In the second paradigm, a long-term progressive climatic trend is overlain by higher-frequency variability (Fig. 2b). Local ecosystems are differentially sensitive to climatic forcing, exhibiting different mixtures of linear and nonlinear responses (Figs 1g,i,k), and site-specific factors strongly govern the local ecological response. Spatial variations in high-frequency climatic events, such as droughts, and stochastic processes, such as fires and other disturbances, can determine when a system experiences a regime shift (Paine, Tegner & Johnson 1998). As a consequence, the rate and timing of ecological change varies widely among sites (Fig. 2d). However, at a regional scale, extreme climate events can simultaneously force multiple local systems past their tipping points, resulting in clusters of abrupt change (Fig. 2f).
Confident attribution of past abrupt ecological changes to internal tipping points is difficult in general and nearly impossible for single sites. Paired palaeoclimatic and palaeoecological records at individual sites can be highly suggestive, if they show abrupt ecological changes in the apparent absence of any abrupt climate changes. However, because the palaeoclimatic history at any single site is always imperfectly known, it is hard to rule out the possibility that abrupt ecological changes observed at that site were forced by an unknown but suddenly changing external driver (Fig. 1a).
Regional networks of high-resolution and well-dated palaeoecological records enable powerful assessments of the causes of abrupt ecological changes (Waller, Street-Parrot & Wang 2007; Fritz 2008; Kuper & Kröpelin 2008; Williams et al. 2010). Specifically, if locally abrupt ecological change is caused by systemic non-reversible thresholds that emerge from the interaction between regional climate forcing and site-specific factors (Fig. 1f–l), then one should observe a ‘temporal mosaic’ of local responses to the regional forcing:
The concept of temporal mosaics is closely related to the spatial mosaics and patterning exhibited among sites within regional systems that are prone to threshold switches between alternate stable states (Rietkerk et al. 2004). Spatial mosaics occur when localized positive feedbacks create self-organized and locally homogenous systems within a regional spatial mosaic. Temporal mosaics occur when these spatial mosaics are subjected to a gradual external forcing, causing the relative balance of stabilizing and destabilizing feedbacks to shift. Because this balance is governed in part by local, system-specific factors, the timing and rate of change will vary among locations within the spatial mosaic, resulting in a temporal mosaic of site-level responses. Inter-site heterogeneity in the timing and rate of ecological change in palaeoecological records can also result from heterogeneous data quality, e.g. low temporal sampling resolution or low precision and accuracy in the chronological controls used to establish a time-scale for palaeo-records (e.g. Grimm, Maher & Nelson 2009). This alternative hypothesis, however, can be tested and potentially rejected by restricting the regional analysis to sites with well-constrained ages and by identifying differences in timing that are too large to be explainable by dating imprecision (Williams et al. 2010).
To illustrate the above points, we review case studies of systems characterized by locally to regionally abrupt regime shifts during periods of progressive long-term climate change. The first case study reviews the response of the Great Plains and surrounding ecosystems in central North America to increasing aridity during the early to middle Holocene, while the second case study reviews the response of the North African Sahara and Sahel to increasing aridity during the middle to late Holocene. These examples were chosen because semi-arid ecoregions provide some of the best examples of ecological thresholds, positive feedbacks and alternative stable states (Wang & Eltahir 2000; Sternberg 2001; van de Koppel & Rietkerk 2004), and locally to regionally abrupt changes are well-documented in these systems during the Holocene (e.g. Grimm 1983; deMenocal et al. 2000; Umbanhowar et al. 2006; Waller, Street-Parrot & Wang 2007; Williams, Shuman & Bartlein 2009).
