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Herbarium specimens reveal the footprint of climate change on flowering trends across north-central North America


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Shifting flowering phenology with rising temperatures is occurring worldwide, but the rarity of co-occurring long-term observational and temperature records has hindered the evaluation of phenological responsiveness in many species and across large spatial scales. We used herbarium specimens combined with historic temperature data to examine the impact of climate change on flowering trends in 141 species collected across 116,000 km(2) in north-central North America. On average, date of maximum flowering advanced 2.4 days °C(-1) , although species-specific responses varied from - 13.5 to + 7.3 days °C(-1) . Plant functional types exhibited distinct patterns of phenological responsiveness with significant differences between native and introduced species, among flowering seasons, and between wind- and biotically pollinated species. This study is the first to assess large-scale patterns of phenological responsiveness with broad species representation and is an important step towards understanding current and future impacts of climate change on species performance and biodiversity.
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Herbarium specimens reveal the footprint of climate change
on flowering trends across north-central North America
Kellen M. Calinger,* Simon
Queenborough and Peter S. Curtis
Department of Evolution, Ecology
and Organismal Biology, The Ohio
State University, Columbus, OH,
*Correspondence: E-mail: kcalin-
Shifting flowering phenology with rising temperatures is occurring worldwide, but the rarity of co-occurring
long-term observational and temperature records has hindered the evaluation of phenological responsive-
ness in many species and across large spatial scales. We used herbarium specimens combined with historic
temperature data to examine the impact of climate change on flowering trends in 141 species collected
across 116,000 km
in north-central North America. On average, date of maximum flowering advanced
2.4 days °C
, although species-specific responses varied from 13.5 to + 7.3 days °C
. Plant functional
types exhibited distinct patterns of phenological responsiveness with significant differences between native
and introduced species, among flowering seasons, and between wind- and biotically pollinated species. This
study is the first to assess large-scale patterns of phenological responsiveness with broad species representa-
tion and is an important step towards understanding current and future impacts of climate change on spe-
cies performance and biodiversity.
Climate change, invasive species, life history, phenological responsiveness, phenology, pollination syn-
Ecology Letters (2013) 16: 1037–1044
Phenology, the timing of key life events, is one of the most sensi-
tive biological indicators of climate change (Pe~nuelas & Filella 2001;
Menzel 2002) and its study has become an important tool for
understanding the impacts of warming from local to global scales.
Shifts in plant phenophases (e.g. budburst, leaf senescence and
flowering) over time, correlated with temperature change, have been
documented worldwide and are consistent with climate change pre-
dictions (Parmesan & Yohe 2003). This evidence of alterations in
plant phenology suggests that increased temperatures may already
be impacting species performance and modifying within-community
interactions (Menzel et al. 2006; Rosenzweig et al. 2008).
However, phenological responses to increased temperature are
highly variable, even among closely related species (Abu-Asab et al.
2001; Fitter & Fitter 2002). For example, in North America, Solidago
rugosa advanced flowering by ~ 11 days °C
, while Solidago graminifo-
lia showed no temperature response (Miller-Rushing & Primack
2008). In the United Kingdom, Geranium pyrenaicum advanced its first
flowering date (FFD) 3 days between 1954 and 2000 whereas
Geranium rotundifolium showed a 6 days delay in FFD over the same
time period (Fitter & Fitter 2002). As a result of such species-spe-
cific differences in phenological responsiveness to temperature
shifts, climate change may non-randomly alter community assem-
blages as species less well adapted to the new temperature regime
decrease in abundance (Willis et al. 2008).
The magnitude of a species’ phenological response to temperature
may have either positive or negative impacts on growth and repro-
duction depending on various interacting environmental factors. For
instance, phenologically responsive species may benefit from greater
early season productivity (Kudo et al. 2008) leading to an increase in
abundance. Mungia-Rosas et al. (2011) suggested that phenotypic
selection favours earlier blooming species in temperate regions.
Alternatively, earlier flowering can increase risk of frost exposure
(Inouye 2008) or may lead to pollinator mismatch, the disconnect
between pollinator availability and flowering as a result of differing
responses to environmental cues (Kudo et al. 2008). Both scenarios
may lead to decreased local species abundance. In the northeastern
US, Willis et al. (2008) found that species with weak flowering
responses to temperature have disproportionately decreased in
abundance over the past 150 years.
Although species-specific phenological responsiveness is likely to
have significant fitness implications with future warming, it is impossi-
ble to assess responsiveness for all species in a given area. However,
certain life history traits may predispose particular functional groups
to be more or less phenologically responsive and we may be able to
use these traits to predict how currently unstudied species will
respond to warming. Further, if plant functional groups do differ in
their phenological responsiveness, leading to increasing variance in fit-
ness, communities may be drastically altered by non-random patterns
of species removal. Thus, the ability to make generalised predictions
based on functional traits would benefit conservation efforts by allow-
ing identification of species most likely to be negatively impacted (in
terms of phenological responsiveness) by temperature increase, as
well as identifying whole communities that may be at risk.
Despite the potential impacts of climate change on species perfor-
mance, few studies have included sufficient species to examine pat-
terns of phenological responsiveness among functional groups
because of the lack of long-term flowering and temperature datasets
that overlap in time and space. The paucity of direct observational
data spanning decades to centuries with broad species representation
has led to key information gaps on species’ and functional groups’
responses to climate change, making generalised predictions of com-
munity responses to climate change challenging. To date, only three
studies have directly evaluated phenological responsiveness to cli-
mate change among plant functional groups (Fitter et al. 1995; Fitter
© 2013 The Authors. Ecology Letters published by John Wiley & Sons Ltd and CNRS
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use,
distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.
Ecology Letters, (2013) 16: 1037–1044 doi: 10.1111/ele.12135
& Fitter 2002; Miller-Rushing & Primack 2008). In these studies,
FFDs of species growing under natural conditions were obtained
from observational records that ranged from over 150 years (Miller-
Rushing & Primack 2008; with several large gaps in the collection
record) to less than 40 years (Fitter et al. 1995). While differences in
phenological responsiveness among plant functional groups were
documented, these studies were conducted over relatively small areas
and do not allow evaluation of flowering trends across broad spatial
scales (but see Lavoie & Lachance 2006).
An alternative to the traditional historic observational dataset is the
analysis of plants preserved in herbarium collections (Primack et al.
2004; Lavoie & Lachance 2006; Miller-Rushing et al. 2006; Robbirt
et al. 2011; Panchen et al. 2012). For example, by extracting flowering
date information on specimens from individual plants collected mul-
tiple times at the Arnold Arboretum from 1885 to 2002 and combin-
ing that data with long-term local temperature records, Primack et al.
(2004) found a significant advancement in flowering phenology asso-
ciated with increasing spring temperatures. To determine the validity
of herbarium-based methods for evaluating phenological responsive-
ness, Robbirt et al. (2011) calculated the flowering response of Ophrys
sphegodes to mean spring temperature (March through May) using
both direct observations of peak flowering time and herbarium speci-
mens. The herbarium specimens were collected between 1848 and
1958 while the observational data set spanned 1975 to 2006. Pheno-
logical response to temperature calculated using herbarium specimens
was statistically indistinguishable from the response calculated from
direct field observations (Robbirt et al. 2011). These initial herbar-
ium-based methods were an important step towards increasing our
ability to assess flowering shifts in areas without historic data sets.
