Reducing predation of conifer seeds by clear-cutting Rubus fruticosus agg. in two montane forest stands

Forest Ecology and Management 126 (2000) 281±290

Reducing predation of conifer seeds by clear-cutting Rubus fruticosus agg. in two montane forest stands Martin Schreiner, Eva-Maria Bauer, Johannes Kollmann* Forstliche Versuchs- und Forschungsanstalt Baden-WuÈrttemberg, Wonnhaldestrasse 4, 79100 Freiburg, Germany Received 5 October 1998; received in revised form 4 February 1999; accepted 18 February 1999

Abstract Natural regeneration of forest trees is negatively affected by dense ground cover of brambles (Rubus fruticosus agg.) in montane forests in southwestern Germany. Seed predation by rodents is one potential factor which may reduce establishment of tree seedlings in these sites. In winter and early spring seed predation of two conifers (Abies alba Mill., Picea abies (L.) H. Karsten) was studied in bramble patches and in adjacent cleared plots (6 m  6 m, respectively) in two montane Abies-Picea stands. Losses of seeds from experimental dishes were higher under bramble vegetation than in the cleared plots in both forest stands in winter. Seed predation was consistently lower for Abies than for Picea. The results were similar in early spring. Predation was reduced after adding conifer litter into the dishes. Rodents were responsible for the seed losses, and rodent abundance was lower in the experimental clearings. The results are discussed with respect to natural succession on clearings and its practical relevance in forestry, i.e. clear-cutting of bramble patches as a measure to reduce seed predation by rodents. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Abies alba; Natural regeneration; Picea abies; Rubus fruticosus; Seed predation; Seed preferences

1. Introduction Post-dispersal seed predation by rodents has serious effects on the population dynamics of many plant species and thus potential consequences for vegetation structure (Crawley, 1992; Davidson, 1993; Hulme, 1998). Seed predation destroys a considerable proportion of the annual seed crop of forest trees (Gurnell,

*Corresponding author. Address: Geobotanisches Institut ETH, ZuÈrichbergstrasse 38, 8044 ZuÈrich, Switzerland. Tel.: +41-1-6324307; fax: +41-1-632-1215. E-mail address: [email protected] (J. Kollmann).

1993), and some authors report that dispersal and regeneration of certain tree species are at least sometimes and in some places inhibited by seed predation (Watt, 1923; Boyer, 1964; Shaw, 1968; Harmer, 1994). In other studies no signi®cant effect of predation was observed due either to a relatively high seed production, prevalent regeneration from a persistent seed bank, or a limiting density of safe sites for germination and/or establishment (Andersen, 1989). Seed and seedling predation by rodents are wellknown problems for foresters (Sullivan et al., 1993), both when seeds are sown in clearings and during natural regeneration in treefall gaps and windthrows (cf. Sullivan, 1979a; Kollmann, 1997). However,

0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 1 0 0 - 0

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today there is still a lack of practical knowledge on species-speci®c differences in seed predation of temperate forest trees and its control by vegetation structure, although ecological research has dealt with seed mortality for about three decades (cf. Janzen, 1971; Price and Jenkins, 1986). Some studies have been conducted in tropical forests (e.g. Schupp, 1988; Forget, 1993), but these ®ndings need to be checked before applying them to the temperate zone. From temperate habitats in North America and Japan several studies have been published about post-dispersal seed predation in old-®elds and adjacent forest stands (Willson and Whelan, 1990; Whelan et al., 1991; Myster and Pickett, 1993), whereas gaps in temperate forests and the effects of microsites are less well studied (Sullivan, 1979a; Boman and Casper, 1995; Kollmann, 1997). Seed losses were often found to be highest under dense vegetation where the small mammals seek protection against predators, as summarized by Hulme (1998). However, to our knowledge this tendency has rarely been checked for its practical relevance in forestry. In central Europe there is an increasing trend to foster natural regeneration of forest stands (Weidenbach, 1992). During natural regeneration, losses of seeds and seedling predation are serious problems which often hinder recruitment of the main tree species. Most studies on effects of rodents on recruitment of trees have focused on planted saplings in clearings where predation is particularly high in grass-dominated sites (BaÈumler, 1992; Sullivan et al., 1993); less information is available about spatio-temporal patterns in seed predation. During the last 20 years foresters in southwestern Germany have observed an increasing cover of brambles (Rubus fruticosus agg.) mainly in lower montane, coniferous forests (Barten, 1997; Schreiner and Grunert, 1998). In dense patches of brambles forest management is notoriously dif®cult and expensive, and natural regeneration of forest trees is much reduced (M. Schreiner, unpublished data). As part of a larger project on consequences of Rubus cover for regeneration of forest trees, this study investigates effects of seed predation and techniques to reduce losses of seed, alternative to the use of rodenticides (BaÈumler, 1990), herbicides (Sullivan et al., 1996), or `diversionary food' (Sullivan and Klenner, 1993). By studying seed removal from

