Light after death : the importance of spectral composition in litter decomposition processes

This dissertation focuses on the effect of sunlight on leaf litter decomposition. Sunlight can affect litter decomposition positively or negatively through the process known as photodegradation. Photodegradation is the ensemble of direct, indirect and mediated mechanisms. Short-wavelength solar radiation, carrying high energy, has the capacity to directly break down relatively stable components of plant tissues, such as lignin and cellulose, through photochemical mineralization causing the release of volatile carbon compounds into the atmosphere. Photochemical mineralization produces more-labile molecules, which can enhance the activity of microbial decomposers through a process known as photofacilitation or photopriming. Solar radiation has also the ability to indirectly alter decomposition through negative effects (photoinhibition) on both the activity and community composition of decomposer organisms. We examined the process of photodegradation under forest canopies in a temperate and a boreal environment. Through two field experiments, we tested the effects of photodegradation on mass loss and carbon content during leaf litter decomposition in each environment (I in France and II in Finland). We also studied these processes under controlled conditions in a filter experiment (II). In France, we performed an additional field experiment, in the same forest as the first, to analyse the effect of photodegradation on microbial assemblages colonizing the litter (III). In these experiments, we employed “photodegradation-litterbags”, bespoke litterbags adapted from classical litterbags used in litter decomposition studies incorporating different types of film filter-material, allowing us to manipulate the spectral composition of sunlight. Finally, we conducted a meta-analysis (IV) to summarise the effect of photodegradation driven by different spectral regions of solar radiation at the global scale, and across different biomes, and to test whether the photodegradation rate is modulated by initial litter traits. This dissertation highlights the importance of blue light as a major driver of photodegradation in a temperate mid-latitude forest understorey, with the potential to enhance both litter mass loss and carbon loss. However, at a higher latitude, the full spectrum of sunlight decreased mass loss, suggesting that the effect of photodegradation is specific to each biome. Forest canopies not only modify the amount of incoming solar radiation and its spectral composition, but also shape the microclimate of the understorey, producing unique combinations of temperature, moisture and snow-pack depth. Hence, each canopy generates novel interactions of solar radiation and other environmental factors which act on leaf litter to determine the photodegradation rate. At both boreal and temperate latitudes, our spectral manipulations revealed the effect of photodegradation to be litter species-specific, with recalcitrant litter experiencing higher rates of photodegradation. In terms of microbial decomposition, we highlighted how blue light, UV-A radiation and green light, act synergistically to shape the structure of microbial decomposer communities, with bacteria tending to dominate in sunlight and fungi in dark conditions. The results of our meta-analysis show that the direction and magnitude of photodegradation are dependent on the spectral region considered. We highlight the crucial role of blue light and UV-A radiation as drivers of photodegradation across biomes. Blue light has a positive effect in enhancing mass loss, while UV-A radiation has a negative effect. Moreover, our meta-analysis shows that the rate of photodegradation at the global level is modulated by climate and ecosystem type; whereby arid and semiarid ecosystems with low canopy cover experience the highest photodegradation rates. On the other hand, initial litter traits failed to predict the rate of photodegradation on the global scale, despite being important at the local level; suggesting that different traits could be important