We reduced early winter snowpack in four experimental plots at the Hubbard Brook Experimental Forest in New Hamphire for 2 years to examine the mechanisms of root injury associated with soil freezing. Three lines of evidence suggested that direct cellular damage, rather than physical damage associated with frost heaving, was the principal mechanism of root injury: (i) decreases in root vitality were not greater on sites with more frost heaving, (ii) in situ freezing damage was confined to first-and second-order roots in the organic horizons rather than entire root systems, and (iii) tensile strength of fine roots was not significantly compromised by experimental stretching to simulate ice lens formation. Although significant differences in the intensity of soil freezing (depth, rate, and minimum temperature) were observed across the plots, no clear effects of soil freezing intensity on root injury were observed. Snow manipulation had no effect on mycorrhizal colonization of sugar maple (Acer saccharum Marsh.) roots. A significant increase in root growth was observed in the second summer after treatments, coincident with a significant pulse of soil nitrate leaching. Through their effects on fine roots, soil freezing events could play an important role in forest ecosystem dynamics in a changing climate.Résumé : Pendant 2 ans, nous avons réduit la couverture de neige au début de l'hiver dans quatre parcelles expérimentales à la forêt expérimentale de Hubbard Brook, au New Hampshire, pour étudier les mécanismes responsables des dommages aux racines causés par le gel dans le sol. Trois types d'indices ont indiqué que des dommages directs aux cellules, plutôt que des dommages physiques dus au soulèvement par la glace, étaient le principal mécanisme responsable des dommages aux racines : (i) la diminution de la vitalité des racines n'était pas plus grande dans les stations où le soulèvement par le gel était plus prononcé, (ii) les dommages in situ causés par le gel étaient limités aux racines de 1 er et 2 e ordres dans les horizons organiques plutôt qu'à l'ensemble du système racinaire et (iii) la résistance à la traction des racines fines n'était pas significativement compromise par un étirement expérimental pour simuler la formation de lentilles de glace. Bien que nous ayons observé des différences significatives dans l'intensité du gel dans le sol (profondeur, taux et température minimum) dans l'ensemble des parcelles, nous n'avons constaté aucun effet évident de l'intensité du gel dans le sol sur les dommages aux racines. La manipulation du couvert nival n'a eu aucun effet sur la colonisation des racines de l'érable à sucre (Acer saccharum Marsh.) par les mycorhizes. Une augmentation significative de la croissance des racines a été observée durant le deuxième été après les traitements. Cette augmentation coïncidait avec une poussée importante de lessivage du nitrate dans le sol. Par leurs effets sur les racines fines, les épisodes de gel dans le sol pourraient jouer un rôle important dans la dynamique des écosystème...

Eight full-scale energy foundations were constructed for a new building at the U.S. Air Force Academy (USAFA). The foundations are being used to demonstrate this technology to the United States Department of Defense, and have several experimental features in order to study of their thermal-mechanical behavior. Three of the foundations are instrumented with strain gages and thermistors, and their thermo-mechanical response during a heating and cooling test were evaluated. For a temperature increase of 18°C, the maximum thermal axial stress ranged from 4.0 to 5.1 MPa, which is approximately 25% of the compressive strength of concrete (estimated at 21 MPa) and the maximum upward displacement ranged from 1.4 to 1.7 mm, which should not cause angular distortions sufficient enough to cause structural or aesthetic damage of the building. The end restraint provided by the building was observed to change depending on the location of the foundation. The heat flux per meter was measured by evaluating the temperatures and flow rates of a heat exchanger fluid entering and exiting the foundations. The heat flux values were consistent with those in the literature, and the foundation with the 3 continuous heat exchanger loops was found to have the greatest heat flux per meter. The transient thermal conductivity of the subsurface measured using the temperatures of the subsurface surrounding the foundation ranged from 2.0 to 2.3 W/mK, which is consistent with results from thermal response tests on energy foundations reported in the literature.

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