Low mortality in tall tropical trees

The dynamics of the tallest trees in tropical forests are of special interest due to their carbon content, canopy dominance, and the large canopy gaps created when they die. Known ecological mechanisms that may influence tall tree survival lead to conflicting predictions. Hydraulic stress and exposure to high winds and desiccation should increase death rates, yet the tallest trees have the greatest access to light and escape damage caused by falling boles and branches. The uncertainty in tall tree mortality rates has been difficult to address due to their low density, which makes mortality rates challenging to estimate accurately. Here, we use a combination of LiDAR remote sensing and field measurements to show that the mortality rate over 8.5 years among individuals >40 m tall in 444 ha of lowland Neotropical rain forest was 1.2% per year, less than half the landscape-scale average for all canopy trees (2.7% per year). The low mortality is likely explained by species-specific traits that decrease the mortality risk and/or ecological advantages of height that outweigh the risks. Regardless of the mechanisms, the low mortality rate has important implications for tropical forest carbon budgets, as we estimated that a single tall individual represents 2-11% of total live aboveground carbon stocks per hectare. Our findings suggest that height-specific dynamics may be surprisingly different from traditional diameter-specific dynamics, emphasizing the importance of extending ecological studies to investigate the role of tree height in forest dynamics.