Plant foraging and rhizome growth patterns of Solidago altissima in response to mowing and fertilizer application

1 The rhizome growth patterns of the clonal plant Solidago altissima were studied in response to mowing and fertilizer application in a field experiment. Rhizome systems of 120 clonal fragments were harvested in the spring of 1993, after 6 years of mowing (1987-92) and 2 years of fertilizer application (1991-92). The results were interpreted in the light of plant foraging theory. 2 The ages of the individual rhizomes comprising each rhizome system were determined and their length, number of offspring rhizomes, branching angles and the height of emerging shoots measured. Rhizomes were then washed, dried and weighed. 3 Persistent rhizomes, i.e. rhizomes that survived more than 1 year, were shorter and less numerous in mown plots and their dry mass was less than 50% of that in unmown plots. Thus, repeated mowing depleted the rhizomes of their stored resources and has been used as a measure to control the further spread of S. altissima. 4 In contrast, rhizomes established in 1992 produced 10% more daughter rhizomes, which were also 23% longer in mown than in unmown plots. 5 There was little evidence that the length, and no evidence that the number of rhizomes, was increased by fertilizer application. Nevertheless, fertilizer application increased the dry mass and decreased the specific length (length per unit dry mass) of rhizomes. 6 Rhizome branching angles were not consistently altered by the treatments. However, there was a significant interaction, i.e. angles increased in response to fertilizer in unmown but not in mown plots. 7  The observed rhizome growth patterns (3–5) could be explained by adaptive growth plasticity, i.e. foraging, in the first year followed by differential survival of long rhizomes in later years. However, the greater length of newly formed rhizomes in mown plots could also have been due to increased soil temperatures caused by the removal of standing biomass and litter, which increased irradiance at the soil surface. This suggests that the observed growth responses of the rhizome systems to mowing and fertilizer application were due to ‘passive growth’ rather than adaptive foraging.