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What makes leaves fall in autumn? A new process description for the timing of leaf senescence in temperate and boreal trees

Periodic Reporting for period 3 - LEAF-FALL (What makes leaves fall in autumn? A new process description for the timing of leaf senescence in temperate and boreal trees)

Reporting period: 2020-02-01 to 2021-07-31

Leaf phenology is a key component in the functioning of temperate and boreal deciduous forests, but little is known about the cues controlling the timing of leaf fall in autumn. Leaf fall is the last stage of leaf senescence, a process which allows trees to recover leaf nutrients. We urgently need to understand the controls timing leaf senescence to improve our projections of forest growth and climate change.
I propose a new general paradigm of the onset of leaf senescence, hypothesizing that leaf senescence is triggered by the cessation of tree growth in autumn. I expect that: (H1) in the absence of growth-limiting environmental conditions, tree growth cessation directly controls leaf-senescence onset; and (H2) in the presence of growth-limiting conditions, photoperiod controls leaf-senescence onset – this prevents trees from starting to senesce too early. In addition I expect that: (H3) the controlling mechanisms of leaf senescence do not differ between young and mature trees; (H4) the dual control of photoperiod and/or tree growth on the onset of leaf senescence is valid among species and genotypes from different latitudes; and (H5) the proposed general paradigm captures the timing of leaf senescence recorded across the major temperate and boreal forested areas of Europe.
The objective is to test these hypotheses with a combination of: (i) manipulative experiments on young trees - these will disentangle the impact of photoperiod from that of other factors affecting tree growth cessation, namely: temperature, drought and soil nutrient availability; (ii) monitoring leaf senescence and growth in mature forest stands; (iii) comparing the leaf senescence dynamics of four major tree species (Fagus sylvatica, Quercus robur, Betula pendula and Populus tremula) in four European locations spanning from 40º to 70º N in Belgium, Spain, Norway and Sweden; and (iv) integrating the new paradigm into a model of forest ecosystem dynamics and testing it for the major forested areas of Europe.
The overall objective is to solve the conundrum of the timing of leaf senescence in temperate and boreal deciduous trees, provide a new interpretation of the relationship between leaf senescence, tree growth and environment, and deliver a modelling tool able to predict leaf senescence and tree growth, for projections of forest biomass production and climate change.

The research is divided into five WPs. After 48 months of the project, WP1 is nearly finished, WP2 and WP3 have been completed, while WP4 has been started and developed.

WP1: manipulative-experiments on young trees in Belgium
Three large experiments have been set-up and used to study leaf senescence and tree growth on young potted trees under different environmental conditions. The first experiment was conducted outdoor, focusing on the impact of nutrients under natural light and temperature but optimal soil water maintained by irrigation (years: 2017, 2018 and 2019; species: all four study species). The second experiment focused on the impact of nutrients, drought and their interaction under light and temperature conditions within greenhouses (years: 2019 and 2020; species: F. sylvatica and B. pendula). The third experiment studied the separated and combined impacts of nutrients, drought, and temperature, using 12 active climate-controlled growing chambers but with natural light (year: 2018; species: F. sylvatica).

WP2: manipulative-experiments on young trees in the other locations
Experiments corresponding to the first experiment of WP1 have been set up and carried in Spain and Norway in 2017, 2018 and 2019.

WP3: monitoring of mature forest stands
We studied leaf senescence and tree growth also on mature forest trees (n=4-12) for each of the four species in Belgium, Spain and Norway (12 stands). Measurements have been performed for the whole growing season (March-November) of 2018 and 2019 and also in the autumn (September-November) of 2017 and 2020. Moreover, for one or multiple years, we studied six additional stands across locations for other comparisons (e.g. between fertile and infertile conditions). Finally, in autumn on 2020, we also investigated one additional stand located in Sweden.

WP4: construction of an European database of forest autumn dynamics
The database comprises phenological data of both leaf- and wood autumn dynamics. We work on this WP since summer of 2020 and the part of the database on the wood dynamics is nearly completed.


Paper 1. “Detecting the onset of autumn leaf senescence in deciduous forest trees of the temperate zone” (Mariën et al. 2019 New Phytologist).
Paper 2. “Timeline of autumn phenology in temperate deciduous tree species” (Dox et al. 2020 Tree Physiology).
Paper 3. “Does summer drought affect the timing of autumn cessation of wood formation in late successional temperate deciduous trees?” (Dox et al. 2021 Tree Physiology).
Paper 4. “Does drought advance the onset of autumn leaf senescence in temperate deciduous forest trees?” (Mariën et al. 2021 Biogeosciences).
Paper 5. “High-resolution X-ray computed tomography: a new workflow for the analysis of xylogenesis and intra-seasonal wood biomass production” (Lehnebach et al. 2021 Frontiers in Plant Science).

Additionally, two ancillary papers of the LEAF-FALL team (Marchand et al 2020 Agricultural and Forest Meteorology, and Marchand et al 2021 Tree Physiology) showed that, for temperate deciduous trees, inter-individual variability in spring phenology of leaves and wood growth dynamics is crucially related to tree characteristics but also to the autumn phenology of the previous’ year. Furthermore, five other papers originating from the LEAF-FALL projects have been submitted to specialistic journals. Finally, four papers are currently in preparations. Among the others, it is worth mentioning the manuscript “What drives the autumn cessation of wood growth in temperate deciduous trees?” (Flores et al) which investigates the environmental controls of wood autumn dynamics using the new European database constructed in WP4.
The project is expected to provide the data (both experimental and modelling simulations) and tools (new methodologies, databases, models) needed to test the initial hypotheses, as well as publications.

In details, the expected key outputs of LEAF-FALL are:
(1) A new general paradigm explaining the environmental control on the timing of leaf senescence in deciduous trees of the temperate and boreal zone;
(2) A new version of the model ORCHIDEE, including the new general paradigm, able to predict leaf-senescence timing and its feedback on forest growth and on the interaction between forest ecosystems and atmosphere;
(3) Unique datasets on the relationships between timing of leaf senescence, tree growth and photoperiod, based on eco-physiological and remote sensing measurements;
(4) A new European database of leaf senescence and autumn wood growth phenology and its environmental drivers based on data from long-term monitoring plots, international networks and the literature;
(5) Several publications with fundamental (e.g. on ecophysiology of leaf senescence) and applied appeal (e.g. new methodological applications) with strong ground-breaking potential.