1
Management and Restoration of
Grasslands on Yellow Island, San
Juan Islands, Washington, USA
Peter W. Dunwiddie1
Abstract
A native grassland dominated by Roemer's fescue
(Festuca idahoensis var. roemeri), great camas
(Camassia leichtlinii), and a diversity of other forbs
has been the focus of a variety of experiments on
restoration techniques, as well as studies tracking
ecological changes since 1981. Investigations in
existing grasslands have primarily focused on
responses of native and non-native species to
prescribed burns.
Following each of 3 burns,
responses of different species are complex, varying in
direction, magnitude, and duration. A second series
of studies has focused on developing effective means
for controlling and removing invading trees and
shrubs, and on limiting non-native grasses and forbs.
Methods have included a variety of manual,
mechanical, and chemical techniques. We have also
tested several approaches for restoring native
grassland species in areas where they had been
excluded by competing woody plant growth. Even
when abundant native seed sources exist in close
proximity, non-native species usually establish more
quickly following removal of trees and shrubs, and
continue to dominate for many years. Out-planting
of propagated plants has proven most effective in
rapidly re-establishing native species.
Greatest
success has been achieved in establishing a dense
fescue matrix that excludes invasive species.
Key words: Camas, fescue, fire, herbicides, longterm studies
Introduction
Native grasslands, prairies, and oak savannahs are rare in
the lowlands of Puget Sound and the Gulf Islands
(Ecoregional Plan). Most were converted to agriculture,
destroyed by development, or overgrown by trees and
shrubs over the last 150 years (Hall and Crawford 1996).
Most that remain are small fragments that have been
degraded by invasive non-native species. Yellow Island
is a 4.5 ha island purchased by The Nature Conservancy
in 1980, and managed as a nature preserve. Native
1
The Nature Conservancy, 217 Pine St., Suite 1100,
Seattle, WA 98101
grasslands dominate the east and west ends of the island
(Fig. 1). They are of particular ecological interest due to
the rich diversity of forbs set in a matrix of
predominantly native grasses.
Since its acquisition by the Conservancy, annual
monitoring and research have primarily focused on the
maintenance and restoration of grassland communities.
Restoration experiments were begun in 1984 to limit the
expansion of shrubs and trees into the grasslands.
Subsequently, studies were initiated to develop methods
for restoring grassland vegetation in areas that were
predominantly occupied by woody species. Restoration
began on a more extensive scale in 1998, and is
continuing up to the present.
The studies conducted on the island's grasslands over
the last 20 years provide insights into many aspects of
the ecology, management, and restoration of this rare
vegetation type. The long duration of several of the
studies affords a unique perspective on community
dynamics. This paper will present data from several ongoing vegetation studies that examine impacts of
prescribed burning, manual control of woody plants, as
well as the use of several herbicides. Results of repeated
treatments, and data on responses of individual species
are synthesized to draw conclusions that may be
applicable to restoration and management efforts in
similar vegetation assemblages elsewhere in the Puget
Sound - Gulf Islands region.
Study Site
The grasslands of Yellow Island are dominated by
Festuca roemeri (Roemer's fescue), Camassia leichtlinii
(Great camas), Ranunculus occidentalis (Western
buttercup), and a diversity of other native forbs and
grasses (Table 1). Several non-native species are also
widespread but generally provide little cover, including
Holcus lanatus (Velvetgrass), Hypochaeris radicata
(Cat's ear), and Rumex acetosella (Sheep sorrel).
Lichens and mosses offer significant cover as well in
many areas, especially prior to burning.
The central portion of the island is dominated by a
forest of Pseudotsuga menziesii (Douglas-fir) and Abies
grandis (Grand fir), with a dense understory of
Symphoricarpos albus (Snowberry) and Holodiscus
discolor (Ocean spray). Other common woody plants on
the island include Rosa nutkana (Nootka rose) and
Prunus cerasus (Sour cherry). There are two Quercus
garryana (Garry oak) trees on the island, neither of
which have successfully reproduced. Invasive nonnative shrubs are absent.
There are no resident mammalian herbivores on the
Yellow Island. Deer occasionally swim over from
adjacent islands, but do not remain due to the absence of
fresh water. Mink and river otters are the only other
mammals that are found in the terrestrial environment.
2
Figure 1. Aerial orthophoto of Yellow Island, showing locations of plots and transects reported in the text.
