Long-term time series of legume cycles in a semi-natural montane grassland: evidence for nitrogen-driven grass dynamics?
Abstract
1. Several dynamic models have shown that dynamics of legumes and grasses can result in periodic behaviour. These oscillations arise due to delays in nitrogen flows coupled with differences in ability to compete for light. However, long-term time series on legume dynamics that could be used to test predictions of these models are almost non-existent.
2. We examine legume oscillations in a semi-natural mountain grassland using a long-term (similar to 30years) data series on aboveground biomass of individual species and on nitrogen and phosphorus content over time.
3. Using autocorrelation analysis, we show that there is a strong periodicity (with period of 8-9years) of legume and grass biomass and nitrogen content in the grass biomass. These three variables are in fairly stable phase shifts relative to each other, with a grass peak followed by a peak in C:N ratio in grasses which is followed by a legume peak. Phosphorus content in either legume or grass biomass does not show synchronous cycling with legume or grass biomass or nitrogen content in grass.
4. Fitting a dynamic linear model to the data showed that legumes affect nitrogen content in grasses, and grass biomass both affects and is affected by nitrogen content. In contrast, there is no negative effect of grasses on legumes, indicating some other process must be responsible for the legume decline. Manuring, which was occasionally applied to the plots, also does not seem to affect the cycling. Second-order term for legumes showed some evidence of self-inhibitory effects in legumes, but phosphorus content in legumes shows no support for phosphorus limitation. The most likely explanation of the legume decline should be sought elsewhere (pathogens, soil biota etc.).
5. Synthesis. Long-term data support the existing the claim that legume dynamics are the key driver of nitrogen dynamics in nutrient-poor semi-natural grasslands. Grasses benefit from the nutrient enrichment due to legume cycling, but are a passive element and do not play a role in legume limitation. Apart from the role of nutrient cycling, these legume-driven nutrient dynamics also constitutes processes by which long-term richness of meadows is maintained.