Understorey light conditions and regeneration with respect to the structural dynamics of a near-natural temperate deciduous forest in Denmark

Forest Ecology and Management 106 Ž1998. 83–95

Understorey light conditions and regeneration with respect to the structural dynamics of a near-natural temperate deciduous forest in Denmark Jens Emborg

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Danish Forest and Landscape Research Institute, Department of Forestry, Hørsholm KongeÕej 11, Hørsholm DK-2970, Denmark Accepted 3 October 1997

Abstract Suserup Skov is a near-natural temperate mixed deciduous forest dominated by Fagus sylÕatica L. Žbeech. and Fraxinus excelsior L. Žash. with some Quercus robur L. Žoak., and Ulmus glabra Huds. Želm.. The forest dynamics in Suserup Skov can be described by the forest cycle, a sequential shift between a series of developmental phases: innovation, aggradation, early biostatic, late biostatic, and degradation phases. Climax microsuccession from ash to beech occurs as an integral part of the forest cycle. The spatial match to the forest cycle is a shifting mosaic of the same phases. The light conditions at the forest floor and the regeneration of beech and ash were studied with regard to the mosaic-cycle. The relative light intensity ŽRLI. and the variation in RLI ŽCV-RLI. were measured in patches representing different phases of the mosaic-cycle. RLI and CV-RLI were compared between phases by analyses of variance, the RLI-model as well as the CV-RLI-model were statistically highly significant. RLI was below 2% in all phases of the forest cycle, except in gaps Ždegradation and innovation phases.. In gaps RLI increased to about 10%, making regeneration of ash and beech possible. CV-RLI was higher under ash dominated canopies than under beech-dominated canopies. Regeneration of ash and beech did not survive at RLI below 2%. At RLI above 3%, regeneration of both ash and beech developed successfully. Successful establishment and development of ash andror beech regeneration only occurred in gaps. Advance regeneration of both beech and ash occurred under smaller, often temporary, canopy gaps. In an examined gap, ash established first, responding with rapid height growth at increasing light levels. Beech was established at the next mast year. Under the ash plants RLI was above 3%, which made the establishment and development of a beech understorey possible. The studied gap represents the beginning of a successional shift from ash to beech within the forest cycle. The results have practical implications for nature-based silviculture, especially concerning choice of regeneration strategy and management of stand structures in order to improve the conditions for regeneration. q 1998 Elsevier Science B.V. Keywords: Forest cycle; Mosaic-cycle; Gap-dynamics; Succession; Climax vegetation; Nature-based silviculture; Fagus sylÕatica; Fraxinus excelsior

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Corresponding author. Tel.: q45-45-76-32-00; fax: q45-45-76-32-33; e-mail: [email protected].

0378-1127r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 1 1 2 7 Ž 9 7 . 0 0 2 9 9 - 5

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J. Emborgr Forest Ecology and Management 106 (1998) 83–95

1. Introduction Sustainable forest management is a major challenge facing modern silviculture. Part of the challenge is to develop nature-based silvicultural models ŽKoop, 1989; Boot et al., 1993; Broekmeyer and Vos, 1993; Schlaepfer et al., 1993; Attiwill, 1994; Bradshaw et al., 1994., including the use of natural regeneration. Light is a key growth factor in combination with water and nutrients, determining regeneration success ŽBurschel and Schmaltz, 1965; Madsen, 1994; Madsen, 1995; Dai, 1996; Madsen and Larsen, 1997.. Natural forest dynamics is a basic reference for development of nature-based silvicultural models ŽKoop, 1989; Broekmeyer et al., 1993; Peterken, 1996; Scherzinger, 1996.. The present study focuses on the light conditions and regeneration patterns with regard to the structure-dynamics of a near-natural forest. Watt Ž1925, 1947. presented the concept of the forest cycle as a basic model of the natural forest dynamics in temperate deciduous forests. The forest cycle describes the continuous shift between a sequential series of developmental phases: innovation, aggradation, early biostatic, late biostatic and degradation phases. The cycle runs over time at any patch of the forest, unsynchronous from patch to patch, creating a shifting mosaic of the developmental phases ŽWatt, 1947; Bormann and Likens, 1979; Oldemann, 1990.. In short, the whole complex in time and space is referred to as the mosaic-cycle ŽRemmert, 1991..

