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Repeated fires reduce plant diversity in low-elevation Wyoming big sagebrush ecosystems (1984 – 2014)


Abstract and Figures

Sagebrush is one of the most imperiled ecosystems in western North America, having lost about half of its original 62 million hectare extent. Annual grass invasions are known to be increasing wild-fire occurrence and burned area, but the lasting effects (greater than five years post-fire) that the resulting reburns have on these plant communities are unclear. We created a fire history atlas from 31 yr (1984-2014) of Landsat-derived fire data to sample along a fire frequency gradient (zero to three fires) in an area of northern Nevada that has experienced frequent fire in this time period. Thirty-two percent of our study area (13,000 km 2) burned in large fires (over 404 ha) at least once, 7% burned twice, and 2% burned three or more times. We collected plant abundance data at 28 plots (N = 7 per fire frequency), with an average time since fire of 17 yr. We examined fire's effect on plant diversity using species accumulation curves, alpha diversity (Shannon's dominance, Pielou's evenness, and number of species), and beta diversity (Whit-taker, Simpson, and Z indexes). For composition, we used non-metric multidimensional scaling. We then used PERMANOVA models to examine how disturbance history, temperature, precipitation, and aridity around the time of the fire affected subsequent community composition and diversity. One fire fundamentally changed community composition and reduced species richness, and each subsequent fire reduced richness further. Alpha diversity decreased after one fire. Beta diversity declined after the third fire. Cover of exotics was 10% higher in all burned plots, and native cover was 20% lower than in unburned plots, regardless of frequency. PERMANOVA models showed fire frequency and antecedent precipitation as the strongest predictors of beta diversity, while time since fire and vapor pressure deficit for the year of the fire were the strongest predictors of community composition. Given that a single fire has such a marked effect on species composition, and repeated fires reduce richness and beta diversity, we suggest that in lower elevation big sagebrush systems fire should be minimized as much as possible, perhaps even prescribed fire. Restoration efforts should be focused on timing with wet years on cooler, wetter sites.
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Repeated res reduce plant diversity in low-elevation Wyoming
big sagebrush ecosystems (19842014)
Department of Geography, University of Colorado Boulder, GUGG 110, 260 UCB, Boulder, Colorado 80309 USA
Citation: Mahood, A. L., and J. K. Balch. 2019. Repeated res reduce plant diversity in low-elevation Wyoming big
sagebrush ecosystems (19842014). Ecosphere 10(2):e02591. 10.1002/ecs2.2591
Abstract. Sagebrush is one of the most imperiled ecosystems in western North America, having lost
about half of its original 62 million hectare extent. Annual grass invasions are known to be increasing wild-
re occurrence and burned area, but the lasting effects (greater than ve years post-re) that the resulting
reburns have on these plant communities are unclear. We created a re history atlas from 31 yr (1984
2014) of Landsat-derived re data to sample along a re frequency gradient (zero to three res) in an area
of northern Nevada that has experienced frequent re in this time period. Thirty-two percent of our study
area (13,000 km
) burned in large res (over 404 ha) at least once, 7% burned twice, and 2% burned three
or more times. We collected plant abundance data at 28 plots (N =7 per re frequency), with an average
time since re of 17 yr. We examined res effect on plant diversity using species accumulation curves,
alpha diversity (Shannons dominance, Pielous evenness, and number of species), and beta diversity (Whit-
taker, Simpson, and Zindexes). For composition, we used non-metric multidimensional scaling. We then
used PERMANOVA models to examine how disturbance history, temperature, precipitation, and aridity
around the time of the re affected subsequent community composition and diversity. One re fundamen-
tally changed community composition and reduced species richness, and each subsequent re reduced
richness further. Alpha diversity decreased after one re. Beta diversity declined after the third re. Cover
of exotics was 10% higher in all burned plots, and native cover was 20% lower than in unburned plots,
regardless of frequency. PERMANOVA models showed re frequency and antecedent precipitation as the
strongest predictors of beta diversity, while time since re and vapor pressure decit for the year of the re
were the strongest predictors of community composition. Given that a single re has such a marked effect
on species composition, and repeated res reduce richness and beta diversity, we suggest that in lower ele-
vation big sagebrush systems re should be minimized as much as possible, perhaps even prescribed re.
Restoration efforts should be focused on timing with wet years on cooler, wetter sites.
Key words: Artemisia tridentata ssp. wyomingensis; biodiversity; Bromus tectorum; cheatgrass; community composition;
re; re frequency; repeated re; sagebrush.
Received 5 October 2018; revised 3 January 2019; accepted 11 January 2019. Corresponding Editor: Debra P. C. Peters.
Copyright: ©2019 The Authors. This is an open access article under the terms of the Creative Commons Attribution
License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Wildre activity has been increasing across the
western United States since the 1980s (Westerling
et al. 2006, Dennison et al. 2014, Westerling 2016,
Balch et al. 2017), and this is leading to concern
among land managers in the U.S. Great Basin
(Miller et al. 2013, Integrated Rangeland Fire
Management Strategy Actionable Science Plan
Team 2016, Chambers et al. 2017). This trend will
likely continue as rising temperatures and more
frequent drought events increase the probability
of re (Krawchuk et al. 2009, Moritz et al. 2012,
Liu et al. 2013), and as these climatic factors com-
bine with increased human ignition pressure
(Balch et al. 2017) and land use change (Bowman 1February 2019 Volume 10(2) Article e02591
et al. 2011) to increase the length of the re sea-
son (Wotton and Flannigan 1993, Jolly et al.
2015). This increased re activity is one con-
tributing factor to the loss of approximately half
of the area of sagebrush (Artemisia tridentata
Nutt.) shrubland communities, which once occu-
pied over 600,000 km
in the western United
States. Much of this land is now dominated by
cheatgrass (Bromus tectorum L.) (Bradley and
Mustard 2008), an introduced annual grass
(Davies 2011). This in turn is initiating a positive
feedback, wherein invading plants increase the
probability of re, and increased re activity
stimulates more annual grass invasion (DAnto-
nio and Vitousek 1992, Brooks et al. 2004, Balch
et al. 2013). The result is a re return interval that
has decreased from a historical range of 100
342 yr for intact sagebrush (Baker 2006,
Bukowski and Baker 2013) to 78 yr in invaded
areas (Balch et al. 2013), to as low as 35yrin
cheatgrass-dominated areas in the Snake River
Plain (Whisenant 1990). This increase in re activ-
ity results in more areas that are burned multiple
times, and the lasting effect this has on plant com-
munitiesbiodiversity and composition is rela-
tively unknown. There are relatively few studies
on the impacts of re after more than 5 yr (but
see Beck et al. 2009, Reed-Dustin et al. 2016), and
fewer still that analyze the impacts of repeated
res in the same location (Miller et al. 2013).
There are at least 40 vertebrate species of con-
servation concern associated with sagebrush
habitats (Rowland et al. 2006), including the
greater sage grouse (Centrocercus urophasianus).
Greater sage grouse depends on sagebrush for its
habitat and has been a management priority by
land managers (Chambers et al. 2017). Optimal
shrub cover for sage grouse is 1525% with over
15% bunchgrasses and forbs (Beck et al. 2009).
Fire is one of the top 2 threats to the greater sage
grouse in the western part of its range (Brooks
et al. 2015), and the loss of sagebrush due to
wildre has contributed strongly to its popula-
tion declines over the past 30 yr (Coates et al.
2016). Land management agencies have linked
re management with long-term conservation
goals focused on sagebrush ecosystems and the
greater sage grouse (Chambers et al. 2017).
There is emerging consensus among research-
ers and land managers that lower elevation
Wyoming big sagebrush (A. tridentata ssp.
wyomingensis Beetle & Young) ecosystems are not
resilient to re (Chambers et al. 2014) and should
be prevented from burning whenever possible,
while higher elevation Mountain big sagebrush
(A. tridentata ssp. vaseyana) ecosystems may still
recover naturally (Hanna and Fulgham 2015) or
with restoration by seeding (Knutson et al.
2014). Several authors have recommended
attempting to reduce the size and frequency of
wildre, and stopping the use of prescribed re
(Whisenant 1990, Baker 2006, Lesica et al. 2007,
Beck et al. 2009), while also reducing grazing
(Shinneman and Baker 2009, Ellsworth and
Kauffman 2013). Others have urged caution with
the use of prescribed re (Davies et al. 2009,
Reed-Dustin et al. 2016, Shinneman and McIlroy
2016). There has been disagreement in the past
about the historical re return interval for
Wyoming big sagebrush. It has been character-
ized as being every 35100 yr (Schmidt et al.
2002), every 100240 yr (Baker 2006), to every
171342 yr (Bukowski and Baker 2013). This dis-
crepancy has important management implica-
tions, leading to disagreement as to which
stressors or disturbances (e.g., grazing, re) need
to be increased or decreased in order to manage
for healthy sagebrush ecosystems. The lower
estimations imply the system is re-dependent
and requires frequent burning in order to persist,
while the upper estimates suggest re sensitivity.
Wyoming big sagebrush assemblages are gen-
erally agreed to be an endangered ecosystem and
re, and the invasive plants that generally colo-
nize afterward are thought to be two major dri-
vers of declining biodiversity in this system
(Davies et al. 2011). Cover of introduced annual
grass species has been mostly observed to be
negatively related to species richness and native
diversity (Davies 2011, Gasch et al. 2013, Bansal
and Sheley 2016), but over a 45-yr period, Ander-
son and Inouye (2001) found that while intro-
duced annual grass cover was negatively
correlated with cover of native species, species
richness was unrelated. While re is strongly cor-
related with annual grass cover in this system at
regional scales (Balch et al. 2013), it has also been
shown to be an unimportant predictor variable
for both exotic cover and species richness in east-
ern Washington (Mitchell et al. 2017).
