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A field experiment informs expected patterns of conifer regeneration after disturbance under changing climate conditions


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Climate changemayinhibit tree regeneration following disturbances such as wildfire, altering post-disturbance vegetation trajectories. We implemented a field experiment to examine the effects of manipulations of temperature and water on ponderosa pine (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings planted in a low-elevation, recently disturbed setting of the Colorado Front Range.Weimplemented four treatments: warmed only (Wm), watered only (Wt), warmed and watered (WmWt), and control (Co). We found that measures of growth and survival varied significantly by treatment type. Average growth and survival was highest in theWtplots, followed by the Co,WmWt,andWmplots, respectively. This general trend was observed for both conifer species, although average growth and survival was generally higher in ponderosa pine than in Douglas-fir. Our findings suggest that warming temperatures and associated drought are likely to inhibit post-disturbance regeneration of ponderosa pine and Douglas-fir in low-elevation forests of the Colorado Front Range and that future vegetation composition and structure may differ notably from historic patterns in some areas. Our findings are relevant to other forested ecosystems in which a warming climate may similarly inhibit regeneration by dominant tree species. © 2015, National Research Council of Canada, All Rights Reserved.
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A field experiment informs expected patterns of conifer
regeneration after disturbance under changing climate
Monica T. Rother, Thomas T. Veblen, and Luke G. Furman
Abstract: Climate change may inhibit tree regeneration following disturbances such as wildfire, altering post-disturbance vegetation
trajectories. We implemented a field experiment to examine the effects of manipulations of temperature and water on ponderosa
pine (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings planted in a
low-elevation, recently disturbed setting of the Colorado Front Range. We implemented four treatments: warmed only (Wm), watered
only (Wt), warmed and watered (WmWt), and control (Co). We found that measures of growth and survival varied significantly by
treatment type. Average growth and survival was highest in the Wt plots, followed by the Co, WmWt, and Wm plots, respectively. This
general trend was observed for both conifer species, although average growth and survival was generally higher in ponderosa pine
than in Douglas-fir. Our findings suggest that warming temperatures and associated drought are likely to inhibit post-disturbance
regeneration of ponderosa pine and Douglas-fir in low-elevation forests of the Colorado Front Range and that future vegetation
composition and structure may differ notably from historic patterns in some areas. Our findings are relevant to other forested
ecosystems in which a warming climate may similarly inhibit regeneration by dominant tree species.
Key words: field experiment, tree regeneration, climate change, wildfire, ponderosa pine, open-top chambers.
Résumé : Les changements climatiques pourraient inhiber la régénération des arbres a
`la suite de perturbations telles qu'un feu,
ce qui modifierait les trajectoires de la végétation après une perturbation. Nous avons effectué une expérience de terrain pour
étudier les effets de manipulations de la température et de l'eau sur la croissance et la survie des semis de pin ponderosa (Pinus
ponderosa Douglas ex P. Lawson & C. Lawson) et de douglas de Menzies (Pseudotsuga menziesii (Mirb.) Franco) plantés a
altitude dans un milieu récemment perturbé du Colorado Front Range (CFR). Nous avons établi quatre traitements : réchauffe-
ment seulement (Wm), arrosage seulement (Wt), réchauffement et arrosage (WmWt) et témoin (Co). Nous avons observé que les
mesures de croissance et de survie variaient significativement selon le traitement. La croissance et la survie étaient en moyenne
les plus élevées dans les parcelles Wt suivies respectivement des parcelles Co, WmWt, et Wm. Cette tendance générale a été
observée chez les deux espèces de conifère même si la croissance et la survie étaient en général meilleures chez le pin ponderosa
que chez le douglas de Menzies. Nos résultats indiquent que l'augmentation de la température et la sécheresse qui y est associée
vont probablement inhiber la régénération qui suit une perturbation chez le pin ponderosa et le douglas de Menzies dans
les forêts du CFR situées a
`faible altitude et que la composition et la structure de la régénération pourraient dans le futur être très
différentes de la configuration historique dans certaines régions. Bien que notre étude mette l'accent sur les forêts du CFR situées
`faible altitude nos résultats sont pertinents pour d'autres écosystèmes forestiers le réchauffement du climat peut de façon
similaire inhiber la régénération des espèces dominantes. [Traduit par la Rédaction]
Mots-clés : expérience de terrain, régénération des arbres, changement climatique, feu de forêt, pin ponderosa, chambres a
Recent studies of vegetation patterns following wildfire have
been motivated by concern that climate change and (or) potential
increases in wildfire severity may alter postfire vegetation trajec-
tories by inhibiting processes of conifer regeneration. In some dry
ponderosa pine forests of the western United States (US), observa-
tions of limited postfire conifer regeneration have led to the hypoth-
esis that forested stands may be replaced by persistent grasslands or
shrublands following fire, at least within portions of burns in
which seed availability is low (Dodson and Root 2013;Keyser et al.
2008;Roccaforte et al. 2012;Savage and Mast 2005). In the Colo-
rado Front Range (CFR) and throughout the western US, more
research is needed to document whether current patterns of post-
fire conifer regeneration are incongruous with historic patterns
and what factors (e.g., increased temperatures and associated
drought, wildfire severity, etc.) explain any significant deviation
from the past. We tackle part of this complicated issue through a
field experiment that assesses the role that variability in temper-
ature and water plays in influencing the growth rates and percent
survival of ponderosa pine (Pinus ponderosa Douglas ex P. Lawson &
C. Lawson) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)
seedlings that were planted in a recently disturbed environment.
Our research will allow land managers to better understand and
prepare for changes in patterns of postfire conifer regeneration,
including potential shifts from forest to nonforest vegetation.
In Colorado, temperatures have risen almost universally across
the state in recent decades and are expected to increase by an addi-
Received 22 January 2015. Accepted 21 July 2015.
M.T. Rother, T.T. Veblen, and L.G. Furman. Biogeography lab, Department of Geography, University of Colorado, Campus Box 260, Boulder,
CO 80309, USA.
Corresponding author: Monica T. Rother (e-mail:
Can. J. For. Res. 45: 1607–1616 (2015) Published at on 30 September 2015.
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tional 1.4–3.6 °C by 2050 (Lukas et al. 2014;Reclamation 2013). Unlike
predictions of changing temperature, uncertainty surrounds how
precipitation regimes might change, and over the last several
decades, there has been no clear trend across the state (Lukas et al.
2014;Reclamation 2013). In the absence of increased precipitation,
higher temperatures are associated with an increased occurrence
of drought due to higher rates of evapotranspiration. Increased
drought has already resulted in ecological change in many forested
communities including higher background tree mortality rates (van
Mantgem et al. 2009;Williams et al. 2013). Researchers have linked
tree mortality to carbon starvation, hydraulic failure, or a combina-
tion of the two, along with other drought-mediated factors such as
insect attack and disease (McDowell et al. 2011;Sevanto et al. 2014).
Trees that do survive climate stress may acclimate in a variety of
ways such as through biomass partitioning (Turner 1997). A number
of studies have shown that conifers allocate more carbon to roots
and (or) sapwood under conditions of elevated temperature, water
stress, or both (Callaway et al. 1994;Delucia et al. 2000;Olszyk et al.
2003). Although this strategy may improve the likelihood of survival
by increasing water access, it may also increase the probability of
death by limiting plant height and increasing susceptibility to mor-
tality by factors such as wildfire, herbivory, and competition for
Land managers and researchers have long recognized the impor-
tance of climate variability in driving patterns of conifer regenera-
tion in dry ponderosa pine forests. For example, many early papers
identified 1919 as an astonishing year for widespread ponderosa
pine regeneration in Arizona, due to high levels of summer pre-
cipitation that followed an excellent seed year (Cooper 1960;
Pearson 1923). This observational evidence of infrequent years of
abundant regeneration coinciding with favorable weather was
supported in later years through the development of large data-
sets of annually resolved establishment dates in a ponderosa pine
forest in Arizona (Savage et al. 1996) and along a grassland–forest
ecotone of the CFR (League and Veblen 2006). These studies both
concluded that in environments lacking recent disturbance, es-
tablishment by ponderosa pine occurs episodically in association
with high moisture availability. More recently, researchers exam-
ined relationships between climate variability and ponderosa
pine regeneration following wildfire (Feddema et al. 2013;Savage
et al. 2013) and found that monthly to seasonal climate conditions
associated with multiple developmental stages of ponderosa pine
(e.g., cone production, germination, etc.) were important for predict-
ing patterns of observed postfire ponderosa pine regeneration. Fur-
ther research is needed to document whether similar relationships
between climate variability and postfire ponderosa pine regenera-
tion hold true in the CFR, given significant differences between the
two areas (e.g., climate regimes, soil characteristics, understory com-
position, genetic provenance of species, etc.).
In addition to climate conditions, fire severity may also influ-
ence patterns of conifer regeneration in dry ponderosa pine for-
ests. Within a given burn, large patches of high-severity fire can
limit regeneration by ponderosa pine and Douglas-fir because these
species disperse seed primarily by wind over relatively short dis-
tances of c. 200 m or less (Bonnet et al. 2005;Haire and McGarigal
2010;Shatford et al. 2007). Additionally, patches of high-severity
wildfire can create altered microclimate conditions such as higher
daily temperature ranges and reduced soil moisture levels due to
blackened soil and absence of vegetation (Montes-Helu et al. 2009;
Ulery and Graham 1993). However, abundant conifer regeneration
following high-severity fire has been documented in ponderosa pine
forests (Ehle and Baker 2003;Haire and McGarigal 2010;Savage and
Mast 2005;Veblen and Lorenz 1986), indicating that high-severity fire
does not universally result in regeneration failure. Additionally, in
the ponderosa pine zone in the CFR, the historic wildfire regime was
mixed severity, meaning that fire effects were varied both within
stands and across the landscape and included low-, moderate- and
high-severity fire (Sherriff and Veblen 2007). Fire severity undoubt-
edly plays an important role in influencing postfire vegetation tra-
jectories in dry ponderosa pine forests, but it is unlikely to be the sole
factor explaining observations of limited conifer regeneration fol-
lowing recent wildfires.
In the present study, we focused on the role that differences in air
temperature and water availability play in influencing postdistur-
bance conifer regeneration in low-elevation forests of the CFR. We
employed open-top chambers and watering treatments to assess
how altered temperature and water availability influenced growth
rates and percent survival of ponderosa pine and Douglas-fir seed-
lings at a site where the aerial biomass was scraped off to expose bare
mineral soil, simulating fire (i.e., “scalping” sensu Kayes et al. (2010)).
Our primary objectives were to (i) examine the effects of manipula-
tions of temperature and water on the growth rates and percent
survival of conifer seedlings, (ii) assess potential differences in
aboveground vs. belowground biomass partitioning by conifer seed-
lings, and (iii) determine whether conifer seedling growth and sur-
vival were dependent on herbaceous and shrub groundcover. We
hypothesized that experimental treatments would result in signifi-
cant differences in growth and survival patterns of both ponderosa
pine and Douglas-fir. Given the semi-arid, low-elevation setting for
the experiment, we expected that increased air temperature would
result in decreased growth rates and percent survival, whereas in-
creased water would result in increased growth rates and percent
survival. We hypothesized that ponderosa pine growth rates and
percent survival may be higher than Douglas-fir growth rates and
percent survival given that the latter species tends to occupy rela-
tively cooler and more mesic sites, although significant overlap of
the two species occurs. Partitioning among aboveground vs. below-
ground biomass of the conifer seedlings was also expected to vary;
we hypothesized that allocation to belowground biomass, as op-
posed to aboveground biomass, would be greater under conditions
of higher water stress. Finally, nonconifer biomass was expected to
vary in response to the experimental treatments; high total nonco-
nifer biomass was expected to be associated with lower growth rates
and percent survival of conifer seedlings due to increased competi-
tion for water.
Materials and methods
Study area
We installed the experiment on a closed section of Heil Valley
Ranch Open Space in Boulder County, Colorado. The research site
was located at 40.15°, −105.32° at an elevation of 1960 m within the
lower montane zone of the CFR. The experimental plot was situated
on a slope with a north–northeast aspect and a gentle gradient,
surrounded by dry ponderosa pine forest. Several juvenile pon-
derosa pine trees were present in the plot prior to experimentation,
indicating that the site was suitable for conifer regeneration. Data
from a nearby weather station (Boulder Station, 1893–2013, Western
Regional Climate Center, 39.99°, −105.27°) show that the mean max-
imum January temperature for the area is c. 7.2 °C and mean maxi-
mum July temperature is c. 30.2 °C. Total annual precipitation is c.
476 mm. Precipitation patterns vary significantly over the course of
the year, as well as interannually. In typical years, peak precipitation
occurs in spring and summer months.
Experimental design
Preparation for the field experiment began in the spring of
2012. A macro plot of 35m×42mwasdivided into a grid of
120 cells of 3.5m×3.5m(Fig. 1). Due to excessively rocky or uneven
surfaces in portions of the macro plot, only 100 of the cells were
selected for use in the experiment. In each of the 100 cells, burn-
ing was simulated by killing all aerial biomass through scraping
away of the vegetation to expose bare mineral soil (i.e., “scalping”
sensu Kayes et al. (2010)). Prescribed fire was not a viable option
because a statewide burn ban was in place at the time. Although the
effects of scalping are not identical to those of burning, an advantage
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of this approach for field experiments is that it creates a more uni-
form disturbance than the patchier effect of prescribed fire. After
scalping was completed, we obtained 700 ponderosa pine and 700
Douglas-fir seedlings from the Colorado State Forest Service seedling
tree program. We then planted fourteen seedlings of ponderosa pine
and Douglas-fir (seven seedlings each) in a hexagonally shaped area
of 2.6 m
within each cell, hereafter termed the micro plot. The
seedlings were from a local genetic provenance and were approxi-
mately 1 year old at the time of planting. We regularly watered all of
the seedlings for approximately 1 month prior to initiating experi-
mental manipulations to help them acclimatize and minimize initial
mortality. A fence that was c.1.8 m tall was installed around the
perimeter of the macro plot to deter entry and subsequent trampling
and (or) herbivory by large mammals such as elk and deer. Following
site preparation and the acclimation period, one of the following
four treatments was assigned randomly to each micro plot: (i)
warmed only (Wm), (ii) watered only (Wt), (iii) warmed and watered
(WmWt), and (iv) control (Co). Treatment types were assigned follow-
ing a restricted random protocol to ensure that minor variability in
soil characteristics, slope, ground cover, and light availability within
the macro plot did not confound the study's outcomes.
For micro plots designated to be warmed (Wm and WmWt), hex-
agonal open-top chambers (OTCs) were constructed and installed
following the methods outlined by Marion et al. (1997). The OTCs
were expected to increase air temperature by c. 1–2 °C (Hollister and
Webber 2000;Marion et al. 1997;Tercero-Bucardo et al. 2007), which
is reasonable given the 1.4–3.6 °C forecasted temperature increase
expected by 2050 in Colorado (Lukas et al. 2014;Reclamation 2013).
More uncertainty surrounds how precipitation regimes might
change in Colorado (Lukas et al. 2014;Reclamation 2013), and over
the last several decades, there has been no clear trend (Lukas et al.
2014). In this study, we simulated increased precipitation through
watering treatments. For micro plots designated to be watered (Wt
and WmWt), weekly watering treatments were implemented to
roughly approximate the upper quartile for total monthly precipita-
tion, based on local instrumental climate data (Boulder Station,
1893–2010, Western Regional Climate Center). To do so, the differ-
ence between the long-term median and upper quartile for monthly
total precipitation was first calculated. Then, we determined what
additional volume of water would be required to raise the median
value to the upper quartile, based on the surface area to be watered.
This calculation led to varying weekly watering requirements for
each month (June, 9.8 L; July, 9.1 L; August, 6.1 L; September, 9.8 L).
However, for feasibility purposes related to water delivery to the site
and the manual implementation of the watering treatments, we
used a consistent watering treatment of 7.6 L·week
per micro plot.
This method does not fully replicate precipitation, as natural precip-
itation falls more variably in terms of both the amount of water in a
single rainfall event and the time between rainfall events. However,
we assumed that our watering efforts would effectively alter mois-
ture availability in similar ways as natural rainfall (i.e., through in-
creased soil moisture). Both warming and watering treatments
occurred only during the growing season (June–September). During
excessively dry periods, additional watering was occasionally deliv-
ered to all 100 micro plots to compensate for excessive drought that
could lead to widespread mortality of seedlings across all treatment
types. It is important to note that growing season precipitation pat-
terns in the study area are highly variable and that excessively dry
periods are not uncommon. Thus supplemental watering during
those periods may have resulted in higher growth rates and percent
survival across all treatment types than would otherwise be ex-
Data collection
We used HOBO automated data loggers (Onset Computer Corp.,
Bourne, Massachusetts) to monitor how the experimental treat-
ments influenced air temperature, relative humidity, and soil
temperature. We used restricted random methods to identify
eight micro plots (two of each treatment type) for monitoring. In
each of these micro plots, we placed (i) one data logger for air
temperature and relative humidity, situated 20 cm aboveground,
near the seedling canopy, and (ii) two data loggers for soil temper-
ature, buried at a depth of 5 cm. Air temperature and relative
humidity data were recorded every 30 min, whereas soil temper-
ature data were recorded every 20 min (the highest allowable
frequency given the data storage limitations of the devices). We
monitored changes in ponderosa pine and Douglas-fir seedling
growth rates and percent survival over the 2-year period. Data
collection of seedling stem height and status as dead or living
occurred at the start and end of both growing seasons (2012 and
2013), for a total of four data collection periods. A seedling was
considered dead if no green needles remained. At the end of the
Fig. 1. Plot design for the study including (A) site location within the Colorado Front Range, (B) layout of randomly located experimental
treatments within the macro plot (1, warmed only (Wm); 2, warmed and watered (WmWt); 3, watered only (Wt); 4, control (Co)), (C) cell, and
the (D) micro plot within the cell in which experimental treatment was applied.
A = 2.6 m
Site location
35 m
42 m
3.5 m
1 m
1 3 3 4 4 4 1 1 4 2
2 2 1 1 4 2 3 3 4
3 4 2 3 3 2 4 3 1 1
1 1 1 4 2 3 3 4 2 2
3 3 1 2 1 3 4 4 3
1 4 2 2 1 1 4 4 2 3
1 3 3 2 2 1 4 2 1
4 4 2 3 4 3 1
1 2 2 2 3 3 4
4 3 1 3 2 1 2 3
4 1 2 4 4 3 2
1 4 2 1
Rother et al. 1609
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second growing season (i.e., 2013), we harvested all biomass in a
subset of the micro plots (n= 80) to calculate the aboveground and
belowground biomass of conifer seedlings, as well as the aboveg-
round herbaceous and shrub biomass (hereafter, “nonconifer bio-
mass”). The nonconifer biomass data were collected to determine
whether experimental treatments influenced nonconifer biomass
and to assess whether competitive effects were important to co-
nifer seedling growth rates and percent survival. All harvested
biomass samples were weighed after they were dried in an oven at
70 °C to remove all water mass.