experimental dishes we focused on the following questions: 1. Which animal species are responsible for seed predation in bramble patches? 2. Can small-scale cutting of brambles reduce seed predation? 3. Are there differences in seed losses for the locally most important forest trees? 4. What is the effect of coniferous litter on the intensity of seed predation?

2. Material and methods 2.1. Study sites We chose two conifer stands at a lower montane altitude in southwestern Germany which were both ca. 100 years old: a silver ®r-spruce forest east of the Black Forest (Oberndorf), and a spruce forest with few silver ®r northeastern of Lake Constance (Ravensburg); silver ®r is Abies alba Mill. and spruce Picea abies (L.) H. Karsten ± only generic names are used subsequently. Similar forest stands are common in the respective regions. The stand in Oberndorf (650± 660 m asl; 488 160 N, 88 330 E) grows on nutrient-rich sandy loam, the stand in Ravensburg (580±590 m asl; 478 440 N, 98 380 E) on slightly acidic moraine loam; the situation of both stands is ¯at (inclination <58). For Oberndorf average annual temperature and annual precipitation are 7.68C and 798 mm, respectively (climatic station Rottweil, 1957±1984), for Ravensburg 8.48C and 974 mm (1951±1984). Although the tree canopy was more or less closed in both forests (Oberndorf 60±90% cover, Ravensburg 55±70% cover; both 35±38 m height), a dense ground vegetation mainly of brambles (Rubus fruticosus agg.) had developed (65±90% cover, 40±60 cm height), which is currently a common phenomenon for similar forest types in southwestern Germany (Schreiner and Grunert, 1998). 2.2. Experimental plots For a pairwise comparison of seed predation in intact and cleared bramble vegetation we established in both study areas eight paired plots in large relatively

M. Schreiner et al. / Forest Ecology and Management 126 (2000) 281±290

homogeneous bramble patches; within a total area of 3 ha in Oberndorf and 4 ha in Ravensburg. The average distance between the plots was 40±50 m. Each paired plot had a total area of 12 m  6 m. One week before the experiment started, brambles were cut down to 10 cm with a brush cutter on one randomly selected half of the plots (6 m  6 m, cut; 6 m  6 m, uncut). In the adjacent subplot the bramble cover was 50±90% in both study areas (average 80%). It would have been ideal to space the paired plots over larger areas to avoid the danger of pseudoreplication (sensu Hurlbert, 1984) within the two stands, or to chose a higher number of different forest stands. Unfortunately, this was not feasible for practical reasons. However, previous studies (Kollmann, 1995, 1997; Kollmann et al., 1998) suggest that seed predation in the eight subplot per stand was rather independent, since differences in predation are mainly controlled by the local vegetation structure. 2.3. Seed dishes For the experiments on seed predation we used green plastic dishes as feeding trays which proved to be reliable in Kollmann (1997) and Kollmann et al. (1998). The dishes had a diameter of 12 cm, slits in the bottom for drainage, and were covered with a transparent PVC screen (15 cm  15 cm), supported 8 cm above-ground by a metal pole. This set-up was identical for both study sites and similar to that used by Kelrick et al. (1986) and Willson and Whelan (1990). A roof was necessary to avoid seed losses by wind, rain or melting snow, and to exclude natural seed rain. In each subplot, nine dishes were placed in a grid pattern with 1.25 m distance between the dishes and 1.75 m distance to the margin of the experimental clearing. Experiments were conducted in winter 1997/1998 and in early spring 1998. The winter experiment started on 1 December 1997 when we exposed ®ve seeds of Abies and ®ve of Picea per dish. The dishes were checked after 2, 10, 22, 38 and 63 days; no observation was possible in Ravensburg after the second night due to heavy snow cover. The spring experiment started on 18 March 1998, with a single observation after 10 days. In this experiment, we aimed to check the previous results in a different season, and additionally to investigate the effects of

