Poplar regrowth rates.  Guggenmoos, S., Wilkinson, M.M. TransAlta Utilities, Distribution Line
Clearance, 1991.

Effective maintenance of a distribution line right of way necessitates a recognition of the problem plant species and an expectation as to the rate of occurrence and growth.  TransAlta’s Distribution Line Clearance group was provided with the rate of occurrence of problem species and implied tree growth rates by Division in the 1985 report on the Distribution Line Clearance Program by Environmental Consultants, Inc..

It was widely observed and accepted that brush regrowth rates after mowing was rapid and appeared to exceed the seedling growth rate.  If this was true, then the maintenance method chosen impacted on future cycles and thus costs.  Two studies were undertaken.  The first was to determine the rate of poplar seedling growth, establishing a baseline, the second to determine the rate of regrowth following mowing with a Hydro Axe.

Work undertaken in 1987 examined the rate of growth of trembling aspen and balsam poplar over a wide geographic area of Alberta consisting of both the aspen parkland area and the boreal forest area (Division 5,6,7).  In 1989 further sampling was undertaken to determine the rate of regrowth of trembling aspen and balsam poplar following mechanical clearing with a Hydro Axe (Division 3,5,6,7).  Sampling was conducted on a random basis working from maps of suggested areas which would meet the criteria.  Growth per year of seedlings of trembling aspen and balsam poplar tended to be quite uniform across all sampling areas.  In fact, though an ANOVA test suggested there were differences in mean growth, these could not be discerned by the Duncan’s
Multiple Range Test at the 5% level.  In examining regrowth after mowing significant differences did emerge.  Trembling aspen regrowth was significantly lower in the sample taken in the Barrhead area than other areas such as Alder Flats and Penhold.  The largest difference, however, occurred between the regrowth after mowing and growth from a seedling.  Mean growth of 77.4 cm and 26.9 cm yields the least significant difference of 50.5 cm.

Growth from seedlings required two to three seasons to achieve the long term mean growth rate of 0.45m/yr..  The growth rate following mowing was substantially higher in the first season (Figure 2).  Further, due to height restrictions imposed by the Vegetation Management Standards and Specifications, it is of interest that the growth rates following mowing had a long-tailed distribution on the high side of the median (Figure 1).  While the growth rate of seedlings increased to the level of the mean in attaining 8m in height, the growth rate after mowing, over a
period of 3 to 4 seasons, dropped to the seedling mean growth rate (Figures 3 & 4).  The height of regrowth on the mowed area expands to approximately 2m more than that of seedlings for the same number of growing seasons.

There are a number of operational implications to these findings.  If foliar herbicide applications are to treat brush at less than 1.5m in height then clearly one must expect to treat this brush in the first growing season after mowing.  Since, the maximum allowable spray height along secondary roadways is 2m, then one should also be prepared to treat this brush in the first growing season. A spray cycle of 3 years is suggested since the herbicide would kill the existing poplar stems to the groundline.  The following year adventitious buds swell and break on the root system.  The second season the root suckers would grow 35 to 175 cm (Figure 1).  The 3’rd season it would again be necessary to spray the regrowth.  When right of ways have been maintained with herbicides, such that regrowth represents seedling invasion, then the spray cycle would be 5 to 7 years on secondary roads with a 2m height limitation (Figure 5).

Where mowing is the maintenance method, we have had an expectation that it would be required on a 9 to 12 year cycle.  The findings support this expectation.  It is also indicated that if we
found a mechanical method which controlled the root system so that regrowth was actually from seed, then the cycle would be extended by 4 years (2m/(.45m/yr)= 4 yr).  This information may
make rotovators more appealing, though the obstacles of maneuverability and travel costs remain as a barrier to their general use as a substitute for mowers.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5