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Little change in the fir tree-line position on the
southeastern Tibetan Plateau after 200 years of warming
Eryuan Liang1, Yafeng Wang1,3, Dieter Eckstein2 and Tianxiang Luo1
1
Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, PO
Box 2871, Beijing 100085, China; 2Department of Wood Science, University of Hamburg, 21031 Hamburg, Germany; 3Graduate University of Chinese
Academy of Sciences, Beijing, 100049, China
Summary
Author for correspondence:
Eryuan Liang
Tel: +86 10 62849380
Email: liangey@itpcas.ac.cn
Received: 19 October 2010
Accepted: 7 December 2010
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doi: 10.1111/j.1469-8137.2010.03623.x
Key words: Abies georgei var. smithii, age
structure, climate change, dendroecology,
recruitment, southeastern Tibetan Plateau,
tree-line dynamics.
• As one of the world’s highest natural tree lines, the Smith fir (Abies georgei var.
smithii) tree line on the southeastern Tibetan Plateau is expected to vary as a function of climate warming. However, the spatial patterns and dynamics of the Smith
fir tree line are not yet well understood.
• Three rectangular plots (30 m · 150 m) were established in the natural alpine
tree-line ecotone on two north-facing (Plot N1, 4390 m asl; Plot N2, 4380 m asl)
and one east-facing (Plot E1, 4360 m asl) slope. Dendroecological methods were
used to monitor the tree-line patterns and dynamics over a 50-yr interval.
• The three study plots showed a similar pattern of regeneration dynamics, characterized by increased recruitment after the 1950s and an abrupt increase in the
1970s. Smith fir recruitment was significantly positively correlated with both
summer and winter temperatures. However, Smith fir tree lines do not show a
significant upward movement, despite warming on the Tibetan Plateau.
• The warming in the past 200 yr is already having a significant impact on the
population density of the trees, but not on the position of the Smith fir tree line.
Introduction
The high-altitude limit of forests, commonly referred to as
the tree line, timberline or forest line, represents one of the
most obvious vegetation boundaries (see a schematic representation in Fig. 1 by Körner & Paulsen, 2004). The treeline ecotone refers to the transition from the timberline to
the treeless alpine vegetation (Körner, 2003). Given the
importance of temperature in controlling the elevation of
alpine tree lines and in constraining tree regeneration and
growth, tree lines are likely to respond quickly to climate
change, showing changes in structure and position (Körner,
2003; Holtmeier & Broll, 2007; Harsch et al., 2009).
Despite the complex ecology of their dynamics, polar and
high-elevation tree lines are generally expected to advance
in response to global warming (Jobbagy & Jackson, 2000;
Kullman, 2001; Malanson, 2001; Grace et al., 2002; Liu
et al., 2002; Dai et al., 2005; Holtmeier & Broll, 2007;
Payette, 2007; Harsch et al., 2009). The easily distinguishable transition between forest and tundra ecosystems allows
for the measurement of shifts in tree-line location (Körner,
2003). The use of dendroecological methods can provide a
rich source of data on tree-line dynamics reaching back for
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several centuries (Payette & Filion, 1985; Lloyd & Fastie,
2003; Camarero & Gutiérrez, 2004). For example, a warm
period in the early Holocene has been associated with the
expansion of the tree line beyond its current position in
northern Eurasia (MacDonald et al., 2000). A growing
body of evidence, accumulated over the past few decades,
has also revealed that the tree line is moving upslope in the
Swedish Scandes, North America and Eurasia (Kullman,
2001; Lloyd & Fastie, 2003; Esper & Schweingruber,
2004; Payette, 2007). In a global database, comprising 166
sites for which tree-line dynamics have been recorded since
AD 1900 (Harsch et al., 2009), no tree lines were retreating,
but tree-line advances were recorded at 52% of sites. In
addition, diffuse tree lines were responding to both winter
and summer warming, and abrupt or Krumholtz tree lines
were responding to winter warming. Upward shifts in tree
lines are often interpreted as evidence of global warming.