Fig. 1. The forest cycle of Suserup Skov occurring through time, in principle at any given place in the forest. In Suserup Skov, one turn of the ideal cycle takes some 280 yr Žafter Emborg, 1996..

The forest cycle in Suserup Skov, a near-natural forest in Denmark, has been modelled and the mosaic of the developmental phases has been mapped in 1993, Figs. 1 and 2 ŽEmborg, 1996.. A full turn of the cycle in Suserup Skov was estimated to last on average some 280 yr. A successional shift Žclimax microsuccession, Forcier, 1975. from Fraxinus excelsior L. Žash. to Fagus sylÕatica L. Žbeech. occurs as an integral part of the forest cycle in Suserup Skov. This successional interaction implies that the canopy usually is dominated by ash in the aggrada-

Fig. 2. The shifting mosaic in Suserup Skov, mapped in 1993 Žafter Emborg, 1996..

J. Emborgr Forest Ecology and Management 106 (1998) 83–95

tion phase and the first half of the early biostatic phase, and by beech during the rest of the early biostatic and the late biostatic phases. The degradation and innovation phases are characterised by the occurrence of gaps in the canopy. The mosaic cycle represents a continuous change in ecological structure, associated with species, growth, architecture, age, size and vitality of the dominant trees. The structural change involves continuing changes of the understorey light intensity largely controlling the vegetation at the forest floor including regeneration of trees. The objective of the present study was to describe the light conditions at the forest floor and the resulting regeneration patterns with respect to the structural dynamics of a natural forest. Suserup Skov is a well documented near-natural forest located on good and fertile forest soil, thus forming a useful reference for the development of nature-based silvicultural models for beech forestry, especially for the better forest soils of the region. 2. Materials and methods 2.1. The site Suserup Skov Ž55822X N, 11834X E. in eastern Denmark has a long history of low human impact under a relatively calm natural disturbance regime ŽFritzbøger and Emborg, 1996.. The study was conducted in a 10.7 ha plot representing the least human influenced part of the forest. The shifting mosaic of the plot is close to a dynamic steady-state condition, and the diameter distribution resembles a negative exponential function ŽEmborg et al., 1996., characteristic of old growth forest ŽBormann and Likens, 1979; Oldemann, 1990; Oliver and Larson, 1990.. Beech dominates the plot accounting for 64% of the basal area, Quercus robur L. Žoak. accounts for 15%, ash 13% and Ulmus glabra Huds. Želm. 6% of the basal area. Oak is represented by a few large, 250–500-year-old individuals, probably remnants from prior periods of grazing. Today, oak does not successfully regenerate ŽEmborg et al., 1996.. Elm typically occurs in the sub-canopy stratum, represented by many small individuals. Most soils have developed on loamy glacial till ŽVejre and Emborg, 1996.. The growth conditions are favourable as indi-

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cated by recorded tree heights of up to 41 m. The climate is cool-temperate, sub-oceanic ŽTroll and Paffen, 1963., the annual mean temperature is 8.18C and the annual precipitation 635 mm with maximum in late summer and fall. 2.2. Light measurements in the mosaic The relative light intensity ŽRLI. and the variation in RLI ŽCV-RLI. were compared between the phases of the mosaic-cycle. The light measurements were recorded in 14 plots during July 1993 ŽTable 1.. The plots were installed in randomly chosen samples Žpatches, Fig. 2. of each phase Žstratified sampling, 2–4 samples per stratum.. The northern endpoint of each plot was randomly defined by throwing a stick over the back from the centre towards the northern part of the chosen patches. Each plot formed a 27 m N–S oriented line in which 10 SKYE SKP-215 quantum censors were mounted 80 cm above ground, or just above regeneration plants taller than 80 cm, if any. The photosynthetic active radiation ŽPAR. was determined as the photon flux density Žmmolrm2rs. in the 400–700 nm wave band. The measurements from each censor was stored in a data-logger and afterwards aggregated and transformed to RLI, expressed as % of full daylight, with reference to two censors recording full daylight. Test measurements showed that the measurements of RLI was not very sensitive to the weather conditions Žsunny vs. cloudy sky.. Measurements were as far as possible carried out from dawn to dusk. There was an extremely high correlation Ž r 2 s 0.999, p s 0.0001, n s 13 representing an almost linear relation y s x . between the RLI calculated from the full data set Žall hours measured. and RLI calculated from matched hours 12–5 p.m. Accordingly the full data set was used for the analyses, regardless that some plots missed a few morning or afternoon hours. The culling level for measurements at dawn and dusk was set to 100 mmolrm2rs under open field conditions Žreference censors.. By an one-way analysis of variance ŽRLI dependent variable, phase class variable. the RLIs of the five phases were compared. The four plots in aggradation phase were further split into two sub-groups, one in which the canopy was dominated by beech ŽA1, A2. and one dominated by ash ŽA3, A4, Tables 1 and 2.. The statistical procedure was basically the

