CHAPTER 2 LONG-TERM EFFECTS OF FIRE FREQUENCY ON HERBACEOUS
2.3 Results
2.3.1 The Nonmetric Multidimensional Scaling (NMS) sample ordination
The ordination of the plots was achieved with a final stress of 0.148 after 27 iterations for the two-dimensional solution. According to the two axes of the two-dimensional NMS sample ordination, species composition over time differed with burning frequency (Figure 2.2). In general compositional change increased with fire frequency.
Dimension1 of the NMS ordination explained 62.7 % of the compositional variation, and distributed the species according to gradients of burning frequency. The extreme right of the diagram was occupied by species occurring in the no burn controls, while those in the extreme left were mostly found in the frequently burnt plots, mainly the annual and biennial burns. Dimension 2 of the NMS ordination was related to other environmental factors influencing species performances, but not accounted for in the experiment. These could include factors such as slope, aspect or soil heterogeneity.
Time after commencement of experiment also appeared to have had some effect, because most of the species noticeably occupied the upper half of the axis in the early years of the study, while some species shifted towards the lower part of the axis in the latter years (Figure 2.3a-c).
Except for a few, most of the species in the no burn plots (1 CON and 2 CON) formed a distinct cluster towards the positive side of the axes, and were generally closer together, indicating that their species composition was less variable than those of the burnt plots. On the other hand, the burnt plots formed less distinct clusters, with more scattered plots, suggesting that burning may have promoted a more variable species composition. The triennial and quadrennial burns resulted in the most scattered plots, followed by the annual and biennial burns, while the six-year burn was the least scattered of the burnt plots, having ordination plots that were almost indistinct from the control plots (Figure 2.2).
The trajectories for each treatment (with replicates kept separate) are presented in Figures 2.3a, b and c respectively. When the trial commenced in 1980, species composition in all the 12 treatment plots was similar, but the variability increased over time, especially in the ten burnt plots. Some of the years that were either too
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close or overlapping have been omitted from the graphs to reduce overcrowding.
There appeared to have been a sharp compositional shift between 1982 and 1985 across all the treatment plots, and after 1988 the variation gradually stabilized (Figures 2.3 a, b and c). The triennial and quadrennial burns produced the longest ordination distances between 1982 and 1983 (Figure 2.3b), followed by the no burn and sexennial burns (Figure 2.3c), while the annual and biennial burns had the shortest distance (Figure 2.3a). Furhermore, some variation was also apparent between the two replications of each treatment, with the highest noticeable variation in the triennial and quadrennial burns (Figure 2.3b).
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Figure 2. 2 NMS sample ordination plots of species composition in a long-term fire trial at Fort Hare research farm between 1980 and 2008.
KEY: 1CON= No burn 1, 2CON= No burn 2, 1ANN =Annual 1; 2ANN= Annual 2, 1BIEN= 1Biennial, 2BIEN = 2Biennial, 1TRI = 1Triennial, 2TRI = 2Triennial, 1QUAD = 1Quadrennial, 2QUAD= 2Quadrennial, 1SIX = 1Sexennial, 2Six = 2 Sexennial.
-2 -1 0 1 2
-2 -1 0 1 2
SAMPLES
1CON 2CON 1ANN 2ANN 1BIEN 2BIEN
1TRI 2TRI 1QUAD 2QUAD 1SIX 2SIX
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Figure 2. 3a NMS trajectories of species composition for the annual and biennial burns of the long-term fire trial.
80 84
85
87 93 96 81 82
83
08 06 04
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Dimension 2
Annual
Rep 1 Rep 2 82
80
08
81
83 85
96
08
80 81 82
83
86
91
01
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Dimension 1 Biennial
Rep 1 Rep 2 88
93
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Figure 2.3b NMS trajectories of species composition for the triennial and quadrennial burns of the of the long-term fire trial
85 83 85
84
82
83
01
93
05 06
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Dimension 2
Triennial
Rep 1 Rep 2 80
80
08
08
80
82 83
84
85
01 80 04
82
83 84 85
92 96 05
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Dimension 1 Quadrennial
Rep 1 Rep 2 08
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Figure 2.3c NMS trajectories of species composition for the sexennial and no burn treatments of the of the long-term fire trial
82
83 85
88 93
95
05 80
82 85
88
93
96 01
05
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Sexennial
Rep 1 Rep 2
90
80 82
83
93
07 05 84
85 80 82
83
84 85
87
97 05 08 -2
-1.5 -1 -0.5 0 0.5 1 1.5 2
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Dimension 2
Dimension 1
No burn
Rep 1 Rep 2
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2.3.2 Species trends in the Nonmetric Multidimensional Scaling A total of eighteen grass species and some unidentified forb and karroid species were
recorded and included in the analyses of this study. Figure 2.4 is the NMS ordination species which were abundant in most of the treatment plots. The results indicate that herbaceous species abundances and distribution were influenced by fire treatments, and the variation increased with time, whereby species spread along gradients of increasing/decreasing fire frequency over time. Cymbopogon plurinodis (though the abundance was generally low), Melica decumbens, Sporobolus fimbriatus, Karroo and Forb species were mostly found in the upper right quarter where treatment plots were mainly the sexennial and quadrennial and no burn controls (Figure 2.4, 2.5b,c).
