Study sites and data collection

Data were collected in stands of pollard and unmanaged (i.e. unpollarded) white willows (Salix alba) in the catchment area of the Thaya (Dyje) river in South Moravia, Czech Republic. Two stands of pollard trees, Vojkovice (49°3′N, 16°36′E) and Krive jezero (48°51′N, 16°43′E), and two stands of unmanaged willow trees, Kanci obora (48°46′N, 16°52′E) and Pastvisko (48°48′N, 16°47′E), were sampled. Vojkovice and Kanci obora are located on publicly accessible land, two other sites are located within protected areas. Site selection was constrained by the fact that pollarding of willows had been regionally common in the past, but was mostly discontinued after the Second World War. Old willows thus bear the signs of previous pollarding, whereas younger trees were (almost) never pollarded throughout the area. The areas where young trees were pollarded, or where older trees were not pollarded were therefore carefully selected. Trees with trunks higher than 0.5 m, whose upper branches had been pruned at least once were recognized as pollards. They were identified by all the main branches sprouting from single part of the trunk, mostly its swollen top. Trees bearing no signs of management were considered unmanaged (Figure 1). For each tree the diameter at breast height (DBH; 1.3 m above ground) and the presence/absence of hollows were recorded. Any cavity with an entrance hole larger than 5×5 cm and with the inner space larger than the entrance hole was recognized as a hollow. Where hollows were not accessible by ladder (>4 m above ground level), binoculars were used to search for hollows in trunks and branches (following Koch 2008 [28]).

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Figure 1. Forms of pollard and unmanaged trees.

A pollard (A) and an unmanaged tree (B), shown with their most common type of tree hollows (grey colored): hollows formed in the upper parts of the trunk as a result of bared heartwood after pruning are common in pollards, whereas hollows formed after a branch fall are the most common hollow type in unmanaged trees.

https://doi.org/10.1371/journal.pone.0060456.g001

To assess the effects of pollarding on tree growth, pollards and unmanaged trees were cored and their growth rates compared. Cores were taken from 10 unmanaged and 10 pollard trees using a steel increment borer (Mora™, Sweden) at the Pastvisko site, where one part of the formerly unmanaged willow stand was pollarded in 2003 (once only). All cores were dried, glued onto a wooden lath, cut off by razor blade and inspected for injuries, reaction wood and other aberrant features. One core was badly broken and another one contained rotten sections that did not allow successful ring width measurements. These two cores were removed from further analysis. For the remaining cores (n = 8 for pollard and n = 10 for unmanaged willows), rings were counted from pith to bark and their widths measured to the nearest 0.01 mm using the TimeTable measuring device and PAST4 software [29]. Ring sequences were cross-dated visually using the pattern of wide and narrow rings, and verified using the PAST4 program.

All necessary permits were obtained for the described field studies; the research was conducted under the permit No. 00356/KK/2008/AOPK issued by the Nature Conservation Agency of the Czech Republic. The data are deposited in Dryad, a publicly accessible digital repository [30].

Data analysis

Relationships between DBH, the probability of hollow presence, and the effect of management (pollard/unmanaged) were tested. The data were analyzed using generalized linear regression models with binomial distribution (logit link) where presence/absence of hollow in the tree was a dependent variable, DBH (ln-transformed) was an explanatory variable (lnDBH), and management type (pollard/unmanaged) was a factor variable (management). An interaction between DBH and management type (lnDBH:management) was added to the model to assess the differences in slope shape between pollards and unmanaged trees. The second regression model where the interaction parameter was excluded was also tested, and then compared with the full model.