On the southeastern Tibetan Plateau, alpine tree lines are
among the highest worldwide (Li, 1985; Miehe et al., 2007;
Opgenoorth et al., 2010), but their dynamics have not been
studied in detail (Luo et al., 2005; Schickhoff, 2005; Li
et al., 2008; Shi et al., 2008). Recent studies have suggested
that warming has caused unprecedented glacial retreat on
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the Tibetan Plateau (Yao et al., 2004). There is also some
photographic evidence of a rapid tree-line advance in accordance with recent glacial retreat in the Hengduan
Mountains on the southeastern Tibetan Plateau (Baker &
Moseley, 2007). In spite of the great potential of this ecosystem for monitoring the effects of climate warming, the
patterns and dynamics of tree lines on the Tibetan Plateau
have received little attention. Considering the negligible
human influence, the Smith fir tree line in the Sygera
Mountains provides a model for the detection of fluctuations in natural alpine tree lines on the Tibetan Plateau.
Therefore, the objective of this study was to examine the
spatio-temporal dynamics of natural Smith fir tree lines in
response to climate change over the past 400 yr in the
Sygera Mountains on the southeastern Tibetan Plateau.
We hypothesized that the upslope movement of Smith fir diffuse tree lines in the Sygera Mountains would be expected to
be proportional to both winter and summer warming (as
reported by Harsch et al., 2009), and that trees would be
expected to grow more densely in response to warming. In
order to detect a distinct tree-line upslope movement signal,
the upslope regeneration (trees with a height ‡ 2 m) beyond
the height of the tree canopy (c. 10 m in the three research
plots) served as a minimum movement criterion.
Materials and Methods
Study area and climate
The study area is located in the Sygera Mountains (2910¢–
3015¢N, 9312¢–9535¢E) on the northwestern side of the
Great Canyon of the Yarlung Zangbo River in southeastern
Tibet (Fig. 1).
Southeastern Tibet is characterized by a semi-humid climate. The south Asian monsoon reaches the Sygera
Research
Mountains through the Yarlung Zangbo River valley,
resulting in ample summer rainfall. Records from the meteorological station in Nyingchi (Linzhi) (2934¢N, 9428¢E,
3000 m asl) (range of 18–25 km from the three plots) show
an average annual precipitation (1960–2009) of 672.7 mm,
71.8% of which is brought by the summer monsoon, prevailing from June to September. July (mean temperature of
15.8C) and January (0.5C) are the warmest and coldest
months, respectively. As recorded by an automatic weather
station at the tree line (4400 m asl), the mean annual
temperatures were 0.56, 0.03 and 0.83C in 2007, 2008
and 2009, respectively, and the annual precipitation values
were 882, 960 and 754 mm, respectively. Snow of 50–
100 cm in depth occurs from November to May. The mean
daily temperature ranged from )15 to 10C from
November 2006 to May 2010, and soil temperature varied
from )5.1 to 11.7C (June 2007–May 2010) at 10 cm
depth.
The temperatures in southeastern Tibet have shown a
clear warming trend in both annual and seasonal means
over recent decades (Liang et al., 2009). As shown for the
meteorological station in Nyingchi, the regional mean and
minimum temperatures in all seasons have increased significantly since 1960 (Liang et al., 2009); the increase in
maximum temperature is significant only in spring
(March–May) and winter (December–February). In addition, annual and summer precipitation values have also
increased in recent decades.
Tree species and tree line
Smith fir, with a pencil-shaped crown, mainly grows on shady
or semi-shady slopes. Pollination occurs in May and seeds
become mature in October of the current year. The seed
cones are sessile, black at maturity and of an ovoid-cylindrical
Fig. 1 Map showing the location of the study sites in the Sygera Mountains (E1, N1 and N2) and of the temperature proxy sites on the
Tibetan Plateau. Numbers 1–4 represent the Dunde, Guliya, Puruogangri and Dasuopu glaciers, respectively.
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shape (4.8–7.8 cm in length and 2.6–3.9 cm in diameter)
based on a random selection of 30 cones in October 2010.
