Plant Ecology & Responses to Global Change

Lab 13: Plant Ecology & Responses to Global Change

Objectives

By the end of this lab, you'll understand:

• How to characterize plant communities (who lives where and in what numbers)

• How to measure species diversity and interpret what it means

• How invasive species impact native plant communities

• How to detect evidence of climate change in plant populations

• How population age structures reveal whether species are thriving or declining

• How microclimate variation shapes where plants can grow in deserts

• Desert-specific ecological patterns and challenges

Background

Plant Community Structure

A plant community = All the plant species living together in one place

Communities are organized in layers:

• Canopy layer: Tall trees (palo verde, mesquite)

• Shrub layer: Woody plants 1-5m tall (creosote, brittlebush, bursage)

• Herbaceous layer: Non-woody plants (grasses, wildflowers)

• Ground layer: Lichens, biological soil crusts

Desert communities differ from forests:

• More open canopy

• Shrubs often dominant

• Large bare ground patches

Seasonal herbaceous layer (ephemeral after rain) 

Global Change Impacts

Two major threats we'll investigate:

1. Invasive Species

Common invasives:

• Red brome (Bromus rubens) - Annual grass

• Sahara mustard (Brassica tournefortii) - Outcompetes natives

• Buffelgrass (Pennisetum ciliare) - Increases fire frequency

• Russian thistle (Salsola tragus) - Tumbleweed

• Tamarisk (Tamarix spp.) - Riparian invader

Impacts:

• Outcompete natives

• Alter fire regimes (grasses fuel fires!)

• Change soil chemistry

• Reduce biodiversity

2. Climate Change

Arizona-specific changes:

• Temperature: +2-4°F since 1970 (Mohave Desert is the hottest place on the planet)

• Precipitation: More variable, intense monsoons

• Drought: Multi-year megadroughts increasing

• Phenology: Earlier spring warming

Evidence we can detect:

• Severe drought stress (widespread)

• Seedling recruitment failure

• Species range shifts (moving upslope)

• Altered flowering times

• Increased mortality events

Modified Habitat Types:

Habitat 1: Desert Scrubland

• Open canopy

• Dominated by creosote, bursage, brittlebush

• Large bare ground patches

• Most accessible habitat

Habitat 2: Riparian/Wash

• More moisture (seasonal or permanent water)

• Mesquite, palo verde, cat claw acacia

• Dense vegetation

• Higher diversity

Habitat 3: Upland/Rocky Slope

• Dry, exposed conditions

• Yucca, agave, cacti

• Shallow soils

• Lower diversity, sparse cover

Materials

Required:

• Measuring tape (30m minimum)

• Meter stick or ruler

• Thermometer (digital recommended)

• Smartphone with:

  • Camera

  • Clinometer app (for tree/shrub heights)

  • Compass app

  • Plant ID app (iNaturalist)

  • Calculator

• Stakes and bright flagging tape

• String (100m+)

• Field notebook and pencils

• Gloves (protection from spines!)

Part 1: Plant Community Structure Analysis

Goal: Systematically document plant communities in three habitat types, revealing composition, structure, and diversity.

Step 1: Select Your Three Study Habitats

Choose contrasting habitats:

Habitat 1: Desert Scrubland (Creosote Flats)

Characteristics:

• Dominated by creosote bush, bursage, brittlebush

• Open spacing (allelopathy—creosote inhibits neighbors)

• Sandy to rocky soils

• Minimal human disturbance

Habitat 2: Riparian Corridor or Wash

Characteristics:

• More moisture (seasonal or permanent)

• Mesquite, palo verde, desert willow dominant

• Denser vegetation

• Higher species richness

Habitat 3: Rocky Upland or Bajada

Characteristics:

• Steep slopes, rocky substrate

• Yucca, agave, cacti, ocotillo

• Shallow soils

• More extreme conditions

Alternative habitat choices:

• Irrigated vs. natural desert

• North-facing vs. south-facing slope (dramatic microclimate differences!)