Post-re communities of introduced annual
grasses are affected by both re frequency and 2February 2019 Volume 10(2) Article e02591
time since re. Cheatgrass cover can increase ini-
tially after re, then stabilize above its pre-re
cover after 25 yr (Reed-Dustin et al. 2016), but
positive linear relationships between time since
re and cheatgrass cover have also been
observed (Shinneman and Baker 2009), as well as
areas where cheatgrass declined and was
replaced by perennial grasses (West and Yorks
2002, Hanna and Fulgham 2015). Pre-re com-
munity composition might explain the inconsis-
tency in results. Cheatgrass can come to
dominate areas with re-intolerant natives post-
re, but in areas with pre-re populations of re-
tolerant species (e.g., Poa secunda J. Presl), these
species can regenerate following re (Davies
et al. 2012, but see Bagchi et al. 2013).
Precipitation, temperature, and aridity affect
both the re occurrence and the subsequent
recovery of plant communities. Unlike most
forested systems in the western United States,
burned area in Great Basin sagebrush systems is
best predicted by antecedent precipitation (Abat-
zoglou and Kolden 2013, Pilliod et al. 2017). Pre-
cipitation also drives the invasion of cheatgrass
into lower elevation sagebrush systems (Cham-
bers et al. 2007), which increases the probability
of re for several years due to the persistence of
the litter it leaves behind (Pilliod et al. 2017).
Cheatgrass invasion increases the continuity of
fuels (Davies and Nafus 2013) and burned area
(Balch et al. 2013), thereby reducing the number
of unburned patches that provide the native seed
sources critical for recolonizing burned areas.
Unburned patches are essential for sagebrush
regeneration as almost every species in this
genus is a seed obligate and the seeds generally
fall no more than 30 m from the mother plant
(Meyer 1994). Once established, a sagebrush
seedling needs to be able to withstand drought
conditions in the summer to survive and be
recruited into the population (Meyer 1994).
Here, we explored how sagebrush community
composition and diversity responded to increas-
ing re disturbance by constructing a re history
atlas and sampling plant communities that
burned zero to three times between 1984 and
2014 in the Central Basin and Range ecoregion.
We constrained soil, ecological site type, eleva-
tion, and climate, and sampled blocks of plots
stratied along a gradient of zero to three res.
Our rst hypothesis was that community
composition would change drastically between
unburned and burned plots, but remain similar
between burned plots of different re frequencies.
This was our expectation because in the Great
Basin there are vast areas of sagebrush which are
generally unburned in the last 30+yr, and burned
areas are almost always completely dominated by
cheatgrass, along with a handful of exotic forb
species and a native grass, P. secunda. These cheat-
grass-dominated areas all appear very similar,
regardless of re frequency (Fig. 1). But we sus-
pected that there would be a signal on plant diver-
sity after multiple res, due to selective pressure
against re-intolerant plants. Thus, our second
hypothesis was that alpha diversity (the Shannon-
Weaver index, Pielous evenness, and the number
of species in a sampling unit), beta diversity (con-
tinuity or turnover of species between plots), and
the extrapolated species richness (with plots
pooled by re frequency) would decrease with
increasing re frequency. Our third hypothesis
was that cheatgrass abundance would have a neg-
ative relationship with plant diversity. Our fourth
hypothesis was that temperature, vapor pressure
decit, and precipitation around the time of the
re would exert a lasting inuence over post-re
community composition and diversity. This is
based on evidence that the effects of introducing
species at the beginning of secondary succession
can be long-lasting (Veen et al. 2018), and in this
system, the assemblage of species that are able to
successfully colonize an area after a re depends
on their abilities to compete for moisture and tol-
erate drought (Meyer 1994).
Study area
We conducted the study in a 13,000 km
region in northern Nevada (Fig. 2). The region
has hot, dry summers and cold, wet winters.
Annual precipitation averages 293 mm, falling
mostly from November to May. Mean tempera-
tures range from 21.8°C in July to 1.4°Cin
December (PRISM Climate Group 2016). The
region consists of mountain ranges that run
northsouth, and the sagebrush ecosystems gen-
erally lie on the lower slopes of the mountains;
our sites ranged from 1272 to 1696 m in elevation
(median: 1458, SD: 99). From 1984 to 2014, 32%
(4096 km
) of the study area burned in large res 3February 2019 Volume 10(2) Article e02591
(over 500 acres) at least once, 7% burned twice,
and 2% burned three or more times (Appendix
S1: Table S1).
Site selection
We used a block sampling design, with each
block containing one site from each of four re
frequencies (zero to three), and all res occurring
at least ve years before the study. We used
geospatial data representing ecosystem state fac-
tors (sensu Amundson and Jenny 1997) to design
a sampling scheme that constrained all other fac-
tors. We used the LANDFIRE (Rollins 2009) bio-
physical setting layer to eliminate all vegetation
types except big sagebrush shrubland. The
LANDFIRE data have 6268% classication
accuracy for shrublands (Zhu et al. 2006). We
used soil data from the Natural Resource Conser-
vation Service to include only areas in the Loamy
810 precipitation zone (Soil Survey Staff, Natu-
ral Resources Conservation Service, United
States Department of Agriculture (USDA) 2016).
We chose this particular zone simply because it
was the most common type in our study area
within the big sagebrush shrubland biophysical
setting. We used the Land Treatment Digital
Library to exclude areas that had undergone
intensive restoration activities (Pilliod and Welty
2013). Excluding private and military land, and
areas more than 5 miles from a road eliminated
impractical plot locations and held human inu-
ence somewhat constant.
Fig. 1. Plot photographs taken from study block 4 in Kings River Valley, west of Orovada, Nevada. We show
these to illustrate the apparent similarity between plots with different re frequencies, and why we thought spe-
cies composition would not change dramatically between one and three res, while also hypothesizing that
diversity would decline. 4February 2019 Volume 10(2) Article e02591
We accounted for additional, unknown distur-
bances such as grazing by using a block sam-
pling design and stratifying our statistical
analyses by these blocks. Long-term grazing data
were not available. Therefore, we assumed that
plots within blocks were close enough together
that they had experienced similar grazing pres-
sure. Additionally, we visually assessed the
impact of grazing on-site, aggregated what
records we could for the allotments in our study
(billed animal unit months (AUM) provided by
the Bureau of Land Management), and normal-
ized AUM by unit area and included these data
in our statistical modeling.
Once we constrained the area to a consistent
sampling space, we used Landsat-derived re
data to stratify the space along a re frequency
gradient. To generate re history maps, we rst
extracted only the values two to four (low, med-
ium, and high severity) from each yearly burn
severity mosaic from the Monitoring Trends in
Burn Severity (MTBS; Eidenshink et al. 2007)
project, as these were the values where one can
be reasonably certain that they actually burned.
Unburned patches and post-re green-up, which
could be caused by a response to re or an
unburned patch, were excluded. To generate re
frequency maps, we reclassied each yearly layer
to a binary grid, and summed all 31 layers. To
avoid areas with less certain re frequencies, we
then converted the MTBS re perimeter polygons
to layers of re frequency to extract only the grid
cells where the frequency from the polygons
matched the frequency from the reclassied ras-
ter grid. To generate last-year-burned maps, we
reclassied each severity mosaic (values two to
four) to the re year, and calculated the maxi-
mum year for the entire time period for each
pixel. To eliminate areas that had burned more
recently than 2014, we masked pixels that
burned in 2015 according to the MODIS MCD64
burned area product (Giglio et al. 2009).
Kolden et al. (2015) have brought up several
shortcomings for the use of the MTBS burn
Inset (C)
Study Area (A)
Fig. 2. The extent of the study area is shown in (A). The striping from the scanner line correction failure from
Landsat 7 is clearly visible, and those areas were avoided in our sampling. Darker shading indicates higher re
frequency. The potential range is 05res, although areas with more than three res were extremely rare (0.2%
of total area). We sampled frequencies 03. The placement of the study area within the Central Basin and Range
ecoregion is shown in (B). A detail of one of the study blocks is represented in (C). 5February 2019 Volume 10(2) Article e02591
severity mosaics, in particular inconsistent
development of class thresholds and a lack of
empirical relationships between the classied
values and ecological metrics. Because we only
used these data to get a more precise estimate of
re occurrence (i.e., we used it to eliminate areas
of uncertainty) rather than using the severity
data as an independent variable for analysis, we
thought it sufcient to use these data in this
state. Another shortcoming that should be noted
is that there is no practical way for us to know
what these sites looked like before the earliest
res in the re record. The fact that our
unburned control plots were all mature sage-
brush is one piece of evidence suggesting these
sites were mature sagebrush pre-re, but we
cannot be 100% certain, and this is a shortcom-
ing of all chronosequence studies (Walker et al.
