Self-shading in cork oak seedlings: Functional implications in heterogeneous light environments

The high self-shading found in Quercus suber seedlings has been interpreted as a feature common for plants growing in high light environments. But many studies reveal that Q. suber has high survival rates under low-light conditions, so a high degree of self-shading could be the consequence of a foliage composed of many small leaves, with no drawbacks for coping with low light. A characterization of the light environment in a Q. suber stand together with a study of photosynthetic parameters of full sunlightexposed (FSLE) and self-shaded (SS) leaves were carried out to tackle this apparent contradiction. Although the number of sunflecks longer than 120 min during the 3 months of measurements was low, the occurrence of at least one sunfleck longer than 120 min per day in the understory of the forest studied was very common. Sunflecks shorter than 30 min promoted an increase in net photosynthesis (A) in FSLE leaves, but not in SS leaves. However, sunflecks longer than 60 min led to a very strong decrease in A and in actual photosystem II efficiency (FPSII ) in FSLE leaves, when compared to sunflecks shorter than 30 min. In SS leaves, changes were, again, negligible. The multi-layered foliage of Q. suber seedlings allowed i) FSLE leaves to obtain the maximum photosynthetic yield for short sunflecks, and ii) SS leaves to increase their contribution to the photosynthesis of the whole plant for long sunflecks, thus, optimizing the use of light by FSLE and SS leaves during short and long sunflecks respectively. Therefore, shoot architecture of Q. suber seedlings involving high levels of self-shading allows to adequately cope with the low but highly heterogeneous light conditions of the understory, particularly when sunflecks of contrasting durations take place as it is frequently the case for evergreen Mediterranean forests