Abstract and Figures

The coastal temperate rainforests of South and North America are part of the most biomass dense forest biome on the planet. They are also subject to rapid climatic shifts and, subsequently, new disturbance processes – snow loss-driven mortality and the emergence of fire in historically non-fire-exposed areas. Here, we compare and contrast Southern and Northern Hemisphere coastal temperate rainforests of the Americas, two of the largest examples of the biome, via synthesis of current literature, future climate expectations and new downscaling of a global fire model. In terms of snow loss, a rapid decline in winter snow is leading to mass mortality of certain conifer species in the Northern Hemisphere rainforests. High-elevation Southern Hemisphere forests, which are beginning to see similar declines in snow, may be vulnerable in the future, especially bogs and high-water content soils. Southern Hemisphere forests are seeing the invasion of fire as an ecological force at mid-to-high latitudes, a shift not yet observed in the north but which may become more prominent with ongoing climate change. We suggest that research should focus on the flammability of seral vegetation and bogs under future climate scenarios in both regions. By comparing these two drivers of change across similar gradients in the Northern and Southern Hemispheres, this work points to the potential for emerging change in unexpected places in both regions. There is a clear benefit to conceptualising the coastal temperate rainforests of the Americas as two examples of the biome which can inform the other, as change is proceeding in similar directions but at different rates in each region.
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Emergent freeze and re disturbance dynamics in
temperate rainforests
BRIAN BUMA,
1
* ENRIC BATLLORI,
2
SARAH BISBING,
3
ANDRES HOLZ,
4
SARI C. SAUNDERS,
5
ALLISON L. BIDLACK,
6
MEGAN K. CREUTZBURG,
7
DOMINICK A. DELLASALA,
8
DAVE GREGOVICH,
9
PAUL HENNON,
10
JOHN KRAPEK,
11
MAX A. MORITZ
12,13
AND KYLA ZARET
4
1
Department of Integrative Biology, University of Colorado, Denver, 1151 Arapahoe St., Denver, Colorado
80204, USA (Email: brian.buma@ucdenver.edu);
2
Universitat Aut
onoma de Barcelona, Cerdanyola del
Vall
es, Spain;
3
Department of Natural Resources & Environmental Science, University of Nevada Reno,
Reno, Nevada;
4
Department of Geography, Portland State University, Portland, Oregon, USA;
5
Coast
Area Research, BC Ministry of Forests, Lands, Natural Resource Operations, and Rural Development,
Nanaimo, British Columbia, Canada;
6
Alaska Coastal Rainforest Center, University of Alaska Southeast,
Juneau, Alaska;
7
Institute for Natural Resources, Oregon State University, Portland;
8
Geos Institute,
Ashland, Oregon;
9
Alaska Department of Fish and Game, Wildlife Conservation Division, Douglas;
10
USDA Forest Service, PNW Research Station;
11
Juneau Greens, Juneau, Alaska;
12
Agriculture and
Natural Resources Division, University of California Cooperative Extension; and
13
Bren School of
Environmental Science & Management, University of California, Santa Barbara, California, USA
Abstract The coastal temperate rainforests of South and North America are part of the most biomass dense for-
est biome on the planet. They are also subject to rapid climatic shifts and, subsequently, new disturbance processes
snow loss-driven mortality and the emergence of re in historically non-re-exposed areas. Here, we compare
and contrast Southern and Northern Hemisphere coastal temperate rainforests of the Americas, two of the largest
examples of the biome, via synthesis of current literature, future climate expectations and new downscaling of a
global re model. In terms of snow loss, a rapid decline in winter snow is leading to mass mortality of certain coni-
fer species in the Northern Hemisphere rainforests. High-elevation Southern Hemisphere forests, which are begin-
ning to see similar declines in snow, may be vulnerable in the future, especially bogs and high-water content soils.
Southern Hemisphere forests are seeing the invasion of re as an ecological force at mid-to-high latitudes, a shift
not yet observed in the north but which may become more prominent with ongoing climate change. We suggest
that research should focus on the ammability of seral vegetation and bogs under future climate scenarios in both
regions. By comparing these two drivers of change across similar gradients in the Northern and Southern Hemi-
spheres, this work points to the potential for emerging change in unexpected places in both regions. There is a clear
benet to conceptualising the coastal temperate rainforests of the Americas as two examples of the biome which
can inform the other, as change is proceeding in similar directions but at different rates in each region.
Abstract in Spanish is available with online material.
Key words: climate change, coastal temperate rainforest, emerging disturbance regimes, re, snow loss.
INTRODUCTION
Climate change is affecting global forests in multiple
ways, often by altering the abiotic conditions forests
experience. Direct effects include increasing water
stress and associated drought-induced tree mortality
(Adams et al. 2009; Holz et al. 2017), CO
2
fertilisa-
tion (Bolker et al. 1995) or lengthening of the grow-
ing season (Cleland et al. 2007), with these effects
leading to altered productivity and/or range shifts
(Krapek & Buma 2018). These changes may be
punctuated, resulting from the crossing of climatic
thresholds that drive major ecological changes related
to species physiological tolerances (e.g. Allen et al.
2010). The existence of environmental thresholds
may also cause shifts in or intensication of distur-
bance regimes (Brooks et al. 2004; Veblen et al.
2011; Buma 2015; Millar & Stephenson 2015),
which can lead to sudden changes in ecosystem type
when disturbance severity or frequency exceeds spe-
cies, community and ecosystem tolerances (Buma &
Wessman 2011). As a result, signicant shifts in eco-
logical relationships such as the establishment of
*Corresponding author.
Accepted for publication February 2019.
© 2019 Ecological Society of Australia doi:10.1111/aec.12751
Austral Ecology (2019) ,
novel functional relationships (Gilman et al. 2010) or
the formation of no-analogue communities (Williams
& Jackson 2007) are widely anticipated.
