Tamm review: Leaf Area Index (LAI) is both a determinant and a consequence of important processes in vegetation canopies

Leaf Area Index, LAI, the total surface area of leaves per unit of ground area (m² m⁻²), is a collective measure of the foliar portion of vegetation canopy structure. The measure underscores the primacy of surface area in many fundamental processes of vegetation-environment interactions. Since its definition by Watson in 1947, LAI has been used to aggregate leaf level characteristics throughout the canopy to the crop or stand level, predict light regimes, assess the total quantity of above ground biomass and estimate the primary production in vegetation among other uses. This property is widely cited as a central parameter in ecosystem or earth system models of production, for schemes to parameterize vegetative surface interactions in climate models and for large-scale estimates of other surface properties.The value of LAI is sensitive to a variety of factors. At a global scale it is related to climate and plant functional type. At a local scale it is affected by weather and site factors such as fertility, stand age, management treatment, disturbance history. It is a difficult quantity to measure, principally because only part of the total is readily apparent. The indirect measurement approaches are affected by several distinct factors: viewpoint effects, occlusion of surfaces and the presence of material that is not green foliage. Much effort has been spent to estimate those hidden parts.Here I review the history of this measure and related ecosystem structure and process concepts, discuss how this structural characteristic has been connected to light distribution, material and energy exchange and used to model a variety of vegetation functions. Although the concept has been applied at the leaf, branch and whole-tree basis, I focus here on canopy and ecosystem scales, mostly in forests.The principal theses are that while LAI is a fundamental characteristic of vegetation and the result of considerable energetic investment by the plants: (1) the absolute value of the index is not always a clear driver of biomass or production, (2) the details of leaf area organization in space and within microclimate gradients is far more important than is the total amount of leaf area and (3) the total LAI can include leaf areas spanning a range of species, strategies and behaviors whose aggregate behavior is a challenge to model. I argue that (1) the value of LAI may be considered more of a consequence than a cause of canopy structural attributes and (2) the total LAI may be considered to have a range of functions, from ecosystem production to sustaining a diversity of biological strategies. Scaling of processes dependent on leaf area is a significant challenge and simple aggregation schemes can be misleading. I suggest that much of the total LAI (beyond about LAI ~ 3) may not be directly relevant for many ecosystem processes but have other important consequences. I illustrate this idea with several examples and suggest some questions related to LAI that might be profitably studied.