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Table 1 Description of the 14 light measurement plots InnoÕation phase I1 Well established regeneration Žabout 10-30 plantsrm2 of ash, mixed with beech, and a few Tilia platyphyllos Scop.. in the gap after the fall of half the crown of an old beech and half the crown of an old oak. I2 Well established regeneration Žtypically ) 100 plantsrm2 .. The upper stratum of the regeneration is dominated by ash, underneath is a well-developed establishment of beech. Aggradation phase A1 Pure beech about 20 m, no sub-canopy strata, no regeneration. A2 Beech about 20 m, admixed with a few elm trees, no sub-canopy strata, no regeneration. Some beech seedlings established in spring 1993 on mineral soil around uprootings, but all plants died again during summer. A3 Ash about 20 m, with slight undergrowth of beech, no regeneration. A4 Ash about 22 m, with some undergrowth of beech and elm, no regeneration. Early biostatic phase E1 Ash about 30 m, understorey of more or less suppressed beech. A few beech trees have reached the upper canopy stratum. No regeneration. E2 Ash of about 30 m, understorey of beech and scattered undergrowth of elm. A few Žtotally less than 10. small regeneration plants of beech were established at the forest floor in the central part of the patch. Late biostatic phase L1 Pure beech up to 40 m, a few suppressed beech trees in the sub-canopy strata, no regeneration. L2 Pure beech up to 40 m, some undergrowth of beech. A scattered group, of weak ash regeneration plants was established, totally about 20 plants - 25 cm high. L3 Pure beech up to 40 m high, lower stratum of 3–5 m high elm undergrowth, no regeneration. Degradation phase D1 Early stage of degradation phase consisting of one, since 10 yr, standing dead beech tree and an old degrading beech Žhaving dropped about one third of its crown.. Scattered regeneration, primarily ash. D2 Degradation phase caused by the fall Ž1992. of an old, broken beech tree, smashing the undergrowth of elm and beech. The understorey trees of elm seem to recover. No regeneration plants or seedlings yet. D3 Completely new degradation phase patch Ž1992r93. caused by the fall of an old stem-rotten ash tree, having overthrown an old beech as well. A small scattered group of ash regeneration was established before the gap occurred. A limited number of beech and ash seedlings established during summer 1993. The crown smash area Žor epicentre. seems to come under control of recovering, smashed elm trees.

same, doing the analysis of variance with six levels of phase instead of five, except that the analysis still produced a test of the five original phases against each other. The analysis of variance was succeeded by paired t-tests Žtwo-tailed. among phases with Bonferroni correction to account for multiple comparisons. The RLI values were first transformed by the arcsine transformation Žnormalising the distribution of relative values, skewed near 0 and 100%, Fowler and Cohen, 1990.. These values were logarithmically transformed to homogenise variation. The variation in RLI within phases is expressed by the coefficients of variation ŽCV-RLI. between the ten censors of each plot. A one-way analysis of variance ŽCV-RLI dependent variable, phase class variable. was carried out, succeeded by paired t-tests

Žtwo-tailed. among phases with Bonferroni correction to account for multiple comparisons ŽTable 2.. 2.3. Light and regeneration measurements in gaps In order to study the regeneration success of beech and ash with regard to the light conditions, three light measurement transects were set up in gaps. Two transects were placed across regeneration patches of ash Ž ash1, ash2 . and one transect across a regeneration patch of beech Ž beech1.. Each transect represented a gradient from successful regeneration in the patch Žgap. centre, through less successful, to poor regeneration in the edge of the regeneration patch. All transects were extended beyond the limit of surviving plants at the gap edge. The transects

