Old Park Trees: A Highly Desirable Resource for Both History and Beetle Diversity

MATERIAL AND METHODS

Study Sites

The inventoried park is situated at Skokloster in southeastern Sweden (N 59°40’, E 17°40’), and the semi-natural, partly grazed wood of Biskops-Arnö is about 8 km (5 mi) southwest of the park. In the context of this study, the most interesting feature of the Skokloster park is an avenue of about 180 lime trees (Tilia cordata) around a baroque garden (Figure 1). The avenue was originally planted in 1684 (National Property Board 2001), and the oldest remaining trees today are probably from that time. However, trees have been continually replaced, so most of them are younger than that. The advanced age of the oldest trees is probably because the trees have been pollarded, which reduces their growth rates but prolongs their lifespan (Slotte 1993). The trees are about 7 m (23 ft) high. Most have hollows (Figure 2) in their trunks, ranging from small, newly formed ones to those so large that the trees have become chimney-shaped. The oldest trees have trunks that consist of a sector of the original circumference. All the trees are still vital and healthy enough to fulfill the purposes of the gardeners who planted them; that is, to form a pleasing avenue in the garden of the castle. Trees, or parts thereof, that are too badly deteriorated are removed. To promote biodiversity, such parts are left to decay in a particular location near the garden so that organisms (including the wood-living beetles) living in them can reproduce in situ (National Property Board 2001).

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Figure 1.

The lime tree (Tilia cordata) alley at Skokloster park, Sweden. It was originally planted in 1684 but as individual trees have become too old they have been continually replaced, which has been beneficial for the biodiversity of rare, wood-living beetles in the park.

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Figure 2.

One of the old lime trees in the Skokloster park that has a large hollow with an opening about 2 m (6.6 ft) above ground.

In other parts of the park are several large, deciduous trees, but they are not nearly as old as those along the lime avenue, so they have much less potential for hosting a diverse saproxylic fauna. However, some of these trees have started to develop hollows, and one horsechestnut (Aesculus hippocastanum) was investigated.

The site used for comparison was Biskops-Arnö, one of the richest sites for saproxylic beetles in southeast Sweden, in which 56 red-listed saproxylic beetle species have been recorded (Nilsson 2001; Jonsell, unpubl). It is an island with an area of about 2 km² (0.8 mi²) in the lake Mälaren, supporting a mixture of arable land, old deciduous forest, and conifer plantations that are about 50 years old. The deciduous forest is rich in old, large, hollow trees, primarily oaks, but with all other species distributed in the region represented. The large trees grew in a fairly open, grazed forest, so they tend to have large crowns and stem diameters (Borgegård 1994). In addition to all the large, hollow trees are several dead and decaying trunks, which provide suitable habitats for saproxylic insects. Today, most of the land with large trees on the island is filling in with younger, deciduous trees (Figure 3), and only some parts are grazed.

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Figure 3.

An old, hollow oak typical of the regrowing sections of the semi-natural, deciduous forest site Biskops-Arnö.

RESULTS

At Skokloster, 647 saproxylic beetles belonging to 67 species were caught (Table 1). In the semi-natural forest of Biskops-Arnö, the corresponding numbers were higher: 1,011 individuals representing 97 species. The number of beetles found in the lime trees in the avenue at Skokloster park (i.e., excluding those found in the horsechestnut tree) was 472, representing 57 saproxylic species, 17 of which were red-listed.

The two sites did not differ much in the frequency of red-listed species: 20 were found at Skokloster park and 24 in the semi-natural forest of Biskops-Arnö. The sum of red-listed species from both sites was 34, implying that ten red-listed species were found at both sites (Table 2).

All the red-listed species, except one found at Skokloster (Monotoma testacea) were saproxylic (Table 2). In total, 14 species—11 from Skokloster park and 12 from Biskops-Arnö’s semi-natural forest—were associated with tree hollows (Table 2). All the other types of dead wood described in Table 2 can be found in old trees, although some of them may also occur in younger trees.

DISCUSSION

The high number of red-listed saproxylic species encountered in the park at Skokloster shows that the park’s high cultural values are accompanied by high biodiversity values. The number of red-listed species was almost as high as the number found in Biskops-Arnö—a site that has long been known to be rich in saproxylic insects (Nilsson 2001; Palm 1959). This was despite the fact that the sample collected in the park (i.e., the number of saproxylic beetles trapped) was one-third smaller than the sample collected in the semi-natural forest. Thus, although the park may seem to be lacking in dead wood at first glance, it hosts a high number of red-listed saproxylic species, all of which live in its old trees.