Data analyses
We analyzed the data from the data loggers to determine how
the experimental treatments influenced air temperature, relative
humidity, and soil temperature. We calculated mean air temper-
ature, relative humidity, and soil temperature for each time step
(e.g., 12:00 AM, 12:30 AM, 1:00 AM) over the course of a day for the
full experimental period (June–September) in both 2012 and 2013.
We also assessed ambient conditions in 2012 and 2013 using local
climate data (Boulder Station, 1893–2011, Western Regional Cli-
mate Center) to compare conditions in those years with the long-
term mean. To assess whether experimental treatments affected
conifer seedling growth rates and percent survival, we pooled
individual tree seedling stem height and survivorship data to the
micro plot level and calculated mean height growth rates and
percent survival for the 2012 and 2013 growing seasons. Height
growth rates were calculated as the mean percent increase in
seedling stem height that occurred from the start to the end of
each growing season, averaged for each micro plot, for each spe-
cies. Percent survival was calculated as the percentage of seed-
lings of each species in a micro plot that survived from the start to
the end of each growing season. Height growth rates and percent
survival for each of the four treatment types were then assessed
using generalized linear models (GLMs) with robust standard er-
rors in Stata software (StataCorp 2015). Robust standard errors
were used to account for clustering in the data due to noninde-
pendence of plots between years (Baum 2006). With regard to our
height growth rate data, log transformations were applied to data
for Douglas-fir to address issues of non-normality. In the case of
the percent survival data, the GLMs applied a logit link function
with a binomial family specified. We also assessed differences in
ratios of root to shoot biomass, as well as nonconifer biomass, by
treatment type, again using data pooled to the micro plot level,
using 2×2 ANOVA.
Temperature and relative humidity data
We observed that air temperature, relative humidity, and soil
temperature varied among treatment types, particularly at mid-
day (10:00 AM to 6:00 PM). Mean midday air temperature in Wm
plots was 4.4 °C warmer than in Co plots in 2012 and 3.2 °C warmer
than in Co plots in 2013 (Fig. 2;Table 1). In contrast, differences in
mean air temperatures among the treatment types for the entire
24-hour day were less pronounced; we observed a 1.63 °C differ-
ence between Wm and Co in 2012 and a 1.38 °C difference between
Wm and Co in 2013. With regard to relative humidity, results were
consistent with expectations that over the course of a day, relative
humidity would typically be lowest at midday, when temperatures
were highest (Fig. 2). Among treatment types, mean midday relative
humidity was lowest in Wm plots in both years. Additionally, we
observed that both mean midday air temperature and mean midday
relative humidity were more variable in Wm and WmWt plots than
in Wt and Co plots in both years, as indicated by the relatively high
standard deviations (SDs) associated with those treatment types. Fi-
nally, we observed that differences in soil temperature were less
pronounced than differences in air temperature and relative humid-
ity, although the Wm plots were associated with higher soil temper-
Ambient climate conditions
In both 2012 and 2013, there were periods during which the
monthly air temperature or precipitation deviated substantially
from the long-term mean (Table 2). Regarding air temperature, all
but one month during the experimental period (July 2013) was
hotter than the long-term mean (1893–2012). Months in which
average temperatures were especially high included June 2012,
September 2012, and June 2013. Mean temperatures during those
months exceeded the long-term mean by more than 1 SD. Addi-
tionally, 2012 and 2013 were notably different from each other,
with higher mean monthly temperatures occurring in 2012 than
in 2013. This general pattern of higher temperatures in 2012 than
in 2013 documented by the climate station data (Table 2) is also
evident in the air temperature data from the field experiment
(Fig. 2;Table 1). Mean and median air temperatures in all four
treatment types were warmer in 2012 than in 2013. With regard to
precipitation, some monthly totals during the experimental pe-
riod were exceptionally low, whereas others were exceptionally
high. June 2012, August 2012, and June 2013 had total precipitation
amounts that were more than 1 SD lower than the long-term mean
(Table 2). These were months during which supplemental water
was applied to the entire experimental plot to compensate for lack
of natural rainfall. In terms of wet periods, July 2012 and Septem-
ber 2013 had total precipitation amounts that were more than
1 SD above the long-term mean. September 2013 is especially
remarkable as during part of that month, flood conditions oc-
curred in Boulder and across a broad stretch of the CFR. Total
monthly precipitation was 46.1 cm (Boulder Station, Western Re-
gional Climate Center), which was approximately 13 SDs above
the long-term mean. The experiment was terminated approxi-
mately 2 weeks after this event, as soon as access was feasible.
Although the study site received an enormous amount of precip-
itation in September, no significant changes were observed (e.g.,
no major erosion, no damage to OTCs or monitoring equipment,
and no significant changes in seedling survival or growth).
Ponderosa pine growth and survival
In both 2012 and 2013, we observed that measurements of
growth and survival for ponderosa pine seedlings varied among
treatment types (Fig. 3). Generally, growth rates and percent sur-
vival were highest in the Wt plots and lower in the Co, WmWt,
and Wm plots, in that order. Differences between Wm and Wt
were especially large. In 2012, the median height growth rate was
5.6% in Wm plots compared with 15.5% in Wt plots, and in 2013,
median height growth rates were 6.2% and 11.7% for Wm and Wt
plots, respectively. Our GLMs revealed numerous significant rela-
tionships (Table 3). In the case of our ponderosa pine height growth
rate GLM, our findings indicate that the Wm treatment resulted
in significantly lower height growth rates compared with the
control (P< 0.001) and the Wt treatment resulted in significantly
higher height growth rates compared with the control (P< 0.05).
Regarding the survival of ponderosa pine seedlings, median per-
cent survival in Wm plots in 2012 and 2013 were 42.9% and 66.7%,
respectively, whereas in both years, the median percent survival
in Wt plots was 100%. With regard to the ponderosa pine survival
GLM, we found significantly lower odds of survival for plots with
the Wm and WmWt treatment (P< 0.001) and significantly higher
odds of survival for plots with the Wt treatment (P 0.01). In terms
of year of experiment, we found significantly lower ponderosa
pine height growth rates in 2013 vs. 2012 (P< 0.001) and signifi-
cantly higher odds of ponderosa pine percent survival in 2013 vs.
2012 (P< 0.001). Finally, results from 2x2 ANOVA indicate that root
to shoot ratios differed by treatment type (F= 11.98, P< 0.001). Root
to shoot ratios were higher in Wm and WmWt plots than in Wt
and Co plots (Fig. 4;Table 4).
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Douglas-fir growth and survival
We observed that Douglas-fir growth rates and percent survival
were typically highest in the Wt plots followed by the Co, WmWt,
and Wm plots, respectively (Fig. 3). This general pattern is the
same as what we observed for ponderosa pine. However, for
Douglas-fir, both height growth rates and percent survival were
lower than for ponderosa pine regardless of treatment type. In
2012, for all treatment types, the median height growth rate of
Douglas-fir seedlings was less than 4%; this is even lower than the
lowest median height growth rate for ponderosa pine that year
(5.6% in Wm plots). The Douglas-fir height growth rate GLM indi-
cated that the watering treatment resulted in significantly higher
height growth rates compared with control (P< 0.05). In terms of
survival of Douglas-fir seedlings, percent survival in both 2012 and
2013 were lowest in Wm plots, with medians of 28.6% and 16.7%,
respectively (Fig. 3). In contrast, Wt plots had median percent
Fig. 2. Mean air temperature, relative humidity, and soil temperature by treatment type for (A) year 1 (2012) and (B) year 2 (2013)
of the experiment. Data were collected for June–September using HOBO data loggers placed in a subset of the plots. Different line types
designate different treatment types: solid lines, warmed only (Wm), long-dashed lines, warmed and watered (WmWt), short-dashed lines,
watered only (Wt), and dotted lines, control (Co).
15 20 25 30 35
20 4030 6050 80
Mean Relative Humidity (%)
(A) 2012 (B) 2013
15 20 25 30 35
12am 4am 8am 12pm 4pm 8pm 12am 12am 4am 8am 12pm 4pm 8pm 12am
Time of Day
Table 1. Midday (10 AM 6 PM) air temperature, relative humidity, and soil temperature by treatment
type during the experimental period (June–September of 2012 and 2013).
Air temperature (°C) Relative humidity (%) Soil temperature (°C)
Mean Median SD Mean Median SD Mean Median SD
Wm 33.2 34.6 4.2 27.4 25.8 5.4 30.7 31.4 3.0
WmWt 32.6 33.6 3.8 29.7 28.4 5.3 29.4 30.3 3.0
Wt 29.3 30.1 2.7 31.5 30.4 4.4 28.5 29.2 3.1
Co 28.8 29.5 2.5 31.6 30.6 4.1 29.6 30.3 2.8
Wm 30.4 31.2 4.0 41.1 39.9 6.3 29.9 30.4 2.7
WmWt 28.8 29.7 3.2 45.4 43.6 6.0 26.3 26.7 1.7
Wt 27.3 28.0 2.7 45.0 43.5 4.7 26.9 27.3 2.1
Co 27.2 27.9 2.8 46.5 45.0 5.1 27.5 27.9 2.2
Note: SD, standard deviation. Treatment types: Wm, warmed only; WmWt, warmed and watered; Wt, watered
only; Co, control.
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survivals of 85.7% and 100% for 2012 and 2013, respectively. Our
Douglas-fir survival model revealed significantly lower odds of
survival for plots with the Wm (P< 0.001) and WmWt (P< 0.05)
treatments. With regard to year of experiment, we found signifi-
cantly higher Douglas-fir height growth rates in 2013 vs. 2012
(P< 0.001) and significantly higher Douglas-fir percent survival in
2013 vs. 2012 (P< 0.001). Lastly, in terms of aboveground and
belowground biomass, results from 2×2 ANOVA indicate that root
to shoot ratios varied by treatment type (F= 16.17, P< 0.001). Root
to shoot ratios were higher in Wm and WmWt plots than in Wt
and Co plots.
