Role of Seed-banks in the Management of Semiarid Rangelands Under Grazing
Investigators (most current known information)
Effects of grazing on the seed-bank and ensuing annual vegetation are habitat-dependent. Grazing changes the overall balance among the functional groups of annual species (grasses, legumes, and forbs) in the seed-bank and in the vegetation of the Wadi, that has greater resources availability. In the less productive habitats, in contrast, grazing did not change or had a less consistent effect on the balance between the different functional groups. Thus, in this semiarid rangeland, quantitative changes in the composition of the seed-bank and vegetation due to grazing are apparently dependent on resources availability in the habitat on concomitant differences in plant size, density, and competition.
Total seed abundance in the seed-bank and total plant abundance in the ensuing vegetation were significantly (P<0.01 and 0.001) correlated in the poorer habitats, but not in the Wadi. Similar trends were found when the different functional groups were subjected to the regression analysis. This difference in correlation patterns between the Wadi and the other habitats suggests that post-germination processes (e.g. interplant competition) determine plant abundance in more productive habitats with higher availability of resources, while in poorer habitats non-dormant seed availability is the limiting factor for plant abundance. However, the relatively low explained variance (low R2) also indicates that other factors, in addition to seed availability, play a central role in determining plant abundance in the low productive habitats (e.g. spatial icroheterogeneity of soil conditions).
Interruption of grazing for 4-5 years did not increase richness and diversity in the seed-bank and in the vegetation. This can be a consequence of lack of nearby sources of other species, or of the need for a longer span of time for an increase in biodiversity. The fact that practically all annual species present in the seed-bank were also present in the vegetation, suggests that in the experiment site the seed reservoir in the soil cannot be a source of species to increase the diversity in the vegetation after the cessation of grazing. Possible sources for additional species are probably far away from the site to affect diversity in such short time scale.
It can be concluded that after a period of 4-5 years without grazing, no qualitative changes (e.g. richness, diversity) occurred in the community of annual plants but clear quantitative changes were observed in the relative balance between palatable and less-palatable species. No correlation was found between diversity indexes (Shannon, Simpson) and biomass of the sample in the different habitats. This may suggest that small changes in richness and diversity not necessarily will lead to variation in primary biomass production.
Hydrothermal models were developed for the germination of the most abundant grasses. These results will be used for the prediction of field germination under different natural combinations of temperature, soil water availability, and salinity.
Article in Journal
Osem Y., M. Sternberg, A. Perevolotsky and J. Kigel. 1999. "Species composition and abundance of annual plants in the seed-bank and in the vegetation in a semiarid region in the northern Negev as affected by sheep grazing." Ecology and Environment 5:180-189 (in Hebrew).
Osem Y., A. Perevolotski and J. Kigel. 2000. "Species diversity of annual-plant communities in a semi-arid Mediterranean region as affected by the interactive action of grazing and small-scale spatial and temporal variation of primary productivity." Presented, MEDECOS 9th international conference on Mediterranean climate ecosystems. Capetown, South Africa.