The influence of environmental factors and management on stands of Phragmites australis. I. Effects of burning, frost and insect damage on shoot density and shoot size.
The viability of a common reed, Phragmites australis, was determined in an experimental field in the Netherlands over a period of 5 years, the aim being to establish the effects of various environmental agents causing damage to the plant. Larvae of the moth Archanara geminipuncta (Haw.) consumed the inside of the upper parts of shoots during May and June. The species was most common in a wet undisturbed area, but the percentage of damaged shoots varied widely between years. It did not occur after spring burning and winter mowing treatments, because the eggs remained on the reed shoots throughout the winter and the larvae did not migrate after hatching in May. Larvae of the moth Rhizedra lutosa (Hb.) ate the inside of young shoots early in May and then attacked the rhizomes. The species pupated in the soil and could only complete its life cycle in a dry environment. It could be eliminated by burning but not by mowing of the old shoots in winter, presumably because the eggs overwintered near the shoot base or in the litter. As a result, the species occurred only in the dry undisturbed area, where it replaced A. geminipuncta in later years, and under dry-mown treatment. The rhizomatous growth habit of the reed determined the reaction to damage, which was essentially the same, regardless of the cause. Shoots whose apical meristem was killed were replaced by several thinner shoots. When early damage occurred (from burning and early ground-frost), replacement shoots arising from underground nodes were formed; late damage (from A. geminipuncta and late ground-frost) gave rise to side shoots formed on above-ground nodes. The progressive destruction of a whole shoot and the adjacent part of the rhizome by R. lutosa larvae probably led to slower formation of replacement shoots. During August and September, progressively more of the thinner and thus shorter shoots in the dense stands died prematurely. The percentage of shoots dying correlated positively with the maximal leaf area index of the stand. It is concluded that this mortality was primarily due to competition by shading. A negative correlation was found between the density and diameter of the shoots in the undamaged plants and those subjected to burning and early frost damage. The relationship followed the same pattern in the period of early emergence and after replacement of damaged shoots. It is argued that this relationship corresponds to a potentially equal yield of shoot biomass over the range of shoot diameters. This implies that damage by burning or early ground-frost gives no depression of competitive ability. On the basis of this relationship, an attempt was made to define damage to the reed stand in an ecological (as opposed to agricultural) sense. It is concluded that A. geminipuncta caused little ecological damage and that the stand could recuperate (i.e. occupy its original space) when damage was slight in some years. Attack by R. lutosa led to serious damage, resulting in an open vegetation vulnerable to invasion by other plant species.