Long-term time series of legume cycles in a semi-natural montane grassland: evidence for nitrogen-driven grass dynamics?

Tomáš Herben, Hana Mayerová, Hana Skálová, Věra Hadincová , Sylvie Pecháčková, František Krahulec

The extraordinary species richness of mountain grasslands often hides less obvious dynamics of individual species, which may appear and disappear over time. One of the conspicuous examples is cycles of legumes (such as clover). Importantly, legumes, in contrast to most other grassland species, can obtain nitrogen directly from the atmosphere and thus such shifts in their abundance can have profound effects on availability of this essential element in the whole system. Although there are number of indirect observations that support the existence of legume cycles, they have never been shown using sufficiently long term data of abundance and nitrogen availability. We were fortunate to have long term observations (~30 years) of legume abundance in a mountain grassland, together with data on soil nitrogen content over the same observation period. Using such unique data, we were able to show that legumes indeed show marked cycles (lasting about 9 years), but also that these cycles are linked to cycles of nitrogen availability and cycles of biomass of grasses. Peaks of legumes, nitrogen and grasses are shifted against each other: a peak of nitrogen availability follows a peak of legumes and is followed by a peak of grasses. This is easily explained by the fact that legumes provide nitrogen that the grasses depend on. The legumes begin to increase when nitrogen availability in the system is low and provide the system with available nitrogen which subsequently leads to increase of grasses. We show that the decline of legumes is not due to competition by grasses, but is driven by unknown other agents (pathogens, soil organisms etc.). After the nitrogen is used up, the grasses begin to decline due to nutrient starvation. Legumes are thus the key driver of nitrogen dynamics in such nutrient-poor semi-natural grasslands. While grasses benefit from the nutrient enrichment due to legumes, they are rather a passive element in the process. Such dynamic processes are important in maintenance of species richness: while individual species come and go, the overall long-term richness of such meadows is maintained.

Image caption: Mountain grassland. Image provided by authors. 
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