According to our weekly wood formation study at the tree
line at E1 in 2007, cambial cell division started in mid-May
and ended in mid-August, whereas cell wall thickening
continued to the middle or end of September. The oldest
individual is c. 400 yr old (Liang et al., 2009, 2010). For
trees with an average age of 180 yr, the mean annual ring
width is c. 1.40 mm (Liang et al., 2010). A successful recruitment is partly related to forest gaps resulting from the
breakdown of rotten stems (Luo et al., 2002).
Smith fir grows from the river valley (c. 3600 m asl) to
the upper tree line, which varies from 4250 m (south-facing
slopes) to 4400 m (north-facing slopes) asl depending on
the topography and site exposure. The tree height diminishes from c. 30 m at 3600–3850 m asl to c. 10 m at the
timberline (the maximum tree heights at N1 and E1 are 10
and 12 m, respectively). The stand density decreases with
increasing altitude. There are no stumps or dead trees above
the current tree line. It is a typical diffuse tree line, without
any flagging of trees or Krumholtz formation.
Along the altitudinal gradient, mixed forests of Smith fir,
Quercus aquifolioides and Picea likiangensis var. linzhiensis are
found at c. 3600 m asl, whereas, from 3700 to 4000 m asl,
mixed forests of Smith fir and Lonicera spp. are prevalent.
Smith fir and lichen (Actinothuidium spp.) communities
dominate from 4000 to 4320 m asl, whereas Smith fir and
Rhododendron spp. communities occur from 4320 to
4400 m asl, and alpine shrubs (Rhododendron spp.) and
meadows are found above 4400 m asl (Ren et al., 2007);
above the tree line, rhododendron shrubs, 2–3 m high, cover
c. 80% of the area.
At the tree-line ecotone, the soil is podzolic (an organic
mat over a gray leached layer) with an average pH value of
4.5. With a denser vegetation cover and damp air, the organic
matter on the surface increases and the leaching action intensifies, resulting in an eluviation of most of the iron and
aluminum and an accumulation of silicon (thickness of
c. 30 cm) (Fang, 1997). The soil is generally covered by
organic matter (c. 5 cm) and moss (from 1.0 to 8.2 cm in
thickness).
The Smith fir tree-line sites selected in the Sygera
Mountains have not been disturbed by animals or humans
(e.g. grazing or logging), probably because of their remoteness and the low human population density. In addition,
the forest understory at the three sites is dominated by
rhododendron shrubs. No grasses are available for yak
grazing. Moreover, we did not find any evidence of disturbance
by insects or fire (Liang et al., 2009). The three sites are
located away from avalanche paths and major rocky outcrops, and the plant community has the potential to shift
without being constrained by major terrain features, such as
talus or cliffs. Thus, the Smith fir tree lines in the Sygera
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Mountains are highly representative of natural tree lines on
the southeastern Tibetan Plateau.
Research plots and sampling
Three sites, one on an east-facing slope (Plot E1: slope angle
13, 2939.468¢N, 9442.596¢E, 4360 m asl) and two on
north-facing slopes (Plot N1: slope angle 10, 2937.918¢N,
9442.136¢E, 4420 m asl; Plot N2: slope angle 15,
2938.47¢N, 9442.462¢E, 4380 m asl), were selected. At
each site, a rectangular plot (30 m · 150 m) was placed on a
topographically uniform area of the alpine tree-line ecotone
to include the timberline and the upper species’ limit of
Smith fir. The longer side (y-axis) of each plot was parallel to
the altitudinal gradient of subalpine forest to alpine shrub
land. For the three plots, the point (x, y) = (0, 0) was located
in the bottom left corner. The altitudes of the lower and
upper parts of the plots were determined by GPS. The location of each Smith fir individual within the plots was mapped
on the xy-plane; the coordinates for a given tree locate the
centre of the main stem to the nearest 0.1 m. The following
measurements were made on all Smith fir trees; diameter at
breast height (DBH) (1.3 m), tree height and diameter of
tree’s canopy along the x- and y-axes. Tree height was determined by a measuring stick if the tree was £ 2 m, and with a
clinometer if the tree was > 2 m. Field work was conducted
in autumn 2010.