Table 1. Frequency of most common species in native
grasslands on Yellow Island. (Non-native species
indicated with asterisk.)
Species
% Freq.
Camassia leichtlinii (Great camas)
Festuca idahoensis (Idaho fescue)
Ranunculus occidentalis (Western
buttercup)
Vicia americana/sativa (Vetch)
Cerastium arvense (Field chickweed)
*Holcus lanatus (Velvetgrass)
Lomatium nudicaule (Naked desert
parsley)
Achillea millefolium (Yarrow)
*Hypochaeris radicata (Hairy cat’s
ear)
Castilleja hispida (Hairy paintbrush)
Luzula campestris (Field woodrush)
Erythronium oregonum (White fawnlily)
*Rumex acetosella (Sheep sorrel)
100
97
97
97
93
87
87
80
80
73
73
73
67
It is likely that prior to settlement of the San Juan
Islands by Europeans in the mid-1800s, Yellow Island
was frequently visited by Native Americans. Several
neighboring islands are known to have been major
collecting sites for camas and other important native
food plants (Gunther 1930). The abundance of Camassia
leichtlinii, Fritillaria lanceolata (Chocolate lily), and
other edible native plants on Yellow Island, as well as
the presence of several shell middens and possible camas
roasting ovens, suggest this island also received similar
use. It therefore seems probable as well that Yellow
Island was frequently burned, a practice that was often
associated with maintenance of camas beds (Boyd 1999).
Several large, open-grown Douglas-fir trees in excess of
250 years old exhibit multiple fire scars, confirming the
past occurrence of at least occasional fire (Agee and
Dunwiddie 1984).
The frequency of fires would have declined in the
latter 19th century as Native American use of the area
declined. As in other Puget Lowland prairies, woody
plants began to invade the grasslands in the absence of
frequent fire (Boyd 1999). An early 20 th-century
photograph of Yellow Island provides independent
corroboration of significant changes in the island's
vegetation over the last century (Agee and Dunwiddie
1984). It reveals a mostly open, herbaceous landscape,
with scattered large trees. The central, forested core of
dense trees and shrubs is not evident, and appears to have
developed primarily in the last 60-80 years. Only one
historical fire has been documented on the island,
sometime in the 1930s.
Yellow Island was settled in the late 1940s by a
couple who lived there until the 1970s. They built
several small cabins and maintained a garden, but appear
not to have significantly altered the natural successional
processes that were occurring. There is no evidence that
they introduced livestock, set fires, or cut trees to a
significant degree. The cabins were built entirely from
3
wood that washed up on the beaches, and there are no
stumps to suggest a history of cutting.
Today the island attracts numerous visitors,
especially during the spring flowering season, which
extends from late March to early June. Impacts from
foot traffic is slight, as visitors are limited to a few
narrow trails. A resident caretaker interacts with the
public, and performs some ecological restoration and
maintenance tasks.
Monitoring and Research Methods
In 1981, three plots were installed in the grasslands to
detect potential invasion by woody species, particularly
Symphoricarpos albus. Data from two of these (Plots 1
and 2) are presented in this paper. Percent cover of all
vascular plants, lichens, mosses, bare ground, and litter
was recorded in a series of ten 1-m2 permanent quadrats
installed in each plot. A wire was lowered 25 times into
each quadrat, and all species that touched the wire at
each pin placement were tallied. These plots later were
incorporated into other studies, as described below.
With only a few interruptions as noted in the figures,
all plots have been monitored annually. To minimize
potential errors introduced by different observers, I have
monitored all plots myself.
Variations due to
phenological differences have been reduced by
conducting all monitoring during the 2nd or 3rd weeks of
May.
Numerous additional plots have been installed since
1981 to investigate various management activities. All
have been monitored using the same 1-m2 quadrats as in
Plots 1 and 2. In 1984, a 2 x 5m plot was installed to
track natural re-establishment of grassland vegetation
following removal of three dense, fast-growing
Pseudotsuga menziesii trees. This plot was monitored
through 1997, and because of its location within the
perimeter of the 1987 and 1996 burns, it also was
exposed to these treatments as well.
