Transcript disturbance
Disturbance and its role in ecosystem structure and function Learning Objectives Lecture Oct 18, 2012 You should be able to: • Describe examples of natural and human disturbances • Discuss the role of disturbance in ecosystem structure and function including biodiversity • Describe the effects of disturbance as influenced by magnitude (intensity), frequency and duration of the disturbance • Discuss differences in disturbances on the east side of the Cascades vs the west side of the Cascades Disturbance influences species diversity and composition • Decades ago, most ecologists favored the view that communities are in a state of equilibrium • This view was supported by F. E. Clements who suggested that species in a climax community function as a superorganism • Disturbance was a problem © 2011 Pearson Education, Inc. • Other ecologists, including A. G. Tansley and H. A. Gleason, challenged whether communities were at equilibrium • Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbances • A disturbance is an event that changes a community, removes organisms from it, and alters resource availability • Disturbance is a natural component of ecosystems © 2011 Pearson Education, Inc. Disturbance • A physical force, agent, or process, either abiotic or biotic, causing a change in an ecological component or system; relative to a specified reference state and system (see Rykiel 1985, Vitousek & White 1981; Bazzaz 1983) Outcomes include: – Existing biomass is reduced in quantity. (See also. Grime 1979.) – Particular populations are selectively eliminated, reduced, added, or expanded. – Matter/energy/information exchange processes are inhibited. – Prevention of natural disturbance (fire exclusion) Disturbance has a spatial aspect Spatial Area affected – actual physical area affected by a disturbance Size and shape Small - Patches - treefall Large – glaciers, volcanoes, wind, fire Disturbances are characterized by: • Magnitude/Intensity – Refers to the severity of the effect • Frequency – Refers to how often the disturbance occurs • Duration – Refers to how long the disturbance lasts Magnitude Intensity – physical force of event Severity – impact on or consequences to the system of interest Frequency Frequency – the number of events per unit of time or probability that event will occurHow often does the disturbance occur? Small scale disturbances occur more frequently than large scale and is related to ecosystem type e.g., Dry forests burn frequently, humid tropical forests rarely e.g., Small forest gaps from tree fall occur more frequently than major windstorms Duration • Effects vary by disturbance duration How long does the disturbance last Recovery from disturbance • Resilience: Ability to recover from disturbances • If the disturbance is so large that a threshold is crossed, ecosystem will not recover to the same state • How much time does it take to recover? • Depends on magnitude, frequency and duration of disturbance • Disturbance creates habitat complexity Intermediate Disturbance Hypothesis Low levels of disturbance allow dominant species to exclude less competitive species Connell 1978 Disturbance Agents • • • • • • • • • • • Insects Diseases Fires Wind Volcanoes Climate change - La Niña, El Niño Drought Floods Glaciers Mass wasting Humans Fire Regimes Vary by Environment Historical Fire Regimes of the Pacific Northwest • High (Lethal) Severity – Infrequent (100+ yrs) and stand-replacing • Mixed (Moderate) Severity – Less frequent (25-75 yrs) and a mix of severities • Low (Non-lethal) Severity – Frequent (5-15 yrs) but low intensity High Severity Patches • Stand replacement • Few survivors, much coarse woody debris • Local scale 5-50 ha Low-severity fire Ponderosa Pine Forest Small trees are a result of fire exclusion Thick Bark Wide Spacing Tall Crowns Yellowstone recovery (a) Soon after fire (b) One year after fire Plant adaptations to fire (most animals flee) • Thick bark • Canopy seed banks - serotinous cones • Soil seed banks – Ceanothus, gooseberries (Ribes) • Sprouting – adventitious buds • Lignotubers • Rapid development – long leafed pine in SE USA Thick Bark • Giant sequoia – Douglas-fir (mature) Ponderosa pine Canopy Seed Bank Cones in Trees or Shrubs • • • • • • • • Lodgepole pine Monterey pine Montezuma pine Sand pine Jack pine Black spruce Aleppo pine Many more Wind Wind effects on trees Crown morphology – flagging, krummholz Stem and root shape –compression and tension wood, buttressing, Windsnap (stem breakage) and windthrow (uprooting) Gap formation 1921 Windstorm – Olympic Peninsula, WA Volcanoes • Mt. St. Helens – 1980 eruption Climate variability Long- time scale (human terms) 1000-1500 AD- Warm period 1500-1700 AD– Little ice age 1700 – Warming Climate variability Intermediate time scale Climate variability Short time scale El Nino/La Nina DROUGHT Reduces growth and kills trees (particularly small trees) Increases stress –makes trees susceptible to bark beetles and root disease Seattle Times Sept. 17, 2007 Poudre River, CO Effects of Disturbances on Ecosystems • Influences all aspects of ecosystems • Changes ecosystem