• Urban park vs. undisturbed desert

Survey A: Tree/Large Shrub Layer

In Arizona deserts, "trees" are often large shrubs or small trees <10m tall

Step 1: Establish Plot

Layout 10m × 10m square plot (100 m²):

1. Choose plot center in representative area

2. Measure 5m in each cardinal direction (N, S, E, W)

3. Mark corners with stakes and flagging

4. Use string to define boundaries (helps visualize)

Photo: Take overview photo from each corner

Step 2: Identify and Measure All Woody Plants >2m Tall

For EVERY woody plant >2m in the plot:

A. Identify Species

Common Arizona "trees":

• Creosote bush (Larrea tridentata) - Can reach 3-4m

• Mesquite (Prosopis spp.) - 3-8m tall

• Palo verde (Parkinsonia spp.) - 5-10m tall

• Ironwood (Olneya tesota) - 5-10m tall

• Desert willow (Chilopsis linearis) - 3-8m tall

• Cat claw acacia (Senegalia greggii) - 2-6m tall

Method:

• Use field guide or ID app

• If unknown, describe and photograph

• Assign species code (e.g., CR = Creosote, ME = Mesquite, PV = Palo Verde)

B. Measure Diameter

For multi-stemmed desert plants:

Method 1: Basal diameter (at ground level)

• Measure circumference of all stems combined at ground

• Calculate diameter: C ÷ 3.14

Method 2: DBH for single-trunked trees

• Measure at 1.4m (breast height)

• Most desert "trees" are multi-stemmed—use Method 1

Recording: Diameter: _____ cm

C. Estimate Height

Method 1: Stick method (simplest)

1. Hold stick at arm's length

2. Back up until stick matches plant height

3. Distance you walked ≈ plant height

Method 2: Clinometer app

• Stand at distance ≈ plant height

• Measure angle to top

• App calculates height

Method 3: Comparison

• Your height: ~1.7m

• Estimate multiples: "About 3 of me tall" = ~5m

Recording: Height: _____ m

Expected heights:

• Creosote: 2-4m

• Mesquite: 3-8m

• Palo verde: 5-10m

D. Assess Plant Health

Rate on 1-5 scale:

• 5 = Vigorous: Full, dense canopy; healthy leaves; active growth

• 4 = Healthy: Normal canopy; few dead branches; good color

• 3 = Fair: Thin canopy; some dead branches; signs of stress

• 2 = Poor: Many dead branches; sparse yellowing foliage; severe stress

• 1 = Dead or Dying: Mostly dead; few/no leaves

Arizona stress indicators:

• Drought-deciduous (dropped leaves)—normal response!

• Branch dieback from tips

• Yellow/brown foliage

• Sparse canopy

Recording: Health: _____ (1-5)

E. Map Plant Positions

Simple method:

• Sketch 10m × 10m plot on paper

• Mark each plant's approximate position

• Label with species code and ID number

• Note orintation (N arrow)

Advanced method:

• Measure distance and bearing from plot center

• Record coordinates for each plant

F. Assign ID Numbers

• Tree/Shrub 1, 2, 3, etc.

• Label consistently in notes and on map

Step 3: Calculate Community Metrics

A. Species Richness

• Count total number of species in plot

• Richness = _____ species

Expected: 3-8 species in desert scrubland

B. Density

• Count total woody plants >2m: _____

• Plot area = 400 m² = 0.04 hectares

• Density = (# plants ÷ 0.04) = _____ plants per hectare

Expected: 50-300 plants/hectare (much more open than forests!)

C. Basal Area

For each plant:

1. Calculate cross-sectional area:

   • Area = 3.14 × (radius)²

   • Radius = Diameter ÷ 2

2. Sum all plant basal areas

3. Total: _____ cm² or _____ m²

Why it matters: Shows how much ground is occupied by plant bases

D. Identify Dominant Species

Which species has:

• Most individuals? _____

• Largest total basal area? _____

• Largest individuals? _____

These dominate the community!

Arizona expectation: Creosote often dominates desert scrubland (60-80% of plants)

Step 4: Data Table for Woody Plants

Survey B: Shrub Layer (<2m tall)

In Arizona, shrubs are THE dominant life form!