Nonconifer biomass
By the end of the second experimental year (i.e., 2013), most
plots had substantial cover by nonconifer biomass, mostly by grasses
and forbs. Among all 100 micro plots, total nonconifer biomass var-
ied widely from 12.9 g·m
to 98.3 g·m
, with an average of 43.3 ±
17.8 g·m
(mean ± SD). Some plots still had substantial amounts of
bare soil at the end of 2013, whereas other plots were completely
vegetated. Although nonconifer biomass varied substantially across
the entire macro plot, no statistically significant relationships were
found through our 2×2 ANOVA (Fig. 5), indicating that the experi-
mental treatments did not explain the observed differences.
Our study provides significant insight regarding the effects of
climate change on post-disturbance vegetation patterns. Our find-
ings suggest that warming temperatures and associated drought
are likely to inhibit postfire regeneration of ponderosa pine and
Douglas-fir in low-elevation forests of the CFR and that future
postfire vegetation composition and structure may differ notably
from historic patterns in some areas. Because we planted tree
seedlings, our experiment focuses specifically on the growth rates
and percent survival of conifer seedlings in the absence of seed
limitation. Masting by ponderosa pine in the CFR is known to be
sensitive to climate (Mooney et al. 2011), and thus research is
needed to investigate how the changing climate may affect seed
production. The experimental treatment in our study that is most
similar to what is expected for the future in the study area is the
Wm treatment, in which temperatures are elevated but the
amount of precipitation does not significantly change. Height
growth rates and percent survival for both ponderosa pine and
Douglas-fir seedlings in Wm plots were much lower than in other
treatment types. Although we think that the Wm scenario most
closely resembles future climate conditions based on model pro-
jections (Lukas et al. 2014;Reclamation 2013), there is consider-
able uncertainty about how precipitation regimes may change in
the CFR. It is possible that elevated temperatures will be accom-
panied by increased precipitation (like the WmWt treatment) or
reduced precipitation (not examined in this study). However, even
if precipitation is to increase in the future, our findings indicate
that ponderosa pine and Douglas-fir growth rates and percent
survival may still be relatively limited, as indicated by the differ-
ence between WmWt and Co treatment types. Changes in pat-
terns of postfire conifer regeneration in lower montane forests of
the CFR have important management implications given the eco-
logical, social, and economic benefits these forests provide (e.g.,
carbon storage, habitat, recreation, timber, etc.).
Effects of experimental treatments on temperature and
relative humidity
The experimental treatments we implemented effectively al-
tered conditions in the plots. With regard to air temperature, the
average increased temperature we achieved was similar to the c.
1–2 °C documented in other studies that used OTCs (Hollister and
Webber 2000;Marion et al. 1997;Tercero-Bucardo et al. 2007) and is
reasonable given expectations of an increase of 1.4–3.6 °C in Colo-
rado in future years (Lukas et al. 2014;Reclamation 2013). Also similar
to other studies, temperature differences between experimental
treatments were most pronounced during midday (Marion et al.
1997;Tercero-Bucardo et al. 2007). The similarity in air tempera-
ture among experimental treatments outside of the midday hours
indicates that the OTCs did not create uniform warming through
time but, instead, resulted in amplified temperatures only when
incoming solar radiation was present. Trapped heat was largely or
completely lost from the OTCs at night. In contrast, warming
conditions associated with human-induced climate change result
in elevated temperatures during both daytime and nighttime.
This is a notable limitation given that warmer nighttime temper-
atures may have significant effects on plant growth rates and
percent survival (Turnball et al. 2002).
In addition to air temperature differences, we also observed
differences in relative humidity among treatment types. Findings
indicated that warmed plots had drier air than plots that did not
receive a warming treatment. Soil moisture was not monitored,
but we assume that soil moisture varied by treatment type be-
cause height growth rates and percent survival of ponderosa pine
and Douglas-fir seedlings were higher in Wt plots than in Co plots.
Regarding soil temperatures, we observed relatively small differ-
ences in soil temperatures among treatment types, suggesting
that air temperature, relative humidity, and soil moisture were
more important than soil temperature in driving differences in
the growth rates and percent survival of ponderosa pine and
Douglas-fir seedlings.
Growth and survival of ponderosa pine and Douglas-fir
Our experiment was situated in the lower montane zone of the
CFR, where moisture limits tree growth. As hypothesized, we found
that increased temperatures generally resulted in lower average per-
cent survival and height growth rates for both ponderosa pine and
Douglas-fir seedlings, whereas increased water resulted in higher
average percent survival and height growth rates for both ponderosa
pine and Douglas-fir seedlings. We interpret lower percent survival
and height growth rates in warmed plots (Wm and WmWt) to be a
consequence of lower moisture availability in those treatment types,
due to increased evapotranspiration. However, we did not monitor
plant physiological responses and thus cannot be certain what mech-
anisms drove the plant responses we observed; further study to that
end would be beneficial. With regard to root to shoot ratios, we
found that the warming treatment (Wm and WmWt) had a signifi-
cant effect on mean root to shoot ratios, with higher root to shoot
ratios in plots that were warmed. Previous studies of conifer biomass
partitioning (Callaway et al. 1994;Delucia et al. 2000;Olszyk et al.
2003) indicate that water-stressed conifers tend to allocate more car-
Table 2. Climate conditions during the exper-
imental period (2012 and 2013) vs. the long-
term record.
2012 2013 Mean SD
Mean temperature (°C)
June 23.4 21.1 19.3 1.7
July 23.8 22.3 22.5 1.5
August 22.9 22.3 21.7 2.3
September 18.9 18.4 17.2 1.3
Total precipitation (cm)
June 1.0 1.5 4.9 3.4
July 12.7 2.6 4.7 3.0
August 0.9 3.6 4.1 3.0
September 5.8 46.1 3.9 3.2
Note: Data are from the Boulder Station, 1893–2011,
Western Regional Climate Center. SD, standard devi-
1612 Can. J. For. Res. Vol. 45, 2015
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bon to their roots to increase their ability to access soil water. This
strategy may reduce the probability of mortality due to water stress
but increase the likelihood of death by drivers where height is ben-
eficial (e.g., wildfire, herbivory, and competition for light). Although
many trends were similar for ponderosa pine and Douglas-fir, we
observed that growth rates and percent survival for Douglas-fir was
universally lower than for ponderosa pine. This finding suggests
that Douglas-fir seedlings were more stressed than ponderosa
pine seedlings. Both ponderosa pine and Douglas-fir can tolerate
dry conditions, but where the two species co-occur, ponderosa
pine is more common in relatively xeric topographic settings (i.e.,
low-elevation, south-facing aspects) compared with Douglas-fir
(Kaufmann et al. 2006;Peet 1981), and our findings are also con-
sistent with experimental work that demonstrated that Douglas-
fir may be more vulnerable to drought stress than ponderosa pine
(Cleary 1970).
Nonconifer biomass
We did not observe significant differences in nonconifer bio-
mass among treatment types, suggesting that altered tempera-
ture and water did not influence the growth rates and percent
survival of forbs, grasses, and shrubs. This indicates that these
understory plants were less sensitive to the experimental treat-
ments we implemented than the ponderosa pine and Douglas-fir
seedlings and thus may be less affected by future climate change in
the lower montane zone. Although there was significant variability
in total nonconifer biomass among the micro plots, this variability
did not correspond to differences in seedling growth rates and per-
cent survival. We interpret this finding to indicate that competitive
effects between nonconifer plant species and ponderosa pine and
Douglas-fir seedlings were not significant in our study. The differ-
ences in temperature and relative humidity created by the experi-
mental treatments, not variability in nonconifer biomass, was the
Fig. 3. Mean height growth rate (%) and survival (%) by treatment type for ponderosa pine and Douglas-fir seedlings for (A) year 1 (2012) and (B)
year 2 (2013) of the experiment. The thick black line inside the box indicates the median, the lines at the outer edges of the box indicate the
upper and lower quartiles, and the lines at the end of vertical dashed lines indicate the maximum and minimum values. The dots indicate any
outliers. Treatment types: Wm, warmed only; WmWt, warmed and watered; Wt, watered only; Co, control.
0 5 10 15 20 25 30
Height Growth Rate (%)
Wm WmWt Wt Co
0 20406080100
Survival (%)
Wm WmWt Wt Co Wm WmWt Wt Co Wm WmWt Wt Co
(A) 2012 (B) 2013
Ponderosa pine Douglas-fir Douglas-fir Ponderosa pine
Treatment Type Treatment Type
Table 3. Generalized linear models (GLMs) of height growth rate and
percent survival for ponderosa pine and Douglas-fir seedlings.
Coefficient (SE) Odds ratio (SE)
pine growth
ln Douglas-fir
pine survival
Wm −4.78 (0.95)*** −0.11 (0.18) 0.16 (0.04)*** 0.16 (0.05)***
Wt 2.41 (0.97)* 0.41 (0.17)* 4.69 (2.10)** 1.77 (0.52)
Wm × Wt −1.71 (1.11) 0.15 (0.17) 0.31 (0.09)*** 0.49 (0.14)*
Year −3.04 (0.62)*** 0.81 (0.16)*** 2.74 (0.49)*** 2.09 (0.42)***
Constant 12.87 (0.87) 0.70 (0.13) 5.49 (1.27) 2.25 (0.52)
Note: Models are based on the combined dataset for both years. Standard
errors (SE) are robust standard errors. Asterisks indicate statistical significance:
*, P< 0.05; **, P< 0.01; ***, P< 0.001. Treatment types: Wm, warmed only;
Wt, watered only.
