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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 2 - 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: 2018-08-01 to 2020-01-31

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.
EXPLANATION OF THE WORK CARRIED OUT PER WP

The research is divided into five WPs. During the first 36 months of the project, the work has been focused on WP1, WP2 and WP3.

WP1: manipulative-experiments on young trees in Belgium
Experiments have been set-up and used to study leaf senescence and growth on young potted trees under different environmental conditions. Setting 1a. An experiment to study the separated and combined impacts of temperature, drought and nutrients on leaf senescence has been conducted in 2018 and 2019 for one model species (F. sylvatica) in 12 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 conducted in 2017, 2018 and 2019 for the all four study species. 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 nutrient conditions has been conducted in 2018 and 2019 on two species (F. sylvatica and B. pendula).

WP2: manipulative-experiments on young trees in the other locations
Experiments corresponding to Setting 1b of WP1 have been set up and carried in Spain and Norway in 2017, 2018 and 2019.

WP3: monitoring of mature forest stands
We selected study trees (n=8) in stands for each of the four species in Spain, Belgium and Norway (12 stands). Additionally, we selected eight stands across locations for other comparisons (e.g. between fertile and infertile conditions).

Measurements have been performed for autumn 2017 and for the whole growing season of 2018 and 2019, for all four study species but focusing especially on F. sylvatica and B. pendula.


RESULTS

Paper 1. “Detecting the onset of autumn leaf senescence in deciduous forest trees of the temperate zone”. Mariën B …… Campioli M (accepted in New Phytologist). In this manuscript we have compared the most common methods used to assess the onset of leaf senescence, we have detected the best methods and further ameliorated them.
Paper 2. “Timeline of autumn phenology in temperate deciduous tree species” Dox I …Campioli M ( accepted pending major revisions in Tree Physiology). In this manuscript we described the timeline of autumn phenology of leaves and wood and found that tree growth cessation occurs concurrently or before to leaf senescence onset.
Paper 3. “Does summer drought affect the timing of autumn cessation of wood formation in late successional temperate deciduous trees?” Dox I …. Campioli M (to be submitted to a specialized journal in spring 2020). This manuscript shows that the drought response of the timing of tree growth cessation in autumn depends on the tree functional type and only ring-porous species are affected.
Paper 4. “Impact of drought on the timing of leaf senescence in temperate deciduous trees” Mariën B …… Campioli M (to submit it to a specialized journal in summer 2020). Based on the manipulative experiment in climate-controlled greenhouses and the comparison of trees in the field for a “normal” (2017) and “exceptionally dry” year (2018), we show the impact of drought on the onset of leaf senescence and autumn leaf dynamics.
Paper 5. “Autumn xylogenesis: roots grow longer than stem and branches”. Dox I …. Campioli M (in preparation). This manuscript shows that woody organs within a tree have different phenology and particularly that woody roots growth longer than aboveground wood.
Paper 6. “Inter-individual variability in spring phenology of temperate deciduous trees is related to tree size and previous year autumn phenology” Marchand LJ……. Campioli M (submitted to Agricultural and Forest Meteorology). We show that, for temperate deciduous trees, inter-individual variability in spring phenology is crucially related to tree growth characteristics but also to the autumn phenology of the previous’ year.
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 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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