Pit & Mound Topography

Drafting..

This page provides descriptive data on the pits and mounds at Sandy Lake and Environs. View Mature/OG Forest discussion of their significance to the forest ecology of Sandy lake and Environs.

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A “pit and mound topography” is found throughout much of the forest at Sandy Lake and Environs and reveals an ‘old process’ of development towards an old growth state interrupted by wind disturbance. It’s a process that can be seen continuing today in blowdown of big trees initiated by Hurricane Juan.

These processes are especially pronounced in this forest close to Atlantic coast where hurricanes coming from the south first make landfall in N.S.

pit and mound

Old Hemlock and yellow birch trees on a mound.
View video on
The Ups and Downs of Pit and Mound Topography
Click on image for larger version

In places, the pit and mound structures occur in a repeated series with the long axes of mounds oriented perpendicular to the direction of wind that blew the trees down; walking across them in the direction of the wind is something akin to swimming or paddling through a set of swells in the ocean.

The mounds were formed by blowdowns of very large trees, which I estimate occurred about 150 years ago at many sites. The mounds are preferred ground for establishment of tree seedlings, and most of the big trees we see today are found on top of mounds.

The pits often develop as vernal (temporary) pools, providing habitat for amphibians and other organisms requiring water for part of their life cycle. As the trees grow old and mounds erode, the tree roots are exposed creating cavities for wildlife between the roots and the trunk.

Direction of Treefall

The direction in which the a tree fell when a mound was formed historically can be inferred from the orientation of mound and the associated pit. In Thirty-Eight, The Hurricane that transformed New England, Stephen Long describes it this way, talking about the field work of plant ecologist Charlie Cogbill:

His fieldwork made him an aficionado of blowdowns, and seventy-five years after Thirty-eight blew through, he could point out signs of its passing. The most common sign is the undulating ground where tip-ips (uprooted trees) transformed the forest floor. The ground in hurricane woods is poskmarked with deep depressions adjacent to correspondingly large mounds. Known as pit-and-mound topography, it looks as though somebody dug a hole and piled the dirt next to it, and it happens anytime the wind uproots a tree. The rootball of a forest tree forms a plate, and when wind knocks the tree over, the roots are wrenched out ofthe ground with only a hinge of roots maintaining it attachment to the ground, A the rest of the roots rip free, they grasp a mass of soil and stone, excavating a hole. The bigger the tree, the bigger the pit.

Over time, the roots and stump decompos, leaving a mound of earth. The pit collects leaves yearly, but the decompose without filling in the hole to any grate extent. Decades after the tree went down, you can stand in the pit, look out across the mound, and that’s the direction that the tree fell. In most tip-ups, the fatal gust of wind catches the trees soil so it keels over in line with the wind direction.

Using that simple guideline, I estimated directions of tree-fall (blowdown) for pits and mounds at the five old forest sites for which we had some ages of the trees, also for recent windfalls at or close to those sites. I chose only the mounds where there is still a distinct pit today, reading the direction from a Silva compass corrected for magnetic declination. I also measured directions of fall for recent blowdowns which were likely initiated by Hurricane Juan. Observations conducted Oct 20 and Nov 1, 2017.

The results are shown in the diagram below.

OG

The directions are consistent with the blowdowns being associated with hurricanes coming from the south. Independently, Bob Guscott, a retired DNR forester, observed very similar patterns for directions of blowdowns (with a N/NW bias) inferred for mounds in the area of Grand Lake (Oakfield, N.S.). Subsequently, we got together and refined the method for inferring direction so it could be applied to situations in which are unsure on which side of a mound the pit occurs. (View video). We also came up with a statistic for describing the frequency of mounds on the landscape (below).

Mound dimensions and density

These are some very preliminary descriptive stats.

Mound Stats

Some Lit

Patterns of pedoturbation by tree uprooting in forest soils
Bobrovsky M.V., Loyko S.V. Russian Journal of Ecosystem Ecology Vol. 1 (1). 2016. A descriptive article with photos. It references classic research by E.V. Ponomarenko who has been working in Nova Scotia recently.

Woodlands shaped by past Hurricanes
By David Dwyer, Forester NSDNR. 1979
“Many of our forest stands in Nova Scotia are a result of past hurricanes. Mounds on the forest floor -the result of uprooted trees – indicate this. The age of trees growing on these mounds give a good indication of when the storm occurred. These stand ages compare well with the written records of past storms…”

Salvaging has minimal impacts on vegetation regeneration 10 years after severe windthrow
by AR taylor et al, Forest Ecology and Management Volume 406, 15 December 2017, Pages 19-27. “The study area was located approximately 50 km east of Halifax”. Includes discussion of pit and mound topography, disturbance regimes etc.

Digital Wind Exposure Map for Nova Scotia
K. Keys et al., 2017 DNR document