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Plant community recovery along a fire chronosequence in the Mojave Desert of southern NevadaE. Cayenne Engel1 and Scott R. Abella1,2
1 Public Lands Institute, University of Nevada Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-2040, e-mail: [email protected] Environmental Studies Department, University of Nevada Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-2040
Acknowledgements• We thank Nick Bechtold, Adria Decorte, Teague Embry, Kate Prengaman, Chris
Roberts, and Sarah Schmid for field assistance.• Sampling of three of the fires was funded through a cooperative agreement
between BLM Las Vegas and the University of Nevada Las Vegas. A weed monitoring effort through the Weed Sentry program, funded through the Clark County Multiple Species Habitat Conservation Plan and a cooperative agreement between Lake Mead National Recreation Area, also supported surveys on some fires
Environmental Studies Department
Objective: Document patterns of post-fire recovery in Mojave ecosystems to better inform land managers about what to expect after fires.
Predictions: Communities that were dominated by creosote pre-fire will recover more quickly than
those dominated by blackbrush. Invasive grass red brome (Bromus rubens) will be more prominent in burned plots,
promoting the “grass-fire cycle” theory.
Key Findings: 1. Pre-fire plant community composition determines the trajectory of recovery. Creosote
and blackbrush dominated communities are colonized by different early successional species, although they share some common abundant species such as globemallow, desert marigold, and virgin river brittlebush.
2. Creosote-bursage communities recover more quickly than do blackbrush communities. However, blackbrush did re-establish in about half of the sites we sampled, which is a phenomenon rarely documented.
3. As with vegetative composition, Bromus establishment post-fire varies with community type. Creosote-bursage communities appear more susceptible to invasion than blackbrush communities. Evidence for the grass-fire cycle in Mojave systems seems limited to creosote-bursage communities, at least in years with low-average rainfall.
Troy Phelps, BLM Las Vegas
Methods: Fire effects monitoring• We sampled 32 fires across Clark County, NV winter 2007 -
spring 2009• Fires were limited to those in Mojave plant communities
including creosote-bursage and blackbrush dominated systems.• Fires occurred 1980 - 2007• Size ranges from ~30 ac – ~82,000 acres• Elevation, topography, and soil vary (soil data not shown)• The inherent variation included in landscape scale analyses is
important to detect generalizeable patterns• In our analyses, we grouped fires by natural breaks, which
correspond to burn decades.
Background:Wildfires are becoming more prevalent across the Mojave desert, particularly in the last 30 years, due in part to increased human presence and increased abundance of exotic grasses which may be linked to wetter years. The last three decades have been wetter than average due to increased winter el niño rain events. Little work has been conducted on the consequences of these fires on native plant communities, successional patterns for natural revegetation, and links to the prevalence of future fire. Therefore, we examined patterns driving fire recovery at the landscape scale with the ultimate goal of understanding the Mojave system in order to inform post-fire management prescriptions.
2004 - 2006
1993 - 19951980-1988
Years of burns
US Forest Service
Bureau of Land Mgmt
Fish and Wildlife Service
National Park Service
1430
13
28
29
12
27
32 15
16
24
4
325
Las Vegas
Nevada
18
26
7
10
21
1920
22112
31 175
8
619
23
ID # Fire NameMean
Elev. (m)Year Burn
Plant Community
Size (acres)
1 1993RedRock 1332 1993 Blackbrush 2402 4322 (Red Rock) 1207 1983 Blackbrush 12503 Arden 1038 2005 Creosote 1154 Blue Diamond Mine 1141 1993 Blackbrush 405 Bonnie Springs 1205 2007 Blackbrush 3906 Bursage 604 1993 Creosote 407 Christmas 919 1993 Creosote 2608 Cohenour 909 1994 Creosote 5509 Cottonwood 1406 1980 Blackbrush 4000
10 Devils Peak 1420 2005 Creosote 57011 Diamond 1051 2005 Creosote 12012 Dry Lake 991 2005 Creosote 220013 Fork 1059 2005 Creosote 8170014 Ghost 1109 1987 Creosote 200015 Hillside 1596 1980 Blackbrush 3016 K389 1378 1986 Blackbrush 12517 Loop 1248 2005 Blackbrush 85018 Miracle 887 1994 Creosote 88019 Overlook2 1192 2005 Blackbrush 6220 Pitt 1036 2005 Creosote 10021 Piute 1327 1995 Blackbrush 11022 Red Rock 1044 2005 Blackbrush 4223 River 708 2005 Creosote 7524 Scenic 1184 2006 Blackbrush 160025 Sloan2 893 2005 Creosote 4326 Spirit 1096 1994 Creosote 9027 Sterling 1676 1988 Blackbrush 145028 Tramp 1049 2005 Creosote 2630029 Vegas 1334 2006 Blackbrush 420030 White Hills 992 1994 Creosote 94031 Willow 1394 1995 Blackbrush 4032 Zipper 1487 1988 Blackbrush 6300
Evidence for grass-fire cycle in Mojave burns
We did not see greater abundance of Bromus rubens immediately following fire in blackbrush communities (fig. 8a). However, within 30 years post-fire Bromus abundance re-established to unburned levels. Creosote-bursage communities showed trends toward greater annual grass cover in burned than unburned communities immediately (fig. 8b). Annual rainfall in the years that sampling was conducted was either average or below-average, therefore we would not expect a great abundance of invasive annual grasses overall. Our data indicate that in years of low-average rainfall, brome is more abundant in older burns than younger burns. There is evidence that post-fire communities may have greater brome abundance than intact communities in years of increased rainfall, but the grass fire-cycle is contingent upon years of abnormally high rainfall, which may be less frequent with impending climate change.
