Subject: BEN # 106

From: [email-masked] (Adolf Ceska)
Date: 13 Jul 1995 07:45:55 -0700

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No. 106                              July 13, 1995

[email-masked]        Victoria, B.C.
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 Dr. A. Ceska, P.O.Box 8546, Victoria, B.C. Canada V8W 3S2
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SCOTCH BROOM (CYTISUS SCOPARIUS) REMOVAL TRIALS
From: Joel Ussery <[email-masked]>

This  report is based on an illustrated presentation made to the
Garry Oak Grassland Rehabilitation Symposium on June 10, 1995.

As part of my master's  thesis  in  Resource  and  Environmental
Management  at  Simon  Fraser  University,  I  conducted a pilot
'adaptive management' experiment to examine the effects  of  two
different techniques and seasons for removing the invasive shrub
Scotch  broom  (Cytisus  scoparius) from Garry oak (Quercus gar-
ryana) grassland or 'meadow' sites. My objective was to  develop
an approach and methodology that would minimize site disturbance
and  subsequent broom regeneration in areas of high conservation
value. Funding for this  pilot  research  was  provided  by  the
Canadian Wildlife Service.

Scotch  broom  is  an invasive exotic leguminous shrub which has
become naturalized over large areas of  the  Pacific  Northwest,
Australia  and New Zealand (Hoshovsky 1991, Zielke et al. 1994).
Broom typically forms dense stands which: 1) are associated with
declines in native forb and grass species diversity and  popula-
tions;  2) produce high numbers of durable seeds which are known
to remain viable in the soil for decades and are  stimulated  to
germinate  by  soil  disturbance and fire; and 3) may facilitate
secondary invasions of nitrophilous exotic grass  species  which
are  also  associated  with  the degradation and displacement of
native herbaceous communities.

Many land management agencies  have  responded  aggressively  to
Scotch  broom  invasions.  Various  methods  and combinations of
manual, mechanized, thermal  (fire),  chemical,  and  biological
control  have  been  tested.  Most  efforts often result in con-
siderable site disturbance and vigorous Scotch  broom  regenera-
tion  from  seed  and  stem  resprouting. Biological control has
proven ineffective in controlling broom in all published reports
to date.

Manual control of Scotch broom in  areas  of  high  conservation
value  appears  to  have focussed on uprooting all broom plants,
despite a well known association between  soil  disturbance  and
Scotch  broom  regeneration  from seed. Presumably uprooting has
been chosen to overcome the ability of Scotch broom to  resprout
>from   cut  stems.  However, Bossard (1991) has demonstrated that
the resprouting of Scotch broom at two sites in  California  was
greatly reduced after the plants were subject to the stresses of
flower and seed production and summer drought. The lowest levels
of  resprouting  at  her  sites  was  recorded in November. Thus
cutting and removing Scotch broom in the appropriate season  may
minimize site disturbance, seed germination, and the resprouting
of cut stems.

In  my  study,  Scotch  broom  was removed from areas of remnant
semi-natural Garry oak 'meadow' in Uplands Park (Victoria -  Oak
Bay)  on  the southeast tip of Vancouver Island. This vegetation
community type is characterized by large well-spaced  oaks  with
an  herbaceous understory on level sites with deep soils (Roemer
1972). In the first year of the study I examined  the  level  of
site   disturbance   from   two   different   removal   methods:
1) uprooting all Scotch broom plants, or 2) clipping  the  above
ground portion of plants over 0.5 cm in diameter and pulling the
rest.  Scotch  broom  at  each  of the four sites was removed by
volunteers the third week of May (1993), after the this  species
had  flowered,  and  in  mid-July  (1993)  before the plants had
dispersed seed. No trials were conducted in November due  to  my
course  work  requirements.  Site disturbance in study plots was
measured by: 1) area of bare soil exposure, 2) soil disturbance,
and 3) area trampled.

The study plots cleared of broom in 1993 were revisited  in  the
summer of 1994 to assess the amount of Scotch broom regeneration
>from   seed  and  stem resprouting. I counted the number of newly
germinated Scotch broom seedlings in each plot and measured  the
diameter  and  height of all relocated cut broom stems in 'clip'
plots, recording any evidence of resprouting.

The herbaceous component of the study  plots  was  dominated  by
varying  combinations  of  the introduced grass species Agrostis
capillaris, Dactylis glomerata, and  Poa  pratensis.  All  plots
contained  at  least  some remnant native forb species. Camassia
quamash, Ranunculus occidentalis, and Lomatium nudicale were the
most constant native species present.

Predictably, soil disturbance was  greatest  from  uprooting  in
both  May  and  July. Likely because of the thick cover of turf-
forming grasses on most plots, the  actual  percentage  of  soil
disturbance was quite small when compared to the overall area of
the  study  plot. The herbaceous vegetation in all removal sites
was heavily trampled. Trampling in May  occurred  before  native
forb and grass species had set and dispersed seed.

The number of Scotch broom seedlings at all sites was highest in
plots where all broom plants were uprooted. However, there was a
large  variation  in  Scotch  broom  seedling numbers (1-330/m2)
between, and within, the four study sites. The highest number of
seedlings occurred in plots with relatively low cover values  of
introduced  grasses.  The  lowest number of broom seedlings, and
native species, occurred in plots  with  high  cover  values  of
Dactylis glomerata.

Less than 1.5% of all clipped Scotch broom stems above 0.5 cm in
diameter  exhibited resprouting in 1994. Resprouting occurred in
broom plants clipped in both May and July 1993 and  was  limited
to  a  subset of the broom plants with small diameter (< 1.0 cm)
stems cut higher than 10 cm above ground level.

