Degradation of Arbuscular Mycorrhizal Symbiosis: A Mechanism Underlying Cynodon dactylon Invasion in Grasslands?

Questions

Degradation of facilitative interactions of native species can play an important role in the establishment and expansion of invasive plants in communities. We evaluated the relationship between the level of invasion of Cynodon dactylon and the arbuscular mycorrhizal colonization of the native Paspalum notatum in Uruguayan grasslands, which were either extensively managed (natural vegetation [NG]) or oversown with exotic legumes and fertilized with phosphorus (NG + LP). Specifically, we investigated whether increasing invasion levels were associated with reductions in P. notatum mycorrhizal colonization, growth, and nutrient content.

Location

Uruguayan grasslands of Río de la Plata grasslands region.

Methods

Two paddocks with 19 and 27 years under NG + LP management were selected contiguous to two paddocks with NG management. In each paddock, we collected nine monoliths (0.2 m diameter × 0.3 m depth) that had P. notatum and increasing percentages of C. dactylon cover, classified as low (0%–10%), medium (30%–50%), or high (70%–90%) invasion levels. After 10 months of uninterrupted growth, shoot mass, phosphorus and nitrogen shoot concentration, and arbuscular mycorrhizal colonization were assessed in P. notatum plants.

Results

Mycorrhization decreased with increasing C. dactylon invasion. This was greater in more intensively managed grasslands (NG + LP). However, lower arbuscular mycorrhizal colonization was not associated with either lower aboveground growth or phosphorus or nitrogen shoot concentration. Furthermore, at low invasion levels, mycorrhizal colonization was similar between NG and NG + LP, despite their contrasting soil P availability.

Conclusions

The presence of arbuscular mycorrhizae in the target native grass P. notatum was negatively associated with the invasion level of C. dactylon. Therefore, mutualism degradation might be a mechanism underlying the success of C. dactylon invading grasslands, particularly those intensively managed, albeit probably not via phosphorus nutritional effects.