A research team led by Prof. WANG Kelin from the Institute of Subtropical Agriculture of the Chinese Academy of Sciences has systematically revealed the long-term legacy effects of human disturbance and how they shape the drought responses of karst forests.

The results were published in Communications Earth & Environment and GIScience & Remote Sensing.

The karst region of southwest China has long been shaped by the combined influences of human activities and climate change, and is one of the most vulnerable ecosystems in China. Under large-scale ecological protection and restoration programs, this region has led the global "greening" trend over the past two decades. However, the region's geology constrains the sustainability of this greening, and positive succession in rocky mountain shrub–grass communities recovering naturally after disturbance is extremely slow.

Persistent historical human disturbance has profoundly altered karst surface landscapes and vegetation structure, leaving difficult-to-erase ecological legacies. The increasing frequency of extreme drought events in recent decades has further heightened the fragility of regional forest ecosystems. Nevertheless, our understanding of when human disturbance began, how it operates, and its influence on karst forest responses to drought stress remains limited. This hinders the precision of forest and grass restoration for controlling rocky desertification and enhancing forest functional resilience after greening.

To address these issues, the researchers conducted an integrative, cross-dimensional study spanning multiple timescales (centennial to decadal) and spatial scales (depression to regional).

To investigate the long-term effects of human disturbance on karst areas, the researchers selected three karst depressions that exhibited mild, moderate, and severe rocky desertification. The researchers collected sediment cores and integrated pollen and phytolith records with radionuclide dating (¹⁴C, ¹³⁷Cs, and ²¹⁰Pb) by combining long-timescale paleoecological analyses with fine-scale satellite monitoring.

Additionally, they coupled sub-meter, high-resolution imagery from QuickBird (0.6 m) and China's Gaofen-2 (GF-2, 0.8 m) with a U-Net deep learning model to precisely count changes in the number of individual trees over the past 20 years. This allowed them to reconstruct vegetation (forest–grass) pattern dynamics in the karst region over the last 500 years.

The results showed that the first appearance of maize pollen and maize phytoliths in the 18th century was highly synchronized across the three depressions, accompanied by an increase in pioneer fern spores (represented by Dicranopteris) and a clear decline in arboreal pollen proportions alongside a rise in herbaceous pollen.

The shift in the tree-to-grass ratio was most dramatic in the severely desertified depression, providing clear evidence that the introduction of maize and subsequent land reclamation activities were key triggers of regional rocky desertification.

Satellite-based analyses further showed divergent recovery trajectories. Tree numbers in the mildly and moderately desertified depressions have exhibited an upward trend over the past two decades, reflecting partial ecological recovery. In contrast, tree numbers in the severely desertified depression have remained nearly unchanged.

The results suggest that intense and persistent human disturbance has created an essentially irreversible historical ecological legacy. Even as human pressures have declined in recent years, natural forest regeneration in severely degraded areas remains limited.

The "green without forest" phenomenon in karst landscapes suggests that once historical degradation exceeds a certain threshold, the capacity for natural forest regeneration may collapse.