Phytoliths as indicators of grassland dynamics during the Holocene from lake sediments in the Ubari sand sea, Fazzan Basin, Libya

2008 ◽  
Vol 39 ◽  
pp. 29-40 ◽  
Author(s):  
Adrian Parker ◽  
Belinda Harris ◽  
Kevin White ◽  
Nick Drake
Keyword(s):  
Lake Sediments ◽  
Growing Season ◽  
Summer Season ◽  
Tree Cover ◽  
C4 Grasses ◽  
The Holocene ◽  
Phytolith Analysis ◽  
Grassland Community ◽  
C3 Grasses

AbstractThis paper presents the first study of phytoliths from the Saharan region. Phytolith analysis shows the potential of reconstructing grassland dynamics during the Holocene, refining our knowledge of palaeoecosystems in the Ubari sand sea, Libya. The lake sediments studied range in age between 3,273–9,440 cal. BP. Tree cover was very low around 3,273–3,436 cal. BP and 6,678–6,796 cal. BP, and the landscape was dominated by C4 grasslands around 6,678–6,796 cal. BP, shifting to a more mixed C3-C4 grassland community around 3,273–3,436 cal. BP. These differences may be caused by the older sample being deposited during the onset of a brief arid period, but a similar phytolith assemblage could arise due to a more pronounced seasonality in the climate, with C3 grasses dominating late and early in the growing season when the climate is cooler and less arid, and C4 grasses becoming dominant in the hot, dry summer season.

scholarly journals A late Quaternary record of eolian silt deposition in a maar lake, St. Michael Island, western Alaska

2003 ◽  
Vol 60 (1) ◽  
pp. 110-122 ◽  
Author(s):  
Daniel R. Muhs ◽  
Thomas A. Ager ◽  
Josh Been ◽  
J. Platt Bradbury ◽  
Walter E. Dean
Keyword(s):  
Lake Sediments ◽  
Late Quaternary ◽  
Geochemical Data ◽  
Unexpected Finding ◽  
The Holocene ◽  
Tundra Vegetation ◽  
Shrub Tundra ◽  
Loess Deposition ◽  
Maar Lake ◽  
Glacial Time

AbstractRecent stratigraphic studies in central Alaska have yielded the unexpected finding that there is little evidence for full-glacial (late Wisconsin) loess deposition. Because the loess record of western Alaska is poorly exposed and not well known, we analyzed a core from Zagoskin Lake, a maar lake on St. Michael Island, to determine if a full-glacial eolian record could be found in that region. Particle size and geochemical data indicate that the mineral fraction of the lake sediments is not derived from the local basalt and is probably eolian. Silt deposition took place from at least the latter part of the mid-Wisconsin interstadial period through the Holocene, based on radiocarbon dating. Based on the locations of likely loess sources, eolian silt in western Alaska was probably deflated by northeasterly winds from glaciofluvial sediments. If last-glacial winds that deposited loess were indeed from the northeast, this reconstruction is in conflict with a model-derived reconstruction of paleowinds in Alaska. Mass accumulation rates in Zagoskin Lake were higher during the Pleistocene than during the Holocene. In addition, more eolian sediment is recorded in the lake sediments than as loess on the adjacent landscape. The thinner loess record on land may be due to the sparse, herb tundra vegetation that dominated the landscape in full-glacial time. Herb tundra would have been an inefficient loess trap compared to forest or even shrub tundra due to its low roughness height. The lack of abundant, full-glacial, eolian silt deposition in the loess stratigraphic record of central Alaska may be due, therefore, to a mimimal ability of the landscape to trap loess, rather than a lack of available eolian sediment.