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coniferous litter. We added one dish per subplot and placed again ®ve Abies and ®ve Picea seeds in each dish. The seeds in ®ve of the 10 dishes per subplot were covered with 4 g of Abies/Picea needles from the two study sites, dried at room temperature for 24 h. Additionally, we established two dishes per subplot to identify the animals responsible for the seed losses: one allowed only predation by insects (birds and rodents excluded by a 1.3 cm wire mesh); a second dish was protected by a wire cage against predation by birds but allowed access of rodents and insects. 2.4. Seed material The conifer seeds were supplied by the state kiln Baden-WuÈrttemberg (Nagold) based on collections from forests near to the two study sites, with 93% and 94% germination in Picea (harvest 1990 and 1992), and 55% and 60% in Abies (harvest 1997 in both regions). Average seed weight for Picea was 7 mg, for Abies 9 mg (n ˆ 1000). 2.5. Identification of rodents We identi®ed the rodent species with 16 snap traps in Oberndorf and Ravensburg, respectively. On each plot one trap was placed in the undisturbed bramble patch and a second in the adjacent cleared area. The traps were set twice for three successive nights in the periods 12±16 March and 3±7 April 1998, i.e. shortly before and after the spring experiment on seed predation. The traps were checked every morning and all rodents were identi®ed with Brohmer (1988). 2.6. Statistical analyses Since predation of the ®ve seeds per species within a dish was not independent, we calculated survivorship as the proportion of dishes left uneaten over time. A dish was considered empty when three or more seeds were gone, because this allowed both for accidental losses and persistence of sterile seeds. The individual subplots were used as units of observation, with nine dishes per subplot in the winter experiment and ®ve dishes in the spring experiment. The statistical comparisons focused on the average number of surviving dishes per subplot after 10 days, because the differences in seed removal were most pronounced at

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and between the two study sites (p ˆ 0.43). Rodents were about twice as abundant under undisturbed brambles than in the experimental gaps. Apodemus sylvaticus was the only species to be found in the open sites.

this time. Since the data were not normally distributed we used non-parametric tests; all statistical analyses followed Zar (1996). 3. Results

3.2. Effects of clearing brambles on seed removal

3.1. Animals causing seed losses

Overall the seed losses after 10 days were signi®cantly lower in the cut subplots than under intact bramble cover for both sites and both conifers in the winter experiment (Wilcoxon signed-rank test: Z ˆ 3.27, p ˆ 0.001; Table 2), with exception of Picea seeds in Ravensburg where predation was exceptionally high. Less than 50% of the dishes survived the second night in subplots with bramble cover in Oberndorf (Fig. 1). At day 10 only 10±15% of the dishes in intact subplots were left over in both sites; seed removal was less intensive in the following weeks. On cleared subplots the losses of seeds were considerably lower: in Oberndorf 50% of the dishes were gone after 22±38 days, in Ravensburg after 10 days. The effect of bramble vegetation was apparently not affected by snow cover which occurred in Oberndorf between days 2±10 and days 38±63.

In both study sites urine, droppings and seed remains in the dishes suggested that rodents were the major granivores, and these traces coincided with high losses of seeds. In the experiments with selectively accessible dishes no seeds disappeared from dishes which were exclusively accessible to insects, and seed predation was not lower when birds were excluded (Wilcoxon signed-rank test: p ˆ 0.56). These results indicate that rodents were the main seed predators, most likely Apodemus sylvaticus L., Clethrionomys glareolus Schreber and Microtus agrestis L. which were caught at the plots in spring (Table 1). These rodents are well-known seed predators (Flowerdew, 1993). We observed no differences in rodent abundance between the two trapping periods in March and April (chi-square test of independence: p ˆ 0.70),

Table 1 Effect of clear-cutting of brambles (Rubus fruticosus agg.) on species composition and abundance of rodents in two montane forest stands Rodent species

Oberndorf

Apodemus sylvaticus Clethrionomys glareolus Microtus agrestis Total

Ravensburg

Brambles intact

Brambles cut

Brambles intact

Brambles cut

5/6 2/2 1/0 8/8

5/3 0/0 0/0 5/3

6/5 1/1 2/1 9/7

5/4 0/0 0/0 5/4

Note: Data are based on two periods of trapping in March and April over three successive nights, respectively (first/second period; eight traps per subplot type and site).