in different biomes. Photodegradation is known to have a role in the carbon cycle, as the process of photochemical mineralization causes the release of volatile carbon compounds into the atmosphere. Therefore, we can expect photodegradation to reduce the amount of carbon sequestered by ecosystems. However, further research is needed to estimate the actual contribution of photodegradation to the global carbon cycle. Moreover, this contribution is likely to be affected by climate change, which modifies environmental factors such as temperature and the amount and pattern of precipitation; these factors together with spectral irradiance determine the photodegradation rate. Overall, our results show that the process of photodegradation has an effect on litter decomposition in the understorey of mid- and high- latitude forests, despite the low irradiance to which litter in these ecosystems is exposed. Blue light appears to be more important than other spectral regions in driving photodegradation in these habitats. However, the photodegradation rate is modulated by both climate and ecosystem type.Tämä väitöskirja korostaa sinisen valon merkitystä valon vaikutuksesta tapahtuvassa karikkeen hajoamisessa keskileveysasteilla sijaitsevan lauhkean vyöhykkeen metsien pohjakerroksessa, mikä voi edistää sekä karikkeen hajoamisnopeutta että hiilen kiertoa. Korkeammilla leveysasteilla kaikki auringonvalon aallonpituudet kuitenkin vähensivät karikkeen hajoamista, mikä viittaa siihen, että valon aiheuttama karikkeen hajoaminen vaihtelee biomikohtaisesti. Metsien latvustot muokkaavat pohjakerrokseen tulevan auringonsäteilyn määrään ja laatuun, mutta ne muovaavat myös pohjakerroksen mikroilmastoa tuottaen ainutlaatuisia lämpötilan, kosteuden ja lumipeitteen syvyyden yhdistelmiä, joilla puolestaan on merkitystä valon aiheuttamaan karikkeen hajoamiseen. Sekä boreaalisella että lauhkealla vyöhykkeellä spektrikoostumuksen manipulaatiot osoittivat että valon vaikutuksesta tapahtuva hajoaminen riippui karikkeen lajista ja oli suurempi hitaasti hajoavaan karikkeeseen. Mikrobihajotustoiminnan osalta havaittiin että sininen valo, UV-A-säteily ja vihreä valo vaikuttivat synergistisesti, muokaten mikrobiyhteisöiden rakennetta niin, että bakteerien osuus korostui auringonvalossa ja sienten valottomissa olosuhteissa. Meta-analyysimme tulokset osoittavat, että valon vaikutuksesta tapahtuva hajoaminen on riippuvainen tarkasteltavasta spektrialueesta. Sinisen valon ja UV-A-säteilyn merkitys valon vaikutuksesta tapahtuvaan hajoamiseen on ratkaiseva eri biomeissa. Sinisellä valolla on positiivinen ja UV-A-säteilyllä negatiivinen vaikutus karikkeen hajoamiseen. Meta-analyysimme osoittaa, että valon vaikutuksesta tapahtuvan hajoamisen nopeuteen globaalilla tasolla vaikuttavat ilmasto ja ekosysteemityyppi; kuivissa ja semiaridisissa ekosysteemeissä, missä on vähän latvuston tarjoamaa suojaa, valon aiheuttamaa hajoamista tapahtuu nopeammin. Toisaalta alkuperäiset karikkeen ominaisuudet eivät ennustaneet tämän prosessin nopeutta globaalissa mittakaavassa, vaikka ne olivat tärkeitä paikallisella tasolla; tämä viittaa siihen, että erilaiset ominaisuudet voivat olla tärkeitä erilaisissa biomeissa. Valon vaikutuksesta tapahtuvalla hajoamisella tiedetään olevan merkitystä hiilen kierron kannalta, koska fotokemiallisen mineralisaatioprosessin seurauksena ilmakehään vapautuu haihtuvia hiiliyhdisteitä. Siksi voidaan olettaa karikkeen valon vaikutuksesta tapahtuvan hajoamisen vähentävän ekosysteemien sitoman hiilen määrää. Tarvitaan kuitenkin lisätutkimuksia, jotta tosiasiallinen vaikutus globaaliin hiilen kiertoon voidaan arvioida. Kaiken kaikkiaan tuloksemme osoittavat, että valon aiheuttamalla prosessilla on vaikutusta karikkeen hajoamiseen sekä keskileveysasteilla että korkeilla leveysasteilla sijaitsevien metsien pohjakerroksessa, huolimatta näiden ekosysteemien karikkeen saamasta alhaisesta säteilymäärästä. Sininen valo näyttää olevan valon vaikutuksesta tapahtuvan hajoamisen edistämisessä muita spektrialueita tärkeämpi näissä elinympäristöissä, mutta hajoamisnopeuteen vaikuttavat myös sekä ilmasto että ekosysteemityyppi