Blackbrush and creosote-bursage communities have different post-fire successional trajectories
ConclusionsCommunity composition
• As predicted, community composition and regeneration rates differed between the two major community types that dominate the middle-elevation Mojave desert ecosystems.
• Creosote communities recovered more quickly in terms of relative species abundance and whole-plot cover than did blackbrush dominated communities.
Blackbrush regeneration• We observed evidence of blackbrush regeneration in approximately half the plots,
up to 20% of original cover within 30 years. However, others exhibited no blackbrush return. More work needs to be done to determine why certain sites promote regeneration.
Bromus invasion (support for the grass-fire cycle)• Blackbrush communities: In years with average or below average rain, Bromus is
less abundant in recently burned sites than in unburned sites. However, abundance does increase with time since burn to reach levels in unburned sites.
• Creosote communities: Bromus abundance in burned sites was greater than in unburned sites, with abundance increasing with time since burn. .
• Earlier reports indicate that blackbrush generally does not re-establish after fire, often with invasive grasses as the most abundant colonizers. We found these patterns to be inconsistent, with Bromus abundance initially lower in burned than unburned sites, and evidence of blackbrush re-establishment in many burned sites.
Figure 1. NMS (non-metric multidimensional scaling) ordinations of blackbrush (brown diamonds) and creosote-bursage (green circles) dominated communities in each of three burn year groupings. The two community types have different post-fire species compositions. Because the communities have very different successional trajectories, we will be presenting blackbrush and creosote data separately. We only have one sample of a fire from the 1980’s in creosote-bursage communities, which prevents statistical analyses with this data.
1980’s Fires
Blackbrush
Creosote
Unburned 1990’s Fires2000’s Fires
Figure 2. Whole-plot cover recovered to nearly 60% of the original cover within about 15 years in blackbrush sites, but did not recover beyond that up to 30 years post-burn. Cover in the single 1980’s creosote site recovered to the equal abundance of unburned sites, with a progressively lower rate of recovery with decreasing fire age.
Post-fire diversity and richness
Blackbrush Creosote
Diversity (H') Richness (S) Diversity (H') Richness (S)
Unburn 1.36 ± 0.09 B 10.4 ± 0.73 A 1.31 ± 0.08 A 7.53 ± 0.80 A
2000s 1.65 ± 0.15 A 8.27 ± 0.98 B 0.98 ± 0.13 B 4.63 ± 1.09 B
1990s 1.59 ± 0.19 AB 9.81 ± 0.88 AB 1.34 ± 0.14 A 7.10 ± 1.20 A
1980s 1.66 ± 0.10 A 12.00 ± 1.62 A 1.40 ± 0 8.00 ± 0
Table 1. In both blackbrush and creosote communities, species richness (number of species per 100 m2 plot) and diversity differ through time. Creosote dominated communities regained species richness to unburned levels within 15 years, whereas blackbrush communities had greater diversity and richness in burned communities due to a lack of dominance of the blackbrush (see fig. 4 and fig. 6)
Table 2. Results from indicator species analyses (ISA) for burned plots in blackbrush and creosote dominated communities. We present the top five indicator species (those with highest ISA values). Higher ISA values represent species associated more with burned sites than with unburned sites within each community type. The ISA value is a metric that describes the % of perfect indication, based on combining values for relative abundance and relative frequency (based on Dufrene and Legendre, 1997).
Which species establish after fires?
Generally, blackbrush vegetation is thought not to regenerate post-fire, at least not within several decades at the earliest. We found a range of re-establishment across our study, with some sites having over 20% of the original cover regenerated within 30 years, and others having no sign of blackbrush regeneration. We will be analyzing soils and other environmental components to work toward understanding which conditions blackbrush establishes in unaided.
Figure 7. (a) Relative blackbrush cover in burned sites compared to adjacent unburned plots (averaged within each burn). (b) Proportion of plots within each fire where blackbrush was present post-fire compared to the proportion with blackbrush presence in unburned plots.