Precipitation including its phase, amount, intensity
and timing is one such climate factor expected to
potentially drive signicant, threshold-like change. The
phase of precipitation (snow or rain) represents only a
small shift in winter temperatures but results in a very
large change in the physical environment that forests
inhabit. For example, the loss of a winter snowpack
results in decreased soil insulation in winter (Groffman
et al. 2001), loss of nival habitat (Pauli et al. 2013),
altered plant communities (Bannister et al. 2005) and
reduced summer streamows (e.g. Mote 2003), among
other factors. Changes in the amount and timing of
precipitation also have major impacts. Several research-
ers have focused on forest health in relation to precipi-
tation-associated physiological stress (Anderegg et al.
2013), winter high ow/summer low ow events (Sur-
eet & Tullos 2013), and erosion and ooding severity
and timing (Klos et al. 2014).
Fire is also a well-known driver of rapid threshold-
like change. Fire is a major disturbance agent in most
of the worlds forests (Rundel 1981; Scott 2000),
burning ~348 Mha annually (Bowman et al. 2009;
Giglio et al. 2013) and inuencing vegetation (He
et al. 2016), soil (Certini 2014), and a host of other
ecological aspects of forest ecosystems. Fire occur-
rence responds rapidly due to climate or anthro-
pogenic activity (Doerr & Sant
ın 2016). Because of its
near ubiquity, re can be difcult to disentangle from
the baseline functioning of the forest (Pausas & Keeley
2009). When excluded via modelling, it is apparent
that whole biome distributions likely result from the
presence (or absence) of re (Bond et al. 2005). Antic-
ipating changes to re regimes especially the emer-
gence of re in areas where it was not historically
present, or rare enough to not be an evolutionary force
is important for management, conservation and pre-
diction of future ecosystem dynamics and functioning.
Both snow loss and re regime shifts are major con-
cerns to forest managers, conservationists, researchers
and culture bearers in forests worldwide. Here, we illus-
trate how the loss of snow and the emergence of re
may act as important agents of change. We focus on
coastal temperate rainforests, a globally important biome
that holds an immense amount of carbon in relatively
intact forests (Keith et al. 2009), where snow loss is pro-
ceeding faster than anywhere else globally and where the
re regime is expected to intensify or even emerge in
areas where it was previously essentially absent.
Objectives
The objectives of this review are to (i) synthesise and
describe the signicance of crossing precipitation and
re thresholds to the ecology and functioning of this
globally important forest biome; (ii) discuss the spa-
tial pattern of potential change within the regions;
and (iii) use the analysis of drivers of the new distur-
bances in this ecosystem to anticipate new dynamics
in other temperate rainforest systems. We hope to
draw attention to the benets of considering the
southern and northern coastal temperate rainforest as
two regions which can inform each other via direct
comparison.
COASTAL TEMPERATE RAINFORESTS: A
FRONTIER OF CHANGE
Coastal temperate rainforests (hereafter CTRFs) are
globally important as the most carbon-dense forested
areas on the planet, containing upwards of 1867 tons
Cha
1
(Australian CTRFs, Keith et al. 2009), a
result of generally low rates of decomposition, low
water stress, moderate climate and relatively long
growing seasons. They provide a multitude of ecosys-
tem services, from signicant cultural resources to
wildlife habitat, and function as the headwaters of
globally signicant sheries (Brandt et al. 2014;
Rodriguez-Echeverry et al. 2018). These regions are
also associated with high endemic biodiversity (e.g.
South American forests) in terms of nonvascular
plants and lichens (DellaSala 2011) and relatively
low levels of human development in many places.
Despite occurring across a wide range of latitudes
(~30°north to south), CTRFs have relatively consis-
tent, moderate climate conditions due to their close
proximity to the ocean (Alaback 1991; DellaSala
2011; Fig. 1). The mild, consistent climatic condi-
tions favour evergreen tree species, ne-scale, infre-
quent disturbances and generally older, late-
successional forests over much of the landscape.
Average annual temperatures range between 4 and
12°C, with annual precipitation from as low as 1.5 m
to as high as 5 m or more in some areas (DellaSala
2011). The historic disturbance regime was domi-
nated by relatively frequent tectonic activity, land-
slides, windstorms, ne-scale tree mortality and
infrequent res (Veblen & Alaback 1996; Buma &
Barrett 2015; Holz et al. 2016). Although observed
and projected absolute warming rates in CTRFs are
not as high as in polar or high-latitude interior
regions, warming temperatures are crossing key cli-
matic and ecological thresholds (Veblen et al. 2011;
Shanley et al. 2015), notably: (i) a phase change from
snow to rain as mean winter temperatures cross the
0°C threshold and (ii) the emergence or increased
role of re on the landscape (e.g. see DellaSala et al.
2018).
The snow-to-rain transition is an abrupt physical
threshold driven directly by temperature. This
doi:10.1111/aec.12751 © 2019 Ecological Society of Australia
2B.BUMAET AL.
fundamental shift in hydrology inuences a variety of
ecosystem properties and underlying processes,
including subnival habitat (Pauli et al. 2013) and
snow disturbance dynamics (Hennon et al. 2016).
Large areas of high-latitude CTRFs are at or near
the 0°C isotherm during winter, meaning that precip-
itation usually falls as snow. This climatological loca-
tion makes CTRF snow regimes especially vulnerable
to a loss of days below freezing given even minimal
warming (Meehl et al. 2004), as illustrated by the
crossing of the snow-to-rain threshold already
reported in portions of the biome (Buma 2018).
Historically, re in CTRFs was very infrequent,
although large in extent, at lower latitudes (~40°
50°) but very rare at higher latitudes (>55°) due to
climatological constraints, especially the relatively wet
summer periods and limited natural ignition sources
(Veblen & Alaback 1996; Kitzberger et al. 2016).
Areas with continuous human habitation had a more
frequent re regime associated with land manage-
ment (Hoffman et al. 2016; M
endez et al. 2016),
though many mid- to high-latitude locations had re
return intervals >100010 000 years (Veblen & Ala-
back 1996; Gavin et al. 2003). At lower latitudes, re
was rare but a signicant driver of landscape pattern
(e.g. Washington State, USA, Agee 1993; Gavin
et al. 2007). Paleoecological records from the higher
latitude portions of North Americas CTRF (>54°N)
have recorded essentially no charcoal since approxi-
mately 7500 years before present and no widespread
res since the Holocene Climatic Optimum (Baichtal
et al. 2008). The anticipated general increase in re
activity at temperate latitudes worldwide, where fuel
is abundant, is tied to lower moisture availability
resulting from predicted higher temperatures,
reduced precipitation and/or longer re season
Fig. 1. The North Pacic coastal temperate rainforest (panel a) and the South Pacic CTR (panel b). The focus of the
comparison is on the perhumid and seasonal zones of both forests. Biome map from DellaSala (2011).
