Comparison of breeding bird assemblages in conifer plantations managed by continuous cover forestry and clearfelling

Forest Ecology and Management 344 (2015) 20–29

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Comparison of breeding bird assemblages in conifer plantations managed by continuous cover forestry and clearfelling John Calladine a,⇑, James Bray a, Alice Broome c, Robert J. Fuller b a

British Trust for Ornithology (Scotland), School of Biological Sciences, University of Stirling, Stirling FK9 4LA, UK British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK c Forest Research, Centre for Ecosystems, Society and Biosecurity, Northern Research Station, Roslin, Midlothian, EH25 9SY, UK b

a r t i c l e

i n f o

Article history: Received 19 November 2014 Received in revised form 3 February 2015 Accepted 11 February 2015 Available online 27 February 2015 Keywords: Bird assemblages Forest management Shrubland Conservation strategy Sitka spruce

a b s t r a c t Continuous cover systems are increasingly advocated for stand management but the implications for biodiversity in European forests, and specifically in plantations of non-native trees, are poorly understood. Timed point counts were used to quantify differences in species richness and abundance of breeding birds supported by conifer plantations (with major Sitka spruce components) under two contrasting management systems in upland Britain: continuous cover forestry (CCF) and clearfelling with replanting (CFR). Each CCF study area was paired with a comparable CFR study area. Sample points within CCF areas were divided into areas with extensive regenerating understorey and areas with none; sample points within CFR study areas were placed within young thicket and pre-thicket stands (trees < 10 years old) and older stands (15–30 years old). Poisson GLMMs were used to identify differences in bird species richness and abundance between the four treatments testing the predictions: (a) CCF can support an enhanced assemblage of forest birds relative to CFR (including mature CFR); and (b) CFR can support a broader range of open habitat and shrubland species relative to CCF (including those with a regenerating understorey). Ranking forest types in descending order of species richness gave: CCF with shrub understorey > CCF without shrubs > young pre-thicket CFR > mature CFR. Many ‘mature forest birds’ were more abundant, or recorded only, within CCF (e.g. blackcap, wood warbler, redstart and hawfinch). A small number of species associated with young-growth (‘shrubland’ and ‘shrub-layer’ species) were most abundant in pre-thicket CFR but a CCF understorey supported some species at densities approaching those found in pre-thicket CFR. Simulations of the effect of increasing the proportion of plantation under CCF indicated for example that a plantation managed exclusively as CCF could support as few as 53% of the willow warblers as one managed exclusively as CFR. A plantation managed exclusively as CCF could support as few as 70% of the lesser redpolls as one managed as CFR, but could support twice as many blackcaps. CCF could be of greater conservation value to many forest birds than CFR. However, CCF may not support such high densities of some species (e.g. dunnock, willow warbler and lesser redpoll) as those found in young growth stage CFR. Forest management that includes some young growth areas alongside CCF could prove to be a strategy that maximises the capacity of a forested landscape to support a greater diversity of bird species. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Managed plantations generally support an impoverished avifauna compared to many more natural woodlands but both species richness and bird abundance can be positively related to their structural complexity (Nájera and Simonetti, 2010). By ‘structural complexity’ we mean the vertical foliage profile; woods with complex structures typically have much low woody understorey ⇑ Corresponding author. Tel.: +44 1786 466560; fax: +44 1786 466561. E-mail address: [email protected] (J. Calladine). http://dx.doi.org/10.1016/j.foreco.2015.02.017 0378-1127/Ó 2015 Elsevier B.V. All rights reserved.

vegetation and a multi-layered canopy. Management regimes operating within plantations can influence that structural complexity, with bird communities supported by the more mature plantations increasingly resembling those supported by native non-planted forests (Moss et al., 1979; Peterken et al., 1992; Hartley, 2002). The most mature plantations can support a number of forest-specialist species (Petty and Avery, 1990; Donald et al., 1997; Marion and Frochot, 2001). However young stages of plantation growth can also support important assemblages of birds that are associated with more open and shrubby habitats (Moss et al., 1979; Bibby et al., 1985; Marion and Frochot, 2001). Some studies

