Arid landscape dynamics along a precipitation gradient: addressing vegetation - landscape structure - resource interactions at different time scales

E. Buis

Research output: Thesisinternal PhD, WU

Abstract

This research is entitled ‘Arid landscape dynamics along a precipitation gradient: addressing
vegetation – landscape structure – resource interactions at different time scales’ with as subtitle
‘A case study for the Northern Negev Desert of Israel’. Landscape dynamics describes the
interactions and feedbacks among landscape structure, resource flows and organisms. This study
focuses on the Northern Negev Desert of Israel, a semi-arid to arid rock desert with local loess
and sand cover. Climate and humans are important driving factors of landscape dynamics here.
Semi-arid and arid regions worldwide, are vulnerable to land degradation and desertification. A
profound knowledge of the processes in these regions can help to avert land degradation and
desertification. The objective of this thesis is to increase the knowledge on landscape dynamics
and its drivers in semi-arid and arid regions by field and model studies in the Northern Negev
Desert. This study can contribute to a sustainable future for the inhabitants of these areas.
• Chapter 1 is the introduction of this thesis and discusses among others the four studied
catchments along a precipitation gradient: Lehavim receives at average 280 mm precipitation per
year, Sayeret Shaked 200 mm yr-1, Halluqim 93 mm yr-1 and Avdat 87 mm yr-1. Of the surface of
Lehavim 53% is covered by vegetation and 15% by bedrock outcrops. The catchment is
intensively grazed by livestock. Sayeret Shaked is covered by a thick layer of homogeneous
loess. Vegetation cover is dense (62%). The catchment is taken out of grazing since 1987. In
Halluqim only 20% of the surface is covered by vegetation. The catchment is very rocky, as
bedrock crops out at 42% of the surface. Avdat, located close by, is much less rocky (7%). Here
22% of the surface is covered by vegetation. Both catchments are extensively grazed.
The thesis can be separated in three parts. In the first part the relationships between landscape
structure and vegetation in the four catchments is studied by statistical analyses. This part gives
insight in the landscape dynamics along a precipitation gradient and provides a system
framework for the remainder of the thesis. The second part focuses on simulating water and
sediment dynamics in the catchments using the landscape evolution model LAPSUS. The model
is adapted to a semi-arid and arid climate, and vegetation cover is incorporated. The interactions
between resource flows and vegetation is studied by model simulations. In the third part the
system knowledge and modelling framework are applied at a longer time scale. Firstly the history
of a valley fill is reconstructed by field observations, after which this valley fill is simulated with
LAPSUS. Additionally the effect of land use on the valley fill development is tested by model
simulations.
Part 1: System framework
• In chapter 2 the controls on functional surface cover types are studied in the four catchments
along the precipitation gradient. First, four functional surface cover types are selected, based on
their unique functionality in terms of water use and redistribution: shrubs, Asphodelus ramosus,
other herbaceous plants and surface crusts (biological and physical). Percentage of surface cover
of these functional surface cover types is estimated, and of bedrock outcrops and loose surface
stones. Additionally, data is collected on soil depth, relative elevation, insolation, slope, profile
curvature and plan curvature. Relations between functional surface cover types and landscape
structure variables are analyzed with descriptive statistics, factor analyses and linear regressions.
The landscape structure variables bedrock outcrop, relative elevation, soil depth and surface
stones explain most of the cover variance in the catchments. In catchments with many bedrock
outcrops, the occurrence of functional surface cover types is best explained by the landscape
structure variables. In catchments with homogeneous soils reaching beyond the root zone,
biological interactions between functional surface cover types are more important. Along the
precipitation gradient the explanatory power of the biological variables decreases with decreasing
precipitation, while the explanatory power of landscape structure variables appears unrelated.
Only in homogeneous semi-arid catchments can regular vegetation patterns develop, in arid and
heterogeneous catchments irregular vegetation patterns dominate.
Part 2: Model framework
• In chapter 3 the process of water redistribution at catchment scale is studied with the landscape
evolution and erosion model LAPSUS. LAPSUS, formerly applied in Mediterranean regions, is
modified to deal with the arid climate of the Northern Negev Desert of Israel. Daily model runs
are used instead of yearly model runs, and the infiltration module is adapted to better represent
the spatial diversity in water availability in an arid catchment. The model is calibrated for
Halluqim and Avdat. First, a sensitivity analysis of the modified LAPSUS was done. Especially
pore volume of the soil appears to have a strong influence on the modelling results. Second, the
capability of LAPSUS to deal with varying surface characteristics was assessed by comparing the
simulated water redistribution patterns in the two catchments with field data. Simulation results
demonstrate that the catchments respond very different to precipitation. Water redistribution is
larger in the dominantly bedrock-covered Halluqim compared to the dominantly sedimentcovered
catchment of Avdat. Consequently, Halluqim has more positions with water
accumulation than Avdat, and can sustain a larger vegetation cover including Mediterranean
species. Finally the modelled infiltration patterns are spatially compared with vegetation cover in
the catchments. The results indicate that there is a broad agreement between infiltration and
vegetation patterns, but locally there is a strong mismatch indicating that part of the involved
processes are still missing in the model.
• In chapter 4 the interactions between resource flows and vegetation is studied and simulated in
