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.