3 Similarity between soil seed bank and aboveground vegetation with increasing
3.3.4 Plant functional types
Plant functional types (PFTs) are described as assemblages of species having certain plant functional attributes in common (Skarpe 1996). The use of PFTs instead of single plant species can be more efficient when searching for general processes or regulating mechanisms in ecological terms such as changes in vegetation composition due to management or succession. We used life form, life span, phytosociological unit as well as classification into woody plants, forbs and grasses (taxonomic group) to build PFTs.
An overview is given in Table 3. The ecological data was extracted from the information system BiolFlor (Klotz et al. 2002). In case species were assigned to more than one functional group (e.g. therophyte and hemicryptophyte), we counted those species proportionally.
3.3.5 Data analysis
Similarity between seed bank and vegetation composition is calculated using the Jaccard index (Jaccard’s coefficient of community) as an asymmetrical binary coefficient of similarity (Legendre & Legendre 1998). It is calculated as:
c b a x a x
J( 1, 2)= + +
where a is the number of species in common and b and c are the number of species occurring only at one site.
We applied linear models (single regression) with the Jaccard index as the response variable and shrub index and proportion of PFTs as the predictor variables. Percentage data were transformed using an arcsine-square root-function. Statistics were performed with the statistic program R (R Development Core Team 2008). Detrended Correspondence Analysis (DCA) as well as Canonical Correspondence Analysis (CCA) were calculated with the program Canoco (ter Braak & Smilauer 2002). Attribute plots with Jaccard index as response variables were generated using generalized linear models (GLM) with quadratic degree and default settings within CanocoDraw (ter Braak & Smilauer 2002).
Table 3 Plant functional traits and their attributes used to classify vegetation data. The
proportion refers to the total number of species and is weighted in case of multiple entries.
Trait and attributes Abbreviation Number of
species
Weighted proportion [%]
Phytosociological unit
Weed communities So1 53 17.3
Shrubs and deciduous forests So2 104 35.0
Dry and semi-dry grasslands So3 74 16.4
Commercially used grasslands So4 117 31.3
Life form
Therophyte LF1 45 16.1
Geophyte LF2 19 6.6
Hemicryptophyte LF3 155 60.2
Chamaephyte LF4 9 2.9
Phanerophyte LF5 34 14.2
Life span
pluriennial LS1 195 78.1
biennial LS2 26 6.3
annual LS3 44 15.6
Taxonomic group
Woody plants G0 33 13.8
Legumes G1 21 8.8
Forbs excl. Legumes G2 116 57.3
Graminaceous plants G3 15 6.3
Grasses G4 33 13.8
3.4 Results
A total of 3487 seedlings germinated from the soil samples, which corresponds to a mean seed density of 4625 seeds m-² (minimum: 796, maximum: 12412). The most abundant species in the seed bank were Hypericum perforatum (20 % of the seedlings), Poa pratensis/trivialis (16 %), Centaurium erythraea (5.1 %) and Epilobium tetragonum (4.2 %). The first two species were also frequent in the aboveground vegetation (H. perforatum 0.25, P. pratensis/trivialis 0.96 of the recorded subplots), whereas the latter two were rare and even absent in some of the plots (C. erythraea 0.01, E. tetragonum 0.04 of the subplots) (Table 4). In total, we found 241 plant species, 38 of which were found exclusively in the soil seed bank, 119 exclusively in the aboveground vegetation, and 84 species in both seed bank and vegetation. The similarity between the soil seed bank and the aboveground vegetation (Jaccard index) ranged between 7.4 and 36.4 % and reached its maximum at intermediately shrub invaded grasslands. This relation could be described by a second order polynomial (Figure 4).
0 2 4 6 8 10
0.100.200.30
Scrub-index
J
Figure 4 Similarity in species composition (Jaccard index) between soil seed bank and aboveground vegetation in different shrub invaded grasslands (high values correspond to a high degree of shrub encroachment), J = -0.0033 x² + 0.034 x + 0.148, R² = 0.28, p = 0.011.
