Boundary-Layer Meteorol (2008) 126:415–431 DOI 10.1007/s10546-007-9246-x ORIGINAL PAPER Spectral Short-circuiting and Wake Production within the Canopy Trunk Space of an Alpine Hardwood Forest Daniela Cava · Gabriel G. Katul Received: 20 June 2007 / Accepted: 13 November 2007 / Published online: 8 December 2007 © Springer Science+Business Media B.V. 2007 Abstract Using synchronous multi-level high frequency velocity measurements, the turbulence spectra within the trunk space of an alpine hardwood forest were analysed. The spectral short-circuiting of the energy cascade for each velocity component was well repro- duced by a simplified spectral model that retained return-to-isotropy and component-wise work done by turbulence against the drag and wake production. However, the use of an aniso- tropic drag coefficient was necessary to reproduce these measured component-wise spectra. The degree of anisotropy in the vertical drag was shown to vary with the element Reynolds number. The wake production frequency in the measured spectra was shown to be consistent with the vortex shedding frequency at constant Strouhal number given by f vs = 0.21 ¯ u/d , where d can be related to the stem diameter at breast height (dbh) and ¯ u is the local mean velocity. The energetic scales, determined from the inflection point instability at the canopy– atmosphere interface, appear to persist into the trunk space when C du a cr h c β ≫ 1, where C du is the longitudinal drag coefficient, a cr is the crown-layer leaf area density, h c is the canopy height, and β is the dimensionless momentum absorption at the canopy top. Keywords Canopy turbulence · Foliage drag · Spectral short-circuiting · Strouhal instabilities · Trunk space · Wake production · Von Karman streets 1 Introduction The spectral properties of turbulence within the trunk space of forested ecosystems are now receiving significant attention because of the proliferating number of applications requiring D. Cava (B ) CNR - Institute of Atmosphere Sciences and Climate, National Research Council (CNR), Lecce, Italy e-mail: d.cava@isac.cnr.it G. G. Katul Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC, USA e-mail: gaby@duke.edu G. G. Katul Department of Civil and Environmental Engineering, Duke University, Durham, NC, USA 123