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 heavily- grazed 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 lightly- shaded 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 States'' (Lorimer, 1989). In Britain, the scarcity of regeneration of Q. robur L. (pedunculate oak) and The paucity and patchiness of successful regenera- Q. petraea (Mattuschka) Liebl. (sessile oak) has been a tion in Quercus spp. has puzzled ecologists and for- cause of concern for over a century (Watt, 1919; esters in many regions. For example, ``by the 1970s, Rackham, 1980; Evans, 1988; Worrell and Nixon, oak regeneration was recognised as one of the most 1991). In Ireland, Q. petraea and Q. robur are major serious silvicultural problems in the eastern United components of the limited area of semi-natural woodland and here, too, oak regeneration is often * Fax: ‡353-1-6081147. conspicuous by its absence. As early as 1936, it was E-mail address: [email protected] (D.L. Kelly). 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 0378-1127(01)00670-3 208 D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226 many oakwoods in Great Britain, oak seedlings are inhibitory substances by ®eld layer species (Jarvis, few, and there is a preponderance of old stems'' (Turner 1964b). and Watt, 1939). To date there have been no systematic Oak seems to be relatively unresponsive to fertiliser studies of oak regeneration in Ireland and few in the addition compared to faster-growing trees such as more oceanic regions of Britain; exceptions include birch (Newton and Pigott, 1991). Nonetheless, the Humphrey and Swaine (1997b) in Argyll (Scotland), poverty of the extensive areas of podzol soils over Old Shaw (1968, 1974) in North Wales and Barkham Red Sandstone in southwest Ireland warranted testing (1978) in Dartmoor. (Oceanic regions are those ``where the hypothesis that nutrient supply is limiting oak there are no sharp extremes of temperature and the regeneration there. rainfall is more or less evenly distributed throughout Experimental studies on oak regeneration have been the year'' (Walter, 1973). The present study was under- mainly short-term and focused on the acorn and young taken as a contribution towards providing a scienti®c seedling. The exclosure study of Pigott (1983) in oak basis for management in Killarney National Park (Off- woodland in the Pennines extended over a 26-year ice of Public Works, 1990) and other Irish woodlands. period, but involved little experimental manipulation. In general, Q. petraea is a species of acid soils and I present a 25-year study, designed to test the hypo- Q. robur of more basic soils rich in mineral nutrients, theses that regeneration of Q. petraea is prevented by but ``both species tolerate an extremely wide range of (i) the presence of a woodland canopy; (ii) competition soils'' (Jones, 1959). There is a strong genetic simi- from the ®eld layer; or (iii) lack of soil nutrients. larity between the two taxa (MuÈller-Stark et al., 1993) Evidence was also obtained to test the hypotheses that and much ecological overlap (Jones, 1959; Ellenberg, it is prevented by (iv) impeded drainage; (v) damage by 1988). large herbivores; (vi) rodent damage; (vii) invertebrate Years of heavy seed production (mast years) occur at damage; or (viii) pathogen attack. irregular intervals in Q. petraea and Q. robur, and their frequency is thought to decrease with increasing latitude, elevation and proximity to the Atlantic sea- 2. Methods board (Jones, 1959; Matthews, 1963). In the Killarney woods, the scarcity of oak regeneration does not derive 2.1. Study area from a scarcity of seed production; copious acorn crops were observed in successive decades. Nor is there any 2.1.1. Location, climate and substratum lack of seed viability; as Shaw (1968) comments for The study was carried out in Tomies Wood in North Wales, high germination rates in Q. petraea are Killarney National Park, County Kerry, southwest associated with an oceanic climate and with the Ireland. The site chosen (grid reference V 908 889) presence of a moss carpet, both of which prevent was in a relatively homogeneous stand of woodland on humidity from falling to lethal levels. In the Killarney level terrain, at an altitude of ca. 70 m. The climate is woods, oak seedlings are plentiful in the year or two extreme oceanic, with cool summers, mild winters and following a mast year. It is the extreme scarcity of a high rainfall with rather little seasonal variation. For established saplings and young trees that constitutes a Killarney town, 6 km ENE of the study site and at cause for concern. 55 m altitude, the mean maximum temperature of the There is con¯icting evidence on the vulnerability of hottest month was 19.3 8C and the mean minimum of oak regeneration to competition. Acorns have sub- the coldest month was 3.1 8C (Met EÂ ireann (The Irish stantial food reserves and oak seedlings in well-lit Meteorological Service), unpublished data for the situations develop large tap-roots leading to an period 1961±1990). Mean annual rainfall at Tomies is enormous root/shoot ratio (4±6 with extremes of ca. 1500 mm (cf. Royal Irish Academy, 1979), which >10, Shaw, 1974). Nonetheless, survival and growth is lower than in most of the Killarney woods. The site rates are diminished by the presence of competing is on a morainic terrace composed of Devonian Old vegetation, whether through shading, root competition Red Sandstone debris. The soil is a stony podzol, with (Shaw, 1974), mechanical smothering (Jarvis, 1964a; a humus layer around 5±6 cm deep overlying a sandy Humphrey and Swaine, 1997a) or the production of 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

Tomies exclosure under Killarney woods over Old canopy (unfertilised plots)a Red Sandstone (general survey)b n 10 25 Depth of samples (cm) 0±5 0±5 pHc 4.50 (0.11) 4.46 (0.09) Total phosphorus (mgmlÀ1)d 237 (33) 164 (20) (n ˆ 13) Total nitrogen (mgmlÀ1) 3960 (176) 3670 (250) Loss-on-ignition (%) 69.7 (6.0) 69.4 (6.0)

a Samples collected in September 1999. b 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).

