Krummholz and grassland coexistence above the forest-line in the Krkonoše Mountains: Grid-based model of shrub dynamics

The coexistence of the contrasting life forms krummholz and grass on the summit plateaux of the Krkonoše Mts., Czech Republic, was studied using a simulation model. A spatially explicit, combined individual- and grid-based simulation model of krummholz dynamics and its interaction with grassland was developed. The field-of-neighbourhood (FON) approach was used to manage the krummholz individual interactions. The model was parameterized using two types of field data: (i) direct measurements of parameter values and (ii) data on the present spatial pattern of the krummholz–grassland mosaic derived from aerial photographs. The latter was compared with simulated spatial patterns and several parameters were estimated based on the patterns that gave the best fit. Two scenarios were explored using the model: without and with disturbance affecting krummholz abundance. The sensitivity of different response variables to most of the model's parameters was estimated under both scenarios. In general the results indicate that krummholz and grassland can coexist as a mosaic in both scenarios for long time under a broad range of parameters. Disturbance may shift the proportion of krummholz and grassland patches, but does not necessarily affect the long-term persistence of the mosaic. The model also predicts irregular fluctuations in the proportions of krummholz and grassland in the long-term equilibrium and a high resilience of this system. A conceptual analytical model that summarizes the dynamics of the system was used to discuss the mechanism of coexistence. It showed that the obvious superiority of krummholz is modified by the ability of grass to reduce sexual reproduction of krummholz shrubs, thus weakening their strict dominance. It was concluded that the competition-colonization (CC) trade-off is the general mechanism allowing the coexistence even of such different contrasting life forms as shrubs and grasses.