Karsten Wesche

Global assessment of the non-equilibrium concept in rangelands

The non-equilibrium concept of rangeland dynamics predicts that the potential for grazing-induced degradation is low in rangelands with relatively variable precipitation. To date, evidence in support of the non-equilibrium concept has been inconsistent. Using a standardized protocol, including a newly developed global map of rainfall variability, we reviewed the incidence of degradation in relation to rainfall variability across 58 published studies. We distinguished between (1) zonal degradation (i.e., degradation independent of water and key resources), (2) degradation in the presence of key resources, and (3) degradation in the presence of water. For studies not affected by proximity to permanent water or key resources, we found strong support for the non-equilibrium concept for rangelands. Zonal degradation was absent at CV (coefficient of variation) values above 33%, which has been proposed as a critical threshold. Grazing degradation was almost entirely restricted to areas with relatively stable annual precipitation as expressed by a low CV, or to rangelands with key resources or water points nearby. To better understand rangeland dynamics, we recommend that future studies use globally comparable measures of degradation and rainfall variability. Our work underlines that rangelands with relatively stable rainfall patterns, and those with access to water or key resources, are potentially vulnerable to degradation. Grazing management in such areas should incorporate strategic rest periods. Such rest periods effectively mimic natural fluctuations in herbivore populations, which are a defining characteristic of non-degraded rangelands occurring under highly variable precipitation regimes.

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.

Effects of altitude and livestock on the regeneration of two tree line forming Polylepis species in Ecuador

Regeneration is known to be limited at many temperate tree lines, but very little data is available on the impacts of altitude and anthropogenic disturbance on regeneration patterns along tropical tree lines. The study focused on the reproductive traits of two Polylepis species in the Páramo de Papallacta in Ecuador along an altitudinal gradient, and involved different intensities of cattle trampling within subsequent altitudinal ranges. We analyzed flowering, fruit set, seed viability, germination, and seedling establishment as well as stand structure of Polylepis incana and P. pauta. The numbers of P. incana inflorescences and seedlings per m² showed a marginally significant decrease with increasing altitude. Mean tree height was significantly lower at higher altitudes, while stem number increased. The number of P. pauta inflorescences also decreased significantly upslope. In both forest types, trampling was found to have a positive impact on seedling abundance, presumably due to the removal of the litter layer. Thus, there was no evidence of negative effects of moderate cattle grazing on both tree line species. However, sapling establishment was minimal inside the forest stands at all altitudes and grazing levels, and we consequently observed a low proportion of narrow stems within all investigated forests. Our results show that, along with vegetative growth limitations of adult trees, important regeneration traits such as seedling and inflorescence numbers are also influenced by altitude, which might contribute to the formation of the upper tree line. Nevertheless, recruitment in the forest interior was low overall indicating that further factors, such as light conditions, affect regeneration of the studied species.

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.

Low genetic variability and strong differentiation among isolated populations of the rare steppe grass Stipa capillata L. in Central Europe

Stipa capillata L. (Poaceae) is a rare grassland species in Central Europe that is thought to have once been widespread in post-glacial times. Such relict species are expected to show low genetic diversity within populations and high genetic differentiation between populations due to bottlenecks, long-term isolation and ongoing habitat fragmentation. These patterns should be particularly pronounced in selfing species. We analysed patterns of random amplified polymorphic DNA (RAPD) variation in the facultatively cleistogamous S. capillata to examine whether genetic diversity is associated with population size, and to draw initial conclusions on the migration history of this species in Central Europe. We analysed 31 S. capillata populations distributed in northeastern, central and western Germany, Switzerland and Slovakia. Estimates of genetic diversity at the population level were low and not related to population size. Among all populations, extraordinarily high levels of genetic differentiation (amova: phi(ST) = 0.86; Bayesian analysis: theta(B) = 0.758) and isolation-by-distance were detected. Hierarchical amova indicated that most of the variability was partitioned among geographic regions (59%), or among populations between regions when the genetically distinct Slovakian populations were excluded. These findings are supported by results of a multivariate ordination analysis. We also found two different groups in an UPGMA cluster analysis: one that contained the populations from Slovakia, and the other that combined the populations from Germany and Switzerland. Our findings imply that S. capillata is indeed a relict species that experienced strong bottlenecks in Central Europe, enhanced by isolation and selfing. Most likely, populations in Slovakia were not the main genetic source for the post-glacial colonization of Central Europe.

Relationships between Population Size, Genetic Diversity and Fitness Components in the Rare Plant Dictamnus albus in Central Germany

An increasing number of studies support the hypothesis that smaller populations face a higher risk of extinction, and declining population sizes are therefore one of the focal points in plant conservation. In small populations, loss of genetic diversity is often related to reduced reproductive fitness. For the rare Dictamnus albus in Central Germany, an earlier study had already confirmed a significant correlation between population size and genetic diversity. In order to assess whether these variables correlate with fitness components, plant height; flower, fruit and seed production; and germination were studied in a total of 11 populations of different size. In the seven populations that were sampled over two consecutive years, differences among populations and among years were tested using a Two-Way ANOVA. Co-linearity among variables was assessed using principal component analysis (PCA), followed by calculating correlations between ordination axes and both population size and genetic diversity. Plant height and flower number were uncorrelated to the other variables and, together with germination, did not show any correlation to either population size or genetic diversity. However, both size and genetic diversity of populations correlated significantly with other PCA axes that reflected reproductive components such as fruit number, seed number, seed fruit ratio, and seed mass. Our results support the idea that reproduction is hampered in small populations and raise concerns over the loss of genetic diversity in D. albus.

