Timeline of autumn phenology in temperate deciduous trees

Abstract

Cessation of xylem formation or wood growth (CWG) and onset of foliar senescence (OFS) are key autumn phenological events in temperate deciduous trees. Their timing is fundamental for the development and survival of trees, ecosystem nutrient cycling and the seasonal exchange of matter and energy between the biosphere and atmosphere, and affects the impact and feedback of forests to global change. A large-scale experimental effort and improved observational methods have allowed us to compare the timing of CWG and OFS for different deciduous tree species in Western Europe, particularly in silver birch, a pioneer species, and European beech, a late-succession species, at stands of different latitudes, of different levels of site fertility, for 2 years with contrasting meteorological and drought conditions, i.e., the low moderately dry 2017 and the extremely dry 2018. Specifically, we tested whether foliar senescence started before, after or concurrently with CWG. Onset of foliar senescence and CWG occurred generally between late September and early November, with larger differences across species and sites for OFS. Foliar senescence started concurrently with CWG in most cases, except for the drier 2018 and, for beech, at the coldest site, where OFS occurred significantly later than CWG. The behavior of beech in Spain, the southern edge of its European distribution, was unclear, with no CWG, but very low wood growth at the time of OFS. Our study suggests that OFS is generally triggered by the same drivers of CWG or when wood growth decreases in late summer, indicating an overarching mechanism of sink limitation as a possible regulator of the timing of foliar senescence.

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

“…The date of bud-burst was defined as the date when 50% of the buds in a tree reached stage 2, combining together top and bottom crown buds to have an indication at the tree level ( Vitasse et al., 2009 ). Leaf autumn phenology was monitored by observing the canopy coloration at day t (Xt) ( Dox et al., 2020 ). In late summer and early autumn, canopy coloration was estimated directly, as a percentage.…”

“…We defined the end of wood formation as the moment in time when latewood differentiation was completed (i.e. the proportion of cells still in the wall-thickening phase was <0.5%) ( Dox et al., 2020 ).…”

Divergent responses of phenology and growth to summer and autumnal warming

“…First, delayed foliar senescence increases photosynthetic carbon gains due to the later degradation of chlorophyll and extends the growing season (source‐driven timing of foliar senescence; de Cárcer et al, 2018; Shi et al, 2015). Second, higher temperatures extend the growth of wood and lead to a later foliar senescence (sink‐driven timing of foliar senescence; Dox et al, 2020). Our results suggest that the larch seedlings could gain a competitive advantage (e.g., more carbon assimilation, longer wood growth) under warmer and favorable hydrological conditions in autumn.…”

“…First, delayed foliar senescence increases photosynthetic carbon gains due to the later degradation of chlorophyll and extends the growing season (source-driven timing of foliar senescence; de Cárcer et al, 2018;Shi et al, 2015). Second, higher temperatures extend the growth of wood and lead to a later foliar senescence (sink-driven timing of foliar senescence; Dox et al, 2020 (Allen et al, 2010). Our manipulative experiment found that seedling mortality was significantly higher for both summer and year-round warming than for the control treatment, especially in the dry 2019 compared to the wet 2018.…”

Inter-tree variability of autumn leaf phenology of European beech (Fagus sylvatica) on a site in Ljubljana, Slovenia

“…Foliar senescence which can be visually observed is accompanied by cessation of xylem and phloem formation by the cambium, which can only be studied under the microscope after tissue sampling and preparation (Čufar et al, 2008;Prislan et al, 2013aPrislan et al, , 2013b. The combination of the two approaches is therefore of great research interest (Čufar et al, 2008;Dox et al, 2020).…”