Transient canopy dynamics and the future of tropical forests

Background/Question/Methods: A major problem at the intersection of basic and applied forest ecology is the role of tropical forests in the global carbon (C) cycle.  Although they represent just 13% of the terrestrial biosphere, tropical forests contribute nearly half of the global biomass C pool.  Atmospheric modeling studies suggest the terrestrial tropics are in net balance with the atmosphere, and do not reveal the large C source anticipated from tropical deforestation.  This implies the existence of a sink to offset deforestation losses, but the location of the sink is controversial.  Our strongest inferences are from plot-based studies that have focused on external drivers, such as ‘CO₂ fertilization’.  However, relatively little is known about endogenous canopy dynamics.  Whether most tropical forests are in equilibrium with contemporary disturbance regimes, and the duration of transient dynamics in response to forest disturbance are poorly understood.  

Results/Conclusions: We used measurements of canopy height from airborne LiDAR remote sensing at two points in time to quantify canopy dynamics in a Hawaiian rain forest landscape.   We developed a hierarchical statistical model to decompose the distribution of canopy height changes into vertical and lateral components.  Our analysis provides estimates of the frequency of disturbance, vertical growth and lateral canopy dynamics.  Analysis of the canopy height transition matrix indicates that this forest landscape is far from the projected steady-state equilibrium.