Periodic Reporting for period 5 - 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: 2023-02-01 to 2024-01-31
Leaf fall is the last stage of leaf senescence, a process which allows trees to recover leaf nutrients. We urgently need to understand the environmental controls timing leaf senescence to improve our projections of forest growth and climate change.
I proposed a new general paradigm of the onset of leaf senescence. I expected 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. In addition I expected 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; and (H5) the proposed general paradigm captures the timing of leaf senescence across the major temperate and boreal forested areas of Europe.
The objective was to test these hypotheses on four major tree species (Fagus sylvatica, Quercus robur, Betula pendula and Populus tremula) in four European locations (in Belgium, Spain, Norway and Sweden) with a combination of: (i) manipulative experiments on young trees (in Belgium, WP1; in the other locations, WP2); (iii) monitoring mature forest stands (WP3); (iv) constructing an European database of autumn tree dynamics (WP4); and (v) integrating the new paradigm into a model of forest ecosystem dynamics (WP5).
We first detected the best methods to assess timing of leaf senescence and tree growth cessation for deciduous forests. Second, we assessed the timeline of leaf senescence and autumn tree growth cessation for the four model species across temperate Europe. This analysis showed a tight coupling between stem tree growth cessation and leaf senescence but not a strong evidence that stem growth cessation drives the leaf senescence onset. Moreover, we discovered that coarse roots of deciduous species continue to growth during winter. Therefore, against H1, we found that tree growth cessation in autumn does not control leaf senescence timing. We observed that photoperiod has a stronger impact on leaf senescence onset than other environmental factors. However, the comparison of years with contrasting climatic conditions (e.g. dry or wet summer), indicated that photoperiod is not the key determinant of leaf senescence onset in unfavorable growth conditions, which was also not expected (H2). The unexpected outcome of H1 and H2 undermined the validity of H5. On the other side, our work did not reveal differences in autumn dynamics between young and mature trees, which confirms H3. Moreover, thought the dual control of photoperiod and tree growth on the onset of leaf senescence was invalid, the existence in autumn of different thresholds to photoperiod and environmental drivers of tree growth across different species and genotypes was confirmed within H4. Our project indicated that to elucidate leaf senescence timing, we need to consider legacy effects from the current and previous year. The project also shaded light on (i) the role that senescence plays in affecting the inter-tree variability of spring budburst and (ii) the environmental controls of stem growth in autumn.
I proposed a new general paradigm of the onset of leaf senescence. I expected 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. In addition I expected 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; and (H5) the proposed general paradigm captures the timing of leaf senescence across the major temperate and boreal forested areas of Europe.
The objective was to test these hypotheses on four major tree species (Fagus sylvatica, Quercus robur, Betula pendula and Populus tremula) in four European locations (in Belgium, Spain, Norway and Sweden) with a combination of: (i) manipulative experiments on young trees (in Belgium, WP1; in the other locations, WP2); (iii) monitoring mature forest stands (WP3); (iv) constructing an European database of autumn tree dynamics (WP4); and (v) integrating the new paradigm into a model of forest ecosystem dynamics (WP5).
We first detected the best methods to assess timing of leaf senescence and tree growth cessation for deciduous forests. Second, we assessed the timeline of leaf senescence and autumn tree growth cessation for the four model species across temperate Europe. This analysis showed a tight coupling between stem tree growth cessation and leaf senescence but not a strong evidence that stem growth cessation drives the leaf senescence onset. Moreover, we discovered that coarse roots of deciduous species continue to growth during winter. Therefore, against H1, we found that tree growth cessation in autumn does not control leaf senescence timing. We observed that photoperiod has a stronger impact on leaf senescence onset than other environmental factors. However, the comparison of years with contrasting climatic conditions (e.g. dry or wet summer), indicated that photoperiod is not the key determinant of leaf senescence onset in unfavorable growth conditions, which was also not expected (H2). The unexpected outcome of H1 and H2 undermined the validity of H5. On the other side, our work did not reveal differences in autumn dynamics between young and mature trees, which confirms H3. Moreover, thought the dual control of photoperiod and tree growth on the onset of leaf senescence was invalid, the existence in autumn of different thresholds to photoperiod and environmental drivers of tree growth across different species and genotypes was confirmed within H4. Our project indicated that to elucidate leaf senescence timing, we need to consider legacy effects from the current and previous year. The project also shaded light on (i) the role that senescence plays in affecting the inter-tree variability of spring budburst and (ii) the environmental controls of stem growth in autumn.
WORK CARRIED OUT
H1. We set up experiments on potted young trees in favourable environmental conditions and monitored mature trees in fertile sites. We investigated experiments for (i) trees subjected to high fertilization (2-4 species, 3-4 years; Belgium, Spain and Norway) and (ii) trees subjected to high fertilization and favourable temperature (2 species, 2 years; Belgium). We monitored mature trees on fertile sites in Belgium, Spain, Norway and Sweden (4 species, 15 sites, up to 7 years). The relationship between leaf senescence and tree growth was determined by measuring autumn leaf chlorophyll and/or coloration and stem xylogenesis. For a limited number of experiments (2 years, Belgium; 1 year, Spain and Norway) branch- and root xylogenesis, as well as fine root elongation, were also measured.