Seasonal patterns in spectral irradiance and leaf UV-A absorbance under forest canopies

Plants commonly respond to UV radiation through the accumulation of flavonoids and related phenolic compounds which potentially ameliorate UV-damage to crucial internal structures. However, the seasonal dynamics of leaf flavonoids corresponding to epidermal UV absorbance is highly variable in nature, and it remains uncertain how environmental factors combine to govern flavonoid accumulation and degradation. We studied leaf UV-A absorbance of species composing the understorey plant community throughout two growing seasons under five adjacent tree canopies in southern Finland. We compared the relationship between leaf flavonol index (Iflav - repeatedly measured with an optical leaf clip Dualex) and measured spectral irradiance, understorey and canopy phenology, air temperature and snowpack variables, whole leaf flavonoid extracts and leaf age. Strong seasonal patterns and stand-related differences were apparent in Iflav of both understorey plant communities and individual species, including divergent trends in Iflav during spring and autumn. Comparing the heterogeneity of the understorey light environment and its spectral composition in looking for potential drivers of seasonal changes in Iflav, we found that unweighted UV-A irradiance, or the effective UV dose calculated according to the biological spectral weighting function (BSWF) for plant growth (PG action spectrum), in understorey shade had a strong relationship with Iflav. Furthermore, understorey species seemed to adjust Iflav to low background diffuse irradiance rather than infrequent high direct-beam irradiance in sunflecks during summer, since leaves produced during or after canopy closure had low Iflav. In conclusion, we found the level of epidermal flavonoids in forest understorey species to be plastic, adjusting to climatic conditions, and differing according to species' leaf retention strategy and new leaf production, all of which contribute to the seasonal trends in leaf flavonoids found within forest stands.Peer reviewe

Testing trait plasticity over the range of spectral composition of sunlight in forb species differing in shade tolerance

Although sunlight is essential for plant growth and development, the relative importance of each spectral region in shaping functional traits is poorly understood, particularly in dynamic light environments such as forest ecosystems. We examined responses of 25 functional traits from groups of 11 shade-intolerant and 12 understorey shade-tolerant forb species grown outdoors under five filter treatments differing in spectral transmittance: (a) transmitting c. 95% of solar radiation (280-800 nm); (b) attenuating ultraviolet-B (UV-B); (c) attenuating all UV; (d) attenuating all UV and blue light; (e) attenuating all UV, blue and green light. Our results show that UV-B radiation mainly affected the biochemical traits but blue light mainly affected the physiological traits irrespective of functional strategy, whereas green light affected both sets of traits. This would suggest that differentiation among suites of functional trait responses proceeds according to light quality. Biomass accumulation was significantly increased by UV-A radiation (contrasting treatment [b] vs. [c]) among shade-intolerant but decreased by blue light among shade-tolerant species; green and red light affected whole-plant morphological development differently according to functional groups. Shade-tolerant species were more plastic than shade-intolerant species in response to each spectral region that we examined except for UV-B radiation. Synthesis. Our results show that differences in the spectral composition of sunlight can drive functional trait expression irrespective of total irradiance received. The different responses of functional traits between functional groups imply that shade-tolerant and intolerant species have adapted to utilize spectral cues differently in their respective light environments.Peer reviewe

Practical Activities Promoting Engagement in Forest Ecology Research

Peer reviewe

UV-screening and springtime recovery of photosynthetic capacity in leaves of Vaccinium vitis-idaea above and below the snow pack

Evergreen plants in boreal biomes undergo seasonal hardening and dehardening adjusting their photosynthetic capacity and photopmtection; acclimating to seasonal changes in temperature and irradiance. Leaf epidermal ultraviolet (UV)-screening by flavonols responds to solar radiation, perceived in part through increased ultraviolet-B (UV-B) radiation, and is a candidate trait to provide cross-photoprotection. At Hyytiala Forestry Station, central Finland, we examined whether the accumulation of flavonols was higher in leaves of Vaccinium vitis-idaea L. growing above the snowpack compared with those below the snowpack. We found that leaves exposed to colder temperatures and higher solar radiation towards the top of hummocks suffered greater photoinhibition than those at the base of hummocks. Epidermal UV-screening was highest in upper-hummock leaves, particularly during winter when lower leaves were beneath the snowpack. There was also a negative relationship between indices of flavonols and anthocyanins across all leaves suggesting fine-tuning of flavonoid composition for screening vs. antioxidant activity in response to temperature and irradiance. However, the positive correlation between the maximum quantum yield of photosystem II photochemistry (F-v/F-m) and flavonol accumulation in upper hummock leaves during dehardening did not confer on them any greater cross-protection than would be expected from the general relationship of F-v/F-m with temperature and irradiance (throughout the hummocks). Irrespective of timing of snow-melt, photosynthesis fully recovered in all leaves, suggesting that V. vills-idaea has the potential to exploit the continuing trend for longer growing seasons in central Finland without incurring significant impairment from reduced duration of snow cover.Peer reviewe