J. Calladine et al. / Forest Ecology and Management 344 (2015) 20–29

have even shown species richness to decline in older plantations (Moss et al., 1979; Marion and Frochot, 2001), however it should be noted that, in these studies, the most mature plantations (older than the pole-stage) were rare or not surveyed. Two alternative (and broadly defined) management systems for plantations are clearfelling with replanting and continuous cover forestry. Clearfelling with replanting (hereafter termed CFR) are where forest coupes are cut down and replanted typically for conifers on 30–60 year rotations, and is how most conifer plantations in Britain (the location of the present study) have been managed to date (Mason et al., 1999). This has resulted in plantations comprising adjacent but discrete coupes of different but uniform aged, and therefore also uniformly structured, stands of trees (hence it also being referred to as an even-aged silvicultural system). Coupe sizes vary from less than 10 ha to several hundreds of hectares, though the largest coupes are now scarce in Britain. Continuous cover forestry (hereafter CCF) is increasingly advocated in Britain (but is more widely established in mainland Europe). This includes management systems referred to as low intensity silvicultural systems, uneven aged silviculture, ‘jardinage’ and ‘Plenterwald’ (O’Hara, 2001; Pommerening and Murphy, 2004; Pukkala, 2006). Felling within CCF managed plantations is more selective with the removal of single trees to small coupes of up to about 0.25 ha in size. If seed sources and browsing pressure permit, then trees can naturally regenerate within the cleared areas, or otherwise by supplementary planting. CCF, as its name implies, retains a relatively continuous forest canopy cover across the plantation extent. Structural diversity amongst the crop trees is at a much finer scale than found within CFR and some trees can grow older, their maximum girth being effectively limited by the ease with which they can be felled and extracted with available machinery. Studies of birds in semi-natural forests of North America, that are either managed through clearfelling or that are selectively felled to maintain uneven aged stands with near continuous cover, identify conflicts of interests between management that favours birds of mature forests and the maintenance of habitats for birds of woody, early successional communities (Costello et al., 2000; Thompson and DeGraaf, 2001; Gram et al., 2003). The birds of CFR managed forests are relatively well known (especially in north-west Europe) but the implications of alternative stand management for biodiversity in Europe is poorly understood. The implications of alternative stand managements in plantations of non-native tree species are similarly poorly understood. Alternative silvicultural systems, that deliver uneven aged stands of trees, have been proposed as a mechanism to improve the value of planted forests for biodiversity in Britain (Kerr, 1999) where there has been a policy shift from clearfell systems to CCF to achieve multipurpose objectives that includes biodiversity (Davies and Kerr, 2011). Environmental accreditation and its associated premium prices for forest products are amongst the encouragement for forest managers to convert management of plantations to CCF, however there has been rather limited evidence of the expected benefits for birds (du Bus de Warnaffe and Deconchat, 2008) and the potential impacts on birds reliant on early successional growth stages perhaps not well considered. An important difference between the two management regimes is the occurrence and distribution of young growth stage trees (saplings). In Europe, areas of low woody shrubs or early growth stages are important habitats notably, though not exclusively, for long distance migrant birds (Helle and Fuller, 1988; Fuller, 2012). Within CFR, pre-thicket and early thicket stage plantations are ecologically equivalent to shrubland (Askins, 2001; Hunter et al., 2001) which occurs as discrete uniform-aged blocks which can be extensive, occupying entire planted or restocked coupes. Within CCF, shrubs (or saplings) tend to occur as smaller clusters and are expected to be more heterogeneous in age and/or structure

21

dependent on the patchiness of the selective felling regime and opportunities for regeneration and tree growth within those felled patches. Within CCF, young trees or shrubs will also occur as an understorey though their density may depend on thinning intensity. Young growth trees or shrubs are infrequent within CFR after canopy closure, typically at about 12–15 years old in British conifer plantations. It is likely that the bird communities found within understorey shrubs/saplings of CCF will differ from those of the more open and extensive patches of young growth CFR (Fuller et al., 2012). Although changes in plantation management from CFR to CCF might be expected to deliver conservation benefits in an enhanced mature-forest avifauna (though this was not demonstrated in a study in Belgium; du Bus de Warnaffe and Deconchat, 2008), there is a possible consequence that the shrubland avifauna could diminish. Data collected from spruce plantations in upland Scotland and Wales are presented that quantify differences in the breeding birds (species richness and abundance) under the contrasting management regimes of CFR and CCF. These are used to assess the predictions: (a) CCF can support an enhanced assemblage of typically mature forest bird species relative to CFR plantations (including the more mature stands within CFR); (b) CFR can support a broader range of open habitat and shrub specialist species relative to CCF managed plantations (including those with a developed regenerating shrub understorey). The relative contributions to bird conservation of the two management regimes are considered and examined using simulations of plantation areas under different proportions of the contrasting management regimes.

2. Methods 2.1. Study sites Suitable study sites in Britain were limited by the availability of CCF-managed plantations that were (a) sufficiently developed for their structure to differ from that of maturing CFR-managed sites, and (b) large enough to be expected to be able to support a bird community with the potential to differ from surrounding CFR plantations. Plantations under transformation to be managed as CCF but where trees were still of uniform age and less than 30 years old were not suitable as they were structurally identical to CFR plots of a similar age. CCF plots that were less than five ha in extent were also considered unsuitable because of the limited bird populations that their restricted size could potentially support. Study areas were also required to include a major Sitka spruce (Picea sitchensis) component to ensure representativeness with the majority of plantations that are likely to become managed as CCF in the uplands of Britain. Within these restrictions, four suitable CCF study areas were identified (Fig. 1). For each CCF study area, a similarly sized CFR study area was selected. Each CFR site was within 15 km of its paired CCF site (in all but one case the distance was <5 km) and was of comparable altitude, aspect and underlying geology. Point counts were used to sample breeding birds within the study areas. Points were at the intersections of a 150 m grid to permit representative sampling of a sufficient area within each management treatment (CCF or CFR) while also ensuring relative independence of data collected from each point (Bibby and Buckland, 1987). Each management treatment was further divided into two sub-categories. Sampling points within CCF sites were

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J. Calladine et al. / Forest Ecology and Management 344 (2015) 20–29

Fig. 1. The location of study areas in Scotland and Wales: (1) Craigvinean (56° 340 N, 3° 400 W); (2) Puck’s Glen (56° 10 N, 4° 580 W); (3) Bowhill (55° 320 N, 2° 550 W); (4) Clocaenog (53° 40 N, 3° 280 W).

divided into areas with an extensive regenerating shrubby understorey (60 sampling points) and areas without or where any regeneration appeared infrequent and patchy (104 sampling points). Areas with a regenerating understorey were areas that had undergone selective felling and sufficient time had elapsed for the regenerating trees to create a shrub layer. Within CFR sites, points were divided into ‘young CFR’, that is areas within young thicket and pre-thicket stands (trees less than 10 years old; 59 sampling points) and older stands (15–40 years old; 112 sampling points). In addition to the occurrence of shrubs (saplings) and tree sizes (or ages) which defined the four management categories, broadleaved trees were more frequent within the CCF managed areas (Table 1).

