species richness, persistence and colonization through the management cycle
A total of 217 plant species was recorded in the survey of forest stands (see Table S1 in the supplementary material), with the minimum and maximum number of species recorded in any one stand being two and 51, respectively. Species richness was highest and most variable (seven to 51 species) in stands 3–15 years old (Fig. 1a). After this period, richness declined to a mean of eight (SD 3·7) at around 25 years. There was a subsequent increase coinciding with the onset of thinning after 25 years; species richness at 40–49 years was approximately two-thirds of that at 5–6 years. The apparent slight decline from 45 to 142 years was non-significant (r = −0·164, P= 0·114, n= 94).
Figure 1. The relationship between stand age and (a) species richness, (b) percentage canopy cover, (c) cover of bare ground and litter and (d) litter depth, ± 1 SD.
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Canopy cover, litter cover and litter depth all increased rapidly from 7 to 8 years, reaching a maximum (95%, 80% and 3·8 cm, respectively) at approximately 25 years (Fig. 1b,d). All three variables then decreased concurrently with the increase in species richness (Fig. 1a). The percentage of bare ground was greatest (40%) in stands aged 1–2 years, but declined rapidly to zero at 7–8 years, remaining zero for the remainder of the growth cycle (Fig. 1c).
The minimal model (Table 1a) showed that species richness decreased with increasing PCA1 score and was significantly higher in stands with higher soil pH. Soil pH was reduced under older Pinus stands, particularly on the two calcareous soil types (rendzina and calcareous brown earth, with stand age as covariate, = 0·224, soil type F1,111 = 22·37, P < 0·001; age F1,111 = 11·88, P = 0·001; B=−0·017 ± 0·005 SE) but less so on the acidic soils (podzol, lithosol, brown earth, = 0·078, soil type F2,184 = 7·90, P= 0·001; age F1,184 = 3·70, P= 0·056; B=−0·005 ± 0·003).
Table 1. Minimal general linear models for species richness and vegetation composition, as defined by the first two axes of a CA performed on all plant species. B shows the direction and magnitude of the effect of each retained variable (original units, unstandardized). Species richness counts were square-root transformed. Removal of the land-use parameter did not significantly affect the pH coefficient in species richness (t322 = 0·20, P= 0·421) or CA1 (t322 = 0·28, P= 0·389) models
| ||B (± SE)||F||d.f.||P|
|(a) Richness ( = 0·38)|
|Model|| ||48·33||4, 321||< 0·001|
|PCA1||−0·46 ± 0·04||123·06||1, 321||< 0·001|
|pH||0·20 ± 0·03||39·22||1, 321||< 0·001|
|Previous land use|| || 3·02||2, 321|| 0·050|
|Arable||0·05 ± 0·19|| || || |
|Heath||−0·18 ± 0·19|| || || |
|Wood|| 0|| || || |
|Without land use: pH||0·23 ± 0·03|| || || |
|(b) CA1 ( = 0·35)|
|Model|| ||43·12||4, 321||< 0·001|
|PCA1||−0·24 ± 0·05||28·60||1, 321||< 0·001|
|pH||−0·28 ± 0·04||62·40||1, 321||< 0·001|
|Previous land use|| ||13·09||2, 321||< 0·001|
|Arable||−0·30 ± 0·20|| || || |
|Heath||0·21 ± 0·20|| || || |
|Wood|| 0|| || || |
|Without land use: pH||0·35 ± 0·04|| || || |
|(c) CA2 ( = 0·37)|
|Model|| ||93·46||2, 323||< 0·001|
|PCA1||0·40 ± 0·05||76·22||1, 323||< 0·001|
|pH||−0·34 ± 0·03||105·70||1, 323||< 0·001|
Stands that were previously arable had more species, and stands that were previously heath fewer species, relative to stands that were previously woodland (Table 1a). The pH of previously arable stands (mean 5·12 ± 1·52 SD, n= 150) was higher than that of previously heath stands (mean 4·00 ± 0·98, n = 158; t306 = 7·76, P < 0·001). However, the coefficient of pH in the model of species richness did not change significantly on removal of land use, indicating that effects of previous land use were largely independent of pH.