The results of this pilot study  suggest  that  clipping  Scotch
broom  plants  low  to  the ground after flowering in May and in
July  will  minimize  soil  disturbance  and  subsequent   broom
regeneration  from  seed  and  resprouting in Garry oak meadows.
However, Scotch broom removal should only  be  undertaken  after
most  native  forb  and  grass  species have set and distributed
seed. Removing Scotch broom before it  sets  seed  should  be  a
priority  when  broom  is  present  in  low densities or is just
beginning to reproduce and the broom  seed  bank  is  relatively
small.

High cover values of exotic turf-forming grasses may inhibit the
establishment  of  Scotch broom but these grass species are also
associated with the decline and displacement of  populations  of
native herbaceous species.

I  believe the findings of this limited pilot project underscore
the importance of making every effort to prevent further  estab-
lishment and spread of exotic species in areas of high conserva-
tion value.

References Cited

Bossard,   C.C.  1990.  Secrets  of  an  ecological  interloper:
      ecological studies on Cytisus scoparius (Scotch broom)  in
      California. PhD Thesis. University of California, Davis.
Hoshovsky,  M.  1991. Elemental stewardship abstract for Cytisus
      scoparius. The Nature Conservancy. Arlington, Virginia.
Roemer, H.L. 1972. Forest vegetation  and  environments  of  the
      Saanich  Peninsula,  Vancouver  Island.  Unpublished Ph.D.
      Dissertation. University of Victoria. Victoria, B.C.
Zielke, K., J.O. Boateng, N. Caldicott, and H.  Williams.  1992.
      Broom  and  gorse in British Columbia: a forestry perspec-
      tive problem analysis. Silviculture  Branch,  Ministry  of
      Forests. Victoria, B.C.

SCOTCH BROOM (CYTISUS SCOPARIUS) REMOVAL STRATEGY
From: Joel Ussery <[email-masked]>

This  strategy  is  based on the results of my literature review
and field research on Scotch broom removal and the points raised
in the Garry Oak  Meadow  Preservation  Society  June  10,  1995
symposium on the Rehabilitation of Garry Oak Grasslands.

 1. Plan  Scotch  broom  removal  carefully. Human and financial
    resources for removing invasive plants such as Scotch  broom
    are quite limited and must be used strategically.

    Start removing Scotch broom from the areas with the greatest
    concentration  of  native wildflowers and grasses and lowest
    broom concentrations. If possible, eliminate small satellite
    populations of Scotch broom from priority areas  before  the
    broom becomes well established. This will prevent a build up
    of broom seeds in the 'seed bank'.

    Establish  a core area of Scotch broom removal which is able
    to maintained by available labour. Taking on too  much  area
    can  lead to rapid re-establishment of broom as regeneration
    exceeds the ability of available labour to remove seedlings.

    Recognize that  follow-up  removal  of  broom  seedlings  is
    required for years after the initial removal effort.

 2. Clip  all  Scotch  broom  plants  in  flower as close to the
    ground as possible after most native  wildflower  and  grass
    species have set and distributed seed.

    Clipping  Scotch  broom  will  minimize soil disturbance and
    regeneration of broom from seed. This is  especially  impor-
    tant  in  rocky  areas  where soils are shallow and broom is
    difficult to uproot.

    Waiting until native plants species have seeded  will  mini-
    mize the negative effects of the trampling of the herbaceous
    vegetation that accompanies broom removal.

    Clipping  broom during the summer and autumn should minimize
    resprouting from the cut stems. During  this  period,  after
    flower  and  seed  production and summer drought, the energy
    reserves of Scotch broom are at the lowest point.

    If Scotch broom is just becoming established in an area,  it
    is  important  to clip the flowering broom plants before the
    pods are ripe and the seeds are distributed.

    Smaller plants (stems < 1.0 cm  in  diameter  at  the  base)
    which  have flowered should be clipped midway up the stem so
    the plant can be uprooted in the winter when soils are moist
    and soil  disturbance  can  be  minimized.  Plants  of  this
    diameter  range  are  most  likely  to resprout from the cut
    stem. The smaller non-flowering Scotch broom plants  can  be
    bypassed in this first removal effort.

    Ideally,  all  clipped Scotch broom plants should be removed
    from the site. Whether piled or spread on the  ground,  dead
    broom  plants  are  a fire hazard and may give off chemicals
    that negatively affect the growth of native plants.

 3. Return to the site during the late fall or winter when soils
    are saturated and uproot remaining Scotch broom  plants  and
    high-cut stems.

    If  there  are  too  many  non-flowering plants to pull with
    available labour, the smaller seedlings can  be  left  until
    the  following  winter. Some of these seedlings may not sur-
    vive the next summer drought.

 4. Revisit the site the following winter to check Scotch  broom
    regeneration from seed and recut any resprouting stems.

    If  there  are  too  many  seedlings  to pull with available
    labour, the smallest seedlings can be left until the follow-
    ing winter. Some of these seedlings may not survive the next
    summer drought.

 5. Continue to revisit the site  each  winter  to  pull  Scotch
    broom  seedlings  until  either  you or the seed bank is ex-
    hausted.

POST SCRIPT - USE OF FIRE

Immediate fire suppression is a general  policy  on  public  and
private  land  in  British  Columbia.  However, selective use of
regular low-intensity prescribed fires after the initial removal
of standing Scotch broom and other major fuel sources  may  'ex-
haust'  the  seed bank by stimulating broom seed germination and
killing young seedlings (MacGillivray, personal  communication).
Native  wildflowers  and  grasses  are well adapted to surviving
such low-intensity fires. Ideally, fires to  kill  Scotch  broom
seedlings  should  be  set during the autumn and early winter to
keep burn temperatures low and minimize  interference  with  the
growth and reproduction of native wildflowers and grasses.