DISTURBANCE AND SUCCESSION IN SIERRAN FORESTSDISTURBANCE AND SUCCESSION IN SIERRAN FORESTS

disturbance- an event that clears away vegetation, opens up space and resources, e.g. fire, hurricane, treefall, disease

time since disturbance

succession- the orderly change in species composition in an area after a disturbance  

disturbance event

early successional species• high growth rate (grow quickly)• shade intolerant (need lots of sun)• Sierra examples: ponderosa pine

deer brush (Ceanothus) resprouting oaks

late successional species• low growth rate (grow slowly)• shade tolerant (grow in shade of other trees)• Sierra examples: white fir

incense cedar

oaks and pines regenerating after clearcut white fir regenerating in shady forest understory

Fire in the Sierras

• historically, frequent ground fires swept through CA forests, removing litter and young trees, maintaining “early successional” forests

• fires were often started by lightning strikes– Native American groups also set fires in order to

maintain the oaks whose acorns they used for food

Fire suppression in the Sierras

• fire suppression was practiced in CA beginning approx. 1900

• in 1960’s, policy changed and controlled burns in CA began

• because of fire suppression incense cedar and white fir are much more common today in CA forests than they were historically

• likewise, black oaks and ponderosa and sugar pines are now less common

increase in dead and down trees, serves as highly flammable “fuel”

fire suppression

in-growth of shade-tolerant trees (incense cedar, white fir) and shrubs

high density forests

increased fungal diseases and beetle attack

increased fire intensity when area burns

increased death of all trees

19002000

frequent, natural, low-intensity fires burned out underbrush and kept pine forests open

100 years of fire suppression have lead to high-density forests and in-growth of shade-tolerant tree species

low intensity fires•a natural part of most California ecosystems•historically started by lightning strikes•burn along ground, typically do not reach the tops of trees•kill only small trees or old, diseased adult trees•burn forest in patches– leave a patchwork of burned and unburned areas•used in “controlled” or “prescribed” burns

high intensity, or “stand replacing” fireshigh intensity, or “stand replacing” fires• result from fire suppression and subsequent buildup of dead wood, “fuel”• reach the crowns (tops) of trees• burn extremely hot• kill most or all trees• are extremely difficult to control• burn and destroy important, nutrient rich soil layers and microorganisms

The Yellowstone Firea high-intensity fire

Fire is important to CA forests:Fire is important to CA forests:

• prevents forests from becoming over-dense and unhealthy

• maintains early successional species:– Ponderosa Pine, Black Oak

• prevents insect disease outbreaks which occur in high density forests & can lead to high-intensity fires

• many California shrubs and trees are dependent on fire for regeneration– Bishop pine, Pt. Reyes: cones won’t open

unless heated in a fire

Wildlife:Wildlife:

• patchy fires maintain a mixture of habitats for wildlife

• early successional shrubs are important source of food for herbivores (deer)

• tall trees killed by fires form snags which are important homes for birds (owls, woodpeckers, sapsuckers)

1995 Point Reyes FireBishop Pine Regeneration

Bishop pine standbefore the fire

Bishop pine stand after the fire new, young Bishop

pines growing 3 yearsafter fire

Other disturbances in the mixed Other disturbances in the mixed conifer forest: conifer forest:

• Bark Beetles

• Root Rot fungus

• White Pine Blister Rust

bark beetles:• Beetle larvae bore through wood,

killing trees or making them more prone to fungal attack

• Typically attack mature trees

Bark beetles

• Most are small beetles- about the size of a rice grain

• Western Pine Beetle attacks Ponderosa and Coulter Pines

• Ambrosia Oak beetles can attack Oaks, Buckeye and tanoak.

• Woodpeckers are natural predators of bark beetles

Forest PathogensForest Pathogens

• native diseases: native diseases: – important to forest health – typically cause small forest gaps– maintain heterogeneous structure in

forest, similar to low-intensity fires

• introduced diseases:introduced diseases: – can spread quickly through forests with

devastating effects

• Heterobasidion annosum: Annosus Root rot • spreads by spores in dense tree stands infecting

freshly cut stumps, wounds• causes disease centers which spread out from

originally infected tree, when infection moves along roots to neighboring tree roots

• spreads at about 3 ft. per year• infected tree’s roots rot and tree falls from wind-

causes gaps/ openings (may be healthy looking!)• major problem in Yosemite! (1960’s cut trees- left

stumps– spread)• May be spread by bark beetles

Fungal diseasesFungal diseases

Root rot

•

White Pine Blister Rust White Pine Blister Rust

• a rust fungus native to Eurasia- introduced North America in 1910 from Europe.

• Hosts – this fungus needs 2 hosts to complete its life cycle: Ribes and a pine

• pine hosts: western white pine, sugar pine, whitebark pine, bristlecone pine, foxtail pine.

• alternate host: all species of the genus Ribes (gooseberrry, currant)

Distribution and damage –

• One of our most serious pathogen outbreaks on conifers

• In North America, it has caused more damage and costs more to control than any other conifer disease

• Since the 1920's, millions of dollars spent on the eradication of the alternate host, Ribes, and thousands of pine stands have been severely damaged

Pine Symptoms 1

Needle Chlorosis

• earliest symptoms = chlorotic spots• Yellowing caused by chlorophyll being

damaged• Spots form on needles shortly after infection

Pine Symptoms 2

• fungus moves into the branches, swellings form and develop into cankers

• the fungus girdles a branch & branch dies

• within a few years (dead needles on branch turn red = blister rust "flag“)

Pine symptoms:Cankers

• the fungus may continue to grow into the trunk, causing a canker that can kill that portion of the tree upward

• cankers often exude large amounts of resin

• tree usually dies from the infection

• approx. 10% of sugar pines are genetically resistant

Cankers (blisters)

Current Distribution

• Introduced to east coast in 1910

• Reaches Sierras in 1961