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Divergence of species responses to climate change

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Divergence of species responses to climate change

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Climate change can have profound impacts on biodiversity and the sustainability of many ecosystems. Various studies have investigated the impacts of climate change, but large-scale, trait-specific impacts are less understood. We analyze abundance data over time for 86 tree species/groups across the eastern United States spanning the last three decades. We show that more tree species have experienced a westward shift (73%) than a poleward shift (62%) in their abundance, a trend that is stronger for saplings than adult trees. The observed shifts are primarily due to the changes of subpopulation abundances in the leading edges and are significantly associated with changes in moisture availability and successional processes. These spatial shifts are associated with species that have similar traits (drought tolerance, wood density, and seed weight) and evolutionary histories (most angiosperms shifted westward and most gymnosperms shifted poleward). Our results indicate that changes in moisture availability have stronger near-term impacts on vegetation dynamics than changes in temperature. The divergent responses to climate change by trait- and phylogenetic-specific groups could lead to changes in composition of forest ecosystems, putting the resilience and sustainability of various forest ecosystems in question.
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Divergence of species responses to climate change
Songlin Fei,
1,2
* Johanna M. Desprez,
1
Kevin M. Potter,
3
Insu Jo,
1
Jonathan A. Knott,
1
Christopher M. Oswalt
4,5
Climate change can have profound impacts on biodiversity and the sustainability of many ecosystems. Various studies
have investigated the impacts of climate change, but large-scale,trait-specificimpactsarelessunderstood.Weanalyze
abundance data over time for 86 tree species/groups across the eastern United States spanning the last three decades.
We show that more tree species have experienced a westward shift (73%) than a poleward shift (62%) in their abun-
dance, a trend that is stronger for saplings than adult trees. The observed shifts are primarily due to the changes of
subpopulation abundances in the leading edges and are significantly associated with changes in moisture availability
and successional processes. These spatial shifts are associated with species that have similar traits (drought tolerance,
wood density, and seed weight) and evolutionary histories (most angiosperms shifted westward and most gymno-
sperms shifted poleward). Our results indicate that changes in moisture availability have stronger near-term impacts on
vegetation dynamics than changes in temperature. The divergent responses to climate change by trait- and phylogenetic-
specific groups could lead to changes in composition of forest ecosystems, putting the resilience and sustainability of
various forest ecosystems in question.
INTRODUCTION
Climate change can have profound impacts on biodiversity and the sus-
tainability of various ecosystems (1,2). Many studies have investigated
the impacts of global temperature rise and suggested that species will
migrate poleward (higher latitude) and upward (higher elevation)
(37). However, climate change incorporates both temperature and
precipitation (8), and the responses to climate change are likely to be
trait-specific (9,10).
In the last 30 years, the mean annual temperature (MAT) in the east-
ern United States has increased about 0.16°C on average, with the north-
ern region undergoing the highest temperature increase (Fig. 1A).
Precipitation patterns have also changed, with an increase of more than
150-mm total annual precipitation (TAP) in the central United States and
a large reduction in the southeast (Fig. 1B). The combined shifts in pre-
cipitation patterns and increasing temperatures have resulted in widespread
droughts, as measured by the Palmer Drought Severity Index (PDSI) in the
southeastern region of the study area (fig. S1), which can be detrimental
to various ecosystems (1113). Therefore, taking only temperature into
consideration is likely to result in an underestimation of the impacts of
climate change on species distributions (1416).
In addition, variations in physiological traits (for example, thermal
tolerance) can result in trait-specific responses to climate change (10,17).
Pollen records indicated that different tree species had differential
patterns of migration and range shifts in response to changes in the late
Quaternary climate (18). These different patterns were likely driven by
trait dissimilarities among species. For example, gymnosperms generally
have slower juvenile maximum growth rate than angiosperms (19), near-
ly all gymnosperms are wind-pollinated whereas many angiosperms are
insect-pollinated, and gymnosperms have greater hydraulic safety
margins than angiosperms in response to drought (11). Tremendous var-
iability also exists within the gymnosperm and angiosperm tree lineages
in response to different environmental conditions because of their varying
functional traits (20). Hence, the realized current and near-term, broad-
scale climate change impacts on vegetation dynamics will likely be trait-
and lineage-specific.
For tree species, upward and poleward shifts have been observed, but
with some inconsistencies. Upward shifts of trees, primarily in response
to temperature change, have been observed in a wide range of studies
(4,21), but downward shifts have also been observed as a result of
tracking moisture (22). Poleward shifts of tree species have also been
observed worldwide but with some exceptions (7,15). For example,
Woodall et al.(23) found that in the eastern United States, the mean
latitude of seedlings shifted greater than20 km north for the northern
species, whereas southern species did not move; Zhu et al.(24)found
no indication of tree range expansion.
