An Anthropogenic, Nutrient-Mediated Ecosystem State-Shift
Collectively, these datasets suggest that upland deforestation by European settlers triggered an abrupt, nutrient-mediated ecosystem state-shift on Titus Bog (Fig. 6). Mineral deposition was linked to land clearance by strong positive correlations between the concentrations of mineral matter and Ambrosia pollen in all peat cores (Fig. 3), a pattern that has been documented before in comparable peatland systems (Hölzer & Hölzer 1998; Martínez Cortizas et al. 2005; Lomas-Clarke & Barber 2007; Hughes et al. 2008). Mineral matter deposited on Titus Bog was most likely derived from nearby upland soils and transported by aeolian processes. Evidence for an aeolian origin of mineral matter in these peat cores includes (i) the lack of stream channels flowing into Titus Bog, (ii) the lack of large slopes on the adjacent uplands that could have facilitated substantial sheet flow into the wetland complex, (iii) the presence of c. 30 m of shrub swamp between the centrally located peatland and surrounding uplands, and (iv) the small particle size of the mineral matter itself (cf. Gorham & Tilton 1978; Santelmann & Gorham 1988). Furthermore, X-ray backscatter data from all three cores indicated that Si, which has long been used as a tracer of dust deposition on peatlands (Chapman 1964; Hölzer & Hölzer 1998; Martínez Cortizas et al. 2005; Lomas-Clarke & Barber 2007; Hughes et al. 2008), was the most abundant element in LOI residue collected within the mineral matter peak and that Si is more abundant above the depth of maximum mineral concentration than below (Fig. 3). Finally, core A, which was collected along the windward margin of the peatland, recorded more dramatic shifts in all measured variables than core B or C, further suggesting that windblown material likely caused nutrient enrichment of the ecosystem.
In all cores, peat deposits above the depth of peak mineral matter and Ambrosia pollen were enriched in N, P and K relative to peat deposits below this level (Fig. 3), consistent with the hypothesis of dust fertilization. P enrichment was especially pronounced, particularly in core A (Fig. 3). Modern studies have shown that windblown dust particles can be composed of up to 0.2% P by weight (Redfield 1998) and often represent the most important source of P replenishment to peatlands (Le Roux, Laverrret & Shotyk 2006). This is especially true in highly disturbed agricultural settings where dry P deposition can reach 50–100 mg m−2 year−1, two to three times greater than less disturbed, forested regions (Redfield 1998).
The post-disturbance shift from Sphagnum-dominated to vascular-plant-dominated plant communities is also consistent with greater nutrient availability on the peatland surface (Rydin & Jeglum 2006). Similarly, expansion of P. strobus on the peatland is consistent with fertilization, especially in terms of P enrichment, as trees can be strongly P-limited in peatland systems and can respond quickly to increased P availability (Rydin & Jeglum 2006). Other studies of peat profiles have documented correlations among palynological indicators of upland land-use change, geochemical properties of the peat deposits and macrofossil records of vegetation shifts. For example, Hughes et al. (2008) demonstrated strong linkages between palynological indicators of pastoral disturbance and marked declines in a particular species of Sphagnum moss on raised bogs in the British Isles and suggested that this species (Sphagnum austinii) was sensitive to aerial deposition of dust particles containing nutrients and/or charcoal.
Testate amoeba communities also underwent marked structural changes coincident with mineral matter deposition, nutrient enrichment and plant community shifts, especially in core A (Figs 5 and 6). Recorded community shifts were not strictly consistent with directional changes towards wetter or drier conditions on the peatland surface, which is not surprising as such directional shifts would not be expected to occur on a hydrologically stable floating peatland (Booth 2010). Rather, data suggest that coincident with the change in plant communities, testate amoeba communities shifted towards those more tolerant of high-magnitude variability in micrometeorological conditions, such as those characterized by high abundances of D. pulex and H. subflava (Sullivan & Booth 2011). Shifts from densely growing Sphagnum mosses to relatively less dense vascular plants would have enhanced micrometeorological variability at the peatland surface, affecting the upper few centimetres where testate amoebae live. Furthermore, food sources for testate amoebae may have changed, due to shifts in the composition of microbial communities, and this may also have contributed to changes in testate amoeba community composition (Jassey et al. 2011). Variation in testate amoeba communities among cores likely resulted from relatively small dissimilarities in the microtopography of coring locations. These differences highlight that core samples generally record very local changes, illustrating the importance of collecting and analysing multiple cores when attempting to reconstruct whole-ecosystem dynamics.
Together, changes in the mineral matter concentrations (Fig. 3) and proportions of highly decomposed plant material (Fig. 4) suggest that nutrient enrichment stimulated microbial decomposition. Reduced peat accumulation rates in the upper portions of these peat cores (Fig. 2) are also suggestive of enhanced decomposition. A post-disturbance increase in rates of peat decomposition is generally consistent with increased P availability, as microbial respiration can be P-limited in nutrient-poor peatlands (Amador & Jones 1993). It is also possible that the expansion of vascular plants yielded less recalcitrant litter than the preceding Sphagnum-dominated peatland community, facilitating relatively high rates of decomposition and rapid recycling of resources (Verhoeven, Maltby & Schmitz 1990; Aerts, Verhoeven & Whigham 1999) (Fig. 7).
