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Instantaneous measurements of photosynthesis are often implicitly or explicitly scaled to longer time frames to provide an understanding of plant performance in a given environment. For plants growing in a forest understory, results from photosynthetic light response curves in conjunction with diurnal light data are frequently extrapolated to daily photosynthesis ( A  day ), ignoring dynamic photosynthetic responses to light. In this study, we evaluated the importance of two factors on  A  day  estimates: dynamic physiological responses to photosynthetic photon flux density (PPFD); and time-resolution of the PPFD data used for modeling. We used a dynamic photosynthesis model to investigate how these factors interact with species-specific photosynthetic traits, forest type, and sky conditions to affect the accuracy of  A  day  predictions. Increasing time-averaging of PPFD significantly increased the relative overestimation of  A  day  similarly for all study species because of the nonlinear response of photosynthesis to PPFD (15% with 5-min PPFD means). Depending on the light environment characteristics and species-specific dynamic responses to PPFD, understory tree  A  day  can be overestimated by 6–42% for the study species by ignoring these dynamics. Although these overestimates decrease under cloudy conditions where direct sunlight and consequently understory sunfleck radiation is reduced, they are still significant. Within a species, overestimation of  A  day  as a result of ignoring dynamic responses was highly dependent on daily sunfleck PPFD and the frequency and irradiance of sunflecks. Overall, large overestimates of  A  day  in understory trees may cause misleading inferences concerning species growth and competition in forest understories with &lt; 2% full sunlight. We conclude that comparisons of  A  day  among co-occurring understory species in deep shade will be enhanced by consideration of sunflecks by using high-resolution PPFD data and understanding the physiological responses to sunfleck variation.

Short-term light and leaf photosynthetic dynamics affect estimates of daily understory photosynthesis in four tree species