1 The spatial pattern of vegetation heterogeneity across different scales m
ay indicate the major environmental controls on vegetation structure. and t
hus guide strategies for the conservation of biodiversity.
2 We analysed cross-scale vegetation patterns in a 90000-km(2) area of natu
ral grasslands in the Flooding Pampa, Argentina. We assessed the contributi
on of regional (latitudinal) and landscape (topographic) patterns of specie
s turnover to overall heterogeneity using data from 749 censuses.
3 A large proportion of the variation in species composition across the ent
ire region was observed at very fine spatial scales (0.1-10 km(2)), associa
ted with subtle topographic features and soil salinity gradients. Latitudin
al variation played a secondary role.
4 Species turnover among stands occupying different landscape positions at
the same latitude was 50% greater than among inventories encompassing two d
egrees of latitude. The fine-grain heterogeneity determined that an area of
10 km(2) was often sufficient to include 50% of all vascular plant species
of the region.
5 Although a large proportion (nearly 70%) of the vascular flora was compos
ed of rare (satellite) species that occurred in less than 10% of the sample
s, few core species (i.e. those occurring in more than 90% of the samples)
were seen at any scale of analysis.
6 Latitude contributed most clearly to variation in species composition amo
ng the zonal communities located in well-drained soils where differences in
relative cover of C-3 and C-4 grasses were seen. However, photosynthetic p
athways still varied more along salinity or topographic gradients than acro
ss regions. A latitudinal pattern in abundance of the different Poaceae tri
bes was also consistent with their climatic classification.
7 Alpha diversity showed a threefold variation among different stands withi
n a landscape: it increased from low topographic positions and high soil sa
linity to high topographic positions and low salinity. However, it was cons
tant among surveys within the region.
8 Nearly 25% of the species were exotic, mostly European, whose invasion wa
s promoted by livestock grazing. Compared with the native species, exotics
were consistently enriched in annuals., particularly forbs, across latitude
, and were less tightly associated with landscape heterogeneity than native