Post-fire blackbrush regeneration
Cre
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-bu
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sB
lack
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sh
C
om
mu
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Tramp fire – 2005
Fire 4322 – 1983
Burned Unburned
Ghost fire – 1988 *
Burned Unburned
Plant community recovery along a 30 year fire chronosequence
We conducted NMS analyses for blackbrush and creosote-bursage plant communities to assess if plant community composition (as calculated from relative cover) returned to that equivalent to the unburned sites. In blackbrush sites, communities were still different after 30 years. However, after just 15 years plant community composition in creosote communities was similar to unburned communities, even though whole plot cover is still lower than in unburned plots (see fig. 2.)
Christmas fire – 1993
Burned Unburned
1993 Red Rock fire
Burned Unburned
Red Rock fire - 2005
Burned Unburned
Dufrene, M. & P. Legendre. 1997. Species assemblages and indicator species: the need for a flexible asymmetrical approach. Ecological Monographs 67:345-366.
Figure 8. Proportion of Bromus cover in burned versus unburned communities in a) blackbrush and b) creosote-bursage communities.
Figures 3 & 5. NMS ordinations with overlaid joint plots for fires in the 1980’s (a), 1990’s (b) and 2000’s (c) year groups. Multiple response permutation procedure (MRPP) analyses for differences among groups were conducted for each plot. Groups are different when p < 0.05. The “A” statistic denotes the difference among groups (burned or unburned), with 1 being completely different, and -1 being identical.
Figures 4 & 6. Relative cover of the most abundant species (defined by natural breaks) in blackbrush (Fig. 4) and creosote (Fig. 6) sites. Relative abundance and time since fire are expressed by decade of burn for unburned plots (a), 2000’s fires (b), 1990’s fires (c), and 1980’s fires (d).
Post-fire whole-plot foliar cover
Blackbrush Creosote-bursage
1980's 1990's 2000's
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1980's 1990's 2000's
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(b)
Creosote
2000's 1990's 1980'sPro
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2000's 1990's 1980'sPro
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Blackbrush
(a)
Unburned
Re
lati
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ov
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(%)
0
5
10
15
20
25
30
1980's Fires
Re
lati
ve c
ov
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(%)
0
5
10
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45
601990's Fires
Re
lati
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ov
er
(%)
0
5
10
15
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30
2000's Fires
Re
lati
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(%)
0
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post-fire regeneration in creosote-bursage communities
MRPP A = -0.03P > 0.05
MRPP A = -0.02P > 0.05
MRPP A = 0.16P = 0.00
1980’s1990’s2000’s
5a) 5b) 5c)
(6a) (6b)
(6c) (6d)
Unburned 2000’s Fires
1980’s Fires1990’s Fires
Unburned
Re
lati
ve c
ov
er
(%)
0
2
4
6
8
10
12
14
404550
1980's Fires
Re
lati
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ov
er
(%)
0
2
4
6
8
10
12
14
16
181990's Fires
Re
lati
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ov
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(%)
0
2
4
6
8
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18
2000's Fires
Re
lati
ve c
ov
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(%)
0
2
4
6
8
10
12
14
16
18
Post-fire regeneration in blackbrush communities
MRPP A = 0.19P = 0.00
MRPP A = 0.20P = 0.00
MRPP A = 0.29P = 0.00
1980’s1990’s2000’s
Burned
Unburned
(4a) (4b)
(4c) (4d)
(3a) (3b) (3c)
Unburned 2000’s Fires
1980’s Fires1990’s Fires
Blackbrush communities
Year Group Species ISA2000's Sphaeralcea ambigua 79.4
Erioneuron pulchellum 74.1Baileya multiradiata 57.8Encelia virginensis 41.4Thymophylla pentachetta 41.4
1990's Erioneuron pulchellum 82.1Sphaeralcea ambigua 76.3Gutierrezia sarothrae 49.9Baileya multiradiata 48.8Encelia virginensis 38.9
1980's Gutierrezia sarothrae 88.7Encelia virginensis 67.5Prunus fasciculata 51.5Sphaeralcea ambigua 49.2
Baileya multiradiata 42.9
Creosote-bursage communities
Year Group Species ISA2000's Sphaeralcea ambigua 73.5
Eriogonum inflatum 16.2Erioneuron pulchellum 16Baileya multiradiata 15.7Gutierrezia sarothrae 9.1
1990's Sphaeralcea ambigua 39.8Krameria erecta 34.1Eriogonum inflatum 31.6Encelia virginensis 28.1Stephanomeria pauciflora 23.5
1980's Brickellia arguta 50Opuntia basilaris 50Stephanomeria pauciflora 50Encelia virginensis 40
Achnatherum speciosum 25