© 2019 Ecological Society of Australia doi:10.1111/aec.12751
TEMPERATE RAINFOREST DYNAMICS 3
(Westerling et al. 2006; Bowman et al. 2009; Moritz
et al. 2012; Abatzoglou et al. 2017). In addition,
increasing temperatures generate higher water decits
(via increased evapotranspiration) even under poten-
tially increased rainfall at higher latitudes. The lack
of historical re exposure combined with a likely
increase in future re activity makes CTRFs poten-
tially vulnerable to novel change.
In sum, the particular climate and disturbance
state spacein CTRFs makes them an ideal biome
for studying how climate change-induced shifts in
winter precipitation phase and the emergence of re
have direct and indirect, long-lasting consequences
for ecosystem structure and function. Coastal tem-
perate rainforests can serve as model systems for
developing predictions about future changes in other
forested regions, which are expected to undergo simi-
lar snow-to-rain and re regime shifts in the future
(Bowman et al. 2014; Holz et al. 2016).
We take advantage of the fact that the two largest
examples of the biome, the North Paciccoastal
temperate rainforest (NPCTR) of North America
and the South Pacic coastal temperate rainforests
(SPCTR) of Chilean South America (27.3 and
12.5 million ha, respectively), together represent
50.7% of all global temperate rainforests (DellaSala
2011). We focus on the perhumid and seasonal
portions, which straddle the snowrain and re-pre-
sence/absence thresholds (Fig. 1; Veblen & Alaback
1996). Parallel characteristics result from adjacency
to the highly moderating, cool Pacicmaritime
environment and continuously wet conditions that
favour dense, contiguous evergreen forests (needle-
leaf in the Northern Hemisphere and broadleaf in
the Southern Hemisphere) and a large abundance
of temperate, peat-accumulating wetlands in areas
of poor drainage. Both have high carbon (C)
stocks, with slightly more in the Northern than the
Southern Hemisphere (NPCTR: 568794 tons
Cha
1
,SPCTR:326571 tons C ha
1
; studies
synthesised in Keith et al. 2009). Both regions of
the CTRF exhibit a gradient of human impacts,
from signicant development/land cover change at
lower latitudes to more intact landscapes at higher
latitudes (DellaSala 2011). While the higher lati-
tude areas are certainly impacted by extensive his-
torical logging, often targeting large trees and
vulnerable portions of the ecosystem (ood plain/ri-
parian zones; Albert & Schoen 2013) with impor-
tant negative impacts on sh, wildlife and habitat
(Beier et al. 2008a), they still retain relatively high
value in regard to cultural resources and biodiver-
sity among other ecosystem services (Brandt et al.
2014; Rodriguez-Echeverry et al. 2018). The long
northsouth latitudinal extent of these CTRFs
means that climate change is asynchronous across
this gradient, with poleward regions warming faster
and equatorward regions experiencing more rapid
precipitation changes (IPCC 2014).
In contrast to many other well-studied biomes
experiencing rapid changes in precipitation regimes
and re dynamics (e.g. Serreze et al. 2000), CTRFs
are useful study systems in that climate change
impacts can be examined concurrently and compara-
tively in both the Northern and Southern Hemi-
spheres. A threshold may be crossed earlier in one
region than the other, providing insights into how the
slower changing area might respond. Comparative
studies can provide a more robust test for hypothe-
sised mechanisms and an indication of potential cli-
mate change impacts on local climatology and
ecosystem responses to these and to other distur-
bances (Alaback 1991). We take advantage of that
cross-comparison in the following discussion.
LOSS OF SNOW AND EMERGING FREEZE
DISTURBANCE
Among the most visible environmental changes asso-
ciated with a warming climate in these two regions
are the reduced depth, extent and persistence of
snow as average winter temperatures cross the rain
snow threshold of 0°C (Fig. 2). In CTRFs, where
precipitation is ample year-round, precipitation phase
and subsequent routing of runoff play important
roles in forest functioning (Bisbing et al. 2016); this
change in phase may be a more signicant shift than
any absolute change in the overall amount or season-
ality. Because the CTRF regions have historically
occurred along the 0°C isotherm in winter, the num-
ber of snow-covered days, there is decreasing faster
than in any other biome (Meehl et al. 2004). Cur-
rently, the rainsnow boundary bisects the NPCTR,
starting at approximately 2000 m by 50
o
N and
reaching sea level around 57
o
N (Shanley et al.
2015). The more maritime southern SPCTR in
Patagonia is generally already above this threshold at
low elevations, and snow is transient in those areas,
though areas at higher latitudes and elevations do
remain snow and ice covered. While, to our knowl-
edge, no research has been done on long-term
changes in snow persistence in those higher elevation
areas of the SPCTR, satellite-based observations in
the Andes just north of the region (from ~28°to
36.5°S) identied signicant declines in duration of
snow persistence, approaching 23% per year, with
the rate of snow loss highest at higher latitudes
(Saavedra et al. 2018). Additionally, various ice caps
in the region have been retreating rapidly over the
last several decades, attributed to warming and sub-
sequent raising of the 0°C isotherm (Davies & Glas-
ser 2012), and this is expected to continue over the
next decades (Fig. 2). Thus, research of climate
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4B.BUMAET AL.
change effects on the rainsnow threshold in the
NPCTR may be instructive to the less studied snow
dynamic SPCTR.