However, these methods require intensive historic sampling of a sin-
gle study site as they do not allow for spatial variation in the tempera-
tures paired with individual herbarium specimens. This intensive
sampling is not characteristic of most locations and therefore leaves
many regions still inaccessible to study.
Here, we introduce a new herbarium-based method that allows
the assessment of flowering phenology in large numbers of species
collected across broad geographical regions. We evaluated the phe-
nological responsiveness to temperature of 141 species recorded
over a period of 115 years across a 116,000 km
region in north-
central North America. We quantified differences in responsiveness
among plant functional groups defined by flowering season, growth
form, pollination syndrome and native status.
We tested a number of hypotheses: (1) spring flowering species
are more responsive to temperature change than those flowering in
early- or late-summer because advancing phenology among spring
flowering species may enhance pollinator availability and increase
access to light prior to canopy closure (Elzinga et al.2007; Kudo
et al.2008); (2) herbaceous annuals are more phenologically respon-
sive than herbaceous perennials or woody perennials because earlier
flowering may benefit annuals through reduced competition for
pollinators and early-season productivity gains during favourable
weather which are crucial for enhancing fitness in their single grow-
ing season (Mungia-Rosas et al. 2011); (3) insect-pollinated species
respond more to temperature change than wind-pollinated species
because insect-pollinated species may respond to selection promot-
ing earlier flowering to maintain pollinator mutualisms (Fitter &
Fitter 2002); and (4) introduced species advance flowering more
than native species to take advantage of currently unoccupied phe-
nological niches (Wolkovich & Cleland 2011).
Our results support previously published patterns of responsive-
ness among different seasons of flowering and growth forms but
differ from previous work with regard to native status and pollina-
tion syndrome. Our analysis of phenological responsiveness among
many species and plant functional groups in a variety of locations is
an important step towards understanding the impacts of climate
change on species performance and biodiversity.
To evaluate flowering phenology, we visually examined specimens
of plant species collected in the US state of Ohio (c. 116,000 km
between 38
and 42
north latitude and 80
and 85
west longitude)
from the Ohio State University herbarium. All specimens with
greater than 50% of flower buds in anthesis were considered at
peak flowering at the time of collection and were included in our
analysis (Primack et al. 2004). However, specimens of the same spe-
cies with identical collection date, location and collector were
included as a single datum to avoid non-independence of samples.
Because we included only those specimens at peak flowering, some
species, such as grasses, were necessarily excluded.
The date of peak flowering (D
) and collection location were
recorded from the label attached to each specimen. From the col-
lection location, specimens were assigned to one of the ten climate
divisions established by the US National Oceanic and Atmospheric
Administration (NOAA) for Ohio (See Appendix S1).
We used average monthly temperature data from the US Histori-
cal Climatology Network (USHCN, Menne et al. 2010) to calculate
the average monthly temperature,
, for each climate division in
each year;
¼ RT
=n ð1Þ
where m is month, j is climate division, k is year, y is a recording
station within a division, and n is the number of recording stations
within division j. Of the 10 climate divisions in Ohio, all but one
had multiple USHCN climate stations. Thus,
is the average of
individual USHCN recording station temperatures within a climate
division. Locations of Ohio’s 26 USHCN stations are situated to
minimise urban heat island effects and have remained constant since
1895, thus making their data particularly useful for climate change
Phenological responsiveness
To determine the effect of temperature on flowering date, or a spe-
cies’ phenological responsiveness (q
, day °C
), we regressed D
against the average temperature of each specimen’s month of flow-
ering and the 3 months prior (
), for that specimen’s year and cli-
mate division of collection. That is,
¼ b
þ q
where D
is the flowering date of specimen i in species x, T
the average temperature of the month of D
and 3 months prior,
and q
is the slope, or temperature effect size, of this relationship.
For example, if a specimen was collected on May 26, 1940, in cli-
mate division 10, it would be paired with the average combined
temperatures of February, March, April and May, 1940 in that cli-
mate division (in this example, 5.21 °C, see Appendix S2). The
regression model for each species was built from specimens col-
© 2013 The Authors. Ecology Letters published by John Wiley & Sons Ltd and CNRS
1038 K. M. Calinger, S. Queenborough and P. S. Curtis Letter
lected in different years and from a variety of climate divisions, with
a minimum of 10 specimens required for each regression. To estab-
lish the temperature averaging period,
, we correlated D
every species with the average temperature of its month of flower-
ing and the 11 months prior (see Miller-Rushing & Primack 2008
and Appendix S2). We found strong correlations between D
the average temperature of the month of D
and 3 months prior,
and thus used
in the regression models. However, as species
flowering in April were not strongly correlated with average January
temperatures, D
for these species were regressed only against Feb-
ruary, March and April monthly average temperatures. Flowering
time of species flowering in late-summer were typically not strongly
correlated with temperature and were also regressed against
While precipitation is often cited as an important driver of phenol-
ogy in tropical environments (Rathcke & Lacey 1985), we did not
assess the impacts of precipitation on phenological responsiveness
as temperature is generally considered the primary variable driving
phenology in temperate mesic environments such as our study site.
A total of 141 species (x = 141) using 5053 specimens (i = 5053)
were included in our analysis with sample sizes ranging from 10 to
235 specimens per species (see Appendix S3).
Functional groups
Each species was characterised based on its season of flowering
[spring (April to May), early summer (June to July) or late summer
(August or later)], pollination syndrome (wind or insect, and faculta-
tive or obligate outcrossing), origin (native or non-native to North
America) and growth form (woody perennial, herbaceous perennial,
herbaceous annual or perennial vine). Information on species’ growth
forms was obtained from the USDA Plants Database [http://www.; Prasad et al. (2007)]. Some species exhibit differing
growth forms under different environmental conditions and were clas-
sified as having multiple growth forms (i.e. herbaceous annual/bien-
nial). Species with a biennial or annual/perennial growth habit were
grouped with annuals (as in Fitter & Fitter 2002, see Appendix S3).
Within pollination syndrome, we classified species as insect- or wind-
pollinated based on an extensive literature search and the USDA Tree
Atlas ( Insect-
pollinated species were further classified as facultative or obligate out-
crossers. Obligate out-crossing species are completely self-incompatible
while facultative out-crossing species are capable of selfing.
All 141 species were classified according to growth form, flower-
ing time and origin. Thirty-eight species could not be definitively
classified with regard to pollination syndrome.
Statistical analysis
We evaluated differences in the strength of phenological response
among functional groups using a linear mixed effects model using
the package lme4 of the statistical software R which uses restricted
maximum likelihood methods to determine effect sizes (R Develop-
ment Core Team 2008; Bates et al. 2012). Because lme4 does not
return P-values, we performed a likelihood ratio test using maxi-
mum likelihood methods for each term in the model to determine
significance (Moore 2010). For each functional group, we included a
covariate in our initial model of flowering date as a function of
temperature, thereby allowing slope to vary with functional trait
(eqn 2). To account for the variation in phenological response
among species, the model included the species level intercept and
slope of
as random effects, thus allowing both the slope and
intercept as a function of
to vary among species.