The effects of pollarding on tree growth rate were investigated by detection of radial growth in pollarded and unmanaged trees. Changes in incremental ring width were compared using a regression model with a repeated measures design. Each core sample (8 pollard, 10 unmanaged) was considered as a single subject, its ring widths being longitudinal measurements. In the model, ring width (ringwidth) was a dependent variable, subsequent years were equally spaced time points (time, continuous explanatory variable), and management type (pollard/unmanaged) was a factor variable (management). Firstly, the pattern in ring width increments for the period from 1990 to 2011 was tested, because from 1990 growth information for all the core samples was available. Secondly, the pattern for the period from 2003 to 2011 was tested, 2003 being the year in which trees were pollarded. In both cases, the interaction between time and management type (time:management) was included in the model. The quadratic effect of time was added to the model, and subsequently removed if not significant. The analyses were performed using R 2.14.2 software [31].

Study outcome and limitations

Our results show that pollarding increases the probability of hollow formation, especially in young trees (i.e., trees with small DBH). In pollard trees, hollows form more frequently and sooner than in unmanaged trees. For instance, in a stand of 100 willows with DBH of 50 cm, there would be ∼75 hollow trees if the trees were pollarded, and only ∼30 hollow trees if the trees were unpollarded (see Figure 4). Since pollarding speeds up hollow formation, it substantially increases the hollow density required for survival of hollow specialists [32].

Pollarding also has other positive effects. Firstly, the hollows in pollards occur in stems rather than in branches, while the latter prevail in unmanaged trees [33]. Stem hollows are larger, contain a greater volume of wood mould, and are thus able to host greater numbers of hollow specialists. Secondly, the bare, mostly sunlit heartwood after pruning is open to colonization by xylophages, including numerous threatened and protected species [23]. Pollards thus provide various deadwood microhabitats from a relatively young age. For example, at one of the sites (Vojkovice), larvae of Osmoderma eremita were found in a pollard <20 years old [10].

Owing to their lower height, pollard trees are less susceptible to branches breaking and falling due to weight imbalance, as long as pruning is carried out periodically. Pollards thus tend to live longer than unmanaged trees, and a single pollard may provide continuum of various deadwood microhabitats for hundreds of years [25]. On the other hand, if pollarding was discontinued, the increasing weight of overgrown branches often results in serious damage - trunk disintegration and the destruction of the tree. This is the reason why abandoned pollard stands suffer from rapid loss of hollow trees and subsequent loss of associated biodiversity [10].

It's not surprising that hollows are mostly present in trees with larger diameters [18], [34], [35]. As the tree ages, the heartwood becomes more susceptible to fungal infections and rot due to branch fall or bark loss [36]. Ranius et al. (2009) [18] provided data on hollow formation in oaks with regard to DBH. Of oaks with a diameter of 80–100 cm, 20–50% contained hollows. In our study, hollow incidence in both unmanaged and pollard willows of the same diameter is much higher than in oaks, probably due to the fact that faster growth and softer wood make willows more prone to damage and infections [4], [37].

During the last century, pollarding was abandoned in most of the study area and also in most of Europe [11], [24], [25], [27], [38]. Because of this, it is difficult to separate the effects of pollarding and tree age. Generally, old trees have been pollarded in the past, whereas young trees never have. In the study area, pollarding has continued locally, but is concentrated on predominantly older trees. As a result of this, our sampling was limited by the absence of stands containing both unmanaged and pollard willows of a similar age. This might possibly be a source of some unintended bias and further studies would therefore be needed to ensure sufficient separation of the age effect.

We found no effect of pollarding on radial tree growth. This is in contrast with the documented decrease in growth rate after pollarding in oaks [39]. Willows, however, may respond to pollarding in a different way than oaks. Also, the sampled trees grew in dense stands, and were probably affected by competition. In easily resprouting, light-demanding willows, reduced competition after pruning (pollarding) may have balanced the negative impact of branch removal. Larger sample size would be needed in order to test such a hypothesis. The poor quality or absence of central heartwood in regularly pollarded older trees also prevented us from assessing the potential effects of regular pollarding on tree growth. This question has yet to be addressed.

Traditional management as a part of landscape history

Ancient landscape in Europe was strongly influenced by human activities over long periods of time [4]. In the past, traditional pruning techniques like pollarding, shredding or coppicing were used for firewood and fodder production [40]–[42], and forests were also used for livestock grazing [43], [44]. As a result, old, open-grown and hollow trees were common features of the landscape.

