Distribution
The species composition and growth of native grass communities
in the Darwin-Kakadu region is primarily determined by plant
available moisture. Seasonality of rainfall, drainage and flooding
regularity all impact on the distribution of native grass species.
Native legumes and forbs form only a minor part of the pasture
communities in this region although they may be locally significant
during their short growing season.
Dominant pastures
The dominant native pasture types are a mixture of fast-growing
perennial medium height grasses grading into medium and taller
species (2-3m) as the rainfall and length of growing season
increases from south to north. Local ground cover can vary
considerably in dominant species and relative density, generally in
response to soil moisture, and to a lesser degree soil fertility.
Major mid grass species include golden beard grass (Chrysopogon
fallax), whitegrass (Sehima nervosum), and black spear
grass (Heteropogon contortus) in areas with annual average
rainfall of 1100 mm or less. Kangaroo grass (Themeda
triandra) and perennial sorghum (Sorghum plumosum) occur
throughout the region, while cockatoo grass (Alloteropsis
semialata), ribbon grass (Crysopogon latifolius) and
Cymbopogon spp. are found in wetter areas. Tallgrass species
common in the north include giant spear grass (Heteropogon
triticeus) and northern cane grass (Mnesithea
rottboelloides). On deep sandy soils annual Sorghum species may
form virtually monospecific stands or mix with sparse tall
perennial species.
Rocky hilltops and sandstone country
Significant areas of the region are not subject to grazing.
These include Kakadu and Nitmiluk National Parks, smaller parks
such as Litchfield and the largely cattle-free areas of Arnhen Land
which lie within this bioregion. A large proportion of this
consists of escarpment country, rocky hilltops and deep pale sands
which support curly spinifex (Plectrachne pungens) and
Triodia spp. Where these pastures do occur on pastoral
leases, they tend to form small or rarely utilised pockets, either
due to terrain or accessibility.
Poorly drained soils
Alluvial plains of the coastal rivers, such as the Adelaide and
Mary systems, support a low to mid grass cover of annual species
which are of limited nutrient value. These include kerosene grass
(Eriachne burkitii), kangaroo grass (Themeda
triandra), Chrysopogon spp. and northern cane grass
(Mnesithea rottboelloides) on river levees and higher ground
within and adjoining the plains.
Coastal floodplains
Pasture composition of floodplain country depends mostly on wet
season flood depth.Shallower zones tend to support a variety of
annual forbs and grasses. Wild rice (Oryza spp.), native
hymenachne (Hymenachne acutigluma) and swamp rice grass
(Leersia hexandra) are found in the intermediate zone, while
native hymenachne will dominate the deepest flooding areas.
Grazing of floodplain country occurs only in the dry season, in
direct contrast to other native pastures which are at their best in
the wet.