We examined spatial heterogeneity of phenological responsiveness
among climate divisions (Appendix S1). We used a linear mixed
effects model with climate division as a covariate added to the ini-
tial model of flowering date as a function of temperature with spe-
cies as a random effect. This model allows the slope to vary
between climate divisions and produces mean phenological respon-
siveness values for each of the 10 climate divisions. We found sig-
nificant variation in responsiveness only among climate divisions 6
and 7 suggesting little spatial heterogeneity in phenological respon-
siveness (Appendix S1, Fig. 2).
We further assessed the impacts of spatial variation in our models
of functional trait heterogeneity in phenological responsiveness by
adding Division, along with species, as random effects to these ini-
tial models. This model allows the slope of
and the intercept to
vary both among species and divisions. Our models were not signif-
icantly altered by adding Division as a random effect, suggesting
that spatial heterogeneity adds no unexplained variation to our func-
tional group analyses (Appendix S1, Figs 3 and 4). Thus, Division
was not included in the final models.
Because of their shared evolutionary history, closely related species
are likely to be more similar than those species less closely related, lead-
ing to non-independence of data and violation of the assumptions of
statistical tests in comparative analyses that do not account for this
autocorrelation (Revell et al. 2008). To test whether significant phyloge-
netic autocorrelation was present in the phenological responsiveness of
species, we first generated a phylogenetic tree of our study species using
the software Phylomatic (Stevens 2004; Webb & Donoghue 2005; see
Appendix S4). The influence of phylogenetic non-independence on
phenological responsiveness was modelled by incorporating the phylo-
genetic covariance matrix in a generalised least squares model. The phy-
logenetic covariance structure was multiplied by a phylogenetic signal
value (k), ranging from 0 (no phylogenetic autocorrelation) to 1 (maxi-
mum phylogenetic autocorrelation), and the log-likelihood of each run
was recorded; from the resulting likelihood surface a maximum likeli-
hood phylogenetic signal value of k was obtained (Pagel 1999). k mea-
sures the degree to which the variation/covariation of traits across a
tree agrees with the Brownian process (Freckleton et al. 2002). A value
of k = 0 implies that phenology is distributed among species at random
with respect to phylogeny. A value of k = 1 indicates that phenology is
phylogenetically conserved; that is, closely related groups have more
similar flowering times than would be expected by chance. Approxi-
mate confidence intervals for the maximum likelihood value of k were
calculated via likelihood ratio tests (Freckleton et al. 2002) on values
derived from the likelihood surface. We used the maximum likelihood
method ‘pgls.profile’ function in the R package ‘caper’ to estimate the
likelihood profile for k for species phenological responsiveness (Orme
et al. 2012, see Appendix S5).
Average spring temperatures (February to May) across Ohio have
increased by 0.9 °C(P 0.01) since 1895 (Fig. 1a) with some areas
experiencing warming of up to 2 °C (Fig. 1b). Seventeen of the 26
counties within Ohio with USHCN weather stations recorded a sig-
nificant warming trend in the spring. Temperature trends for the
late summer to early fall (June to September) were more variable
© 2013 The Authors. Ecology Letters published by John Wiley & Sons Ltd and CNRS
Letter Earlier flowering and climate change in N. America 1039
with no significant state-wide changes over the past 115 years. Ten
counties reported a significant cooling trend since 1895 (average
1 °C) for late summer to early fall, two showed a significant
warming trend (average 0.8 °C), and 14 showed no significant
Phenological responsiveness to temperature change showed a
high degree of variability among species (Fig. 2). Sixty-six species
(46%) showed a significant negative phenological response (i.e.
advancement of flowering date with increasing temperature), while
only two species (1%) showed a significant positive phenological
response (i.e. delay in flowering time with increasing temperature).
Among significantly responsive species, Carduus nutans, an intro-
duced spring flowering perennial showed the greatest advancement
of flowering (12 days °C
) while Monotropa uniflora, a native,
early-summer flowering perennial showed the greatest delay in flow-
ering (5 days °C
). On average, flowering advanced 2.4 days °C
across all species, or 3.7 days °C
among those species showing a
significant negative phenological response. With the increase in
spring temperatures state-wide over the past 115 years, flowering
may already be 13 weeks earlier in particularly responsive species
or in areas of locally greater warming.
Importantly, we found no significant phylogenetic signal in our data
(see Appendices S4 and S5). Thus, we can interpret the impacts of
functional groups on phenological responsiveness independently from
phylogenetic signal. We found significant differences in phenological
responsiveness to temperature among functional groups based on sea-
sonality of flowering, growth form and origin (Fig. 3).
There were strong differences in species’ phenological responsive-
ness based on flowering season. Spring flowering species were sig-
nificantly more responsive to increasing temperatures
(2.5 days °C
= 118.5, P < 0.001, Fig. 3) than either
early- or late-summer flowering species (1.4 days °C
= 6.1, P = 0.014, and 0.6 day °C
= 10.4,
P = 0.001; Fig. 3). Given these differences we then evaluated the
remaining functional group responses separately for spring, early-
summer, and late-summer flowering species. There were few signifi-
cant differences among functional groups in the early- or late-sum-
mer, and thus we focused on between-group differences in spring-
flowering species (Appendix S6, Tables 2.12.3).
Growth form also played a role in a species’ phenological respon-
siveness, with herbaceous annuals and woody perennials showing a
similar, strong negative phenological response to increased tempera-
ture (herbaceous annuals: 2.2 days °C
= 25.0,
1895 1915 1935 1955 1975 1995 2015
Spring temperature anomaly (°C)
y = 0.0008x – 0.546
y = 0.0177x – 1.205
Trumbull county
Figure 1 Spring temperature trends across the state of Ohio (a) and in Trumbull
County (b). Yearly spring (February to May) temperature anomalies were
calculated by subtracting the 115-year (18952009) average for these four
months from yearly averages. Simple linear regression lines were added to
determine shifts in spring temperatures over the 115-year period. Trumbull Co.
has experienced a 2.0 °C average spring temperature increase compared with the
state-wide 0.9 °C average increase.
Figure 2 Variation in phenological responsiveness to changing temperature
among 141 species of plants in Ohio. (a) For each species, the day of year of
maximum flowering (D
) was regressed against the average temperature from
the average month of flowering and the 3 months prior T
. The slope of each
line quantifies the phenological responsiveness for each species. (b) Rank order
of each species’ phenological responsiveness is represented by a point SE
bars. Closed points show a significant (P 0.05) phenological response to
temperature, while open points designate no significant change. The dashed line
indicates 0, or no phenological response. A negative phenological responsiveness
indicates earlier flowering with warming while a positive shift represents delayed
flowering with warming.
