Pine-spruce forests in nature reserves of Saint Petersburg: monitoring results

Large-scale mapping of the vegetation of Saint Petersburg revealed the predominance of pine-spruce and spruce-pine stands among coniferous forests, to which both spruce (Picea abies) and pine (Pinus sylvestris) contribute in different proportions. For the purposes of geobotanical mapping reflecting vegetation dynamics, it is important to establish the dynamic category of such communities in order to clearly determine whether they should be classified in maps legends as spruce or pine formation, or their own category.

This issue has been controversial for a long time. V. N. Sukachev (1908) argued that due to the lack of pine renewal, pine-spruce forests would necessarily turn into spruce ones. G. F. Morozov (1930), on the contrary, considered pine-spruce forests to be a stable formation for such habitats as sandy loam or loam soils of medium richness, without specifying their origin. Both the first and second theories have found their supporters.

The purpose of the article was to study the dynamics of the pine-spruce forests of the studied territory by analyzing changes in all vegetation layers, species composition and structure of the ground cover in 2006–2024 and to establish their dynamic category.

Three permanent sample plots occupied by pine-spruce forests were established for analysis. They are located in the southern taiga subzone, in two Saint Petersburg nature reserves (Fig. 1). Five observations on each sample plot (with intervals of 3–4 years) included complete taxation of the tree and shrub layers as well as the young growth; reveal of the whole species composition and determination of the vascular plant and moss ground species cover; identification of the ecological-coenotic groups and horizontal structure of the lower layers (Smirnov et al., 2006; Khramtsov et al., 2011); calculation of general and partial (separately for vascular plants and bryophytes) ground species turnover (Koch, 1957). NMDS-ordination (Bray-Curtis distance, «year» as the only factor) (Oksanen et al., 2013) was made to reveal if there is a trend in changes of ground species composition and cover.

A brief characteristic of the three community types (Pineto-Piceetum oxalidoso-myrtillosum, Pineto-Piceetum myrtilloso-hylocomioso-sphagnosum and Pineto-Piceetum (oxalidoso-) myrtilloso-sphagnosum) are given in text with detailed information in tables. All of these are located on poor soils (sandy, sandy loam, drained peat). Spruce age ranges between 60 and 100 years, pine— between 80 and 140. General view of the communities is shown in Fig. 2–4.

Results of 15–16 years of observations on the permanent sample plots showed that changes in species composition and horizontal structure in the studied forests (Table 1–6) are mostly related to local natural (windfalls and their consequences) and anthropogenic (ditch cleaning, soil dumping) disturbances; there are no replacements in dominant species.

The ratio of pine and spruce stock within the stand is relatively constant (Fig. 5, 7, 9). Active self-thinning of the stands is observed: in the course of observation, the number of trees has decreased on average by 27 %. Hardwood trees (particularly birch) have been gradually falling out. In the period between observations, 1–2 pine trees and 2–11 spruce ones of different ages die. The resupply of the stands consists of spruce only. Due to the high intraspecific competition, it is very slow (on average, one individual per 4 years). The share of pine in the ratio of pine and spruce trees number is stable at all sample plots; for spruce its share is constant at ГЗ-3, while at two others it has increased due to hardwood trees dieback. Spruce trees are presented in more diameter gradations, than pine ones (Fig. 6, 8, 10).

The overall turnover of the ground species composition over the entire observation period is similar at all sample plots, averaging about 50 %, mainly due to variations in the bryophyte species presence. Results of the NMDS-ordination (Fig. 11) shows that changes in ground species composition and cover are statistically insignificant at all plots (p = 0.17; 0.24; 0.92 for ГЗ-3, КБ-4 and КБ-6 respectively with p = 0.05 significance level).

Data received fit the concept that pine-spruce forests will inevitably transform into pure spruce forests (Sukachev, 1908): spruce mean annual increment is much higher (Table 7), pine undergrowth is almost absent. However, since pine is greater resistant to such catastrophic impacts as fires and windthrows, its presence in stands will be substantial for a long time (taking into account pines current age and under maintaining the protected area regime — at least 40–50 years). The authors believe that spruce and pine trees in Saint Petersburg are in constant dynamic interaction; the restoration of spruce forests might have happen over a long period without fires (more than 200–300 years), but their recurrence near populated areas and highways is much more often (Andreyev, 2003). Our data on overall ground species and vascular plants composition turnover are close to data received by other researchers; as for ground bryophytes, the difference between turnover values is so high that we cannot definitely determine whether our data correspond to “normal” turnover or not.

Consequently, the pine-spruce forests in Saint Petersburg on sandy and sandy loam substrates, as well as on drained peats, may be considered as relatively stable long-term secondary forest stands, which may be shown on maps of actual vegetation as independent units that are not subordinated to conventionally primary spruce forests.