Table 2 Effect of clear-cutting of brambles on seed removal of two conifer species by rodents in the winter experiment Treatment

Surviving dishesa (%) Oberndorf

Brambles intact Brambles cut a

Ravensburg

Abies alba

Picea abies

Abies alba

Picea abies

11 69

1 36

11 2

0 0

Percentage of surviving dishes after 10 days in December (n ˆ 72 for each category; statistical results included in the main text).

M. Schreiner et al. / Forest Ecology and Management 126 (2000) 281±290

285

Fig. 1. Effect of small-scale cuts of Rubus fruticosus on survival of experimental dishes with conifer seeds in Oberndorf and Ravensburg in the winter experiment (start 1 December 1997). Only data on Abies alba are presented (eight plots per site with nine dishes each; means 1 SE).

In the spring, experiment seed removal was again signi®cantly lower in cut subplots (Z ˆ 2.07, p ˆ 0.04; Table 3), but this time the results were not signi®cant for Abies seeds ( p ˆ 0.29) and in Ravensburg (p ˆ 0.34). The overall level of seed predation was not signi®cantly different compared with the winter experiment ( p ˆ 0.15).

3.3. Species-specific differences in seed predation If one pools the data over both sites and both treatments, signi®cantly more Picea seeds (9.4% survival of 288 dishes) than Abies seeds (30% survival) disappeared after 10 days in the winter experiment (Z ˆ ÿ3.58, p ˆ 0.0003; cf. Fig. 2). However, the

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Table 3 Effect of clear-cutting of brambles on seed removal of two conifers by rodents in the spring experiment Treatment

Surviving dishesa (%) Oberndorf

Ravensburg

Abies alba b

Brambles intact Brambles cut

Picea abies c

b

Abies alba nL

L

nLb

Lc

46 54

58 60

0 44

24 48

nL

L

nL

L

14 22

14 34

0 4

2 28

b

Picea abies

c

c

a

Percentage of surviving dishes after 10 days in March (n ˆ 40 for each category; statistical results are given in the main text). Investigation when no conifer litter was added to the dishes. c Investigation when conifer litter was added to the dishes. b

Fig. 2. Seed preferences of rodents for Abies alba compared with Picea abies in the winter experiment, measured as survival of experimental dishes (start 1 December 1997). Only data on cut plots in Oberndorf are included (eight plots with nine dishes each; means 1 SE).

result was not signi®cant in Oberndorf (p ˆ 0.06). After 63 days in total only 0.35% of the Picea dishes survived but still 10% of the Abies dishes. In early spring the rodents again preferred Picea seeds in both sites (Oberndorf: Z ˆ ÿ1.96, p ˆ 0.05; Ravensburg: Z ˆ ÿ4.13, p < 0.0001).

was observed for Abies (p ˆ 0.44). The effect was apparently stronger in cut subplots than in bramble subplots.

3.4. Effects of addition of litter

4.1. Control of seed predation by vegetation structure

Over both sites and both treatments, the addition of litter into the seed dishes reduced only the removal of the smaller Picea seeds signi®cantly (Z ˆ 4.74, p < 0.0001; Table 3), whereas no signi®cant effect

4. Discussion

The main seed predators from the experimental dishes and probably also under natural conditions were small rodents. In the temperate zone post-dis-

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persal seed predation by insects or birds seems to be of minor importance, as shown by selectively accessible dishes in southwestern Germany and south England (Kollmann et al., 1998); Whelan et al. (1991) reported similar results from north America (for a review see Hulme, 1998). Seed predation is probably most intense in sites with high densities of rodents (Sullivan, 1979a; Hay and Fuller, 1981; Kollmann, 1995). Such preferences for certain vegetation structure are controlled by food quality and quantity, and by protection against predators. The latter is higher under dense vegetation, e.g. in patches of scrub, compared with open sites (cf. Simonetti, 1989). The present study revealed consistently higher seed predation in dense bramble patches than in adjacent cleared subplots for both conifer species, both sites and seasons. Snow cover did not obscure this spatial pattern. The spring experiment, four months after the initial clearing of the brambles, documented that reduced predation in the small clearings was not a short-term effect of the disturbance by cutting. We were also surprised that small clearings (6 m  6 m) had such a strong and consistent effect. One reason could have been that the available food was abundant at the time of the study, i.e. that the rodents were not forced to leave the cover for seeking food. Therefore, the spatial pattern in seed predation could change in different years. Our results con®rm earlier ®ndings of higher seed losses in forest stands with a dense herb or scrub layer (Kikuzawa, 1988; Wada, 1993; Kollmann, 1997). In tropical forests in south America, Schupp (1988) and Schupp and Frost (1989) also found higher seed predation in gaps with a dense vegetation compared with adjacent forest with sparse ground vegetation. Similar results on a correlation of vegetation cover and predation were reported for successional scrub in old-®elds and set-aside grassland by Hay and Fuller (1981), Myster and Pickett (1993) and Kollmann and Schill (1996). 4.2. Temporal patterns of seed predation In experiments on seed predation losses are often highest in the ®rst two weeks (Whelan et al., 1991; Boman and Casper, 1995; Kollmann, 1997; Kollmann et al., 1998). The results of this study reveal a similar pattern which is partly explained by decreasing density of seeds. However, the higher attractivity of fresh

