Desert Zonation - Mt & Desert

Desert Zonation matters more about…

soils

• local influence on vegetation

& latitude

• effect on temperature and day length

Desert Zonation: Elevations

GB

• Modoc plateau

  • 4k ft = sagebrush scrub range

• North owens valley

  • 7k ft = sagebrush scrub range

Mojave

• 2k ft = creosote bush scrub range

Colorado Desert

• 1k ft = creosote bush scrub range

Succession in the Desert

Very slow

• early successional species normally

  • have good dispersal

  • short lived

    • i.e. creosote & burr-sage

    • i.e. teddy bear cholla - CD

Succession after Fire - Great Basin

• causes shift in species composition

• favors bitterbrush and perennial grasses

  • resprout from undamaged rootstock

• doesn’t resprout after fire

  • seedlings reestablished within 20 yrs

Mount Saint Helens Eruption

• May 1980 in WA cascades

• ash caused increased soil moisture by inhibiting evaporation

  • increased reflection from white surface increased temp and transpiration rates in vegetative canopy

• GB sagebrush tolerates stress by dropping 75% of leaves

  • water conservation allowed development of flower buds during dry, hot summer

  • volcanism favored GB sagebrush growth

Larva of the Moth

Sagebrush defoliator

• feeds on leaves in early spring and summer

• increasing amount of fuel in form of dead foliage

  • more susceptible to fires

• possible to rejuvenate old stands of sagebrush by precipitating fire

  • recycles mineral nutrients

Creosote Bush Scrub - desert strategies

• only need 1 in of water to flower

• once pollinated, petals turn ¼ 

• small leaves

• waxy/resinous coating

• distinctive odors during/after rain

Creosote Bush - Drought tolerance

Functions well at ~50 bars and up to ~120 bars

• negative water potential measures water stress

• 1 bar is pressure of atmosphere

  • normal agriculture wilts at ~10-15 bars

Critical Water Potential

~70 bars and above, cellular respiration exceeds photosynthesis so the plant will not live long

Creosote Bush - stem elongation

Even under water stress, cells continue division

• after rain, new cells absorb water and elongate

Creosote bush is spaced…

regularly

• believed to be a water soluble chemical that inhibits germination

• absorbs water so well it inhibits germination of other seeds

  • inhibit bur-sage roots only, not its own species

  • allelopathic

Creosote Bush - germination & cloning

• during winter rains = abundant

• many don’t survive due to

  • water stress

  • high ground heat levels

  • germinate near mature plant

• young are vulnerable to heat and drought — only establish in 3-5 yr period of cool, moist weather

• reproduce asexually

  • generate from oldest bush stem in center through underground crown

  • up to 2-9 identical twins

  • center/original plant have taproot but clones do not

  • when center dies, shrub continues expanding until a ring is formed

Creosote bush - keystone species

• food

  • detritus feeders eat fallen leaves

• habitat

  • winds speed up, sand & seeds pile up, animals burrow'

• protection

  • canopy conceals animals from predators

Burr-Sage

• short and gray

• densely branched with small, lobed leaves

• fruit is small and spiny

Great Basin Sagebrush

• dominant community

  • i.e. sagebrush steppe

• aka big basin sagebrush

• joined by bitterbrush on coarse soils

  • tridentata

• in pristine conditions - incl. perennial bunchgrasses

  • grow in clumps not lawns

Alteration of sagebrush

• overgrazing

  • grazing animals consume vegetation faster than it can regrow, damaging land

    • i.e. cattle & sheep

• species shift:

  • sagebrush > bitterbrush

  • perennial bunchgrasses → annual grasses

Native browsers

Animals that feed on leaves, shoots, fruits of trees & shrubs

• pronghorn

• bison

• big horned sheep

Prefer bitterbrush, but never deplete due to low browser density

Bitterbrush

• dark green

• joined by sagebrush on coarse soils

• tridentata

Russian Thistle

• tumbleweed

• encroachment: increase in density and coverage in former grasslands from overgrazing, climate change, and fire supression

• native in Asia

• survives where irrigation of crops provides source of summer water

Rubber Rabbitbrush

Mojave Rubber Brush

• high rubber content of herbage

• common after fires

• nearly leafless

• when present, fine & resin covered

• Autumn bloom = take advantage of pollinators

• wind-disturbed plumed seeds

• good pioneer species

Antelope Brush

• SoCal equivalent of bitterbrush

  • Rose family (Rosaceae)

• joins GB sagebrush

• in spring - covered with yellow & many anthers