Fig. 4. Shoot and root biomass (g·m
) by treatment type for (A)
ponderosa pine and (B) Douglas-fir seedlings. Biomass harvesting
was completed at the end of year 2 (2013) of the experiment.
Treatment types: Wm, warmed only; WmWt, warmed and watered;
Wt, watered only; Co, control.
Shoot Biomass (g/m2)
Root Biomass (g/m2)
Wm WmWt Wt Co
10 0
Wm WmWt Wt Co
Treatment Type
(A) Ponderosa pine (B) Douglas-fir
Rother et al. 1613
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key driver of patterns of ponderosa pine and Douglas-fir growth rates
and percent survival.
Forest management in the context of climate change
Disturbances such as wildfire can act as catalysts of rapid trans-
formation of forested ecosystems, particularly under changing
climate conditions. Although mature trees often survive through
suboptimal environments such as temperatures outside the pre-
ferred range of the species, new germination and establishment
depends on a relatively narrow range of requirements and can be
highly responsive to subtle changes in climate (Hogg and Schwarz
1997;Spittlehouse and Stewart 2003). A general hypothesis of
rapid post-disturbance shifts in vegetation patterns in the context
of changing climate has previously been put forward by numer-
ous researchers (e.g., Enright et al. 2015;Hogg and Schwarz 1997;
Johnstone et al. 2010a;Spittlehouse and Stewart 2003;Turner
2010), yet few studies have tested this expectation (but see Dodson
and Root 2013;Feddema et al. 2013;Hogg and Schwarz 1997;
Johnstone et al. 2010b;Landhausser and Wein 1993;Moser et al.
2010;Overpeck et al. 1990;Savage et al. 2013;Tercero-Bucardo
et al. 2007). Our field experiment directly assessed how tempera-
ture and water availability influence tree seedling growth rates
and percent survival after disturbance and contributes to the
growing understanding that in the context of climate change,
disturbances such as wildfires may result in vegetation patterns
inconsistent with predisturbance patterns.
We demonstrated that ponderosa pine and Douglas-fir seedling
growth rates and percent survival following disturbance was
inhibited by warmer temperatures but that nonconifer biomass
(i.e., grasses, forbs, and some shrubs) was unaffected by the exper-
imental treatments. Historically, conifer regeneration in dry pon-
derosa pine forests nearby and in the CFR was abundant after fire,
as indicated by tree-ring retrospective studies that document pon-
derosa pine and Douglas-fir establishment following wildfires in
the 19th century and earlier (Ehle and Baker 2003;Mast et al. 1998;
Veblen and Lorenz 1986). Our findings suggest that future postfire
vegetation trajectories in lower montane ponderosa pine forests
of the CFR may differ notably from historic patterns. In the ab-
sence of abundant regeneration by ponderosa pine and Douglas-
fir, some previously forested areas may be replaced by persistent
grasslands or shrublands, particularly at lower elevations near the
ecotone, where water stress is highest. Where conifer regenera-
tion does occur, our findings suggest that ponderosa pine may be
more common than Douglas-fir. Given that our study does not
account for seed limitation (removed by planting seedlings) or
periods of extreme drought (removed by supplemental watering
in periods without rainfall), our findings are conservative and
may underestimate the consequences of warmed climate on pon-
derosa pine and Douglas-fir regeneration. Land managers in the
CFR should be prepared for postfire vegetation trajectories that
are incongruent with historic patterns. Difficult decisions will be
required concerning how to manage recently burned, lower mon-
tane forests given expectations of less abundant regeneration by
ponderosa pine and Douglas-fir. For high-priority areas, land man-
agers may choose to adopt resistance strategies (Millar et al. 2007)
to forestall major vegetation changes. Given that wildfire has the
potential to drive rapid vegetation change, fire mitigation prac-
tices such as installing fuel breaks, thinning stands, or using pre-
scribed burning may be appropriate. However, management
strategies such as these can be expensive, time intensive, and of
variable efficacy (Graham et al. 2012;Roccaforte et al. 2010) and
are, therefore, only viable at relatively small spatial scales. Land
managers may also opt for strategies that promote resiliency of
forests to wildfire. Resilient forests are described as those that
return to a similar prior condition after disturbance (Holling 1973;
Millar et al. 2007). Intensive management of the postfire landscape
to promote resiliency may include tree plantings. Our study suggests
that plantings should occur on cooler, wetter microsites such as
north-facing aspects. Additionally, plantings that are timed around
cooler, wetter periods (such as during El Niño conditions in the CFR)
are likely to be most successful. Ultimately, response strategies that
accept the transition of postfire landscapes to new vegetation assem-
blages may be most feasible and practical at broad scales. Such
largely passive response strategies may be ideal for remote areas
where access is limited and costs of active management are high.
One of the most significant impacts of localized climate-induced
transitions from forest to nonforest vegetation is likely to be on
water quantity and quality, which should be an area of priority re-
search (Ebel and Mirus 2014).
In conclusion, the results of our study suggest that under pro-
jected climate change scenarios, post-disturbance regeneration of
ponderosa pine and Douglas-fir may be limited in lower montane
forests of the CFR. Different species assemblages are expected to
emerge as regeneration occurs most abundantly among the spe-
cies best suited to the current climate, rather than those favored
previously. In recently burned areas of high tree mortality, conifer
Table 4. Ratios of root to shoot biomass
by treatment type.
Mean Median SD
Ponderosa pine
Wm 0.43 0.43 0.10
WmWt 0.43 0.42 0.11
Wt 0.36 0.34 0.09
Co 0.35 0.36 0.07
Wm 0.52 0.52 0.09
WmWt 0.46 0.45 0.08
Wt 0.38 0.36 0.10
Co 0.36 0.34 0.10
Note: Data were collected for both ponderosa
pine and Douglas-fir after 2 years of experimen-
tal treatment. Results from 2x2 ANOVA indicate
that root to shoot ratios differed by treatment
type for both ponderosa pine (F= 11.98, P< 0.001)
and Douglas-fir (F= 16.17, P< 0.001). SD, standard
deviation. Treatment types: Wm, warmed only;
WmWt, warmed and watered; Wt, watered on-
ly; Co, control.
Fig. 5. Total nonconifer biomass (g·m
) (i.e., grasses, forbs, and
shrubs) by treatment type based on biomass harvesting completed
at the end of year 2 (2013) of the experiment. The thick black line
inside the box indicates the median, the lines at the outer edges of
the box indicate the upper and lower quartiles, and the lines at the
end of vertical dashed lines indicate the maximum and minimum
values. The circle indicates an outlier. Results of 2×2 ANOVA
indicate that means are statistically equal. Treatment types: Wm,
warmed only; WmWt, warmed and watered; Wt, watered only; Co,
Wm WmWt Wt Co
0 20406080100
Total Nonconifer Biomass (g m-2)
Treatment Type
1614 Can. J. For. Res. Vol. 45, 2015
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regeneration may be restricted to areas that provide suitable mi-
croclimate conditions such as higher elevations and north-facing
slopes. It is possible that a transition to grasslands or shrublands may
occur in some areas following wildfire. Although our study focused
on low-elevation forests of the CFR, our findings are relevant to other
forested ecosystems in which increased temperatures and associated
drought may similarly inhibit post-disturbance regeneration by the
dominant tree species. We expect that many forests worldwide are
currently vulnerable to post-disturbance shifts in vegetation pat-
terns due to climate change, as supported by research in northern
Patagonia (Tercero-Bucardo et al. 2007), the Central Alps (Moser et al.
2010), the eastern US (Overpeck et al. 1990), the western US (Dodson
and Root 2013;Feddema et al. 2013;Johnstone et al. 2010b;Savage
et al. 2013), and portions of Canada (Hogg and Schwarz 1997;
Landhausser and Wein 1993). In areas where climate-mediated shifts
in vegetation following disturbance are anticipated, land managers
will need to act quickly to prioritize areas where they would like to
forestall loss of forested cover following fire vs. areas where the
persistence of alternative vegetation communities is acceptable or
This research was supported by the National Science Founda-
tion (grant nos. 1232997 and 0966472) and the Graduate Research
Fellowship Program. Boulder County Parks and Open Space
(BCPOS) also provided financial and staff support in implementing
and monitoring the experiment. We especially thank N. Stremel,
E. Duncan, and W. Foster for project planning and fieldwork as-
sistance and P. Pendergast for advice regarding statistical analy-
ses. We also thank the anonymous reviewers whose suggestions
significantly improved the manuscript.
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... Food pulses can enable adults to lay a larger clutch, reduce starvation of individual nestlings, and decrease the time that adults need to spend foraging, which can increase the time available for nest defence, and reduce the risk of nest predation (Rastogi, Zanette & Clinchy, 2006). Warmer springs (Apr-Aug) and increased annual rainfall (Sept-Aug) can promote conifer seed masting events in subsequent years (Rother, Veblen & Furman, 2015;Petrie et al., 2016), and such events are predicted to allow chickadees to advance their laying date, and thus increase clutch and brood sizes. However, storm events (precipitation of >10 mm over 24 h) and deep snow in late winter/early spring may reduce the breeding condition of adults, contributing to delayed laying and smaller clutches. ...
... We examined climate variables associated with conifer seed production because mountain chickadees often forage on conifer seeds over winter (McCallum, 1990;McCallum, Grundel & Dahlsten, 2020), and increases in winter food availability can lead to earlier laying dates (e.g., Smith et al., 1980) and increased clutch size in passerine birds (e.g., Broggi et al., 2022). Conifer seed production in dry mixed conifer forests of western North America (in fall) is highly correlated with up to a one-year lag in growing season temperature and soil moisture (Rother, Veblen & Furman, 2015;Petrie et al., 2016); therefore, we calculated 2-year lagged temperature and rainfall for use as predictors in models of mountain chickadee fecundity. We considered rainfall instead of total precipitation because rainfall increases annual soil moisture more consistently than snow due to annual fluctuations in water retention/drainage during snowmelt (Williams, McNamara & Chandler, 2009). ...