Figure 7. Conceptual model summarizing the proposed dynamics that were triggered by deforestation of the surrounding uplands. Boxes and text in black are underpinned by data collected in this study, whereas those in grey represent hypotheses. Data indicate that human deforestation led to dust deposition on the Sphagnum-dominated, nutrient-poor surface of Titus Bog. Elemental data strongly suggest that dust deposition transported nutrients (especially, N, P and K) to the system, and this fertilization likely led to changes in plant communities. Changes in testate amoeba communities were likely caused by some combination of higher microenvironmental variability associated with the more open vegetation canopy, as well as changing food sources caused by broader changes in microbial communities. Macrofossil data documented more decomposition within the post-settlement vascular plant communities than within the pre-settlement Sphagnum-dominated communities. Vascular plant communities may have produced less recalcitrant litter, sustaining high rates of decomposition and rapid nutrient cycling (Verhoeven, Maltby & Schmitz 1990; Aerts, Verhoeven & Whigham 1999) and providing a feedback mechanism to maintain the new ecosystem state.
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While all data indicate that aerial dust deposition fertilized Titus Bog, inferring past nutrient limitation and identifying which specific nutrients led to the ecological changes is challenging. Primary productivity in oligotrophic peatlands is commonly thought to be N-limited (e.g. Bragazza et al. 2006), although others have suggested P-limitation (Walbridge & Navaratnam 2006), potentially in conjunction with K-limitation (Güsewell & Koerselman 2002). Recent work has suggested that many ecosystems, including freshwater wetlands, could in fact be co-limited by N and P, with the addition of both nutrients resulting in strong synergistic effects (Elser et al. 2007).
Nutrient ratios are frequently used to explore the nature of nutrient limitation and ecological processes. In all cores at Titus Bog, C : N ratios were generally consistent with average values reported for bulk peat (Limpens, Heijmans & Berendse 2006) and tracked botanical changes recorded in macrofossil data (Fig. 6). Observed negative excursions in C : N ratios towards the surface were inconsistent with expectations for a decomposition-driven pattern (Kuhry & Vitt 1996) and more likely reflected changes in source material (Dorrepaal et al. 2005). In all cores, N : P ratios exhibited negative shifts coincident with maximum mineral matter concentrations (Fig. 6). The direction of these shifts suggested that Titus Bog may have tended towards P-limitation prior to European settlement (>15 : 1) and that after dust deposition the system may have become more N-limited (<15 : 1) (Walbridge & Navaratnam 2006). More research would be required to systematically test this hypothesis; however, data suggest N, P and K were transported to the surface of Titus Bog, making it likely that increased nutrient availability triggered the dramatic ecological changes, even if the nature of the preceding nutrient limitation is uncertain.
Conclusions and Conservation Implications
Extensive efforts have gone towards predicting responses of C-rich peatland ecosystems to large-scale increases in anthropogenic nutrient deposition (e.g. Bragazza et al. 2006). Results of this study suggest that peatlands in agricultural landscapes may be vulnerable to nutrient enrichment through non-point-source dust deposition, with considerable community-to-ecosystem level consequences. At Titus Bog, dust deposition was associated with reduced peat accumulation rates (Fig. 2), nutrient enrichment (Fig. 3), shifts from Sphagnum moss to vascular plant communities (Figs 4 and 6) and shifts in testate amoeba assemblages (Fig. 5) that likely reflected broader changes in microbial communities. Although speculative, N : P ratios suggest that prior to European settlement, the peatland tended towards P-limitation and that dust deposition associated with land clearance pushed the system towards N-limitation (Fig. 6). Although the mineral fraction of peat deposits never exceeded 20% of total weight, the nutrient inputs were apparently large enough to exceed a critical threshold for this oligotrophic system, leading to a cascade of ecological changes (Scheffer et al. 2001). These results suggest that Titus Bog had limited capacity to resist nutrient-driven changes.
Sphagnum mosses are relatively abundant on the modern peatland surface, suggesting some recovery during the past century and obscuring the dramatic shifts that occurred in the recent past. However, the legacy of dust deposition likely still remains, as vascular plants are substantially more common today than they were prior to the transient disturbance event. Furthermore, the strong spatial patterning of P. strobus (Fig. 1) is apparently unrelated to the developmental history of the peatland (Ireland & Booth 2011), but more likely is related to land-use history in the adjacent upland (cf. Houlahan et al. 2006) and possibly the influence of the dominant westerly surface winds in depositing more dust along the western margins (Santelmann & Gorham 1988). Our results underscore the benefits of including a palaeoecological perspective in developing strategies for the conservation, management and restoration of C-rich peatlands (Vasander et al. 2003; Williams 2011), reaffirm the value of upland buffers along wetland margins and highlight the potential sensitivity of these systems to past and future changes in usage of the surrounding landscape.