In 1987, ten 1-m2 quadrats were established along a
90-m transect (Transect 1) that bisects the grassland on
the eastern end of the island. This transect was set up to
detect changes in response to a prescribed burn of ca. 1
ha that was carried out in late July of 1987. This was the
first fire known to have occurred in this grassland in at
least 50 years. Plots 1 and 2 were in a portion of the
eastern grassland that was left unburned, and served as
reference plots for this burn. A slightly larger prescribed
burn of ca. 1.5 ha was conducted in 1996 that included
all of the area burned in 1987, as well as an additional
area that included Plot 1. Plot 2 continued to serve as an
unburned reference. In 1998, just the quadrats arrayed
along Transect 1 were burned for a third time. All three
burns were conducted in late July or early August. Fire
intensities were not recorded, but all resulted in nearly
complete combustion of all fine fuels. No other
treatments were carried out in these study plots during
this period.
A transect of 35 contiguous 1-m2 quadrats (Transect
2) was established in May, 1998 to track the success of
efforts to restore a forested area to grassland.
Presence/absence was recorded for all species in each
quadrat. Additionally, the number of individuals was
recorded for several species. Prior to installation of the
transect, most trees and all shrubs were cut and removed
from a ca. ¼ ha unit, leaving an open canopy of large
Pseudotsuga trees. Some of the thick needle and twig
litter layer was removed in a burn in September, 1998,
and much of the remainder was removed by raking.
Most shrubs resprouted following cutting; these were
treated with foliar applications or stump-painting of the
herbicides Roundup or Krenite. Revegetation was begun
in March, 1999, and consisted of out-planting nurserygrown Festuca roemeri plugs at a density of ca. 10/m2.
Small numbers of forb plugs, primarily Ranunculus
occidentalis, were also out-planted.
A 28-m transect of 0.2 x 0.5m quadrats spaced at 1m
intervals (Transect 3) was established in 1998 to
investigate techniques for removing invasive non-native
grasses, primarily Dactylis glomerata (Orchardgrass) and
Holcus lanatus. As with Transect 2, presence/absence of
all species was recorded in each quadrat. In April, 1999,
an herbicide (Poast) that selectively impacts a number of
tall grasses was applied. Festuca roemeri plugs were
out-planted in April, 2000 to help re-establish a
graminoid matrix on the site.
Results
Fire Management
Responses of individual taxa to the three burns varied
among one another in direction, magnitude, and duration.
Following the 1987 burn on Transect 1, cover of Festuca
roemeri declined significantly compared to the unburned
plots, and did not recover to pre-burn levels for about 7
years (Fig. 2a). A similar but less pronounced decline
occurred in 1996 following the second burn on this
transect. Fescue cover had only partially returned
towards pre-burn levels when the transect was burned for
a third time in 1998. Cover was once again reduced
following this burn, and has gradually increased since
then. The first burn occurred in Plot 1 in 1996, where
fescue exhibited a dramatic decline that was similar to
the response observed following the first burn on
Transect 1. Here, too, cover has subsequently increased
at a rate of about 10 percent per year over the last 5
years.
A second dominant grassland species, Camassia
leichtlinii, has exhibited a more varied response
following burning (Fig. 2b). In the year following the
first burn on Transect 1, camas cover increased,
contrasting with the decreases that occurred that year in
the two unburned plots. A similar pattern occurred again
following the burn in 1998. However, in 1996, camas
cover declined in both Plot 1 and on Transect 1
following the burn, as well as in the control plot. In the
4
Figures 2a – 2d. Percent cover of Festuca roemeri, Camassia leichtlinii, Ranunculus occidentalis, and Holcus lanatus
in burn and control plots. Transect 1 (solid diamonds) – burns in 1987, 1996, 1998 indicated with black arrows; Plot 1
(open squares) – burn in 1996 indicated with clear arrows; Plot 2 (open triangles) – no treatment.
Festuca r oem er i
C a m a ssi a l ei ch tl i n i i
100
45
90
40
Percen t C o ver
Percen t Co ver
80
70
60
50
40
30
35
30
25
20
15
20
10
10
5
0
0
81 83 85 87 89 91 93 95 97 99 '01
81 83 85 87 89
Ye ar
91 93 95 97 99 '01
Ye ar
Hol cus l a na tus
Ra nuncul us occi denta l i s
20
60
18
16
Percen t Co ver
Percent Cover
50
40
30
20
14
12
10
8
6
4
10
2
0
0
81 83 85 87 89 91 93 95 97 99 '01
81 83 85 87 89 91 93 95 97 99 '01
Ye ar
Ye ar
5
two cases where camas cover increased following
burning, the response persisted for only 1 year.
Photographs of the burned and control sites suggested
that flowering also was stimulated in the burned site.