Germination of fresh and frost-treated seeds from dry Central Asian steppes

We tested the germination of fresh and frost-treated seeds of 26 species of southern Mongolian mountain and desert steppes, covering the major growth forms of woody and herbaceous perennials and short-lived species in the region. In the field, germination depends on rains that are largely restricted to the summer months between June and August. Thus, germination tests were performed at alternating conditions of 10°C in darkness and 20°C in light (12 h/12 h), which correspond to temperatures at the study site in early and late summer. Seeds of both woody and herbaceous perennials germinated well under the chosen conditions and apparently did not require stratification or scarification. In contrast, germination of annual species was mostly below 30%, while seed viability was equally high in all three growth forms. Winter conditions, simulated by freezing dried seeds at −18°C, hardly changed seed germination in the perennial species, but several short-lived species responded with increased germination. Short-lived species are not abundant in the real vegetation, which is governed by perennials. Thus, we conclude that the important species in Mongolian mountain steppes germinate readily without a dormancy-breaking treatment. A review of the available literature revealed that a complete lack of dormancy, or presence of only conditional dormancy, is also widely described for other species of Central Asian deserts and steppes, which is in contrast to data from North American prairies.

The Palaearctic steppe biome: a new synthesis

The Palaearctic steppes range from the Mediterranean basin towards China, forming one of the largest continuous terrestrial biomes. The literature on steppe ecology and conservation is vast but scattered and often not available in English. We provide a review of some key topics based on a new definition of steppes, which includes also Mediterranean steppes and alpine rangelands of the Asian Highlands. Revisiting the terrestrial ecoregions of the world, we estimate that the Palaearctic steppe biome extends over ca. 10.5 million km2. Major chorological regions differ in their macroclimatic niche with a clear distinction between Middle Asia with its winter precipitation and the Central Asian summer-rain regions of the Mongolian plateau and of Tibet. Steppe soils store large amounts of carbon, yet the sequestration potential is debated and depends on land use. Major physiognomic-ecological steppe types include forest-, typical-, desert-, and alpine-steppe, which vary in the importance of grasses, mainly C3 species. The steppes host a specialised fauna, and Middle Asia, Tibet, and especially Mongolia, have large herds of migrating ungulates. The share of pristine and protected sites is low in the steppe regions, with conversion into croplands being the most important land use impact in Europe, Middle Asia, and the Mediterranean, while grazing has a severe impact in some parts of Mongolia and Tibet. There are major gaps in our knowledge on: (1) the effects of climate change on the crucial seasonal patterns; (2) the role of steppe soils in the global carbon budget; and (3) the ecology and distribution of most animal groups except vertebrates.

Recruitment of trees at tropical alpine treelines: Erica in Africa versus Polylepis in South America

Background: Constraints on tree regeneration have been described from several temperate treeline sites, however data from tropical mountains are few. Aim: We test the hypothesis that high altitude treelines are caused by constraints on tree regeneration. We report the results of recent studies on climatic variables and on the reproduction of Erica spp. in east Africa, and compare those with similar studies on treeline stands of Polylepis spp. in Ecuador and Bolivia. Methods: We established altitudinal and horizontal transects comprising sampling plots from forests to surrounding grasslands. Data on microclimate, vegetation structure and reproductive activity were collected in the field, and supplemented by tests on germination requirements in the field in permanent exclosures and in the laboratory. Results: Our measurements demonstrated that the number and severity of frost events and potential water stress were higher near the soil surface (0.5 m) than in free air (2.0 m). Seeds of Erica trimera did not germinate, as the necessary climatic requirements (high diurnal temperature range, but no frost) were rarely fulfilled in the field. Most studied species were capable of clonal growth, and the relative importance of vegetative persistence tended to increase with altitude. These effects were independent of the intensity of land use. Conclusions: The harsh microclimate near the soil surface at tropical treelines may be one reason why recruitment from seed is limited for treeline-forming species. This is important for restoration or conservation schemes as well as predictions of treeline dynamics under global change.

Impact of sowing, canopy cover and litter on seedling dynamics of two Polylepis species at upper tree lines in central Ecuador

Failure of reproduction is hypothesized as being a main reason for the formation of upper tree lines but there is, as yet, little supportive evidence of such. This study investigates the effects of experimental sowing, canopy cover and litter depth on seedling emergence and survival of Polylepis incana and Polylepis pauta at the upper distribution limit of the species in the Paramo de Papallacta, central Ecuador. We established 18 study plots located in the interior, at the edge and at the exterior of closed forests at the upper tree line and analysed seedling dynamics for 1 y following experimental sowing with four different litter treatments. For both species, seedling numbers were significantly higher in the forest interior than outside, and higher in the treatments with the litter layer removed. In P. pauta, sowing significantly increased seedling number, which was more pronounced within and at the edge of the forest than outside. In contrast, there was no impact of sowing on seedling emergence in P. incana. First-year seedling survival and mean growth rate per month were low in both species. Log-linear models did not reveal significant interactions between survival and canopy cover or litter removal. Our data show that seedling recruitment is limited at the upper tree line -presumably due to extreme climatic conditions - which indicates the importance of constraints in sexual regeneration for tree-line formation in central Ecuador.