The regeneration of Quercus petraea (sessile oak) in southwest Ireland: a 25-year experimental study

Forest Ecology and Management 166 (2002) 207±226

The regeneration of Quercus petraea (sessile oak) in southwest Ireland: a 25-year experimental study Daniel L. Kelly* Department of Botany, Trinity College, University of Dublin, Dublin 2, Ireland Received 14 September 2000; received in revised form 25 April 2001; accepted 31 May 2001

Abstract The growth and survival of Quercus petraea saplings were monitored over a 25-year period, in an exclosure in a heavilygrazed oakwood on podzol soil in Killarney National Park. Seedlings were subjected to combinations of two levels of shading (under canopy and in an arti®cial clearing), two levels of competition (weeded and unweeded) and ®ve fertiliser treatments (N, P, NP, Ca and control). Damage by Apodemus sylvaticus (long-tailed ®eldmice) was con®ned to acorns and ®rst-year seedlings, and was not a limiting factor. Invertebrate damage was unimportant. Seedling survival was greatly enhanced within the ungrazed exclosure, even under canopy. In the clearing, following a short-term breach of the fence in the second winter, Cervus nippon (sika deer) browsed 49% of oak seedlings in weeded plots but only 11% in unweeded plots, con®rming that surrounding vegetation cover may exercise a protective effect. In later years, seedling survival was signi®cantly higher in the weeded plots, presumably owing to reduced competition. Seedlings under canopy showed no signi®cant response to fertilisation or weeding; the median proportion surviving per plot was zero within 8 years and all had died within 20 years. In the clearing, the median proportion surviving per plot was 0.33 after 8 years and 0.2 after 25 years. In a portion of the clearing with peaty soil and impeded drainage, seedlings showed reduced performance and signi®cantly increased mortality. Microsphaera alphitoides (oak mildew) attacked seedlings mainly in the clearing; its impact appeared negligible. Fertilisation with P signi®cantly enhanced performance in the clearing. No fertiliser treatment enhanced survival. Fertilisation with N alone led to sharply increased mortality in the clearing. I conclude that, in woodlands in western Ireland, successful oak regeneration is to be expected only in unshaded or lightlyshaded sites where grazing levels are low. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Seedling survival; Exclosure experiments; Tree fertilisation; Shade tolerance; Oakwood management

1. Introduction The paucity and patchiness of successful regeneration in Quercus spp. has puzzled ecologists and foresters in many regions. For example, ``by the 1970s, oak regeneration was recognised as one of the most serious silvicultural problems in the eastern United * Fax: ‡353-1-6081147. E-mail address: [email protected] (D.L. Kelly).

States'' (Lorimer, 1989). In Britain, the scarcity of regeneration of Q. robur L. (pedunculate oak) and Q. petraea (Mattuschka) Liebl. (sessile oak) has been a cause of concern for over a century (Watt, 1919; Rackham, 1980; Evans, 1988; Worrell and Nixon, 1991). In Ireland, Q. petraea and Q. robur are major components of the limited area of semi-natural woodland and here, too, oak regeneration is often conspicuous by its absence. As early as 1936, it was noted of the Killarney Q. petraea woods that ``as in so

0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 1 ) 0 0 6 7 0 - 3

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many oakwoods in Great Britain, oak seedlings are few, and there is a preponderance of old stems'' (Turner and Watt, 1939). To date there have been no systematic studies of oak regeneration in Ireland and few in the more oceanic regions of Britain; exceptions include Humphrey and Swaine (1997b) in Argyll (Scotland), Shaw (1968, 1974) in North Wales and Barkham (1978) in Dartmoor. (Oceanic regions are those ``where there are no sharp extremes of temperature and the rainfall is more or less evenly distributed throughout the year'' (Walter, 1973). The present study was undertaken as a contribution towards providing a scienti®c basis for management in Killarney National Park (Office of Public Works, 1990) and other Irish woodlands. In general, Q. petraea is a species of acid soils and Q. robur of more basic soils rich in mineral nutrients, but ``both species tolerate an extremely wide range of soils'' (Jones, 1959). There is a strong genetic similarity between the two taxa (MuÈller-Stark et al., 1993) and much ecological overlap (Jones, 1959; Ellenberg, 1988). Years of heavy seed production (mast years) occur at irregular intervals in Q. petraea and Q. robur, and their frequency is thought to decrease with increasing latitude, elevation and proximity to the Atlantic seaboard (Jones, 1959; Matthews, 1963). In the Killarney woods, the scarcity of oak regeneration does not derive from a scarcity of seed production; copious acorn crops were observed in successive decades. Nor is there any lack of seed viability; as Shaw (1968) comments for North Wales, high germination rates in Q. petraea are associated with an oceanic climate and with the presence of a moss carpet, both of which prevent humidity from falling to lethal levels. In the Killarney woods, oak seedlings are plentiful in the year or two following a mast year. It is the extreme scarcity of established saplings and young trees that constitutes a cause for concern. There is con¯icting evidence on the vulnerability of oak regeneration to competition. Acorns have substantial food reserves and oak seedlings in well-lit situations develop large tap-roots leading to an enormous root/shoot ratio (4±6 with extremes of >10, Shaw, 1974). Nonetheless, survival and growth rates are diminished by the presence of competing vegetation, whether through shading, root competition (Shaw, 1974), mechanical smothering (Jarvis, 1964a; Humphrey and Swaine, 1997a) or the production of

inhibitory substances by ®eld layer species (Jarvis, 1964b). Oak seems to be relatively unresponsive to fertiliser addition compared to faster-growing trees such as birch (Newton and Pigott, 1991). Nonetheless, the poverty of the extensive areas of podzol soils over Old Red Sandstone in southwest Ireland warranted testing the hypothesis that nutrient supply is limiting oak regeneration there. Experimental studies on oak regeneration have been mainly short-term and focused on the acorn and young seedling. The exclosure study of Pigott (1983) in oak woodland in the Pennines extended over a 26-year period, but involved little experimental manipulation. I present a 25-year study, designed to test the hypotheses that regeneration of Q. petraea is prevented by (i) the presence of a woodland canopy; (ii) competition from the ®eld layer; or (iii) lack of soil nutrients. Evidence was also obtained to test the hypotheses that it is prevented by (iv) impeded drainage; (v) damage by large herbivores; (vi) rodent damage; (vii) invertebrate damage; or (viii) pathogen attack. 2. Methods 2.1. Study area 2.1.1. Location, climate and substratum The study was carried out in Tomies Wood in Killarney National Park, County Kerry, southwest Ireland. The site chosen (grid reference V 908 889) was in a relatively homogeneous stand of woodland on level terrain, at an altitude of ca. 70 m. The climate is extreme oceanic, with cool summers, mild winters and a high rainfall with rather little seasonal variation. For Killarney town, 6 km ENE of the study site and at 55 m altitude, the mean maximum temperature of the hottest month was 19.3 8C and the mean minimum of the coldest month was 3.1 8C (Met EÂireann (The Irish Meteorological Service), unpublished data for the period 1961±1990). Mean annual rainfall at Tomies is ca. 1500 mm (cf. Royal Irish Academy, 1979), which is lower than in most of the Killarney woods. The site is on a morainic terrace composed of Devonian Old Red Sandstone debris. The soil is a stony podzol, with a humus layer around 5±6 cm deep overlying a sandy mineral horizon; its chemistry is typical of the