A third common native species, Ranunculus
occidentalis, exhibited yet a different response to the
three burns (Fig. 2c). In 1987 and 1998, there was no
difference in buttercup cover between the burned and
unburned sites. However, both cover and flowering
(apparent in photographs) increased following the 1996
burn, especially in Plot 1.
The response of non-native species also varied
between burns and among species. Holcus lanatus, a
weedy perennial grass, occurs frequently in some of the
grasslands (Fig. 2d). Cover of this species increased for
a year or two following the 1987 and 1996 burns.
However, in 1998, cover declined the year following the
burn. Overall, this species exhibited considerable interannual variation. Rumex acetosella (Sheep sorrel)
showed a similar pattern, increasing in cover following
the first two burns, but not in 1998 (Fig. 3a). A third
non-native species, Hypochaeris radicata, exhibited no
consistent response following burning (Fig. 3b).
Summarizing the cover data by life form reveals
some consistent patterns. Moss and lichen cover was
reduced by all three burns to near zero (Fig. 3c). Mosses
recovered slowly, but few lichens have returned into the
burned areas. While the dominant native grass (Festuca
roemeri) also repeatedly declined following burning,
native forbs increased in cover following burning when
compared with unburned controls (Fig. 3d).
Collectively, this produced a consistent and pronounced
shift in the balance between forb and grass cover
following burning (Fig. 4a). The ratio of forbs to grasses
increased significantly for about 3 years following the
1987 burn. This pattern was repeated again but to a
lesser degree in 1996, and again in 1998.
The cover of native versus non-native species also
shifted significantly following burning as native species
became proportionally less abundant (Fig. 4b). After the
1987 burn, this ratio declined for 2-3 years before slowly
returning to pre-burn levels after 8-9 years. Similar
responses followed the 1996 and 1998 burns as well.
The ratio of perennial to annual species also
exhibited a marked response to burning (Fig. 4c).
Following the 1987 burn, annuals became relatively
much more abundant for about 3 years. This shift
occurred again after the 1996 burn in both Transect 1 and
Plot 1. As the 1998 burn on Transect 1 followed the
previous burn by only 2 years, the perennial:annual cover
ratio was already low, and therefore showed little
change.
A structural change that burning also imparted to the
grasslands was the creation of bare ground (Fig. 4d).
This change was most pronounced the first year
following each fire.
Grassland Restoration
Establishment and succession of plants in an area where
trees were removed in 1984 was documented for 13
years. Three ca. 15 year old Douglas-firs had been
growing in an open grassland. When they were cut, the
trees were about 10 m tall, with dense "skirts" of
branches close to the ground.
Initial vascular plant
cover was very low, with only a few lilies (Camassia
leichtlinii, Erythronium oreganum) and Festuca roemeri,
which had persisted in the dense shade and litter under
the trees. Species diversity and cover quickly increased
in the open habitat for the next several years (Fig. 5).
Non-native species such as Holcus lanatus, Hypochaeris
radicata, and Rumex acetosella invaded quickly and
have persisted. However, re-establishment of native
grassland species, particularly forbs, has been very slow,
despite the proximity of abundant native seed sources
within 1-2 m. The 1987 prescribed burn appeared to
have little impact on the trajectory of most of the curves
describing succession in this plot.
The ratio of
perennials to annuals, which began to increase rapidly
after 1989, was shifted back to favor annuals following
the 1996 burn, as was observed in the other burned plots.
Re-establishment of native grassland vegetation has
been documented along Transect 2 for 4 years in an area
that was cleared of shrubs and many of the trees in 1998
(Table 2). Species diversity has increased as the number
of both native and non-native species present along the
monitoring transect has grown. Survival of the Festuca
roemeri plugs, which were planted extensively
throughout this area shortly after it was cleared, has been
nearly 100 percent.
Similarly, survival of the
Ranunculus occidentalis plugs in this area was also very
high.
However, many other weedy species have
increased in frequency, including annuals such as
Cardamine oligosperma, Galium aparine, and Stellaria
media, and perennials such as Cirsium vulgare, Dactylis
glomerata, Hypochaeris radicata, Lactuca muralis,
Lathyrus nevadensis, Taraxacum officinale, and Vicia
species. Shrubs have also begun to make their way back
into this area as well.