© 2013 The Authors. Ecology Letters published by John Wiley & Sons Ltd and CNRS
1040 K. M. Calinger, S. Queenborough and P. S. Curtis Letter
P < 0.001; woody perennials: 2.4 days °C
= 0.2,
P = 0.68; Fig. 3). Herbaceous perennials had a weaker phenologi-
cal response of 1.5 days °C
compared with woody perennials
and annuals, although this difference was not significant
= 2.38, P = 0.12). In contrast to the other growth form
groups, perennial vines delayed flowering with increased tempera-
tures although this response was not significant (2.1 days °C
= 4.8, P = 0.028). However, the three perennial vine species
in our dataset were all early- or late-summer flowering species, and
thus growth form was confounded with seasonality for these spe-
cies. Further, there was significant variability in phenological
responsiveness among growth forms during the spring (Fig. 4).
Herbaceous annuals showed the strongest spring flowering
response of 3.4 days °C
= 66.1, P < 0.001), and were
significantly more responsive than herbaceous perennials, which
displayed the weakest response of 2.4 days °C
= 6.9,
P=0.008). Woody perennials shifted flowering 2.9 days °C
and were not significantly different from annuals or herbaceous
perennials (LR
= 1.0, P = 0.325).
There were no significant differences in phenological responsive-
ness among different pollination syndromes across seasons (Fig. 3).
However, pollination syndrome significantly impacted phenological
responsiveness of spring flowering species (Fig 4). Spring flowering
wind-pollinated species had the strongest phenological responsive-
ness of 4.3 days °C
= 49.6, P < 0.001) compared to the
weaker responses of facultative and obligate out-crossing insect-pol-
linated species (facultative: 2.7 days °C
= 6.1, P = 0.014,
obligate: 2.3 days °C
= 9.5, P = 0.002 respectively).
Introduced species were almost twice as responsive in advancing
flowering time with temperature (2.8 days °C
= 26.9,
P < 0.001) compared to native species (1.5 days °C
= 5.9, P=0.015, Fig. 3). The three most phenologically
responsive species in our data set, Datura stramonium
(13.5 days ° C
), Carduus nutans (12.5 days °C
) and Trifolium
pratense (7.6 days °C
) all were non-natives. Further, the effect of
place of origin was even more pronounced among herbaceous
annuals, with introduced species of that group advancing flowering
4.2 days °C
vs. 1.3 days °C
in native species (LR
= 6.87,
P = 0.01, Fig. 5). We found no significant difference between native
and non-native species among herbaceous or woody perennials
(Fig. 5). When comparing only spring flowering species, introduced
species were not significantly more responsiveness than native spe-
cies (introduced: 3.5 days °C
, native: 2.5 days °C
= 2.9, P = 0.08; Fig. 4).
This study is the first to assess large-scale patterns of phenological
responsiveness with broad species representation. Among the 141
species studied, 46% showed significant advancement of flowering
phenology with higher temperatures. Considering the average
0.9 °C temperature increase across Ohio over the past 115 years,
this suggests that climate change is already altering our ecosystems
by causing some species to flower as much as 3 weeks earlier while
others show no change. For example, in Trumbull County, where
average spring temperatures have increased by 2 °C, Phacelia purshii
now flowers 2 weeks earlier while Cypripedium acaule advances flow-
ering by only 1 week and Cardamine diphylla shows no change.
Phenologic responsiveness (d/ºC)
–4 –2 0 2 4 6
Perennial vine
Woody perennial
Herbaceous perennial
Herbaceous annual
Late summer
Early summer
Figure 3 Phenological responsiveness to increasing temperature within different
plant functional groups. Mean 1 SE with the number of species per group
indicated in parentheses. Asterisks indicate a significant difference from the
reference group, which is the topmost group in a sub-panel (P = 0.05).
Phenologic responsiveness (d/ºC)
–6 –5 –4 –3 –2 –1
Woody perennial
Herbaceous perennial
Herbaceous annual
Figure 4 Phenological responsiveness to rising temperature among spring
flowering species separated by functional group. Points indicate group mean
phenological responsiveness with standard error bars; the number of species
included in a group is given in parentheses. Groups that are significantly
different from the reference group (the topmost group in a sub-panel) are
shown with asterisks (P = 0.05).
Phenologic responsiveness (d/ºC)
–6 –5 –4 –3 –2 –1 0 1
Woody introduced
Woody native
Herb. perennial introduced
Herb. perennial native
Herb. annual introduced
Herb. annual native
Figure 5 Differing phenological responsiveness to temperature based on origin
among differing growth forms. Group mean phenological responsiveness is
indicated by points and 1 SE bars. Species numbers per group are given in
parentheses; significant differences between groups are indicated with asterisks
(P = 0.05).
© 2013 The Authors. Ecology Letters published by John Wiley & Sons Ltd and CNRS
Letter Earlier flowering and climate change in N. America 1041
Along with species-specific differences, we found distinct patterns
of phenological responsiveness among functional groups. Our
results are a significant addition to the limited number of studies
worldwide that have evaluated phenological responsiveness at the
functional group level. By expanding our understanding of how
functional traits shape group-level responsiveness in a novel region
as well as across broad spatial scales, we are now better able to pre-
dict species-specific and community scale responses to climate
change. Given the direct reproductive consequences of phenology
shifts, selection for or against phenologically responsive species with
future climate warming may result in an unbalanced loss or gain of
species sharing certain functional characteristics.
Overall, spring flowering species showed higher phenological
responsiveness to temperature than did early- and late-summer
flowering species. Earlier onset of growth and flowering for spring
species may allow increased light availability by extending the
growth and reproductive periods before canopy closure (Kudo
et al. 2008). Fitter & Fitter (2002) also found that spring flowering
species were most sensitive to temperature increase. Kudo et al.
(2008) suggested a link between earlier phenology, higher produc-
tivity, and reproductive benefits for spring flowering species as a
result of greater light availability. However, earlier flowering spring
species may not increase productivity if leaf phenology remains
stable while flower phenology advances. Advanced flowering for
spring flowering species may still improve reproductive success by
allowing more seeds to germinate and grow in more favourable
conditions before summer droughts. The lower responsiveness of
early- and late-summer flowering species to temperature may sug-
gest that these species respond more strongly to other environ-
mental factors such as precipitation. For instance, Jentsch et al.
(2008) found an average flowering advancement of 4 days in
response to 32 days of drought in ten grassland and heath species.
In contrast, in an experimental warming study, Sherry et al. (2007)
found no impacts of water availability on the flowering phenology
of twelve grassland species with the exception of Panicum virgatum.
Overall, precipitation seems to be a stronger factor driving flower-
ing phenology in tropical rather than temperate environments
(Rathcke & Lacey 1985).
We found significantly earlier flowering with temperature increase
in introduced species than in natives, particularly among herbaceous
annuals. Several introduced species showed particularly strong phe-
nological responsiveness including Carduus nutans (12.5
days °C
), Datura stramonium (13.5 days °C
), Trifolium hybridum
(6.4 days °C
) and Trifolium pretense (7.6 days °C
). Wolkovich
& Cleland (2011) suggest that if high phenological responsiveness
among introduced species allows them to adjust more rapidly to
warming climate, they may become a greater invasion risk. Pheno-
logically responsive non-natives may shift flowering to occupy pre-
viously unfilled niches (the vacant niche hypothesis) or by
advancing flowering earlier in the spring before natives flower
(priority effects, Wolkovich & Cleland 2011). However, the link
between phenological responsiveness and invasiveness remains lar-
gely untested. Willis et al. (2010) found that non-native species in a
67 km
region of New England advanced flowering significantly
more with temperature increase than did native species. In addition,
non-native species classified as invasive advanced flowering by
roughly 9 days more than non-native non-invasives over the past
100 years (Willis et al. 2010). Our similar findings indicate that
phenological responsiveness is linked to non-nativeness at small-
scales (as in Willis et al. 2010) but also across much larger spatial
scales. Given previous linkages between phenological responsiveness
and invasiveness of non-natives, our research may indicate that
introduced species flowering throughout the spring and late summer
could utilise flowering advancement to aid invasion. Unlike natives,
introduced species maintain high phenological responsiveness
throughout the late summer, potentially shifting their flowering
before unfavourable autumn weather begins.
