Vegetation responses to mid-Holocene extreme drought events and subsequent long-term drought on the southeastern Inner Mongolian Plateau, China

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Abstract

Ecological responses to climate change are strongly regulated by long-term processes, such as changes in species composition and carbon dynamics in soil; therefore, understanding and predicting these processes require long time scale studies. Based on high-resolution lake sediment records of the Angili Nuur Lake from the forest-steppe ecotone at the southeastern margin of the Asian Gobi, we found three pronounced drought events during the mid-Holocene (∼7.0–6.3 ka BP, BP means “before present”, where present means 1950 and ka = 1000 years) and a subsequent graduate drying trend. During the extreme drought events, lake aquatic system showed strong desiccation and instant salty marsh expansion; whereas regional forest showed strong resilience. During the following long-term drying at millennial scale, broadleaf forest, coniferous forest and steppe successively dominated in this region, in 6.3–4.4 ka BP, 4.0–2.7 ka BP and since 2.7 ka BP, respectively; but the changing dominance between forest and steppe did not match the climate change rate as reconstructed in previous studies, suggesting a lagged and non-proportional change of vegetation type to the range of climate change. The resilience of forest implies a strong buffering effect of biotic and abiotic factors other than climate, probably explained by diverse micro-environments of this mountainous region that allowed forest to survive during the drought spells, and also by gradual soil coarsening that retarded soil field capacity change and nutrient loss. However, under accumulative drying trend, forest was in general replaced by steppe around 2.0 ka BP.

Highlights

► Three dry spells occurred during the Holocene Megathermal in the semi-arid region of China. ► Salty marsh are sensitive to these dry spells whereas regional vegetation not. ► Changing dominance between forest and steppe did not exactly match the climate trends.

Introduction

In the context of recent global warming and drought intensifying (Solomon et al., 2008, Dai, 2011), the understanding of terrestrial ecosystem-climate interactions under rapid climate change is undoubtedly crucial to both scientific research and public (IPCC, 2007). Drought-related ecological degradation, including forest dieback (Breshears et al., 2005, Allen et al., 2010), grassland desertification (Schlesinger et al., 1990, Yang et al., 2005), wetland degradation and lake desiccation (Zhao et al., 2011, Lake, 2011) have been widely reported, especially in the semi-arid regions; however, these studies have mostly been restricted to annual to decadal scales, which might fail to cover the climate change range that current global change is likely to bring (Held, 2012). Moreover, the relative long lifespan of trees and long-term processes of forest regeneration and succession made it difficult to discuss vegetation responses to accumulative climate change on this relatively short time scale (Peng et al., 2011). Therefore, it is important to use palaeo-ecological records, where past droughts have been more severe and persistent than those of instrument-recorded history (Laird et al., 1996, Clark et al., 2002, Chen et al., 2003a, Chen et al., 2003b, Cook et al., 2010, Nelson et al., 2011), to study vegetation responses to climate change over a longer time scale.

Forest and steppe exhibit mosaic distribution at the southern margin of the Asian Gobi desert and surrounding steppe region (Fig. 1), where lies the drought limit of temperate forest distribution range and water availability greatly controls vegetation growth (Walter and Breckle, 2002, Wu, 1980). Meanwhile, it is also the margin of Pacific monsoon influence, with highly fluctuated inter-annual precipitation associated with changing intensity of Pacific monsoon (Clift and Plumb, 2008). Vegetation dynamics in this region is hence sensitive to climate change, especially drought (Piao et al., 2003); but how vegetation type will respond to long-term climate change remains unclear due to complicated interactions among vegetation, climate and soil, including processes in carbon and nitrogen cycling, albedo and snow cover change, dust emission and transportation, as well as aerosol concentration change (Cox et al., 2000, Ridgwell and Watson, 2002, Rotenberg and Yakir, 2010).

A number of studies have reported the middle Holocene drought spells in these region (Chen et al., 2003a, Chen et al., 2003b, Xiao et al., 2006, Xiao et al., 2009, Chen et al., 2003a, Chen et al., 2003b, Chen et al., 2008a), as well as soil development and sand accumulation cycles (Lu et al., 2005) and dust flux changes (Miao et al., 2007, An et al., 2011) during the Holocene. These drought spells provided us an opportunity to study a severe arid period and corresponding vegetation responses on a millennium time scale. However, these studies mainly worked on environment reconstruction, while vegetation responses to climate change are seldom discussed from the ecological point of view. In this study, based on well dated high-resolution lake sediment records, we found several extreme drought events in the mid-Holocene and subsequent long-term drought trend in the mid to late Holocene. Thus, we aimed at exploring following questions: (1) is this mid-Holocene drought only site-specific or of regional importance and what was the possible reason? (2) how vegetation responded to rapid extreme drought events and long-term drying trend, respectively? and (3) how other biotic or abiotic factors have influenced vegetation response to the mid-Holocene droughts?

Section snippets

Data preparation and field sampling

Sediment cores retrieved from Anguli Nuur Lake (∼41°20′N, ∼114°20′E; altitude, 1315 m.a.s.l.; mean annual precipitation (MAP), 380 mm; mean annual temperature (MAT), 3.1 °C) were selected for analysis in this study (Fig. 1, yellow dot). This inland lake lies in the woodland-steppe ecotone of north China, at the southern border of the Asian Gobi, where is also the marginal area of the Pacific monsoon influence (Wu, 1980). The 1138 cm An-S core was drilled near the lowest position of the desiccated

Indication of pollen taxon and chemical elements in the semi-arid region

Thirty-two families or genera of pollen grains were identified in total. RDA results for topsoil and lake surface pollen showed different clusters of pollen composition across vegetation types and illustrated the corresponding indication of labeled families/genera (Fig. 2a). The influences of environmental variables, including mean annual precipitation (MAP), mean annual temperature (MAT) and altitude (ALT) are statistically significant (p < 0.005), suggesting that differences in pollen

Drought events and trends as regional phenomena

Our sediment sequence of the Anguli Nuur showed strong evidence for severe drought events in the mid-Holocene and a subsequent general drying trend. The studied lake lies in the marginal region of the Pacific monsoon influence, where the Pacific monsoon intensity strongly controls summer (annual) precipitation (Clift and Plumb, 2008). The lake basin occupies a shallow depression among small rolling basalt hills with no outlet, which emphasized the influence of evaporation on water table balance

Conclusions

From our study of palaeo-ecological records in the Anguli Nuur lake, we found severe drought events during the mid-Holocene Megathermal and subsequent gradual drying trend. These kinds of warm-induced droughts are likely to have regional significance and are of highly possibility to be enhanced in the semi-arid region under future rapid climate warming. Lake aquatic system and terrestrial forest vegetation showed different sensitivities in response to drought events: lake aquatic system and

Acknowledgments

This study was granted by National Natural Science Foundation of China (No. 41071124, 31021001, 41011120251). We thank Jiangling Zhu and Ji Ren for their assistance in field work, and Dave Chandler for editing the text.

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