ecology and plant communities chapter 27. plant communities community –group of clustered species...
TRANSCRIPT
Ecology and Plant Communities
Chapter 27
Plant Communities
• Community– Group of clustered species associated with
each other– Named after its dominant species and
characterized by its own roster of associated species and their combined architecture
– Each community has features called attributes
Some Attributes of Plant CommunitiesPhysiognomy (architecture)
Canopy cover and leaf area index
Growth forms of dominant species
Spatial pattern
Timing of life cycle events (germination, bud break, flowering, leaf drop)
Species diversity and richness
Productivity
Biomass
Efficiency
Allocation of biomass (roots, stems, leaves, reproduction)
Nutrient cycling
Nutrient demand
Location and size of storage pools
Efficiency
Change over time (succession)
Primary vs. secondary
Progressive vs. retrogressive
Physiognomy
• External appearance of the community, its vertical structure, and the growth forms that dominate each canopy layer– Desert community
• Single canopy layer 10% canopy cover or less (10% of ground is directly beneath foliage of shrubs, 90% is open and unshaded)
– Tropical rain forest• Several overlapping tree layers 100% canopy
cover
Physiognomy
• Leaf area index (LAI) – Total area of leaf surface (one side only) for
all leaves that project over a given area of ground
Species Richness
• Communities differ in number of associated species they contain– Tropical rain forest appears to have greatest diversity
of plant species, up to 365/10,000m2
• Not every species in a community is equally important– Importance of each species can be quantified by
counting the density of individuals (number/unit area), its canopy cover, its biomass, or its frequency of occurrence in community
Biomass and Productivity
• Communities differ in amount of biomass aboveground and belowground– Can be described as the root-to-shoot ratio
• Communities differ in amount of biomass they produce each year (productivity)– Communities with greatest biomass usually
are most productive because they have greater LAI with which to trap solar radiation
Biomass and Productivity
– High biomass communities also tend to live where growing season is longest
• Example – tropical rain forest community annual productivity is 20,000 kg/hectare, desert community annual productivity is 2,000 kg/hectare
Nutrient Cycling
• Process of nutrient uptake, use, and return– Examples
• Nitrogen cycle• Carbon cycle• Sulfur cycle• Water cycle• Phosphorus cycle
The details of each cycle, the rates of nutrient movement within it, and its overall efficiency are different from community to community.
Nitrogen Cycle
Carbon Cycle
Succession
• When stable community is removed by disturbance such as landslide, storm, canopy fire, logging soil surface exposed microenvironment has changed
• Species that first colonize site disturbance usually are not those of the old community– Usually will be seedlings of r-selected species
adapted to open sites
Succession
• Plant succession process of community change at one place over time
• Must occur within a uniform macroenvironment– Generally occurs over an area from 1 hectare to
several square kilometers
• Usually measured over course of several years to several hundred years– Does not occur over time periods shorter than a year – Occurs over time shorter than 1,000 years
Succession
• Stages of succession proceed from pioneer to climax phases
• Pioneer stage– First plants to invade
• Climax stage– End point of succession
Succession
• Primary succession– Occurs on newly exposed ground not
previously occupied by plants– Slower than secondary succession
• Parent materials lack clay particles and essential nutrients
• No bank of plant seeds, bulbs, or rhizomes already in soil
• Secondary succession– Takes place on vegetated land
Succession
• Progressive succession– Changes which make community more
complex and massive, cycles of energy and nutrients more efficient, and microenvironment less stressful
• Retrogressive succession– Reverse of progressive succession– Community becomes simpler and less
massive, cycles of energy and nutrients less efficient, and microenvironment more severe
Comparison of Community Traits During Early and Late Stages of Progressive Succession
Trait Early Stages Later Stages
Biomass Small Large
Architecture Simple Complex
Nutrient pool Soil Vegetation
Mineral cycling Loose Tight
Productivity High Low
Stability Low High
Species diversity Low High
Life history r-selected K-selected
Site quality (microenvironment)
Extreme and not well-developed
Moderate and well-developed
Vegetation Types
• Vegetation refers to dominant growth form, not to dominant species
• Vegetation type– Has two-part name that describes the dominant
growth form and the habitat– Name does not include any information about the
species– Many fewer vegetation types than communities
Vegetation Types
• Most of world’s vegetation types are represented in North America except for extremely arid deserts and several tropical grassland, savanna, and forest types
