The roles of species' relatedness and climate of origin in determining optical leaf traits over a large set of taxa growing at high elevation and high latitude

Climate change is driving many mountain plant species to higher elevations and northern plant species to higher latitudes. However, various biotic or abiotic constraints may restrict any range shift, and one relevant factor for migration to higher elevations could be species' ability to tolerate high UV-doses. Flavonoids are engaged in photoprotection, but also serve multiple ecological roles. We compared plant optical leaf trait responses of a large set of taxa growing in two botanical gardens (French Alps and southern Finland), considering potential constraints imposed by the relatedness of taxa and the legacy of climatic conditions at plants' original collection sites. The segregation of optically measured leaf traits along the phylogeny was studied using a published mega-tree GBOTB.extended.tre for vascular plants as a backbone. For a subset of taxa, we investigated the relationship between climatic conditions (namely solar radiation, temperature and precipitation at a coarse scale) at the plants' original collection site and current trait values. Upon testing the phylogenetic signal (Pagel's lambda), we found a significant difference but intermediate lambda values overall for flavonol or flavone index (I-flav) and anthocyanin index (I-ant), indicating that phylogenetic relatedness alone failed to explain the changes in trait values under a Brownian motion model of trait evolution. The local analysis (local indicator of phylogenetic association) indicated mostly positive autocorrelations for I-flav i.e. similarities in optically measured leaf traits, often among species from the same genus. We found significant relationships between climatic variables and leaf chlorophyll index (I-chl), but not I-flav, particularly for annual solar radiation. Changes in plants' I-flav across microhabitats differing in UV irradiance and predominately high F-v/F-m indicated that most plants studied had sufficient flexibility in photoprotection, conferred by I-flav, to acclimate to contemporary UV irradiances in their environment. While not explaining the mechanisms behind observed trait values, our findings do suggest that some high-elevation taxa display similar leaf flavonoid accumulation responses. These may be phylogenetically constrained and hence moderate plants' capacity to adjust to new combinations of environmental conditions resulting from climate change.Peer reviewe

Transmission of ultraviolet, visible and near-infrared solar radiation to plants within a seasonal snow pack

Sunlight is strongly attenuated by the snowpack, causing irradiance to decrease exponentially with depth. The strength of attenuation is wavelength dependent across the spectrum. Changes in received irradiance and its spectral composition are used by plants as cues for the timing of phenology, and it is known that at shallow depths in the snowpack there is sufficient light for plants to photosynthesize if conditions are otherwise favourable. The spectral composition of solar radiation under snow in the visible region was already determined in the 1970s using scanning spectroradiometers, but spectral attenuation within the ultraviolet region (UV-B 280-315 nm, UV-A 315-400 nm) has not been well characterised because it is difficult to measure. We measured vertical transects of spectral irradiance (290-900 nm) transmitted through a settled seasonal snowpack. The peak transmission of radiation was in the UV-A region in the upper centimetres of the snowpack and transmittance generally declined at longer wavelengths. Given the known action spectra of plant photoreceptors, these results illustrate the possibility that changing UV-A:visible and red:far-red radiation ratios under the snowpack may serve as spectral cues for plants; potentially priming plants for the less stable environment they experience following snowmelt. Array spectrometers open opportunities for rapid and continuous measurement of irradiance in challenging environments, e.g. beneath the snowpack, and capturing changing light conditions for plants. Future research is needed to couple the spectral transmittance of snowpacks differing in their longevity and crystal structure with measurements of the perception and response to radiation by plants under snow.Peer reviewe

The roles of species’ relatedness and climate of origin in determining optical leaf traits over a large set of taxa growing at high elevation and high latitude

Climate change is driving many mountain plant species to higher elevations and northern plant species to higher latitudes. However, various biotic or abiotic constraints may restrict any range shift, and one relevant factor for migration to higher elevations could be species’ ability to tolerate high UV-doses. Flavonoids are engaged in photoprotection, but also serve multiple ecological roles. We compared plant optical leaf trait responses of a large set of taxa growing in two botanical gardens (French Alps and southern Finland), considering potential constraints imposed by the relatedness of taxa and the legacy of climatic conditions at plants’ original collection sites. The segregation of optically measured leaf traits along the phylogeny was studied using a published mega-tree GBOTB.extended.tre for vascular plants as a backbone. For a subset of taxa, we investigated the relationship between climatic conditions (namely solar radiation, temperature and precipitation at a coarse scale) at the plants’ original collection site and current trait values. Upon testing the phylogenetic signal (Pagel’s λ), we found a significant difference but intermediate lambda values overall for flavonol or flavone index (Iflav) and anthocyanin index (Iant), indicating that phylogenetic relatedness alone failed to explain the changes in trait values under a Brownian motion model of trait evolution. The local analysis (local indicator of phylogenetic association) indicated mostly positive autocorrelations for Iflav i.e. similarities in optically measured leaf traits, often among species from the same genus. We found significant relationships between climatic variables and leaf chlorophyll index (Ichl), but not Iflav, particularly for annual solar radiation. Changes in plants’ Iflav across microhabitats differing in UV irradiance and predominately high Fv/Fm indicated that most plants studied had sufficient flexibility in photoprotection, conferred by Iflav, to acclimate to contemporary UV irradiances in their environment. While not explaining the mechanisms behind observed trait values, our findings do suggest that some high-elevation taxa display similar leaf flavonoid accumulation responses. These may be phylogenetically constrained and hence moderate plants’ capacity to adjust to new combinations of environmental conditions resulting from climate change

