The structural characteristics of forest canopies are a defining feature of terrestrial ecosystems and fundamental driver of ecosystem functioning. Forest canopy structure and ecosystem structure-function relationships can be strongly impacted by disturbance, including forest management, with implications for forest resilience to global change drivers. Quantification of canopy structure is important both because of its role as an integrator of ecological factors, and because it provides a basis for studying fundamental structure-function relationships. Canopy structure is most commonly quantified as leaf area index, but vegetation canopies are inherently three-dimensional and the integrated vertical and horizontal arrangement of canopy elements provides a more meaningful representation of the properties of vegetation canopies that affect ecosystem functioning. The lack of a framework for characterizing 3D conceptions of canopy structure has limited research focused on understanding and modeling structure-function linkages. We demonstrate a multivariate, structural trait-based approach to quantifying variation in canopy structure among vegetated ecosystems. Our overall goals were to: 1) quantify a spectrum of potential canopy structural types that characterize temperate forest ecosystems, 2) evaluate effects of natural disturbance and forest management on canopy structure, and 3) assess effects of canopy structure, disturbance, and management on structure-function linkages and resilience in forests.
Results/Conclusions
Our findings demonstrate the potential for deriving ecologically-meaningful canopy structural types and gradients from multivariate canopy structural data and using this information to better understand forest structure-function relationships and how they are affected by disturbance. Our sub-continental forest data set was characterized by 2-3 primary gradients in canopy structure variability and 6 relatively distinct canopy structural types. Our results demonstrated expected separation of open-canopied forests from dense, closed canopy forests and tall, many-layered forests from shorter, single-layered canopies. Our results also illustrated substantial variation (similar in magnitude to that observed at the sub-continental scale) in canopy structure within landscapes and ecosystems related to fine-grain environmental variation, successional processes, and disturbance impacts. We illustrate differing impacts of disturbance on canopy structure (and associated ecosystem processes and functions) related to variable effects of causal agents and directionalities (i.e., top-down vs. bottom-up) of disturbance. Our findings indicate that characteristics of the canopy and disturbance itself together drive resilience of structure-function relationships to disturbance. Our results have implications for isolating the distinct role of physical structure in driving ecosystem functioning, elucidating fundamental ecological mechanisms underpinning ecosystem structure-function relationships, effectively representing canopy structure in models, and promoting forest ecosystem structure and resilience through management.