Influence of sunflecks on the temperature and water relations of two subalpine understory congeners

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Abstract
The temperature and water relations of the herbaceous, understory, congeners Arnica cordifolia and Arnica latifolia were evaluated in relation to the sunfleck dynamics of their respective microhabitats. Arnica cordifolia microhabitats had more frequent, longer, and more intense sunflecks than those of A. latifolia which led to higher leaf temperatures (31°C versus 15°C) and transpirational fluxes (65 μg cm-2 s-1 versus 16 μg cm-2 s-1). Stomatal closure did not occur in response to high leaf temperatures and low stem water potentials during natural sunfleck exposures, even though plants were observed to wilt during midday, especially A. cordifolia. Experimentally, an artificial midday sunfleck of about 165 min caused plants of A. cordifolia not to regain turgor after 8 h in shade compared to a sunfleck duration of about 90 min for plants of A. latifolia. However, these sunfleck intervals occurred naturally only during the early morning and late afternoon when solar intensities were minimal. Also, A. cordifolia populations had over twice as many plants that were sunlit (>40% of total) compared with A. latifolia (<20% of total) at any particular time during a day. The small-scale distribution of both species appears tightly coupled to the sunfleck dynamics of their respective microhabitats due to the lack of stomatal action which would reduce transpiration and improve plant water status under sunlit conditions.