Combined analysis of tree rings and MODIS images to evaluate beech forest productivity along geographic gradients

2020 ◽  
Author(s):  
Luca Di Fiore ◽  
Gianluca Piovesan ◽  
Michele Baliva ◽  
Alfredo Di Filippo
Keyword(s):  
Tree Ring ◽  
Spatial Resolution ◽  
Vegetation Indices ◽  
Growing Season ◽  
Forest Productivity ◽  
Tree Cover ◽  
Multispectral Images ◽  
Continental Scale ◽  
Tree Ring Data ◽  
The Mean

<p>Remote sensing is widely used for monitoring vegetation status and ecosystem productivity. The increasing interest in connecting satellite vegetation indices to actual forest productivity has led to explore their relationship mainly at coarse spatial resolution and continental scale. The aim of this study is to find a connection and predict tree growth using medium resolution multispectral images and tree ring data for a sample of Italian and Austrian beech forests along latitudinal and altitudinal gradients. Beech tree ring data were collected and analyzed during the last 20 years, recording tree positions with a GPS device. MODIS pre-composite 250 m 16 days images (MOD13Q1) from 2000 to 2018 were first re-projected and quality checked using the MODIS quality assessment. Vegetation indices (NDVI and EVI) were extracted within a distance of 750 meters from every site centroid. Only deciduous forests (assessed by Corine Land Cover) with a dense canopy cover (assessed by Global Forest Change tree cover) were selected. Eight different phenology metrics were calculated using a threshold method and a modified one and then correlated with tree ring data (Basal Area Increment, BAI). The overall network and the relationship between metrics were characterized first with a Principal Component Analysis (PCA), and then evaluating the mean phenology, exploring its relationship with environmental variables (elevation, temperature). Last, the model for predicting BAI at every site was calculated for the period 2000-2009 using the metrics as predictors in a multiple linear regression. Two group of metrics were identified from PCA: the first is made of metrics based on dates (named DOY, e.g. start of growing season), the second on the vegetation index values (named VALUE, e.g. peak value,). BAI was modeled using as predictors the highest correlate from each of the two groups of metrics. BAI predictions for every site were generally significant: the 61% of the sites had at least one significant predictor, with a mean R-squared of 0.55 over the 41 sampled sites. DOY metrics were significantly related to altitude and temperature. Because of the wide latitudinal gradient of the study sites, mean annual temperatures showed higher correlations than the altitude with the DOY metrics. The mean growing season was longer for warm sites at low altitude. The relation between multispectral images and beech populations actual growth at medium spatial resolution is consistent even for those sites that are in complex environmental conditions, making possible to predict the annual diameter growth.</p>

Holocene and Last Interglacial cloudiness in eastern Baffin Island, Arctic CanadaThis article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.GEOTOP Publication 2008-0027.

2008 ◽  
Vol 45 (11) ◽  
pp. 1221-1234 ◽  
Author(s):  
Bianca Fréchette ◽  
Anne de Vernal ◽  
Pierre J.H. Richard
Keyword(s):  
Air Temperature ◽  
Growing Season ◽  
Dwarf Shrub ◽  
Interglacial Period ◽  
Vegetation Changes ◽  
Baffin Island ◽  
Last Interglacial ◽  
The Holocene ◽  
Growing Season Length ◽  
Shrub Tundra

This study presents Last Interglacial and Holocene vegetation and climate changes at Fog Lake (67°11′N, 63°15′W) on eastern Baffin Island, Arctic Canada. The vegetation cover is reported as vegetation structural types (or biomes). July air temperature and sunshine during the growing season (June–July–August–September) were reconstructed from pollen assemblages using the modern analogue technique. The vegetation of the Last Interglacial period evolved from a prostrate dwarf-shrub tundra to a low- and high-shrub tundra vegetation. The succession of four Arctic biomes was distinguished from the Last Interglacial sediments, whereas only one Arctic biome was recorded in the Holocene sediments. From ca. 8300 cal. years BP to present, hemiprostrate dwarf-shrub tundra occupied the soils around Fog Lake. During the Last Interglacial, growing season sunshine was higher than during the Holocene and July air temperature was 4 to 5 °C warmer than present. A principal component analysis helped in assessing relationship between floristic gradients and climate. The major vegetation changes through the Last Interglacial and Holocene were driven by July air temperature variations, whereas the minor, or subtle, vegetation changes seem rather correlated to September sunshine. This study demonstrates that growing season sunshine conditions can be reconstructed from Arctic pollen assemblages, thus providing information on feedbacks associated with cloud cover and summer temperatures, and therefore growing season length.

scholarly journals C3 grasses have higher nutritional quality than C4 grasses under ambient and elevated atmospheric CO2

