Heidrun Huber á Josef F. Stuefer Shade-induced changes in the branching pattern of a stoloniferous herb: functional response or allometric effect? Received: 16 August 1996 / Accepted: December 1996 Abstract Shade-induced changes in the branching pat- tern of clonal plants can lead to conspicuous modi®ca- tions of their growth form and architecture. It has been hypothesized that reduced branching in shade may be an adaptive trait, enabling clonal plants to escape from unfavourable patches in a heterogeneous environment by allocating resources preferentially to the growth of the main axis (i.e. linear expansion), rather than to local proliferation by branching. However, such an adap- tionist interpretation may be unjusti®ed if (1) branching frequency is a function of the ontogenetic stage of plants, and if (2) shading slows down the ontogenetic development of plants, thereby delaying branch forma- tion. In this case, architectural dierences between sun- and shade-grown individuals, harvested at the same chronological age, may not represent a functional response to changes in light conditions, but may be a by- product of eects of shade on the rate of plant devel- opment. To distinguish between these two alternatives, individuals of the stoloniferous herb Potentilla reptans were subjected to three experimental light conditions: a control treatment providing full daylight, and two shade treatments: neutral shade (13% of ambient PPFD; no changes in light spectral composition) and simulated canopy shade (13% PPFD and a reduced red:far-red ratio). Plant development was followed throughout the experiment by daily monitoring primary stolon growth as well as branch and leaf initiation. Biomass and clonal ospring production were measured when plants were harvested. At the end of the experiment shaded plants had produced signi®cantly fewer branches than clones grown in full daylight. In all three treatments, however, initiation of secondary stolons occurred at the same developmental stage of individual ramets. Shading signi®cantly slowed down the ontogenetic development of plants and this resulted in the observed dierences in branching patterns between sun- and shade-grown in- dividuals, when compared at the same chronological age. These results hence provide evidence that shade- induced changes in the branching pattern of clonal plants can be due to purely allometric eects. Implica- tions for interpreting architectural changes in terms of functional shade-avoidance responses are discussed. Key words Allometry á Architectural plasticity á Clonal growth á Plant development á Plastochron Introduction Sun- and shade-grown individuals of many plant species may show visually conspicuous dierences in architec- ture (e.g. Warming 1909), caused by environmentally induced changes in branching patterns, internode and petiole lengths and patterns of biomass allocation (Corre 1983 a, b; Slade and Hutchings 1987; Evans 1992; Schmitt and Wul 1993). Such plasticity has often been interpreted as a functional response to changes in the environment, enabling plants to maximise their perfor- mance in spatially and temporally variable habitats. A number of concepts and hypotheses have been formu- lated to explain the proximal as well as the ultimate causes and consequences of plastic changes in plant ar- chitecture in response to shading (Grime 1979; Harper 1985; Grime et al. 1986; Hutchings 1988; Casal and Smith 1989; Ballare 1994; Hutchings and de Kroon 1994). For stoloniferous herbs, the adoption of a more lin- ear growth form by reduced branching under shaded conditions has been interpreted as part of an avoidance strategy enabling horizontally spreading clones to escape from unfavourable resource conditions by giving prior- ity to the extension of the main axis rather than to resource investments into local proliferation. In other Oecologia (1997) 110:478±486 Ó Springer-Verlag 1997 H. Huber (&) á J.F. Stuefer Department of Plant Ecology and Evolutionary Biology, Utrecht University, P.O. Box 800.84, NL-3508 TB Utrecht, The Netherlands fax:+31-30-2518366; e-mail: h.huber@boev.biol.ruu.NL