The effect of initial biomass in manipulative experiments on plants

Mass is often used as a measure of plant performance in manipulative experiments (Eviner et al. 2000; Shen & Harte 2000). Experimental manipulations may be very diverse: from addition of nutrients, to changing abiotic growth conditions, to removing or adding biotic components such as neighbours, herbivores, etc. But, in general, researchers try to clarify the effect of one factor by comparing the biomass of naturally growing plants (or transplants, or planted phytometers) under control and manipulated conditions. Experiments usually start with individuals that are young enough to avoid previous environmental effects, but mature enough to ensure survival and correct identification if several species are used. As a consequence, target individuals in both treatment and control have an initial mass prior to experiments. Established individuals are grown in different conditions and then harvested. The performance of target individuals is assessed by their final dry mass, and the initial value is often ignored in these analyses (Goldberg et al. 1999). However, ignoring the initial mass may conceal the treatment effects. Here we explore this problem, and show how ignoring initial biomass can cause systematic bias, particularly when experimental manipulation reduces plant performance compared with the control. Biomass accumulation during the experiment, or net increase in mass per unit time, can be described as: