Predicting Seed Bank Germination in Semiarid Rangelands Under Grazing
Investigators (most current known information)
Proposal Abstract
The main objectives of the project were:
- To analyze changes in the seed bank of a semiarid rangeland, as affected by year conditions, grazing and habitat (i.e. topographic) characteristics( Israel).
- To analyze the relationships between the composition of the seed bank and the ensuing annual vegetation (Israel).
- To study changed in germination potential of buried seeds as a function of seasonal climatic conditions (Israel).
- Predicting seedling emergence with a hydrothermal time model (USA).
Work in Israel
The long-term study of the changes in the seed bank of a representative semiarid rangeland in the north of the Negev (Lehavim site, 275 mm rainfall/year) initiated I 1995 in IALC project 96R-19 was continued. Effects of year climatic conditions, grazing and topographic conditions determining resource availability (mainly water and nitrogen) on the density of the germinable and total seed bank were studied. Data were analyzed with a functional trait at the plant, dispersal unit and seed levels. Relationships between the composition of the seed bank and ensuing vegetation was analyzed as a function of variation in primary productivity among habitats and years. Findings are summarized in two papers included in this report that were recently submitted for publication.
A four year study on the germination of artificial seed-banks was completed. In most annual grasses the majority of seeds germinated after the first year., with little carry over to the second year (e.g. transient seed bank). Germination fraction was much lower in annual legumes compared to annual grasses, and they produce a persistent seed bank. Initial differences in germination fraction due to date of first irrigation (i.e. rain simulation during autumn, winter and spring) were maintained along the experiment period.
Work in USA
Soil water potential from 3 resistance type sensors and from a Time-Domain Reflectometry (TDR) probe was input into a hydrothermal time model to predict germination of 7 Negev Desert and 4 Great Basin grass collections under different irrigation and drying cycles. Colman cells and Delmhorst gypsum blocks were most sensitive to soil surface drying. All sensors predicted seedling emergence of these rapidly germinating grasses when water potential was high, but incorrectly predicted emergence when water potential decreased rapidly and most species failed to emerge. Water potential sensors or soil heat and water-flow models that provide accurate seed zone water potential may be necessary to more accurately predict field germination using a hydrothermal time model.
Project Objectives
A better understanding of the dynamics of the seed bank and temporal patterns of germination is particularly important I dryland grazing systems in regions with a Mediterranean climate, where annual plants are the main biomass producers for forage; e.g. semiarid regions in Israel, annual grasslands off California. In these regions productivity and biomass quality are affected not only by the availability of the main limiting factors, namely water and minerals, but are also determined by the species composition of the annual vegetation. Variation among years in the relative abundance of different annual species can occur due to:
- Species specific changes in the size and composition of their seed-bank, resulting from the net balance between local seed production, seed rain, germination and granivory.
- Variation in seed dormancy level during burial in the soil (persistent vs. transient seed banks).
- Variation in climatic conditions, such as timing (early vs. late), amount and distribution of rain, as well as temperature at the time of rain effective for germination.
The main goal in this project was to achieve a better understanding of the relationships between the seed-bank and the ensuing annual vegetation as affected by interannual variation in climatic conditions, and to elucidate patterns of dormancy loss in response to rainfall and temperature variation. This knowledge will substantiate future models dealing with the effects of climate change on the vegetation.
Results
Work in Israel
Analysis of changes in the seed bank in a semiarid rangeland. As affected by year conditions, grazing and habitat (i.e. topographic) characteristics.
Data are summarized in the following paper submitted for publication, entitled: The effects of grazing on the seed bank of a semi-arid Mediterranean annual plant community, as related to site productivity and species functional traits.
Summary
- The effects of sheep grazing on the seed bank of a semiarid Mediterranean annual plant community were studied in the northern Negev desert of Israel, during 1995-1999. The seed bank was sampled in autumn, before the rainy season, inside and outside fenced exclosures protecting from grazing. Changes in the seed bank due to grazing exclusion were compared among four neighboring topographic sites (South- and North-facing slopes, Hilltop and Wadi (dry stream) shoulders), differing in vegetation, physical characteristics and soil resources. Primary productivity was highest in the Wadi, but did not different among the less productive site.
