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 w ere studied in response to mowing and fertilizer application in a fiel d experiment. Rhizome systems of 120 clonal fragments were harvested i n 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, numb er of offspring rhizomes, branching angles and the height of emerging shoots measured. Rhizomes were then washed, dried and weighed. 3 Persi stent rhizomes, i.e. rhizomes that survived more than 1 year, were sho rter and less numerous in mown plots and their dry mass was less than 50% of that in unmown plots. Thus, repeated mowing depleted the rhizom es of their stored resources and has been used as a measure to control the further spread of S. altissima. 4 In contrast, rhizomes establish ed in 1992 produced 10% more daughter rhizomes, which were also 23% lo nger in mown than in unmown plots. 5 There was little evidence that th e length, and no evidence that the number of rhizomes, was increased b y fertilizer application. Nevertheless, fertilizer application increas ed the dry mass and decreased the specific length (length per unit dry mass) of rhizomes. 6 Rhizome branching angles were not consistently a ltered 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 exp lained by adaptive growth plasticity, i.e. foraging, in the first year followed by differential survival of long rhizomes in later years. Ho wever, the greater length of newly formed rhizomes in mown plots could also have been due to increased soil temperatures caused by the remov al of standing biomass and litter, which increased irradiance at the s oil surface. This suggests that the observed growth responses of the r hizome systems to mowing and fertilizer application were due to 'passi ve growth' rather than adaptive foraging.