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CER researchers investigate the coexistence of forests and grasslands in Eurasia as part of an international collaboration


Together with researchers from Austria and the US a team from the Vegetation Ecology Research Group of the ELKH Centre for Ecological Research (CER) investigated the factors responsible for the formation of forest-steppes. A significant part of Hungary's natural vegetation is forest-steppe, a biome that is extremely rare to the west of the country and which is only found in a few places in Austria and the Czech Republic. Ecologists have long been engaged with the question of why Hungary has neither closed-canopy forests nor grasslands, but instead has a mixture of the two. The international research group sought an explanation for the cause of this phenomenon. The publication summarizing the results was published in the prestigious scientific journal Biological Review.  

The belt of forest-steppes stretching from Central Europe to the far-eastern part of Russia forms a transitional zone between closed-canopy deciduous forests and open grassy steppes. Viewed from above, a forest-steppe resembles a giant mosaic made up of patches of forest of different sizes mixed with various grasslands. A significant part of Hungary's original vegetation is forest-steppe, a biome that is extremely rare to the west of the country and which is only found in a few places in Austria and the Czech Republic. Ecologists have long been engaged with the question of what factors are responsible for the formation of forest-steppes. Why is it that Hungary has neither closed-canopy forests nor grasslands, but instead has a mixture of the two? An international group of Hungarian, American, and Austrian researchers have attempted to provide a detailed answer to this question for the first time.  

In order to understand the process by which forest-steppes are created experts started with an extremely simplified model according to which only the average climate conditions, among the many environmental factors, were taken into consideration. In mild climate conditions, where there is abundant rainfall and temperatures do not reach extremes – such as in a significant part of western and northern Europe – forests constitute the natural vegetation. Moving towards the interior of the Eurasian continent the amount of precipitation gradually decreases and fluctuations in annual temperatures increase. These are unfavorable conditions for forests and beyond a critical point they preclude their existence, making grasslands predominate. Taking the average climate into account therefore makes it understandable why the forest changes to grassland as one moves towards the interior of the continent. However, it does not yet explain the development of a biome consisting of a mosaic of the two.  

This is why the next step of the research considered not only the average climate, but also its variability. Unusually wet years or decades favor the spread of woody vegetation and solitary trees or even small groups of trees can become established within grasslands. On the other hand, drier periods are more beneficial for grasslands, meaning that as trees dry out they can be replaced by herbaceous vegetation in some areas. As a result of the fluctuations in climate, it is sometimes the forests that are favored and sometimes the grasslands, but neither of them can sustainably prevail over the other, so a mosaic consisting of a mixture of the two is formed. 

The reality is, of course, more complicated than this: when examining the formation of forest-steppes, it is important to take into account topographical factors and soil conditions in addition to the climate. North-facing hillsides and mountains are always cooler and more humid, while south-facing slopes receive more sunlight and are warmer and drier. These factors truly come into their own where forests and grasslands are close to the limit of their distribution: that is, in the forest-steppe zone. Northern slopes in Hungary are usually covered with forests, while southern ones are dominated by grasslands, so that when looking at the whole landscape one sees a mosaic being formed. The mosaic character is also strengthened by the fact that certain soil types favor forests and others favor grasslands. The result is the formation of a forest-steppe.  

The proportion of forests and grasslands is also significantly influenced by both fires and grazing animals. While fires can also occur without human intervention, for example as a result of lightning, humans have been using fire to modify their environment for a long time. A major fire can destroy both the herbaceous plants that make up grasslands and the trees that make up forests. A significant difference, however, is the fact that steppes take a shorter time to restore themselves, as the herbaceous vegetation recovers quickly, in as little as even a few months. Due to the much slower growth of trees, on the other hand, forests can take decades or even centuries to return. Fires can therefore reduce forest cover and create significant patches of grassland even where climatic conditions would otherwise allow the creation of forests.  

The effect of herbivores is also of great importance. Grasses are well adapted to grazing and re-sprout quickly. In the case of woody plants, however, grazing is often fatal: if animals graze on a given seedling several times, it may die, or, in a less drastic case, its growth will slow preventing it from becoming a mature tree. Long ago, the Eurasian forest-steppes were grazed by great numbers of wild horses, wild donkeys, bison, and antelopes. Most of these animals have since been eradicated or driven close to extinction by humanity. Their place has partially been taken over by livestock, such as sheep, goats, and cattle. Although grazing animals can significantly alter the forest/grassland ratio anywhere, their role is particularly important in the relatively rainy northern and western regions of the forest-steppes, including in Hungary. 

The feedback loops that stabilize the existing mosaic pattern also play a role in the long-term survival of forest-steppes. Grass patches, for example, actively resist the invasion of woody plants, thus acting to prevent grasslands from turning into forests over time. Forests, for their part, alter their own environment, such as the microclimate and the soil, in such a way as to make it favor the survival of trees, while hindering the spread of grasslands.  

While the above factors ensured the survival of the forest-steppes for thousands of years, unfortunately most of these ecosystems have since been destroyed. Most of them have fallen victim to the expansion of agriculture over the past centuries. The last remnants of Hungary's forest-steppes have survived only in small protected areas. Climate change, the spread of invasive species and failed forestation initiatives pose new challenges for forest-steppes. Protecting them first requires a change in attitude. The natural vegetation of a significant part of Hungary is made up of a mosaic of forests and grasslands. Creating completely treeless areas is just as wrong and unnatural as excessive forestation efforts are. The black pine, acacia and poplar trees, the latter being of American origin, are species that are completely alien to the Hungarian landscape and do not fit in with Hungary's other flora and fauna. Natural assets of inestimable value are lost when ancient grasslands are planted with these tree species. Grazing, on the other hand, can be considered the continuation of a natural process that has ben ongoing for thousands of years, and one with a significant tradition in our region as well. Therefore, appropriate grazing does not damage the forest-steppe, but, on the contrary, has a pronounced positive effect on landscape diversity and biodiversity. By applying our understanding of ecology it is perhaps not too late to find solutions that allow for economic utilization while also preserving the valuable features of the characteristic Hungarian forest-steppe.