Theimpactsofclimatechangeontreescanbecomplicated,asdif-
ferent combinations of alterations in temperature and precipitation can
result in different impacts (15)and different tree species can have dif-
ferent responses to climate change (25). To complicate things further,
species range shift can also be affected by other nonclimatic factors. In
particular, successional processes, influenced by various types of distur-
bances (or lack of disturbances) along with land use changes, could play
an important role in species recruitment, dispersal, and abundance
change (26,27).
Here, we analyzed the distributions of 86 tree species/groups across
the eastern United States over the last three decades to (i) investigate the
magnitude and directionality of their responses to climate change while
accounting for successional processes and (ii) provide a mechanistic
understanding of the observed spatial shift. Past studies have primarily
focused on the leading and trailing edges of species distributions (14).
We used tree abundance data to quantify species response to climate
change because quantitative changes within species ranges are more in-
formative (6), can be considerable without any observable changes in
overall range extent (28), and are likely to represent intermediate states
in species range shifts (14,29). We hypothesized that (i) young trees
have more prominent responses to climate change than adult trees
and (ii) species that experience similar responses to climate change
share similar physiological traits and evolutionary histories.
1
Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN
47907, USA.
2
Purdue Climate Change Research Center, Purdue University, West Lafayette,
IN 47907, USA.
3
Department of Forestry and Environmental Resources, North Carolina
State University, Research Triangle Park, NC 27709, USA.
4
U.S. Department of Agricul-
ture Forest Service Southern Research Station, Knoxville, TN 37919, USA.
5
Department
of Forestry, Wildlife and Fisheries, University of Tennessee, TN 37996, USA.
*Corresponding author. Email: sfei@purdue.edu
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RESULTS
Spatial patterns of shift
We observed a prominent westward and poleward shift in abundance
for most tree species in the eastern United States during the last 30 years
(Fig. 2). Of the 86 species studied, 73% shifted their abundance centers
westward, of which 65% were statistically significant (P< 0.05), whereas
62% shifted poleward, of which 55% were significant (table S1A). The
median longitudinal rate for species that shifted westward (15.4 km per
decade) was 40% larger than the median latitudinal rate for species that
shifted poleward (11.0 km per decade). The primary direction of species
abundance shift was northwest (37% of the species), and the least com-
mon direction was southeast (2%).
Further examination of tree abundances within the four cardinal
quadrants of each species distribution range indicates that the observed
spatial shifts are primarily due to changes in subpopulations at the
leading edges (Fig. 3). In general, subpopulations in the northwestern
quadrant had the highest rate of poleward and westward shifts (median,
6.9 and 15.7 km per decade, respectively) compared to subpopulations
in other quadrants (Fig. 3, A and B). Moreover, the northwestern sub-
populations within each species range had the highest increase in stem
density (median, 24%), whereas other subpopulations had low increases
(5.6 and 8.1% in the northeastern and southwestern quadrants, respec-
tively) or were nearly flat (0.6% in the southeastern quadrant) (Fig. 3C).
We also observed regional differences in species abundance shifts
(Fig. 4). The majority of species (85%) centered in the warm continental
Northern Hardwood region shifted their abundance poleward at a
median rate of 20.1 km per decade. Species centered in the hot continental
Central Hardwood region shifted primarily westward (83%) at a median
rate of 18.9 km per decade. Similarly, species centered in the subtropical
Southern Pine-Hardwood region also had a primarily westward shift
(77%) at a median rate of 24.7 km per decade. Species centered in the
Forest-Prairie Transition region had a westward shift at a median rate
of 30.0 km per decade.
Sapling-sized trees had a slightly higher proportion but longer dis-
tance in westward and poleward shifts than adults (Fig. 2, B and C, and
table S1A). About 71% of tree species shifted their sapling abundance
westward versus 70% for adult trees (61 and 65% were significant, re-
spectively, at P< 0.05). Similarly, 59% of species shifted their sapling
Fig. 2. Rose diagrams depicting the direction and distance of abundance shifts for 86 species. Spatial shift of species abundance for (A) all stems, (B) saplings,
and (C) adult trees between two inventory periods (1980s and 2010s). Color represents the different distance categories, and wedge width represents the proportion of
species in each direction-distance category.
Fig. 1. Changes in temperature and precipitation across the eastern United States. (A) Changes in MAT and (B) TAP between the recent past (19511980) and the
study period (19812014).
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abundance poleward versus 56% for adults (57 and 58% were signifi-
cant, respectively, at P< 0.05). The median westward and poleward shift
distances for saplings (20.7 and 13.1 km per decade, respectively) were
longer than those for adults (15.6 and 12.5 km per decade, respectively;
table S1A). A paired Wilcoxon signed-rank test demonstrated that the
westward shift distances for sapling and adult tree abundance were sta-
tistically different (P= 0.063).
Associations with climate variables and forest succession
Changes in species abundance during the study period are related to
climatic conditions (Fig. 5). With some exceptions, species located in
higher temperature and precipitation climatic space experienced re-
ductions in abundance, as indicated by the speciesdensity change.
