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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 4 - 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: 2021-08-01 to 2023-01-31

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 60 months of work, WP1 and WP4 are nearly finished, WP2 and WP3 have been completed, while WP5 has been started.

WP1: manipulative-experiments on young trees in Belgium
Four 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 outdoor climate-controlled growing chambers with natural light (year: 2018; species: F. sylvatica). The fourth experiment (ongoing) investigates the separated and combined impacts of nutrients, temperature and modified daylength using 4 active indoor climate-controlled growing chambers with artificial light (year: 2022 and, planned, 2023; species: F. sylvatica and B. pendula).

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, focusing on beech and birch.

WP3: monitoring of mature forest stands
We studied leaf senescence and tree growth also on mature forest trees for each of the four species in Belgium, Spain and Norway (12 stands). Measurements have been performed for the autumn of 2017 till 2020, with additional measurements in the autumn of 2021 in Belgium. Spring and summer measurements have also been performed in most years. Moreover, for one or multiple years, we studied seven additional stands for other comparisons (e.g. between fertile and infertile conditions), comprising also one stand located in Sweden.

WP4: construction of an European database of forest autumn dynamics
The database is focused on wood autumn dynamics, comprising data from our study sites and from other 60 forest sites investigated by 13 European teams in the last 20 years. The first version and analysis of the database is ready, and we are working for its publication. In addition, for our sites, the dataset is supplemented by dataset on autumn leaf phenology, with remote sensing data also available.

WP5: modelling implementation of the findings
All preparatory work for this task has been made.

Multiple datasets on experiments on young and mature trees have been produced, as well as a database of wood phenology at the European level. A total of 13 peer-reviewed articles have been produced, with 6 reported here as example.
1. “Detecting the onset of autumn leaf senescence in deciduous forest trees of the temperate zone” (Mariën et al. 2019 New Phytologist).
2. “Timeline of autumn phenology in temperate deciduous tree species” (Dox et al. 2020 Tree Physiology).
3. “Does drought advance the onset of autumn leaf senescence in temperate deciduous forest trees?” (Mariën et al. 2021 Biogeosciences).
4. “Wood growth phenology and its relationship with leaf phenology in deciduous forest trees of the temperate zone of Western Europe” (Dox et al 2022, Agricultural and Forest Meteorology).
5. “Warming does not delay the start of autumnal leaf coloration but slows its progress rate” (Jiang N et al 2022 Global Ecology and Biogeography).
6. “Severe drought can delay autumn senescence of silver birch in the current year but advance it in the next year” (Dox et al 2022 Agricultural and Forest Meteorology)
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.