Killarney oakwoods, and of Irish acidophilous oak- (cf. Table 3; nomenclature follows Stace (1997)). woods in general (Table 1; cf. Kelly, 1981; Kelly and Most trees in and around the study area were fairly Moore, 1975). At the southeast corner of the site was typical Q. petraea, but some showed characters an area of impeded drainage ca. 100 m2 in extent suggesting introgression by Q. robur (cf. Cousens, which came to occupy ca. 19% of the clearing, in- 1965). In the present study, no attempt is made to cluding most of nine plots. This had a peaty gley soil distinguish introgressed from `pure' Q. petraea. (Table 2) with a mean humus depth of ca. 13 cm. There were no saplings or young trees of any species. The species-poor ®eld layer covered only ca. 2.1.2. Vegetation 5% of the area; the principal species were, in order, Tomies Wood is representative of the semi-natural Pteridium aquilinum, Luzula sylvatica, Agrostis Quercus petraea-dominated woodlands that are capillaris, Oxalis acetosella and Blechnum spicant. widely scattered in Ireland in upland areas over Mosses covered about one-third of the woodland ¯oor. siliceous rock-types (Kelly, 1981; Kelly and Moore, The plant community is representative of western Irish 1975). The wood was largely felled and replanted in acidophilous oakwoods (Association Blechno±Quer- 1805 (Weld, 1807; Radcliff, 1814); it has subsequently cetum); it is described in detail in Kelly (2000). The reverted to a semi-natural condition. At the start of the vegetation in the peaty area was distinct in the experiment, the stand was dominated by oaks 15±18 m dominance of birch in the canopy and the presence of high, established at dates from 1806 to 1861 (as patches of Sphagnum palustre L. estimated by ring-counts from the clearing). Occa- Tomies Wood has been subject to heavy grazing and sional trees of Betula pubescens (downy birch) were browsing pressure throughout the period of this study, present and a patchy understory of mature Ilex mainly by naturalised Cervus nippon Temminck (sika aquifolium (holly) covered ca. 27% of the area deer), also by trespassing sheep. The native Cervus

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

Under canopy In clearing, non-peaty plots In clearing, peaty plots n 17 12 5 Mean water content (w/v) 0.233 (0.014)** 0.453 (0.029)** 0.614 (0.060)** Loss-on-ignition (%)* 58.8 (3.7)b 62.8 (4.3)bc 78.2 (1.7)c

a 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. 210 D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226

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

Species Under canopy In clearing

No. of individuals (haÀ1)a Basal area (m2 haÀ1)b No. of individuals (haÀ1) Basal area (m2 haÀ1)

Q. petraea 175 54.03 82.1 14.16 I. aquifolium 350 10.37 276.2 11.66 B. pubescens 0 0 29.9 1.98 Malus sylvestris 0 0 7.5 0.81 Total 525 64.40 395.7 28.61

a All stems over 2 m high measured. Areas sampled in year 1 (1972): 1340 m2 in clearing (prior to felling), 400 m2 under canopy. b Basal areas calculated from DBH measured at 1.3 m. elaphus L. (red deer) population has a much lower Light level: 98 plots, each 8 m2, were assigned in impact on the wood. two blocks to shaded treatment (under mature canopy: 50 plots) and unshaded treatment (in the clearing: 48 2.1.3. The light regime plots). The blocks were 35 m apart. There was a Using hemispherical canopy photography, Garvey boundary 8 m wide around the unshaded block. Each (1998) calculated a total ``canopy area index'' of 5.4 in plot was separated from its neighbours by a buffer the wooded exclosure, with deciduous leaf area index zone 0.6±1.04 m wide. contributing almost 70% of this. The leaf area index of Fertiliser treatments were `P': ground rock phos- À2 À2 the holly understorey was 1.2±1.5. During the period phate (Ca10(PO4)6(OH)2), 34.5 g m (i.e. 5 g P m , À2 June±September, mean daily penetration of photo- 13.7 g Ca m );`N':sulphateofammonia((NH4)2SO4), synthetically active radiation at 1 m above ground 47 g mÀ2 (i.e. 10 g N mÀ2); `NP': the two preceding À2 level was generally between 0.4 and 0.8 mol m per fertilisers combined; `Ca': gypsum (CaSO4(H2O)2), day, corresponding to a relative transmission of 3±4%; 59 g mÀ2 (i.e. 13.7 g Ca mÀ2); and controls. Fertili- only 9% of days had a mean of 1.0 mol mÀ2 per day or sers were diluted with silica sand in a ratio of 1:10; the greater. Maximal light levels in absolute terms, with ®rst applications were made in mid-April 1972 and daily totals of 4.5 mol mÀ2 per day, were recorded in repeat applications in late March 1973. There were 20 late April and May. A detailed analysis of the light plots for each of the P, NP and O (control) treatments, regime is presented by Garvey (1998). and 19 each for the N and Ca treatments (Table 4). In the ®rst year, only ®ve each of the unshaded plots 2.2. Experimental design received the NP, N and Ca treatments; in the second year all plots received the designated treatments. An exclosure of 1.2 ha was created in April 1972; Within the shaded block, two 5  5 latin square the fence was 2.5 m high, with wire netting of designs were used to assign the ®ve fertiliser 4.5 cm mesh. A clearing was made by felling and treatments to plots. In the unshaded block, fertiliser removing all trees and larger shrubs from an area treatments were assigned to plots in a modi®ed latin ca. 38 m  45 m (0.17 ha) at the southern end of the square design of 6  8 plots. future exclosure. The timber was dragged away by Competition was investigated in a preliminary tractor; this caused some churning and compaction of experiment in year 1 (`pre-weeded' treatment and the topsoil and may have led to an increase in the area controls) and in a main experiment in years 2±4 with impeded drainage. In the clearing, regrowth from (`weeded' treatment and controls). In the `pre- cut stumps, initially vigorous, was removed by weeded' treatment, all vegetation and litter were repeated cutting in successive years until they ceased manually removed from a 10±15 cm radius around to resprout. each planted acorn at the time of sowing. In the The effects of light level, competition and nutrient `weeded' treatment, all surrounding vegetation supply were investigated in a quasi split plot design. 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 1234581012141618202225