2.2. Data collection Timed counts (10 min) were used to sample bird occurrence and abundance at the survey points. Carried out twice in the 2012 breeding season (early survey visits were between 5th April – 4th May and late survey visits 1st – 7th June), most surveys were undertaken in the early mornings (80% between 04:15 and 09:30) when many bird species are most detectable (Bibby et al., 2000). Some later surveys were undertaken (up to 12:00) when logistics,

associated with unsuitable weather, made earlier surveys impractical but only when bird activity remained high. All birds seen and or heard were registered and attributed to one of four distance bands where first detected from the count point (25 m, 50 m, 100 m and >100 m). Singing and displaying by the recorded birds was also noted. Individual birds registered from more than one sampling point were assigned only to the original point of their detection. Birds seen or heard in flight were recorded separately and not attributed to any distance band with the exception of those performing display flights over their breeding territories in which case they were recorded as if in the terrestrial distance band above which they were displaying. The 10 min sampling interval aimed to maximise the likelihood of registering birds within the immediate vicinity but also reduce the risk of counting individuals multiple times (Fuller and Langslow, 1984; Drapeau et al., 1999). Survey effort by individual observers was spread across the four management/habitat types to prevent any systematic bias associated with differing observer abilities.

2.3. Analyses – differences between management treatments Poisson generalised linear mixed models (GLMMs) were used to assess differences in species richness and species abundance

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J. Calladine et al. / Forest Ecology and Management 344 (2015) 20–29 Table 1 A summary of some habitat measures, pooled across all sites, describing tree and shrub structures within the four sampled management treatments. Variable

Treatmenta CCFs

CCFn

CFRy

CFRm

Sitka spruce dominantb (% of points) Norway spruce dominantb (% of points) Other conifer spp. dominantb (% of points) Broad-leaved spp. dominantb (% of points) Height of canopy treesc m (median and interquartile) Height of understorey shrubsd m (median and interquartile) DBHe cm (median and interquartile) N treesf within 5 m (median and interquartile) N shrubsf within 5 m (median and interquartile)

22

26

29

37

25

15

15

16

13

29

17

37

12

23

2

3

25 (25–30)

30 (25–35)

0

20 (15–30)

4.5 (2–5)

1 (0–4)

2 (0–3)

0 (0–2)

25 (10–40) 3 (2–5.5)

30 (20–45) 4 (2–6)

5 (5–10) 0 (0–5)

25 (15–33) 6 (4–10)

20 (11–80)

2 (0–5)

19 (4–31)1 (0–4.5)

a Management treatments: CCFs = CCF with extensive shrubby understorey (tree regeneration); CCFn = CCF where tree regeneration was infrequent and patchy; CFRy = Young stands of CFR (trees < 10 years old); CFRm = older CFR where canopy has closed (trees 15–30 years old). b The dominant species or type of tree within a 5 m radius of the sampling points. c The height of canopy trees within a 10 m radius of the sampling points (at each point estimated to the nearest 5 m). d The height of shrubs (understorey within all treatments except CFRy) within a 10 m radius of the sampling points (at each estimated to the nearest 1 m). e The Diameter at Breast Height (DBH) of the nearest four trees to the sampling point (measured to the nearest 5 cm). f The number of trees (with a DBH > 10 cm) or shrubs (including young trees < 5 m high) within a 5 m radius of the sampling points. Where the number of shrubs exceeded 10, numbers were categorised 10–20, 20–30 etc. Means are derived from the mid points of those estimated categories.

between the four management categories or treatments. For species richness, the number of species recorded within 100 m of a sampling point summed across both survey visits (excluding any recorded as flying over only) was the dependent variable. For species abundance, separate models were used for each species. To permit direct comparisons between treatments, birds recorded more than 50 m from the sampling points were excluded from the abundance analyses. The effective detection radii (EDR; the distance from the sampling point at which as many birds are detected beyond the EDR as remain undetected within it), determined using the program DISTANCE (based on the detection of each species within the three distance bands to 100 m with data fitted with a half-normal detection function; Thomas et al., 2009), were similar between treatments for five tested species that were sufficiently numerous across treatments (Table 2). This is also consistent with the reported effective detection radius for birds within directly comparable habitats (Bibby and Buckland, 1987). The dependent variable in the single-species models was the maximum number of qualifying registrations from either of the two survey visits for each sampling point. For all models, treatment (n = 4 classes) was the fixed variable and study area (n = 4 classes) was included as a random variable to account for the potential influences of

Table 2 The mean Effective Detection Radii in metres (and 95% confidence limits) for five example species within each management treatment as determined using the program DISTANCE. Species

