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Woody plant encroachment into grasslands of the Southern Great Plains

Sam Fuhlendorf, Oklahoma State University, Natural Resource Ecology & Management


Ecological Phantasmagoria: How do grasslands work?

Sam Fuhlendorf Regents Professor and Sarkeys Distinguished Professor Natural Resource Ecology and Management Oklahoma State University


Ecological Phantasmagoria: How do grasslands work?

Sam Fuhlendorf Regents Professor and Sarkeys Distinguished Professor Natural Resource Ecology and Management Oklahoma State University


How do grasslands, savannas and shrublands work?

Fire

1491

Contemporary

Climate

Climate

Wild Herbivores

Complex Landscape Pattern

Domestic Fossil Herbivores Fuels Simplified Agronomic Landscape

Humans and other predators

Humans


Juniperus spp. in the Great Plains— Conservation Priority

Engle et al. 2008


Historical Stocking Rate Decline Sonora, Texas <10 % cover Of Woody Plants

>50 % cover Of Woody Plants

160

Animal Units/Section

140 120 100 80 60 40 20 0 1900

1925

1950

Year

1975

1992


Economic Returns Stillwater, OK. $/acre

Pasture 1

Pasture 2

Prescribed Fire Dates in Pasture 1 No fire in Pasture 2


Lesser Prairie-Chicken Petitioned October, 1995


Increase in trees is 15 times greater on landscapes with declining populations than on landscapes with stable populations- Woodward et al. 1999, Fuhlendorf et al. 2001)


Abundance of Grassland Birds


Emerging biogeochemical perspectives suggest that transitions between grass and woody dominated ecosystems may affect: •  •  •  • 

Climate (Schlesinger et al. 1990) Regional precipitation (Hoffman et al. 2000) Soil respiration (McCulley et al. 2004) Non-methane hydrocarbon (Guenther et al. 1999) •  Nitrous oxide emissions (Martin et al. 2003) •  Ecosystem carbon pools (Wessman et al. 2004)


Eastern Redcedar Pollen Grains/cubic meter of air

Human Health allergies cost $2 Billion a year in US 7000 6000

Peak Concentrations Season Total

5000 4000 3000 2000 1000

1988

1990

1992 Year

1994

1996

Van De Water and Levetin 2001


Objectives •  Review research on ecology/biology of juniper (cedar) that is important to develop a model •  Present a model developed to address the following questions: –  What is the role of fire and grazing in the expansion of fire-sensitive woody plants? –  How does fire, grazing, climate and site productivity interact? –  What are the thresholds associated with vegetation change? –  What fire regime is required to maintain grasslands?


Background on sites •  Low Productivity Site (Sonora, TX) – 50 cm (20 in) PPT – Shallow Soils – Limestone Derived – Southwestern edge of Edward s Plateau – Southern edge of Great Plains – Savanna of Oak/Juniper/Mixed Prairie – Focus on the expansion of Juniperus ashei


Background on sites •  High Productivity Site (Stillwater OK) – 84 cm (33 in) PPT – Shallow Soils – Along edge of Crosstimbers/Tallgrass Prairie – Savanna of Oak/Juniper/Tallgrass Prairie – Plant communities are limited by soil texture- sandstone derived soils= Oak – Focus on the expansion of Juniperus virginiana into grassland plant communities


What do we have from field data? •  Seed production per tree – Chavez-Ramirez 1992, Smeins and Fuhlendorf 1997

•  Seed dispersal by birds – Chavez-Ramirez 1992 •  Seedling germination rates- Smeins and Fuhlendorf 1997 •  Seedling survival response to grazing- Smeins and Fuhlendorf 1997

•  Growth Rates of Juniper - Blomquist 1990, Fuhlendorf 1992, Engle & Kulbeth 1992, Smeins & Fuhlendorf 1997

•  Overstory / understory relationships- Engle et al. 1987, Fuhlendorf et al. 1997

•  Grazing influence on woody plant cover – Fuhlendorf and Smeins 1997, Allred et al. 2012

•  Influence of grazing on herbaceous community – Fuhlendorf and Smeins 1997, Fuhlendorf et al. 2001

•  Influence of weather patterns – Fuhlendorf et al. 2001 •  Effects of fire on juniper mortality – Wright and Bailey 1992


Patch Dynamics (Fuhlendorf et al. 2008)

Mid-Grass

Weather Patterns

Herbaceous Biomass Short-Grass

Grazing

Landscape Dynamics (Fuhlendorf et al. 1996) Seed Dispersal

Fire Intensity Juniper Size and Density Fire Frequency

Seed Production Mortality


100

60

90 80

50

70 40

60 50

30

40 20

30 20

Potential Herb. Biomass

10

Remaining Herb. Biomass Tree Density

0 0

20

40

60

80

Year

100

120

140

10 0

Large Trees/ac (>6m dia.)