Effect of canopy Under canopya 25019212674ND17131061100050 In clearing 194 173 148 133 96 78 73 71 69 63 53 50 46 37 48

Effect of impeded drainage (clearing only) Unimpeded 163 145 125 115 88 74 69 67 65 59 49 46 42 35 39 Impeded 31 28 23 18 84444444429

Effect of weeding (i) Under canopy Unweeded 125 100 66 36 ND 65441100025 Weeded 125 92 60 38 ND 11 8620000025 (ii) In clearing Unweeded 100 85 74 65 41 34 31 31 31 27 22 21 20 15 24 Weeded 94 88 74 68 55 44 42 40 38 36 31 29 26 22 24 Effects of fertilisation (i) Under canopy Control 50 37 28 19 ND 42200000010 Ca 50432513ND11110000010 N 50382514ND32110000010 P 50392212ND54320000010 NP 50352616ND44321100010 (ii) In clearing Control 49 46 43 41 33 26 24 23 23 22 19 19 17 13 10 Ca 3532282419171515151277659 N 3026221995444433219 P 45393229171414141312111010910 NP 35 30 23 20 18 16 16 15 14 13 13 11 11 9 10

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

30±50 cm around each selected oak seedling. `Weed- in the woodland block and in 20 plots in the clearing ing' was carried out at ca. 2-monthly intervals through (P treatments and controls). A programme of trapping the growing season in years 2±4. This treatment was with break-back mouse-traps was carried out in both assigned semi-randomly to groups/rows of plots blocks of plots from mid-March to mid-July of the within each block. `Pre-weeded' and `weeded' treat- ®rst year. ments were each applied to half of the plots receiving Survival of planted seedlings, estimated by plumule each fertiliser treatment in each block. emergence, was 55% in the clearing and 50% under The vegetation of each plot was recorded at the time canopy. In addition, self-sown oak seedlings appeared of fencing; the cover of each vascular plant species in almost all plots. As numbers of surviving planted was estimated to the nearest 5%, and all tree seedlings seedlings were low in many plots, it was decided in the were counted. Acorns for planting were collected in ®rst autumn to combine planted and self-sown Q. petraea woodlands on sandstone elsewhere in seedlings for the remainder of the study. Up to ®ve the Killarney area. Planting was carried out from widely-spaced seedlings were labelled within each 23 March±6 April 1972 (the radicles were already plot for continued monitoring. This made an `initial several cm long and the plumules just appearing). cohort' of 250 seedlings under canopy (5 seedlings in Seventeen acorns were planted in each of the 50 plots every plot) and 194 in the clearing (1±5 seedlings per 212 D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226 plot) (Table 4). The overall ratio of planted to self- from 17 plots each under canopy and in the clearing at sown seedlings in the `initial cohort' was ca. 1:3. the end of the second growing season and soil moisture As the numbers of seedlings per plot varied, the and loss-on-ignition were measured (Table 2). For layout of plots was different within each block and the moisture content, samples of known volume were numbers of plots assigned to each fertiliser treatment weighed moist, then re-weighed after air-drying to differed between blocks, it was considered appropriate constant weight. to analyse the effects of each experimental factor separately by one-way ANOVAs or their non-para- metric equivalents, even though this approach in- 3. Results creased the likelihood of Type II errors. 3.1. Background changes 2.3. Experimental monitoring The wooded sector retained its canopy largely intact The performance of each monitored seedling/ over the 25-year period. Fencing resulted in some sapling was recorded towards the end of the growing basal sprouting of the understorey holly and abundant season. In the ®rst and second years, leaf count was regeneration of holly from seed. In the plots under recorded in all plots; height was also recorded in a canopy, herb cover rose from 6% prior to fencing to subsample of plots in the ®rst year. By destructive 52% after 17 years, then declined to 19% after 26 sampling of seedlings near plot margins, it was found years (Kelly, 2000). Within 10 years L. sylvatica had that in the ®rst year leaf number was clearly cor- become the principal herb species. In the clearing, a related with seedling dry weight (r ˆ 0:72, P < 0:001, closed cover of vegetation was established by the n ˆ 19), but shoot height was not (r ˆ 0:27, P > 0:1). end of the second summer, dominated by A. capillaris, By the third year, shoot height had become clearly L. sylvatica, B. pubescens saplings and Digitalis correlated with seedling dry weight in the clearing purpurea. By the 10th year, the clearing was a closed (r ˆ 0:75, P < 0:01, n ˆ 12), and leaf counting had Betula-dominated scrub, mean height 4.4 m, with ceased to be practicable, so in the third and subsequent much Rubus fruticosus agg. By 26 years, this was a years height was used as the measure of performance. Betula-dominated woodland, mean height ca. 13.5 m; In the canopy plots, leaf number was recorded at each herb cover was ca. 41%, with Luzula comprising 98% monitoring, and height from year 8 onwards. Seedlings of the sward (Kelly, 2000). were also inspected for presence of and damage by In the early years, the vegetation of the peaty herbivores or pathogens. Monitoring was carried out area included much S. palustre, Juncus bulbosus and annually for 4 years under canopy and 5 years in the J. effusus. Growth of tree saplings was markedly clearing, then at 2±3-year intervals thereafter. retarded, and in the second decade this area was open birch scrub with much Luzula, Pteridium and Calluna. 2.4. Soil analyses By 26 years, the height of the birch canopy was no longer signi®cantly lower than in the other plots, and Samples were taken of the top 5 cm of soil from the the vegetation had largely lost its distinctiveness. ten unfertilised plots under canopy (Table 1). The pH was measured on a 1:1 fresh soil: water paste, using a 3.2. Herbivore impact glass electrode pH meter. Loss-on-ignition was mea- sured by combustion at a temperature that was in- 3.2.1. Predation of acorns and young seedlings creased gradually to 550 8C (Grimshaw, 1989). Total The only animals caught during the period of nitrogen was measured by Leco elemental analyser, trapping (March±July 1972) were 57 Apodemus using a modi®cation of the method of Verardo et al. sylvaticus L. (long-tailed ®eldmice). In spite of (1990). Total orthophosphate P was estimated by the trapping, signs of small mammal activity continued molybdenum blue colorimetric test, using a modi®ca- to be observed in the plots. Small mammal diggings tion of the method of Murphy and Riley (1962). In were observed at the site of planting of at least 17 addition, samples of the top 5 cm of soil were collected acorns; in three cases partially-eaten acorns were found. D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226 213

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

Inside exclosure Outside exclosure

All seedlings Mouse-damaged All seedlings Mouse-damaged seedlingsa seedlings

No. of sample plotsb 20 20 15 15 Density of first-year seedlings (29 September 1985) 3.70 (1.32)c 0.35 (0.18) 0.27 (0.12) 0 Density of third-year seedlings (17 November 1987) 0.40 (0.18) 0 Negligible (ND)

a Plumules partly or fully cut off, at ground level b 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).

Diggings were observed only in the `pre-weeded' the exclosure by the start of the next growing season; plots. Successful germination was signi®cantly lower none was recorded in four random 200 m2 plots in in the `pre-weeded' plots (42% plumule emergence by April±May 1972. Inside the exclosure, the effect of mid-July) than in those where the acorns were inserted release from large herbivores was rapidly evident: the with minimum disturbance to litter and moss (58% mean numbers of self-sown seedlings under canopy emergence; P < 0:001, w2 test). Three bitten-off were 2.8 mÀ2 in summer 1972, 2.3 mÀ2 in summer plumules were recorded in mid-May, and by late July 1973 and 1.5 mÀ2 in summer 1974. a further ®ve stems had been bitten through. However, The pattern was repeated after the mast crop of 1984 no signs of damage to the initial cohort of seedlings by (Table 5): by autumn, the density of ®rst-year small mammals were observed subsequent to July of seedlings outside the exclosure was only 7% of the the ®rst year. density inside (P < 0:05). After 2 years, 11% of this The mast year of 1984 provided fresh data on cohort of seedlings still survived under canopy within predation of oak seedlings (Table 5). Plumule damage, the exclosure, whilst seedlings outside the fence had apparently by Apodemus, was recorded for 10% of virtually disappeared. seedlings under canopy within the exclosure. A breach of the exclosure occurred as a result of a tree falling across the fence in the second winter 3.2.2. Invertebrate herbivory (January 1974). A number of sika deer ranged through Various kinds of invertebrate leaf damage were the exclosure over a period of about 2 days frequent but appeared to be of minor impact (chlorotic, (P. Moriarty, personnel communication), concentrat- necrotic or fenestrated patches, nibbled or inrolled ing their activity in the clearing. Feeding concentrated margins, and gall-type swellings). The commonest on Hedera helix, L. sylvatica and Q. petraea (fallen type of damage was `stippling': tiny chlorotic spots branch, basal sprouts and seedlings). The apparent caused by the feeding of green¯y (Aphididae) and leaf order of preference among tree regeneration was hoppers (Cicadellidae sensu lato) (cf. Moore et al., Sorbus aucuparia > Quercus @ Betula ˆ Ilex. Many 1991); in year 3, this was noted in 19% of seedlings birch seedlings in the clearing were over 1 m high, yet under canopy but only 1.3% in the clearing. Only two only a few were lightly browsed. Oak seedlings did not seedlings were ever recorded as having been severely exceed 0.3 m high, yet in the clearing 30% were defoliated, probably by slugs. At no stage did I browsed; about half of the shoot was removed in most observe serious damage by invertebrates to a sig- cases. Seven (4%) died as a direct result, having been ni®cant number of seedlings. more or less uprooted. In the weeded plots, 49% of the oak seedlings were browsed, but only 11% in the 3.2.3. Impact of large herbivores unweeded plots (P < 0:001, w2 test). The luxuriant In spite of the heavy mast crop of 1971, oak surrounding vegetation clearly had a protective effect seedlings were already scarce in Tomies Wood outside on the seedlings in the unweeded plots; R. fruticosus 214 D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226 appeared to be particularly effective. The oak seed- signi®cantly lower in the peaty plots (mean leaf count lings in the plots under canopy were untouched. 10.7) compared to the plots with unimpeded drainage (mean leaf count 13.4; P < 0:05, one-way ANOVA). 3.3. Regeneration under canopy and in clearing Seedlings growing in the most peaty parts were often compared obviously stunted and chlorotic in the early years. Growth of the surviving oaks was evidently retarded, Assessment of seedling performance was compli- although sample size was too low for the differences to cated by high mortality. Survivorship is presented as be statistically signi®cant (Fig. 2b). the median of the proportion of the initial cohort in each plot still alive at a given census (`cumulative 3.5. Effects of weeding survival function' of Fox (1993)). Under canopy most seedlings failed to make signi®cant growth, regardless WeedingÐcarried out in the second to fourth of fertiliser treatment or weeding regime. Mortality yearsÐhad no signi®cant effect on the survival or was high in each year (Table 4, Fig. 1a). Dead, performance of oak seedlings under canopy (cf. withered seedlings were regularly found in the course Table 4). In the clearing, weeding clearly resulted of monitoring, but the immediate cause of death was in increased survival in subsequent years (Table 4, seldom evident. Out of the initial cohort of 250 Fig. 3a). Seedlings in the unweeded plots made fair seedlings, a single sapling survived under canopy for initial growth when surrounded by L. sylvatica or 18±20 years, attaining a ®nal height of 24 cm and a R. fruticosus, but appeared strongly inhibited by leaf count of 10. Leaf count remained static (Fig. 1b). A. capillaris. In year 2, seedlings in the weeded plots Mean height showed a very slow increase, from (mean leaf count 26.5) signi®cantly outperformed 10.3 cm in year 1 to 18.1 cm in year 8. those in the unweeded plots (mean leaf count 19.2; Mortality in the clearing was much lower (Fig. 1a). P < 0:05, one-way ANOVA). The deer incursion in However, the oak saplings were soon overtopped by the second winter led to a reversal; in years 3±5, the faster-growing species, mainly Betula. By year 10, the seedlings in the unweeded plots were signi®cantly surviving oaks had a mean height of 1.75 m, compared taller than in the weeded plots (Fig. 3b). This re¯ected to 4.4 m for the mean height of the birch scrub. The the lower proportion of seedlings browsed where they surviving oaks at year 25 had a mean height of 3.1 m were surrounded by other vegetation. After the and a range of 0.9±7.0 m. By then the birch canopy discontinuance of weeding, surviving oak regenera- was about four times the mean height of the oaks, and tion was subject to strong competition by surrounding all oak individuals were clearly suppressed to some vegetation. In the clearing, young oaks were soon degree. overtopped and suppressed to varying degrees by The majority of seedlings in the clearing produced a saplings of faster-growing trees, mainly B. pubescens. vigorous second ¯ush of shoots in July of each year (`lammas shoots'). In the third year, these were 3.6. Pathogens recorded on 72% of seedlings in the clearing, but only 11% of seedlings under canopy (P < 0:001, w2 test). Microsphaera alphitoides Griff & Moubl. (oak mildew) was the only pathogen of note. Infection of 3.4. Effects of soil moisture regime the seedlings was widespread, but highly variable in space and time. In the ®rst 2 years, infection was much Drainage was clearly impeded in the peaty plots, as more widespread and severe in the clearing (23% of shown by the high sample water content of the topsoil unshaded seedlings severely infected compared to (Table 2), the deeper and more organic humus layer 4% of shaded seedlings in mid-October of year 1 and the presence of S. palustre. Mortality was similar (P < 0:001, w2 test); 14% of unshaded seedlings to that in the rest of the clearing for the ®rst 3 years, but severely infected compared to 0.5% of shaded then showed a sharp increase (Table 4, Fig. 2a). After seedlings in early September of year 2 (P < 0:001, 25 years, only 6% (two individuals) were still alive. By w2 test)). A severe infection was de®ned as one where the autumn of year 1, performance was already several leaves were more or less completely covered D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226 215

