Browser effects on woody plant traits and growth in an East African savanna

Wigley B, Sankaran M, Augustine D, Ratnam J & Kimuyu D

• Useful tool to empirically understand the effects of mammalian herbivores on vegetation

• E.g. woody densities and size class distributions & spp. comp in KNP (Wigley et al. 2014)

• Defence traits & growth in Acacias in HiP (Wigley et al. 2015)

• Limits to recruitment and establishment (e.g. Staver et al. 2012 – Impala; Augustine & McNaughton 2004 – Dik dik).

• Many, many other examples from diverse ecosystems

Herbivore exclosures

• 3 sets of herbivore exclosures set up at Mpala Research Centre, Laikipia, Kenya in the late 1990’s by D. Augustine

• Great opportunity to test what effect 16+ years of herbivore removal has had on savanna woody browse quality, defence traits, plant growth and soil nutrients

MRC exclosures

Questions

1. Has the long-term removal of mammal browsers lead to changes in leaf quality/palatability and plant chemical defences in savanna woody species?

2. Has the long-term removal of mammal herbivores affected soil nutrients and carbon?

3. How are woody plant growth rates affected by the removal of mammal herbivores?

Expectations • Expect higher quality browse with herbivory (more

efficient/faster nutrient cycling, (e.g. Du Toit et al. 1990)

• Expect lower concentrations of chemical defences with herbivory. I.e. carbohydrate demands incurred by rapid regrowth cause limitation of carbon-based secondary metabolite synthesis (e.g. Du Toit et al. 1990; Wigley et al. 2015)

• Expect the removal of herbivores to result in higher concentrations of total soil N, P and carbon OR opposite, i.e. grazing/browsing lawn effect?

• Expect faster growth (height and BA) without herbivores

Study sites: MRC, Laikipia, Kenya, semi-arid savanna (~500 MAR)

Buffalo Baboon cliffs Kopi

Rich faunal diversity at high densities

Trait sampling • Fine-leaved: Acacia mellifera, Acacia etbaica,

Acacia brevispica

• Broad-leaved: Grewia tenax, Balanites pedicellaris, Boscia angustifolia

• Sampled five individuals each from inside and outside of exclosures

• Specific leaf area (SLA)

• Leaf size (ALA)

• Leaf dry matter content (LDMC)

• Leaf nutrients (N, P, C)

• Total phenolics (TP)

• Condensed tannins (CT)

Soil sampling

• 10 reps at three depths (0-5, 10-15, 25-35 cm)

• Inside and outside of exclosures

Growth rate measurements

• Initially surveyed in 1999 by D Augustine

• Resurveyed in 2009 by Augustine, Sankaran and Ratnam

• Resurveyed again in 2016

• Calculated changes in height and basal area for individuals that survived from 1999 to 2009

Leaf N (%) No effect evident

Leaf P Higher inside for some species

SLA Higher (i.e. thinner) outside in fine-leaved Higher inside In broad-leaved

Total polyphenols

Trend of higher inside exclosure for Acacias

Condensed tannins Higher inside exclosure for Acacia brevispica

Low tannin concentrations in broadleaved spp.

Growth rates: Δ plant height

Growth rates: Δ basal area

Soil N All sites 0-5 *** All sites 10-15 ** All site 25-30 NS

Soil N Higher inside exclosures

INSIDE OUTSIDE

Soil C All sites 0-5 *** All sites 10-15 *** All site 25-30 ***

Soil C Higher inside exclosures

INSIDE OUTSIDE

Soil P All sites 0-5 *

All sites 10-15 ***

All site 25-30 NS

Soil P Higher inside exclosures

INSIDE OUTSIDE

Were expectations met? 1) Expect higher quality browse with herbivory

• No difference in leaf N

• P higher inside or no effect

BUT

• SLA higher outside for fine-leaved and lower outside for broad-leaved

• So fine-leaved better quality outside and broad leaved better quality inside

• Different growth and defence strategies

2) Expect lower concentrations of chemical defences with herbivory • TP and CT higher inside in some Acacia spp. no

effect in broad-leaved species

• Higher overall in fine-leaved Acacia species compared to broad-leaved species

• Despite higher levels in Acacia doesn’t seem to deter mammals

• Mostly likely defence against insects

3) Higher growth rates inside?

• Significantly higher increase in plant height inside exclosures

• Significantly higher increase in basal area inside exclosures, except for Acacia etbaica higher outside

• Total soil N, P and C significantly higher inside exclosures (More trees and grass = more leaf litter, more decomposition, more shade, higher soil moisture)

Were expectations met? 4) Soil N, P and C higher in exclosure

INSIDE OUTSIDE

• Results suggest that although soil N is higher in exclosures is not available (lower mineralization rates inside see Augustine & McNaughton 2006)

• High herbivory = lower plant cover outside result in loss of organic matter and nutrients (erosion)

• More available forms of N from animal dung and urine (NH4 and NO3) being lost from outside explaining why no increase in leaf N and P outside

• Also looking at grass communities and traits

Acknowledgements

• MRC and staff

• KWS

• NCBS