The age structure of the tree populations was identified by
a number of methods. In order to obtain a core through the
pith of the tree (or as close as possible to the pith), Smith fir
trees with DBH > 5 cm were cored at breast height using
an increment borer. The age of saplings and seedlings
(height < 2.5 m and DBH £ 5 cm) was nondestructively
determined in the field by counting the terminal bud scars
(internodes or branch whorls) along the main stem (according to Camarero & Gutiérrez, 2004; Batllori & Gutiérrez,
2008). Just outside of the three plots, we collected five seedlings with their roots and counted the number of rings in the
root collar zone in the laboratory. Seedling ages obtained
from the root collars were compared with the ages obtained
from counting the internodes in the field; counting the internodes may underestimate the true seedling age by up to 4 yr.
In order to obtain a more accurate estimate of the age of
Smith fir trees in the two height classes of 1.3–2 m and
‡ 2 m, we determined the age of 20 individuals (located
both inside and next to the plots) by counting the internodes. We found that Smith fir seedlings took 29 ± 5 yr
(standard deviation) (at E1) and 31 ± 7 yr (at N1 and N2)
to reach 1.3 m in height, and 33 ± 5 yr and 34 ± 5 yr to
attain a height of 2 m, respectively. As performed in other
studies (Camarero & Gutiérrez, 2004), we assumed that the
time required for a seedling to reach breast height, or 2 m,
was statistically the same at the three sites.
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Linear regression was used to characterize the relationship
between tree age, tree height and DBH; dead trees or trees
with a rotten centre were excluded from this analysis. The
age of trees with a rotten centre was instead estimated on
the basis of the tree age: DBH relationship. We found one
dead tree at N1 and eight at E1, all located at low elevation;
from these trees, increment cores of the outer part of the
stem or disc were collected and the year of tree death was
identified by cross-dating. Then, their age was estimated
using the relationship between tree age and DBH.
Identification of the tree line and current timberline
The location of the tree line was identified by the presence of
upright trees with a minimum height of 2 m at the highest
altitude (Kullman, 2001; Holtmeier, 2003; Camarero &
Gutiérrez, 2004). A change in the altitudinal position of the
alpine tree-line ecotone includes changes in stand density,
tree growth form, tree height, DBH and tree location. The
simplest descriptor of an upward shift of an alpine tree-line
ecotone is the change in elevation at which the highest (altitude) tree is found. The location of the tree line was
determined over a 50-yr interval. On this timescale, the 5-yr
error associated with age estimation for 2-m tall trees is negligible. Hence, we proceeded to document the variability in
the location of the upper tree line over the past 400 yr in each
study plot. The germination date of large trees was estimated
using the tree ages derived from increment cores (taken at
1.3 m) plus 29 or 31 yr for trees at E1 and N1 ⁄ N2, respectively.
The timberline (location of the highest closed forest)
refers to the maximum elevation at which the coverage with
trees > 5 m tall amounts to at least 30% (Holtmeier, 2003)
(along an area of 30 m (x) · 10 m (y)). The canopy coverage was calculated as p(d1 ⁄ 2 + d2 ⁄ 2) ⁄ 2, where d1 and d2
are the diameters of the canopy measured along the x- and
y-axes, respectively.
Recruitment and tree-line dynamics
The recruitment dynamics of Smith fir (> 3 yr old) were
reconstructed using the age structure of the community for
each plot. The reconstruction of stand densities enables a
comprehensive view of the change in stand dynamics
through time. A large pool of 1–3-yr-old seedlings was not
included because most cannot survive. Temporal recruitment patterns were compared between the sites using the
two-sample Kolmogorov–Smirnov test. Abrupt recruitment
increases were defined when the tree abundance increased by
at least 50% when compared with the preceding age class.
