No. 131 

1962 

LEAF FALL, HUMUS DEPTH, AND SOIL FROST 

IN A NORTHERN HARDWOOD FOREST 

In the mound-and-depression microtopography of the northern hardwood forest, leaves are blown off the mounds and collect in the 

depressions. This influence of microtopography on leaf accumulation is 

responsible for much of the variation in humus depth; and this, in turn, 

affects the formation and depth of soil frost. 

These interrelationships have been studied recently at the Hubbard 

Brook Experimental Forest in the White Mountains of New Hampshire. 

The study site was a second-growth uneven-aged stand that had a basal 

area of 135 square feet per acre, an average tree diameter of 6 inches, 

and a canopy closure of more than 90 percent. 

Leaf Fall 

The amount and variation of leaf fall was determined in the fall of 

1961. Leaves were caught in 10 boxes, each 2 x 2 feet square, scattered 

randomly in three %-acre study plots. On a pound-per-acre basis (ovendry weight) the catch in individual boxes ranged from 2,812 to 3,834 

pounds, averaging 3,348 pounds with a standard deviation. of 449 

pounds, 13 percent of the mean. Most of the leaves fell during October. 

Leaf Accumulation 

Leaf accumulation was measured on 9 ground plots 2 x 2 feet square, 

which were sprayed before leaf fall with a covering of blue paint so 

that new accumulations could be distinguished easily. Three plots were 
located on mounds (12 percent of the area), 3 in depressions (13 percent of the area), and 3 on intervening slopes (75 percent of the area). 

Leaf accumulations were collected for 3 years. Collections were completed each year by November 15. 

Accumulations in the fall of 1961 (to compare with leaf-fall measurements) ranged from a low of 628 pounds per acre (oven-dry weight) 

on one of the mound plots to a high of 8,351 pounds in a depression. 

The mean value was 3,326 pounds, with a standard deviation of 2,253 

pounds, 68 percent of the mean. Thus leaf accumulation was found to 

be about five times more variable than leaf fall. 

The average accumulations for each year, and the averages weighted 

according to proportion of area occupied by each topographic position, 

were as follows: 

Average Accumulation 

Mounds 

Slopes 

Depressions 

1959 1960 1961 M ean 

(pounds) (pounds) (pounds) (pounds) 

1,852 742 1,641 1,478 

2,566 2,658 2,522 2,582 

4,187 6,542 5,815 5,515 

W ekhted Averaxe Accumulation 

Analysis of variance showed that the difference between topographic 

position was highly significant (F = 2 3.36), with non-significant differences between years (F = 0.43) and year-topography interaction 

(F = 1.30). The average annual accumulation of 2,820 pounds per acre 

falls between the 2,425 to 3,020 pounds reported by Chandler1 for mixed 

hardwoods in New York State. The weighted area average for 1761 

was about 500 pounds less than the mean catch of the boxes for that 

year - an unaccountable difference. 

Humus Accumulation 

Humus type and depth were determined at 2,800 points in the experimental area in conjunction with three winters of snow and frost measurements. Mor humus occupied 67 percent of the area, mull 19, and duff 

1 Chandler, R. F. The amount and mineral nutrient content of freshly fallen leaf litter in the 

hardwood forests of central New York. Jour. Amer. Soc. Agronomy 33: 859-869, 1941. 
mull 9; the remainder, 5 percent, was bare mineral soil. The average 

depth of humus according to topographic position was as follows: 

Mor Mull Duff mull 

(inches) (inches) (inches) 

Mounds 1.5 1.4 2 .O 

Slopes 3 .O 2.9 3.5 1 Depressions 5.9 4.2 7.1 

Roughly, humus depth on slopes was about twice that on mounds, and 

depths in the depressions were three to four times greater than the depths I 

I on mounds. Similar proportions were evident in leaf accumulations. 

I 

~ Soil Frost 

The occurrence of concrete frost varied from 23 percent (after a cold 

period without a snow cover) to none. Throughout most of the winter 

snow insulates the soil and restricts frost formation. During three winters 

of frost observations, concrete frost was found at only 4 percent of the 

2,800 sampling points. However, 78 percent of these points where frost 

was found were on mounds where little or no humus had accumulated; 

the remainder were found on slopes; and no frost was observed in 

depressions. The average depth of concrete frost on the mounds was 2.7 

inches, and on slopes 1.6 inches. It should be noted that any overland 

flow from a mound having concrete frost would be absorbed by the 

unfrozen slopes and depressions adjacent to it. 

- GEORGE HART 

RAYMOND E. LEONARD 

ROBERT S. PIERCE