J. Emborgr Forest Ecology and Management 106 (1998) 83–95 Table 2 The statistical design and the overall result of the light measurements of the 14 plots. ‘Phase’ was class variable in two analyses of variance in which respectively ‘RLI’ and ‘CV-RLI’ were dependent variables. ‘Stage’ is an estimate of the developmental stage with regard to the forest cycle, given in years from the beginning of a new cycle Žinnovation phase.. ‘Canopy’ indicates the species Žincluding nones gap. dominating the upper canopy stratum. ‘RLI’ is the relative light intensity Ž% of full daylight.. ‘CV-RLI’ is the coefficient of variation of RLI Phase

Plot Stage Žyr. Canopy RLI Ž%. CV-RLI

InnoÕation I1 I2

3 7

gap gap

Aggradation A1 A2 A3 A4

40 60 30 50

beech beech ash ash

Early biostatic E1 E2

100 120

ash ash

Late biostatic L1 L2 L3

160 250 220

beech beech beech

Degradation D1 D2 D3

265 270 275

Žgap. gap gap

8.97 a

52.1a

9.09 8.84

54.5 49.7

1.61b

21.8 b,c

1.14 2.53 1.26 1.51

11.4 10.3 27.0 38.4

1.66 a,b

30.0 a,b

1.40 1.91

30.2 29.8

1.47 b

15.4 b

1.63 1.62 1.15

5.5 19.8 20.9

7.59 a

38.6 a,c

2.74 7.68 12.34

43.1 33.3 39.3

Ž abc . Same suffixes indicate that differences between phases are not significant Ž t-test, p- 0.05 with Bonferroni correction to account for multiple comparisons., no overlapping suffixes indicate significant differences Ž p- 0.05.. Differences in RLI under beech ŽA1, A2. vs. ash ŽA3, A4. canopies were not significant. CV-RLI was significantly higher under ash ŽA3, A4. than under beech ŽA1, A2..

were 10–20 m long and not established in any specific compass direction Žhowever they all happened to be approximately east–west oriented.. In all transects light measurements Ž10 SKYE quantumcensors. were carried out twice during JulyrAugust 1993. A 1 m = 1 m square plot were installed around each censor. In the ash plots total height and height growth during the previous growing season of all plants were measured Ž Nash1 s 97, Nash2 s 114. in

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the beech plots the total height was measured Ž Nbeech1 s 352.. Mixed regeneration of ash and beech was studied in 1992 in a gap formed 1986–1990. RLI was measured ŽSKYE quantum censors. in ten 1 m = 1 m plots evenly distributed along a 27 m N–S oriented line from the southern edge of the gap to the gap centre. The ages of the ash and beech plants were determined by counting bud-scars on random samples of ash and beech plants from each plot Ž Nash s 101, Nbeech s 125.. For beech, the method probably implied a slight overestimation of the age, because some beech plants shoot twice during one growing season under favourable growth conditions Že.g. 1989 and 1991.. Heights of all plants in each plot were measured Ž Nash s 166, Nbeech s 868.. The average ages of ash and beech were compared by a two-tailed t-test. The effects of age and RLI on height growth were examined by a multiple regression analysis. Ash regeneration formed an upper layer above beech in the gap centre. RLI was measured Ž1993. simultaneously above and below the upper ash layer in the five gap centre plots in order to determine the light ŽRLI. reaching the shaded beech.

3. Results 3.1. Light conditions with regard to the mosaicphases The amount of light reaching the forest floor differed widely among the phases, from less than 2% RLI in aggradation, early and late biostatic phases to about 10% RLI in innovation and degradation phases ŽTable 2.. The analysis of variance RLI-model was highly significant explaining a major part of the total variation in RLI between plots Ž r 2 s 83%, p s 0.006.. The effect of phase was significant Ž p s 0.004. while the effect of species, beech vs. ash Žtested in aggradation phase. was not Ž p s 0.66.. This means that the amount of light reaching the forest floor depends on the particular patch of forests developmental stage Žphase., but apparently not on which species dominates in the upper canopy. The paired t-tests between the phases ŽTable 2. showed that the amount of light reaching the forest

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Fig. 3. Recorded RLIs at single censor level, frequency distributions for each phase of the forest cycle by 1% RLI classes. Aggradation phase is illustrated by two distributions, representing respectively beech- Žplot A1, A2. and ash- ŽA3, A4. dominated canopies. The stitched vertical lines indicate the light limits for survival Ž2% RLI. and successful development Ž3% RLI. of regeneration.