Wren Willow warbler Goldcrest Coal tit Chaffinch

Treatmenta CCFs

CCFn

CFRy

CFRm

45 53 32 48 50

55 56 30 46 48

58 50 34 53 42

58 63 30 42 45

(38–53) (46–60) (29–35) (38–62) (40–63)

(48–63) (48–65) (27–33) (39–54) (39–60)

(47–72) (45–55) (30–38) (36–78) (35–49)

(50–68) (45–88) (28–33) (37–49) (38–54)

a Management treatments: CCFs = CCF with extensive shrubby understorey; CCFn = CCF where tree regeneration appeared was infrequent and patchy; CFRy = Young thicket and pre-thicket stands of CFR (trees < 10 years old); CFRm = older CFR where canopy has closed (trees 15–30 years old).

area-specific variables other than management treatment on the presence and abundance of bird species. Post hoc pair-wise comparisons were used to assess the sources of any significant differences between treatments. Differences between the back-transformed model Least Square Means were used to indicate the magnitude and direction of any statistically significant differences. Principal amongst the aims of these analyses were to assess any differences in the densities of: (a) mature forest birds in the older CFR stands and in CCF managed plantations, for the latter both with and without a shrub understorey; (b) shrub birds within the thicket and younger CFR stands and within the understorey found in CCF plantations. 2.4. A case study of plantation conversion Study sites were restricted by the availability of developed CCF areas. The Clocaenog study area (Fig. 1) represented the most advanced example of a plantation where management changes from CFR to CCF had resulted in structural changes in the plantation. As such, it was arguably more representative of what is expected of CCF plantations in the medium or longer term. Analyses similar to those described above (abundance or species richness as the dependent variable and four treatments as factors) were undertaken for data from that site alone, though using Poisson generalised linear models (GLMs). As only one study area was included there was no need for a random variable to be included in the GLMs. A comparison of the findings for the single site analyses with those for all study sites was used to assess the likely representativeness of the general findings across all sites for plantations that will undergo management conversion from CFR to CCF. 2.5. Simulations of plantation management scenarios Mean densities of birds recorded within each of the management treatments were extrapolated to illustrate some likely influences on the populations of some species supported by plantations under scenarios of different proportions under CCF and CFR management. A hypothetical plantation or group of plantations,

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50 km2 in area was considered, in which 0–100% (in 20% increments) was managed as CCF and the remainder as CFR. Within the CCF managed area(s), two alternatives of proportion with a shrub understorey were considered, 33% and 50%. Similarly within the CFR managed area(s), two alternatives of proportion of young age stands were considered, 33% and 50%. Three different combinations of these alternatives, with CCF in incremental proportions were used to simulate populations of example bird species that could be supported by 50 km2 of Sitka spruce dominated plantation in upland Britain: (a) one-third of CCF has a shrubby understorey and one-third of CFR is thicket growth stage or younger; (b) one-half of CCF has a shrubby understorey and onehalf of CFR is thicket growth stage or younger; and (c) one-half of CCF has a shrubby understorey and one-third of CFR is thicket growth stage or younger. Species densities used in the above simulations were a simple extrapolation of the mean number of registrations of a species recorded within 50 m of sampling point within each management treatment, expressed as birds per km2. Simulations of the effect of increasing the proportion of plantation under CCF management are presented for four example species that were selected (a) for their contrasting associations with the four management treatments and (b) because they were sufficiently numerous within the sampled study areas for data to be considered representative: willow warbler, blackcap, great tit and lesser redpoll (scientific names of birds are given in Appendix A).

breeding territories. There was a statistically significant difference in bird species richness between treatments (Table 3); CCF with a shrubby understorey was the most species rich of the four treatments (back-transformed mean 7.8 species per point; 95% confidence limits 6.2–9.7) followed by CCF without a shrubby understorey (6.9; 5.5–8.5), Young CFR (6.4; 5.1–8.1) and older CFR (6.2; 5.0–7.7). 3.1.1. Mature plantations Amongst the species that were sufficiently numerous for successful convergence of models, the abundances of 11 species found in the more mature plantations differed between management treatments (as suggested by a significant or marginally non-significant (0.05 < P < 0.10) difference in Table 3). These were identified by comparisons between three pairs of treatments: (a) CCF with a shrub understorey and CCF without (Cs:Cn in Table 3); (b) CCF with a shrub understorey and older CFR (Cs:Rm in Table 3); and (c) CCF without a shrub understorey and older CFR (Cn:Rm in Table 3). The presence of a shrub understorey appears associated with higher abundances of four of those species, great spotted woodpecker (marginal significance), blackcap, garden warbler (marginal) and willow warbler. Blackcap, and possibly garden warbler, could be defined as ‘shrub-layer species’ in that they were most abundant in the shrub layer within a mature stand. The apparent association of great spotted woodpecker with a shrub layer is curious but could be a result of an indirect connection whereby tall tree structure that is conducive to the development of a shrub layer is also favourable to great spotted woodpeckers. In the absence of a positive association with a shrub understorey, it is assumed that canopy and tree structure were amongst the principal drivers for the differences in abundances of wood pigeon, wren, goldcrest, blue tit, great tit, lesser redpoll and common crossbill (all these species were more abundant in CCF than in

