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Forest and other ecosystems and plant hosts in EuropeSabine WerresThe following summarizes the influence of some important characteristics of natural or nearly natural ecosystems in Europe on Phytophthora infection and potential spread. Climatic conditionsClimatic conditions greatly influence the biodiversity of an ecosystem, including the range of pathogens. Those Phytophthora species that do not tolerate high temperatures will not establish easily in subtropical zones. P. ramorum (like all other Phytophthora species) is very well adapted to water and needs free water to develop sporangia and zoospores. Because P. ramorum can infect plants both by air and soil, air humidity and soil humidity are of great importance for infection and spread within a tree stand. For example, high air humidity enables P. ramorum to develop sporangia and zoospores on wet leaf surfaces. So, ecosystems adapted to high amounts of precipitation, with rainfall during all seasons or many foggy days per year, offer good conditions for infection of the upper plant parts. On the other hand, ecosystems with periods of high water and with soil conditions that favor stagnant moisture offer excellent conditions for infection from the soil. Water is important for infection and for the spread of Phytophthora species. Rain and fog can carry sporangia and zoospores not only within a tree stand but also to other areas. Furthermore, rain splash brings propagules from the soil surface to the upper parts of the plant. Rivers and other running water systems make up another very important transport system. For example, in Europe the alder Phytophthora species along the river banks and within a river system has been distributed mainly by running water. A similar risk can be expected with P. ramorum. Are the climatic conditions in Europe sufficient for P. ramorum to establish inside and outside nurseries? Inside a nursery, the temperature is perhaps the major limiting factor, because irrigation in the nurseries influences the air and soil humidity. Outside the nursery, both temperature and humidity are of importance. Up to now, P. ramorum has been detected mainly in western and central Europe. Most of this area belongs to the cool-temperature zones with maritime influences in the west and increasing continental climate to the east. As far east as the Ural Mountains, woodland climates dominate with precipitation and moderate temperatures in winter and summer. In the former Soviet Union, steppe climates occur, with little or no precipitation and low temperatures in winter and main of the precipitation and moderate temperatures in summer. Steppe climates also are characteristic of the Anatolian Plateau in Turkey. P. ramorum has been detected mainly on nursery plants but never in forests or other wildland ecosystems. The occurrence of P. ramorum in nursery plants in west and central Europe indicates that the temperatures in winter and in summer in these areas are at least tolerable for P. ramorum. But can P. ramorum tolerate the dry-summer climates with humid winters that are characteristic of the Mediterranean area and of most parts of central and south Spain and Portugal? And can P. ramorum tolerate the oceanic and continental boreal climates in northern Europe with their low winter temperatures? Range of Plant Genera and SpeciesAnother very important factor in Phytophthora infection and spread is the range of plant genera and species within an ecosystem. The range of plants is mainly dominated by climatic and edaphic factors but can also be the result of human influence. Due to the high variations in climatic and edaphic conditions within Europe and sometimes within a single country, there are many different ecosystems, forest populations, and host plants. Predominantly needleleaf evergreen systems can be found mainly in northern Europe, in some regions in central Europe, and very rarely in southern Europe. Predominantly mixed forests are characteristic of central Europe. Predominantly broadleaf forests can be found in nearly all European countries, but predominantly broadleaf evergreen forests occur only in southern Spain. Furthermore, not only are there different categories of forests and of other ecosystems in Europe due to climatic and edaphic factors there is also a wide range of plant families and species within a single ecosystem. For example, Fagus sylvatica, Quercus robur and Q. petraea are the most important broadleaf forest tree species in central Europe. On the other hand, Quercus ilex (Holm oak) is found only in southern Europe (mainly in west Mediterranean countries). The distribution boundary of Holm oak is often used to delineate the Mediterranean region. For another example, Rhododendron species prefer moderate temperatures in summer and winter, not too intense light during the summer, and a high relative humidity and rain all through the year. In addition, Rhododendron species usually like peat soil, but not soil with a high pH and high amounts of calcium carbonate (there are calcium-tolerant Rhododendron species available now, but not in natural stands). Wildland Rhododendron species can be found in different European countries, but are very often limited to regions with a favorable climate and soil. For example, in northern Europe, R. laponicum can be found; in central Europe R. ferrugineum and R. hirsutum occur in the alpine region, while R. tomentosum prefers moorland regions. In southeastern Europe, R. ponticum grows in some areas. First infection studies under controlled conditions with European woody plant species indicate that perhaps many more plant genera and species in the different European ecosystems can be infected by P. ramorum. Most of the European ecosystems are influenced by humans. Due to what we've learned about good forest practice over the centuries and how our ideologies have varied, forest management has changed. Monocultures had once been favored because of industrial needs and related shortages of usable wood, monocultures of Pinus sylvestris and Picea abies were planted as fast growing tree species during the 19th century in many forest areas of central Europe. Today, we know well that Phytophthora species can cause great damage in monocultures. One example is the ink disease of chestnuts (Castanea sativa) in southern Europe caused by P. cinnamomi. Today, most of the European countries favor sustainable forest management according to the guidelines set by the Helsinki declaration, which was approved at the European Ministerial Conference on the Protection of Forests in Helsinki in June 1993. One of the criteria of sustainable forest management listed in the declaration is the maintenance, conservation, and appropriate enhancement of biological diversity in forest ecosystems with mixed stands of broadleaf and conifer trees according to the natural location and the condition of soil, nutrition and water supply. Another important influence on Phytophthora infection and spread to consider is how a plant species conforms genetically to the soil and environmental conditions. Plants growing in their natural (autochthon) habitat will be more resistant to changing environmental conditions, maybe including attacks of harmful organisms, than those growing in other locations: e.g., Norway spruce from coastal regions, which have widely spreading branches, will be damaged if grown in mountain areas with heavy snow. In Europe there have existed regulations (RL 1999/105/EG) concerning the trade of forest tree propagation material for many years. In Germany, for example, regulations clearly state which plant material from which harvest stand is allowed to be planted in which region for each forest species, but not for woody ornamentals or for tree species used as ornamentals. In some European countries, the interest in autochthon woody ornamentals has recently increased, and more and more nurseries now offer autochthon shrubs and trees. RenaturationDuring the last few years, many projects have been established for renaturation of dying ecosystems. In some cases renaturation presupposes flooding, as in former moorlands and swamp forests. The consequence is an increase in soil moisture, which is very favourable for Phytophthora. If the plant species growing in these areas are susceptible towards P. ramorum, renaturation would increase infection and disease development as well as the spread of the pathogen in these ecosystems. Pollution impactsWe still do not know much how acidification and eutrophication from pollution influence P. ramorum infection and disease development in Europe. But from the literature, it is known that high amounts of nitrogen fertilization favor susceptibility of Rhododendron leaves toward Phytophthora species, and that a decrease in soil pH may also favor P. ramorum. References
http://www.minconf-forests.net/ http://www.saveamericasforests.org/europages/legalframework.htm
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