Understanding causes of tree growth response to gap formation: D13C-values in tree rings reveal a predominant effect of light

Carbon isotope ratios in growth rings of a tropical tree species show that treefall gaps stimulate diameter growth mainly through changes in the availability of light and not water. The formation of treefall gaps in closed canopy forests usually entails considerable increases in light and nutrient availability for remaining trees, as well as altered plant water availability, and is considered to play a key role in tree demography. The effects of gaps on tree growth are highly variable and while usually stimulatory they may also include growth reductions. In most studies, the causes of changes in tree growth rates after gap formation remain unknown. We used changes in carbon isotope 13C discrimination (D13C) in annual growth rings to understand growth responses after gap formation of Peltogyne cf. heterophylla, in a moist forest of Northern Bolivia. We compared growth and D13C of the 7 years before and after gap formation. Forty-two trees of different sizes were studied, half of which grew close (\10 m) to single treefall gaps (gap trees), the other halfmore than 40 m away from gaps (controls). We found variable responses among gap trees in growth and D13C. Increased growth was mainly associated with decreased D13C, suggesting that the growth response was driven by increased light availability, possibly in combination with improved nutrient availability. Most trees showing zero or negative growth change after gap formation had increased D13C, suggesting that increased water stress did not play a role, but rather that light conditions had not changed much or nutrient availability was insufficient to support increased growth. Combining growth rates withD13Cproved to be a valuable tool to identify the causes of temporal variation in tree growth.