3. Results 3.1. Differences in species occurrence between plantation treatments A total of 63 species was recorded from sampling points during surveys (Appendix A, which includes indices of relative abundance), the behaviour of 43 of which was indicative of holding

Table 3 Summary of GLMM outputs assessing differences in species richness and abundance between four plantation treatments (Cs = CCF with shrub understorey; Cn = CCF with no shrubs; Rm = CFR mature stands; Ry = CFR thicket and pre-thicket stands). Post-hoc pair-wise comparisons are given where the models indicated significant (P < 0.05) or marginally non-significant (0.05 < P < 0.10) differences (Est = difference in model Least Square Means between pairs of treatments where a negative difference showing that numbers in the second of the pair were greater than in the first). Models did not converge for non-listed species. Model significance F3,

328

P

Pair-wise comparisons (where overall model is significant or marginally non-significant) Cs:Cn Est

Species richness Species Pheasant Wood pigeon Great spotted woodpecker Wren Dunnock Robin Blackbird Song Thrush Blackcap Garden warbler Chiffchaff Willow warbler Goldcrest Coal tit Blue tit Great tit Treecreeper Jay Chaffinch Siskin Lesser redpoll Common crossbill

6.15

<0.001

0.35 4.76 2.63 3.32 10.96 2.01 1.22 0.01 4.29 2.30 1.47 23.33 8.49 3.27 3.52 4.53 7.54 1.11 3.23 3.92 7.01 21.69

0.79 0.003 0.05 0.02 <0.001 0.11 0.25 0.99 0.005 0.07 0.22 <0.001 <0.001 0.02 0.02 0.004 <0.001 0.34 0.02 0.01 <0.001 <0.001

Cs:Rm P

Est

Cs:Ry P

Est

Cn:Rm P

Est

Cn:Ry P

Est 0.11

Est 0.06

0.01

P

0.11

0.05

0.21

<0.001

0.22

<0.001

0.10

0.15 0.73 0.08 0.38

0.55 0.09 0.57 0.29

0.63 0.71 0.32 0.14

0.004 0.09 0.03 0.68

0.86 2.47 0.42 1.04

0.03 0.01 0.01 <0.001

0.63 0.02 0.24 0.52

0.004 0.96 0.06 0.09

1.02 1.74 0.34 1.42

0.003 0.10 0.03 <0.001

0.38 1.76 0.10 0.90

0.28 0.09 0.54 <0.001

1.13 1.30

0.01 0.06

0.73 1.35

1.81 0.00

0.004 1.00

0.41 1.35

0.31 0.06

0.67 1.30

0.31 0.06

1.08 1.35

0.09 0.05

0.05 0.06

0.04

Rm:Ry P

0.81

0.66 0.29 0.14 0.22 0.12 1.14

<0.001 0.01 0.38 0.64 0.78 <0.001

0.40 0.13 0.21 1.89 1.40 1.00

0.02 0.29 0.21 0.01 0.01 0.004

0.58 0.35 0.35 1.92 1.27 2.43

<0.001 0.01 0.09 0.07 0.06 0.02

0.26 0.17 0.06 1.67 1.28 0.14

0.13 0.08 0.63 0.01 0.001 0.46

1.24 0.64 0.50 1.70 1.15 3.52

<0.001 <0.001 0.01 0.10 0.06 <0.001

0.98 0.47 0.56 0.03 0.13 3.42

<0.001 <0.001 0.003 0.98 0.85 <0.001

0.06 0.17 0.37 1.65

0.67 0.40 0.27 <0.001

0.11 0.16 0.57 0.08

0.43 0.43 0.16 0.86

0.34 0.94 0.88 0.77

0.03 <0.001 0.004 0.20

0.05 0.01 0.93 1.73

0.66 0.95 0.009 <0.001

0.39 0.77 0.52 2.42

0.01 0.01 0.04 <0.001

0.45 0.78 1.45 0.70

0.003 0.004 <0.001 0.28

Statistically significant differences are highlighted in bold.

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3.1.3. Scarce species Amongst the less numerous species recorded during surveys (those for which GLMMs did not converge), ten species were recorded exclusively or predominantly within CCF plantations; goshawk, sparrowhawk, stock dove, tawny owl, redstart, wood warbler, willow tit, nuthatch, greenfinch and hawfinch. Three of the less numerous species were recorded predominantly in CFR (cuckoo, whitethroat and linnet). All were most frequently recorded in the young CFR stands and linnet exclusively so. 3.2. Plantation conversion The single-site analyses of data from the only study site to have undergone extensive conversion from CFR to CCF showed comparable levels of statistical significance at both full model and pairwise comparison levels for species richness and for single species, where there were sufficient data for the models to converge. In no cases were there any contradictions with the complete analyses in identifying differences in abundance between treatments for any species.

Treatmenta

Species

Blackcap Willow warbler Great tit Lesser redpoll

CCFn

CCFs

CFRm

CFRy

12 69 33 33

36 (12) 125 (25) 17 (9) 32 (9)

18 (9) 88 (11) 9 (4) 13 (7)

6 (3) 243 (35) 6 (5) 80 (29)

(4) (14) (9) (19)

a Management treatments: CCFs = CCF with extensive shrubby understorey; CCFn = CCF where tree regeneration appeared was infrequent and patchy; CFRy = Young thicket and pre-thicket stands of CFR (trees < 10 years old); CFRm = older CFR where canopy has closed (trees 15–30 years old).

