Extended canopy closure under with mild autumns, and its effect on the understorey growing season

In a new paper just out in Agricultural and Forest Meteorology, we explore how extended canopy closure in oak and birch forests at Lammi Biological Station in southern Finland affects the growing season for common understorey forbs.

Undergraduate student Arthur Daviaud sampling the understorey at Lammi Biological Station for the project which lead to this paper. Arthur was supervised by Maxime Durand (University of Helsinki) and Matthew Robson (University of Cumbria) from CanSEE group.

The paper describes an experiment to test the effects of climate warming which may extend the length of time that canopy trees in a forest keep their leaves into the winter. One puzzle associated with climate change is whether plants growing on the forest floor benefit more from the extended warm periods in the spring and autumn, or whether these warm periods mean that canopy trees retain their leaves for longer and thus restrict the light reaching the ground and available there for photosynthesis.

Autumn colouration of iconic understorey forb Hepatica nobilis
Autumn colouration of iconic understorey forb Hepatica nobilis

In an experiment comparing stands of oak, birch and spruce, we tested the potential for photosynthesis, retention of chlorophyll, and colouration of understorey plant species on the forest floor throughout autumn and the start of winter. Our main finding was that the increase in light received and change in its spectral composition following canopy leaf fall was the main trigger of senescence in the understorey. Those understorey species able to keep photosynthesizing effectively into late autumn were benefitting the most from warmer temperature and an extended closed canopy period.

Spruce scene in summer and winter
Seasonal changes in the boreal spruce forest

Plant species adaptation to high irradiances in the French Alps

Among plant species there are large differences in photoprotection against high irradiance and UV radiation. To explore how these differences are driven by taxonomic relatedness, geographical origin, and local environment we compared a huge database of plant species growing in alpine and boreal botanical gardens.

Hartikainen SM, Robson TM (2022) The roles of species’ relatedness and climate of origin in determining optical leaf traits over large set of taxa from high elevation and latitude. Frontiers in Plant Science 13 . https://doi.org/10.3389/fpls.2022.1058162

Santa Neimane and Twinkle Solanki recording diurnal patterns in leaf optical properties of alpine plants under UV filters

The Station Alpin du Lautaret in the French Alps, is a Research Platform for long-term ecological studies under the framework of Horizon 2020 Transnational Access – who funded our research visit through the French National Centre for Scientific Research, CNRS.


Pedro J Aphalo measures solar radiation; to better understand how reflection from the snow pack affects exposure of plants in the environment.


We collaborated with José Ignacio García Plazaola and Beatriz Fernandez-Marin from the University of the Basque-Country, to study how plants response to the steep increases in UV radiation that they receive on emergence from under snow cover in spring.

By characterising the patterns of response to UV radiation in terms of the photoprotection and UV-screening of plants across a diversity of species, we hope to better understand how and why these response evolved and what environmental cues underpin their induction.

Solanki T, García Plazaola JI, Robson TM*, Fernandez-Marin B* (2022) Comparative Assessment of Changes in Leaf Transmittance in Alpine Plant Species Following Freezing. Photochemical & Photobiological Sciences. https://doi.org/10.1007/s43630-022-00189-0

Fernández-Marín B, Sáenz A, Solanki T, Robson TM, García-Plazaola JI, (2021) Alpine forbs rely on different photoprotective strategies during spring snow melt. Physiologia Plantarum. 172, 1506-1517. https://doi.org/10.1111/ppl.13342

Photodegradation contributes to forest leaf litter decomposition

Qing-Wei Wang and Marta Pieriste inspecting leaf-litter decomposition filters in the understorey site of a Japanese beech forest

The spectral composition of light in a forest gap and understorey through the year affects the rate of photodegradation of senescent leaf litter material across a variety of native plant species. This finding that photodegradation plays an important role in forest litter decomposition  could partially explain the hole in the carbon budget in this ecosystem.

