Effects of tree density on wildlife, grasses, and forage production
Corinna Riginos
Graduate Group in Ecology, University of California, Davis, CA, USA and Mpala Research Centre, Nanyuki, Kenya
Introduction
Experimental Test
Savannas can range from open plains with
scattered trees to dense woodlands. In African
savannas, changes in tree density have been linked
with range management practices. For example, wild
herbivores, especially elephants, often cause a
decrease in tree density, whereas cattle grazing
often causes an increase in tree density.
Since trees and grasses compete for the same
water and nutrients, an increase in tree density may
mean less grass for cattle and wildlife.
At the same time, wild herbivores may avoid
areas with a high number of trees, where predators
are hard to see. This could affect the interactions
between trees and grasses — either preventing or
encouraging further changes in tree density.
My research addresses these issues in an
attempt to disentangle the complex links among
trees, grasses, wildlife, and cattle grazing.
In collaboration with Jacob Goheen and Kari
Veblen, I have set up a large-scale experiment, in
which 60 x 60 m patches of whistling thorn savanna
were either thinned, cleared of all trees, or left
untouched. I will be monitoring these plots for:
•wild and domestic herbivore use
•tree growth, reproduction, and browse damage
•grass productivity and grazing damage
•grass layer species composition
Expected Outcomes
I hope to provide mechanistic answers to the
following questions:
•Do wild herbivores prefer more open areas?
•Is grass productivity lower where there are
more trees?
•Is tree growth and reproduction lower where
there are more trees?
•Does tree density affect grass species
composition?
 Wild herbivores preferred areas with fewer
trees. This was true of almost all of the major
herbivore species, including zebras, Grant’s
gazelles, hartebeests, giraffes, and oryx.
 Zebras avoided areas that had been grazed
more by cattle.
 Out of five dominant grass species, one
(Pennisetum stramineum) was more abundant
where tree density was higher.
 Two other species (Pennisetum mezianum and
Themeda triandra – red oat) were less abundant
where tree density was higher.
Wild herbivore dung piles
From a descriptive survey in the black cotton whistling thorn (Acacia drepanolobium) savanna of
MRC and Jessel Ranch, I found that:
250
200
150
100
50
2
R = 0.6157
0
0
200
400
600
Number of trees
Figure 1. Wild herbivores prefer areas with fewer trees
Themeda triandra biomass
(as a proportion of all grasses)
Preliminary Work
Conclusions
0.5
0.4
0.3
0.2
0.1
2
R = 0.1763
0
0
800
200
400
600
800
Number of trees
Figure 2. Red oat grass is more abundant where there are fewer trees
Conclusions
Acknowledgements
My thanks to: Truman Young, for his advice and support; Kari Veblen and Jake Goheen, for their collaboration in setting up experimental plots; Nick Georgiadis
and Peter Jessel for their permission to work on Mpala Research Centre and Jessel properties; Jackson Ekadeli, Frederick Erii, Patrick Etelej, John Lochukuya,
James Ekiru, Abdikadir Ali Hassan, and Simon Lima for assistance in the field; Dan Kelly for satellite image analysis and GIS work; and to Ken Wreford-Smith,
Kerry Outram and Joe Petters for logistical support. This research is supported by NSF LTREB BSR-03-16402 to T.P. Young, and by an NSF Graduate Research
Fellowship, a Jastro-Shields fellowship, a Ben Madson fellowship, and an NSF Doctoral Dissertation Improvement Grant to C. Riginos.
My preliminary results show that tree density
can have important consequences for both wildlife
and grasses. In the next phase, I will uncover the
mechanisms underlying these patterns and more
fully illuminate the interactions among trees,
grasses, wildlife, and forage productivity.
The results of my research are important for
understanding the consequences of the increases
in tree abundance that continue to occur in
savannas across Africa. Cattle grazing is generally
thought to cause these increases. In order to make
effective, long-term plans for sustainable land
management, we must understand both what
causes tree density to increase, and what the
consequences of these increases are for wildlife
and cattle production.