Vegetation structure
The results of the first Principal Component Analysis (PCA), including
all trees (N=72) showed that vegetation structure starts to change soon
after livestock is excluded. In just one year grass cover equalled that
of long term exclosures, but its height was still shorter. The analyses
retrieved two dimensions, which together explained 78% of the variance.
PC1 (59% of the variance explained) was significantly correlated with
all variables (Table S7) and defined a gradient between trees in farms
with high vegetation cover and height (mostly grass) versus those with a
higher percentage of short grass (i.e. ground). It showed that
vegetation cover was higher beneath the canopies of oaks from which
livestock had been excluded for one or ten years than in control trees.
PC2 (variance explained of 16.7%) was correlated with four variables
(Table S7) and separated areas with high cover of short and high grass
(which corresponded to long-term exclosures) from those with a
prevalence of medium-sized grass (Figure 2A).
The second PCA, performed including only the control oaks (N = 24) from
Farms 1-3 (livestock present), showed that vegetation cover differed
among them at their standard livestock densities (Figure 2B). PC1
(variance explained of 59.6%) was highly correlated with all variables,
and defined a gradient between areas with short grass and those in which
overall grass cover and height were higher. PC2 explained less variance
(15.8%) and was correlated with only two variables that differentiated
areas with high versus medium-sized grass (Table S7). Summarizing, in
Farm 1 the grass beneath the canopies was scarce and shorter than in the
rest, and in Farm 3 there was a high cover of grass which, on average,
was also taller.