Wang QW, Pieristè M, Kenta T, Liu C, Robson TM, Kurokawa H (2020) Photodegradation enhances litter decomposition modulated with canopy openness in a temperate forest. New Phytologist. NPH17022 https://doi.org/10.1111/nph.17022

Enhanced decomposition can occur through direct photochemical mineralisation, but in temperate forests effects of increased temperature and the availability of substrates for microbial decompositions can be even more important. These research supports findings of our meta-analysis, that actions of photofacilitation are highly wavelength dependent and the subtilties of these responses can only be identified through very large scale experimental manipulations of sunlight, as was done in this ambitious experiment.

Wang Q-W, Robson TM, Pieristè M, Kenta T, Kurokawa H. (2022) Photodegradation dynamics below a forest canopy. Science of the Total Environment 820 153185. http://dx.doi.org/10.1016/j.scitotenv.2022.153185

Wang Q-W*, Pieristè M*, Kotilainen TK, Forey E, Chauvat M, Kurokawa H, Robson TM, Jones AG. (2022) The crucial role of blue light as a driver of litter photodegradation in terrestrial ecosystems.  Plant & Soil,  http://doi.org/10.1007/s11104-022-05596-x

Fitness of beech seedlings growing in Finland

We tested how European beech seedlings from across Europe respond to manipulations of light and watering conditions during the growing season in Finland. Our recent paper in Trees: Structure & Function reports on differences among populations receiving combinations of drought, and sun or shade conditions, including gas exchange, water relations, and UV-abs compounds in the leaf epidermis.

Beech seedlings under drought (front) & well watered (back) treatments.

Wang F, Israel D, Ramírez-Valiente J-A, Sánchez-Gómez D, Aranda I, Aphalo PJ, Robson TM. (2021) Seedlings from marginal and core populations of European beech (Fagus sylvatica L.) respond differently to imposed drought and shade. Trees Structure and Function, 35, 53-67.  https://doi.org/10.1007/s00468-020-02011-9


Surviving on the Edge

We are grateful to a grant  from HiLIFE Grand Challenges, Biological Resilience Seed Funding, allowing us to start work on the project: Surviving on the edge – trait syndromes that facilitate northerly species-range expansion.

This project will allow us to utilise a unique field-trial of Fagus sylvatica (European beech) at the University of Helsinki – far beyond the north-eastern limit of its range. Using such trials, we can compare the traits of populations in  new potentially stressful environments, giving us the tools to forecast how they will respond to climate change and potentially exploit more northerly regions of Europe.

In addition to field data, the project will test how trait information can be inserted into species distribution models. We will bring together experts in the field from Helsinki and beyond, to discuss how best to develop such models to provide informed forecasts of future species distributions. Armed with this knowledge, we will consider how policy makers, foresters, and conservation bodies might utilise this information in their work.

Newly thinned Helsinki beech-provenance trial in May 2020; with four populations covering the European range limits.

Do UV-radiation and blue light have a role in photodegradation affecting decomposition in Finnish forests over winter

The rate of decomposition of leaf litter on the forest floor controls the thickness of the litter layer and nutrient recycling within the ecosystem. Decomposition is well known to depend on moisture and temperature which hasten both the microbial and non-microbial breakdown of leaves following their autumn senescence.

In this paper, just accepted in Plant Physiology and Biochemistry we test the extent to which light plays a role in the decomposition of autumn leaves. Photodegradation, the process by which sunlight accelerates decomposition, has typically been considered important only in dry environments. However, Santa Neimane and Marta Pieristè found in this experiment that it can also participate in decomposition during winter and spring prior to forest canopy closure.

The paper also considers how photodegradation acts on leaves of silver birch and European beech. Using spectrally selective filters over leaf litter in the understorey of different stands in Viikki arboretum and under controlled conditions, the distinct roles of UV radiation and of blue light, which facilitates microbial colonisation could be identified.

The experiment also exploited natural UV filters found in the epidermis of leaves as a means to see whether screening UV-radiation from the mesophyll when leaves are orientated with their upper-face upwards would affect the rate of photodegradation.

