Interaction of gap age and microsite type for the regeneration of Picea abies
Introduction
The understanding of regeneration of tree species is important as it ensures the constancy of forest ecosystems. As gaps, i.e. openings in the closed canopy, result in changed environmental conditions such as improved light conditions (Poulson and Platt, 1989), a release from competition (McCarthy, 2001), and the creation of a broad variety of new microsites (Gray and Spies, 1997, Kuuluvainen and Juntunen, 1998, McCarthy, 2001), they play a crucial role for regeneration in all forest types, including tropical forests (Denslow, 1980), temperate deciduous forests (Bruelheide and Luginbühl, 2009), mixed mesophytic forests (Busing, 2005), subtropical broadleaved forests (Barik et al., 1992) and boreal conifer forests (Qinghong and Hytteborn, 1991, Drobyshev, 2001). Gaps may enhance regeneration of tree species, partly through facilitating the establishment of new individuals and partly by promoting the growth of seedlings and saplings established prior to the gap's creation (advance regeneration; Fraver et al., 2008). Advance regeneration is a continuous process that provides a bank of saplings of different ages, and thus forms a type of basic supply with recruits for forest regeneration, while regeneration on newly created microsites in gaps might result in recruitment pulses (Fraver et al., 2008). Thus, many studies have stressed the role of gap-induced microsites – such as snags, stumps, logs and root plates – on seedling establishment (e.g. Kuuluvainen et al., 1998, Zielonka, 2006). The importance of coarse woody debris (CWD) has been particularly emphasized for the natural regeneration of Picea abies (Zielonka and Piatek, 2004, Hunziker and Brang, 2005, Motta et al., 2006, Zielonka, 2006). In particular, decayed logs serve as seedbeds for tree regeneration (Kuuluvainen et al., 1998). However, the existence of CWD is decoupled from gap creation, as dead wood offers optimal regeneration conditions only after several decades, depending on the rate of decomposition, also expressed as decay classes of CWD (Mori et al., 2004, Iijima and Shibuya, 2009). In addition, snags may remain standing for 20 yrs before toppling down (Storaunet and Rolstad, 2002). With increasing degree of decomposition, CWD shows increasingly positive effects on seedling and sapling density (Zielonka, 2006). Thus, recruitment of P. abies steadily increases between a period of 30 and 60 yrs following a tree death (Zielonka, 2006). However, it can be expected that these effects are counterbalanced by a closing canopy with gap age. Runkle (1998) found that after 14 yrs the positive correlation between stem density of saplings and gap age reverted to a negative correlation due to the process of self-thinning. Finally, seed cohorts arriving in older gaps have fewer chances to become established than those that arrived earlier. Overall therefore, it may be assumed that tree regeneration exhibits a more optimum relationship when regarded in respect of gap age.
While most studies on regeneration of Norway spruce have focused on establishment and growth on CWD, as stated above (see the review of Kuuluvainen, 1994), much less attention has been paid to the regeneration of spruce on ordinary regeneration sites, i.e. intact ground including forest floor with moss-covered rocks and boulders (but see Kupferschmid and Bugmann, 2005). The typical approach usually involves a comparison of the number of seedlings on CWD with that on microsites not related to dead wood (e.g. Szewczyk and Szwagrzyk, 1996). For example, according to a study of regeneration patterns in a virgin spruce forest in Sweden, 40% of all spruce individuals shorter than 1.3 m grew on decomposing logs and stumps, which covered only 6% of the forest floor (Hofgaard, 1993). However, different microhabitat types have only rarely been taken into consideration, for example, by comparing biometric data of saplings (e.g. height, number of whorls, growth) between gap-induced microsites, i.e. CWD, and ordinary microsites. One exception is the study of Kupferschmid and Bugmann (2005) on spruce regeneration at four different elevations in montane forests in the Swiss Alps. Differences among microsites in tree-height increment varied with altitude, with reduced growth on CWD compared to the forest floor at the highest elevation, while all other elevations showed no differences between microsites.
The crucial question for tree regeneration is how different microsite conditions might vary with gap age. As outlined above, a time dependency can be expected for gap-induced microsites such as CWD, however, the relative contribution of CWD compared to other microsites over time is far from clear. CWD in any particular gap may be of mixed temporal origin, which can result in the presence of dead wood at different stages of decay (Zielonka and Piatek, 2004). It might well be that microsites differ in their relative contribution to tree regeneration and show different temporal optima. The obvious approach to address this topic would be chronosequencing, i.e. comparing regeneration on microsites in gaps of different ages (gaps varying in the number of years since their creation). Such an approach would also include a space-for-time substitution for microsite development, comparing microsites across different gap ages and thus assessing the importance of microsites over time. There are indications that gap-induced microsites are less favorable long-term regeneration sites than non-gap induced microsites. In a study of tree regeneration on decaying wood and on the forest floor in old-growth mixed forest stands in Poland, Szewczyk and Szwagrzyk (1996) found that the density of spruce seedlings on decaying wood decreased with seedling age, while this was not the case on the forest floor.
The near-natural montane Norway spruce forest at Mt. Brocken in the Harz National Park in Central Germany facilitates the chronosequencing of gaps and offers a broad variety of microsite types, either created by disturbance or ordinary ones. This allowed us to characterize the combined effects of gap age and microsite type on the regeneration of P. abies. We hypothesized that: (1) the density of P. abies recruits shows a unimodal course as a function of gap age; (2) gap-induced microsites contribute most to tree regeneration; and (3) the importance of specific microsite types for regeneration of P. abies changes with gap age.
Section snippets
Study area
The study area comprised 225.18 ha of the near-natural Norway spruce forest in the core zone of the Harz National Park on the north-eastern slopes of Mt. Brocken (Saxony-Anhalt, Germany; 10°37′15″E 51°48′6″N; 900–1050 m asl). Meteorological data showed a mean annual wind velocity of 5.6 Beaufort, a mean annual precipitation of 1727 mm yr−1 and a mean annual air temperature of 3.1 °C (1951–2005; German Weather Service). Owing to a granitic bedrock the soils have pH values around 4, are rich in
Results
At plot level, relative light intensity, measured as PPFD [%], was significantly dependent on gap age class (mixed model, p = 0.0091). This was only due to the significantly lower light intensity in undisturbed plots compared to disturbed plots (contrast, p < 0.0001). Post hoc tests showed no significant differences within disturbed plots of the different gap age classes. Similarly, there was no significant correlation between PPFD [%] and exact gap age determined by the dendrological approach (r2 =
Discussion
With regard to our first question of whether gap age has an influence on the regeneration of P. abies in a montane spruce forest in Germany, the most striking result was that sapling density neither differed between the different categories of gap age, nor showed a linear or unimodal relationship with gap age. Thus, the first hypothesis of an optimum gap age for sapling density has to be rejected. This result does however allow for some interesting conclusions. Firstly, seedling establishment
Acknowledgments
We are very grateful to the team of the Harz National Park for providing aerial photographs and for the permission to work in the core zone. We are thankful to Prof. Dr. F. Schweingruber for his idea to take cores from trees on gap borders. Thanks to Kathrin Kirchner for her help in field and providing information on gap sizes as well as to Annett Baasch and Bea Vonlanthen for their comments on an earlier draft of this manuscript. The help of Daniel McCluskey with the final linguistic check is
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