydraulic stress, reduced photosynthetic capacity, or altered carbon allocation. Spruce’s shallower, less plastic root system and weaker stomatal sensitivity to rising VPD likely compound the problem, and lagged responses to previous-autumn VPD suggest cumulative stress effects mediated through stored carbohydrate reserves.

Soil and site conditions added nuance but did not override the species signal. At the regional scale, differences between mineral and organic soils were weak and statistically non-significant for both species. At the site level, however, local moisture mattered more: Scots pine maintained higher mean growth on wetter lowland plots while showing higher iWUE on dry uplands, whereas Norway spruce grew better on drier, well-drained organic plots than on poorly drained ones, where waterlogging constrains shallow roots and impairs nutrient uptake. The authors suggest that fine-scale hydrological variation, which gridded climate data cannot resolve, modulates drought impacts more strongly than broad soil classifications alone.

The findings arrive amid mounting evidence that atmospheric drying across Europe is unprecedented in the last 400 years, and that recent heat and drought events have damaged forests across the continent. For Finland, where Scots pine and Norway spruce account for roughly 40 and 38 percent of total stem volume respectively, the species-specific divergence has direct management consequences. The authors argue that sustaining boreal forest productivity will require site-adapted strategies that integrate species selection with local soil and moisture conditions: reducing stand density and promoting structural diversity on dry upland pine sites, and reconsidering drainage intensity, thinning regimes, and continuous-cover forestry on spruce-dominated peatlands to buffer trees against intensifying water stress.

Ultimately, the study demonstrates that boreal forest responses to drought are structured primarily by species-specific physiological strategies, further modulated by local hydrology. As warming continues to raise atmospheric moisture demand across northern Europe, the contrast between pine’s gradual physiological adjustment and spruce’s abrupt growth collapse offers both a warning and a roadmap: the forests that survive the coming decades will be those whose water-use strategies, and the management regimes that support them, are matched to the increasingly water-limited climates they face.

Subject of Research: Species-specific growth and water-use efficiency responses of Scots pine and Norway spruce to increasing water limitation in southern Finland

Article Title: Norway spruce shows stronger growth sensitivity and weaker intrinsic water-use efficiency response than Scots pine under increasing water limitation in southern Finland

Article References: Szejner, P., Martínez-García, E., Haakana, H., Heikkinen, J., Rinne-Garmston, K. T., Kilpeläinen, P., Young, G., Sahlstedt, E., Mäkipää, R., & Lehtonen, A. (2026). Norway spruce shows stronger growth sensitivity and weaker intrinsic water-use efficiency response than Scots pine under increasing water limitation in southern Finland. Biogeosciences, 23(20), 7223-7252. https://doi.org/10.5194/bg-23-7223-2026

Image Credits: AI Generated

DOI: 10.5194/bg-23-7223-2026

Keywords: boreal forests, drought, water-use efficiency, tree rings, carbon isotopes, Norway spruce, Scots pine, Finland, climate change, forest management, Norway, spruce