Lowland Polish forest

Eastern Europe is renowned for its old forests, a number of which may well be primary, that is, they extend back to the last glaciation. Arguably the best-preserved lowland deciduous forest is that of Bialowieza (pronounced Bee-ow-a-vey-sha) covering 1300 km2 across the borders of Poland and Belarus. Humans have been in the forest since at least the fourteenth century, using it as a royal hunting forest from the fifteenth century but it appears to have been little altered until large-scale timber extraction began after World War I. Fortunately, in 1921, the Bialowiez a National Park was established in Poland to create a 47 km2 preserve with no further timber extraction or forestry management. What is left is a woodland described by Peterken (1996, p. 73) as 'the largest virgin old-growth stand in lowland Europe' with a full complement oflarge mammals such as deer, bison, wild boar and wolves, and high species richness (see Packham et al., 1992). This has undoubtedly been there for thousands of years, and is probably as natural a woodland as can be found in lowland Europe. Surely, here is a forest that one might expect to be in a stable condition?

Much is known about the forest, largely due to the work of Janusz Falmski (e.g. Falmski, 1986). Despite its naturalness, two things are notable: firstly, the forest is not static but is undergoing a natural cycle of change; and secondly the forest is still recovering from past human intervention.

Figure 9.16 Regrowth area of forest in New England. The ability of woodlands swiftly to recover areas annexed by humans has been remarked by many observers including Rudyard Kipling in his poem The way through the woods. The abandonment of land in New England, however, was remarkable both in its extent and in the rapidity of the subsequent successional processes. This view shows an old wall running through what appears to be an old forest at Harvard Forest, Massachusetts. The fields on either side were used for agriculture from the mid 1700s until the mid 1800s when they were abandoned. The forest is thus just 150 years old at most. (Photograph by Peter A. Thomas.)

Figure 9.16 Regrowth area of forest in New England. The ability of woodlands swiftly to recover areas annexed by humans has been remarked by many observers including Rudyard Kipling in his poem The way through the woods. The abandonment of land in New England, however, was remarkable both in its extent and in the rapidity of the subsequent successional processes. This view shows an old wall running through what appears to be an old forest at Harvard Forest, Massachusetts. The fields on either side were used for agriculture from the mid 1700s until the mid 1800s when they were abandoned. The forest is thus just 150 years old at most. (Photograph by Peter A. Thomas.)

The forest is noted for the rich oak-lime-hornbeam woodland made up mostly of hornbeam Carpinus betulus with smaller proportions of small-leaved lime Tilia cordata, pedunculate oak Quercus robur, Norway maple Acer platanoides, Norway spruce Picea abies and wych elm Ulmus glabra. Hornbeam with varying proportions of Norway maple, lime and wych elm forms the main canopy, 20-25 m above the ground. Above this, a scattering of taller emergent trees of oak and lime reach 30-35 m with even taller Norway spruce reaching over 40 m (Pigott, 1975).

Tree death occurs sporadically through the forests to create gaps, assisted by the wind, at the yearly rate of 200-450 fallen trees per 100 ha (Falinski, 1986). This produces a cyclical change as described in Section 9.2.2 above. Bobiec et al. (2000) have described the cycle as starting with a 'young/pole phase' of seedlings and saplings competing and self-thinning over 60 years to produce a dense canopy of trees up to 35 m high and 40 cm dbh (diameter at breast height). This is followed by an 'optimal phase' from 60-200 years which sees the forest opening up, with 4-8 trees per 100 m2, up to 70 cm dbh. Towards the end of this phase, the canopy begins to thin and the 'regeneration phase' is entered with seedlings appearing below the canopy, increasingly so as gaps form in the canopy. Bobiec et al. (2000) showed that in the parts of the National Park they sampled, the three phases (young/pole, optimal and regeneration) occupy about a third of the area each, and they considered the forest to be in some sort of cyclical balance.

On a smaller scale, earlier work by Pigott (1975) showed that this smooth cycle was not always following the same path. He found that the new groups of even-aged saplings in the young phase tended to be made up of a single species. For example, of 20 groups surveyed, 19 of them were either all hornbeam or lime, with the other one being pure elm. He also found similar groups of pure Norway maple and ash. Several gaps dominated by lime can be seen in the forest cross-section shown in Fig. 9.17. You will notice the odd elm and Norway spruce at the edge of these gaps growing in slightly more shaded conditions.