Observed effects of snow loss
The crossing of the rainsnow threshold can have
signicant ecological effects, especially among species
adapted to reliable winter snow environments. In the
NPCTR, yellow-cedar (Callitropsis nootkatensis)is
experiencing extensive mortality over 9°of latitude
(>400 000 ha; Buma et al. 2017). This species has a
competitive strategy of ne root growth in the early
spring when supplies of nitrogen are abundant in
upper soil layers (Hennon et al. 2016); however, ne
root death can occur during subfreezing weather
events when snow is no longer present (Hennon
et al. 2016). Snow is an effective insulator for soils,
buffering soil temperatures from atmospheric vari-
ability; a lack of snow generally leads to colder soils
in winter months (Groffman et al. 2001) and an
overall increase in soil temperature variability (Jungq-
vist et al. 2014). Even in a warming climate with less
snow, sporadic cold weather events have persisted in
portions of the NPCTR (Beier et al. 2008b; Buma
2018), driving continued tree mortality. Ongoing
mortality is likely to lead to shifts in community com-
position to a smaller suite of species more tolerant of
snow-free winter conditions (Oakes et al. 2014). But,
tree mortality is not the only effect of increasing soil
freezing. Mobilisation of contaminants (Mohanty
Fig. 2. Anticipated shifts in winter snow threshold in the South Pacic CTR (SPCTR) and North Pacic coastal temperate
rainforest (NPCTR) using the HadGEM2-ES (RCP 8.5) climate model/emission scenario. No areas are expected to shift
from above to below freezing. Due to substantial lower elevation/slope areas in the NPCTR, the spatial extent of change is
large. In the SPCTR, shifts are likely along the higher altitudinal range edge throughout the forest, and the inset shows a sub-
section in detail to illustrate this pattern. Climate data from Hijmans et al. (2005) at 1-km resolution, HadGEM2-ES GCM.
© 2019 Ecological Society of Australia doi:10.1111/aec.12751
TEMPERATE RAINFOREST DYNAMICS 5
et al. 2014) and microbial communities (Larsen et al.
2002) can drive signicant changes to nutrient
cycling (Fitzhugh et al. 2001; Urakawa et al. 2014).
These changes, triggered by increasing freezethaw
dynamics in soils, may have signicant downstream
effects as well.
There are multiple pathways by which snow loss
might cause continued plant mortality in the NPCTR
and at higher elevations in SPCTR forests. Climate
change is altering the phenology of forest species, lead-
ing to increased risk of cold-related damage in a war-
mer world (Gu et al. 2008; Rigby & Porporato 2008).
The general ecological strategy of early spring activity
as a means to gain competitive advantage is wide-
spread (Polgar & Primack 2011). Apart from root
freezing, warmer temperatures speed plant develop-
ment earlier each year, making them vulnerable to
frost (Gu et al. 2008). Increased cold damage associ-
ated with climate warming and earlier spring pheno-
logical development is well recognised (Gu et al. 2008;
Inouye 2008) and is typically associated with above-
ground bud mortality. Broadscale damage to sensitive
bud tissue has already been noted in a variety of loca-
tions (Inouye 2008). The threat of root freezing due to
a lack of snow has not been generally quantied out-
side of NPCTR, but it is expected to be a signicant
factor in areas where snow cover will shift from contin-
uous to transient in temperate zones (Bannister et al.
2005). The risk would be highest in areas of the land-
scape prone to shallow rooting habits (e.g. wetlands
and bogs). The most vulnerable species are likely to be
those adapted to early-onset seasonal growth that his-
torically occurred under reliable cover of snow.
It is possible that subfreezing-induced damages
could decline after the transitional period, when tem-
peratures rise above the freezethaw boundary
(Henry 2008, Buma 2018), but this is dependent on
freezing probability corresponding to average temper-
atures as they have in the past. Given the topography
of both regions, with signicantly colder areas located
in close geographic proximity to these ecosystems
(on the eastern sides of the Andes in South America
and Coast Mountains of the United States and
Canada), it is unclear whether that relationship will
hold (Beier et al. 2008b). Areas of historically thin
snowpack and winter temperature slightly below
freezing in the SPCTR should be monitored for plant
stress, root mortality and other emerging dynamics
suggested by the NPCTR decline.
THE EXPANSION OF FIRE
The combination of decreasing winter snowpack
resulting from precipitation phase change, earlier
snowmelt and thaw, and increasing springsummer
evaporative demand (e.g. vapour pressure decit) over
longer, rainfall-free growing seasons is likely to result
in an increase in the prevalence of re in certain por-
tions of CTRFs (Westerling et al. 2006; Littell et al.
2010; Moritz et al. 2012). As noted, the historic
importance of re in CTRFs has been relatively low,
ranging from a nearly nonexistent re regime at higher
latitudes (Gavin et al. 2003) to infrequent, though
stand-replacing, res at lower latitudes (Agee 1993;
Veblen & Alaback 1996; Holz et al. 2012; Walsh et al.
2015; Hoffman et al. 2016; Fig. 3). Although there is
a substantial amount of highly contiguous vegetation
that may act as fuel for a re in CTRFs, vegetation is
typically too wet to burn, and natural ignitions are very
infrequent, as lighting is rare and generally accompa-
nied by rain. Overall, re across both CTRF regions
has historically been driven by the conuence of atmo-
spheric circulation patterns cycling at multiple tempo-
ral scales (Whitlock et al. 2008; Littell et al. 2010;
Holz et al. 2017) and human presence (Hoffman et al.
2016; M
endez et al. 2016).
Under climate change projections, the poleward
portions (>55 °N and >50
o
S) of both the NPCTR
and the SPCTR are expected to experience signi-
cant annual warming, lower snowpack, a potential
increase in springsummer drought (Veblen et al.
2011) and increasing moisture decits (4050% at
~60°N; Hauer 2010). As a result, the equatorward
portions of CTRFs are expected to become more
ammable throughout the re season (e.g. summer;
Fig. 4), leading to more ammable conditions (Littell
et al. 2010; Sheehan et al. 2015). Additionally, war-
mer coastal ocean temperatures are likely to create
the potential for more ignitions via increased light-
ning activity (Garreaud et al. 2014). In several of the
drier and mountainous areas in the SPCTR,
increases in lightning-set res have been observed in
recent decades (Veblen et al. 2011), and increases in
the frequency of dry, warm periods have been linked
to global climate warming and ozone depletion in
Antarctica (Holz & Veblen 2011; Holz et al. 2017).
