Vegetation Response to the Hydro-Climatic Changes During the Late Quaternary

1. Introduction

Climate change is most clearly reflected in vegetation, as it forms an integral and fundamental component of ecosystems that is sensitive to and governed by climatic changes. The global distribution and composition of vegetation are primarily controlled by climate (precipitation, temperature), along with soil characteristics and altitude, although both natural and anthropogenic influences also play a role [1,2,3,4,5,6,7,8,9,10,11,12]. The influence of climate on vegetation is so profound that each climatic zone is characterized by a distinct vegetation type. Understanding climate change and monsoonal variability is a defining issue of our time. Various geological, biological, historical, and archeological proxies provide evidence of past climatic changes and offer insights into potential future trends. Among biological proxies, palynology has proven to be one of the most powerful tools for reconstructing past climates through vegetation dynamics. Therefore, investigating vegetation responses to climate change and monsoonal variability during the late Quaternary is essential. Additionally, identifying the major climatic drivers that influence Indian Summer Monsoon (ISM) rainfall variability remains a critical research focus. The seven papers published in this Special Issue include palaeoecological and palaeoclimatological studies from South Asia, England, and Europe, covering the time period from the Last Glacial Maximum (LGM) to the present. These studies contribute to a broader understanding of past climate dynamics by integrating multiple global climate records.

2. Summary of This Special Issue

This Special Issue’s first paper is by Quamar et al. [13], who utilized multiple proxies—pollen, grain size, and environmental magnetism—to reconstruct vegetation history, assess contemporary climate change, and examine Indian Summer Monsoon (ISM) rainfall variability since the LGM (22,200 cal yr BP) in the central Indian Core Monsoon Zone (CMZ). The study identifies shifts in tropical deciduous forest vegetation, ranging from open formations to dense mixed forests, across five distinct phases: ca. 22,200–18,658 cal yr BP, ca. 18,658–7340 cal yr BP, ca. 7340–1961 cal yr BP, and from ca. 1961 cal yr BP to the present. These variations reflect a transition from a weak ISM during the LGM to a progressively intensified ISM. The study also highlights the influence of broader climatic drivers, with the LGM being associated with orbital variations—specifically, the precession cycle—along with factors such as albedo, Northern Hemisphere ice sheet extent, CO₂, CH₄, other greenhouse gases (e.g., water vapor), and Pacific Ocean warming. Additionally, this study captures the Holocene Climate Optimum (HCO) (ca. 7340–1960 cal yr BP), underscoring the impact of global climatic fluctuations on regional monsoon dynamics.

The paper by Srivastava et al. [14] addresses vegetation dynamics and sea level fluctuations in Southeast India over the past 3000 years (Late Holocene). High-resolution palynological analysis reveals an initial warm and humid phase between 2260 and 1560 cal yr BP, which gradually transitioned to drier conditions from 1580 to 1070 cal yr BP, aligning with the Dark Ages Cold Period (DACP). This was followed by a period of intensified summer monsoon activity between 1090 and 580 cal yr BP, corresponding to the global Medieval Warm Period (MWP). A subsequent cool and dry phase, indicating a weakened monsoon between 580 and 80 cal yr BP, was possibly linked to the Little Ice Age (LIA). From 80 cal yr BP, recent warming was noticed, which could be correlated with the Current Warm Period (CWP; 1800 CE to present). Additionally, the study employed a semi-quantitative aridity, temperature, and moisture index, based on pollen concentration variations, to identify centennial-scale climatic cycles.

Tiwari et al. [15] reconstructed Late Holocene paleolimnological changes in the Arookutty region of the Vembanad wetland, Kerala, India, using diatoms, palynofacies, and grain size analysis, with an emphasis on distinguishing natural influences from anthropogenic impacts. The study identified four distinct depositional phases: ca. 500 BCE to ca. 400 CE, corresponding with the Roman Warm Period (RWP), ca. 450 BCE to ca. 350 BCE, ca. 350 BCE to ca. 50 CE, and ca. 50 CE to ca. 400 CE. Despite the overall dominance of the RWP, the study highlights an extended period of reduced monsoonal activity in the coastal region during a specific interval.

