A second study of the effect of aspen of defoliation by the forest tent caterpillar in Minnesota was conducted 6 years after the conclusion of an initial comprehensive investigation. Results indicate increasing mortality in other than suppressed trees with increasing defoliation intensity. Hypoxylon, Nectria, and insect incidence increased with defoliation intensity. Growth is significantly reduced 1 year beyond the cessation of defoliation in stands where three successive heavy defoliations have occurred. Growth of the dominants increases significantly, however, the third year after defoliation in such stands.

No abstract

The depths of soil freezing and thawing were measured periodically with a Veihmeyer tube in snow removed, litter removed, compacted, compacted‐snow removed, and undisturbed plots during the 1962–63 season near St. Paul, Minn. The soil was a sandy loam to a loamy sand with a high water content when the study began. Cold conditions and minimal snowfall resulted in deeply frozen soil throughout Minnesota this season. At a station near the study site, the mean monthly snowfall and mean daily air temperature were, respectively, 4 to 6 cm and 2 to 5C below normal during January and February.The removal of a snow layer 13 cm or less in depth caused deeper and more persistent soil freezing than did either litter removal or soil compaction. For several weeks, frozen soil averaged 38 cm deeper in snowless plots than in snow‐covered control plots. For about the same period, frozen soil averaged 20 cm deeper in plots without litter than in litter‐covered control plots, and 13 cm deeper in snowless compacted plots than in snowless control plots. The presence of even a shallow snowpack (13 cm) seemed to reduce the individual effects of litter removal and soil compaction on the depth of soil freezing. “Concrete” and “porous concrete” were the types of frozen soil observed in all treatments. The most significant hydrologic effect of deep freezing in snowless plots may have been the 10 additional days required for complete thawing in comparison to snow‐covered plots. However, late in the thawing period, the top of the frozen layer was 60‐cm beneath the soil surface, and it was discontinuous as indicated by complete thawing at some sample points. A discontinuous frozen layer was observed in all treatments 1 to 2 weeks before the complete disappearance of frozen soil. Thawing began earliest in plots without litter.