Permian polar forests: deciduousness and environmental ... et al 2012.pdfof polar forests and perhaps plant physiology at polar lati-tudes. This work is the first comparison of multiple
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Permian polar forests: deciduousness and environmentalvariationE. L . GULBRANSON,1 ,* J . L . ISBELL,1 E . L . TAYLOR,2 , 3 P . E . RYBERG,2 , 3 T . N. TAYLOR2 , 3
AND P. P. FLAIG4
1Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, WI, USA2Department of Ecology and Evolutionary Biology, Natural History Museum and Biodiversity Institute, University of Kansas,
Lawrence, KA, USA3Division of Paleobotany, Natural History Museum and Biodiversity Institute, University of Kansas, Lawrence, KS, USA4Bureau of Economic Geology, University of Texas-Austin, Austin, TX, USA
ABSTRACT
Forests are expected to expand into northern polar latitudes in the next century. However, the impact of
forests at high latitudes on climate and terrestrial biogeochemical cycling is poorly understood because such
forests cannot be studied in the modern. This study presents forestry and geochemical analyses of three
in situ fossil forests from Late Permian strata of Antarctica, which grew at polar latitudes. Stem size mea-
surements and stump spacing measurements indicate significant differences in forest density and canopy
structure that are related to the local depositional setting. For forests closest to fluvial systems, tree density
appears to decrease as the forests mature, which is the opposite trend of self-thinning observed in modern
forests. We speculate that a combination of tree mortality and high disturbance created low-density mature
forests without understory vegetation near Late Permian river systems. Stable carbon isotopes measured
from permineralized wood in these forests demonstrate two important points: (i) recently developed tech-
niques of high-resolution carbon isotope studies of wood and mummified wood can be applied to permin-
eralized wood, for which much of the original organic matter has been lost and (ii) that the fossil trees
maintained a deciduous habit at polar latitudes during the Late Permian. The combination of paleobotani-
cal, sedimentologic, and paleoforestry techniques provides an unrivaled examination of the function of
polar forests in deep time; and the carbon isotope geochemistry supplements this work with subannual
records of carbon fixation that allows for the quantitative analysis of deciduous versus evergreen habits
and environmental parameters, for example, relative humidity.
Received 22 February 2012; accepted 2 July 2012
Corresponding author: E. L. Gulbranson; Tel.: 530-601-1927; Fax: 414-229-4561; e-mail: [email protected]
*Present address: Department of Geology and Geophysics, University of Hawaii, Honolulu, HI, USA
INTRODUCTION
Well-preserved fossils of the extinct plant Glossopteris and
other plants from Antarctica demonstrate that the conti-
nent was vegetated in the late Paleozoic (e.g., Seward,
1914; Schopf, 1970; Francis et al., 1993; Taylor, 1996).
Apparent polar wander paths indicate that Antarctica was
near the South Pole during the Permian (Powell & Li,
1994; Lawver et al., 2008; Domeier et al., 2011; Isbell
et al., 2012), implying that Glossopteris and other flora
grew and thrived near or above the South Polar Circle.
Tree ring studies of permineralized Glossopteris wood in
Upper Permian strata of Antarctica reveal that tree rings
are dominated by earlywood with very minimal amounts of
latewood (1–3 cells wide), suggesting that the unique pho-
toperiod of the polar latitudes promoted rapid cessation of
photosynthesis at the onset of winter (Ryberg & Taylor,
2007; Taylor & Ryberg, 2007). The occurrence of leaf-rich
sedimentary layers has been used to invoke a seasonally
deciduous habit for Glossopteris (Plumstead, 1958; Retal-
lack, 1980; Taylor et al., 1992). Pigg & Taylor (1993)
observed that Glossopteris leaf compressions vary little in
leaf size and suggest that this may reflect periodic leaf drop
owing to environmental stresses similar to the habit of
near the Robert Scott Glacier. The fossil forest in the
upper Buckley Formation at Mt. Achernar (Figs 1 and 2)
is notable in that the preserved tree stumps have diameters
of 9–18 cm with no more than 15 rings per trunk, imply-
ing that this was a stand of saplings or juvenile trees (Tay-
lor et al., 1992). Fossil leaf mats and permineralized peat
from the upper Buckley in the study area suggest that
these polar forests were low diversity and probably did not
have an understory flora (Cuneo et al., 1993). However,
the Mt. Achernar in situ fossil forest does contain evidence
of understory vegetation comprised of herbaceous lycops-
ids, preserved as compression/impression fossils, whereas
the permineralized tree stumps of the forest are attributed
to the Glossopteris seed plant (Schwendemann et al., 2010).
Despite the fact that Glossopteris trees grew at polar lati-
tudes, Taylor et al. (1992) found no evidence of frost rings
in the fossil forest at Mt. Achernar.
