Leaf Spectroscopy Reveals Drought Response Variation in <i>Fagus sylvatica</i> Saplings From Across the Species' Range

Abstract The common European beech ( Fagus sylvatica ), sensitive to prolonged drought, is expected to shift its distribution 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 for the large sample numbers needed to investigate biological variation. We conducted a common garden experiment outdoors, but under controlled watering conditions, with 180 potted 2‐year‐old saplings from 16 beech provenances across the species' range, representing three distinct genetic clusters. Drought stress was simulated by interrupting irrigation and stomatal conductance and soil moisture were used to assess drought severity. We measured leaf reflectance of visible to short‐wave infrared electromagnetic radiation to determine drought‐induced changes in biochemical and structural traits derived from spectral indices and a model of leaf optical properties. We quantified changes in pigmentation, water balance, nitrogen, lignin, epicuticular wax, and leaf mass per area in drought‐treated saplings, revealing differences in likely adaptive responses to drought. F. sylvatica saplings from the Iberian Peninsula showed signatures of greater drought resistance, that is, the least drought‐induced change in spectrally derived traits related to leaf pigments and leaf water content. We demonstrate that high‐resolution leaf spectroscopy is an effective and non‐destructive tool to assess individual drought responses that can characterize functional intraspecific variation among young beech trees. Next, this approach should be scaled up to canopy‐level or airborne spectroscopy to support drought response assessments of forests.