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 ÃP < 0:05 and ÃÃP < 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. 216 D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226

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. D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226 217

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 ÃP < 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 ÃÃP < 0:01; ÃÃÃP < 0:001 (one-way ANOVA).

by mycelium. Infection was markedly more severe on year, infection was low overall (0% severe infection in lammas shoots than on leaves of the spring ¯ush. In unshaded and 2.3% in shaded seedlings, in late both years, level of infection in the clearing showed a August; difference not signi®cant). positive association with performance, as measured by Pustules matching descriptions of the rust Uredo leaf count (P < 0:001, one-way ANOVA). In the third quercus Brond. were observed on the leaves of several 218 D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226

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). D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226 219

Fig. 4. (Continued ).

®rst-year seedlings, both in clearing and under canopy. 4. Discussion They were not re-found in subsequent years and the identi®cation requires con®rmation. 4.1. Herbivore impact

3.7. Effects of fertilisation 4.1.1. Predation of acorns and germinating seedlings The acorn crop is exploited in British woods by a In the canopy plots, no signi®cant effect on either range of birds (especially wood-pigeons and jays, survival or performance was obtained with any of the Worrell and Nixon, 1991), and mammals (especially fertiliser treatments (Table 4). In the clearing, no squirrels, mice, voles, rabbits, deer and badgers, fertiliser treatment signi®cantly enhanced survival, Corbet, 1974). Effective dispersal is ascribed espe- and fertilisation with N alone was associated with a cially to jays (Garrulus glandarius L.), also to other striking increase in mortality (Table 4, Fig. 4a). The P birds (cf. Van der Pijl, 1982; Kollmann and Schill, treatment produced signi®cantly increased perfor- 1996). Evidence that small rodentsÐparticularly mance relative to controls and to N alone (Fig. 4b and long-tailed ®eldmice (A. sylvaticus)Ðare major acorn c). Joint N ‡ P application produced no evidence of consumers is presented by Ashby (1959); Jarvis (1960, an `enhancement' effect relative to P alone. The P 1964b) and Shaw (1968). However, Tanton (1965) treatment may have some long-term effect; 23 years concluded that in mast years mice and voles only after the last application, the mean height in P- removed a small fraction, squirrels and pigeons being fertilised plots was 69% higher than in the controls. the major consumers. The apparent stasis in growth in the control plots In the Killarney woods, heavy pre-dispersal preda- during the period 16±25 years (Fig. 4c) is partly an tion of acorns by wood-pigeons (Columba palumbus) artefact due to mortality: of those trees that survived was noted during mast crops by Smal and Fairley throughout this period, controls made a mean (1980) and by the author. Jays were present in small height increment of 6.9% and P treatments a mean numbers. The rodents A. sylvaticus, Sciurus vulgaris of 12.6%. and Clethrionomys glareolus Schreber (bank vole) 220 D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226 were widespread in the woods. The last-named was defoliation of oak seedlings by phytophagous insects is apparently introduced to Ireland around 1950; it was widely reported from British oakwoods (Watt, 1919; recorded in the Tomies area in 1969 (Fairley and Jarvis, 1960; Shaw, 1968, 1974; Humphrey and O'Donnell, 1970) and was present in moderate Swaine, 1997b) and also from France (Lanier, numbers in Tomies Wood by 1975±1976 (Smal and 1981). Experimental studies found greater seedling Fairley, 1982). However, Apodemus was the only defoliation under oak canopy than in adjacent open rodent trapped within the exclosure. Damage attribu- areas (Shaw, 1968; Humphrey and Swaine, 1997b). table to Apodemus was observed to considerable Shaw (1974) argued that caterpillar defoliation ``is a numbers of acorns and plumules, but none to older major factor, of the same magnitude as light, in seedlings. Examination of stomach contents of Apode- determining the growth and survival of oak seedlings''. mus trapped in another Killarney oakwood found that Both O. brumata and E. defoliaria are common acorns constituted a major part of their diet from the and widely distributed in Ireland (Baynes, 1964). fall of a mast crop well into the following summer T. viridana is apparently less common than in England (Smal and Fairley, 1980). In Tomies, the signi®cantly (Beirne, 1941), yet mass occurrences of this species greater damage in `pre-weeded' plots (where vegeta- and associated defoliation have been recorded from tion and litter were removed from around each acorn at different parts of the Killarney woods, including the time of planting) supports the conclusion of Watt Tomies Wood in 1990 (Bond, personnel communica- (1919) and Jarvis (1964b), that leaf litter may play an tion). Nonetheless, the present study found no evidence important role in concealing acorns from potential of a signi®cant impact of any invertebrate group on predators. Predation of the plumules of ®rst-year oak survival or growth of oak seedlings or saplings. This seedlings by small mammals was also recorded by contrasts with ®ndings for other regionsÐincluding Watt (1919) and Jarvis (1960). Scotland, where Humphrey and Swaine (1997b) found The trapping programme in the early months of the high levels of