Georg Miehe

Mitochondrial and chloroplast phylogeography of Picea crassifolia Kom. (Pinaceae) in the Qinghai-Tibetan Plateau and adjacent highlands

The disjunct distribution of forests in the Qinghai‐Tibetan Plateau (QTP) and adjacent Helan Shan and Daqing Shan highlands provides an excellent model to examine vegetation shifts, glacial refugia and gene flow of key species in this complex landscape region in response to past climatic oscillations and human disturbance. In this study, we examined maternally inherited mitochondrial DNA (nad1 intron b/c and nad5 intron 1) and paternally inherited chloroplast DNA (trnC‐trnD) sequence variation within a dominant forest species, Picea crassifolia Kom. We recovered nine mitotypes and two chlorotypes in a survey of 442 individuals from 32 populations sampled throughout the species’ range. Significant mitochondrial DNA population subdivision was detected (GST = 0.512; NST = 0.679), suggesting low levels of recurrent gene flow through seeds among populations and significant phylogeographical structure (NST > GST, P < 0.05). Plateau haplotypes differed in sequence from those in the adjacent highlands, suggesting a long period of allopatric fragmentation between the species in the two regions and the presence of independent refugia in each region during Quaternary glaciations. On the QTP platform, all but one of the disjunct populations surveyed were fixed for the same mitotype, while most populations at the plateau edge contained more than one haplotype with the mitotype that was fixed in plateau platform populations always present at high frequency. This distribution pattern suggests that present‐day disjunct populations on the QTP platform experienced a common recolonization history. The same phylogeographical pattern, however, was not detected for paternally inherited chloroplast DNA haplotypes. Two chlorotypes were distributed throughout the range of the species with little geographical population differentiation (GST = NST = 0.093). This provides evidence for highly efficient pollen‐mediated gene flow among isolated forest patches, both within and between the QTP and adjacent highland populations. A lack of isolation to pollen‐mediated gene flow between forests on the QTP and adjacent highlands is surprising given that the Tengger Desert has been a geographical barrier between these two regions for approximately the last 1.8 million years.

Tree endurance on the Tibetan Plateau marks the world’s highest known tree line of the Last Glacial Maximum

Because of heterogeneous topographies, high-mountain areas could harbor a significant pool of cryptic forest refugia (glacial microrefugia unrecognized by palaeodata), which, as a result of poor accessibility, have been largely overlooked. The juniper forests of the southern Tibetan Plateau, with one of the highest tree lines worldwide, are ideal for assessing the potential of high-mountain areas to harbor glacial refugia. Genetic evidence for Last Glacial Maximum (LGM) endurance of these microrefugia is presented using paternally inherited chloroplast markers. Five-hundred and ninety individuals from 102 populations of the Juniperus tibetica complex were sequenced at three polymorphic chloroplast regions. Significant interpopulation differentiation and phylogeographic structure were detected (G(ST) = 0.49, N(ST) = 0.72, N(ST) > G(ST), P < 0.01), indicating limited among-population gene flow. Of 62 haplotypes recovered, 40 were restricted to single populations. These private haplotypes and overall degrees of diversity were evenly spread among plateau and edge populations, strongly supporting the existence of LGM microrefugia throughout the present distribution range, partly well above 3500 m. These results mark the highest LGM tree lines known, illustrating the potential significance of high-mountain areas for glacial refugia. Furthermore, as the close vicinity of orographic rear-edge and leading-edge populations potentially allows gene flow, surviving populations could preserve the complete spectrum of rear-edge and leading-edge adaptations.

Highest Treeline in the Northern Hemisphere Found in Southern Tibet

Abstract Three new records of the highest treelines in the northern hemisphere are presented here, based on the definition of a “tree.” The tree species with the highest treeline in the northern hemisphere is Juniperus tibetica Kom. The highest forest stand is located at 4900 m in southeast Tibet. The highest tree stands of Juniperus indica Bertol. in the Himalaya were found in northern Bhutan at 4750 m. Information from such observations is decisive for the development of hypotheses on the limits of tree growth. We compare the present findings with findings on highest treelines in other mountain areas worldwide and draw some conclusions.

