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Here, the arrival of humans c. The conversion of forests in the South Island, presents a striking paradox, that extremely small populations by any estimate — human mtDNA sequences suggest an initial founding population of between 50— females and a low population growth rate of c. Paleoenvironmental records from New Zealand suggest that ecosystem feedbacks during the initial burning period accelerated the rapid rate of forest demise and help explain this paradox [5] , [9].

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In this paper, we examine the rate at which relatively stable forest biomes in New Zealand were converted to open shrublands in the 13 th century, and how forest transitions following early human-set fires in the South Island of New Zealand compares with other regions in the world currently experiencing deforestation. During the IBP, nearly all pollen and charcoal records derived from the drier parts of New Zealand show a distinct decline in pollen from native podocarp tree taxa and to a lesser extent beech forest trees , an increase in pollen from seral taxa particularly bracken fern Pteridium esculentum and grasses Poaceae , and an increase in charcoal e.

However, the age-depth chronologies described in [9] , [14] were based on a small number of radiocarbon dates selected prior to and after the IBP, and consequently, the rate of forest loss remains poorly constrained. To precisely determine the rate at which prehistoric forest transitions occurred following initial human arrival in the South Island, New Zealand, we developed high-resolution reconstructions of vegetation pollen and fire macroscopic charcoal from radiocarbon dated lake-sediment records from two small, closed-basin lakes, Lake Kirkpatrick and Dukes Tarn Fig.

We specifically choose two sites that represent different vulnerabilities to human-set fires: one drier lowland site and a second, wetter high-elevation site. The chronology of these reconstructions and rates-of-change calculations are based on a stratigraphic series of Accelerator Mass Spectrometry AMS radiocarbon dates from the sediment cores Table S1.

Chronology model results were highly convergent, suggesting forest transitions occurred within decades of the first human-set fires Figs. S1 — S5. AD and AD Fig. AD when fire activity decreased. A second increase in fire activity coincided with European arrival ca.

AD Vegetation assemblages changed dramatically following the first fires associated with human arrival. Native trees e.

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S6 for additional pollen information. Tree pollen percentages then declined, recovered again at c. Pollen of exotic taxa Pinaceae, Rumex spp. The interval associated with forest transitions was identified using two criteria: 1 visible change in core lithology, and 2 a rapid decline in pollen percentages of native forest taxa and a coincident increase in pollen percentages of open vegetation taxa.

Colored panels show change in percent of total terrestrial pollen percentages for native trees and disturbance-associated taxa e.

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At Dukes Tarn, charcoal data register initial fire events at c. AD , and , and the pollen data indicate that forests recovered within decades even though fire events occurred periodically once every 50— yrs from the IBP until the last century Fig. AD to see Fig. S7 and [9] for additional pollen information. Non-native plant taxa Pinaceae, Rumex spp. Fires that were recorded during the same time period at both sites Fig. Compared to Lake Kirkpatrick where forest transitions led to open vegetation that persists to the present, native vegetation at Duke's Tarn experienced relatively minor modification and showed partial recovery within several decades of the first IBP fires and full recovery years after the IBP.

Post IBP fires at Duke's Tarn seem to have had less of a lasting impact on the long-term structure and composition of vegetation. Black circles identify peaks in CHAR that are statistically significant from background variation in CHAR and are thus likely to be local fires within 1—3 kms of lake.

Shaded areas highlight time periods when fire events were recorded at both lakes. Statistical analyses of rates of change reflect the rapid transition from native trees to grasses and bracken, evident in changes in pollen percentages Fig. Dissimilarity between pre-human vegetation assemblages and post-IBP vegetation taxa increased from present to the pre-IBP period at both sites but was much more pronounced at Lake Kirkpatrick where dramatic vegetation transitions from forest to open shrublands persisted until present.

Changes in vegetation assemblages at Dukes Tarn are characterized by an initial forest loss, a late recovery of native trees, followed by an increase in grasses and the presence of introduced taxa associated with European colonization. Human-caused forest transitions are documented worldwide, especially during periods when land use by dense agriculturally-based populations intensified [6] , [16] , [17]. However, the rate at which prehistoric human activities led to a persistent biome shift is commonly poorly resolved [18].