We selected seven blocks in our sampling
space in accessible areas where there was a range
of re frequencies and unburned areas for con-
trols within close proximity (0.510 km). Within
each block, we created spatially balanced ran-
dom points (Theobald et al. 2007) for each re
frequency, and sampled one plot for each re his-
tory class within the block. At each block, we
rst sampled the unburned control plot to con-
rm that the area was indeed the correct vegeta-
tion type, and then sampled burned plots. After
navigating to the predetermined coordinates for
each plot, we rst conrmed the physical charac-
teristics (soil type, lack of obvious restoration,
lack of obvious overgrazing) were within the
constraints of our sampling design. If a predeter-
mined point was not suitable (e.g., soil was too
rocky or sandy, an unburned control plot had
obviously burned, or it was the wrong ecological
site type), we referred to georeferenced PDFs of
our re history atlas that we accessed with a sim-
ple application (Avenza Maps https://www.ave on a mobile device and located
nearby areas within the site that were suitable.
When a suitable area was found, we used a ran-
dom number generator to pick a random bearing
and a random distance, and navigated to the
new plot location.
We sampled 28 plots that fell along a gradient
of re frequency (zero to three res; N =seven
plots per frequency) and a range of times since
re (431 yr; mean =17.6, SD =6.6; Fig. 2).
Because most of the re effects research in this
system has been done within ve years of a re,
we aimed to have the time since re of all of the
plots greater than or equal to 5 yr. We encoun-
tered 53 plant species12 were introduced and
41 were native (Appendix S1: Table S2).
Plot establishment
We used GPS to navigate to predetermined
plot locations. Upon arrival, we established a
permanent marker at the southwest corner of the
plot. We recorded the slope, aspect, distance to
the nearest A. tridentata individual or other shrub
species, the topographic curvature of the site
(convex, concave, at), evidence of ecological
restoration, grazing signs, and evidence of past
res. We then delineated a 50 950 meter plot,
and placed pin ags at nine randomly deter-
mined 1 m
subplots within the plot with a mini-
mum spacing of 3 m. Pilliod and Arkle (2013)
found this sampling density sufcient for this
ecosystem, if supplemental methods are used to
estimate disparate functional groups like trees
and shrubs. Hence, we used the point-quarter
method as a supplement to estimate shrub cover
(see Pilliod and Arkle (2013) for detailed
Vegetation sampling
To explore how re frequency inuences com-
munity composition and diversity, we measured
the occurrence and abundance of all species. We
identied and recorded occupancy data for every
species within each subplot, and took a pho-
tograph from nadir with an Olympus Stylus
TG-870 digital camera to be analyzed later for
percent cover.
We used Samplepointsoftware (Booth et al.
2006) to analyze the digital photographs for per-
cent cover. We prepared photographs for analy-
sis by cropping them to the 1 91 m area of the
subplot. Then, we used Samplepoint to overlay a
regular grid of 100 points on each picture, and at
each point identied whether it was litter, bare
ground, rock, dung, or a plant. If it was a plant,
we identied it to species with the aid of the
occupancy data recorded at the plot. These data
were then converted to percent cover. If we
recorded a species as present within the subplot,
but it was missed by the photographic analysis,
we recorded it as 0.5% cover. 6February 2019 Volume 10(2) Article e02591
Environmental data
Aspect was converted to folded aspect (folded
aspect =|180 |aspect 225||; McCune and
Keon 2002). This results in an approximation of
heat load ranging from zero (northeast) to 180
(southwest). Elevation was extracted from 10-m
resolution digital elevation models. The study
sites were situated among six grazing allotments.
To learn how climate before, during, and after
the re event affected the subsequent community
composition and diversity, we extracted monthly
maximum vapor pressure decit, maximum tem-
perature, and precipitation for the years before,
during, and after the most recent re at each plot.
Maximum temperature and maximum vapor
pressure decit were averaged for the entire year
before, during, and after, and precipitation was
averaged for the two winters (NovemberMay)
prior and one after. We used monthly data pro-
vided by the PRISM Climate Group (2016) for all
climate variables. Variables used in modeling are
provided in Table 1. We also sampled soil C and
N (see Mahood (2017) for detailed methods).
Statistical analysis
Community composition and environmental
variables.To analyze how re frequency affects
community composition, we used non-metric
multidimensional scaling (NMDS). We ran a
rank correlation test for re history gradients
against a matrix of relative cover of species per
plot to determine the best hierarchical clustering
method for creating a dissimilarity matrix. We
used this index for NMDS to examine how those
re history characteristics affected the oristic
composition. To assess which species and envi-
ronmental variables had the most inuence on
community composition, we added those vari-
ables to the ordinations using the envtfunc-
tion from Vegan, with 9999 permutations and
stratied by the study block. Then, we grouped
species by their biogeographical origin (i.e.,
native or exotic), and used Tukeys test to assess
how re frequency inuenced native cover, exo-
tic plant cover, and cheatgrass abundance.
Species richness, alpha diversity, and beta
diversity.We created species accumulation
curves grouped by re frequency to assess how
re frequency affected species richness. This is dif-
ferent from alpha diversity in that the species
accumulation curve is estimating number of spe-
cies across all of the sites within each group with
each added plot, as opposed to simply calculating
a diversity index for each plot. We used the sam-
ple-based rarefaction method (Chiarucci et al.
2008, Oksanen et al. 2018, R Core Team 2016). We
used Tukeys honestly signicant difference test
(hereafter, Tukeys test) to see whether different
re frequencies inuenced alpha diversity (the
Shannon-Weaver index, Pielous evenness, and
number of species per plot). There are several
ways to quantify beta diversity, most of which are
grouped into measures of continuityand mea-
sures of gain and loss(Koleff et al. 2003). We
used the Zindex and Whittakersoriginalbeta
diversity index for continuity measures, and
Simpsons index (based on G. Gaylord Simpsons
asymmetric index (Simpson 1943) and modied
by Lennon et al. (2001), not to be confused with
Edward H. Simpsons index (1949)) for a measure
of gain and loss. To see how beta diversity dif-
fered between re frequencies, we modeled the
homogeneity of dispersion of those matrices
(Anderson et al. 2006) and ran pairwise permuta-
tion tests (Legendre et al. 2011) on these models
Table 1. Variables used in PERMANOVA models.
Variable Abbreviation Source
Time since re TSF MTBS
Fire frequency FF MTBS
Maximum vapor
pressure decit
Year of re vpdmax_during PRISM
Year before re vpdmax_before PRISM
Year after re vpdmax_after PRISM
Maximum temperature
Year of re tmax_during PRISM
Year before re tmax_pre PRISM
Year after re tmax_after PRISM
2 yr before re
ppt_2pre PRISM
1 yr before re
ppt_1pre PRISM
After re
ppt_post PRISM
Folded aspect Field
Slope Field
Elevation USGS
Animal unit months
per hectare
AUM_ha BLM 7February 2019 Volume 10(2) Article e02591
with 9999 permutations, stratied by the study
blocks. To assess the inuence of cheatgrass
abundance on alpha and beta diversity, we used
linear mixed models (Pinheiro et al. 2018) with
the study block as a random effect. We included
elevation as a xed effect in addition to cheat-
grass due to its strong correlation with tempera-
ture and moisture availability, and ecosystem
resistance and resilience (Chambers et al. 2014).
We ensured that predictors had no multi-
collinearity using a variable ination factor test
(Fox and Weisberg 2011), and used the partial
coefcient of determination (Jaeger et al. 2016) to
determine the cheatgrass component of the
model. To aid visualization, we removed the par-
tial effects of elevation from the dependent vari-
ables (Hohenstein and Kliegl 2018).
Modeling which re and climate variables drive
post-re composition and diversity.To assess how
pre- and post-re climate, along with soil and
other environmental variables (Table 1) affected
post-re community composition and diversity,
we used permutational multivariate analysis of
variance (PERMANOVA). PERMANOVA uses a
dissimilarity matrix as the response variable and
columns from a separate data frame as the pre-
dictors. It makes the assumption that groups
being modeled have homogeneous dispersions.
If the test is run on groups with heterogeneous
dispersions, it is vulnerable to type 1 error
(Anderson and Walsh 2013). To account for this,
we built multivariate homogeneity of groups dis-
persions (MHGD) models on our community
clustering and beta diversity matrices grouped
by block, re frequency, and burned vs unburned.
We then ran ANOVAs and Tukeystestoneach
model, with Pvalues below 0.05 considered to be
an indication of heterogeneous dispersions. After
removing variables with multicollinearity, we
built PERMANOVA models with both commu-
nity clustering and beta diversity matrices using
an additive model-building process, with 9999
permutations, and stratifying the permutations by
the study blocks, with the aim of producing parsi-
monious models.
Code availability
Data and code to reproduce the analysis are
available at
ff_study and is on the dryad data repository
Community composition fundamentally changed
after one fire
The rank index test showed the Kulczynski
index to have the most consistent high scores
across gradients of re history characteristics, so
we used this index for our hierarchical clustering
and NMDS analyses. Non-metric multidimen-
sional scaling (non-metric t, R
=0.992, linear
t, R
=0.972) showed seven unburned plots
clustered around high abundances of A. triden-
tata, and 18 burned plots clustered around B. tec-
torum (Fig. 3). Two thrice-burned plots were
dominated by exotic annual forbs (Sisymbrium
altissimum L. and Erodium cicutarium (L.) LHer.
ex Aiton), and one was dominated by the native
perennial grass P. secunda (these are the three
thrice-burned plots outside of the burned
ellipse). The ordination showed a clear separa-
tion between burned and unburned plots, but
re frequency was not signicantly correlated
with the ordination, nor were any environmental
For the Tukeys tests of exotic versus native
cover, there were differences between unburned
and burned plots (P<0.05) for both exotic
(increased by 10%) and native cover (decreased
by 20%), and no differences among the burned
plots (Fig. 4A, B). After dividing the mean cover
estimates into native and exotic life form groups
(annual and perennial graminoids and forbs, and
shrubs), we saw lower native shrub cover for
burned plots re (243%), coupled with higher
annual grass cover (414%; Fig. 5).
