Evidence for consistent trait-habitat relations in two closely related violets of contiguous habitat types from a fertilisation experiment

Abstract

Most studies on the adaptive significance of plant traits compared species that differed in inherent growth rate or species of habitats at the extremes of environmental gradients. In the present paper, we compared the response of Viola elatior and V. pumila , two closely related and morphologically similar violets to the experimental variation of nitrogen availability. The species occur in adjacent habitat types, viz. alluvial forest fringes ( V. elatior ) and floodplain meadows ( V. pumila ), that differ in resource availability and disturbance across a relatively small gradient. We tested the hypotheses that, according to their typical habitat, V. elatior should show more traits of a stress tolerant competitive plant, while V. pumila should exhibit more ruderal traits. We carried out a two-year common garden experiment with two levels of fertiliser addition (N5, 5 g N m −2 yr −1 ; N20, 20 g N m −2 yr −1 ), and analysed a number of traits related to relative growth rate (RGR), allocation, and nutrient use. We further did an experiment with seeds from the experimental plants to study variation in germination rate. We found considerable ontogenetic drift in the species during two years of growth. In the first year, both species showed similar mass and growth rate but these were achieved through different trait combinations. In the second year, V. elatior had a higher RGR, produced more biomass per unit nutrient and developed more leaf area per unit plant mass than V. pumila , while the latter produced thinner leaves. V. pumila had a higher capacity for nutrient acquisition, but a large proportion of the captured nutrients were used for reproduction. Germination rate was higher in V. pumila than in V. elatior , independent of the nutrient status of the mother plant, while seeds of V. elatior from N20 plants had a significantly higher germination rate (67%) than seeds from N5 plants (35%). Our data suggested that many of the analysed trait differences seem to be consistent with the requirements of the different habitat types.