Herbaceous Biomass (percent of maximum)

Ungrazed and No Fire


100

60

90 80

50

70 40

60 50

30

40 30 20

Potential Herbaceous Biomass

20

Remaining Herbaceous Biomass Tree Density

10

10 0

0 0

20

40

60

80

Year

100

120

140

Large Trees/ac (>6m dia.)

Herbaceous Biomass (percent of maximum)

Ungrazed with a 10 year fire return interval (FRI)


100

60

90 80

Potential Herbaceous Biomass

50

70

Remaining Herbaceous Biomass Tree Density

40

60 50

30

40 20

30 20

10

10 0

0 0

20

40

60

80

Year

100

120

140

Large Trees/ac (>6m dia.)

Herbaceous Biomass (percent of maximum)

Moderate grazing with a 10 year FRI


So, when productivity is low, even moderate grazing can reduce herbaceous biomass enough to limit the success of prescribed fires in the maintenance of grassland ecosystems. What about cross-site comparisons?


Potential Herbaceous Production (%)

No Grazing 100 90 80 70 60 50 40

Low productivity High productivity

30

No Fire

15

10

7

5

Fire Return-Interval (yrs)

2


Potential Herbaceous Production (%)

Moderate Grazing 100 90 80 70 60 50 40

Low productivity High productivity

30

No Fire

15

10

7

5

Fire Return Interval (yrs)

2


Potential Herbaceous Production (%)

Heavy Grazing 100 90

Low productivity High productivity

80 70 60 50 40 30

No Fire

15

10

7

5

Fire Return Interval (yrs)

2


Fire Return Interval (years)

Fire frequency required to maintain grassland production for 150 years 20 15 10 5 0 Ungrazed

Moderate Moderate Heavy Heavy with 1-year rest with no rest with 1-year rest with no rest

Grazing Treatment


Model assumptions: Grassland Ecosystems


Model assumptions: Grazing is with Cattle and Sheepâ&#x20AC;&#x201C; not browsers- Fuhlendorf 1992 Grazing Treatments Age Class (yrs)

Heavy Browse

Light Browse

Canopy

10-20

0.66 a

1.18 b

Diameter (m)

21-30

1.43 a

2.43 b

31-40

1.41 a

2.69 a

>40

5.56 a

5.41 a

10-20

0.82 a

1.52 b

21-30

1.48 a

2.35 b

31-40

1.51 a

2.67 a

>40

4.46 a

4.37 a

Height (m)


Model assumptions: Homogenous Landscape Patterns suggest Forage vs. Fuel conflict

Fuhlendorf & Engle 2001, 2004, Fuhlendorf et al. 2009, Allred et al. 2011, etc


Breeding(and(Winter(bird(communi2es( respond(to(heterogeneity((

Winter(bird(community(

Breeding(bird(community(

Fuhlendorf et al. 2006, Hovick et al. 2014

33(


Model assumptions: All fires are dormant seasontraditional prescribed burns


Positive Feedback Mechanism Driving Grassland to Woodland Transition

Twidwell et al. 2013, adapted from Langevelde et al. 2003


Twidwell et al 2013


Restoration Fire: helicopter ignition with helitorch & reduced fuel moisture


Summary •  Ashe Juniper on Low productivity site (Sonora) –  Without fire, JUAS gains dominance in about 60 yr –  Without grazing = 15 year fire return interval –  Moderate grazing = 5 year fire return interval –  Heavy grazing = sell the farm? or extreme fire

•  Eastern Redcedar on High Productivity sites (Stillwater) –  Without fire, JUVI gains dominance in about 45 yr –  Without grazing = 10 year fire return interval –  Moderate grazing = 10 year fire return interval –  Heavy grazing = 2-3 year fire return interval


Conclusions •  Fire is the primary driver •  Grazing has little or no direct effect •  Indirect effects of grazing are dependent on site productivity •  Thresholds everywhere •  Weather variation is most important near thresholds •  Alternative= Extreme fires, goats, heterogeneity


Extensive body of research but still important unanswered questions. •  Current knowledge based on short term biology, ecology and management studies •  Unanswered questions are driven by: – long time frame – confounding factors – anecdotal accounts

•  Solutions- systems approach


Model assumptions: Initial Conditions

60

100

X X

50 40

50

30

X

20 10

X

0 Year 10

Year 20

Year 30

Year 40

Year of first fire

0

Biomass Remaining (%; X )

)

70

Large Trees/ac (> 6m dia.;

Moderate grazed with a 10 year FRI and 1-year rest prior to fires- After 150 years

Fuhlendorf 2014 state of the prairie ls  
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