D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226

209

Table 1 Soil characteristics of Tomies Wood exclosure compared to Killarney oakwoods in general

n Depth of samples (cm) pHc Total phosphorus (mg ml 1)d Total nitrogen (mg ml 1) Loss-on-ignition (%)

Tomies exclosure under canopy (unfertilised plots)a

Killarney woods over Old Red Sandstone (general survey)b

10 0±5 4.50 (0.11) 237 (33) 3960 (176) 69.7 (6.0)

25 0±5 4.46 (0.09) 164 (20) (n ˆ 13) 3670 (250) 69.4 (6.0)

a

Samples collected in September 1999. See Kelly (1981). c Means are followed by standard errors in brackets. Differences between paired means are non-signi®cant (one-way ANOVA). d Bulk density of samples was derived from loss-on-ignition using the algorithm of Jeffrey (1970). b

Killarney oakwoods, and of Irish acidophilous oakwoods in general (Table 1; cf. Kelly, 1981; Kelly and Moore, 1975). At the southeast corner of the site was an area of impeded drainage ca. 100 m2 in extent which came to occupy ca. 19% of the clearing, including most of nine plots. This had a peaty gley soil (Table 2) with a mean humus depth of ca. 13 cm. 2.1.2. Vegetation Tomies Wood is representative of the semi-natural Quercus petraea-dominated woodlands that are widely scattered in Ireland in upland areas over siliceous rock-types (Kelly, 1981; Kelly and Moore, 1975). The wood was largely felled and replanted in 1805 (Weld, 1807; Radcliff, 1814); it has subsequently reverted to a semi-natural condition. At the start of the experiment, the stand was dominated by oaks 15±18 m high, established at dates from 1806 to 1861 (as estimated by ring-counts from the clearing). Occasional trees of Betula pubescens (downy birch) were present and a patchy understory of mature Ilex aquifolium (holly) covered ca. 27% of the area

(cf. Table 3; nomenclature follows Stace (1997)). Most trees in and around the study area were fairly typical Q. petraea, but some showed characters suggesting introgression by Q. robur (cf. Cousens, 1965). In the present study, no attempt is made to distinguish introgressed from `pure' Q. petraea. There were no saplings or young trees of any species. The species-poor ®eld layer covered only ca. 5% of the area; the principal species were, in order, Pteridium aquilinum, Luzula sylvatica, Agrostis capillaris, Oxalis acetosella and Blechnum spicant. Mosses covered about one-third of the woodland ¯oor. The plant community is representative of western Irish acidophilous oakwoods (Association Blechno±Quercetum); it is described in detail in Kelly (2000). The vegetation in the peaty area was distinct in the dominance of birch in the canopy and the presence of patches of Sphagnum palustre L. Tomies Wood has been subject to heavy grazing and browsing pressure throughout the period of this study, mainly by naturalised Cervus nippon Temminck (sika deer), also by trespassing sheep. The native Cervus

Table 2 Soil characteristics of canopy and clearing plots in Tomies Wood exclosurea

n Mean water content (w/v) Loss-on-ignition (%)* a

Under canopy

In clearing, non-peaty plots

In clearing, peaty plots

17 0.233 (0.014)** 58.8 (3.7)b

12 0.453 (0.029)** 62.8 (4.3)bc

5 0.614 (0.060)** 78.2 (1.7)c

Samples of top 5 cm, collected on same day in year 2 (5 September 1973). Means are followed by standard errors in brackets. All differences signi®cant at P < 0:01 (Bonferroni post hoc tests). * Differences signi®cant at P < 0:05 (Bonferroni post hoc tests) are distinguished by different superscripts. **

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Table 3 Initial composition of canopy and understorey in Tomies Wood exclosure Species

Under canopy

In clearing 1 a

Q. petraea I. aquifolium B. pubescens Malus sylvestris Total a b

2

1 b

No. of individuals (ha )

Basal area (m ha )

No. of individuals (ha 1)

Basal area (m2 ha 1)

175 350 0 0 525

54.03 10.37 0 0 64.40

82.1 276.2 29.9 7.5 395.7

14.16 11.66 1.98 0.81 28.61

All stems over 2 m high measured. Areas sampled in year 1 (1972): 1340 m2 in clearing (prior to felling), 400 m2 under canopy. Basal areas calculated from DBH measured at 1.3 m.

elaphus L. (red deer) population has a much lower impact on the wood. 2.1.3. The light regime Using hemispherical canopy photography, Garvey (1998) calculated a total ``canopy area index'' of 5.4 in the wooded exclosure, with deciduous leaf area index contributing almost 70% of this. The leaf area index of the holly understorey was 1.2±1.5. During the period June±September, mean daily penetration of photosynthetically active radiation at 1 m above ground level was generally between 0.4 and 0.8 mol m 2 per day, corresponding to a relative transmission of 3±4%; only 9% of days had a mean of 1.0 mol m 2 per day or greater. Maximal light levels in absolute terms, with daily totals of 4.5 mol m 2 per day, were recorded in late April and May. A detailed analysis of the light regime is presented by Garvey (1998). 2.2. Experimental design An exclosure of 1.2 ha was created in April 1972; the fence was 2.5 m high, with wire netting of 4.5 cm mesh. A clearing was made by felling and removing all trees and larger shrubs from an area ca. 38 m  45 m (0.17 ha) at the southern end of the future exclosure. The timber was dragged away by tractor; this caused some churning and compaction of the topsoil and may have led to an increase in the area with impeded drainage. In the clearing, regrowth from cut stumps, initially vigorous, was removed by repeated cutting in successive years until they ceased to resprout. The effects of light level, competition and nutrient supply were investigated in a quasi split plot design.

