ORIGINAL PAPER Understanding causes of tree growth response to gap formation: D 13 C-values in tree rings reveal a predominant effect of light P. van der Sleen • C. C. Soliz-Gamboa • G. Helle • T. L. Pons • N. P. R. Anten • P. A. Zuidema Received: 17 June 2013 / Revised: 20 November 2013 / Accepted: 22 November 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract Key message Carbon isotope ratios in growth rings of a tropical tree species show that treefall gaps stimulate diameter growth mainly through changes in the avail- ability of light and not water. The formation of treefall gaps in closed canopy forests usu- ally 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 13 C discrimination (D 13 C) 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 D 13 C 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 half more than 40 m away from gaps (controls). We found variable responses among gap trees in growth and D 13 C. Increased growth was mainly associated with decreased D 13 C, sug- gesting 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 D 13 C, 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 with D 13 C proved to be a valuable tool to identify the causes of temporal variation in tree growth. Keywords Carbon stable isotopes  Gap formation  Tree growth  Light availability  Water availability  Peltogyne Communicated by S. Leavitte. P. van der Sleen (&)  P. A. Zuidema Forest Ecology and Forest Management, Centre for Ecosystems, Wageningen University, Droevendaalsesteeg 3, 6708 PB Wageningen, The Netherlands e-mail: peter.vandersleen@wur.nl P. van der Sleen Instituto Boliviano de Investigacio ´n Forestal (IBIF), Km 9 carretera al norte, Santa Cruz de la Sierra, Bolivia C. C. Soliz-Gamboa Programa de Manejo de Bosques de la Amazonı ´a Boliviana (PROMAB), Universidad Auto ´noma del Beni, Riberalta, Bolivia C. C. Soliz-Gamboa  N. P. R. Anten Ecology and Biodiversity, Institute of Environmental Biology, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands G. Helle Section 5.2 Climate Dynamics and Landscape Evolution, Helmholtz-Centre Potsdam. Deutsches GeoForschungsZentrum, Telegrafenberg C127, 14473 Potsdam, Germany T. L. Pons Plant Ecophysiology, Institute of Environmental Biology, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands Present Address: N. P. R. Anten Centre for Crop Systems Analysis, Department of Plant Sciences, Wageningen University, Droevendaalsesteeg 1, 6708 PB Wageningen, The Netherlands 123 Trees DOI 10.1007/s00468-013-0961-2