Light quality characterization under climate screens and shade nets for controlled-environment agriculture

Climate screens are typically used inside glass greenhouses to improve control of humidity and temperature, and thus reduce energy expenditure. Shade nets are more appropriate to use, either with or without polyethylene cladding, at locations less-reliant on climate control, but where protection against hail, wind and excessive solar radiation might be needed. In addition, insect screens and nets can be employed to hinder insect pests and other invertebrates entering either type of production environment, and to keep invertebrates used in pest management contained inside. Screens and nets both transmit sunlight in a wavelength-specific manner, giving them the potential to affect plant morphology and physiology. Screens and nets of various colours and nominal shading factors have been described and studied; however, detailed measurements of their spectral characteristics are scarce. We measured solar spectral photon-irradiance and its attenuation by climate screens, shade nets, insect nets, greenhouse glass, and polyethylene covers. Our aim was to elucidate the effects of different patterns, colours, and shading factors, on light quality in production environments. Our measurements reveal that there are large differences both in the fraction of global irradiance attenuated and spectral ratios received under materials that are otherwise superficially similar in terms of their appearance and texture. We suggest that the type of spectral characterization that we performed is required to fully interpret the results of research examining plant responses to different types of screen and net. These data on spectral irradiance would benefit material manufacturers, researchers, growers, and horticultural consultants, enabling material selection to better match the solutions sought by growers and their desired outcomes regarding plant performance.Peer reviewe

Stream quality in small urbanised catchment.

River-length patterns in the chemistry and biology of the Charlton Brook, an unclassified watercourse in northwest Sheffield have been examined. Sampling sites for macroinvertebrates and pollutant analysis were used, in conjunction with Environment Agency General Quality Assessment (GQA) methodologies and hydraulic analysis of the catchment. Sites were strategically located to account for the tributaries and the brook downstream of their confluence, to assess the potential impact from surface water outfalls (SWOs).Variations in GQA parameters indicate a significant drop in quality downstream of the SWOs that discharge to the study watercourse, with a marked drop in biological diversity noted at the onset of urbanisation. The decline in biological quality however, is greater than that suggested by physico-chemical analysis alone. There was a significant inverse relationship between impermeable area and biological diversity.Analysis of polycyclic aromatic hydrocarbons (PAHs) and trace metals in sediment from the watercourse showed significant, yet irregular between site variations. PAHs in conjunction with metals as a function of the PEL-quotient method employed, suggest that all the sites sampled for macroinvertebrates have the potential for being adversely affected by the pollutants contained within the brook's sediment. Mean PEL-quotients suggest that sediment contamination within the brook is indicated at all sites.The potential toxicity of instream metal concentrations was determined using cumulative criterion unit (CCU) scores. CCU analysis highlighted cadmium, copper and lead as the major sources of potential instream toxicity with all sites exceeding the threshold for likely harm to aquatic life.In the absence of different physical characteristics, comparisons of the chemical and biological data indicate that the benthic macroinvertebrate population of such watercourses are adversely affected by the stormwater inputs

Sunfleck properties from time series of fluctuating light

Light in canopies is highly dynamic since the strength and composition of incoming radiation is determined by the wind and the Sun's trajectory and by canopy structure. For this highly dynamic environment, we mathematically defined sunflecks as periods of high irradiance relative to the background light environment. They can account for a large proportion of the light available for photosynthesis. Based on high-frequency irradiance measurements with a CCD array spectroradiometer, we investigated how the frequency of measurement affects what we define as sunflecks. Do different plant canopies produce sunflecks with different properties? How does the spectral composition and strength of irradiance in the shade vary during a sunfleck? Our results suggest that high-frequency measurements improved our description of light fluctuations and led to the detection of shorter, more frequent and intense sunflecks. We found that shorter wind-induced sunflecks contribute most of the irradiance attributable to sunflecks, contrary to previous reports from both forests and crops. Large variations in sunfleck properties related to canopy depth and species, including distinct spectral composition under shade and sunflecks, suggest that mapping canopy structural traits may help us model photosynthesis dynamically.Peer reviewe

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

Fossil pollen and spores as a tool for reconstructing ancient solar-ultraviolet irradiance received by plants : an assessment of prospects and challenges using proxy-system modelling

