«History and age of old limes (Tilia spp.) in Tallinn, Estonia Alar Läänelaid1 & Heldur Sander2 Institute of Geography, University of Tartu, ...»
History and age of old limes (Tilia spp.)
in Tallinn, Estonia
Alar Läänelaid1 & Heldur Sander2
Institute of Geography, University of Tartu, Vanemuise St. 46, Tartu 51014, Estonia
Forest Research Institute & Institute of Zoology and Botany, Estonian Agricultural University,
The historical park areas in Tallinn, Estonia contain many large lime trees (Tilia
spp.) of unknown age. Since the available historical sources on these parks offer
only fragmentary data, we decided to estimate the age of the limes using the treering method and the bark method, along with the data from written documents and maps. Many of the limes grow on former bastions and their origin is related to the history of the fortifications of the city. Twelve lime trees were cored with an increment corer and their tree rings were counted and measured. The age of the trees was estimated by using cumulative graphs of annual increment. The ages of four trees were measured using the bark method of age estimation. This method relies on the counting of bands of phloem fibres in the outer bark. The age estimations of the trees by different methods were compared with historical data on the area. Despite some inaccuracies in the age estimations due to hollow tree trunks, the age estimations by tree rings and the bark method often coincided and was supported by historical sources. The accuracy of the graphical method of age estimation, and especially of the bark method, still needs to be verified in further studies.
Key words: bark, dendrochronology, phloem layers, Tilia spp., tree age.
1 Introduction Trees in many ways are the antithesis of urbanisation. As surrogates for and fragments of nature, they are earnestly desired and yet paradoxically so gravely deficient in cities (Jim 1994). There are a number of functions that trees perform in cities (Mayer 1978).
The trees that have the greatest impact on their environment are the larger, more mature specimens. The old trees as living heritage could be equated with the historical monuments of the city, and be listed for special preservation by amending the existing Antiquities and Monuments ordinance. For privately owned selected large trees, consideration could be given to the allocation of public funds as subsidies for tree maintenance (Wicki 1988; Jim 1994; Nelson 1994).
The most valuable old trees in Tallinn are growing on the old bastions, where parks were established during the second half of 19th century. There are also old trees known to be growing near churches. These trees are important also in the European context. However, the age of the trees cannot be ascertained just by their appearance. Here an attempt will be made to assess the age of the trees by using historical sources as well as by measuring tree rings and the bark rings.
2 The study area Tallinn, the capital of the Estonian Republic, is located in the northern part of the country on the coast of the Gulf of Finland, south of Helsinki (Finland) and to the west of St. Petersburg. Tallinn is the largest Estonian city, with an administrative area of 158.3 square kilometres and about 400,000 inhabitants (in 2001).
There are several historical park areas in the city. Limes (Tilia spp.) are frequent tree species in these parks. The location of the investigated lime trees is shown on the city plan (Figure 1).
Figure 1. Former bastions and churches of Tallinn with sites of investigated lime trees.
The former bastions are generally favourable sites for tree growth, as the climatic and edaphic conditions are relatively stable there. The most serious threats to the trees on these sites are trampling, acts of vandalism, and air pollution. Nevertheless, citizens have, through the centuries, honoured trees of remarkable size. No doubt this positive attitude has played its role in the preservation of the big trees in the city to date.
3 Methods of determining the age of limes Three methods were applied for determining the age of the lime trees: the treering method, the bark method, and the historical method.
The tree-ring method The initial principle of the method is very simple - counting of tree rings in the stem of the trees. The rings are available for counting in cores extracted from the tree trunk with a special age-corer. On the premise that every tree ring corresponds to a growth year, one could ascertain the age of the tree. Pigott (1989) estimated the age of limes from their stem thickness and tree rings.
Actually there are several hindrances in estimating the age of big limes (Tilia spp.) by counting their tree rings. Firstly, limes belong to diffuse-porous tree species.