On the other hand, species located in the lower temperature and pre-
cipitation climatic space experienced increases (Fig. 5A). Among 38
species associated with mean temperature and precipitation less than
the median values (the lower left part of Fig. 5A), 84% experienced an
increase in their total stem density.
Changes in species abundance had a stronger association with mois-
ture (precipitation and drought index) than with temperature (Fig. 5B
and fig. S2). Changes in TAP were positively associated with changes in
species abundance, explaining about 19% of the variability in abun-
dance change (Fig. 5B). The majority of species (80%) that experienced
an increase in TAP over 40 mm in their distribution ranges underwent
an increase in abundance, and all species increased in abundance when
precipitation increased over 60 mm. Not surprisingly, changes in
drought condition (PDSI) were also positively associated with species
abundance changes (r
2
= 0.19; fig. S2A), because of the highly correlated
nature between TAP and PDSI. The association between changes in
temperature (MAT) and species abundance was significant but weak
(r
2
=0.05;fig.S2B).
Forest successional processes, as approximated by stem density and
basal area of all overstory trees, also had influences on the observed spe-
cies abundance shift. Changes in species abundance were positively re-
lated to the changes in overall stem density but negatively related to the
changes in total basal area (Fig. 5C and fig. S2C), indicating that forests
in relatively early successional stages (that is, rapid increases in density
but low accumulations in basal area) had more gains in abundance of
the studied species than forests in late successional stages. A multiple
regression analysis further confirmed that changes in TAP and total
stem density were positively associated with changes in species abun-
dance(tableS2).
Trait-specific shifts
The observed spatial shifts in species abundance are associated with cer-
tain functional traits that are related to physiological tolerance and dis-
persal ability. In general, species that shifted westward had a larger seed
size (P= 0.012) and higher wood density (P= 0.039) than species that
shifted eastward. For species that shifted significantly eastward and
westward, the median value for seed weight is 0.007 and 0.069 g per
seed, and the median value for wood density is 0.47 and 0.51 g/cm
3
,
respectively (fig. S3). In addition, species with different degrees of
drought tolerance had different westward shift rates, where species with
low drought tolerance had the lowest westward shift rate (median rate,
2.6 km per decade) compared to the medium and high drought toler-
ance groups (median rate, 14.3 and 11.6 km per decade, respectively)
(fig. S4).
On the other hand, species that shifted northward had a lower max-
imum annual precipitation (P< 0.01) and lower wood density (P=
0.055) than species that shifted southward (primarily southwest). The
median maximum annual precipitation is 1524 and 2032 mm/year,
and the median wood density is 0.49 and 0.56 g/cm
3
, respectively, for
species that shifted significantly northward and southward (fig. S5). In
addition, wind-pollinated species primarily shifted northward, whereas
animal-pollinated species shifted southward (fig. S6). All other investi-
gated traits (see details in table S3) are not significantly associated with
the observed shifts (P> 0.1), except for maximum elevation of species
distribution, which is marginally significant (P=0.085).
We also detected a small but statistically significant phylogenetic sig-
nal of longitudinal shifts in species abundance (K= 0.023, P= 0.004),
but no such signal for latitudinal shifts (K= 0.007, P= 0.760). In other
words, members of some evolutionary groups exhibit relatively consistent
westward or eastward shifts (Fig. 6). Fifty-three of the 65 angiosperm spe-
cies (81.5%) shifted westward, 34 (52.3%) of which significantly shifted,
−50 0 50 100
Latitudinal shift of species abundance center (km)
NW quadrant
NE quadrant
SW quadrant
SE quadrant
a
ab ab b
−200 −150 −100 −50 0 50 100
Longitudinal shift of species abundance center (km)
NW quadrant
NE quadrant
SW quadrant
SE quadrant
b
ab
b
a
−50 0 50 100 150
Relative density change (%)
NW quadrant
NE quadrant
SW quadrant
SE quadrant
a
ab ab
b
ABC
Fig. 3. Spatial shift of species abundance by subpopulations. (A) Latitudinal shift and (B) longitudinal shift of species abundance center and (C) relativity stem density
change by subpopulations (fourquadrants that are defined by species density center at T1). Positivevalues indicate northward oreastward shift, andnegativevaluesindicate
southward or westward shift in (A) and (B). Different letters represent significant differences according to the Tukey post hoc test using glht in R. NW, northwestern; NE,
northeastern; SW, southwestern; SE, southeastern.
