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.

具有树洞的树木是林地景观中维持生物多样性的关键要素。它们承载着丰富的高度特化生物类群。然而在欧洲，树洞树木已变得稀有且分布局限，与之关联的多数生物类群因此处于衰退或濒危状态。因此，其保护工作的核心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，常栖息于由过往广泛采用的传统营林措施（如矮林作业、截干作业或林间放牧）所形成的生境中。尽管已有零星研究提及此类措施可促进树洞形成，并提升受阳光直射的枯木资源量，但相关效应始终未被量化。本研究通过树木年轮分析，探究了截干柳树与未截干（无人工经营）柳树的树洞发生率，以及截干作业对树木径向生长速率的影响。截干柳树的树洞出现概率显著高于未截干个体，尤其年轻的截干柳树，其树洞形成频率远高于未截干柳树；例如，在胸径（DBH）50厘米的个体中，截干柳树的树洞出现概率约为0.75，而未截干个体仅约为0.3。研究未发现两类树木的生长速率存在差异。由此可见，截干作业可快速促成树洞形成——这类生境通常仅见于老龄树木。因此，截干作业或可成为修复腐木生（saproxylic）境以及保护依赖树洞生存的动物类群的重要手段。若在道路旁或水道沿岸实施截干作业，还可提升腐木生生物的景观连通性。在过往曾实施截干作业的保护地中，恢复截干作业对于防止腐木生生物多样性丧失至关重要。本研究结果表明，积极实施营林管理措施，对于维持枯木微生境的资源量、空间连续性与时间连续性具有重要意义。