Temperature is often a critical factor in controlling tree
species’ recruitment and the tree-line position (Holtmeier &
Broll, 2007; Harsch et al., 2009; Smith et al., 2009). The
meteorological records in Nyingchi (taken since 1960) do
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not date back far enough to evaluate the association between
tree-line dynamics and climate variables. To date, tree-ringbased temperature reconstructions (over the past 400 yr) at
high (Bräuning & Mantwill, 2004; Liang et al., 2009) and
low (Yang et al., 2010) elevations in Nyingchi cannot
capture century-scale temperature variability. However, temperature proxies from tree-ring chronologies on the
northeastern Tibetan Plateau (Liu et al., 2005; Gou et al.,
2008; Zhu et al., 2008; Liu et al., 2009) and from ice core
18
O series (an indicator for summer temperature) obtained
from the glaciers of Dunde (northern Tibetan Plateau),
Guliya (west), Puruogangri (middle) and Dasuopu (south)
(Yao et al., 1996; Thompson et al., 2006) provide a general
picture of temperature variability over the past thousand
years (Fig. 2). These temperature proxies show a general
agreement, characterized by a continuous warming trend
since around the 1820s. The tree-line position and recruitment dynamics over the past 400 yr were evaluated with
respect to these temperature proxies, documenting temperature variability on a coarse scale.
The average September–April temperature (SAT) recorded
at Nyingchi shows a higher correlation with the Climate
Research Unit (CRU)-gridded SAT (the climatic variable in
Fig. 2a) in Wulan (r = 0.64, P < 0.001) (Zhu et al., 2008)
than with the SAT (the climatic variable in Fig. 2b)
recorded from 1961 to 2002 at Zhangye (r = 0.42, P <
0.05) (Liu et al., 2005) on the northeastern Tibetan Plateau.
Therefore, the tree-ring-based winter half-year temperature
reconstruction by Zhu et al. (2008) (Fig. 2a) is used to
represent the long-term temperature situation in Nyingchi.
The integrated ice core 18O series (Fig. 2c) is considered to
be a large-scale indicator of summer temperature for the
Tibetan Plateau.
Results
Age structure of the Smith fir forests
Despite topographical differences, the temporal pattern of
recruitment was not significantly different (at the 0.05 level)
among the three study sites, as indicated by the two-sample
Kolmogorov–Smirnov tests, but showed a continuous reproduction and a high correlation of the decadal recruitment
rates between E1 and N1 from 1751 onwards (r = 0.92,
P < 0.001, n = 26), and between E1 and N2 (r = 0.97, P <
0.001, n = 20) and between N1 and N2 (r = 0.99,
P < 0.001, n = 20) from 1801 onwards (Fig. 3). The oldest
Smith fir trees (in 2010) were 429, 353 and 210 yr at E1, N1
and N2, respectively (Fig. 4). There were more trees
> 100 yr of age at E1 (59 trees) than at N1 (40 trees) and N2
(18 trees). Before the 1950s, the establishment of Smith fir
trees occurred at a low and variable rate.
Trends in the age structure of the tree populations were
similar for the three plots; most striking was the dominance
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(a)
(b)
(c)
Fig. 2 Reconstructed mean temperatures
from the previous September to the current
April for the Wulan area (Zhu et al., 2008)
(a), standardized December–April
temperature-sensitive tree-ring width index
in the Qilian Mountains of the northeastern
Tibetan Plateau (Liu et al., 2005) (b) and
integrated ice core 18O series (an indicator of
summer temperature) from the Dunde,
Guliya, Puruogangri and Dasuopu glaciers on
the Tibetan Plateau (Thompson et al., 2006)
(c). The thick gray curves represent the 100yr low-pass-filtered values and the horizontal
lines represent the long-term means. Z-score
is calculated by dividing the score deviation
(data of each year – mean value of the series)
by the standard deviation of the series.
Fig. 3 Temporal (10-yr interval) dynamics of
Smith fir regeneration within the three
research plots (E1, white bars; N1, gray bars;
N2, black bars) in the Sygera Mountains and
decadal variation in winter half-year
temperature (solid line; see Fig. 2a).
of young trees (established after 1960); they accounted for
81.4%, 84.0% and 81.95% at E1, N1 and N2, respectively
(Figs 3, 4). The age structure approximately follows a
reverse J-shape. This densification trend does not occur only
inside the ecotone, but also above the current tree line. An
abrupt increase in tree recruitment took place in the 1970s,
and the recruitment rate has continued to accelerate since
then. In addition, there are c. 120–200 seedlings (1–3 yr
old) in the three plots. They were not included in Figs 3
and 4, as it cannot be foreseen how many of them will survive in subsequent years.