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whereas 10 of the 21 gymnosperm species shifted westward. At a finer
evolutionary resolution, all 13 red oak species (section Lobatae)had
westward movement, with 53.8% (7 species) being statistically signifi-
cant, whereas 3 of the 9 white oaks (section Quercus) moved east. Mean-
while, all the members of the Fabaceae, Juglandaceae, and Lauraceae,
and most members of the Sapindaceae, moved westward. At the same
time, three of the four Betula species moved eastward. No clear patterns
were observed for the latitudinal shifts among the angiosperms (38 species
shifted poleward and 27 shifted southward), whereas the majority of
gymnosperms (15 species, or 71.4%) shifted poleward (fig. S7).
DISCUSSION
Our results revealed prominent range-wide westward shifts during a
short 30-year period, especially for species in low latitudes. The west-
ward shifts are partially associated with the increased moisture availa-
bility in the western regions of the study area, coupled with decreased
moisture availability in the southeastern regions. Although plants are
unlikely to experience precipitation as a direct impact on physiology,
they can track changes in water availability through multiple mecha-
nisms, such as their ability to deal with drought, as shown in previous
observational studies (3,22) and modeling simulations (30,31). The
Fig. 4. Shift of species meanabundancecenter by ecoprovincesin the eastern UnitedStates duringthe last threedecades. 210, Northern Hardwood region; 220, Central
Hardwood region; 230, Southern Pine-Hardwood region; 250, Forest-Prairie Transition region (44,51). All three species centered in ecoprovince 250 shifted westward (rose
diagram not shown). Si*indicates that the shift is statistically significant (P< 0.05). NS, nonsignificant.
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correlation between changes in drought index and species abundance
further confirms the importance of moisture availability and/or toler-
ance, which is also underscored by the fact that the species that shifted
significantly westward are those with relatively high drought tolerance.
Although the western portion of the study area has increased in mois-
ture availability during the study period, it is still considerably drier
compared to the eastern part (fig. S1). However, species with medium
and high drought tolerance can take the advantage of the increased
moisture availability in the relatively dry region. Future studies using
finermeasuresofphysiologicaltraits such as embolism resistance could
better address moisture-related mechanisms on species response to cli-
mate change.
As expected, we also found evidence of poleward shifts in abun-
dance, which is more prominent in high latitudes where substantial
warming has occurred during the study period. Our abundance-based
poleward shift rate (11.0 km per decade) is similar to a previous esti-
mated rate (10 km per decade) that is based on leading/trailing edges
for forests in the eastern United States (23).Thefactthatthelongitudi-
nalshiftwas1.4timesfasterthanlatitudinalshiftsuggeststhatvegeta-
tion dynamics are more sensitive to precipitation than to temperature,
at least in a near-term time frame, because moisture availability is
considered a critical factor in forest dynamics of eastern North America
(32). When considering the predominantly westward and northward
shifts of tree species, it is important to understand the geographical set-
tings of the study area, because it has hard boundaries to the east and
south (that is, ocean) but soft boundaries to the west and north (that is,
climate limitations). Nevertheless, abundance shifts could still be ob-
served even if species ranges are limited by the ocean.
It is not surprising that saplings have experienced a higher propor-
tion and faster rate in poleward and westward shifts than adult trees,
because new recruitments (that is, young trees) are expected to respond
toclimatechangemorequickly(23,24). The observed differential shift
rates could also be due to the fact that saplings are more sensitive to
droughts in terms of survival than adult trees (25), as substantial drought
was observed in the southeastern region of the study area during the study
period. The differential shift rates among subpopulations in the four car-
dinal quadrants further confirmed that the observed range shift is primar-
ily due to the changes in the leading edges of species distribution ranges,
which agreed with early findings by Woodall et al.(23)ofsignificantpole-
ward shifts of seedlings for most of the northern species in the eastern
United States.
The observed trait- and phylogenetic-specific spatial shift in species
abundance is fascinating (most angiosperms shifted westward whereas
most gymnosperms shifted poleward). There are several mechanisms
that could explain these traits and phylogenetic-specific responses.
The lack of westward shift in gymnosperms might be partially explained
by the fact that gymnosperms have lower maximum growth rates than
angiosperms as a result of their less efficient water transport systems
(19), making them less competitive in the western portion of the study
area, which is drier than the eastern portion (fig. S1). The association
between seed size and directional movement of certain species is also
interesting and is potentially related to the different colonization, tol-
erance, and competitive strategies used by different species (33,34).
However, there are large variabilities in seed size among species,
and seed size is embedded in a complex of many other traits that
together define the life history of a tree species (35). The association
between the observed spatial shift and wood density is also an
interesting one. Low wood density is often associated with fast growth,
whereas high wood density is often associated with high survival (36),
which can partially explain the observed southward and westward
shift of high wood density species. Because the southern region of
the study area experienced droughts and the western part was rela-
tively dry, species with high wood density might have a better chance
to survive.
The predominantly northward shift of gymnosperm trees, along
with all Populus species and most Betula species, is intriguing. The
mechanism for the northward shift could potentially be linked to species
pollination methods because we found that most wind-pollinated spe-
cies (most gymnosperms and some angiosperms) shifted northward
while animal-pollinated species (most angiosperms) shifted southward.
