Extreme weather conditions, always a stress factor for trees, can lead to vitality loss and to the development of visual injury symptoms. Due to exceptional weather conditions the record summer of 2003 affected the forests in many ways. Besides drought damage, the high ozone level also left its mark. This was noticeable in the development of distinct ozone injury symptoms on the leaves of ash, sycamore and beech trees on the northern edge of the Alps. This article deals with these weather conditions, the ozone levels and the diagnosed symptoms in this region.
|Fig. 1 - Ozone injury symptoms (red arrow) on ash, sycamore and beech leaves at the end of August on the northern edge of the Alps. a) Ash on the Kranzhorn (800m above sea level); b) Sycamore on the Laber (1000m above sea level); c) Beech on the Laber (1000m above sea level); white arrow: shading effect: typical for ozone is that symptoms are lacking on the leaf part covered by an another leaf.|
During the vegetation period of 2003 the rainfall in Bayern was well below the long term average. In many areas the combination of high temperatures and intensive radiation led to shortages in the forest water supply. Especially affected by the continuing drought were lower and middle Franken. It was assumed that the northern edge of the Alps was only affected by a slight short term reduction in the water supply. This was confirmed by data from selected precipitation stations as well as recordings from forest climate stations.
During the whole vegetation period in this region rainfall, which at times was exceptionally heavy, was repeatedly recorded. This explains why the soil along the edge of the Alps, especially at higher altitudes was adequately supplied with water until late summer and only in particular places (e.g. steep slopes and shallow floored ridges) signs of water stress or drought damage were made out. Soil samples were taken at the end of August from varying altitudes on the Laber in the Loisach valley (750 or 1060 m above sea level) and from the permanent observation plots on the Kranzhorn in the Inn valley (940 and 1100 m above sea level). With the exception of the top horizons and the samples from the locations at 750 m above sea level the soils were still perceptibly humid.
On the other hand near Freising, where at the end of September dry damage was in places recorded (Fig 2b) on ash leaves, rainfalls were rarer and in total noticeably less (only 287 mm between the beginning of March and the end of September).
During the vegetation period of 2003 in spite of a clear reduction in the emissions of precursor substances of ozone since 1990, extremely high ozone levels and frequent exceeding of critical levels were measured in Germany. This was due to the unusual weather situation with long periods of sunshine and high temperatures. Along the northern edge of the Alps ozone levels were already clearly above the yearly average at the beginning of the vegetation period at the end of April. A period with extremely high levels was registered in the first half of August. Ozone symptoms as a sign of acute injury appear on the leaves of deciduous trees from two weeks and up to 2 months after flushing during ozone rich (hourly mean values over 60 ppb) fine weather periods. Such values were already being reached or exceeded repeatedly in April and again from June to August at the meteorological observatory at Hohenpeissenberg.
Trees regulate the gas exchange with the environment through their stomata. Whilst absorbing carbon dioxide, which is necessary for growth, loss of water and uptake of noxious gases, including ozone, is inevitable. During unfavourable weather conditions the stomata stay mainly closed. High ozone levels are therefore less dangerous during drought stress. They however are inducing injury when they coincide with a good water supply. The foliage of the ash trees showed this impressively at the end of September in locations with varying water supplies near Freising (Fig 2.) Ozone symptoms (bleaching or bronzing between the leaf veins on the upper side of the leaf) were only to be found in riverside sites which were well supplied with water.
A good water supply simultaneously with high ozone levels often occurs at higher altitude on the northern edge of the Alps. This was particularly pronounced in the record summer of 2003. It can be assumed that during the vegetation period of 2003, given the temperatures and radiation favourable for growth at this altitude, an intensive leaf gas exchange took place during high ozone phases. Due to the precipitation situation this was also the case for the ozone period in the first half of August. Our analyses of beech tree- rings from the Kranzhorn confirm this. Compared to the year before, the width of the growth ring was not reduced. On the contrary: many trees showed an increased width of the increment in 2003.
Fig. 2 - Ash leaves from sites with varying water supplies near Freising. Photographed on September 28th 2003:
In the summer of 2003 assessments of ozone injury symptoms were repeatedly carried out at Kranzhorn, Laber, Herzogstand, Hohenpeissenberg and near Freising. To rule out other influencing factors some of these samples were sent to the Swiss Federal Institute for Forest, Snow and Landscape research (WSL) for differential diagnosis. At the end of August at Kranzhorn and Laber typical ozone injury symptoms, which had been previously observed and reported in the field only on the southern edge of the Alps, were already detected on ash, sycamore and beech trees (Fig. 1).
If ozone uptake exceeds the leaf's internal detoxification capacity it leads to a discolouration and dying of single or groups of leaf cells. These become visible on the leaf surface as diffused light green or yellowish or red coloured dots between the leaf veins (Fig. 1c). This discolouration, which is known as stippling, eventually turns brown or black i.e. necrotic. The areas along the leaf veins stay green the longest. A good feature for recognising ozone symptoms is the shading effect, because leaf injury caused by ozone shows a strong dependency on light. Through shading the leaf irradiation is reduced and therefore also stomatal conduction, resulting in a reduced uptake of ozone. The oxidative stress due to ozone inside the leaf is light dependant too. This results in the fact that no characteristic ozone symptoms appear in the areas of leaves overshaded by other leaves. (Fig.1c + Fig. 3)
An increase in punctual necrosis leads to a mottled bronze/silver discolouration. On the foliage of an old beech tree on a southern slope below the Hohenpeissenberg observatory clear ozone injuries at an advanced stage were already to be found at the end of September. A microscopic examination of a leaf cross section showed light dependent cell injury gradients typical for ozone. Whilst the palisade cells were extensively damaged, even partially collapsed, the underlying spongy mesophyll cells showed hardly any damage at all. Insofar as data from all examined tree species was collected from the same locations at the same time, an increase in ozone sensitivity based on characteristic symptoms could be observed in the following order: ash, sycamore and beech (Fig.1)
|Fig. 3 - Ozone symptoms with overshading effect on the leaves of a young beech tree on the northern slope of the Herzogstand (ca. 1000 m above sea level, Sept. 6th 2003). In the lower picture (b) the previously shaded leaf was placed on top.|
The above assessment of visible injury symptoms was carried out in conjunction with a long term interdisciplinary analysis of the vitality of beech forests on the northern edge of the Alps. The frequency and intensity of the symptoms, which occurred in the summer of 2003, underlines that ozone is a stress and injury factor for forests in this area which must be taken seriously. The findings show acute ozone stress in which the unusual weather situation during the vegetation period of 2003 played a crucial role. In the research area the ozone levels had already exceeded critical levels during preceding years (at times extensively) so that a chronic ozone stress can be assumed too. This leads to defence and adaptation reactions resulting in a reduced vitality and an increased intolerance to other stress factors long before the visible injury symptoms appear. New findings point increasingly to the fact that ozone stress could be a possible cause of growth depression of various beech trees in the studied area.
In the vegetation period of 2003, in most of the locations at higher altitude of the northern edge of the Alps, the weather conditions were favourable for the growth of beech trees. Effects that are after effects of the ozone air pollution on growth and tree crown condition, will probably only appear in years to come. This underlines the need for continual monitoring and further research.