Light level: 98 plots, each 8 m2, were assigned in two blocks to shaded treatment (under mature canopy: 50 plots) and unshaded treatment (in the clearing: 48 plots). The blocks were 35 m apart. There was a boundary 8 m wide around the unshaded block. Each plot was separated from its neighbours by a buffer zone 0.6±1.04 m wide. Fertiliser treatments were `P': ground rock phosphate (Ca10(PO4)6(OH)2), 34.5 g m 2 (i.e. 5 g P m 2, 13.7 g Ca m 2); `N': sulphate of ammonia ((NH4)2SO4), 47 g m 2 (i.e. 10 g N m 2); `NP': the two preceding fertilisers combined; `Ca': gypsum (CaSO4(H2O)2), 59 g m 2 (i.e. 13.7 g Ca m 2); and controls. Fertilisers were diluted with silica sand in a ratio of 1:10; the ®rst applications were made in mid-April 1972 and repeat applications in late March 1973. There were 20 plots for each of the P, NP and O (control) treatments, and 19 each for the N and Ca treatments (Table 4). In the ®rst year, only ®ve each of the unshaded plots received the NP, N and Ca treatments; in the second year all plots received the designated treatments. Within the shaded block, two 5  5 latin square designs were used to assign the ®ve fertiliser treatments to plots. In the unshaded block, fertiliser treatments were assigned to plots in a modi®ed latin square design of 6  8 plots. Competition was investigated in a preliminary experiment in year 1 (`pre-weeded' treatment and controls) and in a main experiment in years 2±4 (`weeded' treatment and controls). In the `preweeded' treatment, all vegetation and litter were manually removed from a 10±15 cm radius around each planted acorn at the time of sowing. In the `weeded' treatment, all surrounding vegetation and litter was manually removed within a radius of

D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226

211

Table 4 Oak seedling survival under different treatments, Tomies Wood exclosure Year

No. of plots per treatment

1

2

3

4

5

8

10

12

14

16

18

20

22

25

250 194

192 173

126 148

74 133

ND 96

17 78

13 73

10 71

6 69

1 63

1 53

0 50

0 46

0 37

50 48

Effect of impeded drainage (clearing only) Unimpeded 163 145 125 115 Impeded 31 28 23 18

88 8

74 4

69 4

67 4

65 4

59 4

49 4

46 4

42 4

35 2

39 9

Effect of weeding (i) Under canopy Unweeded Weeded

Effect of canopy Under canopya In clearing

125 125

100 92

66 60

36 38

ND ND

6 11

5 8

4 6

4 2

1 0

1 0

0 0

0 0

0 0

25 25

100 94

85 88

74 74

65 68

41 55

34 44

31 42

31 40

31 38

27 36

22 31

21 29

20 26

15 22

24 24

Effects of fertilisation (i) Under canopy Control 50 Ca 50 N 50 P 50 NP 50

37 43 38 39 35

28 25 25 22 26

19 13 14 12 16

ND ND ND ND ND

4 1 3 5 4

2 1 2 4 4

2 1 1 3 3

0 1 1 2 2

0 0 0 0 1

0 0 0 0 1

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

10 10 10 10 10

46 32 26 39 30

43 28 22 32 23

41 24 19 29 20

33 19 9 17 18

26 17 5 14 16

24 15 4 14 16

23 15 4 14 15

23 15 4 13 14

22 12 4 12 13

19 7 3 11 13

19 7 3 10 11

17 6 2 10 11

13 5 1 9 9

10 9 9 10 10

(ii) In clearing Unweeded Weeded

(ii) In clearing Control Ca N P NP a

49 35 30 45 35

Numbers are total numbers of individuals surviving for that treatment. ND: no data.

30±50 cm around each selected oak seedling. `Weeding' was carried out at ca. 2-monthly intervals through the growing season in years 2±4. This treatment was assigned semi-randomly to groups/rows of plots within each block. `Pre-weeded' and `weeded' treatments were each applied to half of the plots receiving each fertiliser treatment in each block. The vegetation of each plot was recorded at the time of fencing; the cover of each vascular plant species was estimated to the nearest 5%, and all tree seedlings were counted. Acorns for planting were collected in Q. petraea woodlands on sandstone elsewhere in the Killarney area. Planting was carried out from 23 March±6 April 1972 (the radicles were already several cm long and the plumules just appearing). Seventeen acorns were planted in each of the 50 plots

in the woodland block and in 20 plots in the clearing (P treatments and controls). A programme of trapping with break-back mouse-traps was carried out in both blocks of plots from mid-March to mid-July of the ®rst year. Survival of planted seedlings, estimated by plumule emergence, was 55% in the clearing and 50% under canopy. In addition, self-sown oak seedlings appeared in almost all plots. As numbers of surviving planted seedlings were low in many plots, it was decided in the ®rst autumn to combine planted and self-sown seedlings for the remainder of the study. Up to ®ve widely-spaced seedlings were labelled within each plot for continued monitoring. This made an `initial cohort' of 250 seedlings under canopy (5 seedlings in every plot) and 194 in the clearing (1±5 seedlings per

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plot) (Table 4). The overall ratio of planted to selfsown seedlings in the `initial cohort' was ca. 1:3. As the numbers of seedlings per plot varied, the layout of plots was different within each block and the numbers of plots assigned to each fertiliser treatment differed between blocks, it was considered appropriate to analyse the effects of each experimental factor separately by one-way ANOVAs or their non-parametric equivalents, even though this approach increased the likelihood of Type II errors. 2.3. Experimental monitoring The performance of each monitored seedling/ sapling was recorded towards the end of the growing season. In the ®rst and second years, leaf count was recorded in all plots; height was also recorded in a subsample of plots in the ®rst year. By destructive sampling of seedlings near plot margins, it was found that in the ®rst year leaf number was clearly correlated with seedling dry weight (r ˆ 0:72, P < 0:001, n ˆ 19), but shoot height was not (r ˆ 0:27, P > 0:1). By the third year, shoot height had become clearly correlated with seedling dry weight in the clearing (r ˆ 0:75, P < 0:01, n ˆ 12), and leaf counting had ceased to be practicable, so in the third and subsequent years height was used as the measure of performance. In the canopy plots, leaf number was recorded at each monitoring, and height from year 8 onwards. Seedlings were also inspected for presence of and damage by herbivores or pathogens. Monitoring was carried out annually for 4 years under canopy and 5 years in the clearing, then at 2±3-year intervals thereafter. 2.4. Soil analyses Samples were taken of the top 5 cm of soil from the ten unfertilised plots under canopy (Table 1). The pH was measured on a 1:1 fresh soil: water paste, using a glass electrode pH meter. Loss-on-ignition was measured by combustion at a temperature that was increased gradually to 550 8C (Grimshaw, 1989). Total nitrogen was measured by Leco elemental analyser, using a modi®cation of the method of Verardo et al. (1990). Total orthophosphate P was estimated by the molybdenum blue colorimetric test, using a modi®cation of the method of Murphy and Riley (1962). In addition, samples of the top 5 cm of soil were collected

