Introduction

Talus or scree formations develop through mechanical weathering of bedrock and are abundant both in arid zones and on hill and mountain slopes in zones that have experienced periglacial activity. In central Europe, talus formation occurred during the Pleistocene glaciation at low latitudes along vast periglacial zones that faced northern continental and southern alpine glaciers. At present, this weathering activity dominates on high mountains and at high latitudes. However, in some scree formations in subalpine landscapes in central Europe, perennial periglacial-like microclimates still prevail in shallow field depressions, or “ice-hollows,” that are cold all year round. Temperatures fluctuate around 0°C during the vegetative season so that solid ice is often present until late summer or early autumn. Ice hollows have been mentioned in the scientific literature for more than 200 years (e.g., Schaller, 1785; later, Pleischl, 1838; Balch, 1900; and recently, Mareš, 1959; Kubát, 1971; Möseler and Molenda, 1999; Zacharda, 2000b; Gude et al., 2003; among others). However, little scientific research has been carried out on cool screes, as they appear lifeless and are difficult to sample. Consequently, the published information on these sites has not changed for many decades. Until recently, most studies were confined to simple observations of cool and warm air outflows in summer and winter, respectively (Balch, 1900; Mareš, 1959). No precise continuous time series of microclimatic measurements (Hinkel et al., 2001) has been collected in these localities to describe basic seasonal heat transport processes.

Recent European microclimatological studies have identified small discontinuous permafrost-like patches in cold screes with periglacial microclimates. These sites have been found at 300–500 m a.s.l., i.e., about 1500–2000 m below the present limit for permafrost in European mountains (Gude et al., 2003). Biological surveys of these sites have found periglacial relict species of beetles (Molenda, 1996), non-vascular plants (Němcová, 2001), and chelicerates (Zacharda, 1993; Růžička and Zacharda, 1994).

These discoveries have stimulated interest in the ecology of these geo-ecosystems. One of the extant periglacial relicts is the predatory rhagidiid mite, Rhagidia gelida Thorell, 1872. Until the 1980s the mite was known only from the Arctic (Zacharda, 1980), where it was “widely distributed on the northern coast, having been collected in tundra from Cape Beaufort to Tuktoyaktuk” (Strandtmann, 1971). It was later found in central Europe; for example, in deep sandstone gorges and screes with a permanently cool microclimate in Adršpach-Teplice Rocks, NE Bohemia (50°36′N, 16°8′E), and in montane screes in the Krkonoše (Giant) Mountains (50°45′N, 15°40′E), north Bohemia, the Czech Republic, at an altitude of about 1400 m a.s.l. (Zacharda, 1993). Recently, Rhagidia gelida has also been found in two areas of about 100 m² each, at the base of scree slopes with periglacial-like microclimates at 300–500 m a.s.l. in the České Středohoří–highland and Lužické Hory Mountains, north Bohemia. These two new sites are recognized as the southernmost known habitats of disjunct relict populations of this mite.

The main objective of this study was to describe the cool scree habitat and the life cycle of the relict predatory mite Rhagidia gelida as a biological cryoindicator which documents the relict cool, periglacial-like microclimate that prevails in these two screes during the vegetative season. In addition, continuous temperature measurements and geophysical surveys of the internal structure of the screes were undertaken.

Site Description

Twenty different scree slopes in the České Středohoří–highland and the Lužické Hory Mountains were examined during the vegetative season for cool, periglacial-like microclimates (Zacharda, 2000b). Only four sites with such a microclimate were found at the base of the following scree slopes: Boreč hill near Lovosice (50°31′N, 14°2′E), Plešivec hill near Litoměřice (50°33′N, 14°7′E), Kamenná Hůra near Merboltice village (50°42′N, 14°21′E), all in the České Středohoří–highland, and Klíč hill near the Svor village (50°49′N, 14°04′E), in the Lužické Hory Mountains (Fig. 1). All these sites are in north Bohemia, Czech Republic. Subsequent research was focused on the latter two ice-retaining screes as they support populations of Rhagidia gelida mites and seem to be the most typical sites.