floor during the gap-phases Žinnovation and degradation phases. was significantly higher than in the closed canopy phases Žthough not statistically significant in early biostatic phase.. The canopy cover in aggradation, early and late biostatic proved to be homogeneous dense, since no single censor in these phases recorded RLI ) 3% ŽFig. 3.. In contrast to this there was a tremendous variation among single censor measurements in innovation phase Žranging from 1–22% RLI. and degradation phase Žranging from 3–19% RLI.. The analysis of variance CV-RLI model was highly significant explaining a major part of the total variation in CV-RLI Ž r 2 s 90%, p s 0.001.. The effect of phase Ž p s 0.001. as well as the effect of species Žash vs. beech in aggradation phase, p s 0.006. were significant. The variation in light reaching the forest floor,

CV-RLI, was highest in innovation phase ŽTable 2.. The most homogeneous canopy cover Žlowest CVRLI. occurred under relatively young pure beech in aggradation and late biostatic phases ŽA1, A2, L1.. Ash dominated canopies ŽA3, A4, E1, E2. creates a more heterogeneous light climate at the forest floor than beech dominated canopies ŽA1, A2, L1, L2, L3.. This effect of canopy species Žash vs. beech. was specifically tested in aggradation phase, where ash proved to create more heterogeneous Žhigher CV-RLI. light conditions at the forest floor than beech Ž p s 0.006.. The canopies of very old beech trees seems to create more heterogeneous light conditions at the forest floor ŽL2, L3. than younger beech canopies do ŽA1, A2, L1.. These results based on the measurements in the mosaic, representing a ‘snapshot’ of the

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Fig. 4. The upper diagram shows the light levels during one turn of the forest cycle estimated from the light measurement plots. The middle diagram shows the variation in RLI Žexpressed by the coefficient of variance. during one turn of the forest cycle. The solid line indicates the typical course of the forest cycle, including climax microsuccession from ash to beech, the dashed line shows the case of aggradation phase of pure beech. The results from each transect were placed at the x-axis according to its estimated position in the ideal forest cycle Žas indicated, Table 1 ‘Stage’..

present situation in space, can be interpreted in time with reference to the model of the forest cycle ŽFig. 4.. At the beginning of the forest cycle, after the formation of a canopy gap Žinnovation phase. ample light reach the forest floor Žabout 10% RLI.. After some 10–15 yr, the established regeneration controls the area and shades the ground ŽRLI decreases to about 2%.. This extreme darkness at the forest floor continues for some 250 yr Žduring aggradation, early and late biostatic phases. until the light level more or less gradually increases as the canopy begins to open during degradation phase. When the gradual or sudden gap-formation is complete the light level reaches about 10% RLI, representing the beginning of a new cycle of change in forest structure and light conditions. 3.2. Regeneration and light Light levels ranged from 1.5% RLI to 4.5% RLI ŽFig. 5. in the studied transects and regeneration ranged from no Žor poor, hardly surviving. regeneration at the gap edges, to successful, fast growing

regeneration in the gap centres. Only few plants had survived light levels below 2% RLI Ž1.5% for ash1, 2.2% for ash2, and 1.8% for beech1., which from a practical point of view can be considered to be light-limit of survival in Suserup Skov. Above this limit, 2% RLI, the numbers and sizes of seedlings increase with increasing light level. Above 3% RLI, the height increases with increasing RLI, while the numbers are constant, or maybe even decreasing probably due to self-thinning. In all plots receiving more light than 3% RLI there was a fair number of relatively high regeneration plants. The growth rate of ash was studied and found to be closely correlated with the total height Ž r 2 s 0.95, p s 0.0001., basically giving the same picture as Fig. 5. From the results it can be expected that, in Suserup Skov,: Ž1. no regeneration of ash and beech develops at RLIs - 2%, Ž2. only sparse and weak Žas expressed in height growth. regeneration of ash and beech will develop between 2–3% RLI, and that Ž3. regeneration of ash and beech develops successfully Žas expressed in height growth. at RLI ) 3%. The

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J. Emborgr Forest Ecology and Management 106 (1998) 83–95

Fig. 5. Plant density Ža. and height Žb. plotted against RLI. The stitched lines indicate the thresholds for survival Ž2% RLI. and successful development Ž3% RLI. of regeneration. Under field conditions exact thresholds can never be given, the thresholds should be regarded as fairly rough conclusions about regeneration development in Suserup Skov with regard to the light conditions.