Plant biodiversity decreased with each successive
We found a decline in plant diversity at sites
that had burned more frequently. Species rich-
ness estimates declined as re frequency
increased (Fig. 6; Appendix S1: Table S3). The
number of species and the Shannon-Weaver
index were higher in unburned plots, but the dif-
ferences were not signicant, and Pielous even-
ness was not different between frequencies
(Fig. 4CE). All three indexes of beta diversity
followed very similar patterns, so we only report
on Whittakers index here. It was not different
between zero and two res, and lower for thrice-
burned plots (Fig. 4F), meaning that there is less 8February 2019 Volume 10(2) Article e02591
dissimilarity within the group of thrice-burned
plots and more dissimilarity within the other
Alpha diversity and evenness decreased with
cheatgrass abundance
Cheatgrass abundance had a negative relation-
ship with the Shannon-Weaver diversity (P0.05,
partial R
=0.65) and Pielous evenness (P0.05,
partial R
=0.51), a weak negative relationship
with the number of species (P<0.05, partial
=0.24), and no relationship to beta diversity
(P>0.5, partial R
=0.08; Fig. 7, Table 2). Eleva-
tion was important in all models except Pielous
evenness (Table 2).
Different climate and fire variables predict post-
fire composition and diversity
PERMANOVA models showed that re history
and environmental factors inuenced commu-
nity composition and beta diversity differently.
ANOVAs and TukeystestsonMHGDmodels
showed no heterogeneity in groups dispersions
for both beta diversity and hierarchical clustering
(P>0.05 for all models). Community composition
after re was most affected by re frequency, time
since re, maximum vapor pressure decit of the
year of the re, and the interaction between re
frequency and time since re (Table 3, R
The relatively low amounts of variation accounted
for by the individual variables indicate these are
subtle effects. Beta diversity on the other hand was
inuenced most by winter precipitation one and
two years prior to the re, re frequency, and
the interaction between winter precipitation one
year prior and max temperature for the year
after the re (Table 4, R
=0.62). Here, the effect
was more pronounced, as more variation
accounted for by the three most statistically signif-
icant variables (re frequency and precipitation
one and two winters prior to the re).
The purpose of this study is to assess how
Wyoming big sagebrush plant communities
−2 −1 0 1
Fig. 3. Ordination plot of non-metric multidimensional scaling conducted on plant community data using Kul-
czynski hierarchical clustering. Ellipses represent the 95% condence interval around plots grouped by whether
or not they had burned. Species signicantly (P<0.05) correlated with the ordination are shown, with arrows
scaled by the strength of the correlation. Species are listed by their USDA plant codes. ARTRW8 is Artemisia tri-
dentata ssp. wyomingensis; POSE is Poa secunda; ELEL5 is Elymus elymoides; SIAL2 is Sisymbrium altissimum; BRTE
is Bromus tectorum; CETE5 is Ceratocephalum testiculatum; ERCI6 is Erodium cicutarium. 9February 2019 Volume 10(2) Article e02591
respond to being burned repeatedly before
returning to their prior condition. The combina-
tion of a 32-yr re history atlas and the use of the
RRQRR (Theobald et al. 2007) to randomly strat-
ify the sampling blocks over a large area pro-
vides broad-scale statistical inference for the
lower elevation (<1500 m) portion of the Wyom-
ing big sagebrush ecosystem. These lower
elevation sites generally experience higher tem-
peratures and lower soil moisture, and it is well
documented that they have lower resilience after
wildres (Chambers et al. 2014). We did not
detect recovery of Wyoming big sagebrush at our
sites, and also found that while the cover of Bro-
mus tectorum does not change with successive
res, the number of species in the species pool
does decrease and that biodiversity decreases
with cover of B. tectorum. The results of this
study may seem to conict with other recent
studies documenting Wyoming big sagebrush
sagebrush recovery in the Great Basin (Ellsworth
et al. 2016, e.g., Shinneman and McIlroy 2016).
But all of the studies we are aware of showing
sagebrush recovery were conducted at cooler,
wetter sites, where Wyoming big sagebrush is
more resilient after re (Chambers et al. 2014).
Coupling the 30+yr re history atlas created
here with intensive eld sampling offers a
unique opportunity to explore plant diversity
and composition changes in areas that have rela-
tively high re frequencies, such as grass-domi-
nated or grass-invaded areas (Balch et al. 2013).
As annual grass invasions and their alterations to
re regimes are a global phenomenon (DAnto-
nio and Vitousek 1992, Brooks et al. 2004), this
type of study design will be useful for
D. Pielou’s evenness E. Number of species F. Beta diversity (Whittaker)
A. Native cover B. Exotic cover C. Shannon−weaver dominance
0123 0123 0123
0123 0123 0123
Fire frequency
Fig. 4. Alpha diversity (Shannons index of proportional abundance, Pielous index of evenness, and the num-
ber of species per plot), beta diversity (Whittakers indexthe values are a unitless index of dissimilarity), and
native and exotic plant cover, all grouped by re frequency. Shading indicates signicantly different groups as
determined by Tukeys test. 10 February 2019 Volume 10(2) Article e02591
understanding the consequences of changing re
regimes in other regions. Additionally, new algo-
rithms are being developed that will lead to more
accurate and precise re data products (Haw-
baker et al. 2015), leading to more nuanced re
history atlases and thus more precise sampling
straticationsespecially now that burn severity
information can be easily incorporated (Eiden-
shink et al. 2007).
Community composition fundamentally changes
after one fire
In lower elevation A. tridentata ssp. wyomin-
gensis systems, our results show that one re can
convert this shrub-dominated system to one
composed mainly of introduced annual grasses
and forbs, and we demonstrate that this new
state can persist for decades with little sign of
recovery to its prior condition. While almost all
of our burned plots were dominated by cheat-
grass, several thrice-burned plots were domi-
nated by P. secunda or exotic annual forbs (see
Fig. 3, where there are three plots that are out-
side the condence envelope containing all other
burned plots). This corroborates previous work
showing that re can push cheatgrass-invaded
grassland and shrubland communities into those
dominated by cheatgrass, P. secunda, and exotic
forbs, while uninvaded sites, or sites that are
invaded but still have signicant bunchgrass
communities, can persist in a state of native
bunchgrasses and forbs (Davies et al. 2012, Reis-
ner et al. 2013, Condon and Pyke 2018). Other
studies have found that topography can be a
mediating factor, with native bunchgrasses more
likely to persist on steeper, more north-facing
slopes in the face of invasion and disturbance
(Rodhouse et al. 2014, Reed-Dustin et al. 2016).
One hypothesis that we were not able to test in
this study is that increasing re frequency may
select for more re-resilient plant functional
traits. More research is needed to investigate the
relationship between re frequency and func-
tional traits. While it has been demonstrated that
B. tectorum establishes immediately post-re and
can persist in the shorter term (Davies et al. 2012,
Hanna and Fulgham 2015), we show that this
Fire frequency
Percent cover
Origin and life form
Exotic annual forb
Exotic annual grass
Exotic perennial forb
Exotic perennial grass
Native annual forb
Native perennial forb
Native perennial grass
Native shrub
Fig. 5. Percent cover of life form groups, grouped
by re frequency. Of the two most dominant life form
groups, exotic annual grass is >99% cheatgrass, and
native shrub is >99% Wyoming big sagebrush.
Fig. 6. Species accumulation curves for re fre-
quency. Vertical lines represent the conditioned stan-
dard deviation around species richness and are jittered
for visibility. 11 February 2019 Volume 10(2) Article e02591
novel grass state can persist for long periods (i.e.,
>17 yr), corroborating recent work (Reed-Dustin
et al. 2016). If there was recovery, our study
design would have enabled us to detect it, as
Wyoming big sagebrush has been found to
recover from disturbance in as little as nine
(Wambolt et al. 2001) to 20 yr (Shinneman and
McIlroy 2016) following re, and our re history
atlas goes back 32 yr.
Biodiversity decreases with each subsequent fire
Here, we show that over a three decade period
repeated res had long-lasting effects on commu-
nity composition and biodiversity in Wyoming
big sagebrush ecosystems. Species richness
declined with increasing re frequency, but
measures of alpha and beta diversity decreased
after one and three res, respectively (Fig. 4A, B).
Species accumulation curves demonstrated that
repeated res are decreasing the overall pool of
species from which an individual patch might
draw from. So while there may not have been sig-
nicant differences in alpha diversity as re fre-
quency increased, as the number of species each
plot can draw from decreased, this signal mani-
fested itself when beta diversity declined after
three res.
We found negative relationships between
cheatgrass abundance and alpha and beta diver-
sity, as we hypothesized, but no relationship
between cheatgrass abundance and the number
of res. Establishment and dominance of
C. Number of species D. Beta diversity (Whittaker)
A. Shannon−weaver dominance B. Pielou’s evenness
0 20 40 60 0 20 40 60
0 20 40 60 0 20 40 60
Cheatgrass cover
Elevation−adjusted values
Fig. 7. Scatter plots for a) Shannon-Weaver, b) Pielous evenness, c) number of species, and d) Whittakers beta
diversity as predicted by Bromus tectorum cover and elevation. Lines are predictions from linear mixed effects
models with study block as a random effect. The x-axis is cheatgrass cover, and the y-axis is the value of the index
with the effect of elevation removed (Hohenstein and Kliegl 2018). 12 February 2019 Volume 10(2) Article e02591
cheatgrass after re are well documented (Whi-
senant 1990, Balch et al. 2013), and the relation-
ship between re and species richness is clear
from this work. This implies that once an area is
invaded by cheatgrass, the competitive effects
from its increased abundance combine with its
effect on re frequency to exclude species that
either cannot compete for moisture or cannot
survive re. It should be noted that because we
selected sites that had burned at least three times
since 1984, we may have biased our results to be
applicable to only those areas that are susceptible
to initiating a grassre cycle.
