The light environment and growth of C3 and C4 tree species in the understory of a Hawaiian forest.

The light environment in the understory of a Hawaiian forest containing a C4 tree species, Euphorbia forbesii, was characterized using photosynthetic photon flux density sensors connected to portable data acquisition systems and a strip chart recorder, and hemispherical "fisheye" photographs of the canopy. During July 1980, 86 μmol cm2 day1 was received in the understory of which approximately 40% was contributed by sunflecks. The understory received 2.4% of the light reaching the top of the canopy. Nearly all sunflecks had peak photon flux densities greater than 250 μmol m2 s1, but two-thirds were less than 0.5 min in length. The number of minutes of sunflecks received per day at any site was highly variable, depending on cloudiness and the overstory canopy structure. On a relatively clear day a 10-fold difference in the number of minutes of sunflecks was observed between sample sites. Estimates obtained from hemispherical photographs were used to calculate the annual mean potential number of minutes of sunflecks per day received by saplings of Euphorbia and a C3 tree species, Claoxylon sandwicense. The growth of saplings of both species was highly correlated with the estimates of the minutes of sunflecks and was similar for both species. Although C4 photosynthesis is usually found in plants native to high-light environments, it does not appear to confer any disadvantage in terms of growth to Euphorbia forbesii in the low-light conditions of the forest understory.