Critical loads of anthropogenic nitrogen deposition that promote vegetation-type conversion to exotic grassland in coastal

sage scrub and desert

Edith B. Allen, Leela E. Rao,

Robert J. Steers, and Abby G. Sirulnik,

Department of Botany and Plant Sciences and Center for Conservation Biology

Tonnesen et al UCR

Riverside-Perris Plain,Coastal Sage Scrub

Mojave/Colorado Desert

Objectives1. Test responses of native and invasive plants to N fertilization in coastal sage scrub and desert.

2. Compare levels of N that promote increases in invasive grasses and vegetation type conversion.

Coastal sage scrub vegetation receives up to 30 kg N ha-1 yr-1 mainly as dry deposition; frequent fire, loss of native diversity, exotic annual grass invasion

Box Springs Mt. with high N deposition

dominated by exotic annual grasses (Bromus spp.)

Lopez Canyon with low N deposition

dominated by native forbs and shrubs

Soil N gradient from north to south in the Riverside-Perris Plain

0

10

20

30

40

50

60

70

JH MR BG PE MO LM HE LS SM

Site (N to S)

NH4

NO3

LSDNO3

LSDNH4

Coastal sage scrub (CSS) vegetation was fertilized yearly with 60 kg/ha/yr of N following the 1993 wildfire near Lake Skinner, an area of low N deposition

Recovery three years after fire

NATIVE FORBS

0

10

20

30

40

50

60

70

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

% cover

+N

-N

*

*

*

*

% cover of native forbs (69 spp.) and exotic forbs (Erodium spp.) following the 1993 fire and N fertilization.

EXOTIC FORBS

0

5

10

15

20

25

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

% cover

+N

-N

*

* *

Biomass of grasses following the 1993 fire and N fertilization (60 kg/ha/yr). The threshold for fire is 0.5-1.0 t/ha of fine grass fuel (red line).

BIOMASS OF EXOTIC GRASSES

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

BIOMASS (t/ha)

+N -N

*

*

*

*

*

*

*

Soil N concentrations1994-2006

Total extractable soil N 1994-2006

0

10

20

30

40

50

60

70

1994 1995 1996 1998 2000 2001 2002 2003 2006

YEAR

Extractable N (mg/kg)

Series1 Series2+N -N

Air pollutionoutside JoshuaTree National Park (12/2004)12 kg N ha-1yr-1

Schismus barbatainvasion outside JTNP(5/2005)

N deposition (kg ha-1yr-1) measured at experimental N fertilization sites in Joshua Tree NP

12.4 6.2

5.2

3.4

Covington Flat

Wide Canyon

Pine City

Pinto Basin

N fertilizer study sites

Covington FlatsPinus-Juniperus

Wide CanyonLarrea tridentata

Pinto Basin

Pine City

Higher N deposition Lower N deposition

Extractable soil N concentration following N fertilization in July of 2004 (average precipitation) and 2005 (wet year)

Pine City

A

ABC

ABC

ABAB

BCBC

C

0

5

10

15

20

25

30

35

40

45

50

0 5 15 30N-Fertilizer Treatment

Inorganic N (mg/kg)

Covington Flats

A

BCB

BCC C

BC

B

0

5

10

15

20

25

30

35

40

45

50

0 5 15 30

Inorganic N (mg/kg)

2004

2005

Wide Canyon

A

BC

B

BC

BC

CCC

0

5

10

15

20

25

30

35

40

45

50

0 5 15 30

Inorganic N (mg/kg)

2004

2005

Pinto Basin

A

AB

BB

AB

BBB

0

5

10

15

20

25

30

35

40

45

50

0 5 15 30N-Fertilizer Level

Inorganic N (mg/kg)

Exotic grass biomass after two years of N fertilization, 2004

Covington Flats

A

A

0

10

20

30

40

50

N fertilization (kg/ha)

Dry

Wei

gh

t (g

/m2)

A

0 5 30

Pine City

A

B

0

5

10

15

20

25

N fertilization (kg/ha)

A

0 5 30

Wide Canyon

A

B

0

2

4

6

8

10

12

N fertilization (kg/ha)

Dry

Wei

gh

t (g

/m2) AB

0 5 30

Pinto Basin

A

B

0

10

20

30

40

50

60

70

80

N fertilization (kg/ha)

AB

0 5 30

Nitrogen-fertilized plot in Pinto Basin

% cover of exotic and native species in 2005 with N

fertilization

Conclusions1. Exotic annual grasses responded to N fertilizer in

both vegetation types within 2 years. 2. In the desert, native forb cover declined

significantly in an average year with 30 kg/ha N, and in a wet year with only 5 kg/ha added N.

3. CSS is more resilient to N deposition; native forb cover did not decline significantly until the 11th year of fertilization with 60 kg/ha N.

4. Elevated exotic grass biomass above the threshold value of 0.5-1.0 T/ha is most likely responsible for the high incidence of fire in areas affected by N deposition, and fire causes rapid

vegetation type conversion.

Christopher True, Sara Jo Dickens, Heather Schneider, Tracy Tennant, Sheila Kee, Mike Allen, Richard Minnich (UCR)Andrzej Bytnerowicz, Mark Fenn (US Forest Service)Jane Rodgers and many others (NPS)Research funded by National Park Service, National Science Foundation

Acknowledgements