Stand size and crop species were not retained (P > 0·05) in the minimal model. Crop species did not affect the PCA1 score (t324 = 0·12, P = 0·907) and was not confounded with previous land use (Fisher exact test P = 0·359). Plant species richness at years 4 and 5 was not related to the number of broad-spectrum herbicide applications received (partial correlation controlling for pH, R= 0·031, d.f. = 29, P= 0·869).
Only 50 species persisted throughout the management cycle as vegetative plants (Fig. 2) to be recorded in all growth stages. These included pleurocarpous bryophytes (e.g. Hypnum jutlandicum and Kindbergia praelonga) and shade-tolerant perennials (e.g. Dryopteris dilatata and Deschampsia flexuosa; Hill et al. 1999). Of the remaining 167 species, 101 recolonized after felling in stands aged 1–20 years, nine at the pole stage (20–30 years) and 57 in mature (> 30 years) stands after the canopy reopened.
Figure 2. The total number of plant species (with the number of angiosperm species in parentheses) recorded from each stage of the management cycle. Age classes are divided among young (before canopy closure, including restock, prethicket and thicket), pole (closed canopy) and mature (canopy opening, thinned stands, including prefell and retained).
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Those species that attained the highest cover in the stands in which they occurred were widespread (i.e. found in many stands) and those found in fewer stands had lower cover in those stands (Fig. 3). A notable outlier was box Buxus sempervirens, which only occurred in two stands but dominated one of those (66% cover). As B. sempervirens is planted locally for game cover, subsequent analyses of abundance and occupancy excluded this species. The most frequent three species were Holcus lanatus (205 stands), Kindbergia praelonga (195 stands) and Rubus fruticosus agg. (187 stands). The species attaining the highest mean cover in stands where present were Buxus sempervirens (35%), Deschampsia flexuosa (17%) and Holcus lanatus (11%). Thirty-eight species occurred in only one stand, and 132 species in 10 or fewer stands.
Figure 3. The relationship between abundance and occupancy for the 217 plant species recorded in Thetford Forest stands. Each species is plotted according to the number of stands in which it occurred and the mean cover in those stands (log-log scale). = 0·30, F1,216 = 91·71, P < 0·001. The solid line is the best-fit line for non-persistent species with a long-distance dispersal mechanism, the dotted line for non-persistent species without a long-distance dispersal mechanism.
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Seed-bearing vascular plant species that possess a mechanism for long-distance dispersal were not found in a greater number of stands to those lacking such a mechanism, although the difference was close to significant (with, mean 21·0 stands ± 33·6 SD, n= 102; without, 15·0 ± 28·8, n= 76; t-test on log frequency, t1,176 = 1·74, P= 0·083). The slope of the relationship between log frequency and log abundance did not differ between seeding species with (B = 0·74 ± 0·11 SE) and without (B = 0·58 ± 0·13) a long-distance dispersal mechanism (t176 = 0·92, P= 0·18). Within a generalized linear model that assumed a common slope, an additive categorical variable for long-distance dispersal mechanism was again close to significance ( = 0·28, log abundance B= 0·67 ± 0·08, F1,174 = 64·52, P < 0·001; long-distance dispersal B= 0·15 ± 0·08, F1,174 = 3·61, P= 0·059). For those species not present throughout the growth cycle, for which colonization ability was expected to more strongly affect regional frequency, the slope of the relationship between log frequency and log abundance differed significantly between seeding species with (B = 0·59 ± 0·12, n= 83) and without (B = 0·18 ± 0·14, n= 64) a long-distance dispersal mechanism (t146 = 2·21, P= 0·014). No such difference was found for species that persisted throughout the management cycle (with long-distance dispersal mechanism, B= 0·30 ± 0·09, n= 18; without, B= 0·38 ± 0·18, n= 12, t29 = 0·40, P= 0·346).
Most annual and winter annual species were only present at high cover values in the first 5 years after replanting, with the exception of Ceratocapnos claviculata and Galium aparine. The number of annual species per stand decreased slightly (B = 0·06 ± 0·01 SE) but significantly with stand age in the first 20 years ( = 0·09, F1,143 = 14·42, P < 0·001). Herbaceous perennials and trees and shrubs achieved their highest cover after the pole stage, but this was dominated by a few species, and many species had relatively high cover before the pole stage.