Willow Warbler

(a)

Great Tit

Blackcap

7000 6000 5000 4000 3000 2000 1000 0 0%

20%

40%

60%

80%

100%

Proportion CCF Willow Warbler

(b)

Lesser Redpoll

Great Tit

Blackcap

9000 8000 7000 6000 5000 4000 3000 2000 1000 0 0%

20%

40%

60%

80%

100%

Proportion CCF Willow Warbler

3.3. Bird populations with different scenarios of treatment

Lesser Redpoll

9000 8000

Number of birds

3.1.2. Shrubs (including thicket and pre-thicket stands) Three species (amongst those that were sufficiently numerous for successful convergence of models) were more abundant in young CFR stands than within CCF with a shrub understorey (Cs:Ry comparisons in Table 3); dunnock, willow warbler and lesser redpoll. These species appear to show a preference for ‘shrubland’ rather than a shrub layer within a mature forest. The majority of species which could be modelled (11 out of 14) tended to be more abundant in CCF stands with shrubs than in the young CFR stands, however (wood pigeon, great spotted woodpecker, wren, blackcap, goldcrest, coal tit (a marginal difference), blue tit (marginal), great tit (marginal), treecreeper, chaffinch and siskin; Table 3). Amongst the four species that tended to be more abundant within CCF stands and for which their abundance also appeared to be related to the presence of a shrub understorey (see Section 3.1.1 above), great spotted woodpecker and blackcap were more abundant in CCF with shrubs than within the young CFR stands.

Table 4 Bird densities (birds per km2, SE in brackets) derived by simple extrapolation of the number of registrations of a species recorded within 50 m of sampling points within each management treatment used in the simulations of different management scenarios in Fig. 2.

Number of birds

the mature CFR). Of those 7 latter species, goldcrest and common crossbill were both more abundant in stands of CCF without a shrub understorey than CCF stands with regenerating shrub. Two species were less abundant in the CCF stands than in mature CFR; dunnock (a marginally non-significant difference) and treecreeper. For treecreeper there was also a negative association with a shrubby understorey in CCF plots (more abundant in CCF without an understorey).

(c)

Lesser Redpoll

Great Tit

Blackcap

9000

Number of birds

8000

Simulations of the effect of increasing the proportion of plantation management under CCF are based on the observed densities of four species that show contrasting associations with plantation management treatment (Table 4): (a) willow warbler, a long distance migrant apparently associated with shrubs and most abundant within young CFR; (b) lesser redpoll, a resident or more locally dispersive species that was most abundant in young CFR but also seemingly benefiting from the more diverse woodland structures within CCF; (c) great tit, a resident (or locally dispersive) species that was most abundant within CCF stands but not apparently associated with a shrub understorey; and (d) blackcap, a migrant that was most abundant within CCF with a shrub understorey. Within the scenarios presented (Fig. 2), a plantation managed exclusively as CCF could support as few as 53% of the willow warblers as one managed exclusively as CFR (comparing the

7000 6000 5000 4000 3000 2000 1000 0 0%

20%

40%

60%

80%

100%

Proportion CCF Fig. 2. Estimated populations of four example species supported by 50 km2 Sitka spruce dominated plantations in upland Britain under different proportions of CCF management. Three scenarios are presented: (a) one-third of CCF has a shrubby understorey and one-third of young CFR; (b) one-half of CCF has a shrubby understorey and one-half of CFR is young; and (c) one-half of CCF has a shrubby understorey and one-third of CFR is young.

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extremes of scenarios ‘a’ and ‘b’ in Fig. 2). In contrast, a similar comparison suggests that an exclusively CFR managed plantation could support as few as 28% of the great tits as one exclusively managed as CCF. The other two example species suggest that a plantation managed exclusively as CCF could support as few as 70% of the lesser redpolls as one managed as CFR, but could support twice as many blackcaps.

4. Discussion 4.1. Representativeness of study areas Plantations managed as CCF in our study areas supported an enhanced assemblage of birds (more species and many of them more abundantly, including some that are more typical of mature forests) compared to CFR managed plantations, including the older CFR stands. Young CFR managed plantations supported higher densities of a smaller number of bird species that may be typical of shrublands than were found in the understorey of young trees within CCF plantations. However within our study areas, a CCF shrub understorey supported similar densities of some shrub-associated birds to those found in pre-thicket CFR. The number of suitable study areas was limited by the availability of developed CCF plantations that included a major component of Sitka spruce. Therefore it is important to consider the representativeness of those areas and also the likely generality of these findings. The areas managed as CCF included a greater proportion of broad-leaved trees than those managed as CFR. Bird assemblages in conifer plantations can be enhanced by the presence of more tree species, including broad-leaved trees (Bibby et al., 1989; Barbaro et al., 2005; Wilson et al., 2010) and therefore they represent a potentially important and confounding factor influencing the bird communities in addition to management treatment. However, CCF management can encourage tree species diversity (Pommerening and Murphy, 2004) and the greater presence of broad-leaved trees within our study areas is arguably an intrinsic attribute of that management regime. With a few exceptions, CCF supported similar, or only marginally lower, densities of birds associated with shrubs (where conditions permitted the development of a shrub understorey) to those found in young CFR. However, there are other species that have been associated with young growth stage CFR plantations in upland Britain that were not, or very rarely, recorded in our study; for example Black Grouse, Hen Harrier, Short-eared Owl and Whinchat (Shaw, 1995; Madders, 2000; Pearce-Higgins et al., 2007; Wilson et al., 2009; Calladine et al., 2013). These birds are generally species of open and shrub habitats that can find suitable conditions within young pre-thicket plantations. The occurrence of shrubland and open habitat birds in young plantations is influenced by the extent of suitable habitat (the coupe size of the appropriate young-age class trees), connectivity with other open habitats (for example bordering directly onto moorland) and also as to whether the young trees are of first or subsequent rotation plantings (Dettmers, 2003; Askins et al., 2007; Burton, 2007). Our young CFR study areas were all at least second rotation plantings. The vegetative structure of second rotation plantings can be more dominated by shrubs (young trees), have less ground cover (newly planted sites tend to include the ground vegetation of the original open habitat while in subsequent rotation plantings, it has been suppressed, even excluded, by the crop trees) and can support fewer open habitat birds (both numbers of species and population densities) than first rotation plantings (Sweeney et al., 2010). Therefore the rarity or absence from our sampled areas of those species whose requirements are towards the ‘open habitat’ end