Step 1: Establish Plots

Layout THREE 10m × 10m plots (100 m² each) in each habitat:

• Space them across habitat (not clustered)

• Choose representative areas

• Mark corners

Step 2: Survey Shrubs in Each Plot

Shrub = woody plant <2m tall

Common Arizona shrubs:

• White bursage (Ambrosia dumosa)

• Brittlebush (Encelia farinosa)

• Desert marigold (Baileya multiradiata)

• Bursage (Ambrosia deltoidea)

• Jojoba (Simmondsia chinensis)

• Mormon tea (Ephedra spp.)

• Ocotillo (Fouquieria splendens) - technically shrub

For each plot:

A. Identify All Shrub Species

• Walk through systematically

• Record every different species

• Use field guide/app

Recording: Species list: _____, _____, _____

B. Count Individuals by Species

• Count distinct plants

• For clonal species, count clumps

Recording:

• Species 1: _____ individuals

• Species 2: _____ individuals

• etc.

C. Measure Heights

• Measure 3 individuals per species

• Record maximum height

• Calculate average

Recording: Average height: _____ m

D. Estimate Percent Cover

Percent cover = % of ground covered when viewed from above

For each species:

• Look down at plot

• Estimate coverage:  <1%  /  1-5%  /  5-25%  /  25-50%  /  >50%

Total cover often 20-40% (lots of bare ground normal!)

Step 3: Calculate Shrub Metrics

For each habitat (average across 3 plots):

A. Species Richness: _____ species

B. Density: _____ individuals per 100 m²

C. Average Cover: _____%

D. Dominant shrubs: _____

Step 4: Data Table for Shrubs

Survey C: Herbaceous Layer

Step 1: Establish Quadrats

Place FIVE 1m × 1m quadrats randomly in each habitat

Randomization method:

• Walk in random direction

• Every 5 paces, place quadrat

• Or toss quadrat frame over shoulder

Step 2: Survey Each Quadrat

Arizona herbaceous plants:

• Spring wildflowers (March-May): Desert marigold, lupines, poppies, brittlebush

• Grasses: Big galleta, Indian ricegrass, fluffgrass

• After monsoon (Aug-Sept): Summer annuals

For each 1m × 1m quadrat:

A. Identify All Herbaceous Species

Herbaceous = non-woody:

• Grasses

• Wildflowers

• Ferns (rare)

• Seedlings

Recording: Species found: _____, _____, _____

B. Estimate Percent Cover Per Species

Visual guide:

• 0%: Absent

• <1%: 1-2 tiny plants

• 1-5%: Few scattered individuals

• 5-25%: Present throughout, sparse

• 25-50%: Common, covers 1/4 to 1/2

• 50-75%: Abundant

• 75-100%: Dominant, nearly continuous

Arizona note: In dry season, herbaceous cover may be 0-5% total!

C. Count Individuals

• If <20 plants, count exactly

• If >20, estimate

D. Measure Vegetation Height

• Measure tallest plant in quadrat

• Record: _____ cm

E. Note Reproductive Status

• Flowering

• Fruiting

• Vegetative only

• Dormant/absent

Step 3: Calculate Herbaceous Metrics

For each habitat:

A. Mean Species Richness Per Quadrat: _____

B. Total Species Richness: _____ (unique species across all 5 quadrats)

C. Mean Total Cover: _____%

Expected Arizona patterns:

• Spring: 20-60% cover (wildflowers!)

• Summer dry: 0-10% cover

• Post-monsoon: 10-30% cover

D. Evenness:

• High (all species similar abundance)

• Low (one species dominates)

Step 4: Data Table for Herbs

Part 2: Invasive Species Assessment

Goal: Document invasion intensity and impacts on native communities.

Arizona's invasive plant problem is SEVERE—especially fire-promoting grasses!

Common Invasives:

Red brome (Bromus rubens):

• Annual grass

• Forms dense mats

• INCREASES FIRE FREQUENCY (major threat!)

