Wyoming Big Sagebrush (Artemisia
tridentata ssp. wyomingensis) Seedling
Growth and Maternal Plant Stand Position
A. L. Hild
B. Christensen
A. Maier
Abstract—Little is known of maternal plant influence upon seedling characteristics of native shrubs. This study examined influence
of maternal Wyoming big sagebrush (Artemisia tridentata ssp.
wyomingensis) stand position on emergence and growth of seedlings. Seedlings from maternal plants in upslope, core, and
downslope positions were grown in a common greenhouse setting.
Percent germination, height, and canopy volume of seedlings
differed among three maternal plant positions. Emergence of
downslope and core-derived seed occurred later than that of
upslope-derived seed. Canopy volume and height of seedlings from
downslope plants were less than that of core- and upslope-derived
plants after 14 weeks of growth. These results indicate that seed
collection from different positions within a stand can produce
seedlings with divergent growth characteristics. Heritable differences that depend on seed source plant position offer potential for
more refined revegetation efforts when using Wyoming big sagebrush seed.
Little is known of the relative influences of genetics and
environment upon seedlings, although maternal plants
may have great impact (Roach and Wulff 1987). Maternal
plants may undergo selective change via environmental
constraints (Lacey 1991). Within-population variation has
been documented for many herbaceous species (Antonovics
and Schmitt 1986; Ducousso and others 1990; Keeler 1978;
Linhart 1988; Plantenkamp and Shaw 1993). Seedling variability may derive from both environmental and genetic
constraints that limit maternal plant individuals. Shrubland ecotones between shrub stands and adjacent grasslands may provide variable environmental constraints that
alter inheritance of propagules. Ecotones may thus become
a source of genetic variability within shrub stands through
differential selection. Levin (1995) has suggested that outlier plants that are distant from conspecifics may offer
variability in the gene pool that can enhance population
viability. Differences in growth form of hybrid sagebrush in
transition zones between subspecies distributions have been
documented (Freeman and others 1999, this proceedings).
In: McArthur, E. Durant; Ostler, W. Kent; Wambolt, Carl L., comps. 1999.
Proceedings: shrubland ecotones; 1998 August 12–14; Ephraim, UT. Proc.
RMRS-P-11. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Rocky Mountain Research Station.
A. L. Hild is Assistant Professor of Rangeland Ecology and B. Christensen
and A. Maier are Graduate Students in the Department of Renewable
Resources, University of Wyoming, Laramie, WY 82071-3354.
USDA Forest Service Proceedings RMRS-P-11. 1999
Phenotypic display of seedling variability within stand
positions has not been examined for native shrub stands. We
examined the seed crop of three Wyoming big sagebrush
stands with respect to maternal plant positions central or
peripheral to the stand. Seedlings grown in a common
greenhouse setting displayed differential growth depending
on stand position of maternal plants.
Methods _______________________
Study Site
Research sites are located at USDA-ARS Fort Keogh
Livestock and Range Research Labs (LARRL), near Miles
City, Montana, at 46° 22' N latitude and 105° 5' west
longitude. Annual precipitation averages 33.8 cm annually,
with approximately 60% received from mid April to mid
September. Air temperatures range from –10 to 24 °C in
January and July, respectively.
Three stands of Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) were selected; each stand lies on
a gentle (<10%) slope with northern aspect. Within each of
these three Wyoming big sagebrush stands, three relative
stand positions were identified as upslope periphery,
downslope periphery, and core positions. Upslope and
downslope positions were recognized as topographic positions at the margins of each stand. Core positions were then
located intermediate to upslope and downslope positions
and at the center of the stand. We sampled soil cores from
each position at two depths (surface 0-10 cm and 10-20 cm)
in each of three stands. All positions in the three stands were
located on clay loam soils deeper than 50 cm with comparable soil moisture content in surface and subsoil samples.
Stands were approximately 3 km apart from one another.
Data Collection
Within each position, 10 mature flowering sagebrush
plants were randomly selected and marked for study.
These plants (maternal plants) were used as seed sources
for 2 consecutive years (1996 and 1997). We measured
height, width of canopy in two perpendicular measures,
basal diameter, and distance to nearest sagebrush neighbor
for each parental plant. In December 1996 and again in
December 1997, inflorescence was removed from each parental plant and transported to the University of Wyoming
for germination studies. Inflorescences were allowed to
air dry, separated into twigs, leaves, and flower portions.
287
Biomass of each portion was recorded as well as biomass of
20 randomly selected seeds from each maternal plant for
planting. Following the second year of inflorescence collection, spring 1998, all parental plants were harvested for
aging.
In April 1997 and April 1998, we planted 20 seeds from
each maternal plant in a common greenhouse setting. Seeds
were placed on a commercial germination media (2:1:1 parts
peat moss, vermiculite, and pearlite, respectively), watered
and placed under growth lights. For maternal plants that
produced fewer than 20 seeds, we planted all available
seeds. After planting, we recorded number of seeds planted
from each maternal plant, days to seedling emergence, and
percent emergence. Because our goal was to obtain seedlings
from each parent, we replanted seeds of parent plants that
did not have successful seedlings on May 13-15, 1997, and
June 1-3, 1998, and again recorded days to emergence.
Growth of individual seedlings of each successful maternal plant was documented for 26 weeks after emergence.
Beginning 5 weeks after emergence, we recorded the height
and number of leaves of each seedling. After week nine,
seedlings had sufficient growth to cause leaf counts to be
difficult. Consequently, height, canopy dimensions, and
number of branches were recorded, beginning week 14 after
emergence. Canopy of seedlings was recorded as height, two
perpendicular widths of the crown, and total branch length.
