How tree species, tree size, and topographical location influenced tree transpiration in northern boreal forests during the historic 2018 drought

Trees in northern latitude ecosystems are projected to experience increasing drought stress as a result of rising air temperatures and changes in precipitation patterns in northern latitude ecosystems. However, most drought-related studies on high-latitude boreal forests (>50 degrees N) have been conducted in North America, with few studies quantifying the response in European and Eurasian boreal forests. Here, we tested how daily whole-tree transpiration (Q, Liters day(-1)) and Q normalized for mean daytime vapor pressure deficit (Q(DZ), Liters day(-1) kPa(-1)) were affected by the historic 2018 drought in Europe. More specifically, we examined how tree species, size, and topographic position affected drought response in high-latitude mature boreal forest trees. We monitored 30 Pinus sylvestris (pine) and 30 Picea abies (spruce) trees distributed across a topographic gradient in northern Sweden. In general, pine showed a greater Q(DZ) control compared to spruce during periods of severe drought (standardized precipitation-evapotranspiration index: SPEI < -1.5), suggesting that the latter are more sensitive to drought. Overall, Q(DZ) reductions (using non-drought Q(DZ) as reference) were less pronounced in larger trees during severe drought, but there was a species-specific pattern: Q(DZ) reductions were greater in pine trees at high elevations and greater in spruce trees at lower elevations. Despite lower Q(DZ) during severe drought, drought spells were interspersed with small precipitation events and overcast conditions, and Q(DZ) returned to pre-drought conditions relatively quickly. This study highlights unique species-specific responses to drought, which are additionally driven by a codependent interaction among tree size, relative topographic position, and unique regional climate conditions