Network of Strict Forest Reserves as reference system for close to nature forestry in Lower Saxony, Germany
Peter Meyer
Niedersächsische Forstliche Versuchsanstalt, Grätzelstrasse 2, D-37079 Göttingen, Germany.
pmeyer@nfv.gwdg.de
Abstract
Since 1972 continuous research in Strict Forest Reserves (SFR) has been conducted in Lower Saxony. During this time a network of reserves has been established covering all major soil and forest types. Forest structure has been monitored for more than 30 years. In this paper the main principles and problems of researching forest structure in SFRs are outlined and the state of the art, and development of the monitoring program are described and discussed.
Research in SFRs is based on long-term monitoring with non-destructive methods. Continuity of research over long periods of time, concerning in particular assessment methodology, data management and analysis, is a major challenge. However, monitoring methodology has under- gone specific changes with respect to size, number and distribution of plots as well as attributes to be measured. The periodic change in research aims during the course of the SFR research is per- ceived to be a central and specific problem. The existing database serves as a pool of information with growing relevance for close to nature silviculture. Research undertaken in Lower Saxony has inter alia focussed on competition between beech (Fagus sylvatica L.) and light demanding tree species, gap dynamics and deadwood. Corresponding results are presented here, as are the con - clusions drawn for forest management and nature conservation.
The results show that a causal understanding of processes is a prerequisite for translation into management practice. Disturbance regime, forest history and site conditions are identified as major determinants for structural dynamics. The initial situation appears to influence forest dynamics for a long period of time. Thus structural dynamics in SFRs can be considerably different from those in primeval forests.
Keywords: monitoring, beech, competition, disturbance, forest structure, gaps, deadwood
1 Introduction
Nowadays forestry is expected to yield more than timber. Production forestry is increasingly replaced by multi-purpose forestry in industrialised countries. Nature conservation values and economic return play an equally important role. In order to achieve conservation targets the concept of naturalness is a promising approach (SCHERZINGER1996). In addition nat - ural processes are also gaining attention as a tool for optimising forestry: input shall be reduced by their integration into management to a much greater extent.
Consequently profound knowledge about natural processes is needed to ensure that forest development is still in line with targets set. In this context it is critical that many questions relating to natural forest dynamics are not yet sufficiently answered. For example, know -
ledge about competition and regeneration of tree species on different sites, gap dynamics in forest canopies, intrinsic development of forest structure, and the amount and structure of deadwood supply is still rather vague.
Unmanaged forest reserves, so-called “Naturwaldreservate” or “Strict Forest Reserves”
(SFR, European Commission 2000), are regarded as reference sites for natural forest dynamics. Expectations are high that research in SFRs will resolve many questions. They are meant to serve as a substitute for primeval forests, which are very rare in Europe because man has managed forests intensively over hundreds and thousands of years. Since the begin- ning, research in SFRs aimed to enhance close to nature silviculture (HESMER1934) by improving knowledge about forest ecology. Moreover designation and study of SFRs was always closely linked with nature conservation considered reference sites for natural forests.
At present most of the German “Länder” have built up an extensive network of SFRs.
Until now about 800 SFRs have been designated, comprising an area of approximately 28 000 ha. Systematic monitoring has been conducted for approx. 30 years. In the paper presented outcomes of research in SFRs undertaken in Lower Saxony are evaluated.
Answers to the questions “Have expectations been fulfilled or should they be scaled down?”
and “What have we already learnt about close to nature forestry?” shall be sought.
2 Network, methodology and research concept
2.1 Development from 1972 to 2003In Lower Saxony a pool of 63 SFRs comprising an area of 1010 ha was established between 1972 and 1974. The average size of SFRs was 15.8 ha. Nearly all major forest types in north- western Germany were represented (LAMPRECHTet al. 1974). SFRs were solely established in state-owned forests.