Time since fire and vapor pressure deficit drive
community composition
PERMANOVA models showed that re his-
tory and climate variables affect diversity and
Table 2. Results of linear mixed models testing the relationship between diversity indexes and cheatgrass abun-
dance, while accounting for elevation. Study block was the random effect. Partial coefcient of determination
was calculated from Jaeger et al. (2016).
Independent variable
Dependent variable
B. tectorum cover 0.017*** (0.003) 0.008*** (0.002) 0.071*** (0.027) 0.002 (0.001)
Elevation 0.189*** (0.062) 0.046 (0.029) 1.700*** (0.642) 0.065*** (0.021)
Intercept 1.214*** (0.103) 0.652*** (0.052) 8.538*** (1.121) 0.288*** (0.037)
Partial R
,B. tectorum cover 0.65 0.51 0.24 0.08
Notes: BD, Beta Diversity (Whitaker); NS, Number of Species; PE, Pielous evenness; SW, Shannon-Weaver.
Table 3. PERMANOVA results for re history and environmental factors inuencing post-re community
Variable df SumsOfSqs MeanSqs F.Model R
TSF 1 0.1457 0.1457 2.0189 0.0713 0.0082
FF 1 0.2342 0.2342 3.2448 0.1146 0.0485
vpdmax_during 1 0.2243 0.2243 3.1081 0.1098 0.0016
tmax_during 1 0.0952 0.0952 1.3186 0.0466 0.1358
tmax_pre 1 0.0776 0.0776 1.0757 0.0380 0.1790
AUM_ha 1 0.2038 0.2038 2.8232 0.0997 0.2111
TSF:FF 1 0.1246 0.1246 1.7262 0.0610 0.0389
Residuals 13 0.9383 0.0722 0.4591
Total 20 2.0436 1.0000
Table 4. PERMANOVA results for re history and environmental factors inuencing post-re beta diversity
(Whittakers index).
Variable df SumsOfSqs MeanSqs F.Model R
FF 1 0.2110 0.2110 2.4408 0.0783 0.0070
ppt_1pre 1 0.5236 0.5236 6.0581 0.1943 0.0014
tmax_after 1 0.1075 0.1075 1.2438 0.0399 0.0646
ppt_2pre 1 0.3993 0.3993 4.6199 0.1482 0.0134
TSF 1 0.1450 0.1450 1.6779 0.0538 0.2493
Folded aspect 1 0.1105 0.1105 1.2780 0.0410 0.4735
Elevation 1 0.0442 0.0442 0.5108 0.0164 0.9526
ppt_1pre:tmax_after 1 0.1162 0.1162 1.3447 0.0431 0.0255
Residuals 12 1.0371 0.0864 0.3849
Total 20 2.6943 1.0000 13 February 2019 Volume 10(2) Article e02591
community composition differently. Composi-
tion was found to be inuenced by both re fre-
quency and time since re, and high vapor
pressure decit the year of the re. This suggests
that drought stress exerts a signicant inuence
on the particular plant species that will survive
and persist after a re, and this effect can still be
detected decades after the re burned. Shinne-
man and McIlroy (2016) also found that climatic
variables around the time of the re inuence the
eventual composition; namely, winter precipita-
tion the year after the re was benecial for sage-
brush recovery, but winter precipitation 2 yr
later had a negative effect. Elevation and recov-
ery have been shown to be positively related in
this system (Knutson et al. 2014), and most of the
studies showing fast recovery times were done at
higher elevations and latitudes (Wambolt et al.
2001, Hanna and Fulgham 2015, Ellsworth et al.
2016), in areas with long-term grazing exclusion
(Ellsworth et al. 2016), or on sites that were
specically selected because their topographic
position was such that there was potential for
sagebrush recovery (Shinneman and McIlroy
2016). Here, we found that on low-elevation sites,
even after an average of 17 yr, post-re sage-
brush cover was very low (<6%; also see Reed-
Dustin et al. 2016). These differences in recovery
rates (i.e., 920 yr at cooler sites vs no detectable
recovery at hotter sites) could be due to a slow-
ing down of recovery rates as the system loses
resilience with increasing drought stress at hotter
sites, while cooler sites have not yet experienced
sufcient drought stress to hamper recovery
(sensu van de Leemput et al. 2018).
Fire frequency and antecedent precipitation drive
beta diversity
Beta diversity was most heavily inuenced by
re frequency, precipitation for the two wet sea-
sons prior to the re, and an interaction between
antecedent precipitation and maximum tempera-
ture for the year after the re. Antecedent precip-
itation has been shown in other studies to be an
important predictor of re occurrence and
burned area in this system (Abatzoglou and Kol-
den 2013, Balch et al. 2013). Since this is a fuel-
limited system, high precipitation increases ne
fuel loads and continuity (Davies and Nafus
2013), leading to higher re probability, more
homogeneously burning res, and larger extents.
Increased ne fuel loads could also be the driv-
ing factor behind decreasing diversity. Following
highly contiguous and extensive res, there
would be fewer unburned patches as seed
sources, which are essential for the seed-obligate
sagebrush to reestablish quickly (Shinneman and
McIlroy 2016). In addition, Wyoming big sage-
brush is an opportunist in reproduction, setting
most of its seed in wet years (Meyer 1994) during
the short window in early spring when enough
water is available in the soil for plants to uptake
nutrients (Ryel et al. 2010, Schlaepfer et al. 2014).
So, in the years that Wyoming big sagebrush is
maximizing its expenditure on reproductive
resources, increased horizontal fuel continuity of
invasive annual grasses (Davies and Nafus 2013)
(1) increases the probability of burning and (2)
increases interspecic competition for resources
post-re. This may result in a more homoge-
neous post-re landscape populated mostly by
re-tolerant plants.
Management implications
This work adds to the existing body of litera-
ture that suggests that in low-elevation
(<1700 m) Wyoming big sagebrush systems
wildre should be minimized due to the negative
effects of single and repeated res on community
composition and biodiversity. The reality is that
wildre cannot be prevented, but re suppres-
sion policies and practices could be crafted to
maximize the number and size of unburned
patches within burns to increase the probability
that Wyoming big sagebrush and other native
seed obligates recover post-re. These results
also imply that prescribed burning is a risky
proposition with potentially disastrous conse-
quences for biodiversity and ecosystem structure
and function. However, we did not directly
assess the inuence of prescribed res in this
study. Prescribed res typically are conducted at
a cooler time of year outside of or at the shoulder
of the re season, and may have different ecolog-
ical effects due to the phenological stage plants
would be in at this different time of year, as well
as the lower burn severity that would be
expected due to cooler ambient air temperatures
and higher soil moisture. At a cooler, wetter site
where grazing has been excluded since 1994,
Ellsworth et al. (2016) detected the recovery of
sagebrush 17 yr after prescribed res were 14 February 2019 Volume 10(2) Article e02591
conducted in late September 1997, which is the
natural end of the re season at that location.
Two other studies at higher latitudes concluded
that prescribed burning to be an unwise action
even at those wetter sites. Beck et al. (2009) stud-
ied an area in southeast Idaho that was burned
in late August 1989 by prescribed re 14 yr post-
re for its utility in improving sage grouse habi-
tat. They recommended against prescribed res
due to the lack of recovery of sagebrush. Wam-
bolt et al. (2001) found minimal benet to the
herbaceous plant community at 13 sites that had
burned in prescribed res in western Montana,
with little shrub recovery 615 yr after re. Thus,
there is conicting evidence on the use of pre-
scribed res for management objectives even at
cooler wetter sites, providing less optimism for
the use of prescribed res in the lower elevation
portion of the Wyoming big sagebrush ecosys-
tems studied here. Future research could focus
on comparing low-elevation Wyoming big sage-
brush sites that have been burned in prescribed
res in the past paired with nearby areas that
burned in wildres, with particular emphasis on
teasing out the effects of seasonality and burn
Our results from PERMANOVA modeling sug-
gest that the success of post-re restoration efforts
will depend not only on elevation and topo-
graphic conditions (Arkle et al. 2014), but also the
climatic conditions that occur around the time of
the re. This could mean that in a very dry year,
less money is spent on restoration efforts on low-
elevation sites, focusing instead on higher eleva-
tion sites and cooler aspects, and in wet years,
more funding is directed toward those more vul-
nerable low-elevation, southwest-facing sites.
Disagreement on the actual historical re rota-
tion limits our ability to determine whether
Wyoming big sagebrush is re-sensitive or re-
resistant. However, this question may be irrele-
vant given the disruption and interaction
between invasive annual grasses and res. We
demonstrate that when both re and invasive
annual grasses operate in conjunction, sagebrush
is re-sensitive. Moreover, we show that an alter-
nate exotic grass state can persist for 17 yr post-
re even with only a single burn. This makes the
use of prescribed burning problematic, as the risk
of a re-prone grassland establishing after a re
likely outweighs the potential benets of a
prescribed re. Our results are specic to lower
elevation (<1700 m), dryer, hotter Wyoming big
sagebrush sites, and it remains to be explored
how sagebrush at higher elevations and latitudes
responds to increasing re frequency, and how it
will respond under future climate change scenar-
ios. However, if temperatures continue to rise as
projected in this region (Garn et al. 2014), those
areas may also become susceptible to a strong
grassre cycle. Overall, this effort demonstrates
that sagebrush communities are vulnerable to
repeated res (Seipel et al. 2018), which should
be taken into account in land management deci-
sions (Chambers et al. 2017) that attempt to con-
serve or restore these valuable ecosystems, and
the threatened species that they harbor.
