Tag Archives: paleo-ecology

Regime Shifts

The long history of human-environment interactions in China

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In a recent paper, JA Dearing and colleagues (J. Paleolimnology 40: 3-31) use paleolimnological techniques to explore the long-term history of the region around Erhai Lake in Yunnan Province. Lake sediment cores (which can explain catchment vegetation, flooding, soil erosion, sediment sources and metal workings) are complemented by independent regional climate time-series from speleothems, archaeological records of human habitation, and a detailed documented environmental history. The authors integrate these data to “provide a Holocene scale record of environmental change and human–environment interactions.”

They use these data to ask:

  • “How sensitive are the studied environmental system processes to climate and human drivers of change?”
  • “Can we observe long-term trajectories of socio-environmental interactions, or periods of social collapse and recovery?”

The authors identify a number of points at which there were major changes in the human interaction with the landscape, including ~9000 cal year BP, when sediment records show a ‘human-affected environment’, ~4800 cal year BP, when major deforestation for grazing led to the extirpation of forest species and some functional units, and ~2000 cal year BP at the introduction of paddy field irrigated farming, and ~1600 cal year BP at which point surface erosion and gullying were caused by increased exploitation of mountain slopes. They go on to suggest that these records indicate several major ‘periods’ in human-environment interactions in this area:

The earliest of these cases probably represents the dispersion of the population away from the established sedentary agricultural units on alluvial fans to the more inhospitable margins of the lake and the valleys. This perhaps signifies the end of the ‘nature dominated’ phase (Messerli et al.) where society could cause significant modification of the landscape but was still vulnerable to the main risks of drought and flood (though the evidence for climate determinism is weak). In contrast, the introduction of irrigation is associated with a trend of weakening monsoon intensity, increasing numbers of centennial scale dry phases, and population growth. It represents an agrarian society in transition, using technological innovation to raise carrying capacities without increasing greatly the vulnerability to drought or flood. The third period is linked to natural population growth, inward migration and metal extraction brought about by the rise of Nanzhao/Dali as a major center”

The authors then ask at what stage of the adaptive cycle the modern Erhai socio-ecological system exists:

At Erhai, the slow processes of weathering and soil accumulation, in association with vegetation cover held fairly constant by a benign early-mid Holocene climate, were interrupted by fast processes of anthropogenic modification of vegetation. For many centuries, this concatenation of ‘slow–long’ and ‘fast–short’ processes led to a resilient land use-soil system (cf. Gunderson and Holling). But increasing perturbations led to system failure, and we can observe that the late Ming environmental crisis represents the end of the last release phase. Thus, the modern landscape may be approaching a conservation phase (K) characterised by minimum resilience.

Dearing and colleagues explore the meanings of this research for current sustainability and conclude that the main threat to the region is high magnitude-low frequency flooding of the agricultural plain and low terraces, which is exacerbated by:

  1. continued use of high altitude and steep slopes for grazing and cultivation that generate high runoff from unprotected slopes and maintain active gully systems, particularly in the northern basins;
  2. reduction or poor maintenance of paddy field systems, engineered flood defences, river channels and terraces; [and]
  3. increased intensities of the summer monsoon.

This fascinating paper is an excellent example of how historical data sources can be integrated to provide a new perspective on social and ecological change over long periods of time.

Regime Shifts

Climate Change May Transform Fire Regime in Tundra

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arctic tundraPhilip Higuera and collaborators suggests that based on paleo-ecological analysis of past fire regimes, climate change could lead to abrupt shifts in tundra fire frequency as climate change vegetation shifts from herb to shrub dominated tundra.

In their article (Higuera PE, Brubaker LB, Anderson PM, Brown TA, Kennedy AT & Hu FS. 2008 Frequent fires in ancient shrub tundra: implications of paleorecords for Arctic environmental change. PLoS ONE DOI: 10.1371/journal.pone.0001744) the authors write:

… paleorecords from northcentral Alaska imply that ongoing shrub expansion and climate warming will result in greater burning within northern tundra ecosystems. The geographic extent of fire-regime changes could be quite large, as shrubs are expected to expand over the next century in both herb and low shrub tundra ecosystems, which comprise 67% of circumpolar Arctic tundra [10], [15] (Fig. 1). Over this same period, annual temperatures in the Arctic are projected to increase between 3–5°C over land, lengthening the growing season and likely decreasing effective moisture (in spite of increased summer precipitation) [8]. How long might it take for the current shrub expansion to trigger a significant change in fire frequencies? Within the chronological limitations of our records, past shrub expansion and fire-regime changes at each site occurred within a few centuries (Fig. 2). The duration of this shift is consistent with the estimated rate of shrub expansion within a large area of northern Alaska [0.4% yr−1 for ca 200,000 km2; 10]. Based on a simple logistic growth model and the assumption of a constant expansion rate, Tape et al. [10] hypothesize that the ongoing shrub expansion in this region started roughly 125 years ago and should reach 100% of the region in another 125 years. Thus, if fuels and low effective moisture are major limiting factors for tundra fires, we predict that fire frequencies will increase across modern tundra over the next several centuries.

…

Despite these uncertainties, Alaskan paleorecords provide clear precedence of shrub-dominated tundra sustaining higher fire frequencies than observed in present-day tundra. The future expansion of tundra shrubs [10], [16] coupled with decreased effective moisture [8] could thus enhance circumpolar Arctic burning and initiate feedbacks that are potentially important to the climate system. Feedbacks between increased tundra burning and climate are inherently complex [3]–[5], but studies of modern tundra fires suggest the possibility for both short- and long-term impacts from (1) increased summer soil temperatures and moisture levels from altered surface albedo and roughness [24], and (2) the release soil carbon through increased permafrost thaw depths and the consumption of the organic layer [24], [25]. Given the importance of land-atmosphere feedbacks in the Arctic [26]–[28], the precedence of a fire-prone tundra biome should motivate further research into the controls of tundra fire regimes and links between tundra burning and the climate system.

Climate driven changes in vegetation cover across the most northern land surfaces on the planet will likely result in more carbon-releasing fires, according to a study published this week in PLoS ONE. Philip Higuera, currently at Montana State University, and colleagues examined charcoal and pollen samples from Alaskan lakes, which provide a historical record of plant composition and fire frequency between 14000 and 10000 years ago. Back then, the tundra was dominated by extensive thickets of resin birch Betula glandulosa, and the warming climate is likely to see its widespread return to areas currently occupied by somewhat less flammable herbs. The mass of tangled, resin-laden twigs could turn the area into a tinderbox, with the double whammy that such fires encourage vigorous birch regrowth, making it prone to further blazes. The likely consequence is that another source of carbon dioxide will enter the scene, as vegetation and long-frozen soil go up in smoke.

via SCB’s Journal Watch Online