critical loads of anthropogenic nitrogen deposition that promote vegetation-type conversion to...
TRANSCRIPT
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