Control of various non-native grasses through the
application of the herbicide Poast was monitored along
Transect 3 (Table 3). Following application of this
herbicide in the spring of 1998, frequency of several
perennial non-native grasses dropped from pre-treatment
levels, including Holcus lanatus, Dactylis glomerata, and
Poa pratensis. Dominant native perennial forbs of the
grasslands remained unaffected by this herbicide, or
increased in frequency, including Camassia leichtlinii,
Cerastium arvense, and Erythronium oregonum.
However, weedy annual and perennial forbs increased
dramatically as well in response to this treatment,
including Cardamine oligosperma, Galium aparine,
Geranium molle, Hypochaeris radicata, Lathyrus
nevadensis, Veronica arvensis, and several species of
Vicia. Out planting of Festuca roemeri plugs in April,
6
Figures 3a – 3d. Percent cover of Rumex acetosella, Hypochaeris radicata, mosses and lichens, and native forb species
in burn and control plots. Transect 1 (solid diamonds) – burns in 1987, 1996, 1998 indicated with black arrows; Plot 1
(open squares) – burn in 1996 indicated with clear arrows; Plot 2 (open triangles) – no treatment.
R u m ex a ceto sel l a
Hypocha er i s r a di ca ta
20
16
18
14
Percent Cover
Percen t C o ver
16
14
12
10
8
6
12
10
4
8
6
4
2
2
0
0
81 83 85 87 89
81 83 85 87 89 91 93 95 97 99 '01
91 93 95 97 99 '01
Ye ar
Ye ar
M os s e s and Liche ns
Native Fo r b Sp e cie s
60
180
160
Percent Cover
Percent Cover
50
40
30
20
140
120
100
80
60
40
10
20
0
0
81 83 85 87 89 91 93 95 97 99 '01
81 83 85 87 89 91 93 95 97 99 '01
Ye ar
Ye ar
7
Figures 4a – 4d. Ratios of major life form groups, and percent cover of bare ground, in burn and control plots. Transect
1 (solid diamonds) – burns in 1987, 1996, 1998 indicated with black arrows; Plot 1 (open squares) – burn in 1996
indicated with clear arrows; Plot 2 (open triangles) – no treatment.
Native For b:Gr as s Cove r Ratio
Native :No n -Native C o ve r Ratio
100
N ative:N on -na tive R atio
For b:G rass Ratio
12
10
8
6
4
2
10
0
1
81 83 85 87 89 91 93 95 97 99 '01
81 83 85 87 89 91 93 95 97 99 '01
Ye ar
Ye ar
Bar e Gr ound
Pe re nnial:Annual Cove r Ratio
90
80
Percent Cover
Perennial:Annual Ratio
1000
100
10
70
60
50
40
30
20
10
0
1
81 83 85 87 89 91 93 95 97 99 '01
81 83 85 87 89 91 93 95 97 99 '01
Ye ar
Ye ar
8
Figure 5. Floristic changes following removal of trees. 1984 = conditions at time trees were cut. Black arrows indicate
prescribed burns in 1987 and 1996.
40
No. of Species
35
30
No. of Native Spp.
25
Total C over (%/10)
20
15
Native Forb C over
(%/10)
10
Peren:Annual
C over
5
0
84
86
88
90
92
94
96
Year
Table 2. Percent frequency of the more abundant species
in forest restoration area along Transect 2.
1998 1999 2000 2001
Arbutus menziesii sdlings.
Cardamine oligosperma
Cirsium vulgare
Dactylis glomerata
Elymus glaucus
Erythronium oregonum
Festuca idahoensis
Fritillaria lanceolata
Galium aparine
Holcus lanatus
Hypochaeris radicata
0
0
0
9
0
66
0
9
0
37
0
26
17
0
11
0
89
94
3
49
77
6
20
34
17
23
17
86
97
23
66
94
60
20
29
14
23
26
86
94
36
37
100
46
Lactuca muralis
Lathyrus nevadensis
Mosses
Pteridium aquilinum
Ranunculus occidentalis
Rubus ursinus
Senecio vulgaris
Stelleria media
Symphoricarpos albus
Taraxacum officinale
Vicia spp.
3
40
46
11
0
57
0
0
17
0
3
29
66
14
20
9
63
0
26
26
0
43
71
66
29
31
14
66
14
34
29
6
54
46
66
46
31
9
74
0
6
34
11
54
Ave. No. of Spp./Quadrat
Total No. Native Species
Total No. Intro. Species
5
14
6
8
20
9
12
24
18
12
27
14
9
Table 3. Percent frequency of the more abundant species
in Poast treatment site (April, 1998) along Transect 3.
