SUMMARYFlowering in Cynoglossum officinale L. is strongly size-dependent. The probability of flowering increases with the size of the plant. Also, a critical size exists below which the plants do not flower. Plants above this size develop flower primordia during a cold period. During winter it is determined which plants will flower in the following spring and which plants will not. The probability of flowering is not aflFected by nitrogen concentration in the plant or light intensity, when these factors are separated from their effects on the dry weight of the plant. Delay of flowering is a common phenomenon in natural populations of Cynoglossum officinale on coastal sand-dunes at Meijendel (near The Hague, The Netherlands). Over a seven-year period the percentage flowering of rosettes varied between 2 and 25 % • The proportion of rosettes flowering was positively correlated with rainfall in the previous year and was reduced after a very cold winter.
NH 4-fertilization in a natural, undamaged Calluna vegetation did not result in an immediate replacement of Calluna vulgaris by grasses. However, obvious changes were found in plant and soil composition that may lead to a greater stress sensitivity, but also to a lower regeneration capacity and a lower growth of C. vulgaris. Vegetative expansion of Molinia caerulea and Deschampsiaflexuosa was higher in a Calluna vegetation damaged by a severe frost in the winter of 1986/1987 compared to growth in undamaged plots. The higher stress sensitivity of C. vulgaris due to nitrogen deposition may accelerate opening of the vegetation canopy. Once the vegetation has been opened, the negative effects of the high nitrogen input by the lower regeneration capacity and lower plant growth may speed up even more the transition of heathland into grassland.
ores.IvHcL-t.s of \;ii-i()us dcKT-ec-s (if dc-loliatidn on Cyiioi^lo.^siiiii offiriiiiile i\nd Sciwiio jacnbaea were stuilietl untler favouriiblu Krowii-m conditions. Rf«n)wth was found in botli siiccics, but (.lid not o\ erc(>nipc-nsat(.-for defoliation. The physioloKical meebanisms responsible for re«ro\vtb after defoliation were found to relate eitber to an merease in net assimilation rate or to an inerease in relari\e in\estn-n-nt in abosesround parts. Tbe morpbolo^ieal meebanisn-i bebind re^rowtb mvobed an increase m leaf area ratio. In altniost eompletely defoliated plants, a combination of botb meebanisms was found. Tbe time needed to re-establisb tbe same amounl of biomass as control plants diHered between tbe species and between levels of defoliation. Tbese ditterenees in recovery tin-ie after defoliation are discussed in relation to decrees of berbivory found in natural situations.
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