Scott Johnson
The missing half Can we predict how climate change will affect belowground insect-plant interactions?
C l i m a t e c h a n g e
Headline changes are CO2 and temperature
Increasingly known how insect herbivores respond
E l e v a t e d C O 2 a n d p l a n t - i n s e c t r e s p o n s e s
• Most widely reported is trend for higher C:N in tissues
• Dilution effects associated with increased growth and reallocation of N
• Tissues 16% lower in N*
*Stiling & Cornelissen 2007, GCB
• Most insect herbivores are already N limited and so this causes a problem
H o w d o h e r b i v o r e s c o p e ?
• Compensatory feeding, whereby insects eat more to meet their nutritional needs
• Relative consumption rates increase by 17%*
*Stiling & Cornelissen 2007, GCB
• Gut capacity / digestive time
• Ingest more toxins
• Wear out mouthparts
C l i m a t e c h a n g e a n d i n s e c t -p l a n t i n t e r a c t i o n s
• Effects of root herbivores largely unknown?
Collective mass of scarab larvae frequently exceeds that of sheepBritton (1978) Australian Journal of Zoology
Australian pastures comprise C3 and C4 grasses
A u s t r a l i a n p a s t u r e – b i g g e s t h e r b i v o r e s ?
C 3 a n d C 4 g r a s s e s
• Possess different physical and chemical traits that may shape their food quality for insect herbivores
• C4 avoidance hypothesis developed by Caswell (1973). C4 plants represent poorer hosts for herbivores
Plant quality C4 plants C3 plants
Nutrients Low High
Structural carbohydrates
High Low
C:N ratio / quality High / poor quality Low / high quality
C O 2 a n d C 3 - C 4 g r a s s e s
• Rubisco – initial carboxylating enzyme to facilitate assimilation of CO2 into carbohydrates
• Under current CO2 operates below its maximum capacity in C3 plants
• Under elevated CO2 greater responses seen in C3 plants, especially for increased C:N
Prediction 1 C3 grasses
inherently better hosts for root
feeding insects
Prediction 2 C3 grasses show a
greater deterioration in
host plant quality at elevated CO2
Prediction 3 Compensatory
feeding would be triggered on a C3 grass but not a C4
grass
400 600
C:N
0
20
40
60
80
100
400 6000
20
40
60
80
100
CO2 concentration (mol mol-1)
R o o t C : N r a t i o
• C4 grass has higher C:N ratio than C3 grass
• C3 grass shows biggest change (increase) under elevated CO2
C4 grass C3 grass
R o o t c o n s u m p t i o nC
onsu
mpt
ion
(mg
wee
k-1)
0
20
40
60
80
100
120
CO2: F1,4 = 1.75, P = 0.256
Grass species: F1,52 = 14.36, P < 0.001
CO2 x grass species: F1,52 = 4.76, P = 0.034
aa
a
b
400 mol mol-1 600 mol mol-1CO2 conditions
C4 grass C3 grass
Root consumption rose markedly on C3 grasses grown under elevated CO2
P P
$SHUWXUHP P
/ DUYD
7HVWJUDVV
L a r v a l p e r f o r m a n c e
La
rva
l ma
ss (
mg
)
0
100
200
300
400
CO2: F1,4 = 0.13, P = 0.342
Grass species: F1,108 = 5.42, P = 0.024
CO2 x grass species: F1,108 = 8.30, P = 0.006
a a
b
a
400 mol mol-1 600 mol mol-1CO2 conditions
Elevated CO2 did not affect mass on C4 grass but reduced it on C3 grass
Despite compensatory feeding
C4 grass C3 grass
C o u l d r o o t h e r b i v o r e s m o d i f y p l a n t r e s p o n s e s t o e l e v a t e d C O 2 ?
• Common for plants to increase photosynthesis and biomass in response to elevated CO2
• Dependent on improved root function, especially water uptake
• Roots change to support this*:– Root : shoot mass increases by 11%– More lateral roots, highly branched
• What happens when those roots damaged? Can plants maintain physiological responses?
*Rogers et al. 1996; Pritchard & Rogers, 2000
400 600
Pla
nt d
ry m
ass
(g)
-1
0
1
2
3
CO2 concentration
(mol mol-1)
1
Shoots
Roots
*** CO2
*** RD
** CO2*** RD
ab
ab
a
a
c
b
c bc
E u c a l y p t s a n d C O 2
• All eucalypt species tested to date show increased photosynthesis / growth
Control
Root herbivory
• We applied 14 d of root herbivory
R e v e r s a l o f f o r t u n e
Sp
eci
fic le
af
ma
ss(g
m-2)
20
30
40
50
60
* CO2 * RD
aa
b
a
• All expected responses of the eucalypts to elevated CO2 negated / reversed
• Root herbivory reduced photosynthetic rates so the plant could not benefit from elevated CO2
Increased growthIncreased leaf C:NIncreased SLM
400 600
I n c l u s i o n o f i n s e c t h e r b i v o r e s i n c l i m a t e c h a n g e p r e d i c t i o n s
S u m m a r y
• Root herbivores neglected in climate change research – they respond to and mediate the effects of elevated CO2
• C3 grasses superior hosts for root-feeding scarabs but also showed greatest deterioration in root quality under elevated CO2
• Root herbivores displayed compensatory feeding on C3 grasses – this failed to redress lower host quality
• Root herbivores negate / reverse the effects of elevated CO2 on plants because they hinder root function and stop plants exploiting elevated C source
A c k n o w l e d g e m e n t s
Goran Lopaticki, Adam
Frew, Sue Hartley, Kaushal
Tewari
Markus Riegler