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The functional importance of forbs in grassland ecosystems

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Abstract

Grasslands are much more than just grass. Forbs (i.e., the non-graminoid herbaceous component) represent the largest proportion of total species- and functional richness in grassland ecosystems, which secure important ecosystem functions. Here, we present some highlights of only some of the important functions provided by this hyper-diverse plant life form in Africa. Online: https://uknowledge.uky.edu/igc/24/1-2/35/
The functional importance of forbs in
grassland ecosystems
Forbs as grazing indicators
Intense grazing reduces mesic grassland diversity (Fig. 1) by depleting forbs,
especially erect species with their growing points exposed to defoliation and
mechanical damage by trampling. Forbs that consistently decline (i.e., Decreasers)
or increase (i.e., Increasers) with increasing grazing intensity are useful indicators
for calculating a Forb Condition Index for grassland.
Mistbelt grassland: r = -0.806, p < .0001
Sandstone grassland: r = -0.877, p < .0001
0
10
20
30
40
50
60
70
80
90
Native forb species richness (# per plot)
GRAZING INTENSITY →
Fig. 1. Grazing reduces forb species richness in two mesic grasslands in South Africa.
Introduction
Grasslands are much more than just grass. Forbs (i.e., the non-graminoid herbaceous component) represent the largest proportion of total species- and functional richness in
grassland ecosystems, which secure important ecosystem functions. Here, we present some highlights of only some of the important functions provided by this hyper-diverse
plant life form in Africa.
Forbs as nutritious forage
Forbs are important nutritive dietary components of several indigenous African
browsers and mixed-feeders, as well as nutritious food items for livestock. Forb
species, specifically members of the Fabaceae and Convolvulaceae contain higher
levels of phosphorus and crude protein, but lower fibre content than shrubs or
grasses.
Forbs as food and medicinal plants
Some edible grassland forbs also have medicinal properties (Table 2). These are
prone to extinction through ploughing and extensive harvesting of underground
parts from mature and reproductive plants. Such forbs are rich in carbohydrates,
macro- and micronutrients, proteins and vitamins that are essential for human
health.
Species name
Family
Growth habit
Underground
part
Edible plant
part
Part(s) used
for medicine
Albertisia
delagoensis
Menispermaceae
Climber, semi
-
woody
Rhizome
Leaves
Leaves,
rhizome
Ipomoea
plebeia
Convolvulaceae
Climber,
herbaceous
Tuber
Leaves
Ipomoea
wightii
Convolvulaceae
Climber,
herbaceous
Leaves
Limeum
sulcatum
Limeaceae
Perennial, forb
Leaves
Pyrenacantha
grandiflora
Icacinaceae
Climber, semi
-
woody
Rhizome
Leaves
Pyrenacantha kaurabassana
Icacinaceae
Climber, semi
-
woody
Tuber
Leaves
Bulb
Pyrenacantha
scandens
Icacinaceae
Climber, semi
-
woody
Rhizome
Leaves
Riocreuxia
torulosa
Apocynaceae
Climber, semi
-
woody
Rhizome
Leaves
Sesamum
alatum
Pedaliaceae
Annual, forb
Shoot tips
Leaves, seeds
Table 2. Examples of indigenous leafy vegetables with medicinal properties.
Table 1. Hierarchical linear modeling (HLM) comparison of arthropod diversity between sites
with or without indigenous forbs. Different superscripts indicate significant effect sizes (d ≥ 0.5).
Forbs maintain arthropod diversity
Forbs species provide important food sources and habitat for arthropods. When
exogenous soil disturbance leads to a reduction in indigenous forb diversity,
especially in agricultural and urban ecosystems, various measures of arthropod
diversity decrease significantly (Table 1). Arthropod diversity measures are
therefore good indicators to determine the level of disturbance in grassland.
Indigenous forbs
Residual
variance
Transect
variance
F
-
value
P
-
value
Absent Present
27.54a48.15b68.639 20.84
25.633
<0.001
104.6a194.2b
3636.92
982.84
10.604
<0.001
5.729a9.001b1.618 0.67 26.94
<0.001
-Wiener Index 2.622a3.139b0.103 0.061
12.942
<0.001
0.895a0.93b0.004 0.002 4.941 0.016
Siebert, Frances*1; Morris, Craig2; Chamane, Sindiso3; Ntuli, Nontu4; Siebert, Stefan1
Conclusions
There is increasing awareness of the important ecological role of forbs in grassland ecosystems worldwide. Grassland management should, therefore, not only be aimed at
maximizing the productivity of grasses, but also at maintaining the diversity of forb species that provide multiple functions and services.
We strongly recommend the inclusion of forb functional traits and indices for the assessment and monitoring of the functional integrity of grassland ecosystems.
Forbs in grassland ecosystems
provide habitat and forage for
various arthropod species.
Pyrenacantha kaurabassana is one of the many forb species with multiple ecosystem
functions: it is an indigenous leafy vegetable, and also contributes to the total carbon
pool of grasslands through its large belowground tuber.
*1Unit for Environmental Sciences and Management, North-West University, Potchefstroom, South Africa;
2 Agricultural Research Council - Animal Production Institute (ARC-API);
c/o School of Life Sciences, University of KwaZulu-Natal, Pietermaritzburg, South Africa;
3 School of Life Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209,
Pietermaritzburg, South Africa;
4 Department of Botany, University of Zululand, KwaDlangezwa, South Africa
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