Niche partitioning overrides interspecific competition to determine plant species distributions along a nutrient gradient

Changes in some combination of niche availability, niche overlap and the
strength of interspecific interactions are thought to drive changes in
plant composition along resource gradients. However, because these
processes are difficult to measure in the field, their relative importance
in driving compositional change in plant communities remains unclear. In
an Australian temperate grassland, we added seeds of three native and
three exotic grasses to 1,875 experimental plots in a way that allowed us
to simultaneously estimate niche availability, niche overlap and the
strength of pairwise interspecific interactions along a gradient of
nutrient availability, obtained by adding 0, 5 or 20 g/m2 each of
nitrogen, phosphorous and potassium jointly to plots. Niche availability
(the proportion of microsites suitable for establishment and growth) was
generally low and did not vary in response to nutrient addition. Most
species co-occurred along the nutrient gradient by partitioning the
available niche space. Where species interacted due to niche overlap, the
abundance of one species, the native Chloris truncata, was usually
facilitated by other species, with each of the five other species
increasing the niche availability to C. truncata under at least one
nutrient treatment. Chloris truncata also competitively excluded two
species from some but not all sites they could otherwise have occupied.
These outcomes did not clearly differ across nutrient treatments. Our
results show that fine-scale spatial heterogeneity in establishment
microsites can enable species to co-occur via niche partitioning, and
competitive exclusion is rare. This finding contributes to an emerging
picture that niche partitioning is common and frequently a stronger
influence on recruitment outcomes than interspecific competition. The
importance of competition in structuring plant communities may be
overestimated if recruitment processes are overlooked.