Central North America
Central North America east of the Rocky Mountains is a moisture-limited system, characterized by a strong west-to-east gradient of increasing precipitation and soil moisture, and a suite of semi-arid ecosystems (Grimm 2001). Diverse ecological and physical systems in central North America have been strongly responsive to hydrological variations during the Holocene, including shifts in rates of tree growth (Cook et al. 1999); the abundance of C3 and C4 grasses (Nordt, von Fischer & Tieszen 2007) and other herbaceous taxa (Grimm 2001; Clark et al. 2002a); the position of the prairie–forest ecotone (McAndrews 1966; Clark et al. 2001; Williams, Shuman & Bartlein 2009); fire frequency and biomass burnt (Clark 1988; Brown et al. 2005); lake levels, lake salinity and aquatic community composition (Laird et al. 1996, 2003; Shuman et al. 2002a); flood frequency and magnitude in fluvial systems (Knox 2000); and switches in aeolian systems between active and vegetation-stabilized (Forman, Oglesby & Webb 2001; Wolfe, Ollerhead & Lian 2002; Miao et al. 2007). The long-term hydrological trends for this region are well known: central North America became progressively drier until c. 8000–6000 years ago (timing varies within the region). It then became gradually wetter, reaching conditions similar to present, around 4000 years ago (Forman, Oglesby & Webb 2001; Nelson & Hu 2008; Williams et al. 2010). These trends are linked to the early Holocene peak in summer insolation (Fig. 4a), which increased summer temperatures and evaporative demand in the Great Plains (Kutzbach et al. 1998) and may also have suppressed summer precipitation by driving a dynamical shift towards regionally descending vertical air motion and clear-sky conditions (Harrison et al. 2003; Diffenbaugh et al. 2006). The retreat of the Laurentide Ice Sheet (and its collapse 8400 years ago) may also have promoted mid-continental drying by altering atmospheric circulation, moisture advection and the position of storm tracks (Shuman et al. 2002a; Shuman & Donnelly 2006). Higher-frequency drought episodes (with durations of decades to centuries) are superimposed upon these long-term trends (Woodhouse & Overpeck 1998; Clark et al. 2002a; Laird et al. 2003; Miao et al. 2007).
Figure 4. Regional climate drivers of early Holocene aridification in the Great Plains, USA, and the spatiotemporal distribution of abrupt ecological responses (Williams et al. 2010). (a) Temporal trend in June, July, and August insolation for 40° N expressed as differences from present (solid line) (Berger & Loutre 1991) and the rate of change of the area of the Laurentide Ice Sheet (dashed line) (Dyke 2004, P. Clark, pers. comm.). (b) The temporal distribution of locally abrupt changes in central North America, including shifts in the relative abundances of C3 and C4 plants, shifts in tree cove density, and variations in lake level and salinity. (c) The same data as in (b), plotted against longitude, showing a west to east time-transgressive trend.
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In response to these hydrological changes, the prairie–forest ecotone first shifted eastwards during the early Holocene, then westwards during the late Holocene, presumably in response to increasing moisture availability (Williams, Shuman & Bartlein 2009). These ecotonal dynamics were asymmetric, in that the early Holocene conversions of local ecosystems from forest to prairie were rapid (century-scale) while the late Holocene reforestation was gradual (millennium-scale) (Umbanhowar et al. 2006; Nelson & Hu 2008; Williams, Shuman & Bartlein 2009). At some sites, this asymmetry is also evident in δ18O, dust flux, sediment magnetic properties and other abiotic indicators of regional aridity (Nelson et al. 2006; Nelson & Hu 2008), which suggests that in some instances, rapid deforestation at the prairie–forest ecotone was directly caused by extrinsic climate change, perhaps linked to the collapse of the Laurentide Ice Sheet. However, the timing and rates of deforestation were heterogeneous in space and time (Grimm 1983; Camill et al. 2003; Nelson et al. 2006; Umbanhowar et al. 2006). For example, Sharkey Lake and Kimble Pond, in south-central Minnesota, both record decreasing abundances of arboreal taxa during the early Holocene and increasing charcoal influx, but at Sharkey Lake, the rate of charcoal influx is rapid and begins 8000 years ago, while at Kimble Pond, the increase is more gradual and begins a bit later, c. 7500 years ago (Camill et al. 2003). Rates of decrease in arboreal pollen abundances also are faster at Sharkey Lake than at Kimble Pond. Regional syntheses of early Holocene fossil pollen records and other indicators of aridification show spatial variability in the timing of change, with a cluster at c. 8000 years bp (Fig. 4b, Williams et al. 2010).
