Physiological and ecological significance of sunflecks for dipterocarp seedlings

Irradiance is highly dynamic in many plant canopies. Photosynthesis during sunflecks provides 10–90 % of daily carbon gain. The survivorship of tree seedlings in the deeply shaded understorey of tropical rain forests is limited by their ability to maintain a positive carbon bal-ance. Dipterocarp seedlings from the SE Asian rain forestwereusedasamodel system to test novel aspects of the physiological and ecological significance of sun-flecks. First, understorey seedlings experienced leaf temperatures up to 38 8C in association with sunflecks. Under controlled environment conditions, the inhibition of carbon gain at 38 8C, comparedwith 28 8C, was signifi-cantly greater during a sequence of sunflecks (259%), than under uniform irradiance (240%), providing the same total photosynthetic photon flux density (PPFD). Second, the relative enhancement effects of elevated [CO2] were greater under sunflecks (growth 160%, carbon gain 189%), compared with uniform irradiance (growth 125%, carbon gain 159%), supplying the same daily PPFD. Third, seedling growth rates in the forest understorey were 4-fold greater under a dynamic irradi-ance treatment characterized by long flecks, compared with a regimeof short flecks. Therefore, stresses associ-ated with dynamic irradiance may constrain photosyn-thetic carbongain. Additionally, seedling photosynthesis and growth may be more responsive to interactions with abiotic factors, including future changes in climate, than previously estimated. The sensitivity of seedling growth to varying patterns of dynamic irradiance, and the increased likelihood of species-specific responses through interactions with environmental factors, indi-cates the potential for sunflecks to influence regen-eration processes, and hence forest structure and composition