**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