Similar northward shifts of clades were observed in the New England
area during the early Holocene (between 10,000 and 8000 years ago),
where Picea was replaced by Pinus, followed by Betula and then
Quercus species (37). However, the historical process took place over
several thousand years, whereas the observed shift in this study
happened in a few decades, suggesting the impacts of recent climate
5101520
800 900 1000 1200 1400
Temperature at T1 (oC)
Precipitation at T1 (mm)
r2 = 0.84, P < 0.001
y = 34.3x + 691
−20 0 20 40 60 80
−100 −50 050 100
Precipitation change (mm)
Relative density change (%)
y = 0.7x − 15.8
−10 0 10 20 30 40
−100 −50 0 50 100
Total tree stem density change (%)
Relative density change (%)
y = 1.03x + 4.49
AB C
r2 = 0.19, P < 0.001 r2 = 0.13, P < 0.001
Fig. 5. Relationships between climate variablesand species abundance change (shaded area indicates 95%confidence interval). (A) Relative changes in species density
[(Density_T2 Density_T1)/Density_T1 × 100)] in association with species MAT and TAP at T1 within the overlapped species distribution range at T1 and T2. Dot size is proportional
to the relative density change. Red and green represent negative and positive change in the relative density, respectively. (B) Positive association between relative changes in
species density and changes in speciesrespective TAP. (C) Positive association between relative changes in species density and changes in total tree stem density.
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change, along with other nonclimatic factors (for example, land use
change and forest management), on vegetation dynamics.
These trait- and phylogenetic-specific shifts can have profound impli-
cations for the resilience and sustainability of the studied forest ecosys-
tems, because phylogenetic relatedness among species can have strong
influences on community assemblage and composition (38). Because
some closely related species groups (both in functional traits and in evo-
lutionary history) have different responses to the changing climate com-
pared to other groups, the resultant differing spatial shifts among various
groups can lead to the possible marked reduction of some evolutionary
lineages. The reduction or replacement of certain species in a community
can be consequential, because species can have substantially different
effects on ecosystem structure, function, and services, and the impacts
can cascade through a broad range of ecosystem processes (39,40).
Given the broad scale at which our analysis was conducted, a variety
of indirect and nonclimatic factors (for example, fire regimes, invasive
Pinus strobus
Pinus resinosa
Pinus banksiana
Pinus virginiana
Pinus taeda
Pinus rigida
Pinus serotina
Pinus echinata
Pinus elliottii
Pinus palustris
Pinus glabra
Picea rubens
Picea mariana
Picea glauca
Larix laricina
Abies balsamea
Tsuga canadensis
Juniperus virginiana
Thuja occidentalis
T
axodium distichum
Taxodium ascendens
Acer negundo
Acer rubrum
Acer saccharinum
Acer saccharum
Acer pensylvanicum
Aesculus spp.
Tilia spp.
Robinia pseudoacacia
Gleditsia triacanthos
Cercis canadensis
Betula lenta
Betula alleghaniensis
Betula papyrifera
Betula populifolia
Ostrya virginiana
Carpinus caroliniana
Carya spp.
Juglans nigra
Quercus stellata
Quercus lyrata
Quercus macrocarpa
Quercus bicolor
Quercus michauxii
Quercus muehlenbergii
Quercus prinus
Quercus alba
Quercus virginiana
Quercus laevis
Quercus falcata
Quercus laurifolia
Quercus nigra
Quercus phellos
Quercus pagoda
Quercus shumardii
Quercus palustris
Quercus rubra
Quercus coccinea
Quercus imbricaria
Quercus marilandica
Quercus velutina
Fagus grandifolia
Prunus spp.
Ulmus spp.
Maclura pomifera
Celtis spp.
Populus tremuloides
Populus grandidentata
Populus balsamifera
Populus deltoides
Salix spp.
Liquidambar styraciflua
Nyssa sylvatica
Nyssa aquatica
Oxydendrum arboreum
Gordonia lasianthus
Diospyros virginiana
Ilex opaca
Fraxinus nigra
Fraxinus spp.
Platanus occidentalis
Magnolia virginiana
Magnolia acuminata
Liriodendron tulipifera
Sassafras albidum
Persea borbonia
Westward, significant
Eastward, not significant
Westward, not significant
Eastward, significant
Westward, significant
Westward, not significant
Eastward, significant
Eastward, not significant
Fig. 6. Longitudinal species abundance shift mapped on a phylogram representing evolutionary relationships among the 86 study species.
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species, forest management, conservation efforts, and land use change)
could also have influenced the directional trends observed. The fact that
the observed abundance change is positively correlated with the densi-
fication of forests, which is often attributed to diminishing fire frequen-
cy and severity (26), confirms that nonclimatic factors are also
responsible for the observed spatial shift of species abundance. Heavy
infestations of invasive insects, plants, and pathogens could contribute
tospeciesshiftsinotherdirections(41,42). In addition, forest conser-
vation and plantation efforts in the study area such as those by the U.S.