We are grateful for the assistance of Nick Whittemore
and Kathleen Weimer for their assistance in the eld
and in the laboratory. We thank Tom Veblen and Car-
son Farmer for comments on previous versions of the
from three anonymous reviewers, which greatly
improved the paper. Thanks to Max Joseph for help
with the data analysis. We are also very grateful for the
support of the Nevada Bureau of Land Management
and the Central Nevada Interagency Dispatch Center.
This work was funded by the National Aeronautics and
Space Administration Terrestrial Ecology Program
under Award NNX14AJ14G and start-up funding from
the Department of Geography at CU Boulder. Publica-
tion of this chapter was funded by the University of
Colorado Boulder Libraries Open Access Fund.
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2591/full 19 February 2019 Volume 10(2) Article e02591
... A major form of natural disturbance is fire, and it is a key driver of ecosystem structure (Bowman et al., 2009). However, in some parts of the world, there is evidence that fire regimes (sensu Keeley, 2009) are changing (Buma et al., 2013;Goss et al., 2020;Gromtsev, 2002;Johnstone et al., 2016;Mahood & Balch, 2019;Nowacki & Abrams, 2008;Sommerfeld et al., 2018). ...
... The persistence of biodiversity in ecosystems subject to repeated wildfires is a major issue of conservation concern (Enright et al., 2015;Mahood & Balch, 2019;Ward et al., 2020), especially in environments where there have been repeated high severity wildfires in rapid succession . Indeed, a better understanding of patterns of postfire species recovery is critical for designing and implementing effective conservation and resource management strategies (Côté et al., 2016;Foster et al., 2016;Kelly et al., 2020). ...
Full-text available
Aim The distribution and abundance of forest biodiversity can be shaped by multiple drivers, including disturbances like wildfires. We quantified the influence of wildfire severity and bird life history attributes on temporal patterns of bird site occupancy. Location Wet eucalypt forests of Victoria, Australia. Methods We employed a Before, After, Control, Impact experimental design, gathering occupancy data on birds 5 years before, and for 10 years after, a wildfire in 2009. We quantified post‐fire decline and then recovery on sites subject to high‐severity fire, comparing these temporal patterns with those for birds at sites that were unburnt or burnt at moderate severity. We also tested the influence of life history attributes on bird responses to wildfire. Data were analysed using joint species distribution modelling, accounting for imperfect detection. Results We found a two‐way interaction between fire severity and time period for overall bird site occupancy. The largest change between time periods was on sites burnt at high severity where bird occupancy declined immediately after fire followed by a strong recovery. Occupancy patterns remained largely unchanged on unburnt sites. For many individual species, interactions between fire severity and time period were similar to overall species occupancy. On sites subject to high‐severity fire, most species recovered to pre‐fire levels within 6 years. We found no evidence of a three‐way interaction between fire severity, time period, and life history attributes, with all trait groups of birds examined largely recovered to pre‐fire site occupancy levels 10 years post‐fire. Main conclusions The Victorian 2009 wildfires were severe, but their impacts on common bird species were relatively short‐lived, with immediate post‐fire declines mostly reversed within ~10 years. Rapid post‐fire stand regeneration appears a likely driver of these responses and may account for the relatively limited influence of life history attributes on bird species recovery. However, diet influenced bird species occupancy after fire, with nectivores recovering slower than insectivores on sites subject to high severity fire. Our findings may be relevant to other forests types globally where there can be rapid post‐fire vegetation growth and stand regeneration.
... Given that sagebrush seed is only viable for a short duration (Wijayratne & Pyke, 2012) and invasive plant species quickly colonize, there is a narrow 1-2-year window of opportunity for sagebrush to germinate and reestablish naturally after fire (Meyer, 1994;Ziegenhagen & Miller, 2009). Thus, the non-native grassfire cycle, where fires burn the same location repeatedly, can extirpate sagebrush populations and other native perennial plants, leading to a near-complete conversion of the shrubland ecosystem to the one dominated by non-native herbaceous plants Ellsworth et al., 2020;Mahood & Balch, 2019;Shinneman et al., 2021). ...
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Improving post‐wildfire restoration of foundational plant species is crucial for conserving imperiled ecosystems. We sought to better understand the initial establishment of sagebrush (Artemisia sp.), a foundational shrubland species over a vast area of western North America, in the first 1–2 years post‐wildfire, a critical time period for population recovery. Field data from 460 sagebrush populations sampled across the Great Basin revealed several patterns. Sagebrush seedlings were uncommon in the first 1–2 years after fire, with none detected in 69% of plots, largely because most fires occurred in areas of low resistance to invasive species and resilience to disturbance (hereafter, R&R). Post‐fire aerial seeding of sagebrush dramatically increased seedling occupancy, especially in low R&R areas, which exhibited a 3.4‐fold increase in occupancy over similar unseeded locations. However, occupancy models and repeat surveys suggested exceptionally high mortality, as occupancy rates declined by as much as 50% between the first and second years after fire. We found the prevalence of “fertile island” microsites (patches beneath fire‐consumed sagebrush) to be the best predictor of seedling occupancy, followed by aerial seeding status, native perennial grass cover, and years since fire. In populations where no sagebrush seeding occurred, seedlings were most likely to occur in locations with a combination of high fertile island microsite cover and close proximity to a remnant sagebrush plant. These important attributes were only present in 13% of post‐fire locations, making them rare across the Great Basin. Finally, in the absence of fertile islands and remnant plants, seedling establishment was not observed in any unseeded areas, and rarely in seeded locations. Thus, local extirpation of sagebrush could have important, long‐term implications for sagebrush reestablishment following future fires if there are no mature individuals to leave behind fertile islands or serve as remnant individuals. These findings highlight the importance of landscape legacy effects and could help guide where and how big sagebrush restoration is conducted in the future.
... Other global fire perimeter datasets have been produced from satellite-derived burned area products 14,15 , but these are only available in yearly or monthly global shapefiles. Often field-based studies of fire effects require an entire time series over study areas that are only a few hundred km in diameter 16 or a single ecoregion 17 . The end user who wants to understand the fire history for their region would have to download yearly shapefiles with a global extent, clip all of those shapefiles to their area of interest, and then combine them into one shapefile, just to get started. ...
Full-text available
Fire activity is changing across many areas of the globe. Understanding how social and ecological systems respond to fire is an important topic for the coming century. But many countries do not have accessible fire history data. There are several satellite-based products available as gridded data, but these can be difficult to access and use, and require significant computational resources and time to convert into a usable product for a specific area of interest. We developed an open source software package called Fire Event Delineation for python (FIREDpy) which automatically downloads and processes all of the source files for an area of interest from the MODIS burned area product, and runs a spatiotemporal flooding algorithm that converts those hundreds of grids into a single fire perimeter shapefile. Here we present a collection of fire event perimeter datasets for every country on the globe that we created using the FIREDpy software. We will continue to improve the efficiency and flexibility of the underlying algorithm, and intend to update these datasets annually. Measurement(s)Fire event occurrence • growth rate • sizeTechnology Type(s)Satellite fire detectionsSample Characteristic - EnvironmentfireSample Characteristic - Locationglobal Measurement(s) Fire event occurrence • growth rate • size Technology Type(s) Satellite fire detections Sample Characteristic - Environment fire Sample Characteristic - Location global
... Assessing impacts to the germinable seed bank is an important step in evaluating the potential for unforeseen outcomes related to expanded indaziflam use and providing land managers the information they need to accurately assess trade-offs when making treatment decisions. It is highly unlikely that reductions in plant community diversity due to nontarget impacts to native seed banks will be comparable to the striking declines in diversity that typically follow cheatgrass invasion and repeated wildfire ( Young and Evans 1978 ;Whisenant 1989 ;Davies 2011 ;Mahood and Balch 2019 ), but the potential for impacts to native annuals should be considered alongside the reality that their role in community ecology has been underappreciated in the past, largely because they are not often studied or used in restoration ( Forbis 2010 ;de Queiroz et al. 2021 ). ...
Mitigating invasive annual grass impacts is critical to halting native rangeland conversion to fire-prone, annual grass-dominated communities and maintaining the ecosystem services provided by perennial plants. The herbicide indaziflam (Rejuvra, Bayer, Leverkusen, Germany) may allow managers to selectively deplete annual grass seed banks in plant communities that continue to support desirable perennial vegetation, but nontarget impacts are difficult to assess in the small plots typically used in herbicide trials, and the potential for impacts to the seed bank is not well understood. To assess the potential for nontarget impacts resulting from indaziflam treatment, we used modified-Whittaker multiscale vegetation plots to compare diversity (species accumulation) in three treatment plots (73 g ai ha⁻¹ indaziflam) and three control plots in sagebrush-grasslands near Pinedale, Wyoming that are invaded by cheatgrass (Bromus tectorum L.). We also assessed the density and richness of shallow (0- to 1-cm depth) and deep (1- to 5-cm depth) germinable seed banks in treatment and control plots by tracking seedling emergence from seed bank samples during a 20-wk greenhouse study. Vegetation data and seed bank samples were collected during the third growing season after treatment. Treatment did not impact aboveground species diversity, but this contrasted with the results of the seed bank assay; shallow and deep native seed bank density and shallow native seed bank richness were significantly reduced in treatment plots. All impacted species were detected in the aboveground plant community in treatment plots after herbicide application, suggesting that reduced native annual abundance may be temporary. Considering the potential for nontarget impacts to seed banks will help land managers accurately assess trade-offs when making treatment decisions.