of the continuum of shrubland-open country birds might be expected. Further and strong support for the representativeness of the study areas is provided by the similar associations between management treatment and species abundances (and species richness) that were found in both the analyses that included all study sites and that which considered the single longest established site in Wales. That site could be perceived as a good working model for likely changes in avifauna that will be associated with conversions of other areas, at least in the relatively early stages of their development. Arguably therefore, and despite the limited number of qualifying study sites, we have no reason to suggest that the differences in breeding bird populations between the sampled management treatments were not representative where CCF will increase the broad-leaved tree component within the plantation and where young CFR is of restocked planting rather than newly planted areas. 4.2. Conservation implications This study indicates that CCF managed plantations can support an enhanced community of mature forest birds relative to CFR plantations, an expectation that could not be demonstrated between similar silvicultural treatments (of beech and Norway spruce) in the Belgian Ardenne (du Bus de Warnaffe and Deconchat, 2008). Further, our study suggests that some shrubassociated birds which occur within young growth stage CFR plantations can also be supported by shrub understorey within CCF plantations. There are exceptions however, with some species that appear to prefer young CFR (willow warbler, dunnock and lesser redpoll and possibly cuckoo, whitethroat and linnet). These species could be considered towards the shrubland specialist end of a continuum from shrubland to shrub-layer species. Willow warbler, which was one of the most abundant shrubland birds in our study areas, showed marked differences in populations supported by the different simulated management scenarios (Fig. 2). The willow warbler is also a species with a marked contrast in its recent population trends between the north-west of Britain where it has increased and the south-east where it has decreased (Balmer et al., 2013). Local-scale processes such as habitat change have been speculated to be amongst the causal and interacting mechanisms for the spatially variable population trends for that species (Morrison et al., 2010). The felling and restocking of managed forests, policies such as woodland creation and expansion initiatives and reductions of large herbivores in upland north-west Britain are likely to have led to an increase in the extent of shrub cover in recent decades (Gillings et al., 2000). It is a reasonable hypothesis that a regional expansion of shrubland is amongst the processes that have contributed to increasing abundances of willow warblers, and other species associated with shrubs that have been in concurrent decline in southern Britain. Therefore management for shrubland, or otherwise early successional forest growth stages, could improve the conservation status of those species. The provision of extensive areas of young-growth in rotational clear-felled plantations could form a part of a conservation strategy that sought to maintain shrubland bird populations. In North America, and in particular in the eastern United States, there is concern about the declining status of some birds that rely on early successional growth stages of forests, the availability of which has declined in response to reduced areas of (mostly seminatural) forest managed by clear-felling (Costello et al., 2000; Thompson and DeGraaf, 2001). Our simulated scenarios suggest that conversion of plantation management increasingly towards CCF could lead to reduced numbers of some shrubland species (willow warbler and lesser redpoll in our limited example

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scenarios). Given the decline of both these species across Britain as a whole (Balmer et al., 2013), the habitats that can be provided by CFR could prove to be particularly important. If CCF management of plantations was to become more widespread in Britain, there is the possibility of a negative impact on birds which thrive in young growth stage forests, similar to that experienced in the eastern United States. However, the relative disadvantage for many shrubland birds of CCF potentially could be mediated, by a selective felling regime that ensures continuity of a shrub understorey including, or dominated by regenerating crop trees. Critical to this will be the management of herbivores to ensure that browsing intensity is sufficiently low to permit young tree growth (Gill and Fuller, 2007). However, for some species, for example dunnock, willow warbler and lesser redpoll, CCF may not be able to support the densities of birds that can be supported within young growth stage CFR. A further disadvantage of CCF for bird conservation is its unsuitability for birds that favour more open habitats (e.g. Hen Harrier, Short-eared Owl and Whinchat) which can be supported in some young growth stage CFR. Forest management that includes rotation harvesting with associated young growth areas (even-aged plantations or CFR) alongside CCF (uneven-aged plantations) could prove to be an optimal conservation strategy in that a range of conditions will be available for both shrub-dependent

and mature forest bird species. Further, until the potentially confounding influence of the greater presence of broad-leaf trees within CCF stands is tested, it would be sensible to suggest that their retention should be recommended where biological diversity is amongst the management objectives.

