Where wildflowers once bloomed across a
woodland landscape, now mosaics of crops and pastures dotted with
remnant trees represent the quintessential Australian countryside. Only
in the rarest and tiniest of remnants can the original understorey of
grassy Box woodlands still be seen.
We believe that it's important to restore
some large remnants across the range of grassy White and Yellow Box
woodlands to their original condition. It's important simply to augment
the very few existing undisturbed remnants of this once widespread
ecological community; it's important for the many species that survive
only in these types of remnants; and it's important for historical
reasons to show what the country was like before European settlement.
Returning the native perennial groundcover can also make an important
contribution to ameliorating salinity and erosion problems.
]However, returning a modified woodland to
what it once was is not proving easy. Much assistance has been provided
for fencing of remnants, and this is especially valuable for encouraging
tree and shrub regeneration. But the native grasses and wildflowers of
the ground layer don't seem to recover so well. Where seed banks have
been lost, some species will never return without assistance. And even
then, it seems that many of the annual and perennial weeds that have
come to dominate many sites do not disappear once grazing is removed.
|
One reason for this poor recovery may be that
some fundamental conditions (such as soil nutrient levels) in the remnant
have changed, so that secondary and introduced species are now favoured over
the original natives. If this is the case, such changes may be need to be
reversed before restoration efforts can be successful.
As a first step towards
determining effective restoration methods for grassy White and Yellow Box
woodlands, we set out to compare the topsoils of various degraded remnants
with soils of relatively pristine remnants, to see what soil changes occur
when a remnant is degraded.
We found that in ungrazed remnants, dominated by
Kangaroo Grass (Themeda australis) and Snow Tussock (Poa
sieberiana), soils were soft and well aerated, and favourable for plant
growth in most characteristics (e.g. organic matter, total nitrogen,
exchangeable cations and pH). They were low in available nitrogen,
phosphorus and sulphur. Furthermore, soils beneath trees were generally
higher in nutrients, especially in available phosphorus, total nitrogen,
organic matter and potassium, than soils from open areas within ungrazed
remnants. |
Grassy
White Box Woodland restoration trials at 'WIndermere' on the Central Western
Slopes. This site is currently dominated by Red Grass (Bothrio-chloa
macra) and introduced species. We aim to assist the re-establishment
of Kangaroo Grass (Themeda australis), Snow Grass (Poa
sieberiana) and a range of native forbs through seed and seedling
reintroduc-tions in conjunction with treatments such as hot spring burns
and sugar applications that can reduce soil nitrate levels and weed seed
banks. |
Topsoils of modified
or degraded remnants differed from those of ungrazed sites in various
ways, and these differences were related to the current plant species
composition of the remnants.
In partially
degraded remnants dominated by Purple Wire Grass (Aristida ramosa),
Wallaby Grasses (Austrodanthonia spp.) or Corkscrew Grass
(Austrostipa scabra), topsoils were relatively depleted in nutrients,
more acidic and more compacted than undisturbed remnants. We do not know,
however, whether these changes are limiting the return of the original
native dominant grasses and their associated wildflowers, or whether they
simply reflect the effects of livestock grazing that simultaneously
brought about the changes to the understorey.
Topsoils of other types of remnants, dominated
by Red Grass (Bothriochloa macra) or Tall Spear Grass
(Austrostipa bigeniculata), were generally similar to open areas of
ungrazed remnants, and we suspect that these types of sites may be good
starting points for low-input restoration of woodland understorey.
Soils of remnants that
were almost entirely dominated by robust annual and biennial weeds such as
Rye Grass (Lolium rigidum), Black Oats (Avena barbata) and
Paterson's Curse (Echium plantagineum) were well aerated and
relatively high in nutrients. Surprisingly though, they were comparable
with soils beneath trees in ungrazed remnants for most soil
characteristics, including soil phosphorus. The only striking difference
between these two soils was in nitrate levels, which were extremely low in
all ungrazed remnants and high in remnants
dominated by annual weeds. Furthermore, soil nitrate increased
proportionally with weed abundance across all types of degraded remnants
in our study.
From this we suspect
that soil nitrate levels are an important driver of annual weeds in grassy
woodlands. We hope that lowering soil nitrate levels will help to reduce
annual weeds in many remnants, and are currently trying this using
techniques such as adding sugar to the soil (this temporarily ties up
available nitrogen), and hot spring burns to burn off nitrogen in lush
green annuals. At the same time we are adding seed of perennial native
grasses, which we hope will help to maintain lower soil nitrate levels
over the longer term.
We still have a lot to learn about woodland
restoration, but we hope that through studies like this one, we will gain
a better understanding of the processes associated with degradation, and
thus will be better able to enhance the diversity of remnant woodlands in
the future.
References
Prober, S. M., Lunt, I.D. and Thiele K. R.
(2002) Determining reference conditions for management and restoration of
temperate grassy woodlands: relationships among trees, topsoils and
understorey flora in little-grazed remnants. Australian Journal of
Botany 50: 687-697.
Prober, S. M., Thiele K. R. and Lunt, I.D.
(2002) Identifying ecological barriers to restoration in temperate grassy
woodlands: soil changes associated with different degradation states.
Australian Journal of Botany 50: 699-712.
This project was funded by the NSW
Environmental Trust
This article was first published in
Woodland Wanderings, Spring 2003.