Ultraviolet radiation accelerates photodegradation under controlled conditions but slows the decomposition of senescent leaves from forest stands in southern Finland

Depending on the environment, sunlight can positively or negatively affect litter decomposition, through the ensemble of direct and indirect processes constituting photodegradation. Which of these processes predominate depends on the ecosystem studied and on the spectral composition of sunlight received. To examine the relevance of photodegradation for litter decomposition in forest understoreys, we filtered ultraviolet radiation (UV) and blue light from leaves of Fagus sylvatica and Bettda pendula at two different stages of senescence in both a controlled-environment experiment and outdoors in four different forest stands (Picea abies, Pagus sylvatica, Acer platanoides, Betula pendula). Controlling for leaf orientation and initial differences in leaf chlorophyll and flavonol concentrations; we measured mass loss at the end of each experiment and characterised the phenolic profile of the leaf litter following photodegradation. In most forest stands, less mass was lost from decomposing leaves that received solar UV radiation compared with those under UV-attenuating filters, while in the controlled environment UV-A radiation either slightly accelerated or had no significant effect on photodegradation, according to species identity. Only a few individual phenolic compounds were affected by our different filter treatments, but photodegradation did affect the phenolic profile. We can conclude that photodegradation has a small stand- and species-specific effect on the decomposition of surface leaf litter in forest understoreys during the winter following leaf fall in southern Finland. Photodegradation was wavelength-dependent and modulated by the canopy species filtering sunlight and likely creating different combinations of spectral composition, moisture, temperature and snowpack characteristics.Peer reviewe

Solar UV-A radiation and blue light enhance tree leaf litter decomposition in a temperate forest

Sunlight can accelerate the decomposition process through an ensemble of direct and indirect processes known as photodegradation. Although photodegradation is widely studied in arid environments, there have been few studies in temperate regions. This experiment investigated how exposure to solar radiation, and specifically UV-B, UV-A, and blue light, affects leaf litter decomposition under a temperate forest canopy in France. For this purpose, we employed custom-made litterbags built using filters that attenuated different regions of the solar spectrum. Litter mass loss and carbon to nitrogen (C:N) ratio of three species: European ash (Fraxinus excelsior), European beech (Fagus sylvatica) and pedunculate oak (Quercus robur), differing in their leaf traits and decomposition rate, were analysed over a period of 7–10 months. Over the entire period, the effect of treatments attenuating blue light and solar UV radiation on leaf litter decomposition was similar to that of our dark treatment, where litter lost 20–30% less mass and had a lower C:N ratio than under the full-spectrum treatment. Moreover, decomposition was affected more by the filter treatment than mesh size, which controlled access by mesofauna. The effect of filter treatment differed among the three species and appeared to depend on litter quality (and especially C:N), producing the greatest effect in recalcitrant litter (F. sylvatica). Even under the reduced irradiance found in the understorey of a temperate forest, UV radiation and blue light remain important in accelerating surface litter decomposition.Peer reviewe

The importance and direction of current and future plant-UV research : break-out session discussions at the UV4Plants Network Meeting in Bled (April 15th -18th , 2018)

During the 2nd Network Meeting of UV4Plants at Bled (14th–18th April, 2018) the delegates engaged in a group discussion of prescient questions concerning the future of in plant-UV research. The discussion group was tasked to identify the most valuable directions for plant UV research to take, and to create a coherent framework for how to move the field forward. Here, the outcome of these discussions is summarised in sections that follow the composition of discussion groups as ideas taken from a molecular, biochemical and physiological perspective followed by those from an ecological and plant production perspective. In each case, first basic research questions are considered and then applications and methodological considerations are put forward. Finally, some common ground bringing the two perspectives together is discussed, with the aim of solving scaling problems and ways in which the UV4Plants network might be put to good use.Peer reviewe

Solar UV-A radiation and blue light enhance tree leaf litter decomposition in a temperate forest

International audienc

La décomposition de litière dans une forêt tempérée est fonction de la qualité du rayonnement solaire

International audienc