We found that spring flowering wind-pollinated species were
more phenologically responsive than biotically pollinated species. In
contrast to our findings, Fitter & Fitter (2002) found greater
advancement of flowering with increased temperature among bioti-
cally pollinated species with a weaker response among wind-polli-
nated species. Strong phenological responsiveness to temperature
change among wind-pollinated species likely reflects the high benefit
of releasing pollen before trees leaf out in the spring allowing
greater pollen dispersal through the leafless canopy (Rathcke &
Lacey 1985; Whitehead 1969). Bolmgren et al. (2003) found more
synchronous flowering times among a variety of wind-pollinated
species compared to biotically pollinated species, suggesting a strong
response among wind-pollinated species to environmental condi-
tions favouring pollen dispersal (Culley et al. 2002; Bolmgren et al.
2003). Advancing flowering with higher temperatures may be less
advantageous for herbaceous wind-pollinated species than for
woody species that flower in the canopy. Herbaceous wind-polli-
nated species may use over-topping, or growing taller than other
members of the understory, as the primary means of enhancing pol-
len dispersal (Bolmgren et al. 2003).
We found that herbaceous annuals had the highest average pheno-
logical responsiveness in the spring congruent with the results of Fit-
ter & Fitter (2002) and Miller-Rushing & Primack (2008), who both
reported greater flowering shift in annuals compared with perennial
species. With their single growing season, phenologically responsive
annuals may benefit from a rapid onset of productivity and nutrient
uptake, possibly allowing increased nutrient allocation to flowers and
seeds. Earlier flowering annuals may also experience greater pollina-
tor availability by avoiding peak flowering time and thus increase
their reproductive success (Elzinga et al. 2007), although the signifi-
cant advancement of this group may also increase the risk of pollina-
tor mismatch (Kudo et al. 2008). This risk would be particularly
significant for those obligate out-crossing species. Of the 26 annual
species in this study that could be classified regarding pollination syn-
drome, 18 had selfing ability and would likely have relatively little
impact from pollinator mismatch. Along with potential benefits of
flowering advancement, highly responsive annual species may also
run a significant risk of frost damage. Extreme weather events
including unusually high and low temperatures are becoming more
frequent and thus, the risk of late season frosts may increase with cli-
mate change (Inouye 2008). Frost may be a potent selective force
negatively impacting highly responsive annuals as delicate floral struc-
tures are susceptible to frost damage (Inouye 2008).
By combining biological and temperature records, we were able to
study phenological responsiveness of many species across a broad
spatial scale in an area unrepresented by historic observational data
sets. Herbarium collections are common at universities and museums
throughout the Unites States and contain hundreds to thousands of
species collected over long periods of time. Coupled with tempera-
ture data from the USHCN, which has 1218 monitoring stations
throughout the contiguous US, these records are an untapped wealth
© 2013 The Authors. Ecology Letters published by John Wiley & Sons Ltd and CNRS
1042 K. M. Calinger, S. Queenborough and P. S. Curtis Letter
of biological information. By using USHCN temperature data to nor-
malise species’ flowering responses across a large area rather than
using environmental variables specific to limited areas, our method
allows evaluation of phenological responses across much of the Uni-
tes States. Using this method, studies of phenological responsiveness
to climate change in previously unstudied areas may provide crucial
information needed for conservation efforts.
When evaluating phenological responsiveness, particularly with
herbarium specimens, we must consider potential errors associated
with the choice of the target phenophase and varying flowering
duration among species. First-flowering date is commonly chosen
as the target phenophase rather than peak flowering time because
observing plants for the duration of flowering is challenging and
labour intensive. However, assessing FFD can introduce bias
resulting from differing population sizes and sampling efforts
(Miller-Rushing et al. 2008). Our assessment of peak flowering
rather than FFD constrains theses effects on responsiveness calcu-
lations (Miller-Rushing et al. 2008). Evaluation of peak flowering
should also limit uncertainty regarding impacts of flowering dura-
tion on responsiveness calculations by sampling at a specific point
along a potentially broad distribution of flower opening. Further,
Primack et al. (2004) evaluated differences in flowering shifts
derived using herbarium specimens of species with brief (1 week
or less), medium (2 weeks) and long flowering durations (3 weeks
or more). The mean flowering shifts for each of these groups
were not significantly different, indicating that flowering duration
did not impact flowering shift calculations. Through our analysis
of peak-flowering, it is unlikely that our results are biased by flow-
ering duration, population size fluctuations, or variation in sam-
pling effort over time.
Evaluating species-specific and functional-group level variability
in phenological responsiveness is a key step to understanding how
climate change is already altering and will continue to alter ecosys-
tems. Our results suggest that future warming will differentially
impact plant functional groups. As less responsive species and
groups of species are pushed beyond their optimal temperature
ranges, performance may decline, decreasing species abundance.
Coupled with factors such as habitat loss, decreasing pollinator
diversity, introduced species and altered precipitation patterns, a
decline in phenologically non-responsive species could represent a
major impact on biodiversity.
We gratefully acknowledge the tireless support of Don and Manetta
Calinger and Mary Vargo. We also thank Amanda Wubben who
allowed this research to progress more rapidly through her assis-
tance processing data. Finally, we thank Brandon Sinn for his help
accessing herbarium records.
KMC designed the methods, collected data, performed initial statis-
tical analyses and wrote the first draft of the manuscript. SQ com-
pleted phylogenetic analyses, developed linear mixed effects models
in R and wrote the statistics portion of the methods. PSC provided
guidance and support throughout methods development and statisti-
cal analyses. All authors contributed substantially to revision of the
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Additional Supporting Information may be downloaded via the online
version of this article at Wiley Online Library (
Editor, Yvonne Buckley
Manuscript received 10 January 2013
First decision made 11 February 2013
Second decision made 30 March 2013
Third decision made 12 April 2013
Manuscript accepted 6 May 2013
© 2013 The Authors. Ecology Letters published by John Wiley & Sons Ltd and CNRS
1044 K. M. Calinger, S. Queenborough and P. S. Curtis Letter
... As such, changes in the availability of these resources can impact growth forms differentially, resulting in varying impacts to their leaf out and flowering timing. Recent work shows that tree phenology may be shifting more quickly than the phenology of understory forbs in this region (Heberling et al., 2019), though comparisons in phenological change among growth forms has been limited to only a handful of studies (e.g., Calinger et al., 2013;Crimmins et al., 2010Crimmins et al., , 2011Heberling et al., 2019). ...
