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ERC

LEAF-FALL Report Summary

Project ID: 714916
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - 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: 2017-02-01 to 2018-07-31

Summary of the context and overall objectives of the project

Leaf phenology is a key component in the functioning of temperate and boreal deciduous forests. The environmental cues for bud-burst in spring are well known, 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, but different species and genotypes have different thresholds to photoperiod or environmental drivers of tree growth (temperature, soil water or nutrients) according to species traits and characteristics (e.g., temperature sensitivity, drought tolerance, nutrient dynamics); 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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

WORK CARRIED OUT DURING THE REPORTING PERIOD TOWARDS THE ACHIEVEMENT OF THE OBJECTIVES

H1. To test this hypothesis, we set up and investigated (measuring leaf senescence and tree growth) experiments on potted young trees in unfavourable environmental conditions and monitored mature trees in infertile sites. In particular, we set up and investigated pot experiments of (i) one tree species subjected to combined drought, low fertilization and no warming, (ii) four tree species subjected to drought and (iii) four tree species subjected to low nutrients. The first two experiments were conducted in Belgium, while the third experiment was concurrently conducted in Belgium, Spain and Norway (thus on three nearly identical settings). The monitoring of mature trees on infertile sites occurred mainly in Belgium. The impact of photoperiod (time of the year) on the processes studied can be understood repeating the experiments / monitoring for 3 or more years (1.5 years were investigated in the reporting period).

H2. To test this hypothesis, we set up and investigated (measuring leaf senescence and tree growth) experiments on potted young trees in favourable environmental conditions and monitored mature trees in fertile sites. In particular, we set up and investigated pot experiments of (i) one tree species subjected to optimal soil water, high fertilization and warming, (ii) four tree species subjected to optimal soil water and (iii) four tree species subjected to high fertilization. The first two experiments were conducted in Belgium, while the third experiment was concurrently conducted in Belgium, Spain and Norway (thus on three nearly identical settings). The monitoring of mature trees on fertile sites occurred in Belgium, Spain and Norway.

H3. This hypothesis will be tested comparing results of young and mature trees from the experiments / monitoring performed to test H1 and H2 (see above). As the data for comparison will be available only after year 2 of the project, this specific objective was not tested in the first reporting period.

H4. This hypothesis will be tested comparing results of young and mature trees from the experiments / monitoring performed to test H1 and H2 in Belgium, Spain and Norway (see above). As the data for comparison will be available only after year 2 of the project, this specific objective was not tested in the first reporting period.

H5. This hypothesis will be tested in year 4 and 5 of the project.


EXPLANATION OF THE WORK CARRIED OUR PER WP

The research is divided into five WPs each structured around a research infrastructure or tool: WP1: manipulative-experiments on young trees in Belgium; WP2: manipulative-experiments on young trees in the other locations; WP3: monitoring of mature forest stands; WP4: database development for modelling, and WP5: model development and testing. During the first 18 month, as planned, the work has been focused on WP1, WP2 and WP3.

WP1: manipulative-experiments on young trees in Belgium. As planned, experiments have been set-up and tested to study leaf senescence and growth on young potted trees under different environmental conditions. Setting 1a. A full factorial experiment to study the separated and combined impacts of temperature, soil water and nutrients on leaf senescence under natural light conditions has been established in spring 2018 for one model species (Fagus sylvatica) in 16 active climate-controlled small greenhouses. Setting 1b. An experiment to study the impact of nutrients on leaf senescence under natural light and temperature conditions and optimal soil water has been established in spring 2017 for the all four study species (Fagus sylvatica, Quercus robur, Betula pendula, Populus tremula). Setting 2. An experiment to study the impact of drought on leaf senescence under light and temperature conditions within a large passive greenhouse and optimal nutrients conditions has been established in spring 2018 on the four study species.

WP2: manipu

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

PROGRESS BEYOND THE STATE OF THE ART

Several progresses have been made (see section above), but in particular:

(1) Data from 2017 for mature trees in Belgium, Spain and Norway showed that autumn tree growth cessation occurs consistently before or concurrently to the onset of leaf senescence, which is a crucial requirement to the validity of H1 and H2. A manuscript on this topic is under preparation and will be submitted to a peer-reviewed journal in autumn 2018.

(2) We found that inter-tree variability in autumn phenology (i.e. timing of leaf senescence onset and tree growth cessation) and other characteristics of tree growth (e.g. tree diameter, height, age) crucially determine the inter-tree variability in spring phenology. This is important as inter-tree variability of phenology has been thought till now to be primarily related to genetic differences. Thesis publication on the topic: Marchand L. 2018. Inter-individual variability in spring phenology of leaves and wood of temperate deciduous trees. MSc Thesis. University of Antwerp and University of Orleans (the thesis is being reworked to become a peer-reviewed international publication).


EXPECTED RESULTS UNTIL THE END OF THE PROJECT

LEAF-FALL will provide several key outputs, among which:
(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 its environmental drivers based on data from long-term monitoring plots, international networks and the literature;
(5) Several publications with fundamental (e.g., the new general paradigm) and applied appeal (e.g. modelling applications) with strong ground-breaking potential.
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