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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
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)
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
Condensed tannins Higher inside exclosure for Acacia brevispica
Low tannin concentrations in broadleaved spp.
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