Trees in the desert: Reproduction and genetic structure of fragmented Ulmus pumila forests in Mongolian drylands

This paper is dedicated to Werner Hilbig and Eckehart Jäger who both recently celebrated their 75th birthdays. Werner Hilbig laid the foundations of phytosociology in Mongolia and worked on a wide range of questions on the ecology of Mongolian vegetation. He cooperated closely with E. Jäger who shaped our modern understanding of the biogeography of many regions in Eurasia, especially of Mongolia. Both look back at more than four decades of continuous and often joined fieldwork. The authors of the present paper had the great pleasure to participate in some of their activities in the last few years.
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Abstract

The potential natural distribution of deciduous forests in Central Asia is the subject of ongoing discussions. Ulmus pumila (Siberian elm) is the only tree species occurring in southern and south-eastern Mongolia. In the semi-arid Mongolian Gobi, the species is restricted to ravines and beds of semi-temporary rivers. Compared to zonal occurrences in moister northern Mongolia, elm trees in the Gobi were found to be larger, in spite of their slower growth. Recruitment was very rare in the field although germination studies revealed that seeds were viable, survived osmotic stress, and were tolerant of repeated cycles of moistening and drying. Thus, they should be capable of germination in episodically flooded river beds. Fingerprinting revealed that clonal growth is of negligible importance in the Gobi as almost all U. pumila individuals studied constituted separate genets. Given that many trees were <100 years old and must have become established under current climatic conditions, we infer that the current lack of recruitment is likely to be caused by grazing impact. Our data imply that Ulmus pumila could potentially be much more common in the drylands of southern Mongolia and northern China.

Introduction

In 1987, Hilbig published a paper in FLORA that summarised the state of knowledge on an intensively debated issue in Central Asian vegetation ecology: the question of the potential natural distribution of forests in Mongolia. Here, boreal coniferous forests directly border the extensive grass-dominated steppes, while a belt of deciduous forests is hardly developed. This pattern is usually explained by climatic constraints, but Mongolia hosts ancient pastoral societies that have also left an imprint on the vegetation. The relative importance of human impact vs. climatic constraints is most prominently discussed from two aspects of tree distribution: one is the scarcity of trees on south-facing slopes in the forest steppe zone, where northern slopes are covered by dense coniferous forests (mainly Larix sibirica). The other debated issue is the presence of – often small – stands of deciduous forests, with Betula spp. in mountain sites (Jäger, 2005) and Ulmus pumila in lowlands from the forest steppe southwards up to the Gobi desert (Hilbig, 1995).

Blažková (1985) and Hilbig and Knapp (1983) studied stands of U. pumila in the forest steppe and northern steppe zones and concluded that the Siberian elm grows in a wide range of habitats including steep slopes, but also floodplains. Based on phytogeographical evidence, Knapp (1989) inferred that deciduous forests with U. pumila were once much more common in today's grass steppe zone. He pointed to human land use as the main factor responsible for their disappearance. Notes on relict elm trees in the northern Chinese grasslands also indicate the potentially larger importance of the species in the steppes of Central Asia (e.g. Chu et al., 2006, Katoh et al., 1998, Jiang et al., 1999).

The Siberian elm is the only tree species in the Gobi desert of south-eastern Mongolia. Stands there are heavily exploited and usually show no recruitment, as was already noted in the early 20th century (Chapman-Andrews, 1927). Lindeman (1981), who worked specifically on elm stands in southern Mongolia, hypothesised that the lack of recruitment may be related to intensive grazing. All conclusions drawn during the mid-1980s indeed suggest that current stands of U. pumila are mere remnants of potentially much more extensive forests (Hilbig, 1987).

The climate in the Gobi is characterised by very dry winters. Precipitation falls mainly in summer, but even then it is usually below 150 mm. Rains are unreliable and vary strongly from year to year (von Wehrden and Wesche, 2007a). Plant reproduction is equally irregular, and many species do not even flower in years of drought (Wesche et al., 2010). Even where seeds are produced, germination faces a number of constraints. Recent studies on U. pumila demonstrated that recruitment from seeds is possible in the northern Mongolian forest steppe zone (Dulamsuren et al., 2009b), where water availability is much better than in the south (Fig. 1a). No field studies on recruitment have, however, been conducted with respect to the drier conditions of the Mongolian Gobi. Laboratory experiments demonstrated that seed germination of perennial plants of the Mongolian desert steppes (including U. pumila ) needs high temperature, but seeds germinate without any apparent sign of dormancy if moisture and warmth are sufficient (reviewed in Wesche et al., 2006c). In the Gobi, U. pumila is usually found in beds of temporary rivers (Fig. 1b) with irregular water supply and high evaporation. Thus, imbibed seeds should tolerate cycles of drying and wetting, as well as potentially high levels of osmotic stress.

