Jm. Bullock et al., THE EFFECT OF CLIPPING ON INTERCLONAL COMPETITION IN THE GRASS HOLCUS-LANATUS - A RESPONSE-SURFACE ANALYSIS, Journal of Ecology, 82(2), 1994, pp. 259-270
1 The effect of clipping on the outcome of interclonal competition in
the clonaI grass Holcus lanatus was investigated using a response surf
ace analysis in a greenhouse pot experiment. All binary combinations o
f three clones were planted in mixtures at a range of frequencies and
tiller densities from 18.5 to 40 000 tillers m(-2). Two treatments, un
clipped or clipped, were applied in two blocks. After 10 weeks of grow
th the biomass, tiller production and plant mortality of each clone in
each treatment were measured. 2 Plant mortality was low but was incre
ased by increased density and clipping. The plant biomass and tiller n
umber data sets were fitted to a competition model in the form of a no
n-linear difference equation. In all data sets the value of r(2) was o
ver 0.99 and the biomass data sets showed non significant lack of fit
to the model. However, all the tiller number data sets showed signific
ant lack of fit to the model. This was probably due to plasticity in t
iller sizes and in tiller production rates, which also brought about d
ifferences in the parameter estimates for the two variables, biomass a
nd tiller number. 3 Parallel curve analyses showed that there were sig
nificant clonal differences in the parameter estimates for intrinsic g
rowth rates, density responses and equivalence coefficients within eac
h clipping treatment in terms of biomass gain and tiller production. 4
The parameter estimates of each clone were significantly affected by
the clipping treatments. Clipping decreased the intrinsic growth rates
and changed the density responses. In two of the three clone combinat
ions the clipping treatment had significant quantitative effects on th
e equivalence coefficient although there were no qualitative effects.
In the third clone combination the rank order of the equivalence coeff
icients was reversed by clipping. 5 The response surface analyses allo
wed the complete description of competitive interactions under all den
sities and frequencies. The outcome of interclonal competition was aff
ected by the clipping treatment and the relative densities of the comp
etitors. These results may explain the high genotypic diversity of the
study population. Environmental heterogeneity, e.g. in grazing levels
, may vary the outcome of intergenotypic competition over space and ti
me and thus allow clones to coexist.