seedling defoliation by caterpillars at the experiment may have arti®cially enhanced the survival more oceanic of their two study sites. rate of ®rst-year seedlings. The data collected after the mast year of 1984 permit a clearer evaluation of the 4.1.3. Impact of large herbivores role of mice in seedling predation (Table 5). The The underground reserves of the oak seedling are survival of a considerable proportion of this cohort of considered to confer resilience to browse damage seedlings into their third year, within the exclosure, (Shaw, 1974; Rackham, 1980). However, when grazing supports the view that Apodemus are signi®cant pressure rises above a certain level, tree regeneration predators of acorns and ®rst-year seedlings but have practically ceases. In studies of the relations between negligible impact on older seedlings. regeneration and grazing intensity in the New Forest I conclude that mice are not a limiting factor to over a 250-year period, the origin of the three main oak regeneration in the Killarney woods. This echoes surviving cohorts of trees has been dated to periods the ®nding of Shaw (1974) that A. sylvaticus and when grazing was reduced (Peterken and Tubbs, 1965; C. glareolus, although ubiquitous, are ``of little or no Tubbs, 1986). importance'' in relation to oak regeneration in North In Tomies Wood, the exclosure has become an Wales. In mast years, a vast surplus is produced and, as ``island'' of virtually ungrazed vegetation in an over- Mellanby (1968) points out, ``the fact that a particular grazed landscape. The fate of the mast crop of 1984 species eats many acorns may not mean that it is a outside the exclosureÐdecimated within their ®rst major factor in preventing regeneration.'' growing season (Table 5)Ðclearly shows heavy predation of seedlings by large herbivores (principally 4.1.2. Invertebrate herbivory sika deer). Outside the fence, the only self-sown oak Complete defoliation of oak trees is not infrequent in sapling over 0.5 m high observed in the vicinity was Britain, the principal agents being caterpillars of on the upper part of the root plate of a wind-thrown Tortrix viridana (L.), Operophtera brumata (L.) (win- tree. ter moth) and Erannis defoliaria (Clerck) (mottled The 2-day breach of the fence in the second winter umber) (Jones, 1959; Gradwell, 1974). Signi®cant of the experiment provided striking evidence of the D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226 221 partiality of sika deer for oak seedlings. For fallow pioneer species unable to establish itself from seed in deer (Dama dama L.), Peterken and Jones (1989) its own shade''. Evidence for a northwest to southeast found, in broadleaved woodland in the Wye Valley, gradient in shade tolerance in both Q. petraea and that the order of preference among the saplings of the Q. robur is summarised by Krahl-Urban (1959): high main tree species was oak > ash > beech > birch. levels of tolerance are reported for Romania (Dengler, They concluded that deer handicap oak in competition 1944) and Slovenia (Krahl-Urban, 1959) and success- with birch, a ®nding supported by the evidence from ful oak regeneration under relatively light or open tree the Tomies exclosure. The incursion also demon- canopies are reported from various parts of Germany strated that a surrounding growth of vegetation could (Hauskeller-Bullerjahn, 1997; von LuÈpke, 1998) and provide some degree of protection for young oaks The Netherlands (Van Hees et al., 1996). Even in the (cf. Fig. 3b). The most effective cover appeared to Pennines, Pigott (1983) found successful regeneration be R. fruticosus agg. Evans (1988) also noted that of Q. petraea under a canopy of the same species; R. fruticosus may protect oak regeneration against however, this woodland was on a slope facing south- browsing; however, other authors have found that oak east, had a canopy only 5±12 m high and was purely regeneration is poor or absent under Rubus owing to deciduous, hence, the light regime was unusually the heavy shade that it casts (Everard, 1987; Linhart favourable. and Whelan, 1980). In Tomies the Rubus population Light levels under the canopy in the Tomies derived from seedlings which appeared in the ®rst exclosureÐwith 3±4% PAR penetration being typical season after clearance (cf. Kelly, 2000), and adjacent in summer and autumnÐwere considerably lower than oak seedlings had some time to build up root reserves in other temperate deciduous woodlands for which before being overshadowed. Other spiny shrubsÐIlex, comparable data are available (Garvey, 1998). The Prunus spinosa, Crataegus and UlexÐhave been unfavourable nature of the light regime resulted from a reported as protecting oak saplings against browsing combination of the oceanic climate, with its persistent in Britain (Watt, 1919; Shaw, 1974; Rackham, 1980). cloudiness, and the evergreen holly understorey. The negative effects of even a deciduous understorey on 4.2. Shade Quercus rubra regeneration were demonstrated experimentally in mixed hardwood forest in Wisconsin The strikingly negative effects of canopy cover on (Lorimer et al., 1994). seedling survival may be ascribed to insuf®cient A negative effect of canopy trees on oak regenera- penetration of photosynthetically active radiation tion via root competition for water has been demon- (PAR), to root competition for water or nutrients, or strated by trenching experiments in England (Jarvis, to allelopathic effects. Pot experiments that were 1964a) and Germany (Hauskeller-Bullerjahn, 1997). watered to avoid soil moisture stress have con®rmed Ashton and Larson (1996), in a study on New England that light is a major limiting factor to Q. petraea oaks, suggested that