Journal of Vegetation Science && (2016) Soil degradation and feedback processes affect long-term recovery of tropical secondary forests Siew Chin Chua, Benjamin S. Ramage & Matthew D. Potts Keywords Aluminium; Central Catchment Nature Reserve Singapore; Dicranopteris linearis; Environmental filtering; Forest regeneration; Long-lived pioneers; Phosphorus; Plant community diversity and structure; Secondary succession; Short-lived pioneers Abbreviations CnpyO = canopy openness; sdV = mean standard deviation of daily vapour pressure deficits; Tmax = mean daily maximum temperatures; AlSat = aluminium saturation; TEB = total exchangeable bases. Nomenclature Chong et al. (2009) Received 15 March 2015 Accepted 18 February 2016 Co-ordinating Editor: Frank Gilliam Chua, S.C. (corresponding author, scchua@cal.berkeley.edu) 1 , Ramage, B.S. (BenjaminRamage@rmc.edu) 2 , Potts, M.D. (mdpotts@berkeley.edu) 1 1 Department of Environmental Science, Policy & Management, University of California Berkeley, Berkeley, CA 94720, USA; 2 Biology Department, Randolph-Macon College, Ashland, VA 23005, USA Abstract Questions: What processes govern the long-term recovery of tropical secondary forests? Specifically, how are seedling species density, stem density and func- tional groups in older regenerating forests affected by existing trees, the regener- ation environment and distance to seed sources? Location: Central Catchment Nature Reserve, Singapore. Methods: We investigated three primary forest plots and nine 60-yr-old sec- ondary forest plots varying in their degree of recovery. We analysed seedling parameters as a function of the abiotic and biotic regeneration environment as well as distance to primary and mature secondary forests. Results: The secondary forest plots had functional composition distinct from, and Chao-estimated species richness and stem density lower than the primary forest plots. Seedling communities were most strongly associated with the mature tree communities within each plot; associations with other intra-plot variables and distance to potential seed sources were lower but still highly signif- icant. Overall, aside from the mature tree community, the most significant pre- dictors of seedling species composition were fern cover, VPD, soil Al, available P and C:N ratio. Primary forest species were associated with low light environ- ments, low macronutrients and low Al saturation. Secondary forest species were associated with high P, low soil pH and low total exchangeable bases. Long-lived pioneers differed from short-lived pioneers in their association with high soil C: N ratio, low Al saturation, low P and tolerance of a wider light range. Overall, improved seedling recruitment was associated with decreasing fern cover, thin- ner layer of leaf litter and higher light. At the landscape level, species density was inversely related to distance to potential seed sources. Conclusions: Our findings suggest that historical soil degradation and subse- quent feedback processes among the adult trees, seedling communities and the regeneration environment strongly affect forest succession. Nutrient-efficient and Al-tolerant pioneers, such as the fern Dicranopteris linearis, were probably initial colonizers. Chance dispersal and recovery of the regeneration environ- ment subsequently affected the establishment of long-lived or short-lived pio- neers. Persistence of D. linearis and long-lived pioneers slowed forest recovery, while short-lived pioneers improved nutrient recovery and ameliorated the above-ground conditions to facilitate subsequent succession, provided that seeds of primary forest species were present. Introduction Secondary and degraded forests constitute a larger area than primary forests in the tropics today (FAO 2010). This is likely to increase as deforestation and land-use change continues (FAO 2010). Previous research has shown that primary and secondary tropical forests play an important role in conserving biodiversity (Dent & Wright 2009) and sustaining ecosystem functions (Brown & Lugo 1990; Guariguata & Ostertag 2001). However, the ability of sec- 1 Journal of Vegetation Science Doi: 10.1111/jvs.12406 © 2016 International Association for Vegetation Science