7Understory plant species take on different functional strategies, whereby some exploit periods 1 8 of available light in springtime before the canopy closes, and others also benefit from sunlight 1 9 later in autumn when the canopy opens again. These strategies involve understory species 2 0 coordinating phenological events to pre-empt canopy leaf out and to extend their growing 2 1 season beyond canopy leaf senescence, meanwhile accumulating photo-protective pigments 2 2 which mitigate periods of high-light exposure. Canopy closure brings shade to the understory, 2 3 but also causes drastic changes in light quality. Whilst many experiments manipulating spectral 2 4 quality have revealed understory plant responses to the changing R:FR ratio in shade, effect of 2 5 the blue and UV regions have been examined very little. We installed filters attenuating short 2 6 wavelength regions of the solar spectrum in a forest understory in southern Finland, creating the 2 7 following treatments: a transparent control filter, and filters attenuating UV radiation < 350 nm, 2 8 all UV radiation, and both UV and blue light. In eight understory species, representing different 2 9 plant functional types, we repeatedly assessed leaf optical properties to obtain epidermal 3 0 flavonol and anthocyanin contents from leaf emergence in spring to leaf senescence in autumn, 3 1 during both 2017 and 2018. Flavonols responded more to seasonal changes in light quality in 3 2 relatively light-demanding species than in shade-tolerant and wintergreen species; and were 3 3 particularly responsive to blue light. However, anthocyanins were largely unaffected by our 3 4 filter treatments, suggesting that other cues such as cold temperatures govern their seasonal 3 5 variation. UV radiation only accelerated leaf senescence in Acer platanoides seedlings, but blue 3 6 light accelerated leaf senescence in all species measured apart from Quercus robur. In 3 7 summary, seasonal changes in understory solar radiation in the blue and UV regions affected 3 8 leaf pigments and leaf phenology; particularly for more light-demanding species. An increase in 3 9 canopy duration under climate change will extend the period of shade in the understory, with 4 0 consequences for the spectral cues available to understory plants. The resultant reduction in 4 1 blue and UV radiation in shade, could delay leaf senescence in the understory even further. 4 2
Forest understorey plants receive most sunlight in springtime before canopy closure, and in autumn following leaf‐fall. We hypothesised that plant species must adjust their phenological and photoprotective strategies in response to large changes in the spectral composition of the sunlight they receive. Here, we identified how plant species growing in northern deciduous and evergreen forest understoreys differ in their response to blue light and ultraviolet (UV) radiation according to their functional strategy. We installed filters in a forest understorey in southern Finland, to create the following treatments attenuating: UV radiation below 350 nm, all UV radiation (< 400 nm), all blue light and UV radiation (< 500 nm), and a transparent control. In eight species, representing different functional strategies, we assessed leaf optical properties, phenology, and epidermal flavonoid contents over two years. Blue light accelerated leaf senescence in all species measured in the understorey, apart from Quercus robur seedlings, whereas UV radiation only accelerated leaf senescence in Acer platanoides seedlings. More light‐demanding species accumulated flavonols in response to seasonal changes in light quality compared to shade‐tolerant and wintergreen species and were particularly responsive to blue light. Reduction of blue and UV radiation under shade reveals an important role for microclimatic effects on autumn phenology and leaf photoprotection. An extension of canopy cover under climate change, and its associated suppression of understorey blue light and UV radiation, may delay leaf senescence for understorey species with an autumn niche.
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