The relative share of graminoid and forb life-forms in a natural gradient of herb layer productivity

Abstract

The proportional share of graminoid and forb life-form in the herbaceous layer was investigated along a productivity gradient at Laelatu, western Estonia. With an increase in the herbaceous layer standing crop from 43.5 to 723.7 g m2 the graminoid life-form became dominant in total above-ground mass and in species number. Three hypotheses to better explain competitive ability of graminoids were tested: 1) graminoids are able to form higher foliage; 2) they are able to distribute foliage nitrogen in a more beneficial way; 3) they have better nitrogen use efficiency. 21 sample plots 50 x 50 cm were harvested. All above-ground parts of vascular plants were removed by two canopy layers. Vertical separation of layers were made according to the height of haft light interception. A species list was compiled, total and leaf masses and leaf nitrogen content of both life-forms were measured by layer. ANOVA showed that there were no significant differences in vertical distribution of foliage or foliage nitrogen between life-forms in the productivity gradient, and hypotheses 1) and 2) are not supported by our data-set. Hypothesis 3) is approved partly as the nitrogen concentration in graminoid Foliage was 20%, less than in forbs. If one supposes thai nitrogen retention time is equal in both life-forms then graminoids must have higher nitrogen use efficiency when compared to forbs. Although the influence of life-form x productivity interaction on leaf nitrogen concentration was not significant, there was a tendency that difference in leaf mass to nitrogen ratio of the two life-forms increased with increasing incident light. Thus, we can hypothesize that graminoid species dominate in high productive plots where the incident light intensity is also higher due to their better nitrogen use efficiency when compared to forb species.