H2. We set up experiments on potted young trees in unfavourable environmental conditions and monitored mature trees in infertile sites. We investigated experiments of (i) trees subjected to low fertilization (2-4 species, 3-4 years; Belgium, Spain and Norway) and (ii) trees subjected to drought and low fertilization (2 species, 2 years; Belgium). We monitored mature trees on infertile sites in Belgium (3 species, 2 sites, 7 years). The impact of photoperiod on leaf senescence was studied by measuring leaf chlorophyll and/or coloration and comparing across years. An additional experiment was performed (WP1; 2 species, 2 years; Belgium) where the impact of photoperiod on leaf senescence was directly tested by subjecting trees in growing chambers to modified seasonal photoperiod.
H3. We compared results of young and mature trees from the experiments / monitoring performed to test H1 and H2.
H4. We compared the results of the four species studied in the experiments and forest monitoring performed to test H1 and H2 in Belgium, Spain and Norway.
H5. The testing of this hypothesis become unnecessary as the expectations were not met. Additional research was performed to test for alternative hypotheses e.g. the control of budburst timing on senescence timing.
RESULTS
A total of 15 international peer-reviewed articles have been produced, and six additional papers are in preparation. The following key tools were developed: (i) multiple databases of leaf senescence for experimental young trees and mature forest trees, (ii) a new methodology to determine tree growth cessation in autumn based on X-ray CT, (iii) a new European database of autumn tree growth (xylogenesis), and (iv) new modelling frameworks to estimate the timing of leaf senescence and stem growth cessation in deciduous trees in autumn.
The results have been presented in 11 international conferences / events. At least 9 (video)interviews of LEAF-FALL PI and team members on the topic of the project have appeared on the national media of four countries (Belgium, Spain, Switzerland, The Netherlands) (e.g. universiteitvanvlaanderen.be/college/how-do-leaves-know-when-to-fall).
H1. We set up experiments on potted young trees in favourable environmental conditions and monitored mature trees in fertile sites. We investigated experiments for (i) trees subjected to high fertilization (2-4 species, 3-4 years; Belgium, Spain and Norway) and (ii) trees subjected to high fertilization and favourable temperature (2 species, 2 years; Belgium). We monitored mature trees on fertile sites in Belgium, Spain, Norway and Sweden (4 species, 15 sites, up to 7 years). The relationship between leaf senescence and tree growth was determined by measuring autumn leaf chlorophyll and/or coloration and stem xylogenesis. For a limited number of experiments (2 years, Belgium; 1 year, Spain and Norway) branch- and root xylogenesis, as well as fine root elongation, were also measured.
H2. We set up experiments on potted young trees in unfavourable environmental conditions and monitored mature trees in infertile sites. We investigated experiments of (i) trees subjected to low fertilization (2-4 species, 3-4 years; Belgium, Spain and Norway) and (ii) trees subjected to drought and low fertilization (2 species, 2 years; Belgium). We monitored mature trees on infertile sites in Belgium (3 species, 2 sites, 7 years). The impact of photoperiod on leaf senescence was studied by measuring leaf chlorophyll and/or coloration and comparing across years. An additional experiment was performed (WP1; 2 species, 2 years; Belgium) where the impact of photoperiod on leaf senescence was directly tested by subjecting trees in growing chambers to modified seasonal photoperiod.
H3. We compared results of young and mature trees from the experiments / monitoring performed to test H1 and H2.
H4. We compared the results of the four species studied in the experiments and forest monitoring performed to test H1 and H2 in Belgium, Spain and Norway.
H5. The testing of this hypothesis become unnecessary as the expectations were not met. Additional research was performed to test for alternative hypotheses e.g. the control of budburst timing on senescence timing.
RESULTS
A total of 15 international peer-reviewed articles have been produced, and six additional papers are in preparation. The following key tools were developed: (i) multiple databases of leaf senescence for experimental young trees and mature forest trees, (ii) a new methodology to determine tree growth cessation in autumn based on X-ray CT, (iii) a new European database of autumn tree growth (xylogenesis), and (iv) new modelling frameworks to estimate the timing of leaf senescence and stem growth cessation in deciduous trees in autumn.
The results have been presented in 11 international conferences / events. At least 9 (video)interviews of LEAF-FALL PI and team members on the topic of the project have appeared on the national media of four countries (Belgium, Spain, Switzerland, The Netherlands) (e.g. universiteitvanvlaanderen.be/college/how-do-leaves-know-when-to-fall).
(1) Evaluation and improvements of different methods to determine leaf senescence timing
(2) A new method to study xylogenesis and to assess the autumn tree growth cessation
(3) Impact of drought and drought-legacy on leaf senescence timing
(4) Impact of photoperiod on leaf senescence timing
(5) Assessment of the autumn timeline of tree growth cessation and leaf senescence in deciduous temperate trees
(6) Elucidation of intra-tree variability in autumn tree growth: differences among aboveground and belowground organs
(7) New empirical models to simulate the timing of autumn tree growth cessation in deciduous temperate trees based on tree traits and environmental conditions
(8) A new European database of autumn xylogenesis in deciduous trees
(9) The relationship between autumn leaf senescence and next year spring budburst
(2) A new method to study xylogenesis and to assess the autumn tree growth cessation
(3) Impact of drought and drought-legacy on leaf senescence timing
(4) Impact of photoperiod on leaf senescence timing
(5) Assessment of the autumn timeline of tree growth cessation and leaf senescence in deciduous temperate trees
(6) Elucidation of intra-tree variability in autumn tree growth: differences among aboveground and belowground organs
(7) New empirical models to simulate the timing of autumn tree growth cessation in deciduous temperate trees based on tree traits and environmental conditions
(8) A new European database of autumn xylogenesis in deciduous trees
(9) The relationship between autumn leaf senescence and next year spring budburst