Introduction
Coarse woody debris (CWD) has many roles related to the establishment, survival, and growth of tree seedlings.
CWD modifies microsite condition in ways often beneficial to the seedling. These modifications may relate both directly to the physical needs of the seedling, such as light and moisture, or indirectly in being a physical or visual obstacle to
browsing.
We established a small trial to provide us with a basic understanding of the extent of the microsite influence of CWD on seedling survival and growth in the Pothole Creek Study Area. In this trial, seedlings were planted at a range of distances
on both sides of three large pieces of coarse woody debris. Survival and growth were monitored for two growing seasons.
Methods
In early June 1997, three logs of approximately 40 cm diameter and oriented across slope were located on the Pothole Creek Study Area. A 2.5 m x 2.5 m area on each side of the log was scalped to remove initial grass competition leaving duff of
1-2 cm depth. Rows of seedlings were planted parallel to the log at distances of 0.1, 0.5, 1.0, and 2.0 metres on both the up and down slope sides. Each row consisted of 5 seedlings planted 0.5 m apart.
Total height and root collar diameter were measured right away and survival and browsing damage were assessed in late June and July. Survival, height, and browse condition measurements were conducted in October 1997 and 1998.
Results
The seedlings suffered high mortality during the two seasons studied (Table 1). Most seedlings were damaged by animals (Table 2) either by browsing (rabbit) or by girdling (small rodents). Five percent mortality was due to sloughing of the log
bark onto the seedlings closest to the logs. Differences in height growth of both browsed and unbrowsed live seedlings due to distance from the CWD were not discernible, though there is some indication that the seedlings closest to the CWD suffered
less height loss due to browsing (Table 3).
| Table 1. The proportion of the original seedlings that were alive on the measurement date. Proportions based on all individual seedlings, not on plot values. |
|
| Measurement
Date |
Distance
from Log (m) |
Total |
| 0.1 |
0.5 |
1.0 |
2.0 |
| July 1997 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
| Oct 1997 |
1.00 |
0.97 |
1.00 |
1.00 |
0.99 |
| Oct 1998 |
0.30 |
0.17 |
0.23 |
0.37 |
0.27 |
| Table 2. The proportion of the original seedlings found dead or damaged by animals on the measurement date. Proportions are based on all individual seedlings, not on plot values.
|
|
| Measurement Date |
Distance from Log (m) |
Total |
| 0.1 |
0.5 |
1.0 |
2.0 |
| July 1997 |
0.07 |
0.37 |
0.43 |
0.43 |
0.33 |
| Oct 1997 |
0.73 |
0.93 |
0.97 |
1.00 |
0.91 |
| Oct 1998 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
| Table 3. The total height of the remaining live seedlings.
Means and standard errors are based on all individual seedlings, not on plot values. Note that seedling height is a related to height growth, browsing, and mortality.
|
|
Measurement
Date |
Distance from Log (m) |
| 0.1 |
0.5 |
1.0 |
2.0 |
| June 1997 |
25.9 (0.6) |
25.5 (0.6) |
26.1 (0.7) |
27.2 (0.7) |
| Oct 1997 |
20.9 (2.0) |
16.9 (1.9) |
11.7 (1.6) |
14.8 (1.6) |
| Oct 1998 |
16.9 (2.1) |
10.0 (3.8) |
8.4 (2.8) |
19.1 (2.9) |
Discussion
Coarse woody debris had both positive and negative effects on the survival of planted seedlings in this study. CWD appears to be an obstacle (physical or visual) to browsing by rabbits and, presumably, also to ungulates. This should promote
seedling survival. However, in winter the CWD provides shelter for small rodents and those seedlings accessible (e.g., next to CWD) to the rodents could potentially suffer damage. In this study, we used only large and long (> 8m) CWD which is
probably favourable to small rodents. Shorter CWD may provide less favourable habitat for small rodents but still offer obstacle and microsite advantages. Further investigation is needed to confirm the advantages and disadvantages of CWD in
relation to animal damage, as well as microsite modifications. |