Status and dynamics of the Kobresia pygmaea ecosystem on the Tibetan plateau.

Abstract This paper provides information about the distribution, structure, and ecology of the worlds largest alpine ecosystem, the Kobresia pygmaea pastures in the southeastern Tibetan plateau. The environmental importance of these Cyperaceae mats derives from the extremely firm turf, which protects large surfaces against erosion, including the headwaters of the Huang He, Yangtze, Mekong, Salween, and Brahmaputra. The emphasis of the present article is on the climate-driven evolution and recent dynamics of these mats under the grazing impact of small mammals and livestock. Considering pedological analyses, radiocarbon datings, and results from exclosure experiments, we hypothesize that the majority of K. pygmaea mats are human-induced and replace forests, scrub, and taller grasslands. At present, the carrying capacity is increasingly exceeded, and reinforced settlement of nomads threatens this ecosystem especially in its drier part, where small mammals become strong competitors with livestock and the removal of the turf is irreversible. Examples of rehabilitation measures are given.

Impact of Grazing Livestock and Distance from Water Source on Soil Fertility in Southern Mongolia

Abstract The impact of livestock grazing on soil nutrients and vegetation parameters was studied in dry montane steppes of southern Mongolia in order to assess the risk of habitat degradation. Data were collected along transects radiating away from permanent water sources. Dung unit density counts revealed gradients of livestock activity, but utilization belts around water sources overlapped, indicating that pastoral land use affects the entire landscape. Dung unit counts corresponded to gradients in soil nutrient parameters (C, N, P), which significantly decreased with distance from the wells. However, no significant correlation was observed for plant species richness and vegetation composition with distance from water source. This indicates that soil parameters and livestock grazing exert a relatively smaller influence on the vegetation than the high inter-annual variability in precipitation. Therefore, the ecosystem at the study site was found to react in a non-equilibrium way, which suggests that the risk of degradation is low, at least insofar as plant community composition is concerned.

The Significance of Fire for Afroalpine Ericaceous Vegetation

Abstract This paper presents results from studies in 4 mountain regions in East Africa, 2 in Ethiopia (Simen Mountains, Bale Mountains), and 2 in Uganda (Rwenzori Mountains, Mount Elgon). The focus is on the ericaceous vegetation that forms the (upper) treeline ecotone at all sites. There is little evidence for climatic control of the patchy appearance of this belt in all afroalpine environments. Since traces of former fires were observed in all ranges, repeated burning is most probably responsible for the present appearance of the ericaceous vegetation in East Africa. The fires observed were almost exclusively lit by local people, who utilize the afroalpine zone for poaching, livestock grazing, and honey hunting. Although these fires are man-made and not strictly natural, they help to maintain a structurally and biologically diverse environment.

Pliocene intraspecific divergence and Plio‐Pleistocene range expansions within Picea likiangensis (Lijiang spruce), a dominant forest tree of the Qinghai‐Tibet Plateau

A knowledge of intraspecific divergence and range dynamics of dominant forest trees in response to past geological and climate change is of major importance to an understanding of their recent evolution and demography. Such knowledge is informative of how forests were affected by environmental factors in the past and may provide pointers to their response to future environmental change. However, genetic signatures of such historical events are often weak at individual loci due to large effective population sizes and long generation times of forest trees. This problem can be overcome by analysing genetic variation across multiple loci. We used this approach to examine intraspecific divergence and past range dynamics in the conifer Picea likiangensis, a dominant tree of forests occurring in eastern and southern areas of the Qinghai‐Tibet Plateau (QTP). We sequenced 13 nuclear loci, two mitochondrial DNA regions and three plastid (chloroplast) DNA regions in 177 individuals sampled from 22 natural populations of this species, and tested the hypothesis that its evolutionary history was markedly affected by Pliocene QTP uplifts and Quaternary climatic oscillations. Consistent with the taxonomic delimitation of the three morphologically divergent varieties examined, all individuals clustered into three genetic groups with intervariety admixture detected in regions of geographical overlap. Divergence between varieties was estimated to have occurred within the Pliocene and ecological niche modelling based on 20 ecological variables suggested that niche differentiation was high. Furthermore, modelling of population‐genetic data indicated that two of the varieties (var. rubescens and var. linzhiensis) expanded their population sizes after the largest Quaternary glaciation in the QTP, while expansion of the third variety (var. likiangensis) began prior to this, probably following the Pliocene QTP uplift. These findings point to the importance of geological and climatic changes during the Pliocene and Pleistocene as causes of intraspecific diversification and range shifts of dominant tree species in the QTP biodiversity hot spot region.

