Forest floor light conditions in a secondary tropical rain forest after artificial gap creation in northern Borneo

Forest floor light conditions were monitored in a field experiment regarding alternative approaches to canopy gap creation. To establish gaps, three pioneer canopy treatments (canopy tree felling, girdling and untreated control) were combined with two sub-canopy treatments (slashing or untreated control). The canopy treatments were performed within sub-plot circles (radius 5 m) while the sub-canopy treatment was confined to the central parts (2.5 m radius). Pioneer canopy felling and girdling (stripping of the bark) treatments affected 31% (mean value) of the initial mean standing basal area (39 m2 ha−1). Relative photosynthetic photon flux densities (PPFDR) and canopy openness were measured (1.5 m above the forest floor) in the sub-plots before and immediately after treatment and at subsequent intervals of 6, 18 and 30 months. During the study period all treatments resulted in increased PPFDR and canopy openness compared to pre-treatment mean values (PPFDR: 1.8–2.3%, canopy openness: 8.8–10.7%). Felling selected pioneer canopy trees provided distinct but transient shade reduction; mean PPFDR values were 12.5% after 6 months and 8.5% 30 months after initial treatment, while canopy openness averaged 14.2 and 11.8% after 6 and 30 months, respectively. Girdling effects were less drastic but after 30 months forest floor openness and PPFDR averaged values similar to, or slightly higher than the felling treatment. Sub-canopy slashing resulted in 2.4–5.1% higher PPFDR and 2.0–2.4% higher canopy openness compared to the untreated control plots and this effect persisted throughout the study period.