The boundaries of the tree rings of a diffuse-porous tree like lime are often hardly distinguishable even under the microscope. Secondly, an old and slow-growing lime tree can miss rings in some years (at least in some radii of the trunk) which means that the number of counted rings does not necessarily correspond to the actual age of the tree. Thirdly, as mentioned above, big lime trees are mostly hollow; they lack tree rings in the inner portion of their trunk altogether.
In these circumstances we had to elaborate a method for the assessment of the age of thick hollow trees like limes. This method is based not only on counting but also on measuring the widths of the tree rings of the lime tree. The measured ring widths are then cumulatively summarised and the cumulative sums of annual increment are represented as a line graph (Figure 2). The length of the missing part of the radius of the tree trunk (the portion of the radius in the hollow) is calculated from the girth of the trunk. The graph line of the cumulative increment can be extended back to the theoretical centre of the stem (the pith), taking into account the smooth increment tendency. The crossing of the graph line with the abscissa shows the probable onset year of the tree, i.e. the zero age at the height that the core was extracted from the tree trunk. The cores were usually taken from a height of 1.1 to 1.3 m above the base of the tree. As the limes were presumably planted, the established onset years at a height of 1.3 meters should not differ much from the planting year of seedlings of a similar height.
The bark method It has long been known that the thickness of the bark of tree species with nonabscissive bark is related to tree age (Trendelenburg & Mayer-Wegelin 1955).
Besides, the non-conducting outer part of the bark, or the rhytidome, contains layers in some tree species (Esau 1953, 1964). Fritts (1976: 68) notes: The thickness of bark is a function of the trees heredity, the vigour of the tree, and its age. In his voluminous handbook on dendroecology, Schweingruber (1996) also remarks on the growth zones in the phloem and bark of many tree species. It was established that in Central Europe 55 of 77 species exhibit distinct phloem-tree rings (Holdheide 1951, after Schweingruber 1996). However, the above-named authors still remain sceptical about using the counting of bark rings for estimating tree age.
The bark method of establishing the age of limes and oaks was developed by Mart Rohtla, Institute of Cybernetics of Tallinn Technical University (Rohtla 1998; Läänelaid et al. 2001). It is an original method of counting the fine layers of fibres of the phloem in the bark sample of a tree. This method has some disadvantages as well as advantages compared to the tree ring method. The difficulties are that firstly, lime trees can form several layers of phloem fibres during one vegetation period, especially when young. An additional fibber layer may also appear as a reaction to browsing of the tree crown in the park. Secondly, the outermost layers of the bark ribs of old limes are often weathered away and the approximate number of the lost growth layers can only be assessed by the fan-like shape of the converging pith rays in the cross-section of the bark rib.
The main advantage of the bark method over the tree ring method is that, regardless of the hollow tree trunk, its bark can be available for study almost in its entirety. This makes the bark method especially valuable for determining the age of hollow trees. Rohtla determined the age of some big lime trees in Tallinn using both the tree ring and the bark method in parallel.
History of old planted trees In the 17th century, a Dutch type bastion front was founded, which was completed at the beginning of the 18th century. The Northern War between Russia and Sweden (1700-1710) ravaged the whole country, and Tallinn became the centre of a province (gubernia) of the Russian Empire. The Tsarist government of Peter I was not much interested in fortifying Tallinn. In 1710-1721 the Great Coast Gate and Skåne bastions were strengthened. Between 1750-1790, construction works were carried out (Vilbaste 1965; Zobel 1994, 2001).
It has been noted that trees made their first appearance on the fortification plans of Tallinn in 1728 (Kenkmaa & Vilbaste 1966). The plans show that 19 or 20 trees were planted on the Skåne bastion. Zobel (2001) agrees that the trees on Skåne bastion represented on the city map in 1728 were planted during the Swedish period.
The fortification plan from 1728 shows large amounts of trees inside the city wall near churches (the Cathedral, the churches of Niguliste and Oleviste) and along the streets of the old Town. Trees near the Cathedral have obviously been planted after 1684, when the buildings on Toompea Hill perished in fire and were subsequently mostly demolished.
