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Norway spruce's accelerating recession
from Western Hungary: Department of Environmental Sciences, Sopron University, Sopron, Hungary
The zonal occurrence of Norway spruce starts around an altitude of 1000 m on the eastern foothills of the Alps. Mountains along the West Hungarian border nowhere reach that height, but the spontaneous presence of the species in submontane beech forests may be assumed throughout the region, in locations with a favourable microclimate. Since the turn of the century Norway spruce plantings have become common in the beech zone, and even in the low-elevation hornbeam-oak mixed forest belt, where climatic conditions are less favourable. Reasons for the extensive use of the species are partly the same as in most Central European countries: regeneration is easy, early growth is fast and an early economical return can be expected from the production of Christmas trees and poles. An additional reason is the relatively low sensitivity of the species to game damage. While the share of spruce in the national forest area does not exceed 1.5%, in forest districts along the Austrian border (Western Hungary) it is above 20%. Some of the local, 'land race' populations have been introduced into international provenance trials (Köszeg, Szentgotthárd) where they perform well above the average. This points to the value of these populations, justifying special efforts in maintaining them even if their silvicultural future is locally declining. Although off-site plantings were never thought to be maintained for more than one relatively short rotation, the mortality and damages experienced in the last years seem to indicate an irreversible trend of receding of the species even from climatically favourable areas. This fact may serve also as a model case for species migration triggered by climatic change, as predicted for the near future.
Climate and forest health problems Regarding long-term trends, the last 150 years of temperature measurements in Hungary do not reveal a pattern which can be identified as outside the normal range of fluctuation. The last 15 years, however, show a fairly stable trend of increasing annual means. For this time interval, the increase comes close to 1şC (however, the increase appears after an interim of relatively low temperature means). At the same time the frequency of hot days (maximum above 35şC) increased at a much faster pace. Their occurrence jumped from 5-6 days/year to around 25 to 30 days/year. Regarding precipitation, a downward trend is observable both in long-term and short-term datasets since the 1940s. In the last 15 years the decrease of annual rainfall in lower elevations of Western Hungary may be estimated around 50 mm (e.g. from 650 to 600 mm in Szombathely). Favoured by high temperatures and nearly snowless winters, the ever-present infestation of bark beetles (Ips typographus) expanded abruptly in the recent years. Sanitary fellings have been necessary in most years since the 1980s, but infestations appeared sporadically and locally at lower elevations, on drier sites. These centres of mortality expanded in the early 1990s, which had a serious impact on fellings: the amount of sanitary fellings surpassed very quickly the rate of planned final cuts. For example, in the Sopron forest district the mortality of Norway spruce amounted to 470% of the annual cut in 1995, up from 1990 (30%), 1991 (20%), 1992 (30%), 1993 (90%) and 1994 (170%) (Lakatos 1996). A climate-triggered migration of Norway spruce? If the present trend continues, Norway spruce may disappear from the area completely within 5 years, by the year 2002! Such catastrophic events have already struck the species in connection with severe pollution in the infamous Black Triangle (German-Czech border region); however, in this case the role of pollution is regarded as negligible. The triggering effect is the worsening of the climate conditions, a scenario predicted for the next decades for large parts of Europe. Instead of the gradually changing temperature and precipitation means, increase in mortality appears to be linked first of all with the frequency of hot days: it is the appearance of dry and hot periods which dramatically affected the susceptibility and mortality of spruces in the area. The direct connection between climate and mortality may be illustrated by the fact that damages appear earlier and more severely in spruce stands planted in the hornbeam-oak climate, and less in the cooler, semi-humid beech climate (Table 1).
Table 1. Damages connected with bark beetle infestation and mortality in Western Hungary (in percent of investigated trees, Szombathely State Forest, 1994)
Source: Bakó 1995. † 1= no damage; 2= medium damage; 3= excessive mortality.
It is also observable that higher age classes are more susceptible to the damages due to lower growth vigour. After 2-3 years of infestation, bark beetle gradation reaches a stage where climate effects do not play a significant role anymore, and vigorous populations growing on favourable sites are also attacked. The dramatic mortality increase observed with changed weather conditions raises the question: Why is the genetic buffering so weak? although most studies show that intrapopulation genetic variation of forest trees is relatively high, offering a significant range of stability and tolerance in case of condition changes. In the case of Western Hungary it has to be emphasized that the species is already existing at the fringes of its distribution area and has extended considerably beyond it. From other studies (Mátyás 1994) it is known that the reaction norm of productivity shows a very steep downward trend toward warming of the environment. Even relatively small changes in the unfavourable direction (less precipitation, higher temperatures) cause drastic responses in growth; this should apply for mortality as well. Accordingly, the observed phenomena may be interpreted as a model for the northward migration of species, following the expected climate warming.
Lessons for conservation strategy More attention should be provided for populations at the southern fringes of the species, especially at low altitudes, as they are the most exposed to expected climate instability. The change of frequency of extreme weather conditions, coupled with other sources of damage (pollution, pests and diseases) may trigger a rapid change in distribution limits. Marginal populations at the southern limits probably cannot be maintained in the long term through in situ methods and artificial means should be considered for regeneration and conservation of most valuable populations. References
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