results do not indicate major differences in the shade tolerance of ash and beech regeneration. In the gap studied, with mixed regeneration of ash and beech, the light intensity ranged from 4.5% RLI to 17.0% RLI, increasing from the gap edge to the gap centre, Fig. 6. The ash regeneration was considerably higher than the beech regeneration in all plots Ž p s 0.0006. and the ash plants were on average significantly older Ž p s 0.0003. than the beech plants ŽTable 3 The beech plants were on average three years old with only little variation in age, so accordingly the establishment of beech in the gap was

concentrated in the mast year of 1989. The ash plants were on average four years old showing much larger variation than beech. This leads to the conclusions: Ž1. that ash established before beech in the gap and Ž2. that the population of beech plants basically was established during one Žmast. year, while ash was established over a longer period of time. The multiple regression analysis Žheight as a function of RLI and age . for ash showed that RLI had a strong and statistically significant Ž p s 0.002. influence on the height, while age of the plants had less, though statistically significant Ž p s 0.05., influence

J. Emborgr Forest Ecology and Management 106 (1998) 83–95

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centre, even though RLI above the ash plants increased, due to increasing shade effect of ash with increasing amounts of light.

4. Discussion and conclusions 4.1. Light intensity and regeneration success

Fig. 6. RLI Žmeasured above the ash plants. and height of plants in a mixed ash and beech gap-regeneration. The 10 plots represent a light gradient from the gap edge Žleft. to the gap centre Žright..

on the height. The regression analysis for beech showed that the influence of RLI Žmeasured above the ash plants. on height was highly significant Ž p s 0.0003., while the effect of age was non-significant Žwhich is not so surprising regarding that most beech plants were established in the same year.. The result suggest that the height of the plants in a gap to a large extent is determined by the light reaching the plants. In all plots the tallest plant was an ash and in all plots ash on average was higher than beech ŽFig. 6.. In the mid-gap plots ŽFig. 6, plot 6–10. the ash plants clearly formed an upper storey above the beech plants. The amount of light reaching the ash plants was on average 17.9% RLI Žrange 9.8–28.3% RLI., while the average amount of light reaching the beech plants Žbelow the ash. was 6.4% RLI Žranging from 2.3–9.9% RLI.. The amount of light reaching the beech plants in fact decreased towards the gap

Table 3 Ages of ash and beech regeneration in a gap in Suserup Skov. Ages measured in 1992 Plot

Species

n

Age Žyr. Minimum

Maximum

Mean

s

1–10 1–10

ash beech

101 125

2 2

8 6

4.2 a 3.3 b

1.2 0.7

t-test: a, 0.001.

b

different suffixes indicate significant differences, p-

The results from the regeneration transects in Suserup Skov showed Ž1. no major difference in shade tolerance between regeneration of ash and beech; Ž2. that no considerable regeneration established and survived at RLI - 2%; Ž3. that regeneration developed poorly at RLI - 3%; and Ž4. that regeneration of both ash and beech established and developed successfully at RLI ) 3%. Exact general thresholds can never be given under field conditions, but the results seem reasonable compared with other studies: Results from controlled nursery experiments show that beech seedlings can survive at only 1% RLI ŽBurschel and Schmaltz, 1965., but the RLI threshold for survival is much higher in vivo under additional stress from water, nutrients, fungi, insects, competition etc. ŽHarley and Waid, 1955; Burschel et al., 1964; Madsen, 1994.. From a practical forestry point of view 3–4% RLI is considered as a lower limit for natural regeneration of beech ŽBurschel and Huss, 1964; Suner and Rohrig, 1980; Madsen, 1994, ¨ 1995.. Even at 5% RLI under field condition Madsen Ž1995. found that light was the main limiting growth factor. The results concerning regeneration success with regard to light intensity, represents the Žfor the moment. best obtainable indication of the light demand of regeneration in Suserup Skov. The results will be used for evaluating the possibilities for successful regeneration with regard to the structure-dynamics of Suserup Skov. 4.2. Light conditions oÕer time, with regard to the forest cycle The results from Suserup Skov show how the structural dynamics of a near-natural forest, described by the model of the forest cycle, determines the light climate at the forest floor ŽFig. 4.. The probabilities for establishment of regeneration can be