The exact reasons for these single species groups are still unclear but some partial answers are possible. Groups of young lime usually appear under the partial shade of hornbeam (as with the right-hand group in Figure 9.17), regeneration from seed being aided in places by suckering from roots and rooting of branches (Peterken, 1996). As the hornbeam canopy undergoes self-thinning with age (i.e. the shortest trees are eventually outcompeted and die) more light reaches the ground beneath (c. 200-300 kJm-2 day-1) without an open gap appearing in the canopy. As this happens, seedlings of lime, elm and maple are able to establish in the shaded conditions. So, why not mixed groups? Some parts of the answer are straightforward. Each species tends not to establish under its own canopy (the exact reasons for this are still unknown). Also, where nettles are most vigorous it is only the elm (and a few lime which are soon lost) that can establish and grow. Interestingly, Pigott points out that while wych elm of English origin is less shade tolerant than small-leaved lime, elm may prosper because it is the least susceptible to fungal disease and invertebrate herbivores harboured by the nettles. Wild boar (Sus scrofa) may also play a part since seedlings of lime and hornbeam were found to

Figure 9.17 Vertical section through part of the BialowieZa Forest, Poland, showing a strip 2-m wide, passing through two main groups of saplings. T, small-leaved lime Tilia cordata; C, hornbeam Carpinus betulus; U, wych elm Ulmus glabra; P, Norway spruce Picea abies; A, Norway maple Acer platanoides; Q, pedunculate oak Quercus robur; H, hazel Corylus avellana. (From Pigott, 1975. Natural regeneration of Tilia cordata in relation to forest structure in the forest of BialowieZ a, Poland. Philosophical Transactions of the Royal Society of London. Series B 270, fig. 2, facing page 162. Courtesy of the Royal Society.)

Figure 9.17 Vertical section through part of the BialowieZa Forest, Poland, showing a strip 2-m wide, passing through two main groups of saplings. T, small-leaved lime Tilia cordata; C, hornbeam Carpinus betulus; U, wych elm Ulmus glabra; P, Norway spruce Picea abies; A, Norway maple Acer platanoides; Q, pedunculate oak Quercus robur; H, hazel Corylus avellana. (From Pigott, 1975. Natural regeneration of Tilia cordata in relation to forest structure in the forest of BialowieZ a, Poland. Philosophical Transactions of the Royal Society of London. Series B 270, fig. 2, facing page 162. Courtesy of the Royal Society.)

coincide exactly with areas disturbed by boar rooting (possibly due to temporary removal of root competition). It seems, however, that an important cause of single species clumps in these developing gaps is that small variations and combinations of conditions of light, moisture and other factors favour one species over others. It is certainly evident that most groups are single species from the start rather than jostling for dominance once established. There may also be a fair degree of chance in the process such as the proximity of a parent tree and optimal conditions in a gap coinciding with a large crop of fruit. Supporting this idea of the importance of critical combinations of conditions is that in large open gaps there is a greater tendency for groups of seedlings to contain several species; '... conditions are generally favourable and there is no selection for slight differences between the sensitivities of species' (Pigott, 1975, p. 176).

Human influence has played a major role in causing change in the BialowieZ a Forest, where small-leaved lime is more abundant in the central region, which includes the National Park. Pigott highlights earlier work of Paczoski who showed that before 1928 there was a long period with little regeneration of lime; trees between 0.05 and 0.4m diameter were absent. These trees below 0.05 m appear to be the saplings that Pigott found in the 1970s. Paczoski attributed this to the sensitivity ofthe lime to exploitation, the central area having been the least heavily managed. Much of the forest away from the central area had the same herbaceous vegetation, differing only in the much smaller proportion of small-leaved lime and a corresponding increase in pedunculate oak and Norway maple. The gap in the regeneration of Tilia cordata extends backwards from about 1923 to before 1870, a period when the area was maintained as a hunting reserve by the Russian Czars. During this period the number of carnivores was kept low and many deer and European bison were kept for hunting. If this helped cause the failure of regeneration of small-leaved lime it did not prevent that of other trees, and the subsequent reduction in numbers of large mammals, such as wild boar, was influential. Cattle grazing, artificial increases and decreases in deer numbers (and hence grazing pressure), and raking and collection of leaf-litter as compost may also have influenced the situation.

Vigorous regeneration since 1923 has caused the proportion of young trees in the small-leaved lime population to become very high, so the forest still has features initiated by earlier management. In the oak-lime-hornbeam forest small-leaved lime appears to be the potential dominant. The mature trees are long-lived and very tall, and the species is very shade-tolerant when young. Hornbeam Carpinus betulus is shorter-lived and forms a lower canopy; it is likely to become displaced, at least for a time, from its present predominant role. Pedunculate oak Quercus robur is found most commonly in the National Park; it is much less frequent in the whole forest than formerly, having been heavily exploited in the twentieth century, like Scots pine Pinus sylvestris. Norway spruce Picea abies, which is favoured by high numbers of game animals, now controlled, has diminished in numbers since 1950 and is also subject to the influence of acid rain. Climate allowing, the present situation makes it appear that much of the forest will undergo a slow but inevitable change over the next few centuries leading to a dominance of lime with a subcanopy of hornbeam and Norway maple (Peterken, 1996).

Humans are still having an effect on the forest used for timber extraction around the National Park. Bobiec et al. (2000) show that the composition of the forest has been altered; only 19% is now in the optimal (mature) phase while 61-68% is in the young/pole phase.

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