• Biomes– Large, regional, climatically controlled
ecosystems
Major Vegetation Types of North America
Vegetation Types
• Tundra– Word derived from Finnish or Lapp meaning
“marshy hill”– Most of biomass is below soil surface and
shoot-to-root ratio is high– Shrubs are dwarfed, gain height only in
protection of boulders or small hills– Perennials produce many large flowers
Vegetation Types
• Tundra– Most successful reproduction is by rhizomes– Annuals are rare– Covers about 19% or North America’s land
area– Two tundra regions
• Arctic• Alpine
Artic tundra Alpine tundra
Average daily mean temperature during warmest month
10ºC or less 10ºC or less
Growing season Short, 2-3 months Short, 2-3 months
Annual precipitation Less than 25cm Less than 25cm
Thermoperiods More narrow Broader
Summertime solar radiation
Less Greater
Temperatures near soil surface
Lower Greater
Vegetation Types
• Boreal forest– Broad belt of low-elevation conifer forest– Covers 28% of North America– Amount of heat received during growing season is
critical factor for determining location of timberline– Growing season 3-4 months in duration– Temperatures much warmer than in tundra– Annual precipitation 30 to 90 cm, mainly falling in
summer
Vegetation Types
• Boreal forest– Taiga (means “dense forest”) of North
America– Winter temperatures can be lower than in
tundra– Soils are relatively young, acidic, and leached
of nutrients– Vegetation has two-layered architecture
• Trees are slender, short, and relatively short-lived, but are densely packed
Vegetation Types
• Boreal forest– Continuous layer of bryophytes, seedless
vascular plants, and herbaceous perennial angiosperms carpets ground beneath overstory canopy
– About every 200 or more years, disturbances by storms and wildfires affect large areas and set secondary succession in motion
Vegetation Types
• Eastern deciduous forest– Dominated by variety of broadleaf, winter-
deciduous tree species– Growing season 6 months– Precipitation above 100 cm/year– Soils richer in nutrients and less acidic– Covers about 11% of North America
Vegetation Types
• Eastern deciduous forest– Human impact
• Native Americans burned understory• Euro-Americans modified forest by clearing,
selectively cutting, grazing, or accidentally introducing foreign pathogens and weeds
• Forest no longer resembles descriptions by early explorers
Vegetation Types
• Eastern deciduous forest– Striking feature is seasonality– Two intermediate canopy layers between
ground herbs and overstory trees• Scattered shrubs (many in heath family)• Small trees such as dogwood• Many vines grow up through all the tree and shrub
canopies
Vegetation Types
• Grasslands – Woodland
• Grassland with overtopping trees whose canopies cover 30% to 60% of ground
– Savanna• Grassland with overtopping trees that are regularly
present but whose canopies cover less than 30% of the ground
– Steppe• Grassland interspersed with shrubs
Vegetation Types
• Grasslands – Grassland
• Synonym – prairie• Vegetation dominated by herbaceous plants
growing in climate too dry for trees• Trees restricted to areas such as along waterways
or on rocky ridgelines with thin soil• Perennial and annual grasses dominate biomass• Broad-leaved dicot herbs dominate in terms of
numbers of species
Vegetation Types
• Grasslands – Grassland
• Probability of wildfire in same area of land every 1 to 3 years is high
• Favored in areas where growing temperatures are high enough and air humidity low enough to promote transpiration
• Once covered 21%of North America
Vegetation Types
• Grasslands – Grassland
• Areas in North America– Center of North America, from Manitoba to Texas on the
east and from Iowa to the Rocky Mountains on the west– Along the edge of warm deserts in Texas, New Mexico,
and Arizona– Scattered through the intermountain Great Basin (with a
finger extending into the Palouse area of southeastern Washington and outliers in the Willamette Valley of Oregon and north coastal California
– Within the Central Valley of California
Vegetation Types
• Grasslands – Grassland
• Significantly modified by Euro-Americans in the last 200 years
– Most of central grasslands have been cleared and plowed, converted to farmland
– Desert grasslands – overgrazed and fire has been suppressed
– Intermountain, Palouse, and California grasslands have been overgrazed and invaded by aggressive annuals from Eurasia; many hectares also converted to farmland, pasture, and urban sprawl
Vegetation Types
• Desert scrub– Scrub
• Any vegetation dominated by shrubs• Occurs where either precipitation or water storage
capacity of soil is too low to support grassland• Examples include
– Thorn scrub– Chaparral– Desert scrub
Vegetation Types
• Desert scrub– Occurs where annual rainfall is less than 25
cm and pronounced dry season exists every year
– High variation in rainfall from year to year– Usually warm to hot during summer but may
be quite cold in winter
Vegetation Types
• Desert scrub– North American deserts
• Sonoran desert• Mojave of southern California and Nevada• Chihuahua of Texas, New Mexico, and Mexico
– Vegetation • Associated with shrubs in warm deserts may be
