14C
Analysis Confirms the Annual Nature of Sagebrush Wood Layers
Franco Biondi, Scott Mensing, and Scotty Strachan
Sagebrush and Pinyon-Juniper Ecosystems
Abstract
In the Great Basin of North America, big sagebrush (Artemisia tridentata Nutt.) growth rings can
be used to reconstruct environmental changes with annual resolution in areas where there is
otherwise little information available. We tested the annual nature of big sagebrush wood layers
by means of accelerator mass spectrometry (AMS) radiocarbon (14C) dating. Four cross
sections from three sagebrush plants were collected near Ely, Nevada, and analyzed using
dendrochronological methods. Ten 14C measurements were then used to trace the location of
the 1963-'64 “bomb spike”. Although the number of rings on each section did not exceed 60,
crossdating was possible within a section and between sections. Years assigned to individual
wood layers by means of crossdating aligned with their expected 14C values, matching the
location of the 14C peak. This result confirmed the annual nature of growth rings formed by big
sagebrush, and will facilitate the development of spatially explicit, well replicated, proxy records
of environmental change, such as wildfire regime, in Great Basin valleys.
Prior studies have assumed that growth rings of
big sagebrush are annual because they
crossdate. While crossdating is indeed a powerful
tool to assign calendar dates to annual wood
increments, additional, independent verification
should be attempted whenever possible. Since
this verification was absent in the most extensive
studies of big sagebrush growth rings, i.e.
Ferguson (1964) and Cawker (1980), which also
did not include samples from Nevada, we
checked the annual nature of sagebrush wood
layers by means of “bomb” radiocarbon dating.
Among sagebrush ecosystems, big sagebrush
(Artemisia tridentata Nutt.) steppe communities
occupy over 60 million ha from Baja California
and northern Arizona all the way to the Pacific
Northwest and the Dakotas (see map to the
left).
In shrublands, historical records of landscape
changes are limited, and this restricts our
ability to reconstruct, model and predict
vegetation shifts linked to disturbance, climate,
and their interaction.
We used a combination of dendrochronological
crossdating and 14C AMS analysis to answer
the question “Are growth rings of Artemisia
tridentata truly annual?”, which has
immediate implications for the broader issue of
reconstructing environmental changes in semiarid shrublands.
The encroachment of Pinus and Juniperus species into
sagebrush in the Great Basin (see J. Bauer’s photo below)
has been documented for decades, with an estimated 10-fold
increase (from 3 million ha to 30 million ha) in piñon-juniper
(P-J) woodland area within the western U.S. since EuroAmerican settlement. While several factors may have
contributed to this expansion (see diagram below), knowing
the age structure of big sagebrush in Great Basin valleys
should provide information on long-term patterns of
landscape disturbance that would not otherwise be available.
Grazing
2
1
Fire
Suppression
6
Reduced Fire
Frequency
3
P- J Expansion
5
Climate
Change
4
Photo: J. Bauer
Four sections were cut using a chainsaw from
three separate individuals (one had a double
stem) at a site located near Ely, Nevada.
Geographical coordinates for the study area
are 39.30°N, 114.99°W, and 2070 m elevation
(see asterisk in map, and photo of the site on
the left).
Dendrochronological Analysis
Cross-sections had a typical lobate form (see photo on the left) because of the cleft
and irregular stem that is characteristic of big sagebrush, especially in older plants.
The sagebrush sections were sanded and polished until individual wood cells were
clearly visible under a binocular microscope with 10-50x magnification. Calendar
dates were then assigned to growth rings by means of visual crossdating, performed
using skeleton plots.
The number of rings on each section did not exceed 60, and the ring patterns were
relatively uniform, but it was still possible to crossdate growth rings within a sample
and across samples.
Samples were sent for radiocarbon analysis to the National Ocean Sciences AMS
(NOSAMS) facility at Woods Hole Oceanographic Institution. Five samples from each of
two sagebrush sections, corresponding to the estimated years 1955, 1962, 1964, 1966,
1975, were manually collected using a hammer and scalpel (see photos on the right). Each
of these 10 samples was air-dried and weighed. Sample identification codes were assigned
without reference to calendar years or location in the cross section. By doing so, we
guaranteed that the radiocarbon analysis, which was performed in a different laboratory,
could not be influenced by prior knowledge on the chronological sequence of the samples.
Radiocarbon Analysis
Radiocarbon amounts are given using the Fraction Modern (Fm) notation. Fm is a
measurement of the deviation of the 14C/C ratio of a sample from a 1950 standard.
The maximum error reported for the Fm measurements (see table on the left) was
less than 0.6% of the measured value, and therefore negligible compared to the
differences between years. Given the clear boundaries of wood increments in big
sagebrush, the limited number of radiocarbon dates (5 from each of 2 sections) was
adequate to answer the research question.
Measurements of Fm were compared with their expected values from 1950 to 1992,
and years assigned to individual wood layers aligned with their expected 14C values,
matching the location of the 14C peak (see graph below).
After the samples were sent to NOSAMS, all radii that had
been skeleton plotted were measured (see photo below).
Crossdating results are summarized in the table to the right.
Acknowledgments
F. Biondi and S. Strachan were partially supported by the National Science Foundation
(NSF) under Grant ATM-CAREER-0132631. Funding for 14C analyses was provided by the
NOSAMS Research Initiatives Program. The NOSAMS facility was supported by NSF
Cooperative Agreement OCE-9807266.