The early Holocene dynamics of the prairie–forest ecotone thus appear to offer an example of a temporal mosaic, in which the often abrupt, but temporally heterogeneous shift from forest to grassland is caused by strong positive feedbacks among fire, climate and vegetation that promote two alternate stable states within the same climate regime: a forested state, in which a low availability of dry fuels reduces fire severity and forest regeneration is not dispersal-limited, and a grasslands state, in which fires inhibit recruitment and encroachment of woody species and regeneration is further limited by dispersal limitation from scattered forest patches (Grimm 1983, 1984; Umbanhowar 2004). A progressive external forcing will shift the relative balance among positive and negative feedbacks, controlling rates of tree regeneration and fire regime and pushing these systems to new persistent states. Because the threshold for forest persistence will depend on the interaction between fire regime (heavily influenced by local factors and stochastic processes such as fire ignition), the spatial distribution of forest patches, and regional climate change, the timing and rate of deforestation will vary widely among sites (Fig. 1g,j,k, Grimm 1983; Camill et al. 2003; Umbanhowar 2004), as will the relative importance of direct climatic controls on vegetation versus indirect controls manifested via changes in fire regime (Nelson & Hu 2008). However, time-transgressive patterns may be observed within this temporal mosaic because (i) the ecotone is situated on a moisture gradient, and so more arid sites may be nearer to a threshold than less arid sites (Williams, Shuman & Bartlein 2009; Williams et al. 2010), and (ii) as the ecotone shifts, the supply of tree seeds can change exponentially due to metapopulation dynamics at range margins (Levin & Clay 1984). Once the forest-to-grassland conversion is locally complete, the enhanced fire regime and reduced seed rain may prevent woody encroachment, even if climatic conditions are suitable for tree regeneration. Note that if this system was subjected to a large and abrupt external forcing (e.g. a widespread, severe and persistent drought), then the site-level responses could be essentially collapsed into a single synchronous regional-scale change (Fig. 1c,e). For example, post-fire succession from forest to grassland could be synchronized at decadal scales by a widespread, severe event.
Despite strong local-scale heterogeneity, spatial structure in the regional timing and patterns of ecological change can provide insights into the relationship between regional climate change and local response. For example, a synthesis of palaeoecological and palaeohydrological proxies from the Great Plains (Williams et al. 2010) shows that the timing of site responses to aridification was time-transgressive, with western sites beginning to dry before eastern sites. This time-transgression was also apparent in sites with abrupt responses to aridification (Fig. 4c), suggesting that the ecological resilience to aridification was lower for the more xeric western sites, causing them to pass their tipping points before eastern sites. (A possible alternate hypothesis is that the early Holocene decrease in moisture availability propagated eastwards in the central US causing an extrinsic forcing of abrupt change that was time-transgressive, Williams et al. 2010). Superimposed on this regional trend are a cluster of abrupt changes at c. 8000 years ago (Fig. 4b), which roughly corresponds to the collapse of the Laurentide Ice Sheet and the perturbation of the climate system with a meltwater pulse to the North Atlantic (Alley et al. 1997; LeGrande et al. 2006). In central North America, the collapse of the Laurentide Ice Sheet may have forced a step change of the climate system towards a drier state, through shifts in atmospheric circulation patterns and the paths of moisture transport (Shuman et al. 2002a). Thus, the overall picture from central North America is one of progressive aridification during the early Holocene, perhaps accelerated at times by extrinsically driven events such as the 8200 event, which resulted in a temporal mosaic of local response, with sites varying widely in the rate and timing of response.
Figure 5. Summary of North African records showing the prevalence of locally abrupt but regionally asynchronous ecological responses to aridification. (a) Progressive declines in Sudanian woodland taxa at Jikariya Lake, Nigeria, starting c. 8700 years bp (Waller, Street-Parrot & Wang 2007). (b) Abrupt increase in terrigenous dust influx to marine sediment core ODP 538 at 5300 years bp (deMenocal et al. 2000). (c) Abrupt increase in nitrogen isotopes at Lake Bosumtwi, Ghana, interpreted to indicate changes in nitrogen source and cycling (Russell, Talbot & Haskell 2003). Note that this plot is drawn on a depth scale; the rapid shift in δ15N is estimated to occur 3200 years bp (Russell, Talbot & Haskell 2003). (d) Diatom- and chironomid-inferred changes in salinity and pollen abundances for tropical plants at Lake Yoa, Chad (redrawn from Kröpelin et al. 2008).