from 17 plots each under canopy and in the clearing at the end of the second growing season and soil moisture and loss-on-ignition were measured (Table 2). For moisture content, samples of known volume were weighed moist, then re-weighed after air-drying to constant weight. 3. Results 3.1. Background changes The wooded sector retained its canopy largely intact over the 25-year period. Fencing resulted in some basal sprouting of the understorey holly and abundant regeneration of holly from seed. In the plots under canopy, herb cover rose from 6% prior to fencing to 52% after 17 years, then declined to 19% after 26 years (Kelly, 2000). Within 10 years L. sylvatica had become the principal herb species. In the clearing, a closed cover of vegetation was established by the end of the second summer, dominated by A. capillaris, L. sylvatica, B. pubescens saplings and Digitalis purpurea. By the 10th year, the clearing was a closed Betula-dominated scrub, mean height 4.4 m, with much Rubus fruticosus agg. By 26 years, this was a Betula-dominated woodland, mean height ca. 13.5 m; herb cover was ca. 41%, with Luzula comprising 98% of the sward (Kelly, 2000). In the early years, the vegetation of the peaty area included much S. palustre, Juncus bulbosus and J. effusus. Growth of tree saplings was markedly retarded, and in the second decade this area was open birch scrub with much Luzula, Pteridium and Calluna. By 26 years, the height of the birch canopy was no longer signi®cantly lower than in the other plots, and the vegetation had largely lost its distinctiveness. 3.2. Herbivore impact 3.2.1. Predation of acorns and young seedlings The only animals caught during the period of trapping (March±July 1972) were 57 Apodemus sylvaticus L. (long-tailed ®eldmice). In spite of trapping, signs of small mammal activity continued to be observed in the plots. Small mammal diggings were observed at the site of planting of at least 17 acorns; in three cases partially-eaten acorns were found.

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213

Table 5 Oak seedling survival from mast crop of 1984 inside and outside the Tomies Wood exclosure Inside exclosure

No. of sample plotsb Density of first-year seedlings (29 September 1985) Density of third-year seedlings (17 November 1987)

Outside exclosure

All seedlings

Mouse-damaged seedlingsa

All seedlings

Mouse-damaged seedlings

20 3.70 (1.32)c 0.40 (0.18)

20 0.35 (0.18) 0

15 0.27 (0.12) Negligible (ND)

15 0

a

Plumules partly or fully cut off, at ground level Counts in 1 m2 sample plots under canopy: inside the exclosure, placed at centres of control and Ca treatment plots; outside the exclosure, placed at random within 1±8 m distance of fence. c Mean numbers of seedlings per m2 from mast crop of 1984 are followed by standard errors in brackets. Difference between mean numbers of ®rst-year seedlings inside and outside exclosure is signi®cant at P < 0:05 (one-way ANOVA). b

Diggings were observed only in the `pre-weeded' plots. Successful germination was signi®cantly lower in the `pre-weeded' plots (42% plumule emergence by mid-July) than in those where the acorns were inserted with minimum disturbance to litter and moss (58% emergence; P < 0:001, w2 test). Three bitten-off plumules were recorded in mid-May, and by late July a further ®ve stems had been bitten through. However, no signs of damage to the initial cohort of seedlings by small mammals were observed subsequent to July of the ®rst year. The mast year of 1984 provided fresh data on predation of oak seedlings (Table 5). Plumule damage, apparently by Apodemus, was recorded for 10% of seedlings under canopy within the exclosure. 3.2.2. Invertebrate herbivory Various kinds of invertebrate leaf damage were frequent but appeared to be of minor impact (chlorotic, necrotic or fenestrated patches, nibbled or inrolled margins, and gall-type swellings). The commonest type of damage was `stippling': tiny chlorotic spots caused by the feeding of green¯y (Aphididae) and leaf hoppers (Cicadellidae sensu lato) (cf. Moore et al., 1991); in year 3, this was noted in 19% of seedlings under canopy but only 1.3% in the clearing. Only two seedlings were ever recorded as having been severely defoliated, probably by slugs. At no stage did I observe serious damage by invertebrates to a signi®cant number of seedlings. 3.2.3. Impact of large herbivores In spite of the heavy mast crop of 1971, oak seedlings were already scarce in Tomies Wood outside

the exclosure by the start of the next growing season; none was recorded in four random 200 m2 plots in April±May 1972. Inside the exclosure, the effect of release from large herbivores was rapidly evident: the mean numbers of self-sown seedlings under canopy were 2.8 m 2 in summer 1972, 2.3 m 2 in summer 1973 and 1.5 m 2 in summer 1974. The pattern was repeated after the mast crop of 1984 (Table 5): by autumn, the density of ®rst-year seedlings outside the exclosure was only 7% of the density inside (P < 0:05). After 2 years, 11% of this cohort of seedlings still survived under canopy within the exclosure, whilst seedlings outside the fence had virtually disappeared. A breach of the exclosure occurred as a result of a tree falling across the fence in the second winter (January 1974). A number of sika deer ranged through the exclosure over a period of about 2 days (P. Moriarty, personnel communication), concentrating their activity in the clearing. Feeding concentrated on Hedera helix, L. sylvatica and Q. petraea (fallen branch, basal sprouts and seedlings). The apparent order of preference among tree regeneration was Sorbus aucuparia > Quercus @ Betula ˆ Ilex. Many birch seedlings in the clearing were over 1 m high, yet only a few were lightly browsed. Oak seedlings did not exceed 0.3 m high, yet in the clearing 30% were browsed; about half of the shoot was removed in most cases. Seven (4%) died as a direct result, having been more or less uprooted. In the weeded plots, 49% of the oak seedlings were browsed, but only 11% in the unweeded plots (P < 0:001, w2 test). The luxuriant surrounding vegetation clearly had a protective effect on the seedlings in the unweeded plots; R. fruticosus