... Finally, we predict that insect-pollinated species will exhibit greater advancement in first flower dates due to selection promoting earlier flowering to maintain synchrony with pollinators (Prediction 6; Calinger et al., 2013;Fitter & Fitter, 2002). Pollination syndrome is a key factor to explore because of the importance of climate change impacts on plant-pollinator mutualisms (Forrest, 2015;Gérard et al., 2020;Kudo & Cooper, 2019). ...
... In this study, we used only data for species for which we had phenology observations for more than 3 years in each time period and at least 12 observations total for each phenophase. These criteria are similar to those used in studies of regional multi-species phenology based on herbarium specimens (e.g., Calinger et al., 2013;Everill et al., 2014) and single site historical comparisons . The species that lacked species or phenophase cross-over were removed (there were initially 909 species total between the two time periods so they were pared down drastically to meet our minimum requirement). ...
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1. Understanding the breadth and complexity of changes in phenology is limited by the availability of long‐term historical datasets with broad geographic range. 2. We compare a recently discovered historical dataset of plant phenology observations collected across the state of New York (1826‐1872) to contemporary volunteer‐contributed observations (2009‐2017) to evaluate changes in plant phenology between time periods. These multi‐site, multi‐taxa phenology data matched with temperature data uniquely extend historical observations back in time prior to the major atmospheric effects of the Industrial Revolution. 3. The majority of the 36 trees, shrubs and forbs that comprised our analyzable dataset flowered and leafed out earlier in contemporary years than in the early‐to‐mid 19th century. This shift is associated with a warming trend in mean January‐to‐April temperatures, with flowering and leafing advancing on average 3 days/°C earlier. On average, plants flowered 10.5 days earlier and leafed out 19 days earlier in the contemporary period. Urban areas exhibit more advanced phenology than their rural counterparts overall, insect‐pollinated trees show more advanced phenology than wind‐pollinated trees and seasonality and growth form explain significant variation in flowering phenology. The greatest rates of temperature sensitivity and change between time periods for flowering are seen in early‐season species, particularly trees. Changes in the timing of leaf out are the most advanced for trees and shrubs in urban areas. 4. Synthesis: Citizen science observations across two centuries reveal a dramatic, climate‐driven shift to earlier leaf out and flowering. The magnitude of advancement varies across settings, species and functional groups, and illustrates how long‐term monitoring and citizen science efforts are invaluable for ecological forecasting and discovery.
... In Europe, land temperatures have increased c. 1.5°C since 1900 (Luterbacher et al., 2004;Harris et al., 2014;European Environmental Agency, 2020), so the magnitude of overall phenological changes we observed is similar to what would be expected based on climate change and the observed temperature sensitivities (1.5°C × 3.6 d/°C = 5.4 d, vs our observed average shift of c. 6 d). Previous herbarium studies from the temperate zone estimated similar flowering-time advancements of −2.4 to −6.3 d per 1°C temperature increase (Primack et al., 2004;Miller-Rushing et al., 2006;Panchen et al., 2012;Calinger et al., 2013;Hart et al., 2014;Bertin, 2015;Davis et al., 2015;Bertin et al., 2017). Again, most of these studies were from the northeastern USA, and they were often geographically very restricted. ...
Today plants often flower earlier due to climate warming. Herbarium specimens are excellent witnesses of such long‐term changes. However, the magnitude of phenological shifts may vary geographically, and the data are often clustered. Therefore, large‐scale analyses of herbarium data are prone to pseudoreplication and geographical biases. We studied over 6000 herbarium specimens of 20 spring‐flowering forest understory herbs from Europe to understand how their phenology had changed during the last century. We estimated phenology trends with or without taking spatial autocorrelation into account. On average plants now flowered over 6 d earlier than at the beginning of the last century. These changes were strongly associated with warmer spring temperatures. Flowering time advanced 3.6 d per 1°C warming. Spatial modelling showed that, in some parts of Europe, plants flowered earlier or later than expected. Without accounting for this, the estimates of phenological shifts were biased and model fits were poor. Our study indicates that forest wildflowers in Europe strongly advanced their phenology in response to climate change. However, these phenological shifts differ geographically. This shows that it is crucial to combine the analysis of herbarium data with spatial modelling when testing for long‐term phenology trends across large spatial scales.
... Global warming seems to force plant species to shift their ranges in latitudinal and/or elevation gradients, in search of more favorable climatic conditions [4,16], while in certain cases, it can even act as an amplifier of their vulnerability to extinction [13,[17][18][19]. In both hemispheres, plant species tend to migrate towards the poles, while in altitudinal gradient, they shift mostly uphill (e.g., [4,13,16,20,21]), with exceptions of species following opposite directions existing for both patterns (i.e., towards the equator and downhill, respectively (see [4,5]). ...
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Numerous orchid species around the world have already been affected by the ongoing climate change, displaying phenological alterations and considerable changes to their distributions. The fly orchid (Ophrys insectifera L.) is a well-known and distinctive Ophrys species in Europe, with a broad distribution across the continent. This study explores the effects of climate change on the range of O. insectifera, using a species distribution models (SDMs) framework that encompasses different climatic models and scenarios for the near- and long-term future. The species’ environmentally suitable area is projected to shift northwards (as expected) but downhill (contrary to usual expectations) in the future. In addition, an overall range contraction is predicted under all investigated combinations of climatic models and scenarios. While this is moderate overall, it includes some regions of severe loss and other areas with major gains. Specifically, O. insectifera is projected to experience major area loss in its southern reaches (the Balkans, Italy and Spain), while it will expand its northern limits to North Europe, with the UK, Scandinavia, and the Baltic countries exhibiting the largest gains.
... The trends we obtained from herbarium data are comparable to findings of other phenology studies based on long-term field-collected data in the UK and elsewhere in Europe for the same species, with flowering advancing 6-7 days per increase in degrees Celsius (Sparks et al. 2000;Menzel 2003). The importance of long time series for studying the phenological response to climate has long been recognized, and numerous studies have now demonstrated the value of herbarium specimens in recreating long-term phenology data and advancing the study of global change (Primack et al. 2004;Calinger et al. 2013;Davis et al. 2015;Lang et al. 2019). Citizen science data of equal sample size to our herbarium data from a directed phenology monitoring program detected no trends between flowering DOY and climate. ...
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... Because most of the collections are dated and geolocated, they constitute a valuable source of information for (i) determining the proven or potential distribution areas of species [21][22][23], whether native or exotic (dynamics of biological invasions), with direct applications in conservation biology [24], and for (ii) determining the reproductive phenological patterns of species (e.g., date and duration of flowering and fruiting periods) [25][26][27][28]. Research in these fields has been particularly stimulated by questions related to climate change and its effect on the range of species distribution [29,30] or their biological rhythms [31][32][33][34][35][36][37][38][39]. More original aspects have been addressed such as changes over time in (i) herbivory [40,41], (ii) the concentration of isotopes (δC13, δO18) related to water use efficiency or photosynthetic efficiency [42], or (iii) the diversity of endophytic fungi present in leaves [43]. ...