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Holocene aridification in North Africa was externally driven by precession of the Earth’s orbit, which has gradually weakened summer insolation from a peak 11 000 years ago to its current low. Insolation regulates land–ocean heating contrasts and hence the strength of the North African and other subtropical monsoons (Kutzbach 1981). The insolation-driven weakening of monsoonal summer precipitation likely was amplified by positive feedback loops between the atmosphere and the ocean (Kutzbach & Liu 1997) and the atmosphere and land surface (Ganopolski et al. 1998). Earth system models have predicted that the North African grasslands may have abruptly collapsed in response to gradual insolation forcing, either due to positive hydrological coupling between the land surface and the atmosphere (Ganopolski et al. 1998; Claussen et al. 1999; Wang & Eltahir 2000) or by a threshold response of North African vegetation to precipitation variability overlaid on a progressive decrease in precipitation (Liu et al. 2007). Abrupt desertification is supported by many palaeoecological and palaeoclimatic records from the region, most particularly by marine records from western North Africa that show abrupt increases in aeolian dust fluxes c. 5500 years ago (deMenocal et al. 2000). This evidence has led to a widespread view of North Africa as a classic example of a tipping element in the earth system capable of regionally catastrophic shifts between alternate stable state (e.g. Scheffer et al. 2001; Lenton et al. 2008).
A fuller consideration of palaeoecological and palaeoclimatic records from north-western and north-central Africa, however, suggests that the regional history of desertification was considerably more complex than a simple synchronous and abrupt regional shift forced by positive biosphere–atmosphere couplings (Waller, Street-Parrot & Wang 2007; Kuper & Kröpelin 2008). Instead, local rates of response to aridification varied widely in rate and timing, both among sites and among different ecological communities at the same site (Fig. 5). For example, at Lake Yoa in northern Chad, the sequence of vegetation changes at the site strongly indicates that drying began around 5600 years ago and continued until 2700 years ago (Kröpelin et al. 2008). Several abrupt ecological events punctuated this progressive trend, such as abrupt shifts in chironomid and diatom assemblages between 4200 and 3900 years ago, consistent with a rapid salinization of the lake (Kröpelin et al. 2008), and rapid increases in Poaceae and Typha abundances 2700 years ago, consistent with further lake salinization and swamp expansion. At the Manga Grasslands in north-east Nigeria, multiple pollen records drawn from interdune swales show an abrupt onset of more arid conditions, but the timing varies among sites from 5150 to 3150 years ago (Waller, Street-Parrot & Wang 2007). Vegetation changes at Lake Tilla in Nigeria indicate that drying may have begun as early as 7600 years ago (Salzmann, Hoelzmann & Morczinek 2002). A regional synthesis of archaeological records from North Africa indicates that human depopulation began first in the north around 7300 years ago and progressed southward until around 3400 years ago (Kuper & Kröpelin 2008).
Thus, as in the Great Plains, the general pattern is a temporal mosaic of site-level responses to regional aridification, consisting of (i) a consistent shift among sites towards a more arid state with (ii) strong variations among (and within) sites in the timing and pace of change. Abrupt ecological changes are common, but highly variable in timing. The timing is too variable to be explainable by poor chronological precision. Thus, the available data strongly suggest that ecologically abrupt change in Holocene North Africa is governed not by positive atmosphere–vegetation feedbacks operating at the regional scale, but instead by local, site-specific thresholds (Kröpelin et al. 2008). As in central North America, the time-transgressive trend (here recorded in the archaeological data synthesis) suggests that tipping points for the more xeric northern sites were passed before the tipping points for more southern mesic sites (Kuper & Kröpelin 2008). Other regional syntheses of lake-level records suggest that the progressive aridification of North Africa may have been overlaid by episodes of pronounced drought, signalled by clusters of lake-level declines across sites (Gasse 2000). One such episode may have occurred 4200 and 4000 years ago (Gasse 2000), although recent reanalyses at Lake Botswumi, Ghana, suggest that the abrupt changes in lake hydrology may have occurred more recently, c. 3200 years ago (Russell, Talbot & Haskell 2003).