Abstract
We examined the seasonal changes in biophysical, anatomical, and optical traits of young leaves, formed throughout the vegetative season due to sylleptic growth, and mature leaves formed by proleptic growth in spring. Leaf developmental categories contribute to the top-of-canopy reflectance and should be considered when taking ground truth for remote sensing studies (RS). Deciduous tree species, Betula pendula, Populus tremula, and Alnus incana, were sampled from May to October 2018 in an Estonian hemiboreal forest. Chlorophyll and carotenoid content were detected biochemically; leaf anatomical traits (leaf, palisade, and spongy mesophyll thickness) were measured on leaf cross-sections; leaf reflectance was measured by a spectroradiometer with an integrating sphere (350–2500 nm). Biophysical and anatomical leaf traits were related to 64 vegetation indices (VIs). Linear models based on VIs for all tested leaf traits were more robust if both juvenile and mature leaves were included. This study provides information on which VIs are interchangeable or independent. Pigment and leaf thickness sensitive indices formed PC1; water and structural trait related VIs formed an independent group associated with PC3. Type of growth and leaf age could affect the validation of biophysical and anatomical leaf trait retrieval from the optical signal. It is, therefore, necessary to sample both leaf developmental categories—young and mature—in RS, especially if sampling is only once within the vegetation season.
Highlights
Under ongoing climate change, plants are exposed to an ever-increasing load of environmental stress, which may affect the vegetative net primary production and carbon sequestration [1,2,3]
Results are presented in the following sequence: first, seasonal course of anatomical and biophysical traits for three developmental leaf categories separately: juvenile, mature, and senescent; second, leaf traits compared among the juvenile neoformed leaves and mature physiologically active leaves in relation to the time in the vegetation season; seasonal course of anatomical and biophysical traits for all sampled leaves together to demonstrate overall canopy trends in studied leaf traits
Seasonal Course of Anatomical and Biophysical Traits of Juvenile, Mature, and Figure 3a–c shows the seasonal course of selected anatomical traits: palisade parenchyma thickness (PP), spongy parenchyma thickness (SP), and palisade/spongy parenchyma thickness ratio (PP/SP) of both juvenile and mature leaves for all three studied tree species (B. pendula, A. incana, P. tremula)
Summary
Plants are exposed to an ever-increasing load of environmental stress, which may affect the vegetative net primary production and carbon sequestration [1,2,3]. Forests are the most important of all terrestrial ecosystems, representing the greatest terrestrial carbon storage pool [4]. Remote sensing provides a suitable tool for large-scale i.e., spatial temporal monitoring of tree and forest status, as well as carbon fluxes and pools at large scales [5]. Hemiboreal forests represent a transitional zone between boreal and temperate forest biomes [6]. The territory of hemiboreal forests is expected to expand north, gradually replacing boreal forests [7] due to climate change, warming, which is more progressive in the northern boreal region. Hemiboreal forest stands are characterized by greater seasonal variability in forest microclimate, canopy structure, and plant activity when compared to boreal forests [7].
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