Camassia leichtlinii
Cardamine oligosperma
Cerastium arvense
Collinsia parviflora
Dactylis glomerata
Erythronium oregonum
Festuca roemeri
Galium aparine
Geranium molle
Holcus lanatus
Hypochaeris radicata
Lathyrus nevadensis
Lonicera sp.
Poa pratensis
Symphoricarpos albus
Veronica arvensis
Vicia americana
Vicia hirsuta
Vicia sativa
Total Freq. Native Forbs
Total Freq. Intro. Grass
Total Freq. Nat. Grass
1998
1999
2000
2001
85.7
3.6
35.7
10.7
60.7
17.9
7.1
0.0
7.1
71.4
10.7
10.7
75.0
53.6
39.3
0.0
21.4
0.0
7.1
93.1
55.2
37.9
34.5
3.4
24.1
13.8
62.1
41.4
3.4
6.9
72.4
72.4
24.1
48.3
13.8
86.2
0.0
41.4
89.7
55.2
44.8
34.5
17.2
31.0
75.9
82.8
55.2
34.5
27.6
86.2
72.4
37.9
48.3
27.6
82.8
10.3
10.3
96.6
79.3
37.9
0.0
44.8
24.1
69.0
0.0
0.0
27.6
27.6
6.9
75.9
24.1
27.6
10.3
10.3
13.8
17.2
545
90
100
269
97
97
221
214
14
417
31
17
2000 increased the presence of this species, but survival
was somewhat lower than had been observed in other
areas. Over the next several years, the grasses targeted
by the herbicide have gradually increased.
Discussion
The three prescribed burns in the native grasslands
produced responses among species that differed widely
in magnitude, direction, and duration. Festuca roemeri
exhibited one of the most consistent responses, declining
in cover after every burn. The first burn at a site – 1987
in Transect 1, 1996 in Plot 1 – resulted in declines in
cover of 50 percent or more. Subsequent burns resulted
in similar declines in fescue cover, but of lesser
magnitude. These patterns most likely reflect the greater
accumulation of fuels in the previously unburned sites
producing more intense fires. These, in turn, resulted in
greater mortality among fescue plants. Although no
measurements were taken that would allow comparisons
in intensity among the different burns, visual
observations of grass thatch build-up suggested that litter
quantities only began to approach pre-burn levels after
about 5 years.
Reductions in moss and lichen cover, and increases in
bare ground following burning were patterns that also
occurred repeatedly with the three burns. These changes,
together with the loss of litter cover, resulted in
significant changes in vegetation structure after burning.
Bare ground persisted for only about 1 year; litter took
several years to build back up, but mosses and lichens
did not return to pre-burn levels even after 7-9 years.
Spaces between bunchgrasses that had been occupied by
thick accumulations of litter and dense growths of moss
were left bare and open immediately after burning.
The amounts of bare soil, litter, and moss and lichen
covered substrate, which shifted markedly during the
first several years following each burn, were no doubt
also accompanied by changing nutrient and moisture
conditions, although these parameters were not
measured. Collectively, this produced a changing set of
conditions that favored different suites of species. For
the first 3-4 years, annual species were particularly
abundant, and appear to fill in many of the empty spaces
previously occupied by mosses, litter, and some
perennial species. Many of these annuals are forbs and,
combined with an increase in cover of some perennial
forbs, these changes result in a pronounced short-term
shift in the vegetation towards a much greater relative
abundance of forbs. After several years, recovery of the
native fescue gradually shifts the balance back towards a
vegetation dominated by grass and perennials.
While forbs in general were favored over graminoids
by burning, the responses of individual species of native
perennials, such as Camassia and Ranunculus, varied
considerably following the different burns.
This
variability occurred even though all three burns were
conducted at the same time of year under relatively
similar conditions. These observations suggest caution
in extrapolating results from only one or a few burns at a
site to infer how long-term species trajectories may be
affected by reintroduction of a burning regime. Even
after 15 years and 3 burns at Yellow Island, it is difficult
to predict the long-term effects of reintroducing a
frequent, low-intensity fire regime that is thought to
resemble what sustained grassland communities on the
island in prehistoric times. Each burn at a site is
presented with varying quantities of fuel and different
fuel moisture, temperature, and wind conditions, all of
which affect the intensity and severity of the fire and the
subsequent responses of the plant community. How
individual species are affected is further influenced by
numerous other factors, such as their condition at the
time of the burn, the abundance of their seed in the seed
bank, as well by the complex interactions of other
species also present on the site.