The Institute of Silviculture at the University of Göttingen was responsible for research in SFRs. A research concept focusing on forest structure was applied, which based on “core areas”, i.e. 1 ha square or rectangular plots placed subjectively in an SFR in order to represent a certain forest type. DBH, species and height (sampled) of all living as well as standing dead trees above 4 cm DBH were recorded (detailed methodology see LAMP -
RECHT1980). In the 1980’s most core areas were remeasured using the same methodology.
Lying dead trees, if recorded in the first inventory, were also recorded. Research into other areas, such as ground vegetation, soil and humus or fauna was rare.
SFRs regained attention in Lower Saxony in the 1980’s when the political regime aimed to double the area under nature protection. In response the forest administration stated the intention to designate at least 1 %, or about 3200 ha of the state-owned forests as unman- aged SFRs (GRIESE 1989). The responsibility for research in, and selection of SFRs was vested in the Lower Saxony Forestry Research Station.
An extensive process for the selection of new SFRs was commenced and most of the old reserves were enlarged (GRIESE 1997). Those that proved too small or unrepresentative were dropped. The representativeness of the site and the forest type within the study area were the main criteria used in the selection of suitable SFRs. In addition a minimum size of 40 ha constrained selection. By 1998 a transformed network of SFRs had been established (Fig. 1) encompassing 104 reserves with a total area of 4477 ha and an average area of 43 ha per SFR. All main forest types and sites were represented. However climax forests on poor soils are lacking as these sites are mostly stocked with early successional and/or exotic tree species in north-western Germany.
Parallel to the selection of new SFRs the methodology of monitoring forest structure had undergone major changes in the 1990’s. The new concept continues to be applied.
Systematically distributed circular sample plots, 0.1 ha or 0.05 ha in area, were introduced in order to reveal representative results about the entire area within a particular SFR. This in turn made it necessary to establish a permanently marked grid network (width of 100 × 100 m or 50 ×50 m respectively). Old core areas were partly dropped, some new ones estab- lished and many old ones remeasured. In contrast to assessments in the 1970’s and 1980’s deadwood and regeneration < 7 cm DBH are also included. Shrub and regeneration layers are assessed in sub-samples within the circular plots, or the core area. Coordinates of all trees above 7 cm DBH are taken. However the new methodology was found to be time con- suming and costly. Thus, research intensity was graded into three levels (Table 1).
Table 1. Levels of research intensity in Strict Forest Reserves of Lower Saxony.
Level Grid Network Network of circular Core area SFR
(permanently sample plots Number Area
marked)
High obligatory obligatory obligatory 34 1791
Medium obligatory (obligatory)*1 optional 37 1880
Low missing missing optional 33 806
All 104 4477
*1= smaller sample plots and less intensive assessments than in level “high”.
Fig. 1. Current network of Strict Forest Reserves (SFR) in Lower Saxony. Dots indicate SFRs, thin lines indicate growth regions.
Since the 1990’s strengthening cooperation with external scientists has reinforced inter- disciplinary research in SFRs. This has lead to an increase in comparative studies of SFRs and managed forests especially in relation to ground vegetation (SCHMIDT2000; SCHMIDT
and WECKESSER2001; EBRECHTand SCHMIDT 2001; OHEIMB 2003). Zoological studies have also been conducted although less frequently (e.g. KELM1996; FINCH 2000). Overall there are copious studies relating to SFRs in Lower Saxony, comprising reviewed publi - cations, doctoral and diploma thesis. Furthermore site and biotope mapping data as well as data about flora and fauna are available. This wealth of information is extremely hetero - geneous. At the Forestry Research Station, Lower Saxony, an effort is currently being made to revise existing information in order to publish a comprehensive overview of existing results and background information.
2.2 Basic considerations and recent experiences
After 30 years some specific aspects of research in SFRs should be emphasised:
1. It is self-evident that field methodology needs to be non-destructive to ensure that nat - ural processes are not jeopardised by research. However the limits between destructive and non-destructive research are not always indisputable. In Lower Saxony small soil samples as well as buds from regeneration may be taken while coring of trees and exten- sive trespassing, for example, are not allowed.