Recent research has identied the southern SPCTR
as an area where wildres might invadeas soon as
~2039 if ignitions are provided (Moritz et al. 2012).
Downscaled re projections
Unlike global data related to the winter snowrain
temperature threshold (e.g. Meehl et al. 2004; Buma
et al. 2017), comparable, high spatial-resolution re
modelling studies do not currently exist for northern
and southern CTRFs. For the purposes of this syn-
thesis, we used the global modelling framework of
Moritz et al. (2012) to evaluate changes in climate-
driven probability of re across these regions. This
framework integrates global re datasets (NASA
MODIS missions) and environmental covariates
doi:10.1111/aec.12751 © 2019 Ecological Society of Australia
6B.BUMAET AL.
representing re-conducive climate conditions over
the reference period 19712000 to determine re
climate relationships and assess the likelihood of re
under ongoing climate change. We used the frame-
work to build spatial statistical models of relative
changes to re probability that describe the long-
term potential of re occurrence over the period
20712100. Using methods based on Moritz et al.
(2012), we used projected changes in temperature
seasonality, precipitation of the driest month and
annual precipitation to describe potential alterations
in the probability of re over 0.5°latitudinal bands
across CTRFs (Fig. 5). Projected future climate
data were obtained from the WorldClim CMIP5
HadGEM2-ES (RCP 8.5; Hijmans et al. 2005,
updated to CMIP5 in 2017) for the study region
(for full details, see Appendix S1S3).
Because the scope of this synthesis is focused on
relative changes, modelling was limited to the Hadley
climate projection. The Hadley model performed the
best in aggregate when compared to climate-precipi-
tation values for ve other GCMs at the seasonal
level for the North Pacic forest region (SNAP
2009). To check the assumption that the climatere
relationships quantied in Moritz et al. (2012) would
remain valid with the Hadley climatic dataset, corre-
lations between the WorldClim CMIP5 HadGEM2-
ES data (used here) and CMIP3 data used in Moritz
et al. (2012) were computed. We found very high
Spearman rank coefcients for the variables used in
the model (>0.9; Appendix S3) and considered the
climatere relationships from the Moritz et al.
(2012) ensemble re model suitable to assess poten-
tial relative alterations in the CTRFslikelihood of
Fig. 3. Yearly burned area, cumulative burned area (per 0.25
0
) and per cent burned (mean fraction of each pixel burned per
0.25
0
) from 1997 to 2016 in the South Pacic CTR and North Pacic coastal temperate rainforest. Fire statistics come from the
Global Fire Emissions Database version 4 (Giglio et al. 2013).
© 2019 Ecological Society of Australia doi:10.1111/aec.12751
TEMPERATE RAINFOREST DYNAMICS 7
re for the period 20712100 driven by HadGEM2-
ES climate projections. The purpose of this exercise
was not to predict absolute changes in re probability
but rather to identify the portions of each region
likely to see the largest relative change. For further
details associated with this modelling methodology,
see Appendix S1 and Moritz et al. (2012).
While the frequency of dry, warm conditions
increases at low latitudes in both regions (where the
majority of re modelling work has been concen-
trated, for example Littell et al. 2010), the modelling
results suggest that the central latitude portions of
the biome in both hemispheres will see the largest
relative increase in climate-driven re probability.
This is primarily due to projected changes in dry sea-
son precipitation (Fig. 5). The more equatorward
portions of the biome will likely have higher rates of
re due to their higher baseline rates. However, a lar-
ger relative increase in re activity indicates a more
substantial departure from historical norms and is
thus worth noting for future estimates of CTRF
dynamics.
It should also be noted that these models are based
on broadscale climatic trends and reect general cli-
matere relationships; the actual occurrence and
behaviour of res at regional and local scales is a
result of ner scale weather patterns, topo-edaphic
gradients and vegetation-re feedbacks as well.
Effects of re regime changes in the NPCTR
Fires have occurred historically in the southern and
central portions of the NPCTR but with highly
Fig. 4. Projected precipitation change. Relative change in summer precipitation (dened as warmest quarter of the year) by
2070 for the North Pacic coastal temperate rainforest (left) and the South Pacic CTR (right) using the HadGEM2-ES
(RCP 8.5) climate model/emission scenario. Climate data from Hijmans et al. (2005) at 1-km resolution, HadGEM2-ES
GCM (updated to CMIP5, see http://www.worldclim.org).
doi:10.1111/aec.12751 © 2019 Ecological Society of Australia
8B.BUMAET AL.
Fig. 5. Relative change in re probability projected as a function of the major climate drivers in the North Pacic coastal tem-
perate rainforest (NPCTR; top) and the South Pacic CTR (SPCTR; bottom) using the 2070 HadGEM2-ES (RCP 8.5) climate
model/emission scenario. A negative (green or blue) value indicates the driver is expected to change in a direction that reduces
relative re probability, and a positive value (orange or red) indicates an increase in re probability. Agreement between predic-
tors: 100% indicates the three drivers had similar signs (i.e. all were positive), whereas 67% indicates that two-thirds were in
agreement for either an increase (I) or a decrease (D) in re probability. In general, there is strong agreement that re frequency
will increase throughout the SPCTR. Expectations are mixed in the NPCTR, but generally an increase is expected further north
than historical res. The extreme southern portion of the NPCTR is not modelled due to a lack of climate data at the proper
scale; for climate locations used to create the probability graphs, see Fig. S1. Note differences of scale of axes and legends.
© 2019 Ecological Society of Australia doi:10.1111/aec.12751
TEMPERATE RAINFOREST DYNAMICS 9
variable return intervals (3003000+years, Fig. 4)
and spatial heterogeneity due to variable ignitions
and climatic conditions (Hoffman et al. 2018). Thin-
barked, non-serotinous species dominate CTRFs,
and re-adapted species are generally absent (Veblen
& Alaback 1996). In the southern NPCTR (<55
o
N),
where re has an infrequent but more signicant role
(Agee 1993; Tepley et al. 2013; Whitlock et al.
2014), a few seral thick-barked species occur (e.g.