... Availability of mountain pine beetle and lepidopteran defoliators did not explain the variation in our fecundity variables. Moreover, although 2-year lagged temperature and rainfall were expected to increase conifer seed production over winter (Owens, 2006;Rother, Veblen & Furman, 2015;Petrie et al., 2016), we found only evidence relating 2-year lagged temperature to nest survival, and no evidence relating 2-year lagged rainfall to any fecundity variables. It is possible that warmer temperatures 2 years prior to the breeding season contributed to conifer seed production in the intervening year (not measured), and that the increase in conifer seeds contributed to an increase in nest survival through improved winter body condition prior to breeding (Montreuil-Spencer et al., 2019). ...
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Background Examining direct and indirect effects on reproduction at multiple scales allows for a broad understanding of species’ resilience to environmental change. We examine how the fecundity of the mountain chickadee ( Poecile gambeli ), a secondary cavity-nesting, insectivorous bird, varied in relation to factors at three scales: regional weather conditions, regional- and site-level food availability, site-level community dynamics, and nest-level cavity characteristics. We hypothesized that earlier laying dates and higher fecundity (clutch size, nest survival, brood size) would be associated with milder climatic conditions, increased food from insect outbreaks, lower densities of conspecifics and nest predators (red squirrel; Tamiasciurus hudsonicus ), and safer (smaller, higher) cavities. Methods We collected data on laying date, clutch size, brood size, nest fate (success/failure), and cavity characteristics from 513 mountain chickadee nests in tree cavities in temperate mixed coniferous-broadleaf forest in interior British Columbia, Canada, from 2000 to 2011. We surveyed annual abundances of mountain chickadees and squirrels using repeated point counts, and mountain pine beetle ( Dendroctonus ponderosae ) and lepidopteran defoliators by monitoring host trees and by using regional-scale aerial overview forest insect survey data. We used weather data (temperature, rain, snow) from a local Environment and Climate Change Canada weather station. We modeled laying date, clutch size, daily nest survival, and brood size as a function of predictors at regional-, site-, and nest-scales. Results and Conclusions Measures of fecundity varied dramatically across years and spatial scales. At the regional (study-wide) scale, chickadees laid earlier and larger first clutches in warmer springs with minimal storms, and daily nest survival (DSR) increased with a 2-year lag in growing season temperature. Despite a doubling of mountain chickadee density that roughly accompanied the outbreaks of mountain pine beetle and lepidopteran defoliators, we found little evidence at the site scale that fecundity was influenced by insect availability, conspecific density, or predator density. At the nest scale, DSR and brood size increased with clutch size but DSR declined with nest cavity size indicating a positive reproductive effect of small-bodied cavity excavators. Double-brooding, rare in chickadees, occurred frequently in 2005 and 2007, coinciding with early breeding, high food availability from insect outbreaks, and warm spring temperatures with 0-1 spring storms. Our results support the idea that fecundity in secondary cavity-nesting species is impacted directly and indirectly by weather, and indirectly through changes in community dynamics ( via cavity resource supply). We stress the importance of adopting holistic, community-level study frameworks to refine our understanding of fecundity in opportunistic and climate-sensitive species in future.
... Pines are one of the most studied taxa in biological invasions ecology and are among the most widespread and problematic non-native tree species Nuñez et al. 2017). Two common invasive pine species, Pinus ponderosa and Pinus contorta, have strong and relatively well-known climatic controls during establishment and growth in their native range (Hansen and Turner 2019;League and Veblen 2006;Rother et al. 2015;Rother and Veblen 2017). P. ponderosa regeneration depends mainly on moisture availability during spring and fall, which favors pulses of establishment in cooler years (League and Veblen 2006;Rother and Veblen 2017). ...
... P. ponderosa regeneration depends mainly on moisture availability during spring and fall, which favors pulses of establishment in cooler years (League and Veblen 2006;Rother and Veblen 2017). However, once established, it is able to adapt to warmer and drier conditions by increasing its root biomass, being one of the most drought-tolerant tree species in its native range in northwestern USA (Rother et al. 2015). In turn, P. contorta seedlings are tolerant to harsh conditions (i.e., drought and frost) but early survival is inhibited by shade and competition (Rice et al. 2012;Hansen and Turner 2019). ...
... To do this, we calculated the difference between the average monthly precipitation of the previous 20 years and the upper quartile. We made the calculation for the months of September-April and we obtained 4L week per plot (2 m 2 ) (Rother et al. 2015). This water addition resulted in an increase of ca. ...
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Climate change can enhance or hinder the spread of invasive plant species, as climatic conditions influence the population dynamics of invaders in their new range. Pines are one of the most widely distributed and problematic invasive tree species globally. Here we experimentally evaluated how two invasive pine species (Pinus ponderosa and Pinus contorta) respond to simulated climate change conditions in northwestern Patagonia. We conducted a full factorial manipulative field experiment with 3-year-old pine saplings growing in warmed (Open Top Chamber), watered, warmed + watered, and control treatments. In the warmed treatments P. contorta saplings showed lower survival and tended to grow taller while maintaining their root growth. In contrast, P. ponderosa saplings, which did not show differences in survival among treatments, did not vary their aerial growth under warmed conditions, but tended to increase their root-to-shoot ratio. Both species showed similar survival and growth in the watered treatments compared to the control, indicating that the implemented difference in water (ca. 30% during the growing season) had no significant effect. Our findings suggest that a decrease in the invasive capacity (initial stages) of P. contorta may occur under climate change but no significant changes are expected for P. ponderosa. This observed differential response may modify the current relative abundance of invading pine species in northwestern Patagonia and has implications for the response of invasive species to climate change elsewhere.
... Our research showed that HL plots demonstrated reduced daily minimum soil moisture and daily average soil moisture between May and August. Soil temperature varied between HS and HL plots during these months but did not vary significantly across the season as a whole, contrary to what has been shown in previous studies (Maher et al. 2005;Rother et al. 2015). Canopy cover and standing and downed fuels varied by disturbance type and influenced soil temperature, which may have long-term implications for plot suitability for tree regeneration (Donato et al. 2006;Keyser et al. 2009;Marcolin et al. 2019), but did not strongly drive early post-fire tree regeneration trends. ...
... The seedbed conditions at a microsite, including the specific soil moisture, temperature, and forest floor substrate (i.e., the microsite), at a given location and can influence the ability of germination of tree seedlings (Rother et al. 2015, Fig. 1). Our study confirms that seedlings were more prevalent under moderate soil temperatures and increased soil moisture (Petrie et al. 2016). ...
... Furthermore, the reduced soil moisture in HL plots may undermine planting efforts if moisture deficits and drought stress exceed the tolerance of planted seedlings. Early conifer tree regeneration demonstrates vulnerability to extremes, whether that is single day minimum soil moisture or maximum summer temperatures (Rother et al. 2015), thus reducing extreme fluctuations through canopy presence, surface fuels loading, or reducing planting regeneration efforts to more moderate, mid-elevation plots may be critical for long term regeneration success. ...
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Background Wildfires are increasing in size and severity in forests of the western USA, driven by climate change and land management practices during the 20th century. Altered fire regimes have resulted in a greater need for knowledge on best practices for managing burned landscapes, especially in instances where a return to a previous forested ecosystem is desired. We examined a large wildfire from 2018 in southern Colorado to understand how fire severity and post-fire logging influenced stand structure, fuels, vegetation, and soil microsite conditions. Results Two years post-fire and 1 year post logging, there was no difference in understory vegetation response. Logged plots demonstrated lower daily average temperature and minimum soil moisture and higher fuel loading across most fuel size classes relative to unlogged plots, which also corresponded with a loss of dead standing wood and little to no canopy cover. Early post-fire conifer regeneration was low across all plots, but lower soil moisture and higher soil temperature negatively impacted the density of regeneration. Conclusions Successful tree regeneration is mediated by multiple factors from the microsite to landscape scale. Here, we demonstrate the importance of those microsite conditions such as soil moisture and temperature in predicting conifer tree establishment in the early post-fire period. Careful consideration of soil impacts and the associated changes to forest conditions should be taken when conducting post-fire logging to prevent detrimental effects on microsite conditions and forest recovery.
... Regeneration declines have been observed throughout the western US [4,6], and both natural regeneration and human-assisted artificial regeneration are often unsuccessful [5,7]. Severe climate and disturbance events impose environmental conditions that seedlings (<∼ 0-3 y) and juvenile trees are not adapted to survive, and have the potential to restrict the fundamental niche of ponderosa pine forests [8]. ...
... During these seasonal dry periods, rainfall events are often small (≤5 mm), relatively infrequent [18], and are the primary source of moisture recharge in shallow soil layers (0-30 cm depth; Koehn et al. [19]). Naturally growing ponderosa pine seedlings and juveniles have shallow rooting depths and do not acclimate growth to environmental variation [13], and it follows that moderate dry periods are an important component of tree thinning and tree survival [8,20]. Yet there has been little focus on the mechanisms of regeneration until recently [21], and there is need to further define the range of conditions under which seedlings can survive. ...
... Transpiration at unshaded sites was more influenced by 12 cm VWC than 5 cm VWC (Supplementary Table S4). Models for transpiration early in the experiment (days [3][4][5][6][7][8] found that Ts, VPD and VWC were top explanatory variables, whereas later in the experiment (days 13-30) treatment history were the top variables (Supplementary Tables S3 and S4). This corroborates observed Ta and Tc differences between treatment groups, which captures the degree to which transpiration increases latent energy exchange and results in canopy cooling. ...