Department of Agriculture (USDA) Conservation Reserve Program
could also influence the observed spatial shift. Nevertheless, we ob-
served clear broad-scale evidence of the impact of climate change on
forest tree spatial dynamics, where changes in mean annual precipita-
tion alone explained about 19% of the variability in species abundance
change and spatial shift. It could be beneficial for future studies to in-
vestigate the impacts of inter- and intra-annual variability of moisture
availability at finer spatial and temporal resolutions (for example, for
droughts and floods) on vegetation dynamics, given the high likelihood
of more variable precipitation patterns in the future (43). In addition,
because more repeated plot-level measures are becoming available
across the regional scale, we can better study the degree to which climate
change induces new community assemblages owing to the varying
mortality and recruitment and/or competitive release of different trait-
and phylogenetic-specific groups. These repeated measures may even-
tually enable the analysis of more tree species in the eastern U.S. forests,
rather than the selected ones in this study, allowing the detection of
phylogenetic- and trait-specific patterns that might be undetected in the
current analysis.
In summary, trees in the eastern United States have experienced
prominent westward and northward shifts in response to climate
change and successional processes. These spatial shifts are more sensi-
tive to the changes in moisture availability than to changes in tempera-
ture. The observed spatial shifts are associated with species that have
similar functional traits and evolutionary histories. The resultant diver-
gent spatial shifts among various groups can have significant ecological
consequences and possible extinction of certain evolutionary lineages in
some forest communities. Management actions to increase forest eco-
systemsresilience to climate change should consider the changes in
both temperature and precipitation.
MATERIALS AND METHODS
We obtained tree abundance data from the U.S. Forest Inventory and
Analysis (FIA) program from two different inventory periods (Supplemen-
tary Text). FIA provides a long-term data set of forest conditions across all
forest land in the United States, which has been used to study regional spa-
tial and temporal forest dynamics (23,24,44,45). The first inventory (T1)
included in this study was between 1980 and 1995 and varied by state. The
second inventory (T2) was the latest completed inventory, which was
finished in 2015 for most states. The time period between the two intervals
ranged from 20 to 35 years, with an average of 29.5 years (table S4).
Across the study area, a total of 201 species/groups were recorded
during T1, and 240 were recorded during T2. To account for the
variations of sampling protocols between the two inventory periods,
we first applied a set of selection criteria (Supplementary Text), which
resulted in a total of 86 species/groups in our analysis (table S5). Because
a significant portion of the plots were measured during only one of the
two inventory periods, we aggregated plot-level abundance data to the
hexagon level (a spatial tessellation design used by FIA). A total of 2747
hexagons of 1452 km
2
in area, approximately the mean size of eastern
counties, were used as the unit of analysis in our study. Total number of
stems in a hexagon (stems/ha × total forested area) was used to measure
species abundance within each hexagon for saplings [defined as
<12.7 cm in diameter at breast height (dbh)] (24), for adult trees
(>12.7 cm in dbh), and for all trees by species.
To investigate how species abundances shifted between the two inven-
tories, we used the shift distance and direction of the geographic center
weighted by species abundance (Supplementary Text). We conducted a
total of 10,000 random permutations per species to test whether the ob-
served shift was statistically significant (Supplementary Text). Rose
diagrams were generated in the R circular package (46) to illustrate the
overall pattern of abundance shift of both direction and distance for all
species studied. To understand whether the observed spatial shift is due to
the changes in leading or trailing edges, we first subdivided each species
distribution range into four cardinal quadrants on the basis of its abun-
dance center at T1. We then calculated and compared the shift distances
and density changes for subpopulations within each quadrant. To avoid
sampling biases, only species with sufficient sample sizes (30 hexagons
and 300 plots) at either T1 or T2 for each subpopulation were included
in this analysis. A Tukey post hoc test was applied to test whether the
shifts among subpopulations in each quadrant were statistically different.
We then analyzed the relationship between the observed species
shifts in relation to climate variables and forest succession status. For
climate variables, we first calculated MAT, TAP, and mean PDSI in
the recent past (19511980) and during the study period (19812015).
MAT and TAP were calculated using the 4-km spatial resolution cli-
mate data from the PRISM Climate Group (http://prism.oregonstate.
edu/), and PDSI data were obtained from the WestWide Drought
Tracker (www.wrcc.dri.edu/wwdt/), which also had a spatial resolution
of 4 km. These climate variables were then aggregated at the hexagon
level for each period. We used stem density and basal area for all trees
regardlessofspeciesasindicatorsofsuccessionalstatusatT1andT2for
each hexagon. In general, overall stem density decreases whereas basal
area increases as forest progresses from early to late successional stages.