Today, the biota associated with old and/or hollow trees finds refuge mainly in orchards, alleys, parks, pasture woodlands and game reserves rather than in intensively managed forest and agricultural landscape, or strict forest reserves [26], [45]. The importance of such man-made habitats is especially emphasized for the noble chafer beetle Gnorimus nobilis [46], [47] and for Osmoderma eremita [48]. The latter is among the best known European beetles and serves as an indicator and umbrella species for hollow-dependent invertebrates [49]. Both beetles are thermophilic and rarely occur in natural or semi-natural forests that are too shaded [47], [48]. Hollow-dependent invertebrates often have low dispersal abilities, and thus require a high density of microhabitats in space and time [12], [13]. Traditional pollard stands, pasture woodlands, and orchards with large aggregations of sun-exposed hollow trees provide suitable conditions.

Other studies provide evidence for the importance of traditional management practices as they stimulate the formation of open forest features [50]. The cessation of pollarding was found to have negative effects on assemblages of lichens and epiphytic vegetation [25], [38], [51]. Robles et al. (2011) [52] found that the biodiversity of secondary cavity nesting birds was greater in oak forests traditionally managed by extensive grazing and cutting for firewood than in dense forests. Following the same principle as pollarding, coppicing most likely increases the incidence of hollows near the ground, and its cessation thus threatens the violet click beetle Limoniscus violaceus, a rare and highly endangered species, which develops in the wood mould of basal cavities. The beetle can mostly be found in formerly coppiced forests [53], [54] or ancient forest pastures [55].

In Europe, traditional silvicultural practices have contributed to the creation of today's rare deadwood microhabitats, and therefore they helped to preserve saproxylic species over the years. The discontinuation of such practices not only threatens numerous saproxylic organisms in commercial woodland, but it also leads to a peculiar situation in many protected areas: old trees, that currently host populations of hollow-dependent and other specialised saproxylics, were often subject to pollarding or coppicing, that increased dead wood availability. Young trees, on the other hand, were not treated in this way. This is likely to result in the reduction of suitable deadwood microhabitats in the near future.

Conservation practitioners, therefore, need to acknowledge that extant populations of endangered saproxylic species, many of them preferring sunny conditions, very likely depend on microhabitats created by former management practices, since abandoned [5]. Although the insects are still present at numerous sites, they survive only temporarily, as a part of extinction debt created by past management changes [56], [57]. The importance of active management practices is, however, rarely appreciated. Nature conservationists often praise natural, semi-natural and 'old-growth' habitats, and tend to ignore site history and the habitat requirements of endangered organisms. In particular, the 'strict forest reserve' concept [58], [59] based on non-intervention management in woodland habitats is especially damaging when applied to traditionally managed sites [3], [43], [60]–[62]. In Europe, natural disturbance factors such as large herbivores [63] and fires [64] are mostly lacking. Natural succession thus leads to greater canopy closure and causes a transformation from low competition and sunny conditions to higher competition with shady and cold conditions. Old and veteran trees are not able to adapt to the new conditions and subsequently die [4], [5], which leads to the loss of important deadwood microhabitats and therefore to a decline in saproxylic biodiversity associated with these trees.

Future perspectives

The promotion of evidence-based conservation management targeted at saproxylic species is one of the major topics of the currently emerging European Saproxylic Beetle Conservation Strategy [3]. Conservation and retention of hollowed trees as one of the specific habitats are therefore of crucial importance. Creating artificial habitats might be necessary, especially when it takes a long time for the natural production of such habitats [19], or if factors contributing to their formation in the past, e.g. grazing by large herbivores or fires [63], [64], are missing. Active management, therefore, should be adopted to prevent the gradual decimation of saproxylic biodiversity. As shown in the present study, pollarding might play an important role in the restoration of saproxylic habitats. If trees along watercourses, roads and other linear structures in the landscape were to be pollarded, they could well become important habitats and corridors connecting refugia of saproxylic fauna.