The Biology of Coastal Sand Dunes M. Anwar Maun - Inecol

on the basis of increasing biomass attainment
under nitrogen limiting conditions: Ambrosia
artemisiifolia, Achillea millefolium, Chenopodium
album, Agropyron repens, Agrostis scabra, Poa
pratensis, Sorghastrum nutans, Schizachyrium
scoparium (= Andropogon scoparius) and Liatrus
aspera. From these data he concluded that early
successional species grew more rapidly and
acquired more nitrogen from nitrogen-poor
soils than late successional species.
Some of the perennial pioneer species such
as Ammophila breviligulata may receive some
of the nitrogen by their association with diazotrophic
bacteria (Azotobacter) in the rhizosphere
(Ralph 1978; Hassouna and Wareing
1964). Apparently the majority of nitrogen
required by A. arenaria may be acquired by
this means (Wahab 1975). However, the rhizosphere
nitrogen fixation in sand dunes has not
been independently confirmed. For example,
Holton (1980) in his studies on Ammophila arenaria
and Elymus mollis could not detect any
nitrogen fixation in sand and root samples and
was unable to culture nitrogen-fixing bacteria
from root and rhizosphere collections. How
important is nitrogen fixation to primary succession
in sand dune systems? Several studies
show that although occasional nitrogen-fixing
plant species may occur in early, mid and late
stages of succession, their contribution to the
nutrition of other co-occurring plant species
is not significant (Barbour et al. 1985; Lichter
1998), because the number of nitrogen-fixing
species occurring along a coast is very small
and distributed irregularly. The main reasons
for their absence are high energy costs by the
plant in fixing a molecule of nitrogen and a
very high requirement of other nutrients,
especially phosphorous (Sprent 1993), which
are normally in short supply during early
stages of succession in sand dunes.
Nevertheless, several nitrogen-fixing species,
Myrica gale, M. pennsylvanica, Shepherdia
canadensis, Alnus incana and A. rugosa, do
occur at later stages of succession. According
to Lichter (1998) and Olson (1958a) even the
contribution of these species to nitrogen
THE SAND DUNE ENVIRONMENT 35
accumulation is negligible and thus the gradual
build up of nitrogen in the soil depends on
the life history processes of a species (Walker
et al. 1986), acquisition of atmospheric inputs
and build up of organic matter over time.
The acquisition of nutrients by dune plants
is also facilitated by their association with
mycorrhizal fungi. The primary reason for the
increased uptake of macro- and micronu trients
is the exploitation of increased soil volume by
extra-matrical mycelium of the arbuscular
mycorrhizal (AM) fungi. The hyphae are more
efficient than plant roots in the absorption of
soil nutrients. More details are presented in
Chapter 9.
2.12.2 Phosphorous cycle
The second most important nutrient in sand
dunes is phosphorous. Unlike nitrogen, phosphorous
does not occur in the atmosphere as
a gas. The primary source of phosphorous in
sandy substrates is the weathering of min erals
such as apatite (Ca 5(PO 4) 3X where X may be
hydroxide [OH], chlorine [Cl] or flourine [F]).
Since the amount of apatite in sand is very low,
coastal sand dunes are deficient in P (Fig. 2.12).
Nevertheless, atmosphere does make a small
contribution to the overall input of phosphorous
from aerosol spray from oceans, lakes and
rivers, fine dust particles from volcanic ash,
rocks and soil, material of organic origin such
as smoke, ash, fine litter, humus, pollen grains
and combustion of fossil fuels. Estimates
of total phosphorous input from the atmosphere
range from 0.07 to 1.7 kg ha –1 year –1
(Newman 1995). This input is very small, but
it is extremely important for plants in sand
dune systems with little or no phosphorous at
the start of primary succession.
In a sand dune chronosequence along Lake
Michigan, the concentration of phosphorous
in the upper mineral soil increased to a
maximum until about 345 years but a drastic
decline occurred from 345- to 485-yearold
dunes (Lichter 1998). At 2375 years of age
the P levels were even lower than at 25 years,