Next, we calculated relative changes in mean stem density between the
two inventories by species, and we calculated relative changes between
T1 and T2 in total stem density and basal area for all trees and changes
over time in mean MAT, TAP, and PDSI within each speciesover-
lapped distribution range at T1 and T2 (that is, only with hexagons that
contain the target species at both T1 and T2). A multiple linear regres-
sion with a mixed-effects model was performed to test the association
among changes in species-level mean stem density and changes in
MAT, TAP, PDSI, total stem density, and total basal area.
To test whether the observed spatial shifts are trait-specific, we first
constructed a database with two functional trait groups (table S3). The
first group encompassed traits associated with treesability to migrate,
including seed weight, dispersal mechanism, and rate of spread. The
second group encompassed traits associated with treesphysiological tol-
erances, including temperature, precipitation, fire, shade, and drought tol-
erances. Because of the lack of actual precipitation and temperature
tolerance data, we used minimum temperature, minimum precipitation,
and maximum precipitation from species-specific realized climatic niche
as surrogates in our study (see table S3 for variable definition and data
sources). We compared species traits among four classes: significant west-
ward or northward shift, nonsignificant westward or northward shift,
nonsignificant east or southward shift, and significant eastward or south-
ward shift. A Kruskal-Wallis test was used for traits with continuous
variables, and a c
2
test was used for traits with categorical variables due
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to the non-normality of the data. Additionally, a Kruskal-Wallis test for
differences in latitudinal and longitudinal shift rates was used for
categorical plant traits.
We investigated the degree to which longitudinal and latitudinal
shifts were associated with the evolutionary relatedness of tree species
by displaying these shifts on a phylogenetic supertree encompassing the
86 species in the study and by testing for phylogenetic signal in these
shifts. The phylogenetic supertree was pruned from one generated for
397 native tree species occurring throughout the forests of the contigu-
ous48statesandinventoriedbyFIA,asdescribedbyPotterandWoodall
(47). We used the R package phytools (48) to display each specieslongi-
tudinal and latitudinal shifts (across all life stages) on the phylogenetic
tree, with trees in the aforementioned four classes. We then used the
Rpackagepicante(49) to test the degree to which the tree species phylogeny
predicts the ecological similarity of the species with respect to these shifts.
Specifically, we generated the Kstatistic, which compares the observed
phylogenetic signal in a trait (in this case, the longitudinal or latitudinal
shift) to the signal under a Brownian motion model of trait evolution on a
phylogeny (50), and then assessed the statistical significance of the phy-
logenetic signal by comparing observed patterns of the variance of
independent contrasts of each trait to a null model of shuffled taxon labels
across the tips of the phylogeny.
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/
content/full/3/5/e1603055/DC1
Supplementary Text
fig. S1. Average environmental conditions during the study period.
fig. S2. Relationships between changes in species abundance, PDSI, temperature, and total
tree basal area.
fig. S3. Box plots of functional trait values for species that experienced different longitudinal
shifts (E0, nonsignificantly eastward; E1, significantly eastward; W0, nonsignificantly westward;
W1, significantly westward).
fig. S4. Box plots of latitudinal and longitudinal stem density shifts by physiological trait
groups.
fig. S5. Box plots of functional trait values for species that experienced different latitudinal
shifts (N0, nonsignificantly northward; N1, significantly northward; S0, nonsignificantly
southward; S1, significantly southward).
fig. S6. Box plots of latitudinal and longitudinal stem density shifts by dispersal trait groups.
fig. S7. Latitudinal species abundance shift mapped on a phylogram representing evolutionary
relationships among the 86 study species.
table S1A. Summary of shift percentage and distance by directions.
table S1B. Shift of species abundance for all trees during the last three decades.
table S1C. Shift of species abundance for sapling-sized trees during the last three decades.
table S1D. Shift of species abundance for adult trees during the last three decades.
table S2. Linear mixed-effect model parameter estimates and significance for the relationships
between species shift and changes in environmental variables and successional processes.
table S3. Description of functional traits used in the analysis.
table S4. Years during which the first inventory (T1) and second inventory (T2) were completed
for each state.
table S5. Species used in the study and their sample sizes and their mean density at the first
and second inventories.
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Acknowledgments: We thank hundreds of FIA field crew members for the data used in our
study. C. Houser and J. Yoo assisted in compiling the functional traits data and PDSI data.
We thank J. Dukes, M. Jenkins, B. Iannone, G. Nunez-Mir, and two anonymous reviewers for
comments on earlier versions of the manuscript. Funding: This work was supported in part
by funding from USDA National Institute of Food and Agriculture (2013-38420-20517), the
McIntire-Stennis Cooperative Forestry Research Program, Purdue Climate Change Research
Center (1607), and Cost Share Agreement 15-CS-11330110-067 between the USDA Forest
Service and North Carolina State University. Author contributions: S.F. conceived the
research, performed statistical analysis, interpreted the results, and wrote the manuscript.