the loess-covered catchment of Sayeret Shaked. In semi-arid areas vegetation is scarce and occurs
often as individual shrubs on raised mounds. The formation process of these mounds is still
debated. In this chapter the hypothesis that shrub mounds are formed in part of the Northern
Negev Desert by erosion and sedimentation is tested. Height and diameter of shrub canopy and
shrub mounds are measured and micro-morphological techniques are used to reconstruct the
formation process of the shrub mounds. The results suggests that shrub mounds are formed by
accumulation of atmospheric dust and sedimentation of eroded material in the vicinity of the
shrub, as well as by erosion of the surrounding crust. Model simulations are done for single
events and longer time scale (100 years). In the simulations, mound formation appears most
prominent at low shrub density and large shrub canopy diameter. Positive and negative feedbacks
between shrubs and resource redistribution results in a meta-stable landscape. Long-term model
simulations of the current climate indicates that initially formation rate of shrub mounds is high,
but stabilized at lower rates. In dryer and wetter climates mound formation is unlikely to happen,
as respectively too little or too many resources are redistributed, causing a stable or highly
erosive landscape. Mound simulation with LAPSUS is successful and simulated shrub mounds
resemble the actual shrub mounds in Sayeret Shaked. Consequently the model may prove to be
valuable for the modelling of ecohydrological landscape processes in semi-arid areas.
Part 3: Long-term application
• In chapter 5 the interactions between climate change, human occupation and semi-arid
landscape dynamics are studied to increase the insight in the effect of climate change and human
land use. A Late Quaternary valley fill in the catchment of Sayeret Shaked is studied. The
aggradation and incision history is reconstructed based on a transect study. The reconstructed
valley fill is put in a temporal framework by correlation with local climate records and optically
simulated luminescence and potsherd dates. Two Late Pleistocene and four Holocene aggradation
and incision cycles are recognized, of which three in the last 2000 years. Contradictory to the
expected positive relation between amplitude of climate fluctuations and cycles of aggradation
and incision, the Late Holocene cycles are stronger than those in the Late Pleistocene and Early
to Middle Holocene. The most significant cycle coincides with the rise and fall of the Byzantine
Empire and appears related to the higher pressure on the landscape due to human occupation
during that time. Human activity appears to have a strongly amplifying effect on aggradation and
incision phases, which are initially triggered by climate fluctuations. This amplifying effect
occurs only when human occupation crosses a threshold and triggers destabilization of the
landscape. It causes collapse of the ecosystem and increases sediment redistribution.
• Chapter 6 aims to quantify the effect of humans on semi-arid catchments, by reconstructing the
infill history of Sayeret Shaked using LAPSUS. First, the infill history of Sayeret Shaked
between about 800 BC and 800 AD is simulated. Second, three land use scenarios are tested to
quantify the effect of extensive grazing, intensive grazing and intensive grazing combined with
rainfed agriculture. Especially intensive grazing combined with rainfed agriculture leads to strong
landscape dynamics. Extensive grazing causes almost no landscape dynamics, resulting in an
almost stable landscape. The results seem to indicate that this catchment is formed by coevolution
of human and natural induced processes. Rainfed agriculture leads to valley
aggradation by tillage translocation, whereas intensive livestock grazing causes gully incision by
increased slope runoff. Humans appear to be the main driven factor of landscape dynamics in this
semi-arid catchment, much more than climate fluctuations. Only a short time period of strong
human land use can irreversibly alter the development trajectory of a catchment. It is thus of high
importance to manage the land sustainable, both in the present and future, to avoid further
degradation of drylands.
• In chapter 7 the results of the different chapters are combined and the most important
conclusions discussed. The four catchments display very different landscape dynamics, caused by
a high variation in climate, land use and landscape structure. In Lehavim and Halluqim the
landscape dynamics is strongly influenced by the landscape structure, because bedrock outcrops
regulate positions for vegetation grow and stimulate water redistribution. In Sayeret Shaked water
redistribution depends mainly on biological surface cover. In Sayeret Shaked interactions
between shrub and crust patches can, under a more intensive grazing regime, lead to regular
vegetation patterns. When grazing pressure is released the herbaceous plant coverage recovers, as
is happening today. Avdat is a divers catchment, with steep rock outcrop, a flat plateau and a
loess covered wide gully. Though the whole catchment is characterized by a high aridity, each
zone experiences different landscape dynamics.
At a larger spatial scale, in the whole Northern Negev Desert, the most relevant interactions and
feedbacks between landscape structure, resource flows and organisms are related to water
availability and redistribution as well. Since the Late Holocene, the main driving factor of
landscape dynamics is human land use, especially tillage and intensive livestock grazing. Climate
fluctuations seem to have much less influence on the region. The influence of humans, even
confined in a small period in time, strongly affects landscape development in the whole Northern
Negev Desert, causing co-evolution and formation of cultural landscapes.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Veldkamp, Tom, Promotor
  • van Breemen, N., Promotor
  • Boeken, B., Co-promotor, External person
Award date2 Sep 2008
Place of PublicationS.l.
Print ISBNs9789085049463
Publication statusPublished - 2008

Keywords

  • arid lands
  • landscape
  • soil morphology
  • precipitation
  • gradients
  • dynamics
  • water resources
  • israel
  • landscape analysis
  • soil-landscape relationships

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