Introduction

This chapter is about turbulent transfer between a plant community and the atmosphere, especially the transfer of heat, water vapour, CO2 and other scalar entities. We consider the way in which turbulent transfer influences the microclimate within the plant community, in particular the mean scalar concentrations, including temperature and humidity. The second section provides a brief, qualitative overview, to establish the connections between the turbulent transfer process in a plant canopy and exchange processes at both smaller scales, those of individual leaves, and larger scales, those of the entire planetary boundary layer of the atmosphere. Then, with frequent reference to the observed properties of turbulence in plant canopies, the third and fourth sections review two kinds of theory for describing turbulent transfer. The more common Eulerian theories consider the behaviour of the turbulent transfer process at a grid of points fixed in space. Less common, but of increasing importance, are the Lagrangian theories: these describe turbulent transfer by considering the statistical behaviour of the wandering blobs of fluid which actually carry the transferred entity.

Overview

Common experience shows that the air motion within a plant canopy is highly erratic and intermittent. The origin of this behaviour lies in the planetary boundary layer (PBL), the turbulent layer of the atmosphere which extends from the ground to a height of the order of a kilometre (within a factor of three or so).