Effects of Fire and Herbivory on Tree Size Transitions in

Acacia drepanolobiumEric M LaMalfa Preliminary data

11/6/2014

Top-down controls on tree cover• Disturbance: Fire AND/OR Herbivory

Are tree size transitions controlled by herbivory x fire?

Tree cover in savannas

Fire effects: •Topkill •Influences plant-plant intxns

- reduced competition for resources

Grazing effects:•Reduces herbaceous biomass•Influences plant-plant intxns

-Reduced competition for1) unpalatable herbaceous plants2) woody plants

Browsing effects•Reduced woody biomass•Reduced seedling establishment•Influences plant-plant intxns

- Reduced competition for herbaceous plants

Direct fire x herbivory intxns• Herbivore preference for burned areas

Increased crude protein content Decreased lignin

Direct fire x herbivory intxns• Grazing reduces fine fuel load/fire continuity

Increased tree survival

• Browsing reduces heavy fuel and fire tempsIncreased tree survival

Indirect fire x herbivory intxnsFire kills tree-obligate ants

Reduced tree fitness

Increased browse pressure

Decreased ant-defense

Laikipia plateau, Kenya • 1800m • Semi-arid 550-600 mm

Cattle Wildife

Mega-herbivores

Acacia drepanolobium• 95% of tree cover • Nitrogen fixing bacteria

nodules• Highly quality (browse and

herbaceous) • Coppices (re-sprouts after fire

and browse damage)

Trees ProvideNest & FoodCrematogaster sjostedt C. mimosaeC. nigricepsTetraponera penzigi

Domatia

Extra Floral Nectary

Ants ProvidePlant Defense

• High defense against herbivores and insects

• High nectar use• High resistance to fire

• Ants track tree resources during sapling colonization• Trees > 1m (grass height) are rarely observed without ants

Research Questions 1) How do fire and herbivory affect small tree (<1m) size transitions?

2) How do fire and herbivory affect ant colonization of small trees?

Experimental Design – Herbivory

• Kenya Long Term Exclosure Experiment (KLEE) • Fences exclude herbivores (since 1995) • Randomized complete block design

W & WC

0 & C

MW & MWC

0= no herbivores

W= wildlife

C= cattle

M= mega-herbivores

MWC

MWC

MWC

MW

MW

MW

W

W

W

C

C

C

0

0

0

WC

WC

WC

Experimental Design – Fire

• 18 burns, February 2013 • 12 treatments, 3 replicates• 4304 tagged trees

• ant sp., tree size, herbivory• % top-kill

Kimuyu D.M., Sensenig R.L., Riginos C., Veblen K.E., and T.P. Young (2014). “Native and domestic browsers and grazers reduce fuels, fire temperatures, and acacia ant mortality in an African savanna.” Ecological Applications 24(4): 741–749

Post-Fire Trees <1m1

met

er

• 4304 tagged trees • ant sp., tree size, herbivory• % top-kill

Control (no burn)

1 3 5

5 m

30 m

Post fireN = 36 plots27 to 190 trees/ plot1659 trees <1m height

height Main stem length Basal stem length Branch stem length Total stem length Green stem length Main stem diameter Domatia diameterGreen stem diameter Domatia countBasal stem count Green stem count Nectary count (5cm)Ant species

Data

5cm

1m

PRELIMINARY RESULTSHow do fire and herbivory affect small tree (<1m) size transitions?

Un-burned Burned

N = 666 N = 993

5cm

Tree size (<1m)

39 25 233 254

t = -1.6733, df = 1292.361, p-value = 0.09451

t = 16.1482, df = 1013.013, p-value < 2.2e-16

Tree Height Total Stem Length

Ant mutualism rewards (domatia)

t = 3.2196, df = 1000.367, p-value = 0.001325

9.95 7.63

t = -1.7532, df = 1255.261, p-value = 0.07981

9.9 10.5

Ant mutualism rewards (nectar)

t = -10.0173, df = 1598.088, p-value < 2.2e-16

12 31

t = -7.5473, df = 1654.985, p-value = 7.295e-14

0.61 1.24

t = -9.2696, df = 1645.348, p-value < 2.2e-16

1.73 3.33

t = -13.8396, df = 1618.421, p-value < 2.2e-16

0.74 1.33

Preliminary resultsHow do fire and herbivory affect ant colonization of small trees?

Extra Floral Nectary

Un-burned Burned

Green Stem Length

Un-burned Burned

Green Stem Diameter

Nectary Count

Un-burned Burned

Conclusions

1. Mean tree size in the <1m size class is no different 1.5 years following fire.

2. Ant mutualism traits associated with nectar are higher in burned trees.

3. After fire, small trees occupied by C. nigriceps (BBR) and C. mimosae (RRB) & have higher nectar rewards.

Do large top-killed trees produce better nectar rewards?

1 m

eter

Next Steps

Next Steps

Does fire temperature affect tree response?

Low heat > High heatDoes herbivory affect tree response?

Low browse > High browse

ant damage neighbor_tree_1 neighbor_tree_2 neighbor_tree_3 neighbor_tree_4 neighbor_tree_5 neighbor_tree_6 neighbor_ant

Neighbor Data

2m

Next Steps

Does neighbor tree colony affect post fire ant colonization?

Thanks

S.J. and Jessie E. Quinney Foundation

Kari Veblen PhD - AdvisorMellisa Harvey LaMalfa – MutualismVeblen Lab – Rebecca, Kyle, Maike

Correlation matrix

ms_

leng

th

tot_

leng

th

gree

n_le

ngth

mai

n_di

am

dom

atia_

diam

gree

n_di

amdo

mati

a_co

unt

basa

l_co

unt

gree

n_co

unt

nect

ary_

coun

tda

mag

e

height 0.78 0.54 0.13 0.72 0.50 0.07 0.59 -0.14 0.10 0.14 -0.03ms_length 0.56 0.14 0.72 0.55 0.11 0.59 -0.15 0.11 0.16 -0.07tot_length 0.48 0.62 0.54 0.37 0.74 0.36 0.50 0.34 0.08green_length 0.22 0.30 0.50 0.35 0.34 0.80 0.47 0.13main_diam 0.56 0.21 0.67 -0.07 0.25 0.24 0.08domatia_diam 0.36 0.57 0.14 0.28 0.32 0.15green_diam 0.29 0.25 0.52 0.50 0.20domatia_count 0.08 0.33 0.33 0.03basal_count 0.37 0.18 0.19green_count 0.44 0.15nectary_count 0.08