A multi-source remote sensing approach to identify and predict delayed succession in human-dominated tropical landscapes

Abstract

Succession is the process and pattern of changes in communities following a disturbance over time. This process can be disrupted by recurrent disturbances or arrested at an early successional stage even if disturbances cease. Delayed succession can hinder the natural regeneration of vegetation, weakening ecosystem development, functions and services and in severe cases, increase the risk of fires. Despite its ecological significance, delayed succession is typically analysed in a site- or species-specific context, lacking the broad spatial quantification needed for effective restoration planning. Using the tropical vegetated landscapes of Hong Kong as a model system, we identified the delayed succession phenomenon using a multi-source remote sensing approach that integrates satellite-based time-series imagery and airborne-based canopy height data. We then investigated the influence of biotic, abiotic and disturbance factors on the occurrence and spatial variability of delayed succession using XGBoost. Finally, we analysed the drivers of natural regeneration and predicted the recovery trajectories of disturbed grasslands following the cessation of disturbances. Our findings indicate that delayed succession affects 4.2% (30.78 km²) of Hong Kong's vegetated area, predominantly on ridges and mid-slopes. Biotic factors were the primary drivers, with vegetation density (<0.2 m) being the most significant. Fire frequency emerged as a critical disturbance factor; the probability of delayed succession increased sharply after more than three fire instances during the study period. Notably, vegetation density (<0.2 m) shifted from mitigating to promoting delayed succession as fire frequency increased. Upon cessation of post-fire disturbances, natural recovery was more dependent on the proximity of natural woodland remnants than on plantations. Synthesis and applications. Our study challenges the view that fire prevention alone is sufficient for restoring degraded landscapes. We show successful natural regeneration hinges on nearby remnant woodlands acting as crucial seed sources. This research provides a remote sensing framework to quantify the risk of delayed succession where ecosystems fail to recover within a short term. This enables managers to strategically prioritize resources, focusing fire prevention on high-risk sites while directing active restoration like planting to areas with low natural recovery potential. This approach facilitates cost-effective, climate-resilient restoration.