2. The fact that most SFRs were managed forests that were only recently released from management needs to be taken into account. In comparison with the longevity of the development cycle and succession in forests the monitoring period is rather short. The ongoing dynamics will continue to be influenced by the initial man-induced structures for a long period of time (KOOP1989; TABAKUand MEYER1999).
3. The long-term monitoring approach only facilitates explorative data analysis. Objectivity needs to be maintained in order to ensure unbiased results. Because of the abundance of information gathered over decades there is considerable risk of selective data analysis, with results that do not fit expectations being left aside.
4. With ongoing monitoring a growing database will be established. The abundance and heterogeneity of information anticipated becomes a major challenge for researchers. The time needed for updating, proofing and the contemporary analysis of monitoring data is often underestimated. Considerable input will also be necessary to harmonise information within the SFR-network.
5. Forest dynamics are generally closely related to site, as well as many other known and unknown factors. Thus a causal understanding of process is a prerequisite when using SFRs as reference sites for silvicultural or nature conservation concepts. Otherwise the risk of drawing inappropriate conclusions based on single observations is high. The appli- cation of interdisciplinary research is the key to understanding causal relationships.
3 Selected results
3.1 Dynamics of tree species composition
In past decades the natural tree species composition and the spatial distribution of forest types in northwestern Germany has been the subject of intensive discussions (TÜXEN1937;
JAHN1979, 1983, 1984, 1987; LEUSCHNER1998; GRIESE1994; MEYERet al. 2000). The ongoing debate focuses on the role of light-demanding tree species, in particular pedunculate and sessile oak (Quercus robur L., Quercus petraea Mattuschka), in natural forests. Earlier per- ceptions of potential natural vegetation are currently being revised whereby an increasing proportion is assigned to beech forests (Fagus sylvatica L.). This is supported by results from SFR research, which show that the proportion of beech has increased considerably (Fig. 2, MEYERet al. 2000).
However, results need to be interpreted with caution as the monitoring period is still short and exogenous factors, e.g. oak decline or bark-beetle attacks, have led to a decrease in admixed species. Hence, the increase in beech can only be subscribed to asymmetric compe- tition to some extent. Moreover dieback of beech on wet sites, e.g. in the SFR “Friedeholz”
(Fig. 2), indicates site factors, water-surplus in particular, are limiting for beech dominance.
This is also reflected by results from levelling microrelief in a core area of SFR “Landwehr”
combined with spatial tree distribution, which indicate that old and young beech trees thrive mainly on elevated micro-sites where they are less prone to flooding (MEYERet al. 2000).
Limker S trang 100
90 80 70 60 50 40 30 20 10 0
Basal area [%]
Stöber hai
Göh rder Eic
hen Königsbu
che Hünstollen
Haring er Berg
Landwehr Walbec
ker W arte Friedeh
olz Genus : Acer/ F raxinus Carpinus F agu s
Others Picea Quercus
SFR
Fig. 2. Development of tree genus composition in core areas of 9 SFRs in Lower Saxony from first (left column) to last assessment (right column). Monitoring period comprises 19 to 29 years. Three forest types are represented by three SFR respectively: Luzulo-Fagetum by SFR 1 to 3 (starting with “Limker Strang”), Galio odorati-Fagetum by 4 to 6 (starting with “Königsbuche”) and Mixed Oak-Beech- Hornbeam Forests by 7 to 9 (starting with “Landwehr”).
3.2 Gap dynamics in beech forests
To date different viewpoints about the natural structure of beech forests are held. While JENSENand HOFMANN(1996) assume that beech forests tend to be uniformly structured over long periods of time, results from primeval forests show that diameter distribution of beech forests often resemble an inverse J-curve, indicating a high degree of structural diver- sity (KORPEL1995; TABAKU1999; MEYERet al. 2003).