succulent cacti, green-stemmed trees, subshrubs, herbaceous perennials, and ephemerals
Vegetation Types
• Desert scrub– Ground cover may be 50% at maximum and
5% at a minimum– All plants in desert scrub adapted to survive
through extended droughts– Five basic techniques for drought tolerance or
avoidance• Phreatophyte syndrome• Drought-deciduous syndrome
Vegetation Types
• Desert scrub• Evergreen leaves• Succulents• Ephemerals
Basic technique for drought tolerance or avoidance
Description
Phreatophyte syndrome
Deep roots in permanent contact with groundwater; green stems and leaves that are winter deciduous; leaves well supplied with water during hot summer
Drought-deciduous syndrome
Shrubs with shallower roots, retain leaves only during wet season and drop them during the dry season; leaves are thin and energetically inexpensive; can be cast off and remade several times a year
Evergreen leaves
Shrubs (true xerophytes) have evergreen leaves; metabolism is at slow rate all year; some leaves may be shed under prolonged drought; leaves typically small with anatomical features that retard transpiration
Succulents
Store water in vacuoles of large cells; typically exhibit crassulacean acid metabolism; leaves and bodies minimize amount of surface for a given volume or mass (reduces transpiration); shallow root system; appear to avoid drought rather than tolerate it
Ephemerals Live for 6 weeks to 6 months; complete life cycle during wettest, least stressful part of year; avoid drought by remaining dormant as seeds during drought season
Vegetation Types
• Mediterranean – Mediterranean climates and vegetation found
in five locations throughout world• Mediterranean rim of southern Europe, the Middle
East, and northern Africa• Cape region of South Africa• Southern and southwestern Australia• Central Chile• California
Vegetation Types
• Mediterranean – All regions lie between 40 and 32 degrees N
or S latitude– Occupy western or southwestern edges of
continents– Receive 27 to 90 cm of annual precipitation– Minimal frost– Fire-type climates
• Hot, dry summers and cool, wet winters
Vegetation Types
• Mediterranean– Vegetation ranges from forest to woodland to
scrub– Mediterranean scrub
• Called chaparral (means “low-growing”)• Dense, one-layered, about 1 to 3 m in height• Composed of rigidly branched shrubs with small,
hard leaves and extensive root system
Vegetation Types
• Mediterranean– Mediterranean scrub
• Wildfire recurs every 20 to 50 years• Chaparral shrub response to fire
– Some sprout from root crown buried beneath soil surface– Some have hard-coated seeds that are dormant until
cracked by moderately high temperatures– Some have seeds stimulated to germinate by some
active ingredient in smoke
Vegetation Types
• Mediterranean– Mediterranean scrub
• Chaparral community recovers preburn cover and species composition within six years
• Grows on steep slopes with coarse, shallow soils, at elevations below 1,000 m
• Covers about 1% of North America
Vegetation Types
• Pacific coast conifer forest– Most luxuriant, most productive, most
massive vegetation type in world– Often called a temperate rain forest– Dominated by rich diversity of big, long-living
tree species with lower canopies of shrubs, herbs, bryophytes, and epiphytes
– Mild climate, buffered by nearby ocean and summer fog banks
Vegetation Types
• Pacific coast conifer forest– Trees commonly live for 400 to 1,200 years
and attain heights of 100 m– Dominant tree species include
• Coast redwood• Douglas fir• Lowland white fir• Sitka spruce• Western hemlock• Western red cedar
Vegetation Types
• Pacific coast conifer forest– Covers about 3% of North America– Timber volume and value highly significant to
both local human communities and distant corporations
Vegetation Types
• Upland conifer forests– Called montane conifer forests– Cover 7% of North America– Range from 65 to 19 degrees N latitude– Annual precipitation ranges from 60 cm in
lowest elevations to more than 200 cm at highest elevations
Description
Lower montane (low-elevation) forests
Tend to be rather open savannas or woodlands, intermingled with species from adjacent grasslands, Mediterranean woodlands and chaparral, or deserts; frequent wildfires essential to maintenance of some of these communities; significantly degraded by overgrazing and changes in fire frequency; common trees include Pinyon pines, ponderosa pines, junipers
Mid-montane (intermediate-elevation) forests
Typically rich in overstory species such as Douglas fir, white fir, ponderosa pine; variety of shrubs in heath and rose families; seasonally present herbaceous perennials; four-layered forests that often require wildfire to maintain structure
Upper montane and subalpine (highest-elevation) forests
Densest and simplest; experience deepest snow packs; elevation zone where oldest individual plants in world exist (bristlecone pines); dominant genera include fir, hemlock, pine, spruce
Upland Conifer Forests
Aquatic Ecosystems
• Saline wetlands– Wetlands
• Terrestrial sites where upper soil is saturated by saline or freshwater for at least a few weeks of the year