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appeared to be particularly effective. The oak seedlings in the plots under canopy were untouched. 3.3. Regeneration under canopy and in clearing compared Assessment of seedling performance was complicated by high mortality. Survivorship is presented as the median of the proportion of the initial cohort in each plot still alive at a given census (`cumulative survival function' of Fox (1993)). Under canopy most seedlings failed to make signi®cant growth, regardless of fertiliser treatment or weeding regime. Mortality was high in each year (Table 4, Fig. 1a). Dead, withered seedlings were regularly found in the course of monitoring, but the immediate cause of death was seldom evident. Out of the initial cohort of 250 seedlings, a single sapling survived under canopy for 18±20 years, attaining a ®nal height of 24 cm and a leaf count of 10. Leaf count remained static (Fig. 1b). Mean height showed a very slow increase, from 10.3 cm in year 1 to 18.1 cm in year 8. Mortality in the clearing was much lower (Fig. 1a). However, the oak saplings were soon overtopped by faster-growing species, mainly Betula. By year 10, the surviving oaks had a mean height of 1.75 m, compared to 4.4 m for the mean height of the birch scrub. The surviving oaks at year 25 had a mean height of 3.1 m and a range of 0.9±7.0 m. By then the birch canopy was about four times the mean height of the oaks, and all oak individuals were clearly suppressed to some degree. The majority of seedlings in the clearing produced a vigorous second ¯ush of shoots in July of each year (`lammas shoots'). In the third year, these were recorded on 72% of seedlings in the clearing, but only 11% of seedlings under canopy (P < 0:001, w2 test). 3.4. Effects of soil moisture regime Drainage was clearly impeded in the peaty plots, as shown by the high sample water content of the topsoil (Table 2), the deeper and more organic humus layer and the presence of S. palustre. Mortality was similar to that in the rest of the clearing for the ®rst 3 years, but then showed a sharp increase (Table 4, Fig. 2a). After 25 years, only 6% (two individuals) were still alive. By the autumn of year 1, performance was already

signi®cantly lower in the peaty plots (mean leaf count 10.7) compared to the plots with unimpeded drainage (mean leaf count 13.4; P < 0:05, one-way ANOVA). Seedlings growing in the most peaty parts were often obviously stunted and chlorotic in the early years. Growth of the surviving oaks was evidently retarded, although sample size was too low for the differences to be statistically signi®cant (Fig. 2b). 3.5. Effects of weeding WeedingÐcarried out in the second to fourth yearsÐhad no signi®cant effect on the survival or performance of oak seedlings under canopy (cf. Table 4). In the clearing, weeding clearly resulted in increased survival in subsequent years (Table 4, Fig. 3a). Seedlings in the unweeded plots made fair initial growth when surrounded by L. sylvatica or R. fruticosus, but appeared strongly inhibited by A. capillaris. In year 2, seedlings in the weeded plots (mean leaf count 26.5) signi®cantly outperformed those in the unweeded plots (mean leaf count 19.2; P < 0:05, one-way ANOVA). The deer incursion in the second winter led to a reversal; in years 3±5, the seedlings in the unweeded plots were signi®cantly taller than in the weeded plots (Fig. 3b). This re¯ected the lower proportion of seedlings browsed where they were surrounded by other vegetation. After the discontinuance of weeding, surviving oak regeneration was subject to strong competition by surrounding vegetation. In the clearing, young oaks were soon overtopped and suppressed to varying degrees by saplings of faster-growing trees, mainly B. pubescens. 3.6. Pathogens Microsphaera alphitoides Griff & Moubl. (oak mildew) was the only pathogen of note. Infection of the seedlings was widespread, but highly variable in space and time. In the ®rst 2 years, infection was much more widespread and severe in the clearing (23% of unshaded seedlings severely infected compared to 4% of shaded seedlings in mid-October of year 1 (P < 0:001, w2 test); 14% of unshaded seedlings severely infected compared to 0.5% of shaded seedlings in early September of year 2 (P < 0:001, w2 test)). A severe infection was de®ned as one where several leaves were more or less completely covered

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Fig. 1. (a) Oak seedling survival under canopy and in clearing (unfertilised, unweeded plots only). Tomies Wood exclosure, Killarney, southwest Ireland. A signi®cant difference at P < 0:05 and P < 0:01 (Wilcoxon±Mann±Whitney test). (b) Oak seedling performance under canopy and in clearing (unfertilised, unweeded plots only). Bars represent standard errors; all differences signi®cant at P < 0:01 (one-way ANOVA). Leaf counts were discontinued in clearing after year 3.

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Fig. 2. (a) Effects of impeded drainage on oak seedling survival in the clearing, Tomies Wood exclosure, Killarney, southwest Ireland. Differences from year 5 onwards are signi®cant at P < 0:05 (Wilcoxon±Mann±Whitney test). (b) Effects of impeded drainage on oak seedling performance in the clearing. Bars represent standard errors; differences not statistically signi®cant. Height was not recorded in years 1 and 2.

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Fig. 3. (a) Effects of weeding (in years 2±4) on oak seedling survival in the clearing, Tomies Wood exclosure, Killarney, southwest Ireland. A signi®cant difference at P < 0:05 (Wilcoxon±Mann±Whitney test). (b) Effects of weeding (in years 2±4) on oak seedling performance in the clearing. Bars represent standard errors; a signi®cant difference at P < 0:01; P < 0:001 (one-way ANOVA).

by mycelium. Infection was markedly more severe on lammas shoots than on leaves of the spring ¯ush. In both years, level of infection in the clearing showed a positive association with performance, as measured by leaf count (P < 0:001, one-way ANOVA). In the third

year, infection was low overall (0% severe infection in unshaded and 2.3% in shaded seedlings, in late August; difference not signi®cant). Pustules matching descriptions of the rust Uredo quercus Brond. were observed on the leaves of several

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Fig. 4. (a) Fertiliser effects on oak seedling survival in the clearing, Tomies Wood exclosure, Killarney, southwest Ireland. The N treatments differ signi®cantly from controls in years 5±22 (P < 0:05, Kruskal±Wallis one-way ANOVA by ranks). (b) Fertiliser effects on oak seedling performance in the clearing: Ca (gypsum) and P (rock phosphate) compared. Half-bars represent standard errors. The P treatments differ signi®cantly from controls in years 3 (P < 0:01) and 4 (P < 0:05; Bonferroni post hoc tests). (c) Fertiliser effects on seedling performance in the clearing: N (ammonium sulphate) and P (rock phosphate) compared. Tomies Wood exclosure, Killarney, southwest Ireland. Half-bars represent standard errors. The P treatments differ signi®cantly from controls in years 3 (P < 0:01) and 4 (P < 0:05), and from N in year 3 (P < 0:01; Bonferroni post hoc tests).

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Fig. 4. (Continued ).

®rst-year seedlings, both in clearing and under canopy. They were not re-found in subsequent years and the identi®cation requires con®rmation.

