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Thresholds at which multi-species assemblies shift from one ‘stable’ state to another are an accepted part of theoretical and applied ecology (Scheffer et al. 2001; Suding, Gross & Houseman 2004). However, few experimental studies have addressed the critical points at which state transitions occur, thus hindering effective ecosystem management. State transitions are often determined by the population densities of ‘foundation’ species that, upon recruitment, engineer the new ecosystem state (Wilson & Agnew 1992; Jones, Lawton & Shachak 1997; Petraitis & Latham 1999). Thus characterization of the state transition threshold (transitional area) may be obtained by defining what determines the invasion of these species.
In this paper we describe an attempt to quantitatively delimit the transitional area of a heath–scrub vegetation change. Invasion of Betula spp. [both Betula pubescens (Ehrh.) and Betula pendula (Roth)] drives transitions between heath and scrub ecosystems (Mitchell et al. 1997, 1999; Hester, Miles & Gimingham 1991a,b) and threatens the conservation of lowland heath (Harrison 1976; Rose et al. 1999). Betula recruitment is influenced by factors related to both the availability of seed (seed limitation) and the germination and survival of seedlings (safe-site limitation). Many environmental factors, including vegetation density and phosphorus (P) availability, are potentially important axes of the safe site (Manning, Putwain & Webb 2004). However, deriving a general model of Betula invasion is complicated by the considerable landscape-scale heterogeneity of heathland ecosystems. For example, soil phosphorus sorption capacity (PSC) affects P availability (Manning 2002) and displays landscape-scale variation that correlates with patterns of invasion (Chapman, Rose & Basanta 1989).
The overall aim of this research was to extend the site-specific model of Manning, Putwain & Webb (2004) by deriving a general statistical model that quantitatively describes the conditions in which Betula invasion occurs. Such a model would be of greater utility to heathland managers than a detailed but site-specific model. We conducted an experiment identifying Betula recruitment limitations at three geographically distinct sites to assess the universality of the key determinants identified by Manning, Putwain & Webb (2004). Data from the experiments were pooled to formulate a general model describing Betula colonization as a function of relatively simple variables that are applicable to a range of lowland heath environments. It was decided a priori that this would only be attempted if some determinants were identified as universal. Data from independent sites was then used in an attempt to validate the general model. The results of these studies are discussed with reference to both scientific understanding of transitions between alternate ecosystem states and heathland management.
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The results of this study reveal that Betula recruitment is controlled by both seed and safe-site limitation and that vegetation density, vegetation height and P availability can be key axes of the Betula safe site in a range of heathland environments. The findings extend and support the more detailed, but site-specific, results of Manning, Putwain & Webb (2004). The fitted relationships of the general model show that the transitional area of heathland ecosystems is defined by the coincidence of many favourable conditions; the non-optimality of a single important variable (e.g. vegetation density, seed bank density) may inhibit invasion.
The determinants of Betula colonization were broadly similar between sites but their relative importance varied widely. Variables accounting for differences in seedling densities were those varying over a critical range of values, for example seed bank densities at the Dorset site and P availability at the New Forest. Inconsistency in site-specific models may be explained by the use of separate variables representing similar phenomena, lack of within-site variation and site-specificity in the determinants. Seed availability explained seedling densities at all sites, and it probably limits invasion on many heaths, which are typically more isolated from seed sources. Overall, disturbance had a positive effect on seedling recruitment at all sites that operated via several mechanisms, the most important being changes to vegetation. However, disturbance effects, when isolated from vegetation effects, were inconsistent. Between-site differences in Betula response to P resulted from initial limitation and within-site variability in P availability. Low PSC resulted in fairly equal levels of P availability across the Dorset site, while at the New Forest site high PSC generated great differences between control and high-level P addition plots.
Seedling abundance patterns at the Surrey site were more stochastic than at the others. This stochasticity may be linked to the failure of plot-scale measurements to contain the fine-grained resolution that could account for infrequent coincidences of seed and safe site when both are strongly limiting. This concept is supported by the overdispersion of residuals in the models, including the general model, where predicted seedling densities were low.
By describing the constraints on recruitment of a species known to activate a switch between ecosystem states, the general model represents one the first attempts to describe the threshold of transition (e.g. the F2 point of Scheffer et al. 2001) in a general, empirical, quantitative and multi-dimensional fashion. Despite these achievements the model is probabilistic and does not identify a clear threshold between the two alternate states. Although it relies upon the assumption that seedling densities indicate the likelihood of transition, this is valid if the mortality of seedlings > 1 year old displays no density dependence.
The model suggests that low-density vegetation of intermediate height is the most invasible and possibly reflects both low light interception and protection from stresses such as herbivory and frost damage. Invasion is therefore most likely in the early building and degenerate phases of the dwarf shrub cycle and in mixed-species communities containing C. vulgaris damaged by the herbivorous heather beetle Lochmaea suturalis (Thoms.). The contrasting, uninvasible condition is dense and short vegetation, for example the closed canopies of the later building and mature phases. General failure of Betula to penetrate closed canopies is consistent with studies by Gong & Gimingham (1984), Miles (1974) and Marrs (1986). However, Gong & Gimingham (1984) found survival of seedling Betula in all ages of Calluna vegetation, thus supporting the proposal that low-density, effectively stochastic, invasion can occur in generally uninvasible conditions. Such invasion is unlikely to trigger vegetation shifts at larger scales but may increase the likelihood of future occurrence via raised seed bank densities.
