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

The common European beech (F. 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
two-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. Fagus sylvatica saplings from the Iberian
Peninsula showed signatures of greater drought resistance, i.e., 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.