Improved space-for-time substitution for hypothesis generation: secondary grasslands with documented site history in SE-Hungary
Molnár, Zs.; Botta-Dukát, Z.
Space-for-time substitution (SFT) is a common method to study vegetation dynamics; yet, different site histories make it often unreliable. We studied the first 38-yr succession on abandoned fields in a Hungarian steppe landscape, controlling for present and past abiotic conditions, landscape context and disturbance regime. The sites chosen in the landscape appear suitable for a historically improved SFT. The results of the SFT show that the most characteristic features of the local succession are: 1. the very limited colonization of specialist species, though this landscape was expected to be a good propagule source for colonization since there are many species-rich loess grassland patches scattered in the landscape surrounding the old-fields, 2. the development of a closed, low diversity grassland dominated by perennial generalists, 3. the sharply decreasing rate of succession in 25-38-yr-old fields. The old-fields converged in floristic composition with succession. Convergence may be determined by the following factors: 1. low chances for woody species colonization (as a consequence of a poor propagule source for woody species and regular grazing), 2. dominance of Festuca and other generalists that may prevent further colonization and 3. regular sheep and cattle grazing as a homogenizing factor. Hypotheses were generated based on these results that can be used to plan local longterm experiments or observations e.g. to decide whether dispersal limitation or recruitment limitation determines the low colonization rate of specialist species observed in this landscape. Since the 25-38-yr-old fields have a very "characterless" species composition (and lack the distinctive features of loess grasslands), future restoration works will need to focus on the artificial diversification of these secondary grasslands, if the goal is a species-rich grassland that resembles the region's ancient loess grasslands. Improved space-for-time substitution seems to be a useful tool to study local vegetation dynamics. Detailed historical reconstruction could improve the reliability of SFT studies; unfortunately, reconstruction of the history is not an easy task and the site choice is never perfect. The challenge of historical reconstruction may be one reason why SFT studies of sites with documented history are rare. Improved space-for-time substitution may also be an appropriate tool to design restoration projects.