... For example, plants at high elevations should be able to access water at greater depths than plants at lower elevations (Rossatto et al., 2012). Elevation also appears to effect on vegetation dynamics through fire regime (Crouchet et al., 2019;Rodhouse et al., 2020) because low elevations are more prone to exotic grass encroachment, increasing the flammability and probability of persistent fires in these areas (Brooks et al., 2004;D'Antonio and Vitousek, 1992;Mahood and Balch, 2019). Therefore, both climate and elevation are considered local predictors of changes in species susceptibility to extreme events (Crouchet et al., 2019), which ultimately affect vegetation dynamic processes. ...
Full-text available
The Cerrado complex is covered by twelve physiognomies characterized by a gradient of grassland, savanna and forest formations. We assessed the recruitment (RI) and mortality (MI) of individuals and the gain (BG) and loss (BL) of basal area in the Cerrado complex. We used 26 sites classified into savanna (15) and forest formations (11); dry (19) and moist environments (7); non-disturbed (14) and disturbed areas (12) that were recovered from the literature. The median of the vegetation dynamics varied as follows: MI= 3.95, RI = 2.99, BL = 3.08 and the BG = 3.99. MI exceeded RI at 13 sites, and forests formation comprised 70% of the sites with high MI. The median RI was 2.2 times higher in savanna than in forest formations, while it was 1.7 times higher in disturbed areas than in non-disturbed ones. Although the vegetation dynamics have been studied for decades in Cerrado, the studies of disturbances are majority focused on fire and fragmentation effects, indicating that the research should go beyond it. Generalized additive models (GAMs) showed that the MI, RI, BL and BG differ in their responses to three environmental gradients: (1) elevation + temperature, (2) precipitation and (3) precipitation seasonality, even across sites with similar environmental conditions.
... At each of the 20 sites, the ecologist uses a point-quarter sampling technique with four 1 m × 1 m quadrats at which they measure groundcover diversity (see Fig. 4.1a). This type of study design where some kind of biological diversity metric is compared across two or more locations with different times-since-disturbance is not atypical in landscape ecology (e.g., Taillie et al., 2018;Adedoja et al., 2019;Mahood & Balch, 2019) and echoes the space-for-time designs described in Chap. 2. In the imagined case described here, the number of levels of the treatment (of time since fire) is by stand (a = 3), with only one replicate of each (i.e., they do not have multiple stands which have 10, 50 or 100 years since fire available to sample). If the ecologist treated the sample sites per stand as the experimental replicates in their statistical anlaysis, they would likely face accusations of pseudoreplication (Davies & Gray, 2015). ...
... Cheatgrass and other IAGs continue to expand their distribution across the western United States and particularly within the sagebrush biome (at an estimated rate of > 2 300 km 2 /yr; Smith et al. 2021 ), with important ecological consequences ( Jones et al. 2020 ;Rigge et al. 2021 ). Invasions reduce the diversity of native plant communities Mahood and Balch 2019 ), alter fire regimes by increasing the frequency and intensity of fires ( D'Antonio and Vitousek 1992 ;Balch et al. 2013 ), degrade soils and increase erosion risk ( Germino et al. 2016 ), and increase runoff with potentially negative implications for watershed function ( Wilcox et al. 2012 ). Moreover, restoration of ecosystems (e.g., sagebrush communities) invaded by non-native annual grasses is challenging and attempts to reestablish native plant communities before invaders recolonize frequently fail ( Davies and Johnson 2011 ;Svejcar et al. 2017 ). ...
Invasions of native plant communities by non-native species present major challenges for ecosystem management and conservation. Invasive annual grasses such as cheatgrass, medusahead, and ventenata are pervasive and continue to expand their distributions across imperiled sagebrush-steppe communities of the western United States. These invasive grasses alter native plant communities, ecosystem function, and fire regimes, threatening sagebrush ecosystem persistence. Spatial data describing the distribution and abundance of invasive species are often used by resource managers to identify, target, and determine needed interventions. However, there are challenges associated with translating these datasets into management actions. We conducted a review of available spatial products to assess advances in, and barriers to, applying contemporary model-based maps to support rangeland management. We found dozens of regional data products describing cheatgrass or annual herbaceous cover and few maps describing ventenata or medusahead. Over the past decade, IAG spatial data increased in spatial and temporal resolution and increasingly used response variables that indicate the severity of infestation such as percent cover. Despite improvements, use of such data is limited by the time required to find, compare, understand, and translate model-based maps into management strategy. There is also a need for products with higher spatial resolution and accuracy. In collaboration with a multi-partner stakeholder group, we identified key considerations that guide selection of IAG spatial data products for use by land managers and other users. On the basis of these considerations, we discuss issues that contribute to a research-implementation gap between users and product developers and suggest future directions for improved development of management-ready spatial products.
The aim was to characterize fire regimes and estimate fire regime parameters (area burnt, size, intensity, season, patchiness and pyrodiversity) at broad spatial scales using remotely sensed individual‐fire data. Western part of the Palaearctic realm (i.e., Europe, North Africa and the Near East). 2001–2021. Initially, I divided the study area into eight large ecoregions based on their environment and vegetation: Mediterranean, Arid, Atlantic, Mountains, Boreal, Steppes, Continental and Tundra. Next, I intersected each predefined ecoregion with individual‐fire data obtained from remote sensing hotspots to estimate fire regime parameters for each environment. This allowed me to compute annual area burnt, fire size, fire intensity, fire season, fire patchiness, fire recurrence and pyrodiversity for each ecoregion. I related those fire parameters to the climate of the ecoregions and analysed the temporal trends in fire size. Fire regime parameters varied across different environments (ecoregions). The Mediterranean had the largest, most intense and most recurrent fires, but the Steppes had the largest burnt area. Arid ecosystems had the most extended fire season, Tundra had the patchiest fires, and Boreal forests had the earliest fires of the year. The spatial variability in fire regimes was largely explained by the variability of climate and vegetation, with a tendency for greater fire activity in the warmer ecoregions. There was also a temporal tendency for large fires to become larger during the last two decades, especially in Arid and Continental environments. The fire regime characteristics of each ecoregion are unique, with a tendency for greater fire activity in warmer environments. In addition, fires have been increasing in size during recent decades.
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Short-interval and high-severity fires combined are emerging as a catalyst of major reorganization of understory plant communities. In temperate forests of south-central Chile, concern exists about the resilience of threatened Araucaria-Nothofagus forests, including its understory community following extensive and severe fires. In this study we use an array of diversity indices and functional traits as proxies of community resilience. We ask if the species and communities are affected by wildfires, and how these responses are mediated by burn severity and frequency, and other biophysical variables. First, we explore the hypothesis that fire is the major driver of community changes, and that burn severity is the main factor that shifts compositional attributes of communities. Secondly, we hypothesize that a reburn will lead to a greater shift than a single burn in understory compositional attributes, where resprouting species replace obligate seeders, reducing local diversity. We established 120 field plots across a burn severity gradient in two study sites: one affected by a single (burned 2015), and the second by two fires (burned 2002 and 2015), where vascular plant species abundance, among other biotic, abiotic, and topographic variables were estimated. We found that burn (high) severity is the main driver of post-fire understory assemblages, resulting in communities less competitive and heterogeneous, with an increasing number of exotic species. Also, post-fire responses are resulting in communities in which the high abundance of flammable taxa and post-fire resprouter species at the early-seral stage may lead to large-scale transitions from mesic forest ecosystems to dry, open forest and fire-prone shrublands in reburned areas. Our results highlight the ecological importance of short-interval and severe wildfires as leading factors in the transition of post-fire understory communities of Araucaria-Nothofagus forests to a system dominated by post-fire obligate resprouters, where tree species are less represented compromising the recovery of these ecosystems. These findings improve the understanding of the current post-fire processes that affect flammability feedbacks and contribute to a baseline of the current patterns in a world of altered fire regimes.
Adverse weather conditions, particularly freezing or drought, are often associated with poor seedling establishment following restoration seeding in drylands like the Great Basin sagebrush steppe (USA). Management decisions such as planting date or seed source could improve restoration outcomes by reducing seedling exposure to weather barriers. We simulated the effects of management and environmental factors on seedling exposure to post‐germination barriers for bottlebrush squirreltail (Elymus elymoides), Sandberg bluegrass (Poa secunda), and bluebunch wheatgrass (Pseudoroegneria spicata). We combined germination timing models with daily soil moisture and temperature estimates to calculate yearly germination favorability and post‐germination freezing and drought barriers for three planting dates (Oct. 15, Nov. 15, and Mar. 15) and three seed sources or cultivars per species for 5000 sites in each of 40 yrs (water years 1980‐2019). We tested the effects of site environmental variables (elevation, mean annual precipitation, heat load, and clay content) and management choices (seed source and planting date) on germination favorability and barrier occurrence (mean) and variability (coefficient of variation). Seedling exposure to barriers was strongly linked to management decisions in addition to site mean precipitation and elevation. Later fall plantings and seed sources with slower germination (lower mean germination favorability) were less likely to encounter freezing and drought barriers. These results suggest that management actions can play a role comparable to site environmental variables in reducing exposure of vulnerable seedlings to adverse weather conditions and subsequent effects on restoration outcomes. This article is protected by copyright. All rights reserved.