Table 4 Most frequent species in the soil seed bank: the number of seedlings in total (30 plots, 7540 cm²) and per shrub class (5 plots, 1257 cm²) and their frequency of occurrence in the grassland vegetation (counts per subplots).
Number of seedlings Frequency of occurrence in the vegetation
Shrub class Shrub class
Focussing on plant functional types, we found strong differences between the aboveground vegetation and the soil seed bank with shrub encroachment. While the vegetation composition showed clear responses to shrub encroachment in several PFTs, the soil seed bank composition remained more or less stable (Figure 5). With increasing shrub encroachment, the percentage of grassland and weedy species decreased from 0.44 and 0.15 to 0.22 and 0.05, respectively, whereas shrub species and those of deciduous forests increased from 0.20 to 0.62 in the vegetation. Again, the percentage of hemicryptophytes and therophytes decreased (from 0.72 and 0.13 to 0.59 and 0.07, respectively), benefiting phanerophytes (which increased from 0.05 to 0.25).
Accordingly, we found less herbaceous and more woody species with shrub encroachment. Focussing on life span, there was a decrease of annual species in the vegetation (similar to the decrease of therophytes) but no clear trend in the proportion of annual species in the seed bank (Figure 5).
The proportions of plant functional types in the vegetation were tested to predict seed bank-vegetation similarity using linear regression models. A high explanatory power had the proportion of forbs (R² = 0.47), therophytes (R² = 0.44) and annual species, respectively. They were all positively correlated with Jaccard index. Within the phytosociological units we found a positive influence of species of weed communities (R² = 0.27) and a negative influence of species of shrubs and deciduous forests (R² = 0.27) on seed bank-vegetation similarity.
Weed communities depending on shrub encroachment (a higher shrub class corresponds to a higher degree of shrub encroachment). Displayed are percentages of plant species differentiated into phytosociological units, life forms, life span and taxonomic groups.
Canonical Correspondence Analysis shows the alignment of the plots by their vegetation composition constrained by the proportion of PFTs (Figure 6). The significant PFTs were therophyte (LF1), hemicryptophyte (LF3), graminaceous plants (G3), grasses (G4) and the phytosociological units shrubs and deciduous forests (So2) and dry and semi-dry grasslands (So3). The first CCA-axis obviously represents a gradient of shrub encroachment, which is confirmed by the positive correlation with the PFT shrubs and deciduous forests. A fitted model with Jaccard index as response variable (GLM, quadratic degree) showed lowest seed bank-vegetation similarity with high values for the first CCA-axis (Figure 6). This indicates that a higher proportion of species of the PFT shrubs and deciduous forests lead to a lower seed bank-vegetation similarity. In addition, higher proportions of therophytes, hemicryptophytes as well as grasses lead to higher seed bank-vegetation similarity. The results of the CCA corresponded to those of single linear regression models but provided a better overview of the measured variables in condensed form.
-2.0 4.0
-1.52.0 SO3
SO2 LF3
LF1 G4
G3
0.1 0.12 0.14 0.16 0.18 0.2 0.22
Figure 6 Ordination plot (CCA, 1. and 2. axis) of vegetation data. Arrows represent the proportion of plant functional types (for abbreviations see Table 3), curved lines are the visualization of a fitted model with Jaccard Index (similarity between the vegetation and the soil seed bank) as response variables (GLM, quadratic degree).
3.5 Discussion
Contrary to expectations, our results showed a humpback relation between seed bank-vegetation similarity and shrub encroachment. Highest similarity occurred at medium shrub invaded grasslands. So far, several studies found a decreasing seed bank-vegetation similarity with increasing secondary succession (Falinska 1999; Grandin 2001; Kiirikki 1993; Matlack & Good 1990; McGee & Feller 1993; Pierce & Cowling 1991). On the contrary, similarity increased during primary succession (Grandin &
Rydin 1998). In general, lowest similarity should occur where vegetation composition has changed substantially in the recent past. This is true when the vegetation has passed a series of compositionally different successional stages. There could be two reasons for the humpbacked relation we observed. First, there might be different levels of disturbance in grasslands with different shrub invasion. In general, succession can be viewed as a gradient from disturbance-adapted to stress-tolerant vegetation (Davies &
Waite 1998; Grime 1977), and a higher level of disturbance is leading to a higher similarity caused by increased generative reproduction. Secondly, we used shrub encroachment as a measure of succession. It is, however, a hypothetic successional gradient, where later successional stages need not necessarily pass the earlier ones. This may influence the observed relation for seed bank-vegetation similarity. Nevertheless, shrub encroachment is one of the most characteristic results of secondary succession in grasslands and can be seen as an important biotic as well as environmental factor influencing vegetation composition. It affects vegetation composition primarily by competition for light, but also by competition for nutrients and water (Pykälä et al.