2004 ◽  
Vol 10 (9) ◽  
pp. 1565-1575 ◽  
Author(s):  
Raymond V. Barbehenn ◽  
Zhong Chen ◽  
David N. Karowe ◽  
Angela Spickard
Keyword(s):  
Nutritional Quality ◽  
Atmospheric Co2 ◽  
C4 Grasses ◽  
Elevated Atmospheric Co2 ◽  
C3 Grasses

scholarly journals Comparison of modeled and reconstructed changes in forest cover through the past 8000 years

The Holocene ◽  
2011 ◽  
Vol 21 (5) ◽  
pp. 723-734 ◽  
Author(s):  
Thomas Kleinen ◽  
Pavel Tarasov ◽  
Victor Brovkin ◽  
Andrei Andreev ◽  
Martina Stebich
Keyword(s):  
Land Surface ◽  
Forest Cover ◽  
Tree Cover ◽  
Pollen Record ◽  
Carbon Cycle Model ◽  
The Holocene ◽  
Eurasian Continent ◽  
Homogeneous Pattern ◽  
Continental Interior ◽  
Dense Tree

Reproducing the tree cover changes throughout the Holocene is a challenge for land surface–atmosphere models. Here, results of a transient Holocene simulation of the coupled climate–carbon cycle model, CLIMBER2-LPJ, driven by changes in orbital forcing, are compared with pollen data and pollen-based reconstructions for several regions of Eurasia in terms of changes in tree fraction. The decline in tree fraction in the high latitudes suggested by data and model simulations is driven by a decrease in summer temperature over the Holocene. The cooler and drier trend at the eastern side of the Eurasian continent, in Mongolia and China, also led to a decrease in tree cover in both model and data. In contrast, the Holocene trend towards a cooler climate in the continental interior (Kazakhstan) is accompanied by an increase in woody cover. There a relatively small reduction in precipitation was likely compensated by lower evapotranspiration in comparison to the monsoon-affected regions. In general the model-data comparison demonstrates that climate-driven changes during the Holocene result in a non-homogeneous pattern of tree cover change across the Eurasian continent. For the Eifel region in Germany, the model suggests a relatively moist and cool climate and dense tree cover. The Holzmaar pollen record agrees with the model for the intervals 8–3 ka and 1.7–1.3 ka BP, but suggests great reduction of the tree cover 3–2 ka and after 1.3 ka BP, when highly developed settlements and agriculture spread in the region.

scholarly journals Carbon balance of a grazed savanna grassland ecosystem in South Africa

2017 ◽  
Vol 14 (5) ◽  
pp. 1039-1054 ◽  
Author(s):  
Matti Räsänen ◽  
Mika Aurela ◽  
Ville Vakkari ◽  
Johan P. Beukes ◽  
Juha-Pekka Tuovinen ◽  
...  
Keyword(s):  
South Africa ◽  
Carbon Dioxide ◽  
Growing Season ◽  
Tree Cover ◽  
Terrestrial Vegetation ◽  
Wet Season ◽  
Night Time ◽  
Yearly Variation ◽  
Eddy Covariance Method ◽  
Soil Temperatures

Abstract. Tropical savannas and grasslands are estimated to contribute significantly to the total primary production of all terrestrial vegetation. Large parts of African savannas and grasslands are used for agriculture and cattle grazing, but the carbon flux data available from these areas are limited. This study explores carbon dioxide fluxes measured with the eddy covariance method for 3 years at a grazed savanna grassland in Welgegund, South Africa. The tree cover around the measurement site, grazed by cattle and sheep, was around 15 %. The night-time respiration was not significantly dependent on either soil moisture or soil temperature on a weekly temporal scale, whereas on an annual timescale higher respiration rates were observed when soil temperatures were higher. The carbon dioxide balances of the years 2010–2011, 2011–2012 and 2012–2013 were −85 ± 16, 67 ± 20 and 139 ± 13 gC m−2 yr−1, respectively. The yearly variation was largely determined by the changes in the early wet season fluxes (September to November) and in the mid-growing season fluxes (December to January). Early rainfall enhanced the respiratory capacity of the ecosystem throughout the year, whereas during the mid-growing season high rainfall resulted in high carbon uptake.

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