Tree height and DBH were correlated significantly with
tree age; a strong correlation was also found between tree
height and DBH (Fig. 5).
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Tree-line dynamics
The age structure of the tree populations, measured in 50yr intervals, depicts the tree-line dynamics over the past
400 yr. The two timberlines at the north-facing slopes (N1
and N2) are located in a rather similar elevation, higher
than that at the east-facing slope (E1). There was only one
tree higher than 2 m from 1611 to 1660 at E1, and at N1
from 1661 to 1710. From 1711 onwards, the tree lines
in the three plots only showed a slight movement or were
stable for most of the time (Fig. 4).
At E1, the tree-line position was stable from 1761
onwards, but advanced by 3.3 m between 1711 and 1760,
and by 9.7 m between 1611 and 1660 (Fig. 4). Considering
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(a)
(b)
(c)
Fig. 4 Spatio-temporal variability in tree
density and tree-line position (maximum
elevation with live individuals with stems at
least 2 m high) at the three Smith fir alpine
ecotones (a) E1, (b) N1, (c) N2 in the Sygera
Mountains, southeastern Tibetan Plateau.
Each closed symbol represents an individual
that was established during the period
indicated at the top; open symbols represent
trees established during periods previous to
that indicated at the top. Different symbols
correspond to different tree establishment
periods (e.g. triangles = 1661–1710). The
current timberline (forest cover ‡ 30%) is
indicated by a hatched rectangle
(30 m · 10 m).
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(a)
(b)
(c)
Fig. 5 Relationship between Smith fir age,
height and diameter at breast height (DBH)
in the E1, N1 and N2 plots in the Sygera
Mountains.
that a 10-m upslope movement is a minimum criterion for
the identification of a significant advancement, the tree-line
position at E1 has not changed significantly in the past
400 yr. We should also keep in mind that, from 1611 to
1710, there were only one to three trees growing in the plot,
so that the tree-line position cannot be robustly identified.
At N1, the tree line advanced by 16.8 m from 1761 to
1810, when compared with the interval from 1611 to 1710,
displaying a significant upslope movement. However, it
should be considered that this change referred to only one
old tree established before 1611. In addition, altogether,
there were only two trees between 1611 and 1710, precluding a confident analysis for a tree-line shift. From 1711 to
the present time, the tree-line position is considered not to
have changed significantly given a total upslope movement
of 3.6 m.
At N2, there was only one tree from 1761 to 1810. In
comparison with the period 1811–1910, the tree line
moved 9.2 m from 1911 onwards, not showing a significant
shift.
Potential driving forces for recruitment
The decadal recruitment rates were strongly correlated
with the reconstructed mean winter half-year temperature
(Fig. 2a) (r = 0.68, 0.66 and 0.71 for E1, N1 and N2,
respectively; P < 0.001, n = 42 for E1 and N1; n = 20
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for N2) and with summer temperature (Fig. 2c) (r = 0.65,
0.64 and 0.64 for E1, N1 and N2, respectively; P < 0.001).
A 10-yr lag of the decadal winter or summer temperature
showed less significant correlations with the decadal recruitment rate at E1 (r = 0.52 and 0.58, respectively), N1
(r = 0.52 and 0.60) and N2 (r = 0.57 and 0.56) (P < 0.001
for all). Given an insignificant tree-line upslope movement since 1711, the driving forces underlying the tree-line
dynamics were not evaluated statistically.
Discussion
Regeneration dynamics
The age class distribution in our tree-line stands was used as a
rough indicator of the forest state at the landscape scale
(Holtmeier, 2003; Körner, 2003). It followed a reverse
J-shaped curve, which is considered to be characteristic for
a stable forest (Hett & Loucks, 1976). Despite a less significant tree-line advance, a progressively more recent seedling
establishment at higher elevations was evident in the ecotone.