Stivrins and Jarmakovika’s [16] study investigated the role of geological and soil factors in the stability of pine (Pinus), birch (Betula), and alder (Alnus) forests during Holocene climate change in Central Latvia, Northeastern Europe. They reconstructed the vegetation history of the coastal region around Lake Lilaste (Central Latvia) using pollen analysis and radiocarbon dating to assess the impact of climate change on forest composition. The findings reveal that dominant tree species, particularly pine, exhibit remarkable resilience despite significant climatic fluctuations. Pine’s sustained dominance is likely attributed to its adaptation to the sandy, nutrient-poor soils surrounding the lake, while climate change appears to have a more pronounced effect on overall tree biomass. The study further suggests that while future climate variability may pose challenges to vegetation stability, the region’s geological and soil conditions continue to support pine, birch, and alder populations. Human impact, however, has been more prominent in areas farther from the lake. This research highlights the importance of long-term forest dynamics and emphasizes the need to consider soil, geological, and geographical factors in climate change assessments.

For their paper, Nag et al. [17] utilized pollen to reconstruct the Mid-to-Late Holocene vegetation dynamics and climate change of Renuka Lake, Northwestern Himalaya, India. This study suggested the presence of tropical Sal (Shorea robusta) mixed deciduous forests between ca. 7500 and 4460 cal yr BP, indicating a warm and humid climate with a strong Indian Summer Monsoon (ISM). A shift to dry and cold conditions occurred between ca. 4460 and 3480 cal yr BP, marked by the replacement of tropical Sal-mixed deciduous forests with highland taxa, such as Pinus roxburghii and Abies pindrow. The region subsequently experienced alternating warm–humid (ca. 3480–3240, ca. 3060–2680, ca. 2480–2270 cal yr BP) and cold and dry (ca. 3240–3060, ca. 2680–2480, ca. 2270–1965 cal yr BP) phases. A period of strengthened ISM prevailed during ca. 1965–940 cal yr BP, followed by cold and dry conditions between ca. 940 and 540 cal yr BP. From ca. 540 cal yr BP to present, the appearance of moist deciduous taxa alongside dry deciduous and highland species suggests a more moderate climate in the region. These environmental reconstructions were further validated by the Earth System Palaeoclimate Simulation (ESPS) model, providing independent support for the pollen-based interpretations.

Innes and Orton [18] present a novel approach highlighting latitude as a key factor influencing vegetational development in Northeast England during the first (Preboreal) Holocene Millennium. In the North Atlantic region, the transition from the extremely cold Late Glacial Stadial (GS-1) to the temperate Holocene was abrupt, marked by a rapid temperature increase of several degrees. This climatic shift led to the replacement of low-stature, cold-tolerant Stadial vegetation by a succession of tall herbs, heath, and shrub communities, eventually giving way to Betula woodlands of varying density. Pollen records from mid-Yorkshire and the Scottish border reveal considerable differences in the rate of postglacial woodland establishment in the first Holocene millennium. In the Vale of York (mid-Yorkshire), closed Betula woodland developed rapidly, whereas in northern Northumberland, near the Scottish border, the Betula presence remained low for several centuries, with open vegetation persisting and shrub communities—primarily dominated by Juniperus—prevailing. Intermediate sites along this transect indicate a gradual transition in post-Stadial vegetation development, shaped primarily by latitude and altitude. These findings suggest that shifts in vegetation communities occurred progressively along a south-to-north gradient between mid-Yorkshire and the Scottish border.

Using pollen, cluster analysis and the ‘coexistence approach’, Farooqui and Khan [19] reconstructed temperature and precipitation changes over the past 4.3 kyr (ka) from Kundala Lake sediments, Kerala, India. This study analyzed a 120 cm long sedimentary profile from Kundala Lake (1700 m a.m.s.l.), Palni Hills, Kerala, India, investigate climate–vegetation equilibrium over the last four millennia, producing a comprehensive paleovegetation record for the region. The presence of evergreen vegetation between 4.3 and 3.4 ka suggests relics of Middle Holocene vegetation that thrived under a warmer climate. Subsequently, herbaceous and shrub taxa dominated between 3.4 and 2.3 ka and 2.3 and 0.87 ka. A relative increase in the arboreal taxa along with herbaceous taxa was again observed between 0.87 and 0.12 ka. From 1820 CE to present, a few new plant taxa appeared. Based on the ‘coexistence approach’, the Mean Annual Temperature (MAT) was estimated at 22 °C, 15 °C, 15 °C, 20 °C, and 22 °C, respectively, for different phases, whereas the Mean Annual Precipitation (MAP) was 2660 ± 3700 mm between ca. 4.3 and 0.12 ka, decreasing to ~1750 mm from 3.4 to 2.3 ka. Pollen evidence also indicates short-term cooler spells during the 16th–17th centuries CE, aligning with globally documented periods of cooler and arid conditions that began around ca. 5.0–4.0 ka.