Data from: Is active management the key to the conservation of saproxylic biodiversity? Pollarding promotes the formation of tree hollows

Trees with hollows are key features sustaining biodiversity in wooded landscapes. They host rich assemblages of often highly specialised organisms. Hollow trees, however, have become rare and localised in Europe. Many of the associated biota is thus declining or endangered. The challenge of its conservation, therefore, is to safeguard the presence of hollow trees in sufficient numbers. Populations of numerous species associated with tree hollows and dead wood are often found in habitats that were formed by formerly common traditional silvicultural practices such as coppicing, pollarding or pasture. Although it has been occasionally mentioned that such practices increase the formation of hollows and the availability of often sun-exposed dead wood, their effect has never been quantified. Our study examined the hollow incidence in pollard and non-pollard (unmanaged) willows and the effect of pollarding on incremental growth rate by tree ring analysis. The probability of hollow occurrence was substantially higher in pollard than in non-pollard trees. Young pollards, especially, form hollows much more often than non-pollards; for instance, in trees of 50 cm DBH, the probability of hollow ocurrence was ~0.75 in pollards, but only ~0.3 in non-pollards. No difference in growth rate was found. Pollarding thus leads to the rapid formation of tree hollows, a habitat usually associated with old trees. It is therefore potentially a very important tool in the restoration of saproxylic habitats and conservation of hollow-dependent fauna. If applied along e.g. roads and watercourses, pollarding could also be used to increase landscape connectivity for saproxylic organisms. In reserves where pollarding was formerly practiced, its restoration would be necessary to prevent loss of saproxylic biodiversity. Our results point to the importance of active management measures for maintaining availability, and spatial and temporal continuity of deadwood microhabitats.树洞树木是林地景观中维持生物多样性的关键生境要素，它们承载着丰富且通常高度特化的生物群落。然而在欧洲，树洞树木已变得稀少且分布局限，与之相伴的众多生物类群因此面临种群衰退或濒危的处境。因此，这类生物的保护挑战在于保障足够数量的树洞树木存续。诸多依赖树洞与枯木的物种种群，常栖息于曾被广泛采用的传统营林措施（如矮林抚育、截干作业或牧场经营）所形成的生境中。尽管已有零星研究提及这类营林措施可促进树洞形成，并增加向阳枯木的可获得性，但相关效应从未得到量化验证。本研究通过树木年轮分析，探究了截干柳树与未受干扰（非截干）柳树的树洞发生率，以及截干作业对柳树径向生长速率的影响。结果显示，截干柳树的树洞出现概率显著高于非截干个体，其中幼龄截干柳树的树洞形成频率尤为突出；例如在胸径（DBH）为50厘米的个体中，截干柳树的树洞出现概率约为0.75，而非截干柳树仅约为0.3。本研究未发现两组间生长速率存在显著差异。由此可见，截干作业可加速树洞形成——这类生境通常仅见于老龄树木。因此，截干作业有望成为恢复枯木栖生（saproxylic）生境、保护依赖树洞的动物类群的重要手段。若在道路旁或水道沿岸开展截干作业，还可提升枯木栖生物种的景观连通性。在曾开展截干作业的保护地中，恢复截干措施对防止枯木栖生生物多样性丧失至关重要。本研究结果凸显了主动营林措施在维持枯木微生境可获得性及其空间与时间连续性方面的重要意义。