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evaluated for each phase of the forest cycle, from information of the regenerations demand for light, the amount of light ŽRLI. and the variation in light ŽCV-RLI. reaching the forest floor. At the beginning of the cycle Žduring innovation phase. the RLI is high, about 10%, and regeneration is expected to establish and develop successfully according to the results. As the regeneration grow up and fill the gap Žduring aggradation phase. the RLI decreases to less than 2% full daylight, at which level it remains for about 250 yr throughout early and late biostatic phases. No regeneration is expected during this long period of relative darkness, except for small Žtemporary. patches of hardly surviving Žadvance. regeneration in places where the RLI exceeds 2%, mostly expected to happen towards the end of the late biostatic phase. As the canopy starts to degrade, during degradation phase, the RLI increases to about 10%, and successful establishment and development of regeneration is to be expected. The model of the forest cycle ŽEmborg, 1996. includes climax microsuccession from ash to beech as an integral part of the forest cycle. The solid line scenario in Fig. 4 shows the light development during the typical forest-cycle including climax microsuccession from ash to beech, characterised by a high CV-RLI in aggradation and early biostatic phases, compared with the alternative scenario Ždashed line. showing pure beech in aggradation phase. The RLI in the two scenarios are at the same level, though ash is not known to form very dense canopies ŽGardner, 1975.. The obvious explanation is that beech Žand elm. forms a sub-canopy stratum, utilising the light penetrating the upper canopy of ash ŽTable 1, A3, A4.. The ash dominated canopy is of more heterogeneous structure resulting in a low RLI and a high CV-RLI at the forest floor ŽTable 2, Fig. 4.. As beech gradually displaces ash during early biostatic phase Žsolid line scenario., the canopy structure homogenises to a dense beech canopy characterised by a low CV-RLI. Towards the end of late biostatic phase, CV-RLI gradually increases as the beech canopy starts to degrade, forming small gaps. In such gaps RLI might exceed 2% or even 3%, allowing establishment of ash andror beech in small patches. Advance regeneration Ži.e. regeneration in the absence of larger gaps. of both ash and beech occurs in Suserup Skov ŽTable 1, plot E2, L2, D1,

D3., representing a competitive advantage Žshould gaps occur.. Unlike beech, the initial shade tolerance of ash regeneration is known to decrease with ager size of the plants, as typical for many gap specialist species ŽGia, 1927; Mayer, 1980; Oldemann, 1990.. Shade tolerance in youth, exemplified by ash, could be an adaptation of a gap-specialist species to slow gap formation, improving the chances of survival in youth under harsh light conditions. 4.3. Light conditions in space, with regard to the forest mosaic The spatial interpretation of the results is that Suserup Skov forms a mosaic of light and dark places in the forest floor, dominated by a dark matrix ŽRLI - 2%. covering some 92% Žaggradation, early and late biostatic phases. of the mosaic ŽFig. 2., assuming steady state ŽEmborg, 1996.. In this dark matrix, small light patches occur Žgaps, RLI ) 3%. accounting for some 8% Ždegradation and innovation phases. of the mosaic area ŽFig. 2.. Small patches of intermediate RLI Žas represented by single censor recordings of 2–3% RLI, Fig. 3. are found scattered over the matrix Žespecially in patches representing the termination of late biostatic phase., in which advance regeneration may establish. This general pattern of darkness is reflected in the herb flora, being completely dominated by perennial species adapted to utilise the light in early spring before foliation, like Anemone spp., Mercurialis perennis L. and Corydalis bulbosa ŽL.. DC. 4.4. Gap specialist species in climax forest Ash is a gap specialist in the climax forest, with pioneer features, while beech is a typical shade tolerant species ŽIversen, 1967; Grubb, 1977; Etherington, 1982; Finegan, 1984; Oldemann, 1990.. A small-scale pollen analysis from the site indicate that ash and beech have co-occurred in Suserup Skov, ever since beech invaded about 2500 yr BP ŽGina Hannon and Richard Bradshaw, personal communication, 1996.. Grubb Ž1977. has described several examples of species co-existence in ‘climax’ forest by the concept of the ‘regeneration niche’, involving speciesspecific features like seed production, seed dispersal, establishment and growth rhythm of plants.