seedlings in gaps bene®ted not regeneration (Jarvis, 1964a). Increased shading only from increased light but also from increased leads to lower root/shoot ratios in Q. petraea and moisture during spring and summer as a result of Q. petraea  Q. robur seedlings (Ovington and reduced root competition. It is possible that the MacRae, 1960; Jarvis, 1964a; Humphrey and Swaine, seedlings in the non-peaty part of the Tomies clearing 1997a); this may be expected to reduce their resilience experienced a similar double bene®t (cf. Table 2); in to grazing and other kinds of damage. the moist, cool climate of southwest Ireland, competi- Some British studies suggest that oak has a lower tion for water may be of limited signi®cance, but shade tolerance in more oceanic regions. In an 8-year cannot be discounted. study of Q. petraea regeneration in North Wales, Shaw (1974) found that light levels of ca. 30% of full daylight 4.3. Waterlogging are required for longer-term survival, and that levels of 50% or more are required if reasonable growth rates are The high mortality and retarded growth of the to be achieved. In the upland oakwoods of Dartmoor, oak seedlings on peaty soil (Fig. 2a and b) supports Barkham (1978) suggests that Q. robur ``behaves as a the general view that Q. petraea is intolerant of 222 D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226 waterlogging (Jones, 1959; Ellenberg, 1988). A nega- 26-year study of Pigott (1983), on fenced oak-birch tive effect of acid peat on Q. petraea seedling woodland in the Pennines, differs in recording development was also noted by Ovington and MacRae intermittent recruitment of oak saplings subsequent (1960). The effect may be through a combination of to the initial cohort. anoxia and nutrient de®ciency: in Tomies the topsoil in the peaty plots had a signi®cantly lower ash content 4.5. Pathogens as well as being subject to waterlogging (Table 2). M. alphitoides (oak mildew) is considered to have a 4.4. Competition damaging impact on oak regeneration in Britain (Murray, 1974) and Denmark (Haugh, 1934). Oak regeneration is clearly vulnerable to competi- Although Q. petraea is regarded as less susceptible tive inhibition from herbaceous and low woody species than Q. robur (Jones, 1959; Murray, 1974), mildew has in the early years, including Deschampsia ¯exuosa been found to have a serious impact on Q. petraea (Jarvis, 1964b), Holcus mollis (Shaw, 1974) Molinia seedlings growing under shade in English oakwoods caerulea (Becker and Levy, 1983) and P. aquilinum (Jarvis, 1964a; Shaw, 1974). The fungus was appar- (Watt, 1919; Humphrey and Swaine, 1997a). Some ently unknown in Europe prior to 1907; Rackham species may have inhibiting effects yet may provide (1980) opined that ``the arrival of mildew probably some protection for oak seedlings against browsing, made oaklings succumb to a degree of shade which notably R. fruticosus agg. (see above) and Pteridium they would formerly have survived''. (Barkham, 1978). In the Tomies clearing, the deer Monitoring in Tomies Wood was somewhat incon- incursion con®rmed that a surrounding vegetation sistent in timing, but adequate to demonstrate that cover could have a protective effect on oak seedlings. severe infection was rare on seedlings under canopyÐ However, in the absence of further grazing, the which is the situation in which it would be most likely negative effects of competition are demonstrated by to be seriously damaging. In the ®rst- and second-year the enhanced seedling survival in the weeded plots seedlings in the clearing, infection was often severe, from the ®fth year (Fig. 3a). A comparable long-term but the positive association between performance and response was reported in a study of the effects on level of infection indicated that it did not constitute a Q. petraea regeneration of removing herbaceous signi®cant handicap. There was clearly a relation vegetation by repeated herbicide application (von between the development of lammas shoots and the LuÈpke, 1987); in clearings, height growth was greater virulence of mildew attack, as also reported by Haugh in treated plots even 4 years after herbicide application (1934) and Jones (1959). was discontinued. I conclude that mildew is not a signi®cant deterrent B. pubescens had become the principal competitor to oak regeneration in the Killarney oakwoods. in the Tomies clearing within the ®rst decade of the Powdery mildew infections increase plants' suscept- study. It increased its lead in succeeding decades: the ibility to abiotic stresses such as drought (Smith et al., ratio of height of birch canopy to mean height of 1988), and the apparently slight impact in these woods surviving oaks rose from 2.5:1 in year 10 to ca. 4:1 in may perhaps be ascribed to the mitigating in¯uence of year 25. All established saplings of both oak and birch the moist, mild climate. belonged to the same initial cohort; there was no successful recruitment of oak from subsequent mast 4.6. Effects of fertilisation years. In their 40-year study of natural regeneration in felled areas in broadleaved woodland in the Wye The seedlings under canopy were apparently too Valley, Peterken and Jones (1989) also found that all light-starved to respond to fertilisation (cf. Table 4). oak saplings seemed to belong to the same cohort and Shaw (1974) found that Q. petraea seedlings grown on that subsequent heavy masts gave rise to no recruits. a base-poor woodland soil at 10% of full daylight These results conform to the model of succession showed net losses in P and Krelative to the contents of based on initial ¯oristic composition (Egler, 1954) the acorn; he concluded that ``seedlings in less than rather than to the classic ``relay ¯oristics'' model. The about 30% full daylight were simply unable to balance D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226 223 losses of potassium and phosphorus through leaf fall, of the growth of oak by fertiliser addition is dif®cult to and other minor causes such as root death, by achieve in practice (Evans, 1986a,b). Fertilisation suf®cient uptake''. would clearly be inappropriate to the management of Oak seedling performance in the clearing showed a National Parks or Nature Reserves. positive response to rock phosphate (P) but not to Oak regeneration is adversely affected by water- calcium sulphate (Ca) or ammonium sulphate (N) logged, peaty soil. The rise in water tables consequent (Fig. 4b and c). The lack of response to calcium on forest clearance may help to explain the failure of sulphate con®rms that the positive response to the P oak to recolonise areas that have been deforested, treatment is to orthophosphate and not to calcium ion. especially in regions of strongly oceanic climate. These ®ndings echo those of Newnham and Carlisle Phytophagous invertebrates and fungal pathogens (1969) for typical oak and oak/ash woods in Cumbria: were both present, but neither had a signi®cant impact they found that the soils yielded adequate N but on oak regeneration. Both have emerged as signi®cant insuf®cient available orthophosphate for maximum factors in British and Continental studies; these growth of ®rst-year seedlings of Q. petraea and contrasts warrant further investigation. Q. robur. A similar response to P but not to N, using Oak is vulnerable to competition from a range of soil from the Killarney oakwoods on sandstone, was herb, shrub and tree species. The clearing with which obtained in experiments with ®rst-year seedlings of the experiment began is now a young birchwood, and Rhododendron ponticum (Cross, 1973, 1975) and it remains to be seen whether the surviving oak I. aquifolium (Cross, 1973). saplings will succeed in becoming a signi®cant The negative effect of N on seedling survival in the component of the future canopy. clearing was striking (Table 4, Fig. 4a). Newton and Oak regeneration was precluded by the presence of Pigott (1991), investigating the growth of Q. robur a closed oak canopy accompanied by a patchy seedlings on infertile woodland soils, recorded a evergreen understorey; this was almost certainly due number of negative effects of fertiliser application: to insuf®cient penetration of photosynthetically active both N and Kbrought about signi®cant decreases in radiation. foliar P concentration; N, P and N ‡ Kled to sig- Oak regeneration is virtually precluded where ni®cantly reduced levels of ectomycorrhizal infection. woods are subject to heavy grazing pressure. In Shaw (1974) records that oak seedlings planted in Killarney National Park, oak saplings (outside exclo- British woodlands showed a marked initial growth sures) are largely restricted to the proximity of roads response to a general NPKfertiliser, but that this and tracks, where browsing animals are more or less rapidly disappeared and was replaced by a signi®cant deterred by frequent human disturbance. A concen- increase in mortality. The effect was most marked for tration of young oaks along woodland rides has also seedlings growing in D. ¯exuosa and was attributed to been noted in woods in eastern England (Rackham, increased competition from that species. However, in 1980). It is clear that a vital condition for the Tomies, no evidence was found of increased compe- conservation of native oak is the achievement of titive inhibition, predation or disease in N-fertilised suf®ciently low densities of large herbivores. As plots. Evans (1988) warned for Britain, ``many semi-natural woods in the uplands are failing to regenerate and gradually disappearing because of grazing pressures''. 5. Conclusions The achievement of successful regeneration of oak in existing gaps and open areas would seem to provide a Height growth of young Q. petraea was clearly measure of the true `carrying capacity' for large stimulated by fertilisation with orthophosphate, herbivores of a wooded region such as the Killarney although survival did not differ signi®cantly from National Park. controls. Soil fertility on podsolised soils over parent I conclude that the pre-condition for regeneration of material derived from Old Red Sandstone may, oak in western Ireland and other oceanic regions is a therefore, be limiting. This ®nding has potential combination of suf®ciently high light levels and signi®cance for sylviculture, although improvement suf®ciently low grazing levels. 224 D.L. Kelly / Forest Ecology and Management 166 (2002) 207±226

Acknowledgements Cross, J.R., 1973. The ecology and control of Rhododendron ponticum L. with special reference to the Killarney National Park. Ph.D. Thesis, Trinity College, University of Dublin, The ®eldwork for this study has been largely funded Ireland. by National Parks and Wildlife (DuÂchasÐThe Cross, J.R., 1975. Biological ¯ora of the British Isles: Rhododen- Heritage Service, Department of Arts, Heritage, dron ponticum L. J. Ecol. 63, 345±364. Gaeltacht and the Islands) and its predecessor Dengler, A., 1944. Waldbau auf oÈkologischer Grundlage, 3rd organisations. I thank in particular Mr. M. Neff and Edition. Springer, Berlin. Egler, F.E., 1954. Vegetation science concepts. Part I. Initial Dr. A. Craig for sustained support, also Mr. D. ¯oristic composition, a factor in old-®eld vegetation develop- Kelleher, Mr. P. O'Leary, Mr. P. Dower and other staff ment. Vegetatio 4, 412±417. of the Killarney National Park, and Mr. P.J. Bruton. Ellenberg, H., 1988. 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