Fjölrit RALA - 05.12.1999, Síða 131

KevínF. O’Conngr etal.
129
Following the earlier conversion of tall tussock grasslands to short grasslands, some
changes in grassland composition and structure continued under pastoral use. Adven-
tive grasses, especially Poa pratensis, Anthhoxanthum odoratum, Holcus lanatus and
later, Agrostis capillaris, gradually increased with oversowing and grassland distur-
bance by livestock, especially in the montane and lower subalpine zones, accompa-
nied by forbs such as Crepis capillaris and Hypochaeris radicata. These adventive
herbs have generally improved the pastoral resource, but they have been less success-
ful invaders of open grasslands at higher altitude. In recent years they, along with na-
tive inter-tussock herbs, have tended to decline in the face of later-invading species of
Hieracium, especially H. pilosella, H. lepidulum and H. praealtum. In drier sectors of
the high country in particular, an important and continuing vegetation change under
continuing pastoral use has been reduction in density of tussock grasses and of inter-
tussock vegetation. Although this depletion or reduction in vegetative cover was most
dramatically shown in semi-arid central Otago in the Elymus grasslands, it has also
occurred in sub-humid climatic sectors, and even in humid sectors among Chiono-
chloa grasslands and in Festuca and Poa grasslands derived írorn them. In some of
these grasslands, especially but not exclusively on steep terrains, surface soil erosion
has occurred following exposure to erosive agents, especially soil ffost and wind.
The influence of animal behaviour and diet selection
In interpreting such changes in botanical composition of tussock grasslands, it is as
diffícult now as it was for Cockayne (1919, 1920a, 1920b, 1920c) to ignore evidence
for the differentially selective grazing pressure of sheep. Differential acceptability to
livestock may also affect the physiognomy of tall tussock grasslands. With the excep-
tion of Chionochloa macra, sometimes grazed, all the tall tussocks are unacceptable to
sheep, except as seedlings or as herbage recovering from fire. Cattle have sometimes
been used in recent years instead of buming, to open up the canopy of C. rigida on
shady slopes. Such transformations had not been achieved in a century of fire and
sheep grazing, sheep being loath to sustain grazing pressure on shady terrain, even
after buming.
When sheep diet has been determined in unimproved grasslands by direct observa-
tion or by microscopic analysis of cuticle fragments recovered from faeces, many of
the more abundant plant species in the grassland have been found to be uneaten or to
contribute very small fractions of the selected diet. For example, Poa cita and Dis-
caria toumatou have been very rarely recorded in diet, despite their abundance in
many communities. This apparentiy rejected condition likewise belongs to many of
the abundant inter-tussock piants. Conversely, in other studies, sheep’s diet has been
found to have been composed almost exclusively of plant species which occurred only
in small areas of naturally or culturally enhanced nutrition. In some other studies in
which vegetation has been examined under continued grazing as well as with grazing
excluded, differences in seedling abundance in favour of the ungrazed areas indicate
that continued grazing may be sometimes serving to arrest vegetation recovery that
might otherwise occur.
It is widelv accepted that diet selection by sheep may be affected by grazing pres-
sure, the ratio of animal demand to forage supply at a particular time. This has been
corroborated in research studies. Research has aiso shown that sheep diet may be as