structure: e.g., plant biomass, animal and plant species composition, forest succession • Influences processes (function) such as decomposition and nutrient cycling and water flow and primary productivity • Not necessarily good or bad, just is Human-Caused disturbances • • • • • • • • Air pollution Fire (prevention and ignition) Introduced plants, insects and diseases Forest management Urbanization Road construction Mining Climate change AIR POLLUTION 1. Gaseous air pollutants Ozone 2. Acid Rain and Fog – pH, NOx, SO2 Most are human-caused, but some are natural (e.g., volcanoes produce SO2) Ozone damage to ponderosa pine in southern CA Acid Rain • pH < 5.3 • Mostly sulphuric acid (H2SO4) and nitric acid (HNO3) • Lakes/streams are usually neutral but dropped 2 pH units (100X more acidic) killed fish • Direct adverse effect on vegetation • Soils: aluminum is mobilized Acid rain damage to a Czech forest Fire Exclusion Removed Fire • Effective suppression of wildfire began ~1900 in most U.S. pine forest • Fire killed small trees • Therefore fire was bad Pine Forest - 50 Years of Change Low Fire Hazard 1909 1927 High Fire Hazard 1938 1948 With Frequent Fire: With Fire Exclusion: Fires Get Bigger and More Severe • Fire decline until after World War II • Mechanization/Air support help • Fuel buildup becomes too widespread for control of all fires Natural recovery of lodgepole pine forests 5 years after the Yellowstone wildfire. Note carpet of young seedlings. Introduced plants Introduced plants mostly from ornamental or agricultural sources Kudzu – in southeastern U.S. Pacific Northwest Scotch broom – clearcuts and disturbed areas Himalayan blackberry – clearcuts, young forests English Ivy- gardens Japanese Knotweed – riparian areas What do the areas where plants have ‘invaded’ in the PNW have in common? Kudzu • Grows very rapidly • Deep roots prevent soil erosion • Medicinal uses include treatment of alcoholism Forest Management • Clearcutting, thinning, fertilization (including biosolids), monocultures, genetic selection, fire suppression all influence forest productivity • In many cases insect and disease problems get worse with forest management Others • Urbanization – continuing to occur • Road construction – urban and rural 380,000 miles of road in U.S. National Forests alone. Strongly affects wildlife and fish (culvert failure and sedimentation). Wildlife bridges and tunnels. • Mining • Climate change – will influence forests in the future – not sure how. Species will shift on the landscape. Forest Health along the I-90 corridor Urban/suburban Fire – none/rare Insects – DF beetle, aphids, Balsam Woolly Adelgid (BWA) Pathogens – root diseases, stem decay, cankers, Hazard Trees Western WA Forest Fire – infrequent Insects – DF beetle Pathogens – root diseases, Stem decay (old-growth), Dwarf mistletoes Hazard Trees Eastern WA Forest Fire – frequent Insects – beetles, defoliators Pathogens – root diseases, dwarf mistletoes stem decay, Hazard trees EASTERN WASHINGTON MORTALITY CAUSES WESTERN WASHINGTON MORTALITY CAUSES WA- East vs. West • West- modified disturbance regime – clearcuts on short rotation – removal of debris, structure, nutrients • East- highly modified disturbance regime – fire suppression- too much fuel, tree species shift – grazing- grass species shift, more fire prone – too many trees of “wrong” species- vulnerable to disease, fire and insects • Insects- WA- East vs. West – West- pests are there, but not very damaging • hemlock looper, Douglas-fir beetle, tussock moth – all natives acting like natives – East- many seriously damaging pests • spruce budworm, pine beetles do most damage – natives, but acting like invasives WA- East vs. West • Interactions – West- native root rots and insects such as Douglas-fir beetle tend to interact: rots weaken trees, beetles kill them- picking off sick and weak – East- fire used to interact with insects and diseases by keeping trees healthy and killing inappropriate species, but no more- all trees at risk of being sick and weak: good insect, fungus food. WA- East vs. West • Take home message – West- plenty of problems, but the forest is still generally able to recover from disturbance; maintains many of it historical, balanced interactions; and is relatively healthy on different scales. – East- historical interactions are gone; forests are no longer able to tolerate native insects and fire; unhealthy on all scales. SOLUTIONS (for “unnatural” disturbances) a. Not easy - many factors involved. Took years to create; how many years to fix? b. Thinning helps reduce fuel loading, reduces bark beetles, mistletoes and some root diseases; but no market for small diameter trees. Biofuels perhaps. Who pays? c. Reintroduction of fire; may work in some ecosystems (high natural high frequency) not others (low natural fire frequency). d. Social factors play a big role - property damage, human life, how people feel about forests; population increase. e. Greater concern about introduced diseases and insects Do human caused disturbances affect ecosystems more than natural disturbances? What do you think? What things would you consider?