Natural forest dynamics and late successional stages

Most temperate natural forests underlie cyclic age dynamics with subsequent development stages (KORPEL’ 1995, STÖCKER 1997). Different from boreal forests, large-scale, stand-replacing disturbances are comparably rare in Central Europe (FISCHER et al. 2012).

Consequently, the different forest development stages occur in small-scale mosaics comprised of unevenly-aged patches differing in the tree vitality. The ageing and dieback of small tree groups or single trees leads to the formation of canopy gaps in advanced development stages (HOLEKSA & CYBULSKI 2001). Light availability, microclimate and soil conditions below canopy gaps can strongly deviate from neighbouring closed-canopy forest (HOLEKSA 2003, NADKARNI &SUMERA 2004). Therefore, the variations in size and age of gaps in natural forests can influence the diversity and abundance of forest ground species (KIRCHNER et al. 2011) as well as epiphytes on trees (COOTE et al. 2007). In contrast to comparisons between managed and unmanaged forests, the importance of overmature and decaying tree stands for plant diversity within natural forest dynamics has rarely been addressed yet (VON OHEIMB et al. 2004, STÖCKER 1997, UOTILA & KOUKI

2005). The significance of advanced and late successional stages in forests has been shown for epiphytes (KUUSINEN & SIITONEN 1998, HAUCK 2011). Few studies suggest a comparably low significance of overmature and decaying forest patches for ground vegetation diversity (ZUKRIGL 1982, VON OHEIMB et al. 2004, JONÁŠOVÁ &PRACH 2008).

In many forest ecosystems, the response of epiphytes as well as ground vegetation to senescent and decaying tree stands is difficult to estimate, as the forest areas have not been solely affected by forest management. Other site factors include the input of different elements by agriculture and industry, e.g. pollution by atmospheric nitrogen or sulphurous deposition (ELLENBERG &LEUSCHNER 2010, HAUCK et al. 2012, HRUŠKA et al. 2012). As both direct and indirect human influences, such as forestry and air pollution, affect forest vegetation (HEDL 2004), the investigation of unmanaged forests, particularly overmature and naturally declining stands, is necessary for the separation of these effects (HAUCK

2000). The impacts of pollution-induced substrate acidification, heavy-metal influx and related forest dieback on both ground vegetation and epiphytes have been intensively studied (BUSSOTTI &FERETTI 1998, HAUCK 2000). Possible reverse effects under low S02

or nitrogen deposition have rarely been explored yet (STRENGBOHM et al. 2001, SUJETOVIENE &STAKENAS 2007). This especially applies to cryptogamic epiphytes, which are strongly affected by atmospheric pollutants, leading to markedly different diversity and distribution patterns across different levels of pollution (HAUCK 2005). Studies under high levels of air pollution have revealed the higher diversity of lichens and bryophytes on damaged and dead trees compared to live trees (HAUCK et al. 2002). This has been relativized by studies under lower pollution levels (HAUCK 2005). Further studies have to clarify the actual importance of late successional stages, and the differences in the epiphyte diversity of live, overmature and decaying trees in natural, lowly-polluted forests.

Recent research activities in natural, old-growth forests do not only include the diversity patterns of forest organisms, but also other ecosystem services (HÜTTL et al. 2000, WIRTH

2009). With regard to global change, the role of natural forests as carbon sinks is widely discussed (VINSON et al. 1996). Though the productivity, and with it, the carbon uptake declines in late forest development stages old tree stands can still be important carbon sinks (ZHOU et al. 2006, DOLMAN et al. 2010). Carbon is not only accumulated in the biomass, but also in the soil (ZHOU et al. 2006, GLEIXNER et al. 2009) Therefore, much more than managed forests, old-growth forests could serve as a carbon sink (KNOHL et al.

2009). Consequently, a better knowledge on carbon stocks in over-mature and decaying

forest development stages, together with biodiversity assessments would certainly underline their exceptional importance.

In Central Europe, studies on natural forest vegetation and natural forest ecosystem services are strongly hampered. Protected forests, which have been excluded from management, make up less than 0.4 % of the total forest area (PARVIAINEN 2005). Old-growth forests are rare and long-term management continuously affects the structure, site factors and plant diversity even in forests, where the management has ceased (GRAAE &

HESKJÆR 1997, LAMEDICA et al. 2010). Many relevant studies have been performed in managed or otherwise anthropogenically disturbed forests. Most studies in natural forests have focussed on single plant groups and their responses to forest structure-related habitat traits (THOMSEN et al. 2005, MONING et al. 2009, KIRCHNER et al. 2011) rather than differences in the plant diversity of different forest development stages. Gaps in the knowledge of diversity patterns and other functions of senescent and decaying forest stands are particularly critical for the numerous conflicts between nature conservation and forest economics (EID et al. 2002, YOUNG et al. 2005, BOUGET et al. 2012). Both the exclusion of single large trees from management as well as the retention of larger old-growth patches from logging implies economic burdens for forest stakeholders (BERGSENG et al. 2012). To support the protection of forest species diversity and ecosystem services against the demands of commercial forestry, it is necessary to elucidate the ecological importance of the late forest development stages.

While earlier successional stages of natural forests are roughly included in the age classes of managed forests, common rotations do not admit over-mature and decay stages (STÖCKER 1997, KUULUVAINEN 2009). Therefore, the present study focussed on the dynamics of an unmanaged, old-growth forest ecosystem with special emphasis on the late successional stages. The investigations included both ground and epiphyte vegetation.

Additionally, the forest structure in the different development stages was analysed, including aspects of forest history and biogeochemical features of natural stand structures providing epiphyte habitats. The main objectives of this thesis were to

(1) Outline the importance of late successional stages for the diversity and species richness of the forest vegetation,

(2) assess the diversity patterns of deadwood-inhabiting vegetation and the significance of deadwood to the plant diversity in natural forests,

(3) examine the effect of stand history and tree age on the plant diversity.

Additional studies were to address

(4) the ecological response of epiphytic lichens to site conditions in natural forests and (5) the possible function of senescent and decaying tree stands as carbon sinks.