Co-Evolution of the Willow (Genus Salix) and Humankind with Particular Emphasis on Estuarine and Delta Systems- Juniper Publishers
Archaeology & Anthropology- Juniper Publishers
Introduction
The willow (genus Salix) thrived in the floodplain environment since the upper cretaceous period, e.g. fruiting Salix catkins were found in the Pipe Creek Valley, Dakota (Figure 1). The life history of Salicaceae
is closely related to riverine habitats. Therefore, characteristic
traits have evolved. Efficient
seed production is followed by establishment on exposed riverine
sediments and fast growth. High bending capacity and breaking resistance
make Salix shrubs and trees resilient to river currents and
waves [1], erosion and sedimentation processes [2]. Moreover, plant
parts resprout vigorously after fragmentation by physical disturbance
and flooding [3].
Ancient civilisations developed settlements along
major flows. Willows naturally occurring in floodplains were used for
dwellings along the Euphrates more than 10.000 years ago and for
construction, tool handles, hoes and ploughs during the dynasty of Ur in
Mesopotamia [4]. Willows provided flexible twigs and branches for
furniture, fences for shelter, and fish traps. Willow baskets for food,
supply and for storage may have been among the first articles
manufactured by human hunter-gatherer communities [5]. The Food and
Agriculture Organization of the United Nations FAO [4] highlights
willows as “Trees for Society and the Environment” serving humankind
since the dawn of history.
Global Distribution
Salix is a globally distributed species rich
genus that can be divided into two ecological groups, (i) non-alluvial
species of forest, rocks, wetlands with stagnant water and prostrate
shrubs of tundra and alpine habitats, and (ii) narrow-leaved alluvial
tree and large shrub species adapted to floodplains used for their
flexible shoots [6]. The largest tree Salix species belong to the widely distributed and cultivated willows of the floodplain environment (Figure 2). The white willow (Salix
alba L.) forms softwood floodplain forests throughout Europe up to
North Africa, Asia Minor and western Asia, and is worldwide planted near
human habitations and on riverbanks. The Black
willow (Salix nigra M.) forms extended forest stands along large
rivers in North America, whereas S. humboldtiana W. is the only
Salix species native to South American floodplains [7]. Salix
phylogeny is complex due to frequent interspecific hybridization.
Hybridization accompanied by polyploidy likely played a major
role in modern Salix species evolution in genetically variable
populations in disturbed floodplains and in cultivation [1].
Estuarine and Delta Systems
In South America, S. humboldtiana settles on non-tidal sandy
deposits up to where the lower Parana Delta propagates in the
Rio de la Plata estuary. Estuarine islands have been subjected
to land use change since decades and today willow-plantations
cover extended tidal freshwater wetlands [8]. Tidal freshwater
wetlands were occupied by humans since they developed in
coastal plain estuaries during the last 6.000 years. Ports were
established because these sheltered locations were the most
inland point in the estuary that could be reached by ships and
where river discharge prevents salt water intrusion, but flat
topography allows tides to determine the geomorphology and
hydrodynamics [9].
Along the US Atlantic coast, downstream located oligohaline
marshes were transferred into tidal freshwater forested
wetlands due to colonial land clearance 300-500 years ago
shifting sediment into tidal wetlands, resulting in increased
accretion. More recently, tidal freshwater forest declines due
to tidal inundation and increasing salinity from sea level rise
[10], e.g. in the Mississippi Delta where the Black willow
forms extended forest stands. Similarly, S. humboldtiana
may be substituted by some ten-thousand hectares of tidal
forest plantations consisting of hybrid-willows (S. alba and S.
babylonica L. (syn. S. matsudana Koidzumi) genotypes exist in
the Rio de la Plata Delta. Salix babylonica grows originally in
sandy river valley depression in arid-semiarid regions of China
with pendulous branches. The weeping willow is one of the most
widely used tree in horticultural and environmental application
[7] eventually due to adaptation to changing water levels.
North Sea Region
In the North Sea Region, vast tidal freshwater wetlands
containing willows still occur along the Scheldt, Belgium, the
Weser and the Elbe, Germany, in De Biesbosch and along the
Oude Maas in the Netherlands [11]. Most European TFW were
utilized by humans for similar reasons as in North America due
to the availability of water, nutrient-rich soils for agriculture and
the abundance of fish, shellfish, mammals, and plants for food
and shelter. However, humans may have also been negatively
impacted by flooding and mosquito-borne diseases [9]. Willows
extract large quantities of water due to high evapotranspiration
rates (phreatophyte-type of vegetation), and from historical
references it is reported that willow plantations in malaria
affected areas were the most effective for drying up the earth
and thus sheltered human settlements [12]. The pulverized
bark of Salix alba is even said “its having the properties of the
Peruvian bark” and similar effects on “the agues” [13]. The
“Marsh-Fever” occurred in brackish coastal and estuarine areas
in North-West Europe connected to the occupational history.
These areas were exploited by Ertebolle and Swifterbant people
since the 5th millennium BC. Subsequent sedentary communities
utilized fertile marshes and estuarine forests for pasturage and
agriculture. The coastal malaria vector Anopheles atroparvus
found hibernation chances in human settlements, stagnant
brackish water provided sites for breeding in oxbow lakes and
health risks increased in embanked polders with high-days from
1500-1750 AC [14].
Severe storm surges damaged dikes during that time. This
lead to dike reinforcement along the Elbe estuary whereas
pollard willows were planted along the dike for protection
against surge and ice-scour (Figure 3). The willow plantations
led to sediment accretion and prepared the foreland for the use as
meadows and pastures. In addition, willow rods were harvested,
used for fascines and groynes, and since 1800 until 1950 an
extended willow culture for basketry, furniture is reported for
most of the Haseldorfer Marsch [15], one of Europe`s largest
tidal freshwater wetlands. However, after a major flood in 1962 a
new constructed dikeline lead to the loss of approximately 75%
of the former TFW area in the Haseldorfer Marsch. De Biesbosch consisted of approximately 8,000 ha TFW but lost vast areas
due to the construction of a storm surge barrier. However, still
more than 1.500 ha TFW exist including bulrush (Bies = Dutch
word for bulrush; Bosch = bush) at lower and willow forests and
coppices at higher elevations [11]. Bulrush and willows enhance
sediment deposition and floodplain landform construction
controlling ecosystem changes in space and in time [2]. Thus,
creation of vegetated tidal wetlands in suitable locations for
ecosystem-based coastal defence is recently proposed as an
economic and ecologic supplement to conventional engineering
[16].
These findings suggest that the floodplain willow is not only
key resource and develops floor for life but evolves valuable and
characteristic traits due to human cultivation what leads to the
call “Make me a willow cabin at your gate” to implement tidal
willow forest plantation and restoration [3].
Acknowledgement
The author likes to thank the colleagues at Royal Netherlands
Institute for Sea Research Yerseke, NL, for fruitful discussion on
the topic and Matthias Michalczyk, Friedrich-Wilhelm University
Bonn, Department of Geography, for valuable comments that
helped to improve the manuscript. We further thank the editors
for the invitation to contribute to the journal.
I further wish to thank my colleagues in the Department
Wetland Sciences and Engineering at University of Maryland for
inviting me as a guest scientist to study the Chesapeake Bay tidal
wetlands and the Genus Salix in Maryland and Washington DC.
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