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A better knowledge of tree vegetative growth phenology and its relationship to environmental variables is crucial to understanding forest growth dynamics and how climate change may affect it. Less studied than reproductive structures, vegetative growth phenology focuses primarily on the analysis of growing shoots, from buds to leaf fall. In temperate regions, low winter temperatures impose a cessation of vegetative growth shoots and lead to a well-known annual growth cycle pattern for most species. The humid tropics, on the other hand, have less seasonality and contain many more tree species, leading to a diversity of patterns that is still poorly known and understood. The work in this study aims to advance knowledge in this area, focusing specifically on herbarium scans, as herbariums offer the promise of tracking phenology over long periods of time. However, such a study requires a large number of shoots to be able to draw statistically relevant conclusions. We propose to investigate the extent to which the use of deep learning can help detect and type-classify these relatively rare vegetative structures in herbarium collections. Our results demonstrate the relevance of using herbarium data in vegetative phenology research as well as the potential of deep learning approaches for growing shoot detection.
... Each specimen contains a wealth of information including geographic occurrence data, phenotype, genotype, phenological status, and biotic interactions (Funk, 2003;Heberling and Burke, 2019). Collectively herbarium specimens are analyzed for studies in taxonomy, systematics, floristics, ecology, phenology, conservation, and global environmental change (Funk, 2003;Calinger et al., 2013;Willis et al., 2017;Lang et al., 2019;Albani Rocchetti et al., 2021). ...
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Herbarium sheets present a unique view of the world's botanical history, evolution, and biodiversity. This makes them an all–important data source for botanical research. With the increased digitization of herbaria worldwide and advances in the domain of fine–grained visual classification which can facilitate automatic identification of herbarium specimen images, there are many opportunities for supporting and expanding research in this field. However, existing datasets are either too small, or not diverse enough, in terms of represented taxa, geographic distribution, and imaging protocols. Furthermore, aggregating datasets is difficult as taxa are recognized under a multitude of names and must be aligned to a common reference. We introduce the Herbarium 2021 Half–Earth dataset: the largest and most diverse dataset of herbarium specimen images, to date, for automatic taxon recognition. We also present the results of the Herbarium 2021 Half–Earth challenge, a competition that was part of the Eighth Workshop on Fine-Grained Visual Categorization (FGVC8) and hosted by Kaggle to encourage the development of models to automatically identify taxa from herbarium sheet images.
... Although in some plant species, petal closure is apparently independent of specific external regulation as it occurs at any time of the day, in most species petal movement is repeatable showing a relationship with the time of day, such as dayblooming flowers (close flowers at dusk or night, i.e., nyctinasty) and night-blooming flowers (close flowers at dawn), which are widely reported and explored (reviewed by van Doorn & Kamdee, 2014;van Doorn & van Meeteren, 2003). Most of the previous works pointed out that opening and closure of flowers was regulated by several suspected regulatory mechanisms, such as internal circadian rhythm (Trivellini et al., 2016;Yon et al., 2016), light (Bai & Kawabata, 2015;Trivellini et al., 2016), temperature (Calinger et al., 2013;He et al., 2006), moisture (Magalhaes & Angelocci, 1976;Peter et al., 2004;Von Hase et al., 2006), and endogenous hormone (Ke et al., 2018). ...
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Repeatable floral closure with diurnal rhythms, that is, flower opening in the morning and closing in the evening, was widely reported. However, the rhythm of flower opening in the morning but closing in the midday received much less attention. Gentianopsis paludosa, Gentianaceae, has an obvious petal movement rhythm opening in the morning and closing at noon at northeast of the Qinghai-Tibetan Plateau. In this study, we examined the effects of temperature (T), relative humidity (RH), and illumination intensity (II) on G. paludosa's flower closure. Furthermore, we monitored the environmental changes inside and outside of the flowers, aiming to test the effect of floral closure on the stability of microenvironment inside the flower. Finally, we artificially interrupted temporal petal closure and investigated its effects on reproductive fitness. The results showed that high/low temperature contributed more to the flower closure than low RH, while illumination intensity had no significant effect on it. The medium temperature, relative humidity and illumination intensity (environmental conditions at 10:00) did not delay flower closure when flowers at pre-closing period or stimulate reopen when flowers full closed. Floral closure provided a stable temperature condition and a higher RH condition inside the flower. Meanwhile, compulsive opening and delayed closure of flowers decreased the seed-set ratio while no effect was found when flowers were forced to close. We conclude that endogenous rhythm regulates floral closure. The rhythm of petal movement providing a stable microenvironment for G. paludosa, increasing the seed production and saving energy from flower opening maintenance, which might be an adaptive strategy to against unfavorable environmental conditions.
Herbarium specimens provide a critical source of phenological data that can be used to identify the direct and indirect drivers of variation in flowering date within and among species. Specimen-based phenological research in California has been accelerated by digitization efforts such as the California Phenology Network, which has scored and archived the phenological status of over 1.4 million specimens to date. Using this new data source in the Consortium of California Herbaria's CCH2 data portal, we obtained data from 993 specimens of the iconic California Poppy, Eschscholzia californica Cham., along with climate data representing all collection sites. Our goal was to determine how long-term and interannual climate variation affect flowering dates, and whether the magnitude of phenological sensitivity to climate varies across the species' range. We found that specimens collected from chronically warm or dry sites flowered relatively early, and flowering date was more sensitive to long-term mean temperature than to long-term mean precipitation. Independent of these effects of long-term conditions, flowering date in E. californica was sensitive to interannual variation in seasonal precipitation, but the direction of this effect depended on the season in which the precipitation occurred. Specimens sampled from sites experiencing warmer-than-average springs in the year of collection flowered 2.73.3 days earlier for every 1C increase in spring temperature relative to long-term mean spring temperature. The magnitude of these effects, however, varied across the range of E. californica, with greater sensitivity to temperature in relatively cooler regions and no discernible sensitivity in relatively warm regions. Consistently, California Poppies exhibited significant phenological advancement over the last 120 years, but this advancement was restricted to the cooler portions of its range. Our results provide one of the first accounts of intraspecific variation in both phenological sensitivity to climate and the magnitude of phenological shifts over time, and demonstrate that, for a single species, location- or population-specific estimates of phenological sensitivity or of temporal trends in phenology might not accurately predict phenological responses to climate change in other locations throughout its range. In this study, we highlight the utility and promise of herbarium specimens for addressing novel questions about the phenological responses of plants to climate trends.