Vegetation patterns on Mount Everest as influenced by monsoon and föhn

The vegetation of Mt Everest is described by means of the dominant plant formations and characteristic features of biotopes. Climatic data givenin connection with weather observations show evidence that the extreme asymmetry of the altitudinal vegetation belt on the south and north slope is induced by heavy rainfall on the south slope and the desiccating effect of the Himalaya föhn in the valleys of the north slope. Biotope shift from hypsozonal distribution on the south slope to extrazonal distribution on the north slope is described, the patterns of the actual timber line are discussed in order to reconstruct the natural upper forest limit, and regressive plant successions during the last 400 years of mans impact are summarized. The dominant vegetation pattern of the alpine belt is compared with that in the European Alps. On the arid north slope alpine steppe communities occur up to 5 500 m. The highest altitudinal vegetation belt and the highest plant communities at 5 960 m are dominated by periglacial processes. The highest records of flowering plants (6 100/6 200 m) and lichens (7 400 m) are discussed in light of the present knowledge on high-altitude vegetation ecology.

On the connexion of vegetation dynamics with climatic changes in High Asia

Abstract Within the scope of research on environmental changes in High Asia indications of desiccation and thermal oscillations are described. They refer to different periods and are caused by different climatic factors. Distribution patterns of forest constituents with disjunctions in the Central and Eastern Himalayas are explained with changes in monsoonal rainfall presumably during the Holocene. In Southern Tibet the plant relicts of one of the younger Holocene phases of higher humidity (“Kobresia pygmaea age”) have been widely destroyed by the Himalaya-fohn, giving way to semiarid alpine steppe on stone pavements. Field observations of a recent intensification of gelifluction processes in the southeastern part of the Tibetan Plateau is in accordance with a cooling period between the late 1950s and the late 1970s. The obviously decreased vigour and lacking regeneration of the climax vegetation in the alpine belt becomes conceivable under the presumption that the respective cover of alpine turf is a relict of the climatic optimum which slowly but increasingly deteriorated since then. A decrease of plant vigour at climatically sensitive vegetation borders (upper treeline, drought limit of forests, transition zone between alpine Cyperaceae mats and the free gelifluction belt, transition zone between humid alpine mats and alpine steppe) gives evidence of diminishing winter precipitation during the last 30 to 40 years in the Karakorum.

Sacred Forests in Tibet: Using Geographical Information Systems for Forest Rehabilitation

Abstract The treeless desertlike environments of southern Tibet are assumed to be naturally unsuitable for forests. Yet, climatic conditions do allow for the growth of indigenous trees in Lhasa and many parts of southern Tibet, even where there is no high groundwater table or irrigation. This was discovered and proven in a Sino–German research project launched in 1997. The project made an inventory of forest relics, correlated residual tree stands with climatic data, and successfully cultivated nonirrigated indigenous junipers and cypresses. The eroded semidesert landscape of southern Tibet appears to have a huge potential for reforestation. The area with a potential for tree growth was investigated using the Geographical Information System known as GRASS (Geographical Resource Analysis Support System). Reforestation measures could meet the heavy demand for timber and firewood, help combat erosion on overgrazed slopes, and restore the degraded pastures. Grazing must be excluded on reforestation plots. Simultaneously, rangelands may regenerate after overgrazing. The optimum duration of the ungrazed period varies with altitude, humidity, soil conditions, and the degree of degradation. Successional trends observed on exclosure plots suggest that the drier the climate and the lower the initial degree of herbaceous vegetation cover, the longer the ungrazed period will be beneficial for pasture regeneration. Challenges in research and practice resulting from these preliminary results are highlighted.