J.M.D. and K.M.P. performed statistical analysis, interpreted the results, and wrote the
manuscript. I.J., J.A.K., and C.M.O. performed statistical analysis, interpreted the results, and
assisted in manuscript revisions. All authors approved the manuscript. Competing
interests: The authors declare that they have no competing interests. Data and materials
availability: All data needed to evaluate the conclusions in the paper are present in the paper
and/or the Supplementary Materials. Additional data related to this paper are available in the
Purdue University Research Repository, an archive (doi:10.4231/R7FX77FC; doi:10.4231/
R7B8564P) and may be requested from the authors.
Submitted 4 December 2016
Accepted 15 March 2017
Published 17 May 2017
10.1126/sciadv.1603055
Citation: S. Fei, J. M. Desprez, K. M. Potter, I. Jo, J. A. Knott, C. M. Oswalt, Divergence of species
responses to climate change. Sci. Adv. 3, e1603055 (2017).
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Songlin Fei, Johanna M. Desprez, Kevin M. Potter, Insu Jo,
Divergence of species responses to climate change
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... The distribution for the future climatic scenarios indicated an increase in the habitat of T. grandis in the northern and eastern parts of the study area, which agrees with an earlier study by Balakrishnan et al. (2021) reporting that the distribution pattern of T. grandis in central India may shift in the ensuing years. Such a shift in distribution due to climate change in tropical tree species has also been reported in numerous studies (Fei et al. 2017;Ghosh et al. 2021;Tiwari et al. 2021). With respect to decline in distribution pattern of the species, Gopalakrishnan et al. (2011) reported that about 30% of grids of their study would negatively impact due to climate change under A2 and B2 SRES scenarios. ...
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Temperate (broadleaved) deciduous forests occur in eastern North America, western and central Europe, and eastern Asia. In these regions, temperatures are expected to increase by up to 5°C by 2100, and increases in precipitation may be offset by increased evaporation, leading to decreases in soil moisture. One likely response to climate change will be shifts in germination phenology that may have important repercussions in the population dynamics of species and community structure/composition. However, the regeneration response of species to climate change will be dependent on interactions with biotic, like herbivory, and nonclimatic abiotic factors, such as atmospheric nitrogen deposition. Geographic range shifts of a species will depend on seed production and dispersal, germination, seedling survival, soil conditions, and interactions with other species already present. Future research needs to focus on how germination and seedling survival will interact with other seed and species functional traits in response to the complexity of a changing climate, such as extreme climatic events.
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Department of Geography, University of Wisconsin-Madison, 550 North ParkStreet, Madison, WI 53706, USAThe influence of climate on forest change during thepast century in the eastern United States was evalu-ated in a recent paper (Nowacki & Abrams, 2014)that centers on an increase in ‘highly competitivemesophytic hardwoods’ (Nowacki & Abrams, 2008)and a concomitant decrease in the more xerophyticQuercus species. Nowacki & Abrams (2014) con-cluded that climate change has not contributed sig-nificantly to observed changes in forest composition.However, the authors restrict their focus to a singleelement of climate: increasing temperature since theend of the Little Ice Age ca. 150 years ago. In theirstudy, species were binned into four classifications(e.g., Acer saccharum – ‘cool-adapted’, Acer rubrum –‘warm-adapted’) based on average annual tempera-ture within each species range in the United States,reducing the multifaceted character of climate into asingle, categorical measure. The broad temperatureclasses not only veil the many biologically relevantaspects of temperature (e.g., seasonal and extremetemperatures) but they may also mask other influ-ences, both climatic (e.g., moisture sensitivity) andnonclimatic (e.g., competition).Understanding the primary drivers of forest changeis critically important. However, using annual tem-perature reduces the broad spectrum of climaticinfluence on forests (e.g., Jackson & Overpeck, 2000;Jackson et al., 2009) to a single variable. Tsuga canad-ensis illustrates one example of the complex interac-tion between trees and temperature. In the southernpart of its range, Tsuga canadensis growth is weakly,but positively correlated with early growing-seasontemperature. However, this relationship becomesstronger and shifts to later in the season toward thenorthern part of its range (Cook & Cole, 1991). More-over, Tsuga canadensis growth is significantly andnegatively correlated with just May temperaturesduring the current growing season in the northeast-ern United States (Cook, 1991; Cook & Cole, 1991;Vaganov et al., 2011), while in the southeastern Uni-ted States it is strongly and negatively correlatedwith summer (June–August) temperatures (Hart et al.,2010). Trees can also be sensitive to diverse and ofteninteracting climate variables at various stages of theirlife cycles (Jackson et al., 2009). Interactions betweenprecipitation and temperature are clearly important(Harsch & Hille Ris Lambers, 2014; Martin-Benito &Pederson, accepted), and often lead to counterintui-tive responses. For example, some plant species thatwould have been expected to move north and ups-
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