Detecting the onset of autumn leaf senescence in deciduous forest trees of the temperate zone

Abstract

Summary Information on the onset of the leaf senescence in temperate deciduous trees and comparisons on its assessment methods are limited, hampering our understanding of autumn dynamics.We compare five field proxies, five remote sensing proxies and two data analysis approaches to assess leaf senescence onset at one main beech stand, two stands of oak and birch and three ancillary stands of the same species in Belgium during 2017 and 2018.Across species and sites, onset of leaf senescence was not significantly different for the field proxies based on chlorophyll leaf content and canopy coloration, except for an advanced canopy coloration during the extremely dry and warm 2018. Two remote sensing indices provided results fully consistent with the field data. A significant lag emerged between leaf senescence onset and leaf fall, and when a threshold of 50% change in the seasonal variable under study (e.g. chlorophyll content) was used to derive the leaf senescence onset.Our results provide unprecedented information on the quality and applicability of different proxies to assess leaf senescence onset in temperate deciduous trees. In addition, a sound base is offered to select the most suited methods for the different disciplines that need this type of data.

Inter-individual variability in spring phenology of temperate deciduous trees depends on species, tree size and previous year autumn phenology

“…The first one is the ‘Park of Brasschaat’ (PB), the second one is the military domain ‘Klein Schietveld’ (KS), a Natura2000 reserve. The study areas are at the distance of 9,5 km, in a region with topography flat and without any features possibly causing significant climatological differences ( Mariën et al., 2019 ). Both areas encompass planted (oak and beech) and unplanted stands (birch) on sandy soil, with low management activity.…”

“…The onset of leaf senescence was defined as the point in time (breakpoint) when the seasonal canopy coloration sharply increased due to the start of the natural process of leaf senescence (before this point slight coloration already happened due to slow build-up of leaf stress damage). Details about the determination of the leaf senescence onset at the study sites can be found in ( Mariën et al., 2019 ).…”

Does drought advance the onset of autumn leaf senescence in temperate deciduous forest trees?

“…The loss of canopy greenness was observed for the same tree individuals and four additional tree individuals per species and site (thus for 32 trees in total). The two forests and their meteorological conditions are described in detail by Mariën et al (2019), which also showed a lack of site effects on the autumn chlorophyll dynamics for the tree species studied here. To have a larger statistical sample, the data of the two beech stands (also of similar age and stem diameter) were aggregated.…”

“…In principle, the onset of autumn leaf senescence could be derived from the CCI or loss of canopy greenness. However, Mariën et al (2019) recently showed that the latter method cannot be used under severe drought stress. Therefore, two phenological variables were considered to describe the autumn canopy dynamics: the onset of autumn leaf senescence derived from the CCI (the onset of autumn leaf senescenceCCI) and the onset of the loss of canopy greenness.…”

“…For each tree, we defined the onset of autumn leaf senescence and the onset of loss of canopy greenness as the date by which the variable of interest started to decline substantially in early autumn. These dates were calculated using piecewise linear regressions and are represented by the breakpoints resulting from these analyses (Menzel et al, 2015;Mariën et al, 2019;Xie and Wilson, 2020). The piecewise linear regressions were performed using R/DPLYR and R/SEGMENTED (Vito and Muggeo, 2008).…”