Our study plots were characterized by increasing recruitment after the 1950s. It was also shown that the stand density
had increased at the upper elevation sites since 1950. Many
studies have demonstrated substantial increases in tree-line
population density during the 20th century in both highlatitude and high-elevation sites in the Northern Hemisphere
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(Payette & Filion, 1985; Szeicz & Macdonald, 1995; Camarero
& Gutiérrez, 2004; Esper & Schweingruber, 2004; Baker &
Moseley, 2007; Danby & Hik, 2007; Kullman, 2007;
Shiyatov et al., 2007; Hallinger et al., 2010). However, these
densifications of tree-line stands commenced in the first half
of the 20th century. In contrast with our findings, poor
recruitment after the 1950s occurred in the central Tianshan
Mountains, northwest China (Wang et al., 2006), and a
seedling establishment deficit was observed after the 1970s
and 1980s at alpine tree lines in southern America (Villalba
& Veblen, 1997). The Smith fir regeneration patterns and
dynamics in the Sygera Mountains may reflect an influence
of the regional climate in southeastern Tibet. In addition, we
should clarify that trees that died c. > 200 yr ago might have
been rotten. Thus, there might have been more trees growing
in the earlier periods than identified by the reconstructed
population density for the earlier periods.
The synchronous recruitment trends at these three treeline sites suggest that Smith fir regeneration has been driven
by a common external factor, such as climate. The most severe
bottlenecks for tree recruitment at tree lines tend to be seed
production, seedling emergence and survival (Körner, 2003).
At the alpine tree-line ecotone, both winter and summer
temperatures are often key constraints on tree recruitment
(Lloyd & Fastie, 2003; Holtmeier & Broll, 2007; Harsch et al.,
2009). A growing body of literature has also demonstrated
that tree population density at tree lines can respond quickly
to rising temperatures (Camarero & Gutiérrez, 2004; Esper
& Schweingruber, 2004). In our study, Smith fir recruitment, measured on a decadal scale, was also strongly
correlated with mean summer and winter temperatures. This
seems reasonable considering the harsh winter conditions
at the natural Smith fir tree lines in the Sygera Mountains.
Severe soil frosts during cold winters were considered to
be critical factors in the control of seedling survival by
causing needle and shoot desiccation or fine root mortality
(Tranquillini, 1979; Körner, 2003; Kullman, 2007). As a confident support, Kullman (2007) found a strong and positive
link between winter temperatures and survival rates of
Pinus sylvestris tree-line populations in the Swedish Scandes.
The second half of the 20th century was the warmest
period on record during the past 1000 yr on the Tibetan
Plateau, probably stimulating a rapid pulse of viable seed
production, dispersal, germination, seedling establishment
and survival at the Smith fir tree lines of the Sygera
Mountains. In addition, enhanced precipitation in association with recent warming in the study area could also have
lowered the mortality rate. The abrupt change in the population age structure observed in the 1970s may also be
indicative of the importance of a positive feedback occurring
between trees and site-specific environmental conditions,
which could influence the spatial patterning of subsequent
tree establishment (Alftine & Malanson, 2004; Bekker,
2005; Batllori & Gutiérrez, 2008).
2011 The Authors
New Phytologist 2011 New Phytologist Trust
Research
Tree-line movement
The Smith fir tree-line position did not display a proportional upslope movement in response to climatic warming
on the Tibetan Plateau (Liu et al., 2005; Thompson et al.,
2006; Zhu et al., 2008), in contrast with our hypothesis.
Considering that the oldest Smith fir tree is c. 400 yr old,
the entire sequence from AD 1611 to 1950 might be viewed
as the consequence of a low recruitment into the pre-existing
forest during the 200-yr-long Little Ice Age period
(AD 1600–1820) after a rather long warm period (AD 1100–
1600). If the forest established between AD 1100 and 1600
was of mixed age, we would expect a gradual loss of trees
with little replacement over the next 200 yr. The average
temperature from 1600 to 1820, reconstructed from tree
rings, appeared to be 0.5C lower than the long-term average, and thus would possibly have lowered the recruitment
zone over that time. As an apparent confirmation of this, a
continuous recruitment of trees began again at the three
study sites at c. AD 1800 and a lack of trees from 1611 to
1760 at N2. The once higher altitude trees may have been
lost after these were recruited. The oldest tree recorded in
this study was actually found close to the current timberline, and the difference in the early placement of the tree
line among the three sites is almost certainly a consequence
of random survivorship. For an altitudinal lapse rate of
0.6C per 100 m, the upward movement in the past 400 yr
has been a total of 10 m in altitude or 0.06C at E1, a total
of 19.2 m in altitude or 0.12C at N1, and a total of 9.2 m
in altitude or 0.06C at N2, where the low placement of the
previous tree line may have masked upslope movement.