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Shrubs, bunchgrasses and biological soil crusts (biocrusts) are believed to contribute to site resistance to plant invasions in the presence of cattle grazing. Although fire is a concomitant disturbance with grazing, little is known regarding their combined impacts on invasion resistance. We are the first to date to test the idea that biotic communities mediate the effects of disturbance on site resistance. We assessed cover of Bromus tectorum, shrubs, native bunchgrasses, lichens and mosses in 99 burned and unburned plots located on similar soils where fires occurred between 12 and 23 years before sampling. Structural equation modeling was used to test hypothesized relationships between environmental and disturbance characteristics, the biotic community and resistance to B. tectorum cover. Characteristics of fire and grazing did not directly relate to cover of B. tectorum. Relationships were mediated through shrub, bunchgrass and biocrust communities. Increased site resistance following fire was associated with higher bunchgrass cover and recovery of bunchgrasses and mosses with time since fire. Evidence of grazing was more pronounced on burned sites and was positively correlated with the cover of B. tectorum, indicating an interaction between fire and grazing that decreases site resistance. Lichen cover showed a weak, negative relationship with cover of B. tectorum. Fire reduced near-term site resistance to B. tectorum on actively grazed rangelands. Independent of fire, grazing impacts resulted in reduced site resistance to B. tectorum, suggesting that grazing management that enhances plant and biocrust communities will also enhance site resistance.
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How does type of disturbance alter plant community composition when an invasive species with high intrinsic population growth rate is present? The sagebrush steppe in Montana, USA (45.593° N and 111.628° W, 45.595° N 111.831° W). The sagebrush steppe is a cold semi-arid steppe dominated by the native shrub Artemisia tridentata Nutt., native bunchgrasses, and has been invaded by the non-native winter annual Bromus tectorum L. We assessed the effect of fire and soil disturbance, due to bulldozing to create a firebreak, on the resilience of plant communities, and their resistance to invasion by B. tectorum. Plant species richness and species composition were monitored for three years at two sites post-fire and firebreak construction. Burned plant communities were resilient and had similar native grass cover and native species richness compared with the unburned sites after three years. Soil disturbance from firebreak construction resulted in species composition that was distinct and had lower native grass cover. Type of disturbance also affected the communities’ resistance to B. tectorum. Bromus tectorum cover was similar in burned and unburned areas, but increased by up to three times and remained high where soil disturbance occurred, suggesting a shift to an alternative state. In this northern portion of the sagebrush steppe, communities with native plant cover were resilient to fire but not soil disturbance, which facilitated B. tectorum increase and a transition to an alternative state. In areas of high native plant cover, management tactics should avoid soil disturbance. This article is protected by copyright. All rights reserved.
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Larger, more frequent wildfires in arid and semi-arid ecosystems have been associated with invasion by non-native annual grasses, yet a complete understanding of fine fuel development and subsequent wildfire trends is lacking. We investigated the complex relationships among weather, fine fuels, and fire in the Great Basin, USA. We first modeled the annual and time-lagged effects of precipitation and temperature on herbaceous vegetation cover and litter accumulation over a 26-year period in the northern Great Basin. We then modeled how these fine fuels and weather patterns influence subsequent wildfires. We found that cheatgrass cover increased in years with higher precipitation and especially when one of the previous 3 years also was particularly wet. Cover of non-native forbs and native herbs also increased in wet years, but only after several dry years. The area burned by wildfire in a given year was mostly associated with native herb and non-native forb cover, whereas cheatgrass mainly influenced area burned in the form of litter derived from previous years’ growth. Consequently, multiyear weather patterns, including precipitation in the previous 1–3 years, was a strong predictor of wildfire in a given year because of the time needed to develop these fine fuel loads. The strong relationship between precipitation and wildfire allowed us to expand our inference to 10,162 wildfires across the entire Great Basin over a 35-year period from 1980 to 2014. Our results suggest that the region's precipitation pattern of consecutive wet years followed by consecutive dry years results in a cycle of fuel accumulation followed by weather conditions that increase the probability of wildfire events in the year when the cycle transitions from wet to dry. These patterns varied regionally but were strong enough to allow us to model annual wildfire risk across the Great Basin based on precipitation alone.
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A range of indicators have been proposed for identifying the elevated risk of critical transitions in ecosystems. Most indicators are based on the idea that critical slowing down can be inferred from changes in statistical properties of natural fluctuations and spatial patterns. However, identifying these signals in nature has remained challenging. An alternative approach is to infer changes in resilience from differences in standardized experimental perturbations. However, system-wide experimental perturbations are rarely feasible. Here we evaluate the potential to infer the risk of large-scale systemic transitions from local experimental or natural perturbations. We use models of spatially explicit landscapes to illustrate how recovery rates upon small-scale perturbations decrease as an ecosystem approaches a tipping point for a large-scale collapse. We show that the recovery trajectory depends on: (1) the resilience of the ecosystem at large scale, (2) the dispersal rate of organisms, and (3) the scale of the perturbation. In addition, we show that recovery of natural disturbances in a heterogeneous environment can potentially function as an indicator of resilience of a large-scale ecosystem. Our analyses reveal fundamental differences between large-scale weak and local-scale strong perturbations, leading to an overview of opportunities and limitations of the use of local disturbance-recovery experiments.
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The economic and ecological costs of wildfire in the United States have risen substantially in recent decades. Although climate change has likely enabled a portion of the increase in wildfire activity, the direct role of people in increasing wildfire activity has been largely overlooked. We evaluate over 1.5 million government records of wildfires that had to be extinguished or managed by state or federal agencies from 1992 to 2012, and examined geographic and seasonal extents of human-ignited wildfires relative to lightningignited wildfires. Humans have vastly expanded the spatial and seasonal "fire niche" in the coterminous United States, accounting for 84% of all wildfires and 44% of total area burned. During the 21-y time period, the human-caused fire season was three times longer than the lightning-caused fire season and added an average of 40,000 wildfires per year across the United States. Human-started wildfires disproportionally occurredwhere fuel moisture was higher than lightning-started fires, thereby helping expand the geographic and seasonal niche of wildfire. Human-started wildfires were dominant (<80% of ignitions) in over 5.1 million km2, the vast majority of the United States, whereas lightning-started fires were dominant in only 0.7 million km2, primarily in sparsely populated areas of the mountainous western United States. Ignitions caused by human activities are a substantial driver of overall fire risk to ecosystems and economies. Actions to raise awareness and increase management in regions prone to human-started wildfires should be a focus of United States policy to reduce fire risk and associated hazards.
1. Using data on the spatial distribution of the British avifauna, we address three basic questions about the spatial structure of assemblages: (i) Is there a relationship between species richness (alpha diversity) and spatial turnover of species (beta diversity)? (ii) Do high richness locations have fewer species in common with neighbouring areas than low richness locations?, and (iii) Are any such relationships contingent on spatial scale (resolution or quadrat area), and do they reflect the operation of a particular kind of species-area relationship (SAR)? 2. For all measures of spatial turnover, we found a negative relationship with species richness. This held across all scales, with the exception of turnover measured as β sim. 3. Higher richness areas were found to have more species in common with neighbouring areas. 4. The logarithmic SAR fitted better than the power SAR overall, and fitted significantly better in areas with low richness and high turnover. 5. Spatial patterns of both turnover and richness vary with scale. The finest scale richness pattern (10 km) and the coarse scale richness pattern (90 km) are statistically unrelated. The same is true of the turnover patterns. 6. With coarsening scale, locations of the most species-rich quadrats move north. This observed sensitivity of richness 'hotspot' location to spatial scale has implications for conservation biology, e.g. the location of a reserve selected on the basis of maximum richness may change considerably with reserve size or scale of analysis. 7. Average turnover measured using indices declined with coarsening scale, but the average number of species gained or lost between neighbouring quadrats was essentially scale invariant at 10-13 species, despite mean richness rising from 80 to 146 species (across an 81-fold area increase). We show that this kind of scale invariance is consistent with the logarithmic SAR.
Many grassland biodiversity experiments show a positive relationship between biodiversity and ecosystem functioning, however, in most these experiments plant communities are established by sowing and natural colonization is prevented by selective weeding of non‐sown species. During ecosystem restoration, for example on abandoned fields, plant communities start on bare soil, and diversity is often manipulated in a single sowing event. How such initial plant diversity manipulations influence plant biodiversity development and ecosystem functioning is not well understood. We examined how relationships between taxonomic and functional diversity, biomass production and stability develop over 16 years in non‐weeded plots sown with 15 species, 4 species, or that were not sown. We found that sown plant communities become functionally similar to unsown, naturally colonized plant communities. However, initial sowing treatments had long‐lasting effects on species composition and taxonomic diversity. We found only few relationships between biomass production, or stability in biomass production, and functional or taxonomic diversity, and the ones we observed were negative. In addition, the cover of dominant plant species was positively related to biomass production and stability. We conclude that effects of introducing plant species at the start of secondary succession can persist for a long time, and that in secondary succession communities with natural plant species dynamics diversity‐functioning relationships can be weak or negative. Moreover, our findings indicate that in systems where natural colonization of species is allowed effects of plant dominance may underlie diversity‐functioning relationships. This article is protected by copyright. All rights reserved.