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seeds might play an additional role, since seeds are much more dif®cult to ®nd after 2±3 weeks of exposition to natural conditions (Price and Jenkins, 1986). Thus, the ®rst two weeks after dispersal might be most critical for seed survival. If during this time period they become covered by coniferous litter or disappear in the humus layer, the chances for survival are higher as shown by the litter addition experiment; similar observations were reported by other authors (Fagus sylvatica, Watt, 1923; Quercus petraea, Shaw, 1968). In central Europe the main period of litter fall in Abies alba is July±August, in Picea abies October±November, and shedding of seeds in mixed stands partly overlaps with this period (Abies, October; Picea, January±April; Rohmeder, 1972). Clearly, the experiments on predation were done in the most important time period, since seeds will not germinate before spring. Litter of the two conifers and other tree species may reduce the level of predation in late autumn and winter. The result of similar seed predation in winter and spring supports previous observations (Kollmann, 1997; Kollmann et al., 1998) where predation was only slightly higher in spring. It is unlikely that our results were caused by rodent trapping before the spring experiment, because species composition and abundance of rodents were nearly identical before and after the spring experiment. Certainly, much higher rates of predation are to be expected in summer and autumn (cf. Boman and Casper, 1995; Kollmann, 1997; Kollmann et al., 1998), which may re¯ect temporal shifts in the population density of rodents. In the temperate zone rodent populations often undergo seasonal changes due to the time of reproduction (BaÈumler, 1992; Jedrzejewski and Jedrzejewska, 1996). However, seed predation in summer and autumn will not affect the studied conifers whose seeds are only viable in winter. 4.3. Seed preferences The rodents demonstrated species-speci®c selectivity among the two conifers, and the seed preferences were consistent among seasons and sites. Preferences for Picea seeds are surprising since these seeds are smaller than those of the Abies, and larger seeds are often preferred (Hulme, 1998). However, germinability was much higher in Picea seeds which may explain

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the observed selectivity of the rodents, since intact seeds were also preferred in previous experiments on predation of 12 ¯eshy-fruited species in early-successional forest (Kollmann et al., 1998). Additionally, seeds of the Abies are reported to be less attractive for rodents because of the higher turpentine content (Schwerdtfeger, 1981, p. 263). 4.4. Effects of seed predation on regeneration of forest trees Certainly, most seeds which disappeared from the experimental dishes were destroyed by the rodents, although at least some were carried away for future consumption (cf. Jensen and Nielsen, 1986; Miyaki and Kikuzawa, 1988; Vander Wall, 1990). If by chance these seeds survive, then the rodents have contributed to secondary dispersal which might bene®t the regeneration of the forest trees. However, in most cases seed predation has detrimental effects (e.g. Hulme, 1996 for Taxus baccata). The observed spatial differences in predation clearly reduce regeneration of trees in bramble patches as observed in dense vegetation of forest gaps (cf. Wada, 1993). Additionally, Rubus cover may change the microclimate, the soil moisture and nutrient availability, with clear consequences for germination and establishment of forest trees (Lautenschlager, 1997). Besides the effects of vegetation structure, the probability of seed survival depends on the time interval between dispersal and germination, although the results of this study indicate that the predation risk is highest in the ®rst two weeks after dispersal when seeds are fresh. As described above, seed rain of Abies is highest in October (Picea, January±April), whereas germination of both species occurs in spring. Thus, the losses to seed predators could be higher in Abies, although this temporal effect is at least partly compensated by the lower attractivity of Abies seeds. Unfortunately, little is known about the seasonal effects of predation in the context of the temporal patterns of seed rain and germination. The functional effect of the species-speci®c differences in seed predation also remains to be investigated. Signi®cant consequences for recruitment and colonization of new sites in forests are to be expected, although results from the literature indicate that differences in germination and establishment may also be

