Canopy complexity's role in (re)shaping carbon cycling following disturbance and with age: Do new observations support old theories?

Canopy physical and biological complexity is integrally linked to carbon (C) cycling processes. Canopy complexity is shaped and reshaped by the interplay between disturbance and succession, and, in turn, confers lasting effects on biogeochemical cycling. Until recently, empirical studies tended to focus separately on documenting changes in canopy complexity and C cycling with little attempt to explicitly join the two, but new experiments and data are providing novel opportunities to examine, inform, and challenge long-standing theories on canopy complexity-C cycling relationships. We will discuss how canopy complexity and C cycling observational data from research networks and site-level experimental manipulations are refining theory. We specifically evaluate, using observations, the veracity of the following theoretical assumptions: 1) Tree mortality reshapes canopy structure and leads to declines in ecosystem C balance; 2) Disturbance-related declines in C sink strength are proportional to the extent of severity and associated structural change; 3) Changes over ecosystem development in physical and biological canopy complexity parallel one-another. Observational data provide mixed support for theories, suggesting `one-size-fits-all' theoretical frameworks should be refined to reflect a more nuanced understanding of canopy complexity-C cycling interactions. Age-related tree mortality does modify complexity, but without substantially reducing forest C uptake. Disturbance severity and reductions in C uptake are non-linearly related, owing to offsetting increases in complexity-driven resource-use-efficiency. Lastly, we find that changes in physical, but not biological, complexity of a forest canopy are strikingly conserved across ecosystem development regardless of disturbance history and community composition, suggesting that leaves and branches may optimize their arrangement to maximize the acquisition and efficient-use of resources.