The inventoried species are associated with a large variety of different microhabitats, some of which are found only in old trees. The manner in which trees develop old-tree microhabitats has not been studied in detail, but circumstantial observations indicate that they tend to be at least 100 years when such habitats first appear. Some of the beetles were associated with types of decaying wood, which may occur in younger trees but is more frequent in old trees. Two additional beetle species are associated with sap flow from the trunk, which may occur in younger trees but is more common in older ones. The single species associated with compost heaps (Monotoma testacea) may have been caught because it used the mold within the hollows, which is essentially a type of compost. There were no other compost habitats within at least 100 m (300 ft) of the trap. In conclusion, all the microhabitats listed in Table 2 can be found within an old tree population, but few are found in a young one.

Around Skokloster were larger areas containing old deciduous trees in previous times, as there were around most estates. Today, only small amounts of such land remain. Instead, there are numerous spruce plantations, many originating from converted deciduous land. Similar developments have occurred on Biskops-Arnö, where about half of the forested ground was planted with spruce 30 to 50 years ago (Borgegård 1994). Among the spruces, it is still possible to find large, struggling, old trees competing with the conifers, if they are not already dead. Thus, the abundance of suitable habitats for organisms that depend on old trees has declined considerably, and these organisms now have a greatly increased risk of extinction. The continued presence of old trees in the park is therefore probably more important these days for animals that are dependent on old trees.

The exceptional age of the trees has probably helped conserve high numbers of red-listed species in the park in two ways. First, because the avenue was planted as early as 1684, the trees had already reached an age of more than 100 years by the early 1800s—a time when the landscape was richer in old trees. At that time, the pool of species that was able to colonize such habitats was probably larger than it is today, and the avenue may have provided a refuge for a fauna that have declined or even disappeared in the surrounding landscape. Second, as trees age, the diversity of the microhabitats they provide tends to increase. The same is true for the avenue when considered as a whole. As the trees age, increasingly large variations in appearance between the tree individuals develop. Skokloster park contains trees with a wide spectrum of hollows, ranging from small, newly formed ones to those so large that only a sector of the original trunk remains.

The species of the investigated trees may have had some influence on the abundance and diversity of the beetle fauna, but it is unlikely to have been very strong. Lime is not one of the most highly ranked Swedish tree species in species-richness of the saproxylic fauna associated with it, although it does host a relatively high proportion of specialists (Palm 1959; Jonsell et al. 1998). In Skokloster park, three of the beetle species found are known to be specifically associated with lime (Table 2) and, more specifically, with a certain Ascomycetous fungus, Biscogniauxia cineriolilacina, which grows on lime trees (Palm 1956; Jonsell and Eriksson 2002). Oak, which was the most common tree on Biskops-Arnö, is the richest tree species for saproxylic beetle species in Sweden (Palm 1959). However, the effect of the tree species decreases as the wood decays (Jonsell et al. 1998), and the fauna become much more strongly influenced by the fungal flora. For insect species living in tree hollows, the structure and nature of the hollow are the most important factors. Although neither the fungal flora nor the characteristics of the hollows are independent of tree species, the old-tree structures in the park trees are much more important for maintaining high biodiversity than the species to which the trees belong.

MANAGEMENT IMPLICATIONS

From a biodiversity point of view, the most important and urgent task is to take care of the oldest trees and keep them alive as long as possible. Pollarding is likely to be beneficial in this context, for three main reasons. First, it is likely to preserve the old trunks, together with all the microhabitats described in Table 2. Second, the branches are unlikely to become too heavy and long for the old trunks to support, thus reducing the risk that their weight will eventually tear them apart. Third, the retarded growth associated with pollarding may increase the likelihood that the trees will grow old (Rackham 1976; Slotte 1993).

However, even if everything possible is done to prolong the life of the trees, they will eventually die. In parks, they will be replaced before their natural death, for either aesthetic or safety considerations. In such cases, it is important to remember that the various components of the fauna will continue to need appropriate habitats. Every summer the new generation of insects requires suitable sites to lay eggs and for the larvae to develop. Therefore, to replace all the old trees in a park in a single operation would be catastrophic for the park’s biodiversity, especially because there are unlikely to be many recolonizations. In the surrounding landscape, which was presumably the source of the beetles that first colonized the park, the number of old trees has decreased drastically.

Therefore, the avenue must be replaced in a stepwise manner. So far, the alley at Skokloster park has been replaced tree by tree when individual trees have become too old (National Property Board 2001). There may, however, be problems with the establishment of newly planted trees if their roots have to compete with the root systems of older trees. If so, replacing sections of the avenue may be the best option, provided steps are taken to ensure that there is always a high number of trees with biodiversity-promoting structures. Because such structures are found on trees that are at least 100 years old, this replacement has to be a long-term project—as is the conservation of old cultural values.

CONCLUSIONS

This inventory shows that there are good opportunities for both cultural and biodiversity values to be maintained at high levels in parks. The high biodiversity value in Skokloster park is primarily attributable to management practices in the park that have preserved the large avenue of trees in such a way that many of the trees are now several hundred years old.