Life-history categories differed in their location on CA axes one and two (Fig. 5 and Table 2b). Annuals, acrocarpous bryophytes and semelparous perennials tended to have lower CA1 and CA2 scores, consistent with the low CA2 scores of restock and prethicket stands, while iteroparous herbs were widely scattered. In contrast, bryophytes had relatively high CA1 scores, acrocarpous bryophytes had low CA2 scores and pleurocarpous bryophytes high CA2 scores. Woody plants also had high CA2 scores.
Figure 5. Species ordination scores on CA axes 1 and 2 for different life-history categories, showing minimum convex polygons.
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Previous land use significantly affected vegetation composition, as represented by CA1 score. In minimal models also including PCA1 and soil pH (Table 1b), stands that were previously arable had lower CA1 scores, and stands that were previously heath had higher CA1 scores, than stands that were previously woodland. The coefficient of pH did not change significantly on removal of the land-use term, suggesting that effects of previous land use were not artefacts of covariance with pH.
Considering just the more frequent species (found in 20 or more stands), many showed a significant bias in their distributions towards stands that were previously either arable or heathland (Table 3), although none was entirely restricted to a single land-use type. Nineteen of the 28 frequent species that showed a significant bias were present at all stages of the life cycle. Considering vascular species for which Ellenberg values were available, the tendency to occur in more stands that were previously heath than were arable was strongly negatively related to Ellenberg values for soil reaction (i.e. having lower pH) and soil fertility (less nitrogen-rich) [R (reaction) Rs = −0·353, P < 0·001, N (fertility) Rs = −0·375, P < 0·001, n= (181)].
Table 3. Species that were recorded in 20 or more stands that occurred in a significantly different (greater or fewer) proportion of heath than arable stands (chi-square test with Bonferroni correction to P≤ 0·002). Species are arranged in order of the ratio of the proportion of stands occupied on previously heath land to the proportion of stands occupied on previously arable land (163 heath stands, 151 arable stands). n, total number of stands occupied; *, species found in all stand age classes. Ellenberg scores (Hill et al. 1999): N (fertility) ranges from 1, indicating the most infertile soils, to 9, the most fertile; R (reaction) ranges from 1, very acidic, to 9, very base-rich soils
|Species||Heath:arable ratio||n||Chi-square||P||Ellenberg score|
|Galium saxatile*||2·99|| 55|| 15·90||< 0·001||3||3|
|Pinus sylvestris (from seed)*||2·85||53||14·07||< 0·001||2||2|
|Campylopus introflexus||2·49||59||12·75||< 0·001||–||–|
|Deschampsia flexuosa*||2·37||140||37·97||< 0·001||3||2|
|Hypnum jutlandicum*||1·75||150||19·19||< 0·001||–||–|
|Pteridium aquilinum*||1·51||171||14·84||< 0·001||3||3|
|Rubus fruticosus agg.*||0·62||187||22·87||< 0·001||6||6|
|Urtica dioica*||0·49||167||38·08||< 0·001||8||7|
|Galium aparine*||0·36||91||27·29||< 0·001||8||7|
|Glechoma hederacea*||0·35||59||14·88||< 0·001||7||7|
|Geranium robertianum*||0·29||79||28·83||< 0·001||6||6|
|Poa trivialis*||0·26||45||17·37||< 0·001||6||6|
|Crataegus monogyna*||0·25||36||14·43||< 0·001||6||7|
|Hedera helix||0·19||33||15·96||< 0·001||6||7|
|Dactylis glomerata*||0·19||49||28·53||< 0·001||6||7|
|Acer pseudoplatanus*||0·17|| 35|| 19·33||< 0·001||6||6|
The species found more often on previously arable stands did not include many arable weeds. Galium aparine and Reseda lutea are arable weeds, but are also associated with a range of open habitats, and Galium aparine with hedgerows. Instead, most species were associated with higher soil fertility (e.g. Stachys sylvatica and Urtica dioica), and some were associated with woodland or hedgerows (e.g. Acer pseudoplatanus, Crataegus monogyna, Hedera helix and Geranium robertianum). The species found more often on stands that were previously heath included species associated with infertile and acidic soils, such as Galium saxatile and Deschampsia flexuosa.