Does soil change cause vegetation change or vice versa? A temporal perspective from Hungary

Ecology, April, 1997 by K.J. Willis, M. Braun, P. Sumegi, A. Toth

INTRODUCTION

Modelling of the long-term relationship between climate and vegetation change has received much attention over the past few decades (cf. Wright et al. 1993). In comparison there have been relatively few models developed to describe the long-term relationship between climate change, vegetation change, and soil development. Those that have been developed (Iversen 1958, van der Hammen et al. 1971, Birks 1986, Pennington 1986), suggest that soil development was probably a limiting factor to the postglacial increase in temperate trees in northern Europe. Estimates of a time delay up to 1500 yr before temperate trees could colonize have been attributed to soil maturation processes (Pennington 1986). However, these models mainly concentrate on northern Europe where ice covered much of the landscape during the full glacial and the development of soils would have involved the colonization and primary succession of bare rock (cf. Matthews and Whittaker 1987, Crouch 1993). But what about regions that were not covered with ice during the full glacial? Much of central and southern Europe remained unglaciated and although the predominate vegetation was steppe-like communities including grasses, Artemisia, and Chenopodiaceae, there were also a number of coniferous and deciduous trees in refugia (Bennett et al. 1991, Willis 1992, 1994, 1996). Although soils in these regions would have been cold (in some cases semi-permanently frozen) and nutrient poor, it is likely that podzols covered many areas of central and southern Europe during the full glacial as in the modern boreal forest. The impact of postglacial climatic warming on soil development of these regions is therefore significantly different. For example, during the late glacial, vegetation in Hungary was dominated by coniferous forest (Willis et al. 1995). Early in the postglacial a rapid change to deciduous woodland occurred. Climatic change was the ultimate driving force behind this change, but how was this related to vegetation change and changes in the soils? Present-day coniferous forests in central Europe produce an acidic litter and are usually situated upon acidic podzols typically in cool humid regions; mixed deciduous woodland produce a calcium-rich litter and are situated upon brown-earth soils (Russell 1961, Wild 1993). Did the change in climate therefore result in a change from one soil type to another, which in turn resulted in a change in forest composition or did the vegetation change first and subsequently alter the soil? How long did these soil formation processes take? And what mechanisms were involved in the development of a brown-earth soil from a podzol?

This study attempts to address some of these questions through the palaeoecological analyses of a sedimentary sequence from Kis-Mohos To in northeastern Hungary. Palaeoecological techniques of pollen analysis and geochemistry on an 8.79-m sedimentary sequence from Kis-Mohos To provide a detailed record of changes occurring on the slopes around the lake basin since the late glacial. Concentrating only on the sediments covering the time period from [approximately]14 6008000 cal. yr BP (prior to anthropogenic activity), results from this study suggest that it is possible to detect the processes and triggering mechanisms involved in the transition from podzol to brown-earth soil with postglacial climatic warming. We present a model for this mechanism and discuss the relationship of changes in the soils to climate and vegetational change.

METHODOLOGY

Study area

The study area is in the Kelemer region of northeastern Hungary. This region, in stark contrast to the Hungarian plain ([approximately]60 km southwest), is composed of undulating hills and valleys formed in alluvial sands and gravels that were deposited during the Tertiary period. Within one of these valleys (at an altitude of 310 m above sea level) there is a small Sphagnum peat bog ([approximately]60 m diameter), Kis-Mohos To (20 [degrees] 24 [minutes] 30 [seconds] E, 48 [degrees] 24 [minutes] 40 [seconds] N), from which an 8.86-m sedimentary sequence was collected.

The Kis-Mohos To peat bog [ILLUSTRATION FOR FIGURE 1 OMITTED] has no inflowing or outflowing streams and is probably hydrologically maintained by the continental climate of the region, which provides relatively high amounts of precipitation (650-700 mm/yr) with a maximum in June and a minimum in February and a mean annual temperature of 9.5 [degrees] C (January: -3 [degrees] C; July: 20 [degrees] c) (Kakas 1960). The microclimate in the valley is cooler (summer: 16 [degrees] C, winter: -6 [degrees] C) and moister (P. Sumegi, personal communication, 1995). Slopes surrounding the bog are covered with an acidic nonpodzol brown forest soil upon which is situated a mixed deciduous woodland dominated by Quercus cerris and Carpinus betulus.

Analyses

Two continuous, undisturbed sedimentary sequences (8.86 m) were obtained from the Kis-Mohos To basin using a 5 cm diameter modified Livingstone piston corer (Wright 1967) and the following analyses were carried out.