• Outcompetes native forbs

Sahara mustard (Brassica tournefortii):

• Winter annual

• Large (>1m tall)

• Allelopathic (chemically inhibits natives)

• Prolific seed producer

Russian thistle (Salsola tragus):

• Tumbleweed

• Spreads seeds while tumbling

• Tolerates disturbed sites

Buffelgrass (Pennisetum ciliare):

• Perennial grass

• Extreme fire promoter

• Threatens saguaros, desert ecosystems

• Spreading north from southern AZ

Mediterranean grass (Schismus spp.):

• Small annual grass

• Ubiquitous in disturbed areas

Tamarisk (Tamarix spp.):

• Riparian invader

• High water use

• Forms dense thickets

Step 1: Identify Invasive Species

In each habitat:

A. Search Systematically

• Walk transects through habitat

• Note ALL invasive species

• Use regional list: https://www.nps.gov/subjects/invasive/plants.htm

B. Record for Each Invasive:

• Scientific name: _____

• Common name: _____

• Life form: Annual grass  /  Perennial grass  /  Forb  /  Shrub  /  Tree

• Native range: _____

Step 2: Map Invasive Distribution

For each invasive:

Method 1: Sketch map

• Draw habitat outline

• Mark invasion patches

• Estimate patch sizes

Method 2: GPS

• Record waypoints at patch centers

• Note patch extent

Step 3: Quantify Invasion Intensity

Within each habitat:

A. Percent Cover by Invasives

Estimate across entire habitat:

• 0%: None present

• <1%: Few scattered individuals

• 1-10%: Present but not dominant

• 10-25%: Becoming problematic

• 25-50%: Seriously invaded

• >50%: Completely invaded

Recording: Invasive cover: _____%

B. Density

Count in 5 quadrats (same ones from Part 1C):

• Invasive individuals per m²: _____

• Compare to native density: _____

• Ratio: Invasive:Native = :

C. Size/Age Structure

Sample 20 individuals of dominant invasive:

Classify:

• Seedlings (<10 cm): _____ individuals

• Juveniles (10cm-1m): _____ individuals

• Adults (>1m or reproductive): _____ individuals

Interpretation:

• Lots of seedlings = Actively spreading!

• Only adults = Established but not recruiting

Step 4: Document Invasion Impacts

Compare invaded vs. non-invaded quadrats:

A. Native Species Richness

From your Part 1C data:

• Invaded quadrats (>25% invasive cover): Avg _____ native species

• Non-invaded quadrats (<5% invasive cover): Avg _____ native species

• Difference: _____ species lost

B. Structural Changes

Compare:

• Vegetation height: Invaded _____ cm vs. Non-invaded _____ cm

• Total cover: Invaded _____% vs. Non-invaded _____%

• Bare ground: Invaded _____% vs. Non-invaded _____%

C. Fire Risk Assessment (CRITICAL in Arizona!)

Grass invasions CREATE fire risk:

Fuel continuity:

• Continuous - Invasive grass connects widely-spaced natives (HIGH RISK!)

• Patchy - Some connections (MODERATE RISK)

• Isolated - No connections (LOW RISK)

Fuel load:

• Estimate dry grass biomass: Light  /  Moderate  /  Heavy

Native desert has NATURAL fire return interval of 100-1000+ years. Invasive grasses reduce this to 3-5 years, killing native cacti, trees, shrubs!

Step 5: Identify Invasion Vectors

How did invasives arrive?

Distance from probable sources:

• Roads/trails: _____ m

• Buildings/development: _____ m

• Washes (water dispersal): _____ m

Likely vector:

•  /  Vehicle transport (seeds in tire treads)

•  /  Water dispersal

•  /  Wind dispersal

•  /  Escaped cultivation

•  /  Animal dispersal (livestock)

Step 6: Assess Reproduction

For each invasive:

A. Seed Production

• Estimate: None  /  Low  /  Moderate  /  High  /  Very high

B. Vegetative Spread

• Rhizomes (underground stems)

• Root suckers

• Fragmentation

• None (seeds only)

Step 7: Calculate Invasion Severity Score

Rate each invasive (1-5 scale):

1. Cover: 1 = <1%, 5 = >50%: _____

2. Density: 1 = rare, 5 = dominant: _____

3. Distribution: 1 = one patch, 5 = throughout: _____

4. Impact on natives: 1 = minimal, 5 = severe: _____

5. Fire risk (Arizona-specific): 1 = none, 5 = extreme: _____

Total severity score: _____ (5-25)

• 5-10: Low severity

• 11-17: Moderate severity

• 18-25: High severity (management priority!)