In the South Island of New Zealand, the arrival of Polynesians resulted in dramatic forest loss, with conversion of nearly half of native forests to open vegetation by a small number of sparsely distributed populations [10]. Paleoenvironmental records suggest that this conversion occurred within decades to centuries of human arrival but the speed at which site conversion took place once burning commenced has been unclear until now.

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The speed at which vegetation conversion occurred is likely even more rapid than estimates based on age-depth chronologies alone because they incorporate joint uncertainty originating from both AMS radiocarbon dating and radiocarbon calibration distributions.

Despite inherent limitations in calculating the rate of past vegetation change from lake-sediment records, forest loss in New Zealand was rapid by any measure. These rates of deforestation parallel those made for the eastern North Island where dry, lowland podocarp forests transitioned to bracken fern shrubland within decades of the first human-set fires whereas loss of more mesic, upland forests took a century or longer [19]. For most of the South Island, fire impacts on forested ecosystems were ecologically insignificant throughout the Holocene, in that changes in the structure and composition of vegetation were poorly related to the occurrence of fire [20].

The widespread and mostly one-way shift from forest to open shrubland that occurred with the arrival of Polynesians, and then following European arrival, was unprecedented in the Holocene, even after extreme disturbance events e. This was the case for Lake Kirkpatrick and many other sites throughout similarly dry forests of eastern New Zealand including the eastern North Island [21].

The higher-elevation site, Dukes Tarn, experienced equally rapid vegetation change, but declines in forest taxa at this site were less pronounced. This is most likely because this site is wetter and because the deeply-incised forested drainage basin south of the tarn has provided an ongoing refuge from fire, contributing significant wind-transported pollen to the tarn sediments. Differences in the extent to which fire activity and vegetation changed following human arrival at Lake Kirkpatrick and Dukes Tarn may be explained by variation in the intensity of human activity at these two sites; the most parsimonious explanation, however, centers on differences in rainfall and inferred fuel moisture at these two sites and is supported by previous results comparing a network of sites from the South Island [9].

Dynamic Global Vegetation Models that estimate rates of prehistoric deforestation at a regional to global scale suggest that the extent and pattern of preindustrial deforestation in most regions was strongly linked to population densities, inherent landscape productivity and lifestyles and technological innovation [16] , [17]. Estimates of population density and reconstructions of environmental change in these models are derived from archeological and paleoenvironmental records, and in most regions, rates of deforestation are poorly resolved.

Our high-resolution chronology of environmental change from the South Island, New Zealand indicates rapid forest transitions occurred during the IBP during a time of exceedingly low population numbers and limited use of technological innovation i. Why then were New Zealand's forests particularly vulnerable to fire?

It was only when human-set fires expanded the proportion of flammable early-seral vegetation on the landscape that widespread forest transitions were possible [5]. Positive feedbacks between the fire sensitivity of the vegetation, introduction of human-set ignitions, and changes in landscape flammability with the spread of early-seral vegetation offer an explanation for these rapid transitions. Perry et al. The model estimated that conversion of the forest to shrubland could have occurred in 50— years if fires were targeted both spatially in highly-flammable early-seral vegetation and temporally every few decades to maintain early-seral vegetation.

Forest transitions occurring at present in a number of regions around the world bring new attention to conditions and feedbacks that drive rapid transitions between seemingly stable states [25] — [28]. Analyzing global tree cover and fire activity data, Staver et al. Hirota et al. A number of mechanisms create positive feedbacks that trigger and maintain these multiple stable states.

Landscapes with high canopy cover of mature forests are resilient to increased fire activity because they maintain fuel moisture levels that suppress fire and often lack continuous understory fuels for carrying fire [3].

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Dry landscapes reinforce open conditions by supporting low fuel moisture and species with plant functional traits that promote fire. This has been shown to be particularly true for forests where flammability decreases with forest age. Kitzberger et al. The New Zealand example suggests that targeted ignitions alone where sufficient to initiate feedbacks promoting high landscape flammability and increased fire activity. Today, the vulnerability of temperate forests to rapid transitions is evident in a number of regions. For example, alterations to fuel and microclimate conditions following logging activities in evergreen forests of the northwestern US increase post-fire flammability, ultimately increasing fire severity such that forest recovery is delayed or non-existent [31] , [32].

