Which factors influence the occurrence and density of tree microhabitats in Mediterranean oak forests?
Highlights
► We studied nine types of tree microhabitats in three managed forests. ► We examined changes in tree microhabitats with tree and stand characteristics. ► The richness of microhabitats per tree is influenced by diameter, vitality, and species. ► Microhabitat types are poor predictors of other microhabitat types. ► Time since last cutting strongly influence the density of microhabitats per stand.
Introduction
In many parts of the world, forest managers have to balance wood production with biodiversity conservation over the long term (Secretariat of the Convention on Biological Diversity, 2010). Stand structure, the composition of tree species, deadwood volume and tree microhabitats (i.e., microhabitats that grow on trees) are important components of conserving biodiversity in forests (Grove, 2002, Hunter, 1999, Paillet et al., 2010). Many species depend on tree microhabitats for food, shelter and breeding habitat (Michel and Winter, 2009, Winter and Möller, 2008). For example, dead branches are food sources for saproxylic insects and fungi (Jonsell and Nordlander, 2002, Vanderwel et al., 2006). Cavities provide habitats for breeding birds, mammals, and invertebrates but also lichens and bryophytes (Fritz and Heilmann-Clausen, 2010, Lučan et al., 2009, Parsons et al., 2003, Ranius, 2002). Many forest bats nest or roost behind cracked and loose bark (Pénicaud, 2000, Russo et al., 2004, Vonhof and Gwilliam, 2007). Other microhabitats, such as conks of fungi or ivy, are homes to insects and provide potential nesting and foraging sites for birds (Bässler et al., 2012, Dajoz, 2005, Mitchell, 1973). Tree microhabitats support a large food web and may have an important role in the functioning of forest ecosystems (Aitken and Martin, 2007, Martin and Eadie, 1999). Some species linked to tree microhabitats are also of major concern for biodiversity conservation, such as some saproxylic insects (e.g., Limoniscus violaceus – Gouix and Brustel, 2011), birds (e.g., the White-backed Woodpecker Dendrocopos leucotos – Lehikoinen et al., 2011), and bats (e.g., Barbastella barbastellus – Russo et al., 2010).
Surprisingly, the links between trees, stand characteristics and tree microhabitats remain poorly explored compared to other research areas related to biodiversity indicators (Michel and Winter, 2009, Vuidot et al., 2011). Recently, several studies investigated the occurrence of tree microhabitats by comparing managed and unmanaged forests (Larrieu et al., 2012, Michel and Winter, 2009, Vuidot et al., 2011, Winter and Möller, 2008). However, the distinction between ‘managed’ and ‘unmanaged’ forests may involve a large array of silvicultural practices. For example, two managed forests may have contrasting management histories, leading to different structural characteristics and biodiversity levels (Bengtsson et al., 2000, Burger, 2009, Summerville, 2011).
Silvicultural practices tend to convert forests into simplified ecosystems (with reductions in species tree diversity, vertical stratification, and age structure) compared to unmanaged forests (Paillet et al., 2010). However, forest management also has the potential to maintain natural structural complexity and support the development of microhabitats (Bauhus et al., 2009, Larrieu et al., 2012). Although microhabitats are believed to play a key role in the conservation of biodiversity in managed forests (Michel and Winter, 2009, Winter and Möller, 2008), the factors that underlie their occurrence and density remain poorly known. To our knowledge, only Michel and Winter (2009) have explored the role of management history and stand structure in the occurrence of tree microhabitats in Douglas fir forests (USA). Yet, better understanding of the factors that allow the development of microhabitats in managed forests is of strong interest for managers and decision-makers, especially in Europe, where most forests are subject to silvicultural treatments (Bengtsson et al., 2000, Vanbergen et al., 2005).
The Mediterranean region is considered a priority area for biodiversity conservation (Cuttelod et al., 2008, Millennium Ecosystem Assessment (MEA), 2005, Myers et al., 2000). Biodiversity conservation in Mediterranean forests is the subject of several research programs (see for example the on-going program ‘Mediterranean Integrated STudies at Regional And Local Scales’ (MISTRALS, 2012); however, to our knowledge, research on the factors underlying the occurrence of tree microhabitats has never been conducted in Mediterranean forests (Vuidot et al., 2011).
The overall objective of our study is to test the effect of tree and stand characteristics (time since last cutting and stand structure) on the occurrence and density of tree microhabitats in managed forest stands in the Mediterranean region, specifically southern France. We distinguished between the tree and stand levels following the Vuidot et al. (2011) approach: at the tree level, the objectives were to (a) investigate the relationships between tree characteristics (species, vitality, and diameter) and tree microhabitats, and (b) determine co-occurrences among tree microhabitats. At the stand level (c), we examined the effects of time since last cutting (i.e., the number of years since the last cutting) and stand structure on the density of microhabitats. We showed that managed forests have the potential to conserve microhabitats, and we discuss measuring and promoting microhabitats from the perspective of forest managers.
Section snippets
Study area
The studied stands were located in southern France, in Saint-Paul-lès-Durance (44°41′16″N/5°42′30″E, surface area 1415 ha, mean altitude 300 m), Ribiers (44°13′55″N/5°21′26″E, area 126 ha, alt. 800 m), and Saint-Vincent-sur-Jabron (44°10′18″N/5°44′33″E, area 90 ha, alt. 850 m). Climate conditions were typical of the Mediterranean region with a long sun exposure (2500–2800 h annually) and warm summers (mean temperature in summer = 27 °C; annual mean = 12 °C) (Météo France, 2012). Annual precipitations
Influences of diameter, vitality and tree species on tree microhabitats
The model with the lowest AIC was the complete additive model without interactions (Appendix C). The richness of microhabitats per tree was influenced by diameter (GLM, F1,1618 = 236.58, p < 0.001), vitality (GLM, F1,1618 = 58.57, p < 0.001), and species (GLM, F3,1618 = 35.99, p < 0.001). We found a positive relationship between the tree diameter and the richness of microhabitats (slope estimate (SE) of the variable ‘tree diameter’ = 0.14 (0.01)). Living trees supported a lower richness of microhabitats than
Influence of diameter, vitality and tree species on tree microhabitats
We found a positive relationship between tree diameter and the richness of microhabitats, which confirmed several previous observations in contrasting ecological contexts (Larrieu et al., 2012, Michel and Winter, 2009, Vuidot et al., 2011). However, this relationship may not be systematic, as Winter and Möller (2008) found no significant relationship in managed or recently unmanaged stands. Generally, it appears that larger trees host more microhabitats than smaller ones, most likely due to
Acknowledgements
We are very grateful to Emmanuel Cosson (Groupe Chiroptères de Provence) and Olivier Ferreira (ONF), who developed the inventory protocol for microhabitats. Many thanks to the officers of the ONF, Olivier Ferreira and Jérôme Guyot, who performed the surveys of microhabitats, and Rémi Bonardo, Jean-Pierre Viguier, and Jeanne Dulac for their helpful discussions on microhabitats in Mediterranean forests. Funding was provided by the Commissariat à l’Energie Atomique (prefectorial decree no 200863-5
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