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Juvenile tree survival will increasingly shape the persistence of ponderosa pine forests in the western United States. In contrast to severe pulse disturbances that induce widespread adult and juvenile tree mortality, moderate periods of low rainfall and warm temperatures may reduce forest persistence by killing juvenile trees at the seedling stage. Intensification of these periods in a changing climate could therefore increasingly restrict both natural regeneration and artificial regeneration of planted seedlings. We conducted a controlled field experiment at a single site in the Front Range of Colorado, USA, to determine the responses and survival of 3 Colorado subpopulations of
... Altered structure can change the likelihood of a disturbance, the properties of a disturbance, and the capacity of the system to recover after a disturbance (Brooks et al., 2004). Global climate change can also directly affect the magnitude of disturbances (Parks & Abatzoglou, 2020) and act as a demographic filter that affects how ecosystems recover after disturbances (Davis et al., 2019;Rother et al., 2015) via impacts on adult plant survival and seed dispersal (Davis et al., 2018;Eskelinen et al., 2020). The combined effects of global change forces on structure, function, and disturbance can cascade and interact. ...
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A key challenge in ecology is understanding how multiple drivers interact to precipitate persistent vegetation state changes. These state changes may be both precipitated and maintained by disturbances, but predicting whether the state change is fleeting or persistent requires an understanding of the mechanisms by which disturbance affects the alternative communities. In the sagebrush shrublands of the western United States, widespread annual grass invasion has increased fuel connectivity, which increases the size and spatial contiguity of fires, leading to post‐fire monocultures of introduced annual grasses (IAG). The novel grassland state can be persistent, and more likely to promote large fires than the shrubland it replaced. But the mechanisms by which pre‐fire invasion and fire occurrence are linked to higher post‐fire flammability are not fully understood. A natural experiment to explore these interactions presented itself when we arrived in northern Nevada immediately after a 50,000 ha wildfire was extinguished. We hypothesized that the novel grassland state is maintained via a reinforcing feedback where higher fuel connectivity increases burn severity, which subsequently increases post‐fire IAG dispersal, seed survivorship, and fuel connectivity. We used a Bayesian joint species distribution model and structural equation model framework to assess the strength of the support for each element in this feedback pathway. We found that pre‐fire fuel connectivity increased burn severity and that higher burn severity had mostly positive effects on the occurrence of IAG and another non‐native species, and mostly negative or neutral relationships with all other species. Finally, we found that the abundance of IAG seeds in the seedbank immediately post‐fire had a positive effect on the fuel connectivity 3 years after fire, completing a positive feedback promoting IAG. These results demonstrate that the strength of the positive feedback is controlled by measurable characteristics of ecosystem structure, composition and disturbance. Further, each node in the loop is affected independently by multiple global change drivers. It is possible that these characteristics can be modeled to predict threshold behavior and inform management actions to mitigate or slow the establishment of the grass‐fire cycle, perhaps via targeted restoration applications or pre‐fire fuel treatments. This article is protected by copyright. All rights reserved.
... BCDF stands with sparse understory, although unlikely to burn at high intensity, are not ideal restoration sites because these areas are more arid. Conifer seedling establishment is sensitive to precipitation patterns (Varmola et al., 2000;Brown and Wu, 2005;Rother et al., 2015) so drier sites may have a lower chance of longterm survival of outplanted seedlings. By contrast, canyons and valleys are ideal restoration sites particularly if oaks are present, due to the increased access to water, which will be necessary for sapling survival. ...
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Big-cone Douglas-Fir ( Pseudotsuga macrocarpa , hereafter BCDF) is an endemic, fire-adapted conifer found throughout the mountains of southern California. Because recent large high intensity wildfires have resulted in loss of BCDF, understanding how environmental factors, such as topography, fuels, climate, and weather, impact BCDF survivorship is important for informing restoration and conservation efforts. Here, we used randomForest (RF) and accumulated local effects (ALE) plots to examine how environmental variables contribute to the occurrence of both fire refugia and high fire-induced mortality of BCDF stands during two large wildfires. Additionally, we explored how the influence of these variables changed between the use of two different response variables: (1) visually-assessed mortality evaluated through estimation of canopy survival using Google Earth imagery and (2) RdNBR. This comparison allows us to evaluate the potential that RdNBR overestimates BCDF mortality because it is highly indicative of understory conditions post-fire, rather than direct changes to BCDF trees. We found that pre-fire fuel was one of the most influential variables contributing to both fire refugia and high mortality; sparse and oak dominant understories contributed to fire refugia, while chaparral contributed to high mortality. We also found that the role of certain variables was not consistent across the two fires. For example, areas of the landscape with hotter temperature and higher vapor pressure deficit (VPD) during the fire experienced high BCDF mortality in the Zaca Fire, but had the inverse effect in the Thomas Fire. Lastly, we found that our two metrics of response resulted in significantly different classification of BCDF stands: RdNBR resulted in more stands being classified as high intensity and fewer low severity/unburned areas, supporting our concern that it can overestimate high severity impact in some ecosystems. However, the two model types resulted in relatively similar explanatory environmental variable selections, although different rankings.
... Descriptions of management responses to VTC from workshop participants along with case study examples Management response Description Case study examples Reverse change Actively try to reverse change via: • Coupled thinning and prescribed fire treatments to reduce fuel loads and fire severity and promote fire-dependent species and ecosystem recovery (Stephens et al. 2009) • Planting or seeding pre-VTC species • Removing or managing new or undesirable species (e.g., non-native grasses and shrubs that may increase fire frequency and/or severity) • Fire suppression to reduce fire extent and allow for recovery time • Preventing post-disturbance soil loss to sustain ecological functions 1. Klamath Reservation, southern Oregon 2. Southern Front Range, Colorado 3. Laguna Mountain, CaliforniaObserve change Take no active intervention measures and adopt monitoring to assess ecosystem trajectory over time. This approach may be most appropriate where there is:• Limited management capacity (e.g., high upfront and maintenance costs of active intervention, limitations to access in sites such as those in wilderness or roadless lands)(Rother et al. 2015; Aplet and Mckinley 2017) • High uncertainty of unintended consequences of active intervention (e.g., one workshop participant noted that "sometimes doing something is worse than doing nothing")(Landres 2010). This approach is consistent with restoration paradigms emphasizing a spectrum of approaches to spread risk (Aplet and Mckinley 2017). ...
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Background Forest and nonforest ecosystems of the western United States are experiencing major transformations in response to land-use change, climate warming, and their interactive effects with wildland fire. Some ecosystems are transitioning to persistent alternative types, hereafter called “vegetation type conversion” (VTC). VTC is one of the most pressing management issues in the southwestern US, yet current strategies to intervene and address change often use trial-and-error approaches devised after the fact. To better understand how to manage VTC, we gathered managers, scientists, and practitioners from across the southwestern US to collect their experiences with VTC challenges, management responses, and outcomes. Results Participants in two workshops provided 11 descriptive case studies and 61 examples of VTC from their own field observations. These experiences demonstrate the extent and complexity of ecological reorganization across the region. High-severity fire was the predominant driver of VTC in semi-arid coniferous forests. By a large margin, these forests converted to shrubland, with fewer conversions to native or non-native herbaceous communities. Chaparral and sagebrush areas nearly always converted to non-native grasses through interactions among land use, climate, and fire. Management interventions in VTC areas most often attempted to reverse changes, although we found that these efforts cover only a small portion of high-severity burn areas undergoing VTC. Some areas incurred long (>10 years) observational periods prior to initiating interventions. Efforts to facilitate VTC were rare, but could cover large spatial areas. Conclusions Our findings underscore that type conversion is a common outcome of high-severity wildland fire in the southwestern US. Ecosystem managers are frontline observers of these far-reaching and potentially persistent changes, making their experiences valuable in further developing intervention strategies and research agendas. As its drivers increase with climate change, VTC appears increasingly likely in many ecological contexts and may require management paradigms to transition as well. Approaches to VTC potentially include developing new models of desired conditions, the use of experimentation by managers, and broader implementation of adaptive management strategies. Continuing to support and develop science-manager partnerships and peer learning groups will help to shape our response to ongoing rapid ecological transformations.
... Altered climate conditions can favor different species once reorganization begins, if they are better adapted in terms of climate tolerance to persistent emerging conditions. For instance, field experiments evaluating seedling survival and growth under experimental warming indicate that many tree species could soon lose the capacity to regenerate in areas currently occupied by conspecific adults (Tercero-Bucardo et al. 2007, Rother et al. 2015. The degree to which these differences are projected to increase with increasing climatic warming becomes more pronounced with reduction of the buffering effect of the forest canopy, which may be expected under increasing rates of forest disturbance (Dobrowski et al. 2015, Wolf et al. 2021. ...
