Impact of grazing on range plant community components under arid Mediterranean climate in northern Syria

Abdoul Aziz Niane (2013). Impact of grazing on range plant community components under arid Mediterranean climate in northern Syria; Doctorate thesis, Wageningen University, Wageningen, The Netherlands, 209 pp., with references, with English and Dutch summaries Rangelands represent 70% of the semi-arid and arid Mediterranean land mass. It is a natural habitat for millions of people whose livelihood depends on animal husbandry. The revolutionary developments in the animal husbandry and veterinary medicines resulted in exponential increases in human and livestock population living on and from dry lands. To respond to population growth, urbanization and transportation means and expanded road networks, land reform and rural development policies forced nomads to adopt semi to fully sedentary lifestyles with disintegrated traditional community structures and organizational frameworks. Under these demographic changes coupled with national and international border crossing restrictions, there was an escalation in opportunistic cultivation, and excessive exploitation of the scarce and slowly renewable vegetation cover resulting in a steep decline in the primary production components of the rangeland ecosystems. In an attempt to stop and reverse the degradation process, large-scale re-vegetation programs based on transplanting and reseeding with perennial shrubs, resting and grazing management systems were devised and implemented. This study aimed to evaluate the impacts of the rehabilitation programs on the above-ground vegetation cover and soil seed bank replenishment in the Syrian rangelands. The underlying assumption of the rehabilitation program is that with a well-established perennial plant cover and proper grazing management, top soil is stabilized, soil moisture, nutrients and seed bank are replenished, organic matter is accumulated and microorganisms’ activity is promoted resulting in greater abundance, species richness and diversity of annuals. To test the above hypothesis, field and controlled environment based studies were carried out with quantitative data collection and processing on plant species abundance, richness and diversity of above-ground vegetation and soil seed bank for fully protected rotationally and continuously grazed areas of 10 rangeland sites in northern Syria for three consecutive seasons. In addition to the use of frequentist statistical approaches for species diversity estimation in the studies, the Bayesian method was explored. Moreover, the crucial issues of seed quality in re-vegetation were tackled through a study on seed viability and longevity in Salsola vermiculata L. , which is a highly palatable, well adapted and widely used species in the arid Mediterranean rangeland rehabilitation programs. The major findings are indicated below. Above ground vegetation cover The vegetation cover data analysis using ANOVA showed that overall plant density consistently declined from full protection to rotational and then continuous grazing in 9 out of the 10 sites studied, whereas the trends of change in species richness and diversity were not consistent. Pair-wise comparison showed that plant density, species richness and diversity were lowest for the overall plant population under rotational grazing in which plant density of perennial grasses was highest. This suggests that rotational grazing has reshaped the composition of the plant communities under the study areas by promoting the perennial grasses that are more arid rangeland adapted and ecosystem significant. Higher plant density under rotational grazing and similarity in species diversity under the three grazing treatments for perennial grasses imply that a longer period of rehabilitation and/or probably incorporating inter-seasonal rotational grazing and variation in herbivore types into the current intra-seasonal rotational grazing system will be required to cross the line of no return in plant community composition optimization through the prevalent arid Mediterranean rangelands rehabilitation programs. Soil seed bank assessment The soil seed bank data analysis using ANOVA showed no significant differences in the overall physical and germinable soil seed bank size and diversity along the grazing gradient. However, there was a significant grazing-by-site interaction for both and a significant grazing-by-year interaction for germinable seed bank size showing that the change in seed bank size is moderated by physical and environmental characteristics and human-induced disturbances. Continuous grazing treatments for some sites were located near agglomerations of people and animals, main roads and water points. Under such conditions the more disturbance-adapted ephemerons and non-palatable plants with limited constraints for seed setting dominated and this resulted in a larger soil seed bank under continuous compared to rotational and full protection grazing treatments. For the more human-induced disturbance distanced sites, the soil seed bank was at larger or similar under full and/or rotational compared to continuous grazing. Results from pair-wise comparisons showed a simultaneous decline and surge in physical and germinable soil seed bank size of annuals and those of perennials under the grazing treatments over sites. This suggests relative differences in root competition and gap exploitation characteristics among plant functional groups; these differences could be considered indicative to rangeland status and a guide to vary herbivores in order to maintain optimum plant species diversity in the target rangelands. Soil seed banks with no seeds of perennial grasses generated 208 seedlings m -2 of germinable soil seed bank under continuous grazing. This is probably due to seed setting failure resulting from overgrazing compensated by vegetative reproduction. The widely used phanerophytes in the rangeland rehabilitation program had a physical soil seed bank of 59.7 to 119 seed m -2 and a zero germinable one. This shows high complementarity between physical and germinable seed testing methods for rangeland monitoring. Similarity indices High Morisita-Horn and Sørensen similarities were recorded between the quadrat and point intercept measurements of the above ground vegetation and with each of physical and germinable soil seed banks. However, the similarity indices of the above ground vegetation measurements were higher with the germinable soil seed bank than with the physical soil seed bank. This suggests that the germinable soil seed bank is more suitable for monitoring arid Mediterranean rangeland than the physical soil seed bank. Correlation coefficients Based on the coefficients of determination (CDs), the variation in plant density and seed bank size accounted for a significant portion of the variations in spcies richness but not of the diversity indices. However, plant density and species diversity consistently and significantly declined during the season with the lowest mean annual precipitation showing the crucial role of precipitation in the dynamics of the yet active soil seed banks of the study areas. The CDs for the germinable soil seed bank size tested under optimum soil moisture with species richness were also significant throughout the grazing treatments reflecting the dependence of seed bank dynamics on soil moisture. For the physical soil seed bank (PSSB), CDs of its size with species richness were only significant under rotational grazing implying positive impacts of grazing management on soil seed bank replenishment. Morover, the CDs of PSSB richness with the diversity indices were only significant for Singletons but not for Shannon and Simpson. This is attributable to the fact that the Singleton index is more sensitive to rare species than Shannon and Simpson. The non-significant correlations between plant density and species diversity reflect a need to incorporate inter-seasonal rotational grazing and herbivore variation to the current intra-seasonal rotational grazing for greater plant community integrity. Phytogeographic analysis Using two above and two underground vegetation data collection methods, a total of 137 species, including 102 annuals and 35 perennials from 36 families of 11 chorotypes, were recorded. The number of species recorded were 56, 66, 68 and 98 from physical seed extraction, point intercept, quadrat and grow out test, respectively. These results showed the superiority of the growing out test over the three other methods. With its easiness and relative flexibility of application in terms of time and space, the grow out test seems to be the best method for arid Mediterranean rangelands monitoring and assessment of rehabilitation impacts in which the frequency of good rainy season is one out of four years. Conclusions  The rotational grazing component of the rangeland rehabilitation program resulted in a change in plant community composition shown by an increase in low proportional abundance perennial grasses with greater arid Mediterranean rangeland adaptation.  Continuous grazing reduced plant density but not richness and diversity, indicating that maximum diversity is not a sign of rangeland health and integrity. This also suggests that inter-seasonal rotational grazing and herbivore variation could probably improve the shaping effects of grazing on the arid rangeland rehabilitation programs.  Capturing more species and higher similarity indices with the above ground measurements, the simple and flexible, germinable soil seed bank test seems to be a good monitoring and evaluation method for arid Mediterranean rangelands. However, results from the tedious and less accurate physical seed extraction method could be crucial to capture the species with long seed dormancy.  Larger Bayesian estimates of diversity, smaller standard errors, lower p-values and more significance of differences in diversity between grazing treatments compared to the frequentist approaches were observed, thus indicatin