Acknowledgements Susan Holoran, Colin Everett and Andrew Francis (BTO) helped with fieldwork and Colin Edwards (Forest Research) identified and advised on suitable study areas. For access, we are grateful to Dave Williams (Forestry Commission Wales), Peter Fullerton, Rob Coope, Fraser McDonald (Forestry Commission Scotland), Jim Colchester and Gary Morrison (Buccleuch Estates). The project was funded by the Department for Environment Food and Rural Affairs (Defra), Forestry Commission GB, Forestry Commission Scotland, Forestry Commission England and Forestry Commission Wales. It formed part of a wider study investigating woodland structure and birds undertaken by a consortium consisting of the British Trust for Ornithology, Forest Research, RSPB and the University of Nottingham. We appreciate the comments from two anonymous referees.

Appendix A The mean number of registrations per ten sampling points per survey visit recorded in each four plantation treatments (Cs = CCF with shrub understorey; Cn = CCF with no shrubs; Rm = CFR mature stands; Ry = young CFR stands). Species marked ⁄ are those for which behaviour indicative of holding a breeding territory was recorded. Note six species (wildfowl, shorebirds and gulls) are excluded as they were recorded as transients in flight only and not associated with the habitats considered. Species

Scientific name

Cs

Cn

Ry

Rm

Black Grouse Pheasant Hen Harrier Goshawk Sparrowhawk Buzzard Stock dove Wood pigeon Cuckoo Tawny owl Green Woodpecker Great spotted woodpecker Swallow House Martin Tree Pipit Meadow Pipit Grey Wagtail Pied Wagtail Wren Dunnock Robin Redstart Blackbird Fieldfare Song Thrush Mistle Thrush Blackcap Garden warbler Whitethroat Wood warbler

Tetrao tetrix Phasianus colchicus Circus cyaneus Accipiter gentilis Accipiter nisus Buteo buteo Columba oenas Columba palumbus Cuculus canorus Strix aluco Picus viridis Dendrocopos major Hirundo rustica Delichon urbicum Anthus trivialis Anthus pratensis Motacilla cinerea Motacilla alba Troglodytes troglodytes Prunella modularis Erithacus rubecula Phoenicurus phoenicurus Turdus merula Turdus pilaris Turdus philomelos Turdus viscivorus Sylvia atricapilla Sylvia borin Sylvia communis Phylloscopus sibilatrix

0 1.25⁄ 0 0.08 0.08⁄ 0.50⁄ 0 4.33⁄ 0.17⁄ 0 0.08⁄ 1.67⁄ 0 0 0.33⁄ 0.08 0 0.08 14.83⁄ 1.83⁄ 8.33⁄ 0.75⁄ 2.33⁄ 1.25 4.08⁄ 1.83⁄ 2.17⁄ 0.58⁄ 0.17⁄ 0.17⁄

0.05 1.68⁄ 0.05 0 0 0.91⁄ 0.05⁄ 6.15⁄ 0.34⁄ 0.05⁄ 0.05⁄ 1.54⁄ 0.10 0.34 0.38⁄ 0.10 0 0 16.15⁄ 1.25⁄ 9.76⁄ 0.38⁄ 2.36⁄ 0.14 3.32⁄ 1.06⁄ 1.59⁄ 0.24⁄ 0.10⁄ 0.24⁄

0 2.12⁄ 0 0 0 0.42 0 5.51⁄ 1.36⁄ 0 0.08⁄ 0.85⁄ 0.51 0 1.02⁄ 1.02 0.08 0.25 12.03⁄ 5.68⁄ 8.05⁄ 0.17 4.15⁄ 6.53 5.00⁄ 1.69⁄ 0.59⁄ 0.59⁄ 0.93⁄ 0

0 1.29⁄ 0 0 0.04 0.22⁄ 0 4.73⁄ 0.85⁄ 0 0 1.16⁄ 0 0 0.13⁄ 0 0 0.04 11.38⁄ 1.56⁄ 8.44⁄ 0.27⁄ 2.77⁄ 0 4.33⁄ 1.12⁄ 1.21⁄ 0.27⁄ 0 0

(continued on next page)

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Appendix A (continued)

Species Chiffchaff Willow warbler Goldcrest Spotted Flycatcher Long-tailed Tit Willow tit Coal tit Blue tit Great tit Nuthatch Treecreeper Jay Jackdaw Rook Carrion/Hooded Crow Raven Chaffinch Brambling Greenfinch Goldfinch Siskin Linnet Lesser redpoll Crossbill Bullfinch Hawfinch

Scientific name Phylloscopus collybita Phylloscopus trochilus Regulus regulus Muscicapa striata Aegithalos caudatus Poecile montanus Periparus ater Cyanistes caeruleus Parus major Sitta europaea Certhia familiaris Garrulus glandarius Corvus monedula Corvus frugilegus Corvus corone/cornix Corvus corax Fringilla coelebs Fringilla montifringilla Carduelis chloris Carduelis carduelis Carduelis spinus Carduelis cannabina Carduelis cabaret Loxia curvirostra Pyrrhula pyrrhula Coccothraustes coccothraustes

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