Step 8: Data Tables

Invasive Species Inventory:

Impact Comparison:

Part 3: Climate Change Indicators

Goal: Detect evidence of climate change impacts in plant populations.

Arizona is a climate change hotspot! Temperature increases here are 2× global average.

Observation A: Drought Stress Indicators

Mohave County is experiencing intensifying drought

Step 1: Survey for Stress Symptoms

Select 25+ individual plants across multiple species:

• 5+ individuals per species

• Mix of size classes

• Various microhabitats

Focus on:

• Palo verde

• Mesquite

• Creosote

• Brittlebush

• Ocotillo

Step 2: Assess Stress Symptoms

For each individual:

A. Drought-Deciduous Response

Many Arizona plants drop leaves in drought (NORMAL!):

Check:

• Fully leafed - Normal for season

• Partially leafed - Some drought response

• Leafless - Full drought-deciduous

• Dead - Brown branches, no life

Timing context:

• Spring (March-May): Should be fully leafed

• Summer dry (May-June): Partial leaf drop normal

• Monsoon (July-Sept): Should re-leaf

• Winter: Some species naturally deciduous

Recording: Leaf status: _____

B. Branch Dieback

Dead branch tips indicate severe stress:

Estimate % of branches with dieback:

• 0% (healthy)

• <10% (minimal)

• 10-25% (moderate)

• 25-50% (severe)

• >50% (critical)

Recording: Dieback: _____%

C. Bark Damage

Severe stress causes:

• Vertical cracks in bark

• Exposed wood

• Bark sloughing off

Recording: Present  /  Absent

D. Epicormic Sprouting

Stress response: Dormant buds on trunk sprout

Recording: Present  /  Absent

Step 3: Assess Soil Moisture

At each survey location:

1. Dig to 15 cm (6 inches)

2. Squeeze soil:

   • Dry: Powder, won't hold shape

   • Slightly moist: Forms weak ball

   • Moist: Forms ball, slightly damp

   • Wet: Water visible

3. Days since last rain: _____

Expected in Mohave County: Soil dry most of year!

Step 4: Calculate Stress Percentages

For each species:

• % showing severe dieback (>25%): _____%

• % leafless (if should be leafed): _____%

• % with bark damage: _____%

Compare species:

• Most stressed: _____

• Most resilient: _____

Step 5: Data Table

Observation B: Evidence of Extreme Weather

Recent Monsoon Flooding

Look for:

• Debris lines on shrubs/trees (high water mark)

• Sediment deposits around plant bases

• Scoured areas (soil/plants washed away)

• Vegetation knocked over

Measure: Height of debris line above ground: _____ cm

Extreme Heat Damage

Look for:

• Sunscald on south/west-facing bark (white patches)

• Leaf scorch (brown, crispy edges)

• Stem tip death on cacti

Winter Freeze Damage (Rare but Increasing)

Look for:

• Blackened cactus tissue

• Mushy, collapsed pads/stems

• Dead branch tips on cold-sensitive species

Observation C: Phenology Shifts

Phenology = Timing of life events

Document unseasonable activity:

For 20+ plants:

• Flowering in wrong season? (e.g., winter blooms when should be spring)

• Leafing out early? (January instead of March)

• Fruit ripening off-schedule?

Recording:

Part 4: Population Structure Analysis

Goal: Determine if populations are growing, stable, or declining based on age structure.