Given that c. 2C warming has taken place in the past
400 yr (Fig. 2), the safest conclusion has to be that there
was no significant alteration in tree-line position over this
period. On the other hand, an exact number (e.g. a total
tree-line advance of 10 m in altitude at E1), derived from
the arbitrary tree-line position in the study plots, should not
be over-interpreted.
Temperature is generally considered to be the primary
determinant for tree-line location; however, only strong and
long-term climatic changes are able to produce significant
changes in vegetation patterns in alpine regions (Körner,
2003; Holtmeier & Broll, 2007). Strong ecological inertia of
Smith fir forests may counteract the influence of warming on
the tree-line position, being consistent with other alpine tree
lines at low or mid-latitudes (Cuevas, 2000; Cullen et al.,
2001; Cui et al., 2005; Dalen & Hofgaard, 2005; Wang et al.,
2006; Green, 2009). At some alpine tree lines, the stand
density increased, but the tree line did not advance significantly in the 20th century (Luckman & Kavanagh, 1998;
Klasner & Fagre, 2002; Camarero & Gutiérrez, 2004). In
the Alps, an altitudinal rise in tree occurrence was closely
related to changes in land use, such as abandonment of
pasture (Gehrig-Fasel et al., 2007; Wieser & Tausz, 2007).
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There have also been reports that recent warming has caused
the consolidation of pre-existing tree populations, rather
than changes in the absolute position of the tree line (Payette
& Filion, 1985; Danby & Hik, 2007). Thus, a lag response
of alpine tree-line position to climate change has been widely
reported. It may be a result of local site conditions, species’
traits and feedback effects, as suggested by some alpine treeline studies (Lloyd & Fastie, 2003; Dalen & Hofgaard,
2005; Batllori & Gutiérrez, 2008).
Seed dispersal and a lack of disturbance are possible limiting factors for Smith fir tree-line movement. Smith fir seeds
become mature in October and fall to the ground in
November and December. Based on observations by our
automatic weather station, the maximum wind speed is
c. 3 m s)1 during this period, which is insufficient to blow
the seed far away from the crown. The absence of windflagging trees at the tree-line ecotone also indicates that the
average wind force is not strong. According to Yao et al.
(2008), 95% of the seeds fall within the vertical projection of
the tree crown, and only 5% can reach 2 m away from the
crown, 2% of which can germinate. The lack of animals for
the seed dispersal of Smith fir is probably another limiting
factor for tree-line upslope movement. Feedback effects may
also constrain the response of the Smith fir tree-line position
to climate warming. Further studies are necessary to explore
why there has been no significant expansion of the highelevation Smith fir tree line in response to global warming.
Conclusions
Although the Smith fir stands at the tree line in the Sygera
Mountains have become increasingly dense because of an
increasing number of seedlings after the 1950s, the tree-line
position has only moved slightly and insignificantly upslope
in response to climate warming. The lack of understanding
of the spatio-temporal dynamics of Smith fir tree lines precludes a robust statistical analysis of the underlying causes
of a tree-line change. Therefore, the current results should
not be over-interpreted, but remain noteworthy because of
the significant warming trend in the study area. In order to
obtain a more general picture of trends in recruitment and
tree-line dynamics, further tree-line studies across different
topographical aspects in the Sygera Mountains are needed.
Acknowledgements
This work was supported by the National Basic Research
Program of China (2010CB951301), the Knowledge
Innovation Program of the Chinese Academy of Sciences
(KZCX2-YWQN111) and the National Natural Science
Foundation of China (40871097). We thank Prof. Ian
Woodward and the reviewers for useful comments and
suggestions, and E. B. M. Drummond at the University of
New Phytologist (2011) 190: 760–769
www.newphytologist.com
British Columbia for her assistance with English language
and grammatical editing of the manuscript.
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