Att restaurera forna tiders beståndsstruktur : ett exempel från Jämtgaveln

Sammanfattning: In Sweden today so-called "prescribed nature conservation burning" is performed due to the change in silvicultural law, certification and an increased knowledge regarding the field of fire in the forest landscape. The aim is to re-create structures and qualities made by the fire regimes of the past, though on a much smaller geographical scale. The structures and qualities to be re-created are for example broad leaf stands, trees with large diameter, heterogeneous stands regarding variables such as age and diameter. If we increase our knowledge of the constitution of forests characterized by fire, then we will be able to set goals for what achievements to reach both on landscape scale and stand scale. The aim of this paper is to try to describe how a stand from the past was composed regarding mainly the spatial arrangement and compare it with the stand of today in the same place. This to be done by using relict material, that is stumps and dead standing trees. The study only concerns the stand level, not the landscape level. The stand from the past - henceforth called the relict stand - is from an approximate time in the past. This time can be fixed to somewhere between the last forest fire and the large-scale cuttings which most likely took place during the period 1850-1870 (Kardell 1977, Linder 1988). This relict stand is reconstructed and compared to the current stand - henceforth called the current stand. Since this study is performed on a stand which was selectively cut before a prescribed burning for nature conservation purposes, the stand remaining after the cutting is also described. It is described with the same variables for the relict and as for the actual stand. The purpose of the cutting was not solely aimed to fulfill the nature conservation goal; the cut timber was a part of the payment from the county board to the forest company for the creation of the nature reserve. Jämtgaveln is situated in the province of Medelpad and in the county of Västernorrland, just on the border of the province and county of Jämtland. The area is in the northern part of the muncipality of Ånge, about 15 kilometers north of Borgsjö village (figure 3). The study area covers 22 hectares and is situated in the central part of the Jämtgaveln nature reserve, some kilometers south west of the lake Värsjön and north of the small lakes called Bodmyrtjärnarna. The Jämtgaveln nature reserve - created in 1993 - covers 3 000 hectares of which 2 000 hectares is are productive forest land (that is, capable of producing more than one cubic meter of wood per year). Most areas in Jämtgaveln have a quite high frequency of forest fires throughout history. The period between forest fires have been 60 to 80 years at most (Linder 1988). Inspite of its peripheral location Jämtgaveln has been exploited for its timber resources. The lack of living trees with fire scars shows that the recent generation of pine trees have been cut. In some parts of Jämtgaveln the selective cuttings of the 1870's and the thinnnings of 1930's and 1940's have been very carefully performed or not performed at all. The absence of larger silvicultural actions during the recent 40 to 50 years have resulted in several stands in Jämtgaveln regaining some of their original structures - structures like a virgin forest (Linder 1988). Study of the fire history and the age structure of the current stand have been performed over the whole study area. Study of the stand structure is based on data within four random plots of 0,1 hectares each, that is a diameter of 17,84 meters. The years of forest fire occurences where established by using dendrochronological methods on living tree, as well as on stumps and dead standing trees from the relict forest generation. Establishing the age of the current stand was done by random samples over the whole study area. The study of stand structure is also based on the data from four random 0,1 hectare plots. The variables registrered where their position within the plot, diameter at two decimeters above the stump for all trees with a diameter of more than 6 cm including bark and tree species only for living trees. Other observations such as coal, marks from axes, decomposition etc where also taken. The stumps from the relic stand are assumed to be Scotch pine trees since stumps from Norwegian spruce would have decomposed after such a long time space. The most obvious difference one can see is the relation between number of stems and basal area when comparing the relic stand to the current stand left after the selective cutting (figure 10). The number of stems is the same for both stands but the grundyta is four or five times larger for the stand. This is because of the larger share of large (more than 45 cm) trees in the relic stands, seen in the diagram over diameter distribution (figure 14). Trees with a diameter of more than 45 cm are rare in the current present-day stand. A large portion of trees in the higher diameter classes were cut since there was a need to gain capital for the establishment of the nature reserve. Unfortunately quite a few of the trees that were not cut have blown down after the cutting (figure 7). The conclusion that the relic stand would have an aggregated distribution, according to the hypothesis from the start, is not possible to determine when looking at the results of the kNN-analysis (tabel 2). The kNN-value is the same for the current stand left after the cuttings. Conclusions about aggregation should not be drawn when looking at the pine trees left after cutting since the number of trees are too few. A visual interpretation of the plot maps (figure 16 and 17) give the same impression as the kNN-analysis. By looking at the plot maps one can also state that what was earlier mentioned about the higher share of larger diameter trees in the relic stand compared to the current stand seems reasonable. The hypothesis of this study is that the share of trees with a large diameter is high in a fire regimed stand and that these larger trees are grouped together, aggregated. This would mean having a wider diameter distribution in the relic stand compared to the current stand, a larger share of large trees and that these larger trees would not be in an even formation. The results of this study confirms the assumption of a larger share of large trees and a wider diameter distribution but does not support the assumption of an aggregation of larger trees in the relic stand. The origin of this study was the question whether it would be possible to imitate the fire regimes of the past - by actively using prescribed burning, maybe in combination with selective cuttings. Though the later defined aim of this study was not to answer this very wide issue, but might there be something in the results that can be of use? There is a lack of large trees of high age in the forest landscape of today (Linder &Östlund 1992), that is trees that have survived one or several fires. When performing a prescribed burning for nature conservation reasons it is of great importance to leave many of the larger trees standing if one choses to cut selectively before burning. The losses in these diameter classes can be large both from the actual burning and afterwards when exposed to strong winds (Jonsson 1997). When it comes to aggregation the aim of leaving large trees and trees clustered in areas which wont be as strongly burned ore not burned at all, such as wet areas, also conclude reaching an aggregation though it was not the main goal. The main goal should be a sufficient survival of standing surviving trees either already large or with the potential to grow large and old over the coming decades and centuries.