The apparent success of the general model in predicting seedling densities at the model validation sites suggests that the identity and influence of predictor variables is constant over a reasonably wide range of heathland conditions. Its relatively accurate prediction was surprising as many predictions relied upon extrapolation of the fitted relationships. The most likely explanation for the apparent accuracy of these extrapolations is that the extrapolated variables had values resulting in low expected seedling densities. Tall vegetation heights, for instance, result in low predicted seedling densities. However, the extent to which these relationships remain constant over even greater spatiotemporal ranges of biotic, climatic and edaphic conditions is unknown, as is the model's capacity to predict accurately high Betula seedling densities. Minor inaccuracy of predictions at the low-seed availability Arne site also highlights inadequacy in the model; because seedling density is not restricted to seed bank density, seedling densities are overestimated in optimal safe-site plots with low seed input. It is also clear that the factors retained in the model, and their relative effect size, reflect not just their importance but also their variance within the study sites and during the year in question. The small effect size of soil water content, for instance, may reflect low variability at the study sites rather than its importance in the more variable range of natural heathland conditions. In summary, reasonable confidence can be held in the qualitative conclusions that can be drawn from the model but its value in quantitative prediction is yet to be fully confirmed.
In synthesizing the findings of this and previously published research it can be tentatively concluded that invasion is most likely where vegetation is sparse and of intermediate height, where propagule supply is plentiful, where P availability (Pox) is > 100 µg g−1, and where herbivores are absent; active management of such sites should be considered a priority by conservation bodies. It is important to note, however, that there is no single set of invasible conditions but rather a suite of combinations that occupies a narrow area of multi-dimensional space. Because non-optimality of a single factor can preclude invasion, managers seeking to halt invasion have several simple strategies available to them. Perhaps the most practical are tree removal and, where this is not possible, for example in invaded habitats utilized by nightjars Caprilmulgus europaeus (L.), the management for building-phase vegetation by mowing and regular burning. However, before detailed and fully informed management recommendations are made it is important to know which invasible conditions occur in unmanipulated heathland environments and how the identified factors vary at the larger spatial scales to which management is applied.
Management regimes and inherent regional differences, for example geology and climate, will alter the susceptibility to invasion. Regular burning, for example, returns the ecosystem to the heath stability domain, after its endogenous movement towards the vulnerable degenerate state, as it promotes nutrient loss, maintains short, dense vegetation and destroys Betula seed. Infrequent burning events, however, are likely to initiate invasion as dwarf shrub rootstocks are destroyed and large quantities of P are released (Bullock & Webb 1995). Mammalian herbivores also influence Betula invasion through both direct consumption and via impacts on nutrient cycling and vegetation structure. Browsing animals will have strong direct negative effects upon scrub colonists but may do little to influence physical properties. In contrast, grazing animals are less selective and so probably have smaller direct effects on Betula colonists (Bokdam & Gleichman 2000). Strong indirect effects of grazing animals may be expressed via high biomass consumption, which influences vegetation structure, nutrient supply rate and the spatial pattern of these determinants. The net balance of these influences may depend greatly upon the density, type and spatiotemporal distribution of the animals, but will, in most cases, be negative.
The large-scale correlation between soil PSC and scrub invasion (Chapman, Rose & Basanta 1989) may be explained by direct effects of PSC on P availability, but is more likely to represent indirect influences. Heathlands on low PSC soils have a low productivity that slows gap formation and P accumulation (Chapman, Rose & Clarke 1989). By implication a greater proportion of vegetation will be in the invasible degenerate state after management cessation in high PSC regions. A second implication is the raised probability that natural or accidental fires will occur during periods of the cycle, in which fire shifts conditions towards the transitional area. Severe burn events are particularly likely to trigger invasion in high PSC areas as large amounts of P may be retained within the topsoil. For these reasons we suggest that the intensity of management for short, compact vegetation and low P conditions, by grazing, mowing or burning, should be increased in high PSC regions.
As there has been no direct test we tentatively conclude that an interaction between management regimes and PSC controls heath–scrub transitions. This is best exemplified by the widespread invasion of the Surrey heaths (Harrison 1976). Early abandonment of traditional management appears to have interacted with high PSC to result in invasion that, by increasing the propagule density of the region, further increases the likelihood of invasion in remaining heaths. Dorset, in contrast, is typified by low PSC, later abandonment, more intensive management and, therefore, slower rates of invasion.
Petraitis & Latham (1999) present two models of ecosystem state transition, one in which the foundation species of the state alters the environment and out-competes that of the original state in a continuous process, and a second in which disturbance stimulates rapid transition to the alternate state. Transition to Betula scrub occurs via both mechanisms: invasion and exclusion of susceptible dwarf shrub vegetation via a positive feedback, and through severe disturbances including fires and beetle outbreaks. Thus heathland vegetation dynamics can be viewed as applicable to both the continuous and discontinuous models described by Briske, Fuhlendorf & Smeins (2003), even at a single, spatiotemporal scale.