The Kamenná Hůra olivine basalt scree slope is located in the Kamenná Hůra Nature Reserve, which covers an area of 56.7 ha. The bare, open screes on the slopes of the Kamenec massif (519 m a.s.l.) are among the largest screes in the České Středohoří–highland (Fig. 1). They originated through mechanical weathering of a tertiary basalt plateau overlying chalky clay. The climate is mild, with an average annual temperature of about 8°C, and January and July average air temperatures of −2 to −3°C and 16°C, respectively. Several natural ice hollows are found at 300 m altitude at the base of the large northern scree field (Fig. 2). Accumulations of bedrock boulders about 40–80 cm diameter are densely covered with pads of many species of nonvascular plants. Among these plants are subarctic or subalpine liverworts (Gymnomitrion corallioides and Diplophyllum taxifolium) and moss (Polytrichum alpinum), and mountain liverworts (Lophozia hatcheri, L. sudetica, and Tritomaria quinquedentata) and moss (Bryum alpinum) (Němcová, 2001). Norway spruce (Picea excelsa), birch (Betula sp.), and rowan tree (Sorbus aucuparia) grow on the periphery of the ice hollow.

A large, free, open phonolite scree is found on the SW slope of Klíč Mountain (759.5 m a.s.l.) Nature Reserve in the Lužické Hory Mountains (Fig. 1). The climate is wet and mild to cool, with an average annual temperature of 6–7°C, and January and July average air temperatures of −4 to −3°C and 16°C, respectively. The scree slope originates from mechanical weathering and rockfall activity and consists of bedrock blocks 40–80 cm in diameter. There is a distinct field depression, where outflows of cool air are perceivable to a man, particularly in hot summers, at the SSW base (524 m a.s.l.) of the scree. The entire ice-hollow area is covered with lichen and moss pads consisting of species such as the reindeer lichen Cladonia rangiferina and C. uncinalis; the liverworts Calypogeia integristipula, Diplophyllum taxifolium, Lophozia hatcheri, and L. sudetica; and the mosses Grimmia donniana, G. incurva, Polytrichum alpinum, P. formosum, Racomitrium sudeticum, and R. heterostichum (Sýkora, 1972; Němcová, 2001). Small, dwarfish Norway spruce trees (Picea excelsa) and rowan trees (Sorbus aucuparia) grow on the periphery. This habitat resembles tundra (Billings, 1973; Danks, 1979). Botanists consider both Polytrichum alpinum and Grimmia incurva as subarctic-subalpine species of mosses that often indicate cool sites such as ice hollows when found in low-altitude screes (Němcová, 2001). On the NW and SE outskirts of the ice hollow, the vegetation pattern changes to a Vaccinio-Abietion plant community, with Norway spruce and rowan trees, and the Calamagrosti arundinaceae-Fagetum plant community, respectively (Sýkora, 1972).

Continuous temperature measurements, geophysical estimates of scree ice content within the screes, and long-term collections of invertebrates were performed at both sites.

Methods and Material

Results

Discussion

Recent European microclimatological studies have identified small discontinuous permafrost-like patches in cold screes with periglacial microclimates. Though permafrost-like conditions in the sense of modern usage in Canada and U.S.A., i.e., “ground (soil and/or rock) that remains at or below 0°C for at least two years” (French, 1996), have been verified by numerous underground temperature measurements in several scree slope systems, by observations of ground ice in mid- and late summer, and by geophysical surveys, it has not been definitely resolved whether these sites do fit within the conventional definitions of permafrost or not (Gude et al., 2003). Complementary long-term series of temperature measurements and studies on latent heat processes in the ice-retaining screes are still needed.

During the European Pleistocene glaciation there was a vast periglacial zone between the Alps and the northern continental ice sheet. This zone was subject to severe nival climate and permafrost with associated upper active layer cryogenic processes such as intensive frost weathering, solifluction, and growth of ice wedges. Similar conditions still prevail in the Arctic (Ives, 1974; Schrott, 1999). The predatory mite R. gelida frequently occurs in the Arctic and was also collected in Barrow, Alaska (Strandtmann, 1971), in high arctic tundra meadow on the Arctic Coastal Plain where surface temperatures reach +8–13°C during the summer (MacLean and Pitelka, 1971; Hinkel et al., 2001). The temperature regimes of the surface organic layer in the Klíč and Kamenná Hůra scree ice hollows are strikingly similar to those near Barrow, Alaska. In the Arctic the mite has only been collected in localities with permafrost (Fig. 7) and periglacial microclimate. Direct observations in the Kamenná Hůra and Klíč ice hollows show that this species is strictly limited to areas where the substratum surface temperature is +3.8 to +10.0°C. It is difficult to believe that this strictly cold, stenotherm mite (Danks, 1979) could have migrated into the ice hollows during the Holocene either from some mountain refugia, e.g., screes of the Krkonoše Mountains, or across vast temperate areas from the Arctic. The only plausible explanation is that R. gelida is a glacial or, strictly speaking, periglacial (sensu Molenda, 1996) relict that has survived in the periglacial-like microclimate of the ice hollows, at least from the height of the last Pleistocene glaciation, when most of Europe was tundra (Matthews, 1979).