In beech forests the spatiotemporal distribution of canopy gaps plays a vital role in regeneration success and therefore determines forest structure to a large degree. Gap dynamics have been studied in three Beech-SFRs on acid soils based on aerial photographs.
Photographs suitable for gap analysis (Scaled 1:6000 to 1:12 5000, mainly CIR, SFR in approx. mid position) date back to late 1970’s and early 1980’s. The monitoring period com- prises 17 to 24 years (Table 2).
The SFRs “Limker Strang” and “Vogelherd” are situated in the Solling Mountains at an elevation of 400 to 500 m a.s.l. The SFR “Lüssberg” is situated in the lowlands of Lower Saxony at approximately 100 m a.s.l. The SFRs are 143 (“Limker Strang”), 174 (“Vogel - herd”) and 183 (“Lüssberg”) years old respectively.
Once the SFRs were released from management the majority of gaps originated from a heavy storm in 1972. A small number were the result of earlier cuttings.
Gap dynamics in all SFRs studied are rather similar (Table 2). The gap area decreased considerably within the monitoring period (Fig. 3) though severe storms post 1972 also occurred and single trees have been killed by Fomes fomentarius L. The observed stands show a high capacity to respond to the major 1972 disturbance event. The average gap size is equivalent to the crown area of a dominant beech tree, indicating that primarily single tree gaps were formed.
While beech regeneration has established within gaps, the density of canopy increased in the remaining area, resulting in density-dependent mortality of suppressed trees. Thus struc- tural dynamics proceeds in two contrasting directions. On the one hand gaps contribute to structural diversity by enhancing regeneration. On the other hand forest structure tends to become more homogeneous where degree of canopy closure increases, which occurs over the greater part of the SFR area.
Furthermore light availability within gaps is steadily reduced by lateral gap closure.
Average values of lateral closure range from 7.4 cm/a-1to 14.4 cm/a-1. Assuming a gap is circular, an average sized gap achieves closure after about 50 years. It is unlikely that regen- eration will achieve canopy height within the “lifespan” of one gap because assessments show regeneration did not exceed a diameter limit of 7 cm 30 years after gap formation (compare RUNKLEand YETTER1987). Given that rate of gap formation continues to be as low as in the monitoring period, forest structure will become increasingly homogeneous over large areas of the SFRs studied.
Table 2. Main results of a canopy-gap analysis in three beech-SFRs in Lower Saxony based on aerial photographs.
SFR Year Gap area Number of Average Maximum
[%] gaps/ha gap size ± s gap size
[m2] [m2]
Limker Strang 1982 11.0 8.4 131 ± 237 2252
1992 4.8 5.3 91 ± 125 849
2000 3.0 3.1 97 ± 123 815
Vogelherd 1982 14.6 17.0 86 ± 91 636
1989 6.0 8.7 69 ± 58 253
1999 5.3 8.1 66 ± 54 245
Lüßberg 1977 17.6 10.8 163 ± 318 3179
1990 10.8 8.7 123 ± 208 1740
2001 8.0 7.6 105 ± 170 1356
Fig. 3. Gap pattern dynamics derived from aerial photographs in the SFR “Limker Strang” from 1982 to 2000.
1982 1992 2000
3.3 Deadwood
At present deadwood is widely recognized as a key factor of biodiversity in forests. This is reflected in the Helsinki-process (MCPFE 2003), where deadwood has been adopted as an indicator of sustainable forest management. Nevertheless structure and dynamics of dead- wood, in different forest types, needs further clarification. In this context research in SFRs can yield important results.
In most of the old reserves a considerable accumulation of deadwood can be observed since management has ceased. After two to three decades 30 m3/ha or more of deadwood had accumulated (MEYER1999). Deadwood over 7 cm (diameter at butt end) largely orig - inated from dead trees. Broken crowns and branches were less important. Main causes of tree death were storms and density-dependent mortality. In addition, oak decline and attacks by bark beetles (spruce) or Fomes fomentarius L. (beech) contributed to deadwood accumulation. Senescence seldomly occurs as most of the SFRs in Lower Saxony are rather young compared to maximum ages tree species are able to reach.
