Unexpected ecotone dynamics of a sand dune vegetation complex following water table decline

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Abstract

Aims

Central Hungarian inland dune ranges harbor heterogeneous grassland vegetation with an extensive network of ecotones, arranged perpendicular to topography-driven hydrologic gradients. The area suffers from severe aridification due to climate change and local anthropogenic factors, which have led to a dramatic decline of the water table. As a result, groundwater is no longer reachable for low-lying plant communities; thus, we expect they are bound to undergo profound changes. This study investigates how the plant communities respond to this changing environment over time by monitoring ecotones, since they are frequently the hotspots of ecosystem change. We monitored five ecotones along permanent belt transects for 15 years to characterize their dynamic response, and to identify the internal structural changes of the plant communities the ecotones delimit.

Methods

Ecotones were delineated with the split moving window technique. The dynamics of two ecotone parameters, location and contrast, were analyzed with linear regression models incorporating two independent variables: study year as a measure of time since the loss of groundwater, and precipitation as a possible driver of inter-annual variations. The internal changes of the patches separated by the ecotones were analyzed using plant functional groups.

Important Findings

Precipitation had no detectable effect on the ecotone descriptors, but study year influenced ecotones in an unusual fashion. The position of the ecotones appeared to be very stable in time; their dynamics are stationary, not directional as we predicted. The contrasts had clear tendencies; two ecotones disappeared, one new one was formed and two ecotones showed no trend. The internal changes of the patches over time were dramatic, showing a shift toward more xeric and more open plant assemblages in most stretches of the transects. Thus, the dynamic response of the vegetation was not patch expansion vs. shrinking, but fusion vs. division, which profoundly restructured the vegetation pattern. Analysis of plant functional groups revealed that the trends of the ecotone contrasts could be traced back to internal changes of the patches and not to processes within ecotones. Hence, in situations where stationary ecotone dynamics prevail, ecotone position may be a poor indicator of the effects of strong directional environmental changes. However, in this study we show that ecotone contrast can serve as a sensitive tool for monitoring landscape pattern transformations in these cases. Also, this highlights the long-term nature of ecotone responses, which can have implications in landscape planning and restoration measures.

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