Cross-Canopy Coupling In A New England Forested Mountain Valley

To advance knowledge of how a forest uses water, energy, and carbon dioxide, especially in a warming climate, it is critical to understand wind patterns above and below the forest canopy. Turbulence can create a coupled wind regime that moves resources through the canopy, though horizontal flows below the canopy can also produce significant fluxes of resources. Defining regimes of coupled and decoupled winds will help gain a better understanding of how resources are circulating and being used in a forest. This study aims to determine when coupled and decoupled winds are occurring in the Hubbard Brook Experimental Forest.

The Hubbard Brook Experimental Forest (HBEF) is an 8,700 acre, temperate mixed deciduous and conifer forest located in North Woodstock, New Hampshire, USA. The HBEF is also a mountain-valley system. It has a heterogenous canopy roughly 20 m AGL and sparse undergrowth. From 25 May - 16 June 2022 and 18 May - 9 June 2023, two eddy-covariance systems gathered 3-D wind speed and direction, water vapor and CO₂ concentrations, temperature, and relative humidity at the top of a 30-m flux tower and on the tower at 6 m AGL. Synoptic-scale maps were acquired from the NWS WPC’s Surface Analysis Archive of United States (CONUS) Analyses to compare with the flux tower data.

Nighttime above- and below-canopy winds often followed westerly mountain flows, while daytime above-canopy winds were often driven by synoptic-scale winds. Two coupling metrics, one using the standard deviation of vertical velocity (σ_w) and one using friction velocity (u*), were used to delineate coupled and decoupled regimes. A below-canopy σ_w coupling threshold value was found to be 0.105 m/s. Daytime and nighttime below-canopy u* coupling threshold values were found to be 0.080 m/s and 0.090 m/s respectively. Times with values above these thresholds were considered coupled. Both u* and σ_w agreed that coupling occurs more frequently during the day. However, the σ_w metric suggests a secondary peak at night, with brief decoupling during the transition periods of sunrise and sunset. Further research and additional data are needed to increase certainty in our results.