A test of the compensatory continuum: fertilization increases and below-ground competition decreases the grazing tolerance of tall wormseed mustard (Erysimum strictum)

Contrary to the general expectation, the compensatory continuum hypothesis proposes that grazing may not always affect plant performance adversely. Instead, the effects may vary from negative (undercompensation) to positive (overcompensation), depending on the local availability of resources and the intensity of competition experienced by individual plants. We tested this hypothesis in a common garden experiment by growing tall wormseed mustard, Erysimum strictum, under a factorial design involving simulated grazing (0, 10, or 50% of the main stem clipped), supplemental fertilization and below-ground competition. The results supported the hypothesis. On an average, fertilization increased and competition decreased plant performance. Overcompensation was only observed among the fertilized plants growing free of competition. Simulated grazing increased seed yield 1.6 (10% clipping) and 1.4 times (50% clipping) as compared to unclipped plants when the plants were grown with fertilization and without competition. In contrast, clipping did not significantly increase seed yield in the plants grown without fertilization and/or with competition. The breakage of apical dominance provides a proximate mechanism of these regrowth responses. This is consistent with the fact that most plants (85%) had an unbranched shoot architecture in our study population. However, it is not clear why E. strictum has a relatively unbranched architecture in natural populations. We briefly discuss the alternative ecological factors - competition for light, adaptation to herbivory and optimal timing of flowering as a bet-hedging strategy in monocarpic plants - which might maintain unbranched architecture in this species.