Analyzing the influence of climate and environmental factors on growth rates of secondary tropical forests using NASA GEDI spaceborne LiDAR

Tropical secondary forests in the Amazon are able to sequester carbon up to 20 times faster compared to old-growth forests and potentially play a significant role in mitigating climate change. At the same time, recent studies suggest that the secondary forests might be more sensitive to climate extremities such as droughts. Nonetheless, how these forest responses vary within different age groups and across environmental gradients - such as rainfall and length of dry season - is poorly understood. The NASA GEDI spaceborne LiDAR allows extraction of large-scale three-dimensional forest structure information - including canopy height and plant area density - from space. In this study, we extracted spaceborne LiDAR-derived relative heights for secondary forests within the Amazon basin and investigated i) how growth rates of secondary forests vary across distinct age groups and ii) the influence of different environmental and climate factors on growth rates. For determining the study plots, initially 100 x 100 m grids of secondary forest areas, homogenous within a 5-year age interval, were selected based on a secondary forest map created from Landsat imagery data from 1985 through 2018. Plots where there were no overlaps of high quality GEDI shots were removed. Using a space-for-time substitution approach, we created a 30 year time series of secondary forest data from the available 3 year (2019-2021) GEDI dataset. Our results confirm that height growth rates of early age secondary forests (< 15 years) are higher than older secondary forests on average. Distance from the equator and land surface temperature were also found to affect the growth rates. We further explore the carbon sink potential of early age secondary forests under changing climate conditions and evaluate its capability to offset ongoing carbon emissions from anthropogenic forest disturbances.