Simple models of competition for one and two resources have been well
investigated for both nitrogen and water. Ecosystem models of nitrogen and
water, including their interactions are also well known, but competition and
ecosystem processes have rarely been considered together. Using a simple
ecosystem model of the nitrogen and water cycles including the dependence
of the nitrogen cycle on soil moisture I examine the outcome of competition
for nitrogen. This model shows that when there are species specific effects
on soil moisture, the competitive outcome for nitrogen can be coexistence
or alternative stable states.
The last century has seen widespread invasion of Californian grasslands by
annual grasses and forbs from the southern Mediterranean region. Both nitrogen
and water have been shown to be limiting in this system. The native
perennial grasses deplete the soil moisture and nitrogen to a lower level
than the invasive annual grasses, suggesting that natives should be better
competitors. Parameterising my model for a Californian grassland system
I show that in fact the invasive annual grasses are the superior competitor
for nitrogen at lower soil moisture when nitrogen competition is mediated
by soil moisture. The competitive outcome at current rainfall levels in California
is limited to competitive exclusion by the annuals or coexistence.
Climate change caused by anthropogenic emissions of CO2 is already affecting
communities and ecosystems. Global climate models predict changes in
both mean amount and distribution of precipitation. I investigate the effect
of changing precipitation distributions on both ecosystem and community
processes in nitrogen limited systems. There is wide variation in ecosystem
response to increased variation in rainfall. The outcome of N competition
was more predictable, with the best competitor at lower soil moisture being
consistently favoured with increasing rainfall variability. This might prove
another problem in restoring native grasses in Californian grasslands.