Variation in phytolith formation in Asteraceae species in Israel: The effects of rain and grazing

Background/Question/Methods

Phytoliths are microscopic silica bodies formed in plants, which among other functions are thought to have a defensive role against herbivores.  Phytoliths are most abundant in the Poaceae and other closely related families, in which they occur in very high numbers due to seemingly unique active silica uptake, transport and deposition mechanisms.  Studies of the formation of phytoliths in Poaceae species have shown general positive dependencies on water availability and grazing intensity.  However, phytolith formation patterns in non-Poaceae species are seldom studied because of the lower phytolith concentrations.   Furthermore, most studies so far were carried on under controlled conditions, which do not necessarily accurately represent the natural reality.  We investigated the variation in phytolith concentrations in 19 Asteraceae species from Israel, in five sites with mean annual rainfalls ranging from 80 to 900 mm.  In four of these sites plants were sampled in both grazed and ungrazed plots.  The main purpose of the study was to determine whether more phytoliths are formed in these species under higher water availability and grazing intensity.  Both spiny and non-spiny species were studied in order to investigate whether phytolith formation is influenced by another physical defence.

Results/Conclusions

We have observed little consistent dependencies of phytolith concentrations per g dry matter in the Asteraceae on the studied variables.  While most non-spiny species usually form more phytoliths under higher water availability conditions, the spiny species were shown to have more diverse and complicated patterns.  There is also no consistent dependency of phytolith concentrations on grazing intensity.  In fact, in 12 out of 21 cases, plants formed more phytoliths under ungrazed conditions, in opposite to the vast majority of observations on Poaceae species.  Phytolith concentrations did not differ between spiny and non-spiny species, although the relation to water availability does show some spininess-related variation.  In conclusion, these measurements show clear differences in phytolith formation patterns between Poaceae and non-Poaceae species.  Moreover, our finds in general do not fit what could have been expected if phytoliths were an important antiherbivory defence mechanism in the Asteraceae, probably because of much lower phytolith concentrations in the Asteraceae compared to the Poaceae.