The upper Buckley Formation is primarily composed of
volcaniclastic sedimentary rocks with terrestrial facies of flu-
vial and lacustrine affinity (Isbell et al., 1997; Isbell and
MacDonald, 1998), but coal also occurs within this part of
the formation and it is notable that coal is absent in Ant-
arctica between the top of the Buckley Formation and the
upper portion of the overlying Triassic Fremouw
Formation (Retallack et al., 1996). The Buckley Formation
has local variations in grain size (Barrett et al., 1986; Isbell
et al., 1997). Some regions are predominantly sand rich,
but coeval stratigraphic intervals in other areas are com-
posed of thinner sandstone beds interbedded with thicker
beds of shale and silty sandstone containing either carbona-
ceous plant detritus or non-carbonaceous plant compres-
sions (Briggs et al., 2010).
Glacial deposits of the late Paleozoic ice age do not
occur in Antarctica after (i.e., younger than) the Early
Permian (Isbell et al., 2003, 2008, 2012); however, glacia-
tion is interpreted to have existed at lower paleolatitudes in
eastern Australia until the Late Permian (Fielding et al.,
2008). Therefore, it is likely that the Late Permian time
interval in which these forests developed represents a tran-
sitional climate period from the icehouse(s) of the late
Paleozoic to the greenhouse state of the Mesozoic.
METHODS
Measured stratigraphic sections (Fig. 2) were made at
Graphite Peak, Wahl Glacier, and Mt. Achernar noting
sedimentary structures, sediment composition, and fossils.
Three permineralized stumps and root material were
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Mil
ler
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Skaar Ridge
Mt. AchernarGordon Valley
Mt. Falla
Fremouw Peak
Lamping peak
Mt. Sirius
Wahl Glacier
Keltie Glacier
5 S
Graphite Peak
BE
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ange
Permian fossil forest locality studied herein
Permian sections studied in 2010-2011
Triassic sections studied in 2010-2011
Law Glacier
P o l a r P l a t e a u
Permian fossil forest locality with Triassic strata
Permian and Triassic sections studied in 2010-2011
LatePermian
paleoflow
300 Ma
275 Ma
250Ma
polar wander (Domeier et al., 2011)
EastAntarctica
Africa
India
Australia
S.Amer.
studyarea
300Mapola
rcirc
le
C
B
A
Fig. 1 Map of the study area within Antarctica. (A) Present-day map of the study area, gray areas represent exposed rock of the Transantarctic Mountains.
Symbols denote studied locations during the 2010–2011 austral summer season; star symbols represent fossil forest localities studied herein. Large arrow
denotes general Late Permian paleoflow direction compiled from paleocurrent measurements throughout the Beardmore Glacier region. (B) Map of Antarctica
showing position of study area on the continent. (C) Paleogeographic reconstruction showing the position of the study area (star) relative to the rest of
Gondwana (gray areas). The most recent paleo-polar wander path for the studied time interval (Domeier et al., 2011; Isbell et al., 2012) is shown for
Fig. 2 Stratigraphic columns at Graphite Peak, Wahl Glacier, and Mount Achernar. Only the thickness that contains the fossil forest and the contact between
the upper Buckley and Frewmouw fms. are shown. Grain size on the x-axis increases from left to right.
tion of permineralized wood from Mt. Achernar used for isotope ratio mea-
surements. (B) Tree ring d13C values, gray shading indicates where two
samples were taken from the same ring, other values likely represent one
d13C value for the entire ring. Arrows point to region in the stump that the
data points correspond to. Dashed vertical lines denote tree ring bound-
aries. Note peculiar increase in d13C values toward the exterior of the sam-
ple (to the right).
A B
Fig. 8 Carbon isotope ratios for the permineralized wood specimen from Mt. Sirius. (A) Individual data points (circles) were used to calculate a 3-point run-
ning average (black line). Photograph (inset) of the section of permineralized wood used for sampling. Sample widths are indicated by the widths of the alter-
nating black and gray bars, with 10 samples for rings 1 and 2, sample numbers are shown for ring 1 for reference. Note that sample widths <0.1 mm
(Table 3) are difficult to show clearly. (B) Idealized carbon isotope trends within tree rings for a deciduous tree and an evergreen tree generalized from a glo-
bal compilation of high-resolution isotope records of modern wood (modified from Schubert & Jahren, 2011). The Mt. Sirius sample displays carbon isotope
trends that are more similar to a tree of a deciduous habit, having a three-phase carbon isotope trend, than an evergreen habit that has a prominent positive
d13C value peak yet similar d13C values at the beginning and end of a growth ring.