Of particular concern on Yellow Island was the
response of non-native species to burning. Several of the
10
annual species that increased after burning are nonnative, but they generally persisted for only a year or two
before largely disappearing. Overall, non-native species
increased after burning, and it is not clear yet whether
they would persist at elevated levels over the long term,
or if they will eventually decline to pre-burn levels in the
absence of repeated fires. It is likely that, as with native
taxa, individual species will respond differently.
It takes many years of careful observation and
experimentation to accurately predict how repeated fires
of different intensities, intervals, and seasons will shape
the composition and structure of native plant
communities at a site. However, patterns observed over
the last two decades on Yellow Island suggest several
generalizations that may be expected to occur in other
lowland fescue-dominated grassland sites where frequent
burn regimes are contemplated. These include the
following:
1) Reduction in cover and dominance of Roemer’s
fescue,
2) Reduction in the extent and density of moss and
lichen ground cover,
3) Greater dominance of many forb species,
4) Increased presence of annual species, and a
relative reduction in dominance by perennials,
and
5) Likely increase in presence and dominance of
some non-native taxa.
Future burning experiments on Yellow Island will
focus on clarifying how repeated fires affect vegetation
composition and structure. Particular attention will be
directed towards developing fire regimes that do not
result in significant increases in non-native taxa.
Evidence from Yellow Island experiments suggests
that restoration of native grassland vegetation is unlikely
to be very successful simply by removing invasive
woody species, even when native species occur in close
proximity.
Without active measures to assist the
establishment of native taxa, aggressive non-native
species are likely to quickly occupy the site, and are
capable of persisting, perhaps indefinitely.
Dense
plantings of fescue plugs were successful in rapidly
providing a ground cover that appeared to limit
establishment of non-native species, at least for several
years. More experimentation is needed to determine
whether non-natives can be reduced over the long term,
and whether a diversity of forbs can be established
within the grass matrix.
Selective use of herbicides to control shrubs and nonnative grasses have produced mixed results. One or two
spot applications of Roundup or Krenite have generally
been successful in preventing regrowth of cut shrubs and
trees. Applications of Poast controlled non-native
grasses for several years, but it remains to be seen
whether this will persist. Other non-native forbs quickly
took advantage of the reduced competition; their longerterm persistence also remains to be determined.
Yellow Island has presented a unique opportunity to
study methods for maintaining and restoring a rare
grassland community. Its existence as an isolated island
managed as a nature preserve has facilitated a variety of
experiments that are beginning to provide longer-term
perspectives on the roles of fire in maintaining lowland
grasslands.
It has also provided opportunities to
investigate the use of other techniques to restore
grasslands that have become degraded in the absence of
fire. The Nature Conservancy intends to continue to
conduct monitoring and research on the island to refine
restoration techniques, as well as to understand the
longer-term changes that occur with repeated
management actions.
Acknowledgments
Financial assistance to conduct the research and
restoration work on Yellow Island has been provided by
numerous donors to the Nature Conservancy, and by
several grants from the U.S. Fish and Wildlife Service.
Many individuals assisted with the installation of
research plots, collection of data, and restoration of
portions of the island. Fayette Krause and Eliza
Habegger assisted with many of the investigations and
restoration efforts. Caretakers on the island have been
especially helpful, including David Duggins, Meghan
Dethier, Ruthie Johns, Gary Sale, Linnea and Glen
Pritchard, and Phil and Kathy Green. Members of the
Nature Conservancy prairie restoration crew have done
the bulk of the cutting, clearing, and planting; Ian
Hannah, Dan Grosboll, and Ron Pratt deserve particular
mention for their efforts over the years. Finally, I would
like to thank the many volunteers have contributed
hundreds of hours to the restoration of Yellow Island.
Literature Cited
Agee, J.K., & Dunwiddie, P.W., 1984. Recent forest
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Boyd, R. 1999. Indians, fire, and the land in the Pacific
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Crawford, R. C., and H. Hall. 1997. Changes in the
south Puget prairie landscape. Pages 11-15 in P. V.
Dunn and K. Ewing, editors. Ecology and
Conservation of the South Puget Sound Prairie
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Gunther, E. 1930. Ethnobotany of Western Washington:
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Washington Press, Seattle.