Figure 4. Mean direct (a, c) and higher-order interaction (b,
d) effects grouped per species in the most parsimonious models at zero
and maximum drought index. To calculate mean direct interactions, the
average of bidirectional direct interactions is taken. Positive values
indicate facilitative effects, while negative values indicate
competitive effects. Colors refer to functional group of the species
(red: non-N-fixing Forb, green: Grass, blue: N-fixer). Higher-order
interaction is abbreviated by HOI. Non-significant interactions were
excluded. For full species names, see methods section 3.1.
Species identity determined the nature of interactions. Out of the four
species involved in competitive interactions on average at maximum
drought index, three were grasses in both direct interactions and HOIs
(Fig. 4c,d). Forbs (incl. N-fixers) were on average more involved in
facilitation relative to their species number, especially at maximum
drought index (Fig. 4). Furthermore, Ellenberg’s moisture and nutrient
indices correlated with average interaction effects. At maximum drought
index, direct interactions became more negative with increasing affinity
to nutrient rich environments (p = 0.02; Fig. S5b), while HOIs became
more negative with increasing hydrophilicity (p = 0.04; Fig. S5a). No
correlations were found between average interaction strength or number
and average species biomass (Fig. S6, Table S5).
Interaction networks became stronger and more negative at maximum
drought index (Fig. 5). Net interactions (sum of direct interactions and
HOIs) were generally most positive on average for N-fixers, more
negative for non-N-fixing forbs and most negative for grasses (Table 1).
Moreover, the majority of net interactions became negative for grasses
and non-N-fixing forbs at maximum drought index, while the majority of
net N-fixer interactions remained facilitative. Both at zero and maximum
drought index, N-fixers were proportionally most involved in net
interactions, followed by forbs and grasses (Table 1). Average net
interaction strengths did not change noticeably with increasing drought
index within functional groups (Table 1).
Table 1. Number and average strength of all significant
positive and negative net interactions (sum of direct interactions and
HOIs) involving grasses, (non-N-fixing) forbs or N-fixers at zero and
maximum drought index, respectively. Non-significant interactions were
excluded. See Figure 5 for the complete network overview.