The best overview of the trees growing in the city centre as of the early 19th century is given by the city plan of 1825, prepared by Üprus (1965). The plan shows many private and church gardens, trees on the bastions, and one City Park (founded in 1822). 145 trees were growing on eight bastions and 67 of them on the following three former bastions: Skåne bastion - 39, Swedish bastion - 17, and Ingermanland bastion - 22 trees. We do not know if the trees originating from 1728 were still growing on Skåne bastion at that time or not.
The trees on the bastions have probably been planted during the period 1750when the bastions were restored and renovated (Kenkmaa & Vilbaste 1965).
Unfortunately, there is no data available about the planting of the trees. Trees are not depicted on the city plans of that period.
After the demilitarisation in 1857, some of the ramparts were preserved, some levelled, and some turned into green or residential areas.
According to the data of Viirok (1930), trees were still growing on only three former bastions in 1928 - the Skåne (since 1884 park of Rannamägi), Swedish (since 1862 park of Lindamägi), and Ingermanland (since 1862 park of Harjumägi) bastions, with a total of 55 old limes; 34, 10 and 11 trees growing on each, respectively.
In 2001, 23 of these limes were still obviously growing, 10, 7 and 6 trees on the respective bastions. Of course, it is difficult to guess which trees on Skåne bastion originate from 1825.
4 Results Let us explain how, using the cumulative graphic line and the growth rate of younger limes, we assessed the age of a lime tree. Here are some examples to illustrate the method.
Figure 2 serves as a good example of how the graphical method of age assessment works. Small-leafed lime (Tilia cordata L.) No. 10, next to the Cathedral, yielded a series of 146 tree rings. From the perimeter of the trunk, 342 cm, the radius of the trunk was calculated as 54.4 cm. To get the radius of the xylem or wood, the
Figure 2. Cumulative annual increment of trunk radius of common lime (Tilia cordata L.
10, growing next to the Tallinn Cathedral. Dashed lines - extrapolated growth models.
thickness of the bark was subtracted: 54.4 - 4.0 = 50.4 cm. The length of the raw sample core, or the cumulative sum of the measured tree-ring widths in it, 34.9 cm, gives us the existing portion of the radius. As the last ring, formed in 1999, located at the end of the radius of the tree trunk, we can easily find the radius of the missing inner part of the trunk: 50.4 - 34.9 = 15.5 cm. This is the distance we have to fill with tree-rings of hypothetical widths. First, assuming nearly constant annual increment of wood throughout the lifetime of the lime tree, a linear line was drawn as an extension of the cumulative curve to the x-axis (the fine dotted line A). We know that the annual increment of trees at a younger age is greater than at an older age, hence the number of wider tree-rings in the same radius of tree trunk is smaller at a greater distance from the centre of the trunk. Therefore, the linear graph obviously overestimates the age of the lime tree.
The question arises how big the annual increment of wood in limes at a younger age is. To answer to this question, we also sampled six younger lime trees in Tallinn.
These European limes (Tilia × europaea L.) grow on the same bastions and near the churches together with the older trees. They have been planted there later, between the older trees; they have no hollow in the trunk and their increment cores extended very close to the pith. Two of these younger trees grow on the Ingermanland Bastion, one on the Skåne Bastion, one on the Swedish Bastion, one near St. Johns church and one next to the Niguliste church. The cumulative growth curves of these limes look similar (Figure 3): during the first thirty to sixty years of their life radial increment is higher and then stabilizes smoothly at a lower level.
Some exceptional growth can be seen in lime No. 17, growing on Ingermanland Bastion: the increment increases again in the last decades of the life of the tree.
This feature is probably caused by changes in the light conditions of the tree.
Figure 3. Cumulative increment of limes (No.
7, 9, 17, 18, 19, and 22) at a younger age. Abscissa age of tree in years, ordinate - radius of the tree trunk.