4. Discussion

3.7. Effects of fertilisation

4.1.1. Predation of acorns and germinating seedlings The acorn crop is exploited in British woods by a range of birds (especially wood-pigeons and jays, Worrell and Nixon, 1991), and mammals (especially squirrels, mice, voles, rabbits, deer and badgers, Corbet, 1974). Effective dispersal is ascribed especially to jays (Garrulus glandarius L.), also to other birds (cf. Van der Pijl, 1982; Kollmann and Schill, 1996). Evidence that small rodentsÐparticularly long-tailed ®eldmice (A. sylvaticus)Ðare major acorn consumers is presented by Ashby (1959); Jarvis (1960, 1964b) and Shaw (1968). However, Tanton (1965) concluded that in mast years mice and voles only removed a small fraction, squirrels and pigeons being the major consumers. In the Killarney woods, heavy pre-dispersal predation of acorns by wood-pigeons (Columba palumbus) was noted during mast crops by Smal and Fairley (1980) and by the author. Jays were present in small numbers. The rodents A. sylvaticus, Sciurus vulgaris and Clethrionomys glareolus Schreber (bank vole)

In the canopy plots, no signi®cant effect on either survival or performance was obtained with any of the fertiliser treatments (Table 4). In the clearing, no fertiliser treatment signi®cantly enhanced survival, and fertilisation with N alone was associated with a striking increase in mortality (Table 4, Fig. 4a). The P treatment produced signi®cantly increased performance relative to controls and to N alone (Fig. 4b and c). Joint N ‡ P application produced no evidence of an `enhancement' effect relative to P alone. The P treatment may have some long-term effect; 23 years after the last application, the mean height in Pfertilised plots was 69% higher than in the controls. The apparent stasis in growth in the control plots during the period 16±25 years (Fig. 4c) is partly an artefact due to mortality: of those trees that survived throughout this period, controls made a mean height increment of 6.9% and P treatments a mean of 12.6%.

4.1. Herbivore impact

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were widespread in the woods. The last-named was apparently introduced to Ireland around 1950; it was recorded in the Tomies area in 1969 (Fairley and O'Donnell, 1970) and was present in moderate numbers in Tomies Wood by 1975±1976 (Smal and Fairley, 1982). However, Apodemus was the only rodent trapped within the exclosure. Damage attributable to Apodemus was observed to considerable numbers of acorns and plumules, but none to older seedlings. Examination of stomach contents of Apodemus trapped in another Killarney oakwood found that acorns constituted a major part of their diet from the fall of a mast crop well into the following summer (Smal and Fairley, 1980). In Tomies, the signi®cantly greater damage in `pre-weeded' plots (where vegetation and litter were removed from around each acorn at the time of planting) supports the conclusion of Watt (1919) and Jarvis (1964b), that leaf litter may play an important role in concealing acorns from potential predators. Predation of the plumules of ®rst-year oak seedlings by small mammals was also recorded by Watt (1919) and Jarvis (1960). The trapping programme in the early months of the experiment may have arti®cially enhanced the survival rate of ®rst-year seedlings. The data collected after the mast year of 1984 permit a clearer evaluation of the role of mice in seedling predation (Table 5). The survival of a considerable proportion of this cohort of seedlings into their third year, within the exclosure, supports the view that Apodemus are signi®cant predators of acorns and ®rst-year seedlings but have negligible impact on older seedlings. I conclude that mice are not a limiting factor to oak regeneration in the Killarney woods. This echoes the ®nding of Shaw (1974) that A. sylvaticus and C. glareolus, although ubiquitous, are ``of little or no importance'' in relation to oak regeneration in North Wales. In mast years, a vast surplus is produced and, as Mellanby (1968) points out, ``the fact that a particular species eats many acorns may not mean that it is a major factor in preventing regeneration.'' 4.1.2. Invertebrate herbivory Complete defoliation of oak trees is not infrequent in Britain, the principal agents being caterpillars of Tortrix viridana (L.), Operophtera brumata (L.) (winter moth) and Erannis defoliaria (Clerck) (mottled umber) (Jones, 1959; Gradwell, 1974). Signi®cant

defoliation of oak seedlings by phytophagous insects is widely reported from British oakwoods (Watt, 1919; Jarvis, 1960; Shaw, 1968, 1974; Humphrey and Swaine, 1997b) and also from France (Lanier, 1981). Experimental studies found greater seedling defoliation under oak canopy than in adjacent open areas (Shaw, 1968; Humphrey and Swaine, 1997b). Shaw (1974) argued that caterpillar defoliation ``is a major factor, of the same magnitude as light, in determining the growth and survival of oak seedlings''. Both O. brumata and E. defoliaria are common and widely distributed in Ireland (Baynes, 1964). T. viridana is apparently less common than in England (Beirne, 1941), yet mass occurrences of this species and associated defoliation have been recorded from different parts of the Killarney woods, including Tomies Wood in 1990 (Bond, personnel communication). Nonetheless, the present study found no evidence of a signi®cant impact of any invertebrate group on survival or growth of oak seedlings or saplings. This contrasts with ®ndings for other regionsÐincluding Scotland, where Humphrey and Swaine (1997b) found high levels of seedling defoliation by caterpillars at the more oceanic of their two study sites. 4.1.3. Impact of large herbivores The underground reserves of the oak seedling are considered to confer resilience to browse damage (Shaw, 1974; Rackham, 1980). However, when grazing pressure rises above a certain level, tree regeneration practically ceases. In studies of the relations between regeneration and grazing intensity in the New Forest over a 250-year period, the origin of the three main surviving cohorts of trees has been dated to periods when grazing was reduced (Peterken and Tubbs, 1965; Tubbs, 1986). In Tomies Wood, the exclosure has become an ``island'' of virtually ungrazed vegetation in an overgrazed landscape. The fate of the mast crop of 1984 outside the exclosureÐdecimated within their ®rst growing season (Table 5)Ðclearly shows heavy predation of seedlings by large herbivores (principally sika deer). Outside the fence, the only self-sown oak sapling over 0.5 m high observed in the vicinity was on the upper part of the root plate of a wind-thrown tree. The 2-day breach of the fence in the second winter of the experiment provided striking evidence of the

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partiality of sika deer for oak seedlings. For fallow deer (Dama dama L.), Peterken and Jones (1989) found, in broadleaved woodland in the Wye Valley, that the order of preference among the saplings of the main tree species was oak > ash > beech > birch. They concluded that deer handicap oak in competition with birch, a ®nding supported by the evidence from the Tomies exclosure. The incursion also demonstrated that a surrounding growth of vegetation could provide some degree of protection for young oaks (cf. Fig. 3b). The most effective cover appeared to be R. fruticosus agg. Evans (1988) also noted that R. fruticosus may protect oak regeneration against browsing; however, other authors have found that oak regeneration is poor or absent under Rubus owing to the heavy shade that it casts (Everard, 1987; Linhart and Whelan, 1980). In Tomies the Rubus population derived from seedlings which appeared in the ®rst season after clearance (cf. Kelly, 2000), and adjacent oak seedlings had some time to build up root reserves before being overshadowed. Other spiny shrubsÐIlex, Prunus spinosa, Crataegus and UlexÐhave been reported as protecting oak saplings against browsing in Britain (Watt, 1919; Shaw, 1974; Rackham, 1980). 4.2. Shade The strikingly negative effects of canopy cover on seedling survival may be ascribed to insuf®cient penetration of photosynthetically active radiation (PAR), to root competition for water or nutrients, or to allelopathic effects. Pot experiments that were watered to avoid soil moisture stress have con®rmed that light is a major limiting factor to Q. petraea regeneration (Jarvis, 1964a). Increased shading leads to lower root/shoot ratios in Q. petraea and Q. petraea  Q. robur seedlings (Ovington and MacRae, 1960; Jarvis, 1964a; Humphrey and Swaine, 1997a); this may be expected to reduce their resilience to grazing and other kinds of damage. Some British studies suggest that oak has a lower shade tolerance in more oceanic regions. In an 8-year study of Q. petraea regeneration in North Wales, Shaw (1974) found that light levels of ca. 30% of full daylight are required for longer-term survival, and that levels of 50% or more are required if reasonable growth rates are to be achieved. In the upland oakwoods of Dartmoor, Barkham (1978) suggests that Q. robur ``behaves as a

