Empirical series of journal h-indices: The JCR category Horticulture as a case study

Two types of series of h-indices for journals published in the field of Horticulture during the period 1998-2007 are calculated. Type I h-indices are based on yearly data, while type II h-indices use cumulative data. These h-indices are also considered in a form normalised with respect to the number of published articles. It is observed that type I h-indices, normalised or not, decrease linearly over a period of ten years. The type II series, however, is not linear in nature: it exhibits partly a concave shape. This proves that the journals (in Horticulture) do not exhibit a linear increase in h-index as argued by Hirsch in the case of life-time achievements of scientists. In the second part of the paper, an attempt is made to study the relative visibility of a journal and its change over time, based on h-indices of journals. It is shown that: the h-index over the complete period 1998-2007 of the journal Theoretical & Applied Genetics (h = 62) is much higher than that of all other journals in the field the relation between the number of publications and the type II h-index for the whole period is not an exact power law (as it would have to be if the Egghe-Rousseau model were applicable) in order to study the dynamic aspects of journal visibility, a field-relative normalised h-ratio is defined to monitor systematic changes in the field of Horticulture. Except for two journals, the Pearson correlation coefficient for yearly values of this field-relative normalised h-ratio indicates that there is no systematic change of the performance of the journals with respect to the field as a whole.