This research formed part of Santa Neimane’s undergraduate thesis on Erasmus exchange in Helsinki. It complements the subsequent long-term study of decomposition in a temperate beech forest published this summer, Pieristè et al. 2019, which considers the dynamics of photodegradation in leaves of tree species spanning a continuum of succession.

Read more in the papers:

Pieristè M, Neimane S, Nybakken L, Solanki T, Jones AG, S, Forey E, Chauvat M, Ņečajeva J, Robson TM. (2020) Ultraviolet radiation accelerates photodegradation under controlled conditions but slows the decomposition of leaf litter from forest stands in southern Finland. Accepted in Plant Physiology and Biochemistry. PLAPHY5920. https://doi.org/10.1016/j.plaphy.2019.11.005

Pieristè M, Chauvat M, Kotilainen TK, Jones AG, Aubert M, Robson TM, Forey E. (2019) Solar UV-A radiation and blue light enhance tree leaf litter decomposition in a temperate forest by accelerating photodegradation rate. Oecologia, 191(1), 191-203. https://doi.org/10.1007/s00442-019-04478-x

Understanding plants’ readiness to grow beneath the snowpack

A quick start to the growing season for plants emerging from the snowpack can provide them with a fitness dividend in locations where the summer is short.

In our latest paper, Transmission of ultraviolet, visible and near-infrared solar radiation to plants within a seasonal snow pack, just out in a Special Issue of PPS devoted to plant UV responses, we measured how much sunlight plants receive under snow and how the properties of the snowpack affect the irradiance spectrum.

Changes in the spectral composition of light under the snowpack may help plants perceive the onset of spring and physiologically prepare for exposure to the full sunlight and cold temperatures they will experience once they emerge.

This field work involved spending some very cold clear days hiding in deep snowpacks! 

Springtime recovery of Vaccinium vitis-idaea leaves above and below the snow pack

In Arctic and alpine environments warming temperatures are expected to result in longer growing seasons and to encourage growth, but snow will melt faster and more will fall as rain. This means that the protective winter blanket of snow cover may no longer be present to hide plants from the extremes of cold that periodically occur. Whether plants can overcome this paradox to benefit from the increased sunlight and warmth above the snow, while resisting the greater fluctuations in temperature, will depend on their physiological capacity to cope with the changing conditions.

Is the paper, Solanki et al., 2018, published in a special issue of Plant Physiology and Biochemistry on UV-cross protection, we explore the ecophysiological response of Vaccinium hummocks to snow cover over the course of a year in central Finland.

Twinkle Solanki taking Dualex measurements in central Finland

We focus on the role played by UV-absorbing compounds in protection against high light and low temperature combinations as shoots emerge from under snow in the early spring.

Solanki T. et al., 2018 UV-screening and springtime recovery of photosynthetic capacity in leaves of Vaccinium vitis-idaea above and below the snow pack Plant Physiology and Biochemistryhttps://doi.org/10.1016/j.plaphy.2018.09.003

Allocation to root growth can determine the survival of Mediterranean oaks during seasonal drought

The range of Holm oak (Quercus ilex), green; cork oak (Q. suber), purple; and their overlap, brown. Circle sample site, and triangle the field experiment.

Complex trade-offs in allocation to growth can determine the success of oak species where their ranges overlap.

This is highlighted by our paper Ramírez-Valiente et al., (2018), where higher root investment under seasonal drought by cork oak gave it an advantage over Holm oak, despite our prior expectations that the latter species is more drought tolerant.

Acorns germinating under controlled conditions prior to planting in the experimental plot

José-Alberto Ramírez-Valiente, Ismael Aranda, David Sanchéz-Gómez, Jesús Rodríguez-Calcerrada, Fernando Valladares, T Matthew Robson; Increased root investment can explain the higher survival of seedlings of ‘mesic’ Quercus suber than ‘xeric’ Quercus ilex in sandy soils during a summer drought, Tree Physiology, , tpy084, https://doi.org/10.1093/treephys/tpy084