Actually, M. decumbens was completely eliminated from the annual, biennial burns, and was almost absent in the triennial burns as well. Conversely, Themeda triandra was most dominant in the annual and biennial plots and became less abundant as fire frequency decreased (Figure 2.5a). Several species such as Digitaria eriantha, Eragrostis species, Panicum stapfianum and, Sporobolus africanus were also abundant in the top left quarter where frequent burns were applied, which also occurred during the first five years of the experiment (Figure 2.3a-c, 2.4).
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Figure 2.4 NMS ordination of species composition in a long-term fire trial at FortH Hare research farm
KEY: CYNDA= Cynodon dactylon, CYMPL = Cymbopogon plurinodis, DIGER = Digitaria eriantha, ERCUR= Eragrostis curvula, ERCL = Eragrostis chloromelas, ERAOB = Eragrostis obtuse, EUSMUT = Eustachys mutica (changed to Eustachys paspaloides), HETCON = Heteropogon contortus, KAR= All Karroo species, MELDE= Melica decumbens, MICAF = Microchloa caffra; PANEQ = Panicum aequinerve, PANMAX = Panicum maximum, PANSTAP = Panicum stapfianum, SPOAF = Sporobolus africanus, SPOFI= Sporobolus fimbriatus, THTRI = Themeda triandra.
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
PANMAX PANSTAP
SETNEG
THTRI
PANEQ
CYMPL
MELDE CYNDA
DIGER
EUSMUT ERCL
ERAOB ERCUR
SPOAF
SPOFIM
FORBS KAR
HETCON MICAF
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Figure 2.5a NMS ordinations for mean abundances for Panicum stapfianum (left) and Themeda triandra (right) across the burning treatments
-2 -1 0 1 2
-2 -1 0 1 2
0 2
4 6 8 10 12 14
16
SAMPLES
1CON 2CON 1ANN 2ANN 1BIEN 2BIEN
1TRI 2TRI 1QUAD 2QUAD 1SIX 2SIX
-2 -1 0 1 2
-2 -1 0 1 2
0
5 10 15 20 25 30 35 40
45 50
55 60
SAMPLES
1CON 2CON 1ANN 2ANN 1BIEN 2BIEN
1TRI 2TRI 1QUAD 2QUAD 1SIX 2SIX
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Figure 2.5b NMS ordinations for mean abundances for Digitaria eriantha (left) and Sporobolus africanus (right) across the burning treatments
-2 -1 0 1 2
-2 -1 0 1 2
0
5 10 15
20
SAMPLES
1CON 2CON 1ANN 2ANN 1BIEN 2BIEN
1TRI 2TRI 1QUAD 2QUAD 1SIX 2SIX
-2 -1 0 1 2
-2 -1 0 1 2
0
0 1 0.5
1.5 2 2.5 3 3.5 4 4.5
SAMPLES
1CON 2CON 1ANN 2ANN 1BIEN 2BIEN
1TRI 2TRI 1QUAD 2QUAD 1SIX 2SIX
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Figure 2.5c NMS ordinations for mean abundances for Melica decumbens (left) and Cymbopogon plurinodis (right) across the burning treatments.
-2 -1 0 1 2
-2 -1 0 1 2
5 10
15 20
25
SAMPLES
1CON 2CON 1ANN 2ANN 1BIEN 2BIEN
1TRI 2TRI 1QUAD 2QUAD 1SIX 2SIX
-2 -1 0 1 2
-2 -1 0 1 2
1 2 3
4 5 6 7
8 9 10 11
SAMPLES
1CON 2CON 1ANN 2ANN 1BIEN 2BIEN
1TRI 2TRI 1QUAD 2QUAD 1SIX 2SIX
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