Ultraviolet-B radiation (UV-B, 280-315 nm) constitutes less than 1% of the total solar radiation that reaches the Earth's surface but has a disproportional impact on biological and ecological processes from the individual to the ecosystem level. Absorption of UV-B by ozone is also one of the primary heat sources to the stratosphere, so variations in UV-B have important relationships to the Earth's radiation budget. Yet despite its importance for understanding atmospheric and ecological processes, there is limited understanding about the changes in UV-B radiation in the geological past. This is because systematic measurements of total ozone and surface UV-B only exist since the 1970s, so biological or geochemical proxies from sediment archives are needed to reconstruct UV-B irradiance received at the Earth surface beyond the experimental record. Recent developments have shown that the quantification of UV-B-absorbing compounds in pollen and spores have the potential to provide a continuous record of the solar-ultraviolet radiation received by plants. There is increasing interest in developing this proxy in palaeoclimatic and palaeoecological research. However, differences in interpretation exist between palaeoecologists, who are beginning to apply the proxy under various geological settings, and UV-B ecologists, who question whether a causal dose-response relationship of pollen and spore chemistry to UV-B irradiance has really been established. Here, we use a proxy-system modelling approach to systematically assess components of the pollen-and spore-based UV-B-irradiance proxy to ask how these differences can be resolved. We identify key unknowns and uncertainties in making inferences about past UV-B irradiance, from the pollen sensor, the sedimentary archive, and through the laboratory and experimental procedures in order to target priority areas of future work. We argue that an interdisciplinary approach, modifying methods used by plant ecologists studying contemporary responses to solar-UV-B radiation specifically to suit the needs of palaeoecological analyses, provides a way forward in developing the most reliable reconstructions for the UV-B irradiance received by plants across a range of timescales.Peer reviewe

Freezing induces an increase in leaf spectral transmittance of forest understorey and alpine forbs

Evergreen plants growing at high latitudes or high elevations may experience freezing events in their photosynthetic tissues. Freezing events can have physical and physiological effects on the leaves which alter leaf optical properties affecting remote and proximal sensing parameters. We froze leaves of six alpine plant species (Soldanella alpina, Ranunculus kuepferi, Luzula nutans, Gentiana acaulis, Geum montanum, and Centaurea uniflora) and three evergreen forest understorey species (Hepatica nobilis, Fragaria vesca and Oxalis acetosella), and assessed their spectral transmittance and optically measured pigments, as well as photochemical efficiency of photosystem II (PSII) as an indicator of freezing damage. Upon freezing, leaves of all the species transmitted more photosynthetically active radiation (PAR) and some species had increased ultraviolet-A (UV-A) transmittance. These differences were less pronounced in alpine than in understorey species, which may be related to higher chlorophyll degradation, visible as reduced leaf chlorophyll content upon freezing in the latter species. Among these understorey forbs, the thin leaves of O. acetosella displayed the largest reduction in chlorophyll (−79%). This study provides insights into how freezing changes the leaf optical properties of wild plants which could be used to set a baseline for upscaling optical reflectance data from remote sensing. Changes in leaf transmittance may also serve to indicate photosynthetic sufficiency and physiological tolerance of freezing events, but experimental research is required to establish this functional association.Peer reviewe

Differences in the leaf functional traits of six beech (Fagus sylvatica L.) populations are reflected in their response to water limitation

Patterns of intraspecific variation in functional traits have been widely studied across plant species to find out what general suites of traits provide functional advantage under specific environmental conditions. Much less is known about this variation within tree species and, in particular, about its relationship with performance variables such as photosynthetic rates under water deficit. Nevertheless, this knowledge is fundamental to understand the adaptive potential of drought sensitive tree species to increased aridity as predicted in the context of climate change. Intraspecific variation in photosynthetic performance and other leaf functional traits in response to water availability were examined in a glasshouse experiment using seedlings of six European beech populations. The physiological response of seedlings to a “water stress” treatment was compared to a “control” treatment along an experimental cycle of progressive soil water deficit and recovery. We found evidence of intraspecific variation in beech's photosynthetic performance and other leaf functional traits in response to water availability. We also detected intraspecific variation in leaf-level tolerance of water deficit and phenotypic plasticity to water availability suggesting a pattern shaped by both regional and local scale effects. The Swedish population was particularly sensitive to water deficit, being the only population showing impaired photochemical efficiency under the experimental water deficit. Leaf-level tolerance of water deficit was related to PNUE, but not to other functional traits, such as WUE, SLA or leaf nitrogen content, that have been described to vary across species in adaptation to drought tolerance. Our results support the idea that general trends for variation in functional traits across species do not necessarily reflect a similar pattern when observed at the intraspecific level. The observed functional variation between beech populations reaffirms the importance of local adaptation to water deficit in the context of climate changePeer ReviewedClimate changeGas exchangeIntraspecific variabilityNitrogen contentPhotochemical efficiencyPhotosynthesisSpecific leaf areaStomatal conductanceWater-use efficiencyWater stressPublishe