6. Changes in horizontal structure of the herbaceous layer in the years 1998-2003

Jerzy Solon

Horizontal structure of the herbaceous layer over the last five years underwent insignificant changes. The number of types of sinusia decreased by two on 41 POPs, while it increased at least by two types on 54 POPs. A decline in spatial variability of sinusia by 0.2 was noted on 50 POPs, while on 61 POPs it increased by 0.2 or more (Fig. 9).

An in-depth analysis of the trend and intensity of changes points to insignificant differences between groups of plant communities. It seems that all communities from the class Vaccinio-Piceetea show slight changes in the number of siniusia when compared to the communities from the class Querco-Fagetea. More distinct decrease in the number of sinusia was noted in the communities from the class Quercetea robori-petreae, while in substitutional communities the number of sinusia increased. Slight differences in the change of the number of sinusia were not reflected in the changes of the variability index - no significant differences were found between mean values (Table 7).

In addition to the general changes in the number of sinusia, an important issue is the permanence of their occurrence on 4 metre squares located within POPs. In this case, the herbaceous layer underwent more distinct changes. Only on 34 POPs, the total similarity (reflected in the number of squares on which the type of sinusium was identical in both measurement periods) was greater than 80 squares, while on as many as 29 POPs the number of squares was lower than 20 (Fig. 9).

However, when partial similarity (when mixed sinusia with a single common element occur in both measurement periods e.g. sub-shrub - moss and grass - sub-shrub) is taken into consideration the similarity was found on more than 80 squares on 108 POPs. The number of such squares was less than 40 only on 8 POPs.

The changes in the distribution on the squares set up on 32 POPs entailed changes in the dominance of the sinusium type. Most frequent was the disappearance of the dominance of moss (11 POPs) and grass sinusia (6 POPs). An increase was noted in the number of POPs dominated by the dicotyledonous plant sinusium (by 10) and sub-shrub sinusium (by 8). Changes in the dominance of particular sinusium types were most frequent on the POPs located in oak-hornbeam stands (on 7 out of 20 POPs) and mixed coniferous stands (on 10 out of 32 POPs), less frequent - in pine stands from association Dicrano-Pinion (only on 2 out of 41 POPs).

The changes of the sinusium type resulted from changes in the abundance of a given species particularly of the dominant species such as Vaccinium myrtillus which was found on 102 POPs. On the whole, a decline in the Vaccinium myrtillus population size observed on 25 POPs, and an increase on 77 POPs was as a result of changes in the herbaceous layer vegetation that underwent in the last five years.