BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Gutierrezia microcephala | Threadleaf Snakeweed 



GENERAL BOTANICAL CHARACTERISTICS : 
Threadleaf snakeweed is a small, native, resinous, perennial desert
subshrub that is typically less than 1.6 feet (0.5 m) in height and less
than 3.3 feet (1 m) in canopy diameter.  It has many erect herbaceous
stems which arise from woody basal stems and die back annually
[16,27,30,40].  The herbaceous stems are photosynthetic, giving the
plant a high photosynthetic capacity [24].  A mature plant with an 8- to
12-inch (20-30 cm) canopy diameter has 100 to 200 photosynthetic stems
[30].  The seedling taproot becomes more extensive and diffuse with age
[24].  Root length of mature plants averages 6.9 inches (17.4 cm) [25].
Threadleaf snakeweed generally lives less than one decade; maximum age
observed in a New Mexico grassland was 18 years [30].

Threadleaf snakeweed exhibits some degree of summer deciduousness.  In
the Mojave Desert in western Arizona, total threadleaf snakeweed canopy
in early August was 54 percent of the maximum biomass observed in spring [8].


RAUNKIAER LIFE FORM : 
   Chamaephyte
   Phanerophyte


REGENERATION PROCESSES : 
Threadleaf snakeweed produces many seeds [30,33].  Wind dispersal of
seeds is inefficient because of a highly reduced pappus, so most seeds
fall within a few meters of the parent plant [30].  Seeds are dormant
and require a 4- to 6-month afterripening period.  After 6 months,
maximum germination occurs at a constant temperature from 59 to 77
degrees Fahrenheit (15-25 deg C) with an 8-hour light period.  Light
enhances germination, suggesting that germination is favored at or near
the soil surface [22].  In a New Mexico grassland, seeds germinated
abundantly where the established vegetation had been experimentally
removed [30].  Germination is also enhanced by winter and spring
precipitation [29].

Seedlings produce a single stem and a taproot.  A yearling has three to
four stems.  Several years are required before seeds are produced [30].

The duration of threadleaf snakeweed seed viability in soil is unknown
[25].  Parker [30] found an average of 24.2 viable seeds per square
meter in the soil of an arid grassland site in New Mexico.  Seedling
abundance was not noticeably depressed in the spring following a year in
which insect herbivory destroyed the seed crop [30].

Parker [30] found considerable year-to-year fluctuation in plant
recruitment.  Threadleaf snakeweed is vulnerable to local extinction
because seedlings frequently succumb to drought, grasshopper
defoliation, and competition from grasses [30].  In an arid New Mexico
grassland, seedling survival was five times greater where competing
vegetation had been removed than on undisturbed sites.  In a year of
abundant grasshoppers (Hesperotettix viridis), predation on threadleaf
snakeweed seedlings was reduced when mature threadleaf snakeweed plants
were nearby because the grasshoppers generally hopped from one mature
threadleaf snakeweed canopy to another and rarely moved among seedlings
at ground level [29].

According to Mayeux and Leotta [22], threadleaf snakeweed reproduces
solely by seed.  One reference from a study in New Mexico described a
very low rate of sprouting from the root crown after mature plants were
clipped to ground level [29].


SITE CHARACTERISTICS : 
Threadleaf snakeweed occurs in desert climates [8].  In southwestern
Utah, threadleaf snakeweed is preferentially associated with
xeroriparian sites which have higher annual moisture than uplands but
running water only on infrequent occasions [23].  It primarily occurs on
well-drained, sandy, gravelly, or rocky soils [4,16,28,41].  It is often
found in washes or on adjacent slopes [8].  Threadleaf snakeweed often
predominates on southern exposures with shallow rocky soils where
grasses are not well established [4].

Elevational ranges by state are as follows:

Arizona		3,500 to 6,500 feet (1,100-2,000 m) [18] 
California	5,900 to 8,200 feet (1,800-2,500 m) [16]
Colorado	4,000 to 8,000 feet (1,200-2,400 m) [12]
Texas		2,500 to 6,000 feet (760-1,800 m) [31]
Utah		2,800 to 6,000 feet (850-1,830 m) [40]

Although the ranges of the two perennial snakeweeds overlap, they appear
to occupy slightly different niches.  Threadleaf snakeweed was more
prevalent than broom snakeweed on drier upper slopes along a vegetation
transect in the Chihuahuan Desert, New Mexico [41].  However, in
southwestern Utah, threadleaf snakeweed occupied the lower slope and
floodplain while broom snakeweed occupied the upper slope and ridgetop [3].


SUCCESSIONAL STATUS : 
Facultative Seral Species

Threadleaf snakeweed is probably intolerant of shade since it rarely
occurs in the presence of a woody overstory [25].  It pioneers on
disturbed sites and overgrazed rangeland [28,20,31,38].  In San
Bernardino County, California, threadleaf snakeweed was present in low
numbers on a pipeline construction zone 12 years after the disturbance
[38].

Perennial snakeweed populations are often cyclic, dying back when
conditions for seedling establishment are unfavorable and then
reappearing when conditions are favorable [24].  Prolific seed
production and buried seeds permit opportunistic seedling recruitment
during favorable periods.  Once established on a site, threadleaf
snakeweed may persist numerous generations despite a short life span,
poor seedling recruitment during some years, and intense competition
from grasses [30].


SEASONAL DEVELOPMENT : 
New terminal growth begins on basal stems from January through March.
Flowering is initiated in the spring and early summer, but plants become
dormant during summer drought and do not bloom until late summer or
fall.  Twig dieback occurs in late fall after flowering [8,33].
Flowering phenology depends on yearly climatic conditions; blooms last
longer in wet years [33].  Germination occurs in early spring [30].

Related categories for Species: Gutierrezia microcephala | Threadleaf Snakeweed