– Tidal wetlands or salt marshes• Coastal meadows subject to periodic flooding by
the sea• Vegetation is usually a single, low-growing, nearly
closed layer of perennial herbs
Aquatic Ecosystems
• Saline wetlands– Tidal wetlands or salt marshes
• Soil crowded with rhizomes and roots• Traits of successful species include
– Succulence– Asexual reproduction by rhizomes– Aerenchyma tissue in stems and roots
• Ecological functions of wetlands– Biological filter for runoffs from land– Nursery for young of many aquatic animals
Aquatic Ecosystems
• Saline wetlands– Tidal wetlands or salt marshes
• Annual productivity as great as that of tropical rain forest
– Herbaceous plant tissue shed each year into water where it fuels extensive food chain
Aquatic Ecosystems
• Rocky intertidal ecosystem– Along exposed coasts that receive full brunt of
wave action– Only a few flowering plants
• Example: surf grass
– Many seaweeds and a few kelps attached to rocks
Aquatic Ecosystems
• Neritic zone– Rocky shelf always covered with water but
shallow enough to admit enough sunlight to support attached algae that grow along bottom
• Pattern of distribution reflects differing tolerances among algal species to low light
– Many kelp found here, especially in cool-temperate oceans
Aquatic Ecosystems
• Freshwater wetlands– Found along the shorelines of lakes, rivers,
seeps, and springs– Trees, shrubs, herbs occupying habitat must
be tolerant of occasional flooding– Often have fast growth rates– Produce abundant wind-distributed seed– Capable of vegetative reproduction
Aquatic Ecosystems
• Freshwater wetlands– Economically important as filter of eroded soil
and nutrients that would otherwise enter adjacent aquatic ecosystems and degrade them
– Emergent aquatic plants (rooted in water but part of plant body extends above water surface) found at shallow margins
• Sedge, cattail, water lily
Aquatic Ecosystems
• Freshwater wetlands– Deeper water
• Submerged or floating flowering plants are common
• Deep lake water may stagnate into zones– Epilimnion– Thermocline– Hypolimnion
Aquatic Ecosystems
• Freshwater wetlands• Deep lake water may stagnate into zones
– Epilimnion» Relatively warm, sunlight is intense enough to
support large phytoplankton population» High oxygen level
– Thermocline» Narrow transition zone where temperature declines
rapidly with depth» Serves as barrier to any mixing between upper
epilimnion and lower hypolimnion– Hypolimnion
» Extends to bottom of lake
Conservation Biology
• Conservation – Originally meant a rate of natural resource
consumption that would result in sustained, continued existence of that resource far into future
– Now usually means restricted use, nonuse, or preservation of some natural resources
Conservation Biology
• Relatively new science that studies impact of human societies on nonhuman landscape
• Question asked by conservation biologists– Can a growth-oriented, technological culture
coexist with its surrounding natural systems?
Conservation Biology
• Conservation biologists investigating ways to measure sustainability– How do we know when plant or animal
population is sustaining itself?– How do we measure biotic diversity?– How do we design parks so that the
probability of extinction for any rare population is as low as possible?
Conservation Biology
• Conservation biologists investigating ways to measure sustainability– How do we restore degraded habitats and their plant
and animal communities?– How do our technological activities interweave with
the biosphere in unexpected ways to magnify into global stresses (acid rain, ozone depletion, climate change, pollutants carried through food chains), and how might we best modify these technological activities to reduce the stress?
Ecosystem Restoration
• Some vegetation types and entire ecosystems cannot be conserved because they have been largely changed by human activities– Exotic weedy plants and animals invaded and
became widely established• Reduced abundance or eliminated some native
species
– Domestic livestock have caused severe surface erosion
Ecosystem Restoration
– Dams no longer permit seasonal fluctuations in volume of water in rivers
– Fire suppression management has led to forest thickening, loss of species richness, and epidemic tree mortality during period droughts
Ecosystem Restoration
• Ecosystem restoration– Active management techniques used to reverse the
human-caused changes and to bring back the previous ecosystem
– Advantages of restoring habitat, vegetation type, or ecosystem
• Restored system tends to be self-maintaining, requires less in terms of human attention
• Ecosystem services increase– Soil stabilization, protection against catastrophic fire, filtering of
contaminants from agricultural runoff, provision of maximum biodiversity, moderation of temperature
Ecosystem Restoration
• Restoration techniques imitate natural successional or disturbance processes
• Has been somewhat successful in wetlands, grasslands, and certain forest types
• Will require much more experimentation and accumulated wisdom
Ecosystem Restoration
For now, the best action is to conserve what remains.