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