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The seed production of beech is strongly periodical Žmast. with heavy seeds ŽBurschel et al., 1964; Holmsgard ˚ and Olsen, 1960; Watt, 1925.. In contrast to beech, ash produces seed in most years with light wind-dispersed seeds ŽWatt, 1925; Mitchell, 1974.. These differences in the regeneration niches of ash and beech, result in a high probability of ash establishing before beech in gaps ŽTable 3.. Peet and Christensen Ž1987. concluded that a one year headstart of a species in a gap represented a considerable competitive advantage. In accordance with this ash, in Suserup, formed the upper layer after a few years of growth in the examined gap ŽFig. 6.. But still RLI below the ash stratum was sufficient Ž) 3%. for development of an understorey of beech. The examined gap represents the beginning of a climax microsuccession from ash to beech. The successional interaction between ash and beech continues during aggradation and early biostatic phases in Suserup Skov ŽEmborg, 1996.. Climax microsuccession ŽFig. 1. is well known from other studies of temperate forests ŽWatt, 1925, 1947; Remmert, 1987, 1991; Wissel, 1991; Forcier, 1975.. The regeneration niches of beech and ash, and climax microsuccession from ash to beech offer possible explanations for the long-term co-existence of ash and beech in Suserup Skov. At first glance it seems contradictory that regeneration of beech and ash do not show major difference in shade-tolerance, while at the same time beech typically forms a sub-canopy stratum below ash in Suserup Skov. Part of the explanation is that ash, unlike beech, looses its initial shade tolerance with agersize. Another part of the explanation is that many different species-specific features, together, distributes the tree-species across a spectrum ranging from typical pioneers, over gap-specialists, to typical climax species. Features like frequency of seed production Žmast., seed dispersal, height growth rate, branch and canopy architecture separate the ecological profile of ash from the profile of beech ŽOldemann, 1990.. 4.5. Implications for forest management The near-natural forest of Suserup represents a basic reference for nature-based silviculture of beech

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and mixed beech forests in the region. Several features of the Suserup Skov structural dynamics represents cluesrinspiration for forest management, as discussed below. Ž1. Suserup Skov is formed by a mixture of interacting tree-species of differing ecological features. Most managed beech stands in the region are relatively pure, which implies some limitations on the regeneration possibilities. The managers depend on mast years before they can start regeneration and final cutting of the stand. In Suserup Skov ash, as a gap specialist species, is almost ever present, able to colonise new gaps. This flexibility of the natural forest could be mimicked in silviculture by introducing and actively managing ash into the beech stands. Ž2. The light conditions of Suserup Skov is characterised by extreme darkness throughout most of the forest cycle suddenly succeeded by a short period of ample light during the gap phases. Traditionally beech management implies quite heavy thinning of the usually one-layered stands towards the end of rotation, in order to increase the diameter increment. This stand structure and thinning procedure opens the canopy and alters the light climate at the forest floor allowing grasses and more light-demanding herbs to invade before regeneration of the stand is started. Sometimes massive invasion of mostly undesired tree species Že.g. Acer pseudoplatanus L.. or shrubs Že.g. Rubus fruticosus L.. occur. Such invasion of trees, herbs, grasses etc. often hampers the establishment of the desired regeneration. The micro-climate and soil conditions for regeneration could be improved, and more control over the regeneration process could be obtained Ž1. by introducing a period, e.g. 10 yr, of relative darkness before regenerating the stand, and Ž2. by building up a sub-canopy stratum during the life of the stand. Retention of suppressed trees and stronger early thinnings could be means to introduce and maintain a sub-canopy stratum in order to alter the forest floor light conditions, and in order to increase the silvicultural flexibility and control during the regeneration process. This points to a more active management of the stand aimed at creating favourable conditions for regeneration, not just during the early stages of the regeneration process as pointed out by Madsen and Larsen Ž1997., but even many years before the regeneration process is actually started.

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Acknowledgements This project was supported by The National Forest and Nature Agency and The Danish Academy of Science. I am indebted to Sorø Akademi for protecting Suserup Skov and for permission to work there. Thanks to Morten Christensen for good discussions and help with figures, to Jacob Heilmann-Clausen for good hours spent in field work, to Henrik Stryhn for advice on the statistics, and to J. Bo Larsen, Henrik Vejre, Ruth Bejder and three anonymous referees for comments on the manuscript.

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