In week 12 seedlings were transplanted from six-pac pots
into plastic-lined PVC tubes (16 cm diameter x 100 cm tall)
containing a soil mix of 2:1 parts potting soil and washed
masonry sand to allow for deep root growth, and monitoring
was continued. After seedlings were 1 year old, they were
removed from PVC containers and planted into a field
setting.
Experimental Design
Data for parental plants were subjected to analysis of
variance (SAS 1997) appropriate to a randomized block
design; with each stand as a block and parental position
(downslope, core, and upslope) treatments. Seedling data
were assessed for treatment (maternal plant stand position)
effects among weeks in a repeated measures analysis
(weeks = 5, 6, 7, and 9 for number of leaves; 5, 6, 7, 9, 14, 18,
22, and 26 for seedling height; 14, 18, 22, and 26 for seedling
canopy width, and branch lengths). Results are reported as
F ratio probabilities and all data were assessed for conformance to assumptions of the analysis of variance.
Results ________________________
Maternal Plants
Maternal plants did not differ in canopy size or basal
diameter by position. Upslope position parental plants
were shorter (P <0.03) than core or down-position plants
(mean heights were 45.5, 55.5, and 53.6 cm for upslope, core,
and downslope parental plants, respectively). Additionally,
as might be expected, distance to nearest neighboring sagebrush plants was least between core plants (P <0.01 mean
distance 46.2 cm), farthest between downslope parental
plants (mean distance 134.4 cm); upslope plants were intermediate in distance (77.4 cm) to nearest neighboring sagebrush and not different than core-position plants.
Biomass of inflorescence, biomass of 20 seeds, and number of seeds planted did not differ between positions in either
year of the study (P >0.66 and 0.25, respectively). Total
inflorescence biomass of maternal plants was greater in
1996 than in 1997 collections (P <0.01) with means of 13.0
and 8.8 g per maternal plant, respectively. Number of
successful parental plants (producing at least one seedling)
was not different by position or year (table 1).
Seedlings
In 1997 and 1998, 130 and 338 seedlings, respectively,
were monitored. Both number and percent emergence of
seedlings differed between years depending on position
(position by year interaction P <0.01 and 0.01 for number
and percent emerged, respectively). Total number of seedlings and percent emergence (table 1) were both greater for
downslope and core positions in 1998 than from upslope
positions. In 1997, number and percent emergence did not
differ between positions, and emergence was less than 25%
for any position in both years.
Core-position seedlings emerged later than did downslopeposition seedlings in both years (P <0.02). Core seedlings
emerged 17.1 days after planting while the mean number of
days to emerge were 13.5 and 14.9 for downslope and
upslope positions, respectively.
In 1997, branch length increased with week (P <0.01)
but did not differ by position (P >0.74). Seedling heights in
1997 differed (P <0.01) by position after week nine.
Seedlings derived from downslope maternal plants were
shorter than seedlings derived from maternal plants in the
other two positions. Additionally, canopy volume of
Table 1—Successful maternal plants and seedling emergence by position in 1997 and 1998.
Downslope
Successful maternal plants (total)
Percent germination (mean)*
Seedlings emerged (total)
12
3.6a
23
1997
Core
12
7.7a
66
Upslope
Downslope
7
7.6a
41
19
25.4b
155
1998
Core
15
24b
145
Upslope
6
6a
38
*Mean within years with the same letters do not differ, P >0.05, LSD.
288
USDA Forest Service Proceedings RMRS-P-11. 1999
Figure 1—Canopy volume of seedlings derived from maternal plants in three stand positions by
week after emergence. Means within a position with the same lower case letters, or within a week
with the same upper case letters, do not differ, P >0.05, LSD.
downslope-derived seedlings was less (P <0.01) than that of
seedlings from the other two positions by week 18 following
their emergence (fig. 1). Growth of the second cohort of
seedlings continues to be monitored in 1998.
Discussion _____________________
This study is limited by the growth of seedlings in a
controlled greenhouse rather than in a field setting. However, the common growth environment restricts the possibility that seedling growth differences noted here are the
result of environmental conditions. Thus, our study supports the notion that differences noted in seedling growth
may derive from genetic inheritance.
Production of seedlings from core and downslope maternal plants was much greater in 1998 although their success
varied greatly by year. Additionally, reduced canopy of
downslope-derived seedlings relative to seedlings from maternal plants in the other two positions suggests the existence of within-stand variation. Explanations of the differences in growth of seedlings are difficult to tie to biological
constraints on maternal plants, yet the presence of growth
differences by position suggests that growth potential of
seedlings within a stand may provide variety within stand
gene pools. Future genetic studies are needed to verify our
results. However, these growth differences may result in
an array of relative competitive abilities of sagebrush seedlings. Given the possible genetic differences of seed by virtue
of maternal plant locations within sagebrush populations,
seed collection efforts for specific goals may be enhanced. It
remains to be seen, however, whether greenhouse growth
trends are persistent under competitive field conditions.
USDA Forest Service Proceedings RMRS-P-11. 1999
Acknowledgments ______________
This study was funded by USDA-ARS Cooperative Agreement No. 58-5434-7-103. We thank Dr. Rod Heitschmidt and
the staff of the Fort Keogh Livestock and Range Research
Lab in Miles City, Montana, for providing access to the
research sites and facilitating our sampling efforts. Especially helpful were the many students who assisted with
greenhouse activities.
References _____________________
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