Douglas-r, Pseudotsuga menziesii) that can survive
low-to-moderate re intensities (Agee 1993).
Anticipating the effects of both increasing and
emerging re (at lower and higher latitudes, respec-
tively) is critical (Littell et al. 2010). Theory suggests
that in wet systems, where ammability declines as
forest structure develops, the introduction of re-
conducive conditions can lead to a cycle of increasing
re extents and subsequent widespread ecological
changes. This positive feedback occurs because
increases in the spatial extent and connectivity of the
more ammable, early seral vegetation after each re
event lead, subsequently, to more extensive res.
Given sufcient ignition opportunities and a climate
conducive to periodic re (Perry et al. 2012), rapid
and persistent threshold-like changes can occur when
forests at the landscape scale cross critical am-
mable-connectivity thresholds and any re event is
likely to spread over the majority of the landscape.
Increased NPCTR ammability associated with early
successional species can lead to positive re-vegeta-
tion feedbacks (Agee & Huff 1987) due to highly
ammable early successional ne fuels that dry
rapidly even in the relatively short re season com-
mon to the regions climate. Thus, the emergence or
acceleration of re regimes is a signicant concern
and changes projected here should be considered
conservative estimates. Recent events in mesic forest
stands in the southern portions of the NPCTR (e.g.
Eagle Creek Fire in the Columbia Gorge Scenic Area
in Oregon and Norse Peak Fire in Washington, from
a human ignition) remind us that the transformation
of these forests by re is likely as a potential result of
the warming regional climate. To this point, the his-
torical and already-underway expansion of re into
the SPCTR is instructive for the NPCTR.
The expansion of re in the SPCTR
The expansion of re is well documented in the
SPCTR. Historically, humans were the ignition
source for most SPCTR res (Holz et al. 2016), with
ignitions occurring primarily at the warmest and dri-
est equatorward edge. Since the 1970s, there has
been an increase in lightning-ignited res in the
northern and central regions of the SPCTR (Veblen
et al. 2011), attributed to environmental shifts
associated with climate change (Thompson et al.
2011, IPCC 2014, Garreaud et al. 2014). As biomass
is not limiting and the climate is becoming more
conducive to re, the occurrence of re is increas-
ingly ignition limited (Paritsis et al. 2013). Emerging
res tied to climate change and increasing variability
in climate have already been transforming ecological
composition, structure and function, particularly in
re-sensitive Pilgerodendron forests (Holz & Veblen
2009; Bannister et al. 2012). These species are
mostly re sensitive, and the landscape has been rela-
tively nonammable historically but is becoming
less so today.
In addition to climate change driving increases in
re probability, tree plantations in the SPCTR have
the potential to increase re risk. Pine and euca-
lypts plantations, like other plantations in the
southern NPCTR (Zald & Dunn 2018), facilitate
re spread due to homogeneous patch structure
and connectivity, which in turn can result in higher
re frequency and severity (McWethy et al. 2018;
Paritsis et al. 2018). While many historical and
recent large res in Chile occurred just north of
the CTRF region in the more Mediterranean cen-
tral valley, there are plantations of Eucalypts in the
SPCTR as far south as 42
0
on Isla Chilo
e, and at
the Patagonian dry forest/steppe ecotone lodgepole
pine (Pinus contorta var. latifolia), a re-associated
species has established from plantations even fur-
ther south (at least 45.5
0
; Taylor et al. 2017).
There are concerns that invasion by re-adapted
species may alter the water balance, fuel type (Tng
et al. 2012) and fuel structures (Cobar-Carranza
et al. 2014) of the region.
When res do occur, they can cause signicant
changes to forests and have the potential to initiate
positive feedbacks that drive further increases in re
frequency (e.g. Paritsis et al. 2013; Taylor et al.
2017). In the SPCTR, invasion of shade intolerant
Sphagnum species can result in subsequent waterlog-
ging of the habitat due to an overall decline in evapo-
transpiration (D
ıaz et al. 2007). Sphagnum mosses,
which dominate wetlands in the NPCTR as well,
acidify substrates, outcompete tree seedlings, trans-
form the plant community and potentially lock the
system into an alternative stable state (Kitzberger
et al. 2016; Zaret & Holz 2016). Preliminary results
suggest that: (i) water table and substrate interact
and best explain patterns in post-re tree seedling
abundance and plant community; (ii) small seedling
abundance is best explained by water-table height
and plant community type (Zaret & Holz 2016); and
(iii) ne-fuel bric peats are more likely to support a
high-frequency, low-severity re regime given an
amenable climate for re resulting in a positive feed-
back between re and vegetation ammability (Holz
2009, Kitzberger et al. 2016).
doi:10.1111/aec.12751 © 2019 Ecological Society of Australia
10 B. BU M A ET AL.
This observation of a rapid ecosystem shift trig-
gered by the emergence of re and explained by the
expansion of more ammable, early seral vegetation
coupled with increasingly favourable climates for re
is clearly instructive to the NPCTR. It also echoes
observations from paleoecological reconstructions in
temperate New Zealand (Perry et al. 2012) and
recent events in Tasmanian temperate rainforests,
where unprecedented res in 2015 and 2016 burned
re-sensitive trees as well as vast tracks of peatland
and alpine vegetation (~105 000 ha) during the driest
season on record (Marris 2016). That the largest rel-
ative change in re probability is not anticipated at
the equatorward edge of the biome, but rather in the
middle and at higher latitudes, is an unexpected nd-
ing and suggests research should consider emerging
re regimes beyond the drier, equatorward extents
typically considered where adaptations to re are
minimal and resilience potentially low. The story of
re emergence clearly illustrates the value of linking
the NPCTR and the SPCTR in a single analysis, as
physical drivers are changing in similar directions,
but at different times, in both areas the emergence
of re in the south is a valuable case study for the
forests in the north.
