Natural regeneration and successional development of lower montane forests in northern Thailand: Decadal patterns of structural, compositional, and demographic change

Abstract

Long-term evidence of structural, compositional, and demographic change during natural regeneration of tropical montane forests is limited in Southeast Asia. We analyzed six censuses (2011–2021) in two 1-ha naturally regenerating lower-montane forest stands in northern Thailand. Designated NFR-17y and NFR-37y, the stands were 17 and 37 years old in 2021. A 1-ha natural forest surveyed in 2021 was the static compositional reference. Species richness and basal area increased significantly in both regenerating stands, whereas stem density declined. Although NFR-17y had the highest richness in 2021, it remained less similar in composition to the natural forest than NFR-37y. Bray–Curtis dissimilarity based on relative abundance or importance value index declined significantly in both stands (all BH-FDR q ≤ 0.0167), indicating stand-specific convergence. Mortality and recruitment fluctuated among five census intervals, but no monotonic trend was detected. Among species meeting the ≥ 20-stem criterion, functional-group differences in mortality were not supported after false-discovery-rate adjustment, and recruitment did not differ among groups within either stand. Climax-species recruitment was higher in NFR-17y than in NFR-37y. Mean diameter growth rate was higher in NFR-17y for pioneer and climax species, whereas relative growth rate was higher for all three groups. Structural development and compositional convergence therefore occurred concurrently, but high richness did not indicate recovery of natural-forest composition. Because each condition was represented by one plot, these findings are stand-specific rather than replicated effects of forest age. Natural regeneration was associated with continuing structural development and compositional convergence over the monitoring decade.