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pioneer species unable to establish itself from seed in its own shade''. Evidence for a northwest to southeast gradient in shade tolerance in both Q. petraea and Q. robur is summarised by Krahl-Urban (1959): high levels of tolerance are reported for Romania (Dengler, 1944) and Slovenia (Krahl-Urban, 1959) and successful oak regeneration under relatively light or open tree canopies are reported from various parts of Germany (Hauskeller-Bullerjahn, 1997; von LuÈpke, 1998) and The Netherlands (Van Hees et al., 1996). Even in the Pennines, Pigott (1983) found successful regeneration of Q. petraea under a canopy of the same species; however, this woodland was on a slope facing southeast, had a canopy only 5±12 m high and was purely deciduous, hence, the light regime was unusually favourable. Light levels under the canopy in the Tomies exclosureÐwith 3±4% PAR penetration being typical in summer and autumnÐwere considerably lower than in other temperate deciduous woodlands for which comparable data are available (Garvey, 1998). The unfavourable nature of the light regime resulted from a combination of the oceanic climate, with its persistent cloudiness, and the evergreen holly understorey. The negative effects of even a deciduous understorey on Quercus rubra regeneration were demonstrated experimentally in mixed hardwood forest in Wisconsin (Lorimer et al., 1994). A negative effect of canopy trees on oak regeneration via root competition for water has been demonstrated by trenching experiments in England (Jarvis, 1964a) and Germany (Hauskeller-Bullerjahn, 1997). Ashton and Larson (1996), in a study on New England oaks, suggested that seedlings in gaps bene®ted not only from increased light but also from increased moisture during spring and summer as a result of reduced root competition. It is possible that the seedlings in the non-peaty part of the Tomies clearing experienced a similar double bene®t (cf. Table 2); in the moist, cool climate of southwest Ireland, competition for water may be of limited signi®cance, but cannot be discounted. 4.3. Waterlogging The high mortality and retarded growth of the oak seedlings on peaty soil (Fig. 2a and b) supports the general view that Q. petraea is intolerant of

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waterlogging (Jones, 1959; Ellenberg, 1988). A negative effect of acid peat on Q. petraea seedling development was also noted by Ovington and MacRae (1960). The effect may be through a combination of anoxia and nutrient de®ciency: in Tomies the topsoil in the peaty plots had a signi®cantly lower ash content as well as being subject to waterlogging (Table 2). 4.4. Competition Oak regeneration is clearly vulnerable to competitive inhibition from herbaceous and low woody species in the early years, including Deschampsia ¯exuosa (Jarvis, 1964b), Holcus mollis (Shaw, 1974) Molinia caerulea (Becker and Levy, 1983) and P. aquilinum (Watt, 1919; Humphrey and Swaine, 1997a). Some species may have inhibiting effects yet may provide some protection for oak seedlings against browsing, notably R. fruticosus agg. (see above) and Pteridium (Barkham, 1978). In the Tomies clearing, the deer incursion con®rmed that a surrounding vegetation cover could have a protective effect on oak seedlings. However, in the absence of further grazing, the negative effects of competition are demonstrated by the enhanced seedling survival in the weeded plots from the ®fth year (Fig. 3a). A comparable long-term response was reported in a study of the effects on Q. petraea regeneration of removing herbaceous vegetation by repeated herbicide application (von LuÈpke, 1987); in clearings, height growth was greater in treated plots even 4 years after herbicide application was discontinued. B. pubescens had become the principal competitor in the Tomies clearing within the ®rst decade of the study. It increased its lead in succeeding decades: the ratio of height of birch canopy to mean height of surviving oaks rose from 2.5:1 in year 10 to ca. 4:1 in year 25. All established saplings of both oak and birch belonged to the same initial cohort; there was no successful recruitment of oak from subsequent mast years. In their 40-year study of natural regeneration in felled areas in broadleaved woodland in the Wye Valley, Peterken and Jones (1989) also found that all oak saplings seemed to belong to the same cohort and that subsequent heavy masts gave rise to no recruits. These results conform to the model of succession based on initial ¯oristic composition (Egler, 1954) rather than to the classic ``relay ¯oristics'' model. The

26-year study of Pigott (1983), on fenced oak-birch woodland in the Pennines, differs in recording intermittent recruitment of oak saplings subsequent to the initial cohort. 4.5. Pathogens M. alphitoides (oak mildew) is considered to have a damaging impact on oak regeneration in Britain (Murray, 1974) and Denmark (Haugh, 1934). Although Q. petraea is regarded as less susceptible than Q. robur (Jones, 1959; Murray, 1974), mildew has been found to have a serious impact on Q. petraea seedlings growing under shade in English oakwoods (Jarvis, 1964a; Shaw, 1974). The fungus was apparently unknown in Europe prior to 1907; Rackham (1980) opined that ``the arrival of mildew probably made oaklings succumb to a degree of shade which they would formerly have survived''. Monitoring in Tomies Wood was somewhat inconsistent in timing, but adequate to demonstrate that severe infection was rare on seedlings under canopyÐ which is the situation in which it would be most likely to be seriously damaging. In the ®rst- and second-year seedlings in the clearing, infection was often severe, but the positive association between performance and level of infection indicated that it did not constitute a signi®cant handicap. There was clearly a relation between the development of lammas shoots and the virulence of mildew attack, as also reported by Haugh (1934) and Jones (1959). I conclude that mildew is not a signi®cant deterrent to oak regeneration in the Killarney oakwoods. Powdery mildew infections increase plants' susceptibility to abiotic stresses such as drought (Smith et al., 1988), and the apparently slight impact in these woods may perhaps be ascribed to the mitigating in¯uence of the moist, mild climate. 4.6. Effects of fertilisation The seedlings under canopy were apparently too light-starved to respond to fertilisation (cf. Table 4). Shaw (1974) found that Q. petraea seedlings grown on a base-poor woodland soil at 10% of full daylight showed net losses in P and K relative to the contents of the acorn; he concluded that ``seedlings in less than about 30% full daylight were simply unable to balance