The percentage of deadwood seems to be significantly lower in pure beech forests than in mixed oak forests (Fig. 4). This may be due to the higher decomposition rate of beech wood.
Moreover the proportion of lying deadwood is at least two to three times higher than the proportion of standing deadwood (Fig. 4). In beech forests particularly snags have already broken down after some years. High turnover seems to be characteristic for this deadwood compartment.
Results show that, even in one forest type, the amount and structure of deadwood may be very variable in space and in time (see also MEYER1999; MEYERet al. 2003). Dynamics depend on different factors. For instance, the high percentage of deadwood in SFR 26 is mainly due to oak decline while in SFR 46 density-dependent mortality in the understorey is a major source of deadwood (Fig. 4). Therefore, in addition to disturbance regime and site con ditions, initial stand structure and forest history may influence deadwood dynamics to a large degree.
Fig. 4. Percentage of deadwood (proportion of living and dead stand > 7 cm DBH) in core areas of 9 SFRs in Lower Saxony in ascending sequence. Numbers and forest types of reserves are indicated as follows: 30 = “Lüssberg”, 02 = “Franzhorn”, 14 = “Hünstollen”, 58 = “Lohn”, 41 = “Vogelherd” ‚ 46 =
“Königsbuche”, 53 = “Friedeholz”, 26 =“Landwehr”; LuFa = Luzulo-Fagetum, GaFa = Galio odorati- Fagetum, StCa = Stellario-Carpinetum, FaQu = Fago-Quercetum.
25
20
15
10
5
0
Dead wood [%]
30 LuFa 02 LuFa 14 GaFa 58 GaFa 41 LuFa 02 StCa 46 GaFa 53 StCa 26 FaQu SFR
Type: Lying Standing
4 Discussion
Upon close consideration the results presented show that monitoring in SFRs can either help to develop and refine silvicultural and conservation concepts or to reveal contradictions within existing concepts.
Results about tree species dynamics are an example of the latter. They show that conflict may arise between the nature conservation aim to cease management on the one hand and to enhance biodiversity on the other hand. Obviously a high degree of naturalness and a high degree of biodiversity can be contradictory. For instance, as monitoring results confirm, beech forests tend to develop towards mono-specific stands (JAHN1979, 1983; LEUSCHNER
1998; MEYERet al. 2000). Thus there is the risk of local extinction of admixed species in SFRs or other protected areas without management.
An example of the refinement of concepts includes reference values for deadwood share and structure. Results from SFRs point to the necessity to differentiate between forest types.
In beech forests, for example, different views about the characteristic amount of deadwood are held. The amount reported by KORPEL(1997) and SANIGAand SCHÜTZ(2001) from Slovakian primeval beech forests as well as modelling results of WINTERand RADEMACHER (2003) are much higher than results from Albanian primeval beech forests (TABAKU1999;
MEYERet al. 2003). As the results from SFRs indicate the amount of deadwood is consider- ably smaller in beech forests than in forests consisting of tree species with wood that decom- poses slowly. Therefore lower amounts in Albanian primeval beech forests seem to be reasonable. In-depth studies of processes related to deadwood in SFR, in particular input and output rates, should be conducted to gain a causal understanding of deadwood dynamics.
Furthermore the predominance of lying wood components and the high turnover of snags cast doubt upon recent targets of the silvicultural program for Lower Saxony state forests as well as many regulations for forested nature protection/conservation areas. Here the values of standing dead trees in old stands are set at 5/ha (state forests in general) and 10/ha (nature protection areas) irrespective of forest type. In view of the results from SFRs (MEYER1999) it may not be realistic to maintain these targets in all forest types.
The results concerning gap dynamics contribute to the understanding of natural struc- tures in beech forests but at the same time point to limitations in transferring results directly to managed forests.