Lindenmayer et al. While fire occurred in these forests in the past, it was typically infrequent, and eventually followed by conditions that promoted robust recruitment of mountain ash seedlings. Likewise, Cochrane et al. In southern South America, the combination of increased anthropogenic fires and herbivory of native seedlings by introduced species is rapidly facilitating the replacement of mesic Nothofagus forests with fire-prone shrubs [34].

Lacking fire, these forests would likely recover but Perry et al. These recent forest transitions demonstrate the strong influence of human-initiated positive feedbacks that increase landscape flammability and highlight the inherent vulnerability of seemingly stable temperate forests to rapid biome and fire regime shifts. Widespread forest transitions are predicted for the future in regions with similar age-flammability and low-ignition characteristics [36].

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The timing of an abrupt change from forest to open shrubland following human-set fires at two sites in the South Island of New Zealand is well-constrained in sediment cores using high-resolution chronology models, suggesting widespread deforestation occurred within decades of the first anthropogenic fires in the most vulnerable drier regions.

The fire history of New Zealand is an example of how a seemingly stable ecosystem can pass natural tipping points and experience persistent transformations. The rate at which anthropogenic forest loss occurred was rapid, taking place within decades following the initial arrival of a small founding population. The rapidity of the response was partly facilitated by positive feedbacks created by the introduction of a new and frequent ignition source by humans.

In addition, the dominant forest taxa lacked natural adaptations to fire e. The newly created fire-prone landscapes persisted even when fire activity decreased after the IBP. As with temperate forests experiencing similar transitions today, simply removing fire from New Zealand landscapes could eventually lead to forest recovery. However, recent examples suggest that a confluence of factors that degrade biophysical conditions, impede seedling regeneration, remove source populations and dispersal agents and introduce fire-adapted weedy species will continue to hinder native forest recovery in New Zealand and elsewhere.

Even in the absence of fire, management of landscapes to promote open vegetation, agriculture, and exotic forestry plantations promises to reinforce conditions that prevent regeneration of native species. The rapid rate of forest transitions witnessed in New Zealand years ago portends potentially rapid transitions in biomass-rich regions experiencing similar land-use-disturbance interactions and feedbacks and highlights challenges to native forest recovery. The study regions lie in the southeastern hill country and southern alpine region of the South Island of New Zealand, an area that ranges between and m elevation Fig.

The climate is cool year-round and moderately wet with most of the precipitation occurring in winter. During the Holocene the vegetation of the region was dominated by closed-canopy broadleaf evergreen forests [20]. Natural fires occurred in areas of forest in the dry southeastern region of New Zealand but were characterized by long intervals between fires [37]. The original forest extent and composition are inferred from pollen records, isolated surviving forest stands and remnant wood [38] , [39].

Lake Kirkpatrick lies within the Lake Wakatipu catchment. Dukes Tarn is approximately 9. Phyllocladus, Dracophyllum longifolium, Hebe spp. Deeply-incised drainages supporting native forest remnants border Dukes Tarn to the south whereas open vegetation surrounds the tarn on rolling terrain to the north.

Lake sediment cores were obtained from Lake Kirkpatrick and Dukes Tarn using a polycarbonate tube Klien core. Cores were split at University of Minnesota's LacCore facility for charcoal and pollen analysis. A cm sediment core was retrieved from Lake Kirkpatrick in and a cm sediment core was retrieved from Dukes Tarn in Core lithology for Lake Kirkpatrick consisted of light green gyttja 0—27 cm depth below surface , gray silty clay 27—29 cm, dark green gyttja 29— cm, light gray silty clay — cm, and light green gyttja — cm.

Core lithology for Dukes Tarn consisted of light green gyttja 0—42 cm, silty clay 42—43 cm, light green gyttja 43— cm, dark silty clay with charcoal — cm, and light green gyttja — cm. Pollen analysis followed the preparation methods of Moore et al.

Pollen counts exceeded terrestrial pollen grains and bracken Pteridium spores. Percentage calculations were based on this terrestrial sum excluding other ground ferns and tree fern spores as they tend to be over-represented. We followed the protocol of McGlone and Wilmshurst [10] to detect initial human impact on the vegetation and grouped pollen taxa into tall forest taxa predominantly Fuscospora spp.

These groupings were used to identify the transition from closed forest to open shrubland and grassland. High-resolution charcoal analysis followed methods of Whitlock and Larsen [41].