2005; Tilman 1985).
Calculated seed bank-vegetation similarity depends on the proportion of shared as well as the ratio of the number of species. The maximum of the Jaccard index is defined as:
B A Jmax= /
where A and B are the numbers of species and A ≤ B. In agreement with other studies, we found the seed bank less diverse than the corresponding vegetation. A decline in the number of seed bank species, with constant numbers of species in the vegetation, limits the calculated similarity. In our investigation, there was no significant effect of the ratio of the number of species on the similarity, but it should be accounted for if high similarities are expected.
Several plant functional types have been discussed in the context of seed bank-vegetation analyses, including life span (perennial-annual ratio) (Peco et al. 1998), seed
mass (Donelan & Thompson 1980; Grandin & Rydin 1998), strategy type sensu Grime (Pellissier et al. 2004) and seed longevity. Annual plants produce mostly high amounts of small seeds and they often form a persistent seed bank. Furthermore, they are often linked to the strategy type R (ruderals). Accordingly, we found a positive correlation between proportion of annual plants (which is similar to the life form therophyte) and seed bank-vegetation similarity. Peco et al. (1998) pointed out a similar result, as they found the ratio of number of perennial species and number of annual species able to predict the seed bank-vegetation similarity. In contrast, they could proof this result only for autumn collected and not for spring collected seed bank samples. This was attributed to the loss of seeds over the winter period. Nevertheless, life span (annual) is probably a good indicator for seed bank-vegetation similarity.
We also tested the plant functional type life form, phytosociological unit and taxonomic group for correlations with seed bank-vegetation similarity. Due to the gradient of shrub encroachment in the present study, there was a high variability in the proportion of the PFTs concerning shrubs: phanerophytes (life form), woody plants, shrubs and deciduous forests (phytosociological unit). To avoid autocorrelations, we used single regressions as well as CCA with forward selection of the variables to explain the seed bank-vegetation similarity. Besides the life span (annual and pluriennial), we found well fitted models with the proportion of forbs and therophytes as explanatory variables.
They were positively correlated with the Jaccard index. The high influence of the proportion of forbs might be explained by their tendency to have persistent seeds (Peco et al. 1998) as well as by the negative correlation between the proportion of forbs and the proportion of woody plants. Woody species, especially trees and shrubs, are often found to be little represented or absent from the soil seed bank (Brown & Oosterhuis 1981; Davies & Waite 1998; Donelan & Thompson 1980; Thompson et al. 1997; Warr et al. 1994).
In addition, the CCA pointed out the negative effect of species of the sociological unit shrubs and deciduous forests and the positive effect of therophytes, hemicryptophytes and grass species on the seed bank-vegetation similarity. While a predominance of grass species and hemicryptophytes is characteristic of grassland vegetation, the first CCA axis can be regarded as a gradient of decreasing seed bank-vegetation similarity from grassland to shrub and forest vegetation. The change in vegetation composition is obviously a better indicator for succession than the establishment of shrubs and trees only.
In summary, we could not verify the hypothesis that seed bank-vegetation similarity decreases with shrub encroachment. However, we found evidence for decreasing similarity with changing vegetation composition from grassland to shrub and forest vegetation. Furthermore, the proportion of the PFTs therophyte (life form), annual plants (life span) and forbs seem to be useful as predictors of seed bank-vegetation similarities in shrub-encroached grassland.
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4 Herbage mass and nutritive value of herbage of extensively managed