This research confirms the observations made in many temperate broadleaved forests that single tree gaps are formed (RUNKLEand YETTER1987; YAMAMOTO1989; CANHAM
et al. 1990). High rates of gap formation and/or slow closure of small gaps will lead to diverse forest structures as observed in primeval beech forests (DENGLER 1931; KORPEL 1995;
TABAKU 1999; MEYER et al. 2003). In contrast fast gap closure and/or low rates of gap formation will lead to uniform structures. Though gap patterns are similar in SFRs and primeval forests (TABAKUand MEYER1999), the underlying processes seem to be different:
in contrast to assumptions about primeval forests low rates of gap formation and high rates of closure were observed in the SFRs. The similarity can be subscribed to the outstanding disturbance in 1972 from which the SFRs are recovering rather fast, thereby undergoing a phase similar to the gap pattern of primeval forests. Forest structure is significantly less diverse in SFRs than in primeval forests because gap proportion is constantly decreasing.
However increasing stand age and heavier disturbances may lead to an increasing pro - portion of canopy gaps in the future.
The high capacity of the canopy to respond is typical for beech forests and may be the main explanation for the formation of mono-layered stands in the optimum stage. In man- aged forests it is inevitable that most of the trees will be harvested during this stage because wood quality devalues rapidly later on. Thus the transfer of concepts gained from highly
structured natural beech forests with a complete development cycle to managed forests with a shortened cycle ending at the optimum stage is questionable. Nevertheless diverse struc- tures in beech forests remain a reference point for nature conservation.
Moreover the results of gap dynamics reveal that single and group tree harvesting is a close to nature technique. However, this will only lead to structural diversity under specific conditions. On the one hand gap formation must be fast enough to allow for the continuous development of regeneration. On the other hand it must be so slow and spatially hetero - geneous that there is no uniform development of regeneration. Aside from these consider - ations it is uncertain whether small-scale development of regeneration yields satisfactory plant quality (HUSSand CONRAD2000). These considerations show that direct translation of results from SFRs or natural forests may contradict economical/silvicultural targets.
5 Conclusions
The fact that monitoring methodology has changed significantly over three decades points to a deficiency of target-oriented research in SFRs. In most cases methodologies designed for and applied in primeval forests were adopted (LEIBUNDGUT 1993) although their suitability for long-term monitoring in SFRs was not proven. Target-oriented research is an intrinsic problem in the long-term monitoring of SFRs. Changing research issues over time often result in adaptations of the methodology, at the risk of losing continuity. For instance deadwood, at present a highly relevant topic, was not considered in the first assessments. In order to overcome this problem MEYER(1997) suggests a methodological approach that is based on monitoring the basic processes of population dynamics. Here, the aim of monitoring SFRs is to build up a pool of time series information with great relevance for forest ecology especially in the long-term, instead of adapting the methodology to incorporate immediate topical issues.
The results presented reveal that previous research in SFRs has already yielded insights into forest ecology, which are relevant for close to nature forestry as well as nature conser- vation. Furthermore they are specific in that they would not have been obtained from other sources or methodological approaches. This confirmes that long-term monitoring in SFRs makes sense. However, well-founded results from long-term monitoring can not be expected in the short-term. Given that monitoring as well as proper data management and analysis continues, growing relevance of results from SFRs can be anticipated.
Moreover it is self-evident that long-term monitoring in SFRs is only one branch of research needed to enhance silviculture and nature conservation. Further, experimental approaches are indispensable for hypothesis testing. Rather than relying on one approach the wise combination of different approaches may yield optimal results.
Long-term monitoring in SFRs is a substitute for research in primeval forests only in so far as understanding of natural processes is concerned. Because forest history and the initial stand structure influence forest dynamics for a long period of time structural dynamics can be considerably different from those in primeval forests. This must be taken into account if results are applied in forest management and nature conservation.
Acknowledgments
I would like to thank Helen Desmond for revising the language of this paper and for giving important conceptual advices.
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Accepted February 2nd, 2005