Woody perennials rarely show sexual recruitment in the Gobi, and many populations survive by clonal persistence (Gunin et al., 2003, Wesche et al., 2005b). Clonal growth may last for centuries or even millennia, allowing plants to survive unfavourable climatic periods. In Central Asia, the most prominent examples are stands of Populus euphratica, which form clonal structures covering more than 120 ha in the Taklamakan (Vonlanthen et al., 2010). The Siberian elm and the related U. minor are known to form root suckers and thus at least also have the potential for clonal growth (Meusel et al., 1965). Prolonged clonal growth usually leads to pronounced genetic structuring and is risky, as it may result in loss of gene diversity (Honnay and Bossyut, 2005) with detrimental effects for reproduction.

Stands of U. pumila are of interest for a range of ecological questions. One topic is the potential landscape structure of the Gobi with its implication on vegetation and climate history; another fundamental question refers to tree growth under harsh conditions. Elms could be used for restoration in landscapes where trees and taller shrubs are otherwise scarce (Katoh et al., 1998, Shi et al., 2004). They provide important breeding sites and shelter for a number of bird species (e.g. Aegypius monachus, Falco amurensis, Falco subbuteo, Falco tinnunculus, and Milvus migrans), and offer shade for threatened mammals such as gazella and Asiatic wild ass. In the present paper, we therefore provide an updated map of the global natural distribution range, and give data on regional occurrences of U. pumila in the Gobi of southern and south-eastern Mongolia. This serves as background information to specifically address the following questions:

  • How are populations of U. pumila structured in the Gobi desert, and is there evidence of less successful recruitment than in the more favourable climates of northern Mongolia?

  • Are U. pumila trees capable of flowering under current climatic conditions and is germination/establishment possible in the field?

  • Does sexual or asexual/clonal recruitment prevail?

    The latter aspect was studied using RAPD fingerprinting, which also allowed us to assess genetic structure:

  • Are there differences in gene diversity between stands of northern and southern Mongolia and do the Gobi populations show a more pronounced genetic structure and evidence of fragmentation effects?

Section snippets

Ecology, growth and distribution of Ulmus pumila

Ulmus pumila L. (Ulmaceae) belongs to the same section (Ulmus) as the European U. glabra Huds. em. Moss and U. minor Mill. em. Richens, but has smaller leaves at 1–3 cm (max 8 cm). Siberian elms need reliable groundwater supplies as they hardly reduce transpiration under periods of high radiation and they neither tolerate stagnant water nor high salt contents (Blažková, 1985, Lindeman et al., 1994). In Central Asia, flowering occurs in May, fruits are shed in June and seeds are usually

Structure of elm stands

Ulmus pumila grows as a single-stemmed tree, but multi-stemmed trees are also common – up to 50% of all individuals had several stems (no apparent latitudinal trend). Shrub-like forms were also widespread. A high fraction of trees in the Gobi produced resprouts at their stem bases (20–64%), and root suckers also commonly formed (0–37%). Resprouts and suckers were slightly less common in the two northern populations (0–20 and 0–10% respectively).

The size class distribution of the Borzongiyn Gobi

Stand structure and sexual reproduction

Wood coring revealed that Siberian elms can become considerably older than 200 years, which is in line with earlier reports (Lindeman, 1981, Lindeman et al., 1994). Indeed, U. pumila trees seem to get much larger in the Gobi compared to the moister parts of Mongolia. Lack of nitrogen has been inferred as one of the main constraints in the Gobi (Lindeman, 1981), as this in combination with the aridity seems to reduce tree growth, but also fungal diseases. Nevertheless, bacterial wetwood, which

Conclusions

Our data show that elms occur at dozens of sites in the Gobi where potentially suitable habitats such as drainage lines are commonplace. Elm woodlands, however, are actually rare. Mature trees are apparently connected to the groundwater, and thus flower even in a relatively dry climate. Seeds are usually abundantly produced and will germinate if sufficient water has flown together in the riverbeds and ravines that elms usually occupy. Such suitable climate conditions by no means occur every

Acknowledgements

We thank A. and M. Stubbe for pointing us to some of the most extensive elm woodlands in southern Mongolia. Work in Mongolia would not have been possible without the support of our Mongolian colleagues B. Oyuntsetseg and U. Tuvshin. Katja Wittig, Katrin Ronnenberg, Heike Zimmermann and Michael Beckmann helped with data collection in the field and/or the laboratory. Frank Schlütz gave valuable comments on pollen dispersal. Danny McCluskey polished our English. Our work in the Gobi was financed

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