zones of high and persistent disturbance (i.e., streams). A
common progression of diversity and tree densities in the
Pacific northwest trend from bare alluvium to mixed shrubs
and thin alders of high density (approximately
100 000 ind. Ha�1) on terraces adjacent to active stream
channels, whereas the oldest landscapes (>700 years) of
bogs or mires occupied by black spruce with densities rang-
ing from 500 to 2000 ind. Ha�1 (Fig. 9B,C, Viereck et al.,
1993). The observed self-thinning in the Permian, how-
ever, is opposite of the modern, where Permian tree matu-
rity increases (and density decreases) closer to the area of
high potential disturbance and is similar to younger high-
latitude fossil forests of Cretaceous age in the Antarctic
peninsula region (Falcon-Lang et al., 2001; Falcon-Lang,
2004). This trend is possibly related to intense competition
for resources in a persistently disturbed (e.g., flooding)
environment that was unsuitable for understory vegetation
in the region. Moreover, as trees age, tree mortality can be
induced by intense competition for light, water, and nutri-
ents (Cao et al., 2000; Falcon-Lang, 2004). Forest canopy
structure is an important determining factor in resource
competition and future work to assess the growth form of
glossopterid trees at different latitudinal gradients, as well
as the ecology of different late Permian wood morphogen-
era in Antarctica, may elucidate competition for light as
opposed to nutrients or water.
The sedimentary rocks that host in situ fossil forests at
Graphite Peak, Mt. Achernar, and Wahl Glacier are inter-
preted as paleosols as they meet the principle criterion for
soil, which is to support plant life (Soil Survey Staff,
2010). The paleosols at each locality are gray to whitish
gray (Figs 3B–F; 4A,B; 6B,D), contain some degree of
bedding or sedimentary fabric, including leaf compressions
on bedding planes, and have a silt texture. These paleosols
lack field-based indicators indicative of seasonal changes
in moisture content (Kraus & Aslan, 1993), that is,
60
40
20
0
80
100
0 500 1000 1500 2000 2500 3000
GP
WG
LP
LP MA
Levee/splayDistal floodplain/
lacustrine
Tree Density (trees ha–1)
Bas
al A
rea
(m2
ha–
1 )
Bas
al A
rea
(m2
ha–
1 )
A
Curio Bay
Gordon Valley
Axel Heiberg
Decreasing tree density
Increasing forest maturity/age
Mt. AchernarWahl Gl./Graphite Pk.
C
D Lamping Pk.
60
40
20
80
100
6543210Log10 Tree Density (trees ha–1)
B
Distal
Splay
Modernfloodplain
position
Dis
tal
Spla
y
river
Braided streamLake
Shr
ubs
Ald
erW
hite
sp
ruce
Bla
cksp
ruce
Bog
Late Permian forests
Decreasing tree density
Increasing forest maturity/age
Perm
ian
flood
plai
npo
sitio
n
Glossopterids
Lycopsids
Modern boreal forests
Fig. 9 Paleoforestry calculations and modern comparison. (A) Basal area and tree density estimates for Permian fossil forests (green circles): Graphite Peak
(GP), Wahl Glacier (WG), Lamping Peak (LP), and Mount Achernar (MA). Lamping Peak estimates are from Knepprath (2006). Additional polar forest locali-
ties are displayed by the blue square symbols: Gordon Valley (Middle Triassic, Cuneo et al., 2003), Curio Bay (Middle Jurassic, Pole, 1999), and Axel Heiberg
Island (Eocene, Francis, 1991; Basinger et al., 1994; Greenwood & Basinger, 1994). (B) Basal area and tree density measurements for a modern boreal forest.
Data from Viereck et al. (1993). Blue square symbols correspond to early succession forest near a river. Red circles correspond to mature forest on terraces
and within bogs. Gray symbols indicate the Late Permian forestry data for reference. (C) Illustration of ‘self-thinning’ within a modern boreal forest (modified
from Viereck et al., 1993) where tree density decreases from left to right and forest maturity increases from left to right. (D) Interpretation of Late Permian
paleoenvironments and forest structure at Wahl Glacier, Graphite Peak, Lamping Peak, and Mt. Achernar. Self-thinning is interpreted to be the exact opposite
trend from the modern, that is, toward areas of high disturbance in the Late Permian.
in permineralized samples. Such detail can provide impor-
tant insights into the habit of arborescent plants and implies
that additional environmental variables, such as water stress,
could be quantified from such ancient ecosystems.
ACKNOWLEDGMENTS
We are especially grateful to Drs. Neil Tabor and A. Hope
Jahren for assistance and generosity in the use of their lab-
oratory facilities; and Mr. Bill Hagopian and Dr. Scott Me-
yers for assistance with stable isotope ratio measurements.
Danielle Sieger is thanked for her assistance with the mea-
sured section at Mt. Achernar. We thank Brian Staite for
his help with logistics, mountaineering, and for sharing
with neophyte Antarctic researchers his >2 decades of
experience working on ‘the ice.’ Fixed wing transport was
provided by the 62nd and 446th Airlift Wing of the U.S.
Air Force, stationed at Joint Base Lewis-McChord, WA,
the 109th Airlift Wing of the New York Air National
Guard, and Kenn Borek Air Ltd. This work was supported
by funding from NSF grants ANT-0943935 and ANT-
0944532 to the University of Wisconsin-Milwaukee, and
ANT-0943934 to the University of Kansas.
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