THE EMERGING BIOME EDGE
Much of the concern regarding climate change-dri-
ven mortality centres on changes at the trailing
(lower latitude or elevation; Parmesan & Yohe 2003)
or leading edges (higher latitude or higher elevation;
Mason et al. 2015) of species and biome distribu-
tions. Generally, discussion in temperate regions has
focused on increasing temperatures and declining
precipitation on the southern boundary, as both are
known to strongly structure species range edges and
biome extents. At broad scales, species and biomes
do generally track long-term climate conditions such
as mean winter temperatures or annual water bal-
ance, though there may be lag after major climatic
shifts (e.g. Ice Ages) due to slow migration rates
(Krapek & Buma 2018). In the future, however,
there is the potential for thresholds in speciestoler-
ances to be crossed elsewhere within the current
range of a forest ecosystem, because climate warming
and precipitation changes are not synchronous and
occur at different rates. Edgesof climatic tolerance
may emerge within central portions of a range due to
the intersection of climatic trends with important
physical thresholds or biological tolerances.
The NPCTR and SPCTR regions demonstrate sig-
nicant, climate change-driven changes occurring near
the geographic middle of a biome not just on the
lower latitude portions. Loss of snow is causing the
most signicant ecological changes in the geographic
middle of the NPCTR, where snow was reliably pre-
sent but winter mean temperatures were near 0°
(Buma et al. 2017). Higher elevations of the SPCTR
and surrounding ecosystems are likely to be similarly
susceptible in the future, as snow loss (although mini-
mal in absolute terms) is reported in the central por-
tion (Fig. 2). Further research on the role of snowpack
changes in determining species ranges is a signicant
need (Pauli et al. 2013), either via direct mortality as
in the case of yellow-cedar or via interactions with
other stressors and disturbance agents (e.g. Poulos
2014). Similarly, the highest increase in relative re
likelihood is in the geographic middle latitudes of the
biome in both hemispheres, where re was historically
rare and species are not well adapted to re (this anal-
ysis, Fig. 5). Fire may emerge in unexpected loca-
tions, and the emergence rather than simple
intensication is also a major research need, espe-
cially in landscapes where theory suggests rapid trans-
formations due to seral changes in ammability.
Temperate rainforests, by virtue of their long lati-
tudinal extent, exemplify these emerging edge phe-
nomena at both the species and ecosystem levels.
This suggests that climate change monitoring in a
variety of regions should focus not only on leading or
lagging edges, but also emerging edges driven by cli-
matic shifts like precipitation phase.
CONCLUSIONS
The objectives of this synthesis and review were to
draw attention to emerging disturbance phenomena
in the coastal temperate rainforests of the Pacic
Coast, discuss the causes and effects of those phe-
nomena and utilise the cross-hemispheric comparison
to enable more general predictions about change
than can be done from single-system studies. This
comparative examination of climateecosystem rela-
tionships across hemispheres of the CTRFs provides
a framework within which to hypothesise the nature,
geographic location and potential effects of emergent
disturbances within similar systems.
The counter-intuitive nature of the processes being
observed root freezing due to warming and re in
wet forests makes their prediction more difcult but
the broad nature of the changes underway underlines
the signicance of these emergent trends. In particu-
lar, the sensitivity of forests to root freezing mortality
in areas where snow will become transient should be
investigated in other systems, especially those prone
to late spring cold events. Emergence of re, or
increases in re frequency and/or intensity, may con-
strain survival and self-replacement of dominant spe-
cies, leading to long-lasting shifts in community
composition or landscape structure, or the establish-
ment of alternative ecosystem states. Both are
© 2019 Ecological Society of Australia doi:10.1111/aec.12751
TEMPERATE RAINFOREST DYNAMICS 11
resulting in community simplication by acting as l-
ters selective removal of freeze-susceptible trees and
selection for more re-tolerant species; longer term
implications of this shift clearly need more research.
Temperate rainforests are undergoing novel change
largely driven by climate change. Change in these
systems is particularly important given the two
regions we examined are well recognised for their
global biodiversity importance and relative intactness,
which provides opportunities to proactively respond
to emerging conditions relative to highly disturbed
areas. Their role as major storehouses of carbon at
the global scale underlines the importance of these
shifts. Finally, the use of coastal temperate rainforests
as early indicators of change is valuable and can lead
to predictive capabilities for similar functional groups
and responses in forests elsewhere.
ACKNOWLEDGEMENTS
This paper emerged from an invited symposium at
the International Association for Landscape Ecology
in Portland, Oregon, 2015. The authors would like
to thank all participants in that session for their
inputs.
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SUPPORTING INFORMATION
Additional supporting information may/can be found
online in the supporting information tab for this
article.
Appendix S1. Fire modelling.
Appendix S2. Red dots depict the xycoordinates
used to extract the values of each bioclimatic variable
in each region.
Appendix S3. Spearman rank correlation coef-
cient between WorldClim CMIP5 HadGEM2-ES
(RCP8.5; Hijmans et al. 2005) and the CMIP3 cli-
mate projections used to build the ensemble re mod-
els in the Moritz et al. (2012) global re framework.
© 2019 Ecological Society of Australia doi:10.1111/aec.12751
TEMPERATE RAINFOREST DYNAMICS 15
... While IRA fish and wildlife habitat values have been documented on the TNF [40], our study is the first to quantify the C Land 2022, 11, 717 14 of 18 stock value of IRAs, which contain over half the entire C stock on the TNF. Importantly, the C stock within IRA POGs (and POGs generally) are likely to remain relatively stable compared to the interior of Alaska and the southern extent of the North Pacific coastal temperate rainforest biome subject to more extreme climate change [41][42][43]. ...
... The maritime climate and intact forests of the TNF have climate refugia properties compared to more extreme climatic zones in the interior of Alaska and temperate rainforests further south [41][42][43], thereby offering a relatively stable C reservoir. However, due to declining late-season snow cover that prevents late-winter root freezing, yellow-cedar is experiencing a range contraction, and is a climate-sensitive focal species [44]. ...
... Those areas should be candidates for proforestation [37] to restore carbon stocks over time. Thus, a climate-smart strategy centered on sequestration and accumulation of C is generally essential to addressing the climate crisis [37] and would offer co-benefits, including a host of ecosystem services derived from C dense forests [48] as well as potential climate refugia [41][42][43]. ...
