Community structure across grazing regimes
Wet/dry, grazing intensity, and year were all able to explain a
significant amount of the variation in overall vascular plant community
structure in the PERMANOVA (P <= 0.002 for each; Table 2;
Figure 2). Post-hoc analysis revealed that the effect of grazing
intensity on community structure was significant between plots with
light and pulse grazing (Bonferroni adjusted p = 0.003) and between
plots with light and press grazing (Bonferroni adjusted p = 0.003), but
not between plots with press and pulse grazing (Bonferroni adjusted p =
0.27; Figure 2b). The effect of year on community structure was only
significant between plots in years 2004 and plots 2007 (Bonferroni
adjusted p = 0.006; Figure 2c).
In addition to the variation in each of the above metrics of overall
plant richness, there was also substantial variation among plots in the
mean species richness of shrubs (mean ± SD (range): 3.55 ± 1.84 (0.11 –
9.00) species), herbs (11.05 ± 5.22 (1.56 – 22.56) species), and
graminoids (6.15 ± 2.63 (1.44 – 12.56) species). The number of shrub
species was independent of year (F3,128 = 1.87, P =
0.14) and the grazing:year interaction (F6,122 = 0.11, P
= 0.99), but varied with grazing intensity (F2,128 =
30.27 P < 0.0001) and wet/dry (F1,128 = 44.63,
P < 0.0001); lightly grazed plots had more shrub species than
plots with pulse (P < 0.0001) and press grazing (P <
0.0001; Figure 3a), and dry plots (lsmean ± SE: 4.23 ± 0.33 species) had
more shrub species than wet plots (2.77 ± 0.33 species). The number of
graminoid species varied with year (F3,128 = 13.11 P
< 0.0001), as plots from 2004 (lsmean ± SE: 7.84 ± 0.54
species) had more species of graminoids than plots from 2005 (6.00 ±
0.54 species; P = 0.0005), 2006 (5.74 ± 0.54 species; P = 0.0001) and
2007 (5.08 ± 0.55 species; P < 0.0001), but was independent of
wet/dry (F1,128 = 0.05, P = 0.82) and the grazing:year
interaction (F6,122 = 0.30, P = 0.93). Complementing the
effect of grazing intensity on shrub species richness, the species
richness of graminoids also varied with grazing intensity
(F2,128 = 23.24, P < 0.0001) as lightly grazed
plots had fewer graminoid species compared to plots with pulse (P
< 0.0001) and press grazing (P < 0.0001; Figure 3b).
In contrast, the species richness of herbs did not vary with wet/dry
(F1,128 = 2.70, P = 0.10), year (F3,128= 2.30, P = 0.08), grazing (F2,128 = 0.15 P = 0.86;
Figure 3c) or the grazing:year interaction (F6,122 =
0.07, P = 0.99).