To date, most herbarium-based studies of phenological sensitivity to climate and of climate-driven phenological shifts fall into two categories: detailed species-specific studies vs. multi-species investigations designed to explain inter-specific variation in sensitivity to climate and/or the magnitude and direction of their long-term phenological shifts. Few herbarium-based studies, however, have compared the phenological responses of closely related taxa to detect: (1) phenological divergence, which may result from selection for the avoidance of heterospecific pollen transfer or competition for pollinators, or (2) phenological similarity, which may result from phylogenetic niche conservatism, parallel or convergent adaptive evolution, or genetic constraints that prevent divergence. Here, we compare two widespread Clarkia species in California with respect to: the climates that they occupy; mean flowering date, controlling for local climate; the degree and direction of climate change to which they have been exposed over the last 115 yr; the sensitivity of flowering date to inter-annual and to long-term mean maximum spring temperature and annual precipitation across their ranges; and their phenological change over time. Specimens of C. cylindrica were sampled from sites that were chronically cooler and drier than those of C. unguiculata, although their climate envelopes broadly overlapped. Clarkia cylindrica flowers 3.5 d earlier than C. unguiculata when controlling for the effects of local climatic conditions and for quantitative variation in the phenological status of specimens. However, the congeners did not differ in their sensitivities to the climatic variables examined here; cumulative annual precipitation delayed flowering and higher spring temperatures advanced flowering. In spite of significant spring warming over the sampling period, neither species exhibited a long-term phenological shift. Precipitation and spring temperature interacted to influence flowering date: the advancing effect on flowering date of high spring temperatures was greater in dry than in mesic regions, and the delaying effect of high precipitation was greater in warm than in cool regions. The similarities between these species in their phenological sensitivity and behavior are consistent with the interpretation that facilitation by pollinators and/or shared environmental conditions generate similar patterns of selection, or that limited genetic variation in flowering time prevents evolutionary divergence between these species.
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The flowering phenology of plants is influenced by the unique set of environmental variations, and therefore, elucidation of important driving factors is important. The study area of Dhirkot (western Himalaya, Pakistan) is explored to record the interactions among the flowering phenology of the vascular plants and current climate along the temporal gradient from March-2015 to February-2018. A total of 38 randomly selected representative sites were visited to record the timing of flowering response and compared with mean monthly climatic data. Multivariate classification and ordination tools were used to analyze the data. The results revealed that majority (185 spp; 68%) of plant species passes through thier flowering phase in the month of July. Canonical correspondence analysis (CCA) results depicted that about 63.7% of the phenological variations were explained by the monthly explanatory climatic variables, and mean minimum temperature, precipitation, wind speed and soil moisture were significantly (p-adj. <0.05) important. Pseudo-canonical correlation of the first three CCA axes was found higher than 0.8 which depicted that the selected variables were important determinants. This study concluded that predicted future temperature increase might alter the phenological responses, and prove to be devastating for valuable plant species of this unique and very delicate western Himalayan ecosystem.
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Maximum likelihood or restricted maximum likelihood (REML) estimates of the parameters in linear mixed-effects models can be determined using the lmer function in the lme4 package for R. As for most model-fitting functions in R, the model is described in an lmer call by a formula, in this case including both fixed- and random-effects terms. The formula and data together determine a numerical representation of the model from which the profiled deviance or the profiled REML criterion can be evaluated as a function of some of the model parameters. The appropriate criterion is optimized, using one of the constrained optimization functions in R, to provide the parameter estimates. We describe the structure of the model, the steps in evaluating the profiled deviance or REML criterion, and the structure of classes or types that represents such a model. Sufficient detail is included to allow specialization of these structures by users who wish to write functions to fit specialized linear mixed models, such as models incorporating pedigrees or smoothing splines, that are not easily expressible in the formula language used by lmer.
1. A data set of 36 years (1954-1989) of observations on first flowering dates (FFD) of 243 species of angiosperms and gymnosperms in one locality in southern central England is presented and analysed. 2. Individual FFDs ranged from 1 January to 17 August, and species varied considerably in the standard deviation of their FFD. The most variable species were mainly annuals and there was a negative relationship between mean FFD and variability, early-flowering species being the most variable. 3. For 219 of the 243 species, it was possible to fit regression equations for FFD to some set of monthly mean temperatures of the preceding months. These fits were generally best for woody plants and geophytes. February temperature was overall the most important determinant of flowering time. Sixty per cent of species flowering between January and April were affected by temperature 1-2 months before flowering; for summer (May onwards) flowering species, temperatures up to 4 months previously were important. 4. High spring temperatures advanced flowering by a mean of 4 days per degree. In contrast, both spring- and summer-flowering species were retarded in flowering by high temperatures in the previous autumn. 5. These relationships were used to simulate the effects of climatic warming: an overall increase of 1-degree-C in each month would advance flowering in some species and retard others, by as much as 6 weeks. Retarded species were early-flowering, advanced species late-flowering. These results suggest a high degree of dependence of flowering time on temperature, and the variation between species implies that responses to climatic warming may be difficult to predict.
Studies of the aerodynamics of particle transport and capture suggest that the conditions most propitious for effective wind pollination are as follows: (1) production and release of large numbers of grains; (2) both anthers and stigma exposed; (3) grains falling within a certain size range (20-40μ); very small grains will be dispersed readily but cannot be captured efficiently by the stigma, large grains have a high terminal velocity, hence will settle too rapidly; (4) exine of grains thin (or with large air spaces), sculpture smooth; (5) the stigma should present much surface area to capture grains, but, since collection efficiency will decrease with increasing stigma diameter, the increase in surface should be accomplished through the evolution of a complex stigma with many branches of small diameter; (6) individuals of the species should not be too widely spaced in the vegetation; (7) the vegetation should be open in structure or deciduous; thus there will be few obstructions to transport during at least one portion of the year; (8) flowering must be closely coordinated by relatively unambiguous environmental stimuli; (9) pollen release (hence flowering) should coincide with the most favorable time of the year for transport (low probability of precipitation, adequate winds and turbulence, deciduous season). The geography of wind pollination in the angiosperms can be understood best in terms of these prerequisites for efficient transport of pollen. Anemophily is very infrequent in the tropical rain forest, becomes more frequent as one moves to more seasonally variable environments, and is frequent in northern temperate forests. The environment of the tropical rain forest is unsuitable for anemophily: (1) species diversity is very high, hence individuals of the same species are apt to be widely spaced; (2) the forest is densely structured, hence wind velocities are low and there are many obstacles to transport; (3) there is no leafless season; (4) rainfall is frequent throughout the year, hence transport will be limited; (5) there are few unambiguous stimuli which can coordinate flowering; (6) potential animal pollinators are abundant. These conditions do not exist in northern temperate forests, or in prairies, or in savannas. It is possible that the evolution of wind pollination in the angiosperms paralleled the evolution of the deciduous habit. Axelrod has suggested that the deciduous habit evolved in response to seasonal drought as angiosperms migrated into lower middle latitudes during the early Cretaceous. Both the deciduous habit and the physical conditions existing just peripheral to the tropics would favor the evolution of wind pollination. An examination of seasonal environments in the tropics at present and the fossil record of deciduous and wind pollinated angiosperms supports this contention.
Community ecologists have long recognized the importance of phenology (the timing of periodic life-history events) in structuring communities. Phenological differences between exotic and native species may contribute to the success of invaders, yet a general theory for how phenology may shape invasions has not been developed. Shifts toward longer growing seasons, tracked by plant and animal species worldwide, heighten the need for this analysis. The concurrent availability of extensive citizen-science and long-term datasets has created tremendous opportunities to test the relationship between phenology and invasion. Here, we (1) extend major theories within community and invasion biology to include phenology, (2) develop a predictive framework to test these theories, and (3) outline available data resources to test predictions. By creating an integrated framework, we show how new analyses of long-term datasets could advance the fields of community ecology and invasion biology, while developing novel strategies for invasive species management. Although we focus here on terrestrial plants, our framework has clear extensions to animal communities and aquatic ecosystems as well.