Leaf spectroscopy reveals drought response variation in Fagus sylvatica saplings from across the species’ range

The common European beech ( F. sylvatica ) is sensitive to prolonged droughts, and its natural distribution is expected to shift with climate change. To persist in novel environments, young trees rely on the capacity to express diverse response phenotypes. Several methods exist to study drought effects on trees and their diverse adaptive mechanisms, but these are usually destructive, and challenging to scale to the large sample numbers needed to investigate biological variation.

We conducted a common garden experiment outdoors, but under controlled watering conditions, with 180 potted two-year-old saplings from 16 beech provenances across the species’ range, representing three distinct genetic clusters. Drought stress was simulated by interrupting irrigation. We measured leaf reflectance of visible to short-wave infrared electromagnetic radiation. We determined drought-induced changes in biochemical and structural traits derived from spectral indices and a model of leaf optical properties and assessed intraspecific response diversity.

We quantified changes in pigmentation, water balance, nitrogen, lignin, epicuticular wax, and leaf mass per area in drought-treated saplings from different genetic clusters, revealing differences in likely adaptive responses to drought. F. sylvatica saplings from the Iberian Peninsula showed signatures of greater drought resistance, i.e., the least trait change in response to the drought treatment. Stomatal conductance and soil moisture were used to assess drought severity.

We demonstrate that high-resolution, broad-range leaf spectroscopy is an effective and non-destructive tool to assess individual drought responses that can characterize functional intraspecific variation among young beech trees.