of considerable importance (Kollmann and Schill, 1996). Spatio-temporal con¯icts between subsequent stages in the life cycle of plants are well known (Schupp, 1995), and seed predation may be insignificant where other stages of recruitment are limiting (Andersen, 1989; Hulme, 1998). However, these questions can only be answered when experiments are set up that exclude rodents over several years (e.g. Heske et al., 1993), or by experiments with `diversionary food' as described by Sullivan (1979b) and Sullivan and Klenner (1993). 5. Conclusions Relatively small cuts of brambles already reduce seed losses to rodents in coniferous forests and may increase the chances for natural establishment of forest trees. We assume that the effects of larger cuts or any widespread reduction in Rubus cover would even be more dramatic; repeated clear-cuts in the growing season are also more ef®cient in reducing plant vigour than the winter cuts of the present experiment (Schreiner and Grunert, 1998). However, besides increased seed predation under Rubus cover other site factors may limit germination, seedling establishment and growth, and it seems likely that in mown subplots germination and seedling survival are higher than under bramble cover (Gill, 1992; Ida and Nakagoshi, 1996). To enhance natural recruitment of forest trees on clear-cuts or in natural gaps, gap vegetation should be cut at least locally and the soil should be disturbed to reduce abundance of rodents in these sites (Harmer, 1994), especially in autumn when seed rain and postdispersal predation are particularly high. Soil disturbance and cutting of the dominant plant species could also increase the diversity of gap vegetation. However, in many places in North America, contrasting the situation in central Europe, herbicides are used to reduce Rubus abundance. Herbicides provide both protection from herbivores and growth bene®ts for tree seedlings (Lautenschlager, 1995). In addition herbicide treatments have none of the negative consequences (erosion, nutrient losses, activation of the Rubus soil seed bank) which are associated with soil disturbance. If forest management aims to foster plant diversity in gaps, herbicide treatments could also be

M. Schreiner et al. / Forest Ecology and Management 126 (2000) 281±290

considered because they often increase plant species richness (Sullivan et al., 1996). Acknowledgements We are indebted to Prof. W. Schumacher and Dr. E. Aldinger who supported the study with a grant from our research institute (FVA). The responsible forest of®cers in Oberndorf and Ravensburg (W. Bauer, Dr. R. Bosch, R. Frank and U. Sperlich) kindly helped with the set-up of the experiments. Dr. E. Kublin provided statistical advice, and M. Karopka solved several technical problems. Two anonymous referees made helpful comments on an earlier version of the manuscript. References Andersen, A.N., 1989. How important is seed predation to recruitment in stable populations of long-lived perennials? Oecologia 81, 310±315. BaÈumler, W., MaÈuseschaÈden in Forstkulturen. Anz. SchaÈdlingskde., Pflanzenschutz, Umweltschutz 63, 52±55. BaÈumler, W., 1992. UnkrautbekaÈmpfung und MaÈuse in Forstkulturen. Anz. SchaÈdlingskde., Pflanzenschutz, Umweltschutz 65, 141±143. Barten, R., 1997. Brombeere (Rubus fruticosus) ± ein Problem im Forst? Forst u. Holz 7, 190±192. Boman, J.S., Casper, B.B., 1995. Differential postdispersal seed predation in disturbed and intact temperate forest. Am. Midl. Nat. 134, 107±116. Boyer, W.D., 1964. Longleaf pine seed predators in southwest Alabama. J. Forestry 62, 481±484. Brohmer, P., 1988. Fauna von Deutschland. Quelle and Meyer, Heidelberg, p. 586. Crawley, M.J., 1992. Seed predators and plant population dynamics. In: Fenner, M. (Ed.), Seeds. The Ecology of Regeneration in Plant Communities. CABI, Wallingford, pp. 157±191. Davidson, D.W., 1993. The effects of herbivory and granivory on terrestrial plant succession. Oikos 68, 23±35. Flowerdew, J., 1993. Mice and Voles. Whittet Books, London, p. 243. Forget, P.M., 1993. Postdispersal predation and scatterhoarding of Dipteryx panamensis (Papilionaceae) seeds by rodents in Panama. Oecologia 94, 255±261. Gill, R.M.A., 1992. A review of damage by mammals in north temperate forests. 3. Impact on trees and forests. Forestry 65, 364±388. Gurnell, J., 1993. Tree seed production and food conditions for rodents in an oak wood in southern England. Forestry 66, 291± 315.

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