- Total seed bank density varied between 2700-19700 seed m-2. Topographic sites differed in their seed density under grazing as well as in the response of their seed bank to grazing exclusion. Seed bank density under grazing was lower in the South facing slope compared to the other sites. Fencing increased seed density by 78%, 51%, and 18% in the Wadi, South and North facing slopes, respectively, but had no effect in the Hilltop. significant differences in seed density among years occurred only between the driest and rainiest years in the ungrazed subplots of the Wadi and South-facing slope.
- Of the 40 most common annual species, representing 84% to 90% of total seed bank in different sites, the seed bank density of 30 species was affected by protection from grazing, with 17 increasers, 9 decreasers and 4 site-dependents (species that decrease in the Wadi, but increase in the other sites). 21 species exhibit larger or opposite changes in the high productive Wadi, compared to the three other less productive sites.
- Changes in the seed bank in response to grazing exclusion were analyzed in relation to plant, dispersal unit and seed functional traits. Changes in seed density were strongly related to size traits (categorized as large, medium, small and tiny size groups). Grazing exclusion increased seed density of the large plant group, large dispersal unit group and the large seed groups, while species with medium and small size traits showed intermediate or no responses. These trends were particularly strong in the Wadi.
- Size traits (plant, dispersal unit, seed) were strongly correlated to each other, thus making it difficult to evaluate the relative importance of each trait in the response of species to grazing and site conditions. It is suggested that the relative importance of plant size vs. seed size in the response to grazing varies along the productivity gradient.
- The relationships between other traits, i.e. dispersal mode, seed shape and germinability, and seed bank changes in response to grazing exclusion were less clear. Generally, fencing increased seed density of several telechoric species and decreased seed density of antitelechoric species. These trends were probably due to the association between dispersal mode and species size traits. Fencing increased seed density of species with round seeds and low germinability (mainly legumes), while seed density of species with elongated seeds and high germinability (mainly grasses) was generally unaffected or even decreased by fencing.
- Differential effects of grazing exclusion on the seed bank density of neighboring topographic sites were mainly due to variation in the level of resource availability among sites. In the more productive Wadi, dominance of small species under grazing is replaced by dominance of large species after fencing. It is suggested that in the less productive sites, in which average productivity was similar, differential responses of the seed bank to fencing among sites were due to spatial heterogeneity in resource availability. Higher frequency of deeper, more resourceful patches in the slopes compared to the Hilltop resulted in higher abundance of larger species in the slopes, hence increasing susceptibility to grazing in these sites.
Outcome
Articles in Journals
Osem, Y., A. Perevolotsky and J. Kigel. 2004. "Site productivity and plant size explain the response of annual species to grazing exclusion in a Mediterranean semi-arid rangeland." Journal of Ecology 92:297-309.
Osem, Y., A. Perevolotsky and J. Kigel. 2002. "Grazing effect on diversity of annual plant communities in a semi-arid rangeland: Interactions with small-scale spatial and temporal variation in primary productivity." Journal of Ecology 90:936-946.
Presentations
Kigel, J., Y. Osem and A. Perevolotsky. 2004. "Similarity between seed-bank and ensuing vegetation is productivity dependent in a semi-arid Mediterranean annual plant community." Presented, Joint session MEDECOS 10th international conference and seed ecology 2004 international meeting. Rhodos.
Kigel, J., Y. Osem and A. Perevolotski. 2002. "Species diversity in annual plant communities: Small scale interactions between productivity, grazing and plant size." Presented, Annual meeting of the ecological society of America. Tucson AZ.
Two papers related to the seed bank work have been submitted to journals and a third paper dealing with the role of the shrubs in this rangeland is under preparation.