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losses of potassium and phosphorus through leaf fall, and other minor causes such as root death, by suf®cient uptake''. Oak seedling performance in the clearing showed a positive response to rock phosphate (P) but not to calcium sulphate (Ca) or ammonium sulphate (N) (Fig. 4b and c). The lack of response to calcium sulphate con®rms that the positive response to the P treatment is to orthophosphate and not to calcium ion. These ®ndings echo those of Newnham and Carlisle (1969) for typical oak and oak/ash woods in Cumbria: they found that the soils yielded adequate N but insuf®cient available orthophosphate for maximum growth of ®rst-year seedlings of Q. petraea and Q. robur. A similar response to P but not to N, using soil from the Killarney oakwoods on sandstone, was obtained in experiments with ®rst-year seedlings of Rhododendron ponticum (Cross, 1973, 1975) and I. aquifolium (Cross, 1973). The negative effect of N on seedling survival in the clearing was striking (Table 4, Fig. 4a). Newton and Pigott (1991), investigating the growth of Q. robur seedlings on infertile woodland soils, recorded a number of negative effects of fertiliser application: both N and K brought about signi®cant decreases in foliar P concentration; N, P and N ‡ K led to signi®cantly reduced levels of ectomycorrhizal infection. Shaw (1974) records that oak seedlings planted in British woodlands showed a marked initial growth response to a general NPK fertiliser, but that this rapidly disappeared and was replaced by a signi®cant increase in mortality. The effect was most marked for seedlings growing in D. ¯exuosa and was attributed to increased competition from that species. However, in Tomies, no evidence was found of increased competitive inhibition, predation or disease in N-fertilised plots. 5. Conclusions Height growth of young Q. petraea was clearly stimulated by fertilisation with orthophosphate, although survival did not differ signi®cantly from controls. Soil fertility on podsolised soils over parent material derived from Old Red Sandstone may, therefore, be limiting. This ®nding has potential signi®cance for sylviculture, although improvement

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of the growth of oak by fertiliser addition is dif®cult to achieve in practice (Evans, 1986a,b). Fertilisation would clearly be inappropriate to the management of National Parks or Nature Reserves. Oak regeneration is adversely affected by waterlogged, peaty soil. The rise in water tables consequent on forest clearance may help to explain the failure of oak to recolonise areas that have been deforested, especially in regions of strongly oceanic climate. Phytophagous invertebrates and fungal pathogens were both present, but neither had a signi®cant impact on oak regeneration. Both have emerged as signi®cant factors in British and Continental studies; these contrasts warrant further investigation. Oak is vulnerable to competition from a range of herb, shrub and tree species. The clearing with which the experiment began is now a young birchwood, and it remains to be seen whether the surviving oak saplings will succeed in becoming a signi®cant component of the future canopy. Oak regeneration was precluded by the presence of a closed oak canopy accompanied by a patchy evergreen understorey; this was almost certainly due to insuf®cient penetration of photosynthetically active radiation. Oak regeneration is virtually precluded where woods are subject to heavy grazing pressure. In Killarney National Park, oak saplings (outside exclosures) are largely restricted to the proximity of roads and tracks, where browsing animals are more or less deterred by frequent human disturbance. A concentration of young oaks along woodland rides has also been noted in woods in eastern England (Rackham, 1980). It is clear that a vital condition for the conservation of native oak is the achievement of suf®ciently low densities of large herbivores. As Evans (1988) warned for Britain, ``many semi-natural woods in the uplands are failing to regenerate and gradually disappearing because of grazing pressures''. The achievement of successful regeneration of oak in existing gaps and open areas would seem to provide a measure of the true `carrying capacity' for large herbivores of a wooded region such as the Killarney National Park. I conclude that the pre-condition for regeneration of oak in western Ireland and other oceanic regions is a combination of suf®ciently high light levels and suf®ciently low grazing levels.

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Acknowledgements The ®eldwork for this study has been largely funded by National Parks and Wildlife (DuÂchasÐThe Heritage Service, Department of Arts, Heritage, Gaeltacht and the Islands) and its predecessor organisations. I thank in particular Mr. M. Neff and Dr. A. Craig for sustained support, also Mr. D. Kelleher, Mr. P. O'Leary, Mr. P. Dower and other staff of the Killarney National Park, and Mr. P.J. Bruton. For much helpful advice I thank Professor W. Watts, under whose supervision this study commenced, and professor D. Jeffrey. I thank Mrs. C. McCoy for assistance in acorn-planting and Dr. J. Cross for carrying out the monitoring for a period. For kind hospitality, I thank Mrs. M. Doody, Mr. D. Doody, Mr. W. Quirke and Dr. H. Twomey. I thank Ms. G.T. Higgins and Ms. S. McNamee for soil analyses. For statistical and computational advice I am indebted to Dr. S. Waldren, Dr. P. Dowding and Dr. D. Lynn. For helpful information and comments on the draft paper I thank Mr. K. Bond, Dr. P. Dowding, Ms. G.T. Higgins, Dr. F.J.G. Mitchell, Dr. G. Peterken and Mr. G. Smith. References Ashby, K.R., 1959. Prevention of regeneration of woodland by ®eld mice (Apodemus sylvaticus L.) and voles (Clethrionomys glareolus Schreber and Microtus agrestis L.). Quart. J. For. 53, 228±236. Ashton, M.S., Larson, B.C., 1996. Germination and seedling growth of Quercus (section Erythrobalanus) across openings in a mixed-deciduous forest of southern New England, U.S.A. For. Ecol. Manage. 80, 81±94. Barkham, J.P., 1978. Pedunculate oak woodland in a severe environment: Black Tor Copse, Dartmoor. J. Ecol. 66, 707±740. Baynes, E.S.A., 1964. A Revised Catalogue of Irish Macrolepidoptera Butter¯ies and Moths. E.W. Classey, Hampton, Middlesex. Becker, M., Levy, G., 1983. Installation et dynamique d'une population de semis de cheÃne en milieu hydromorphe sous l'in¯uence de divers facteurs (lumieÂre, reÂgime hydrique, compeÂtition herbaceÂe). Oecol. Plant. 4, 299±317. Beirne, B.P., 1941. A list of the microlepidoptera of Ireland. Proc. R. Ireland Acad. 47B, 53±147. Corbet, G.B., 1974. The importance of oak to mammals. In: Morris, M.G., Perring, F.H. (Eds.), The British oak: its history and natural history. Botanical Society of the British Isles and E.W. Classey, Faringdon, Berkshire, pp. 312-323. Cousens, J.E., 1965. The status of the pedunculate and sessile oaks in Britain. Watsonia 6, 161±176.

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