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... Non-growing season climate and interactions with extreme events can also drive change. Warming winter temperatures, are leading to reduced snowpack levels (Groisman et al., 2004;Knowles et al., 2006), with consequences for water supply from spring snowmelt runoff (Stewart et al., 2004) and species that rely on persistent snow cover in late-winter as protection from thaw-freeze events (Buma et al., 2019). Species sensitive to winter climate include yellow-cedar [Callitropsis nootkatensis (D. ...
... However, across coastal temperate rainforests globally, warming temperatures are crossing key climatic and ecological thresholds (Veblen et al., 2011;Shanley et al., 2015). Along the Pacific coast of North America, average annual temperatures range from 4 to 12 • C (DellaSala, 2011), and importantly winter temperatures exist near the rain-tosnow threshold (−2 to 2 • C) (Buma et al., 2019). Therefore, although changes in temperature in coastal regions have not been as extreme as other high-latitude regions, small changes in temperature have had significant impacts (Buma, 2018;Buma et al., 2019). ...
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... The Pacific Northwest (PNW) contains the largest seasonal temperate rainforests in the world [22] and despite only slight increases in the area burned (before 2020), major shifts in future fire activity have been projected [28][29][30][31]. Discerning these trends is difficult owing to the episodic nature of wildfire events [32], the size and severity of these disturbances [33], and striking climatic gradients. ...
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... In southeast Alaska, southwest New Zealand, and Chilean Patagonia, these forests are amongst the densest and biomass-rich biomes worldwide (DellaSala, 2011). These forests experience frequent disturbances (Johnstone et al., 2016) such as earthquakes, windstorms, volcanic eruptions, and landslides (Buma et al., 2019;Korup et al., 2019;Sommerfeld et al., 2018); such forest disturbances, in turn, alter the susceptibility to slope failure in feedback (Buma & Johnson, 2015;Scheidl et al., 2020). The susceptibility to shallow landslides can increase the following deforestation because of limited root reinforcement (Schwarz et al., 2010;Sidle, 1991), altered soil infiltration, and permeability rates. ...
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Planted and invading non-native plant species can alter fire regimes through changes in fuel loads and in the structure and continuity of fuels, potentially modifying the flammability of native plant communities. Such changes are not easily predicted and deserve system-specific studies. In several regions of the southern hemisphere, exotic pines have been extensively planted in native treeless areas for forestry purposes and have subsequently invaded the native environments. However, studies evaluating alterations in flammability caused by pines in Patagonia are scarce. In the forest-steppe ecotone of northwestern Patagonia, we evaluated fine fuels structure and simulated fire behavior in the native shrubby steppe, pine plantations, pine invasions, and mechanically removed invasions to establish the relative ecological vulnerability of these forestry and invasion scenarios to fire. We found that pine plantations and their subsequent invasion in the Patagonian shrubby steppe produced sharp changes in fine fuel amount and its vertical and horizontal continuity. These changes in fuel properties have the potential to affect fire behavior, increasing fire intensity by almost 30 times. Pruning of basal branches in plantations may substantially reduce fire hazard by lowering the probability of fire crowning, and mechanical removal of invasion seems effective in restoring original fuel structure in the native community. The current expansion of pine plantations and subsequent invasions acting synergistically with climate warming and increased human ignitions warrant a highly vulnerable landscape in the near future for northwestern Patagonia if no management actions are undertaken.
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The Southern Annular Mode (SAM) is the main driver of climate variability at mid to high latitudes in the Southern Hemisphere, affecting wildfire activity, which in turn pollutes the air and contributes to human health problems and mortality, and potentially provides strong feedback to the climate system through emissions and land cover changes. Here we report the largest Southern Hemisphere network of annually resolved tree ring fire histories, consisting of 1,767 fire-scarred trees from 97 sites (from 22 °S to 54 °S) in southern South America (SAS), to quantify the coupling of SAM and regional wildfire variability using recently created multicentury proxy indices of SAM for the years 1531–2010 AD. We show that at interan-nual time scales, as well as at multidecadal time scales across 37–54 °S, latitudinal gradient elevated wildfire activity is synchronous with positive phases of the SAM over the years 1665–1995. Positive phases of the SAM are associated primarily with warm conditions in these biomass-rich forests, in which widespread fire activity depends on fuel desiccation. Climate modeling studies indicate that greenhouse gases will force SAM into its positive phase even if stratospheric ozone returns to normal levels, so that climate conditions conducive to widespread fire activity in SAS will continue throughout the 21st century. fire scars | climate modes | AAO | synchrony | warming
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In an era of rapid climate change, understanding the natural capacity of species' ranges to track shifting climatic niches is a critical research and conservation need. Because species do not move across the landscape through empty space, but instead have to migrate through existing biotic communities, basic dispersal ecology and biotic interactions are important considerations beyond simple climate niche tracking. Yellow-cedar (Callitropsis nootkatensis), a long-lived conifer of the North Pacific coastal temperate rainforest region, is thought to be undergoing a continued natural range expansion in southeast Alaska. At the same time, yellow-cedar's trailing edge is approaching its leading edge in the region, due to climate-induced root injury leading to widespread mortality over the past century. To examine the current dispersal capacity of yellow-cedar at its leading range edge, and potential for the species' leading edge to stay ahead of its trailing edge, we characterized recent yellow-cedar stand development near Juneau, Alaska, and surveyed the spread of yellow-cedar seedlings just beyond existing stand boundaries. Despite suitable habitat beyond stand edges, stand expansion appears limited in recent decades to centuries. Large quantities of seed are germinating within stands and just beyond boundaries, but seedlings are not developing to maturity. Furthermore, c. 100-200-year-old yellow-cedar trees are located abruptly at stand boundaries, indicating stand expansion is in a period of stasis with a last pulse at the end of the Little Ice Age climate period. Vegetative regeneration is common across stands and may be an adaptive strategy for this long-lived tree to persist on the landscape until conditions are favourable for successful seedling recruitment, leading to an overall punctuated migration and colonization of new landscapes. Synthesis. Species ranges do not always respond linearly to shifting climatic conditions. Instead, successful colonization of new habitat may be tied to episodic, threshold-related landscape phenomena, dispersal ability, and competition with existing plant communities.