Aboveground Biomass and Nutrient Dynamics of Calluna vulgaris and Molinia caerulea in a Dry Heathland

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Abstract
I tested the hypthesis that perennial plant species which replace each other when nutrient availability increases, are characterized by increasing productivity and nutrient (N and P) turnover. A two-year study was conducted in adjacent Dutch dry heathland stands dominated by the evergreen shrub Calluna vulgaris (characteristic of low-nutrient conditions) and the perennial deciduous grass Molinia caerulea, respectively. Molinia replaces Calluna when nutrient availability increases. Annual litter production (per unit of biomass) was higher in Molinia. However, aboveground nutrient turnover (annual nutrient loss per unit of biomass) of Calluna exceeded that of Molinia, because of a lower efficiency of nutrient-retranslocation from senescing plant parts. Aboveground productivity of Calluna was slightly lower or exceeded that of Molinia. Thus there is a discrepancy between the hypothesis and the results. Maximum aboveground productivity of Calluna and Molinia in Dutch heathlands is twice as high as in similar British stands. Data are presented which suggest that this is caused by higher atmospheric N-deposition levels in The Netherlands. It is concluded that the replacement of Calluna by Molinia in The Netherlands is the combined result of increased levels of nutrient availability and factors reducing the fitness of Calluna (heather beetle attacks, damage by frost).