As perennial coolness is a prerequisite for the relict mite R. gelida, it is plausible that the periglacial-like microclimate that causes permafrost-like patches appeared with scree formation during the Pleistocene. In other words, the relict mite can be considered as an indicator that a perennial cool microclimate has persisted in these ice-retaining screes throughout the Holocene.

Geographically and climatically isolated ice hollows are thought to be island-like refugia for relict species of landscape biota (Hellwig, 1999). Nekola (1999) designated North American cool algific talus slopes located in the driftless area (Paleozoic plateau) in the Upper Midwest, U.S.A., as paleorefugia. They show a similar microclimatic scree slope phenomenon to that described above and support extant colonies of the periglacial relict candidate land stenotherm snails (Frest, 1991). The occurrence of these snails indicates the periglacial-like microclimate in the ice-retaining talus slopes. These paleorefugia represent habitats that are older than the surrounding biological matrix and support communities or populations unable to survive elsewhere in the landscape.

Some of the cool, ice-retaining scree slopes in the Czech Republic, which support populations of the mite R. gelida, also support spiders such as Bathyphantes simillimus, Leptyphantes tripartitus, Diplocentria bidentata, Latithorax faustus, or Wubanoides uralensis (Růžička and Zacharda, 1994; Růžička, 1999) and nonvascular plants with a boreo-alpine pattern of distribution (Němcová, 2001). These sites seem to fit Nekola's concept of paleorefugia, and the above-mentioned taxa can be also considered relicts, although in the case of the spiders or non-vascular plants, presence can also be explained by airborne travel.

The question then arises: Do these local, disjunct, scree-inhabiting populations differ from each other and the present boreal or Arctic populations? This question could be answered using molecular biology.

It should also be mentioned that in the Czech Republic the mite R. gelida occurs only in the northern part of north to NE Bohemia (Fig. 1). This corresponds to approximate borders of the maximum Pleistocene continental glaciation. In contrast, up until now this mite has never been collected in any cool screes in central and south Bohemia or south Moravia. For example, R. gelida does not occur in a cool phonolite scree on Boreč hill near Lovosice (50°31′N, 14°2′E), i.e., only about 15 km in a direct line from the southernmost occurrence of this mite in scree located at the foot of Plešivec hill near Litoměřice, although the periglacial microclimate here resembles that in the Klíč and Kamenná Hůra study sites. Similarly, R. gelida is absent from the cool screes of ice caves in the Dyje River valley, south Moravia (Růžička, 1996). This mite has never been collected from the subnival alpine zone of the Austrian Ötztal Alps in the vicinity of Obergurgl, Tirol, with a severe alpine climate and permafrost (Haeberli and Patzelt, 1982), where assemblages of rhagidiid mites were studied at an altitude of 2000–3000 m a.s.l. in 1993–1994 by Zacharda (2000a). Thus, the ice hollows at the foot of the Plešivec hill near the town of Litoměřice (50°33′N, 14°7′E) are currently the southernmost known locality of this species in central Europe.

The disjunct populations of R. gelida in the ice hollows still exhibit seasonal biological adaptations correlated with an adverse winter period. Univoltine mites are cold stenotherms which are only active and multiply during the summer. Currently it is not known why the mite prefers a cool habitat. It could be a lack of competition because no other species of rhagidiid mites have been collected in this habitat, or the species may be more than cool tolerant and may have the ability to move, feed, multiply, etc. at low temperatures. Probably it is freezing intolerant, i.e., cannot survive ice formation within its body, but can survive ice formation around it in its habitat. However, neither dormancy nor cold-hardiness (Danks, 1979) has been studied in this mite.

Conclusions

Periglacial microclimates occur at bases of scree slopes located at the subalpine altitude of only 300–600 m a.s.l. in the Czech Republic, central Europe. Particular geo-ecosystems supporting disjunct populations of the cool tolerant relict species of nonvascular plants and invertebrates have evolved in these cool places. Among these, the predatory prostigmatic mite R. gelida is the periglacial relict and can be considered a biological cryoindicator of the periglacial microclimate. The mite R. gelida is univoltine. This is an adaptation to adverse climatic conditions typical of extant species of invertebrates that live in the Arctic. As perennial coolness is needed for R. gelida to survive in these habitats, it is concluded that the scree slope patchy permafrost-like habitat in the study sites is long-lasting, and perhaps has persisted in modified form since the Pleistocene.