Arizona concern: Drought may prevent seedling establishment

Step 1: Select Study Species

Choose 3 species:

• 1 common desert tree/shrub (mesquite, palo verde)

• 1 rare or sensitive species (ironwood, desert willow)

• 1 invasive (if present)

Step 2: Sample Population

For each species:

• Locate 20-30 individuals

• Spread across habitat

• Include all sizes visible

Step 3: Classify by Size/Age Class

For each individual:

Seedlings:

• Height <0.5m

• Age: <2 years typically

• Most vulnerable to drought

Juveniles:

• Height 0.5-2m

• Age: 2-10 years

• Established but not reproductive

Adults:

• Height >2m

• OR reproductive (flowering/fruiting)

• Age: 10+ years

Recording: Size class: _____

Step 4: Record Measurements

For each:

• Height: _____ m

• Basal diameter: _____ cm

• Health (1-5): _____

Step 5: Create Size-Frequency Histograms

For each species:

Count:

• Seedlings: _____

• Juveniles: _____

• Adults: _____

Create bar graph:

• X-axis: Size class

• Y-axis: Number of individuals

Interpret shape:

• Pyramid (many seedlings → few adults) = Growing population

• Rectangle (similar across classes) = Stable population

• Inverted pyramid (few/no seedlings → many adults) = Declining population

Step 6: Analyze Population Status

For each species:

1. What is the population structure?

   • Growing / Stable / Declining

2. Is regeneration occurring?

   • Are seedlings present?

   • What's the seedling:adult ratio?

3. What might limit recruitment?

   • Browse pressure (deer eating seedlings?)

   • Lack of light (too shady?)

   • Competition (from other species?)

   • Drought (seedlings dying?)

   • Poor seed production?

4. What is the long-term prognosis?

   • Will this species persist here?

   • Is intervention needed?

Step 8: Data Tables

Size Class Data:

Population Summary:

Part 5: Microclimate Measurements

Microclimate = Local environmental conditions at small scales

Even within one habitat, microclimates vary:

• Sunny slopes vs. shady ravines

• Under trees vs. in gaps

• Wet seeps vs. dry ridges

These differences control where plants can grow!

Step 1: Select Measurement Locations

Choose 5+ locations in EACH habitat (15+ total across all 3 habitats):

Vary locations to capture diversity:

• Sun vs. shade

• Wet vs. dry

• Slope vs. flat

• Under canopy vs. in gap

• North-facing vs. south-facing slope

Step 2: Measure Environmental Variables

At each location:

A. Air Temperature

Measure at three heights (or use general temperatures from a weather app):

• Ground level (at soil surface)

• 1 meter height (typical herb height)

• 2 meter height (above herb layer)

Method:

• Wait 2 minutes for equilibration

• Record in °C

• Shade thermometer from direct sun!

B. Relative Humidity (Use a weather app)

C. Wind Speed

Estimate on scale:

• Calm: Leaves not moving

• Light: Leaves rustling slightly

• Moderate: Branches swaying

• Strong: Whole trees moving

D. Light Intensity

Use a light meter app:

• Measure in lux or foot-candles

• At plant height

E. Soil Temperature

• Insert thermometer to 5 cm (2 inch) depth

• Wait 2-3 minutes

• Record

F. Soil Moisture

Use squeeze test:

• Dry / Moist / Wet / Saturated

• (See Part 3C.3 for details)

Step 3: Create Microclimate Maps

For each habitat:

1. Sketch map showing locations measured

2. Label each point with key data:

   • Temperature

   • Light level

   • Moisture

3. Draw isolines (lines connecting similar values)

4. Shade areas to show patterns

   • Hotspots vs. cool spots

   • Wet vs. dry zones

Step 4: Compare Within vs. Between Habitats

Calculate variation:

Within-habitat variation:

• In forest, how much does temperature vary across locations?

• Range = Maximum - Minimum

• Standard deviation (if you know how)

Between-habitat variation:

• How different is forest vs. grassland vs. edge?

• Compare means

Which is greater?

• If within-habitat variation is HIGH → microhabitats important!

• If between-habitat variation is HIGH → habitats are distinct!

Step 5: Correlate with Vegetation

Look for patterns:

• Do certain species occur only in coolest spots?

• Do species richness correlate with microclimate?

• Are invasives associated with certain microclimates?

Step 6: Data Table