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Vliv světla a sucha na fotosyntézu a vodní provoz podrostních bylin temperátních doubrav
Vitásek, Roman
Woody plants, especially trees, affects the site microclimate. With their physiological manifestations, they influence hydrological and light conditions and, consequently, the composition and abundance of species from the plant community. A dense tree layer lowers the air and even more the surface temperatures. Its mitigating effect is most pronounced on hot summer days. While transpiration lowers air temperature, trees compete with understory for water. While the shade lowers surface temperature of understory, plants, as autotrophic organisms, need enough light for their survival. Therefore, the question arises, which density of forest canopy is optimal for the survival of understory during the global change that brings along increased temperatures and elevated levels of drought. I investigated how understory herbs cope with drought under simulated sparse and dense forest canopy. The focal species, European wild ginger (Asarum europaeum L.) and liverwort (Hepatica nobilis Schreb.), were divided into four groups: two of them were fully watered and two of them droughted. Each of the water regimes were divided in two groups, simulating sparse or dense canopy of woody overstory. The physiological response of plants (such as the rate of light-saturated photosynthesis, stomatal conductance, intrinsic water use efficiency, concentration of non-structural carbohydrates and water potential) on manipulated light and drought conditions was monitored under various levels of drought stress. The mortality was always higher under deep shade than under light conditions, despite different water management strategies of stomatal regulation. The European wild ginger has characteristics of an isohydric type of plant. During drought, it copes with the negative water potential by reducing stomatal conductivity, at the expense of the ability to actively fight water stress by creating assimilates. Under favorable conditions, this species stores starch that allow it to survive even with a significantly reduced rate of photosynthesis. The liverwort is more anisohydric in its behavior. With the increasing intensity of drought and increasing transpiration, it keeps the stomata open and by active photosynthesis it creates soluble carbohydrates for the production of osmotically active secondary metabolites. It has a very low level of stored starch, which can have a negative effect during prolonged drought. The results indicate that light plays an important role for plant survival under water stress. The changing climate brings along not only increased temperatures but often increased levels of summer drought. Therefore, growing in the understory of the dense forest does not always help the plant to survive warming climate.
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