Ecosystems are dynamic systems with complex responses to environmental variation. In response to pervasive stressors of changing climate and disturbance regimes, many ecosystems are realigning rapidly across spatial scales, in many cases moving outside of their observed historical range of variation into alternative ecological states. In some cases, these new states are transitory and represent successional stages that may ultimately revert to the pre-disturbance condition; in other cases, alternative states are persistent and potentially self-reinforcing, especially under conditions of altered climate, disturbance regimes, and influences of non-native species. These reorganized states may appear novel, but reorganization is a characteristic ecosystem response to environmental variation that has been expressed and documented throughout the paleoecological record. Resilience, the ability of an ecosystem to recover or adapt following disturbance, is an emergent property that results from the expression of multiple mechanisms operating across levels of organism, population, and community. We outline a unifying framework of ecological resilience based on ecological mechanisms that lead to outcomes of persistence, recovery, and reorganization. Persistence is the ability of individuals to tolerate exposure to environmental stress, disturbance, or competitive interactions. As a direct expression of life history evolution and adaptation to environmental variation and stress, persistence is manifested most directly in survivorship and continued growth and reproduction of established individuals. When persistence has been overcome (e.g., following mortality from stress, disturbance, or both), populations must recover by reproduction. Recovery requires the establishment of new individuals from seed or other propagules following dispersal from the parent plant. When recovery fails to re-establish the pre-disturbance community, the ecosystem will assemble into a new state. Reorganization occurs along a gradient of magnitude, from changes in the relative dominance of species present in a community, to individual species replacements within an essentially intact community, to complete species turnover and shift to dominance by plants of different functional types, e.g. transition from forest to shrub or grass dominance. When this latter outcome is persistent and involves reinforcing mechanisms, the resulting state represents a vegetation type conversion (VTC), which in this framework represents an end member of reorganization processes. We explore reorganization in greater detail as this phase is increasingly observed but the least understood of the resilience responses. This resilience framework provides a direct and actionable basis for ecosystem management in a rapidly changing world, by targeting specific components of ecological response and managing for sustainable change.
... Davis et al., 2019). With global temperatures expected to increase by ≥2.5 • C over the next 50 years (IPCC, 2014) and summer VPD levels in the southwest to increase significantly (Ficklin and Novick, 2017), both inhibiting seedling regeneration (Rother et al., 2015), leveraging microclimate refugia created by small-scale vegetative shade, seedlings may be able to establish in areas that would otherwise have experienced ecological state-change because of the pervasive impacts of global climate warming (De Frenne et al., 2013). However, the capacity of microclimate refugia created through biotic means to persist in a warming and drying climate is uncertain (Davis et al., 2019). ...
High-severity wildfire in arid regions has caused ecological state change, transforming previously forested areas into shrublands. This dramatically alters the microclimatic conditions, which can exceed the climatic tolerance of tree seedlings, rendering the likelihood of returning post-wildlife landscapes to their previous state relatively low. Characterizing microclimatic variability across severely burned landscapes could allow for identifying locations where seedling survival is more likely. We used a combination of small unmanned aircraft system imagery, satellite data and in-situ microclimate data recordings, together with a machine learning approach, to model monthly near-ground minimum, mean and max temperature as well as relative humidity and vapor pressure deficit in a previously forested area, which is now dominated by two different shrub species. Spatially explicit models predicted recorded microclimate well (r = 0.73 to 0.97), and model projections highlight that at any given location in the hottest month, the solar buffering capacity of existing vegetation can alter the maximum temperature by ∼12 °C, increase relative humidity by ∼20% and reduced vapor pressure deficit by 0.3mbar relative to open areas. By harnessing these microclimate refugia, the success rate of reforestation efforts in post-wildfire landscapes could be substantially increased and mitigate seedlings from climate warming at local scales.
... Our study corroborates others reporting low root:shoot ratios for juvenile ponderosa pines (Kolb et al. 2016, Dixit and, and we report that juvenile rooting depth may actually be shallower than estimated in previous studies (Petrie et al. 2017). Based on our findings, we propose that juvenile ponderosa pines may be less able to compete for belowground resources (Elliott and White 1987) and survive in unfavorable semiarid locations and environmental conditions than previously estimated (Johnson et al. 2011, Rother et al. 2015. In post-disturbance environments, successful regeneration will become increasingly difficult due to the aboveground-focused growth strategy employed by juvenile ponderosa pines (Kemp et al. 2019), which may maximize carbon gain at the expense of reducing stress tolerance (Augustine and Reinhardt 2019). ...
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Abstract Juvenile tree survival will play an important role in the persistence of coniferous forests and woodlands in the southwestern United States (SWUS). Vulnerability to climatic and environmental stress declines as trees grow, such that larger, more deeply rooted juveniles are less likely to experience mortality. It is unclear how juvenile conifers partition the aboveground and belowground components of early growth, if growth differs between species and ecosystem types, and what environmental factors influence juvenile carbon allocation above‐ or belowground. We developed a novel data set for four juvenile conifer groups (junipers, piñon pines, ponderosa pines, firs; 1121 juveniles sampled, 221 destructively) in three height classes (
Technical Report
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This report is a synthesis of climate science relevant for management and planning for Colorado’s water resources. It focuses on observed climate trends, climate modeling, and projections of temperature, precipitation, snowpack, and streamflow. Climate projections are reported for the mid-21st century because this time frame is the focus of adaptation strategies being developed by the State of Colorado and other water entities.
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In the Colorado Front Range, disturbances and climatic variation influence stand structure of ponderosa pine (Pinus ponderosa) along the lower timberline ecotone. Over the past 100 years there has been a shift to a greater density and extent of ponderosa pine at the forest-grassland boundaries. Ponderosa pine regeneration at lower timberline appears to be influenced by fires in the 1860s and decreased grazing pressure in the 1970s-1980s. Climatic variation may also have influenced age structure, even though analyses of age structure at a 10-year class scale prevented the detection of climatic influences occurring at a finer scale. These changes in disturbance regimes, possibly together with moister springs/early summers, created favourable conditions for the increase in density and extent of ponderosa pine at lower timberline ecotone.
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Projected effects of climate change across many ecosystems globally include more frequent disturbance by fire and reduced plant growth due to warmer (and especially drier) conditions. Such changes affect species - particularly fire-intolerant woody plants - by simultaneously reducing recruitment, growth, and survival. Collectively, these mechanisms may narrow the fire interval window compatible with population persistence, driving species to extirpation or extinction. We present a conceptual model of these combined effects, based on synthesis of the known impacts of climate change and altered fire regimes on plant demography, and describe a syndrome we term "interval squeeze". This model predicts that interval squeeze will increase woody plant extinction risk and change ecosystem structure, composition, and carbon storage, especially in regions projected to become both warmer and drier. These predicted changes demand new approaches to fire management that will maximize the in situ adaptive capacity of species to respond to climate change and fire regime change.
The Fourmile Canyon Fire burned in the fall of 2010 in the Rocky Mountain Front Range adjacent to Boulder, Colorado. The fire occurred in steep, rugged terrain, primarily on privately owned mixed ponderosa pine and Douglas-fir forests. The fire started on September 6 when the humidity of the air was very dry (≈ <7%) and the winds were steadily blowing in the range of 15 miles per hour and gusting to over 40 miles per hour. These conditions prevailed for most of the first day when the fire burned approximately 5,700 acres and destroyed 162 homes. Because of the windy conditions, aircraft could not be used until late that first day. The first responders concentrated on evacuating the occupants of the 474 homes in the fire vicinity. No public or firefighters were injured during the course of the fire. This outcome was directly related to the excellent preparedness of Boulder County and, in particular, the Sheriff's Department and the local fire districts. Fuel treatments had previously been applied to several areas within the fire perimeter to modify fire behavior and/or burn severity if a wildfire was to occur. However, the fuel treatments had minimal impact in affecting how the fire burned or the damage it caused. After the initial day of intense burning and 4 additional days of relatively benign fire behavior, the Fourmile Canyon Fire had burned 6,181 acres and become one of the most damaging fires in Colorado's history. This report summarizes how the fire burned, the damage it caused, and offers insights to help the residents and first responders prepare for the next wildfire that will burn on the Colorado Front Range.
The influence of dry climates on white spruce (Picea glauca (Moench) Voss)) regeneration was examined by conducting surveys of seedlings and small trees that had regenerated naturally at 100 farm shelterbelts and plantations in southern Saskatchewan, Canada. The sites surveyed were located along a climate moisture gradient extending from the relatively moist boreal forest, across the aspen parkland, to the semi-arid prairie grasslands. Natural regeneration was greatest at sites in the boreal forest and northern aspen parkland, decreased in the southern aspen parkland, and was negligible in the grassland zone. Furthermore, the few seedlings found in the drier zones were usually in poor condition. Similar results were obtained for the introduced Colorado spruce (Picea pungens Engelm.) and Scots pine (Pinus sylvestris L.). It s concluded that the present climate of the southern parkland and grassland is too dry to permit natural regeneration of white spruce and other conifers. If increases in atmospheric CO2 levels lead to a drier future climate in the southern boreal forest of western Canada, the ability of conifers to regenerate naturally may be significantly reduced.
Climate change is expected to increase disturbances such as stand-replacing wildfire in many ecosystems, which have the potential to drive rapid turnover in ecological communities. Ecosystem recovery, and therefore maintenance of critical structures and functions (resilience), is likely to vary across environmental gradients such as moisture availability, but has received little study. We examined conifer regeneration a decade following complete stand-replacing wildfire in dry coniferous forests spanning a 700 m elevation gradient where low elevation sites had relatively high moisture stress due to the combination of high temperature and low precipitation. Conifer regeneration varied strongly across the elevation gradient, with little tree regeneration at warm and dry low elevation sites. Logistic regression models predicted rapid increases in regeneration across the elevation gradient for both seedlings of all conifer species and ponderosa pine seedlings individually. This pattern was especially pronounced for well-established seedlings ( >= 38 cm in height). Graminoids dominated lower elevation sites following wildfire, which may have added to moisture stress for seedlings due to competition for water. These results suggest moisture stress can be a critical factor limiting conifer regeneration following stand-replacing wildfire in dry coniferous forests, with predicted increases in temperature and drought in the coming century likely to increase the importance of moisture stress. Strongly moisture limited forested sites may fail to regenerate for extended periods after stand-replacing disturbance, suggesting these sites are high priorities for management intervention where maintaining forests is a priority.