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