Modeling free convection in bryophytes using temperature distributions within the canopy

Non-vascular bryophytes cover soil surfaces in many boreal and peatland environments, contributing to water and energy exchange within the ecosystem. Prior research has led to a good understanding of how conductance to water vapor relates to wind speed (i.e., under forced convection), but little is known about mechanisms that govern free convection under still conditions. Lab studies show that temperature gradients exist within bryophyte canopies, with individual canopies ranging 4-5 °C due to evaporative cooling effects. Also, significant relationships were found between temperature ranges and vapor pressure deficits (VPD) that related to conductance as temperature gradients can cause air density differences that drive free convection. The purpose of the present study is to determine if such relationships exist in the field and alter an existing model of convection from bryophyte layers to account for free convection at low wind speeds.  To determine if temperature gradients exist within bryophyte canopies in the field, a portable 3D thermal imaging system was developed to map shoot temperatures in 3D space.  The distribution of shoot temperatures at two field sites in central and upstate New York was analyzed to determine temperature gradients and temperature ranges within canopies of Hylocomium splendens, Rhytidiadelphus triquetrus and Polytrichum commune and compared with VPD. The temperature range and VPD data collected in the field formed a continuum with the data collected under laboratory conditions, although the VPD was lower in the field.  Using this combined relationship, an existing model of bryophyte conductance to water vapor under forced convection (Launiainen et al. 2015, Ecological Modeling 312:385) was altered to include free convection. The altered model accounts for free convection controlled by VPD influencing temperature ranges within the canopy at low wind speeds (<0.6 m/s). In this range, conductance was four to six-fold larger than the Launiainen model due to the temperature differences within bryophyte canopies causing free convection. A full model is presented accounting for conditions where free and forced convection dominate conductance to water vapor.