Disturbance history and historical stand dynamics of a seasonal tropical forest in western Thailand
Disturbances influence forest dynamics across a range of spatial and temporal scales. In tropical forests most studies have focused on disturbances occurring at small spatial and temporal scales (i.e., gap dynamics). This is primarily due to the difficulty of reconstructing long-term disturbance histories of forests in which most tree species lack annual growth rings. Consequently, the role of past disturbances in tropical forests is poorly understood. We used a combination of direct and indirect methods to reconstruct the historical disturbance regime and stand development patterns in mature and regenerating seasonal dry evergreen forest (SDEF) in the Huai Kha KhaengWildlife Sanctuary in western Thailand. Direct estimates of long-term establishment and growth patterns were obtained from 12 tree species that form annual growth rings as a consequence of the region's strong intra-annual rainfall seasonality. Indirect estimates of establishment patterns were obtained from analyses of stand structure and individual tree architecture and application of ageestimation models to 10 dominant canopy-tree species using demographic data from a largescale, permanent forest-dynamics plot.
The combination of direct and indirect methodologies revealed a complex disturbance history in the seasonal evergreen forest over the past 250 years. In the mid-1800s, 200-300 ha of forest were destroyed by a catastrophic disturbance, which led to the synchronous establishment of many of the trees that presently dominate the forest canopy. Since then widespread disturbances of variable intensity have occurred at least three times (1910s, 1940s, and 1960s). These disturbances created discrete temporal pulses of establishment in small to large gaps in the forest matrix across several square kilometers. Background mortality and gap formation were evident in every decade since 1790, but these varied in intensity and frequency.
The SDEF retains a distinct structural and floristic legacy from the catastrophic disturbance of the mid-1800s. The single-age cohort that established after the disturbance has developed a complex three-dimensional structure as a consequence of differences in interspecific growth patterns of the canopy-tree species and subsequent disturbances of moderate and low intensity. While no single methodological approach provided a complete picture of the disturbance history and stand development patterns of the seasonal evergreen forest, taken together they offered new insights into the long-term dynamics of a primary tropical forest. In particular, the study highlighted the role of disturbance at multiple spatial and temporal scales and varying intensities in determining the structure and composition of a complex, species-rich tropical forest and raises important questions about the role of rare, catastrophic events on tropical forest dynamics.