Ecology and Plant Communities Chapter 27. Plant Communities Community –Group of clustered species associated with each other –Named after its dominant.

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.