Effects of biological soil crusts and drought on emergence and survival of a Patagonian perennial grass in the Monte of Argentina

Biological soil crusts are widely distributed in arid and semiarid regions. They have an important eco- logical role, especially by modifying physical and chemical properties of soils. Biological crusts may also modify seed germination and seedling establishment. The effects vary widely according to the type of crust and the vas- cular plant species. The objective of this study was to determine the effect of moss-dominated biological soil crusts on the emergence, biomass and survival of Poa ligularis Nees ex Steud. under different irrigation regimes. We col- lected seeds of P. ligularis and biological soil crusts composed of two species of mosses： Syntrichia princeps （De Not.） Mitt and Ceratodon purpureus （Hedw.） Brid. from an area in the Monte of Argentina. The result showed that seedling emergence of P. ligularis was higher in treatments with bare soil than in soil covered by crusts, and also in those with watering to field capacity. Mean emergence time was higher in treatments with bare soil and watering to field capacity. Seedling biomass also showed significant differences between treatments. These results suggest that biological soil crusts dominated by mosses do not promote P. ligularis emergence, although they would not affect its survival.

Seed rain and soil seed bank compensatory roles on Nassella tenuis(Phil.)Barkworth seedling recruitment in ungrazed and grazed sites

In semi-arid lands,vegetation is distributed in shrub patches immersed in a less vegetated interpatch matrix.Grazing affects perennial grass seed bank through a decrease in seed rain and an increase in seed predation and soil compaction.Nevertheless,some species with anchorage mechanisms in their seeds might overcome this,such as Nassella tenuis(Phil.)Barkworth.This is an important species in grazing paddocks because it has an intermediate palatability and its relatively tolerant to grazing.These characteristics allow N.tenuis to increase its abundance in grazed sites.Our objective was to assess how grazing affects the key palatable species from seeds to seedlings:i.e.,seed rain,soil seed bank,and seedling recruitment in different microsites along a windward-leeward transect across shrub canopy.We hypothesized that:(1)the negative effects of grazing on N.tenuis fructification are reflected in its seed rain,soil seed bank,and seedling recruitment,especially in interpatches;(2)Nassella tenuis seed rain reduction,soil compaction by cattle in grazed sites,and removal of seeds by wind decrease its soil seed bank,especially in microsites exposed to the predominant wind;and(3)the decrease in N.tenuis soil seed bank and cover increase in annual species in grazed sites have negative effects on its seedling recruitment,especially in microsites exposed to predominant wind.We placed seed traps,collected soil samples,and monitored seedling recruitment in different locations around shrub canopy to address our hypotheses.Also,we established a manipulative experiment in which we sow N.tenuis seeds and followed its recruitment in different microsites.We compared the seed rain,soil seed bank,natural seedling recruitment,and sown seeds recruitment of N.tenuis between grazed and ungrazed sites.We analyzed differences between microsites along a windward-leeward transect across shrubs patches.Seed rain and soil seed bank had the same density in patches and interpatches both in ungrazed and grazed sites.But seed rain was higher,and soil seed bank was lower in ungrazed sites than in grazed sites.Almost all under-canopy microsites showed greater soil seed bank abundance and natural seedling recruitment in ungrazed sites.Sown seeds recruitment was the same between grazed and ungrazed sites,but it showed protective effects of shrubs in leeward microsites under grazed sites.As a conclusion,seed rain and soil seed bank are complementary under grazed sites.

Rehabilitation of Semi-Arid Grasslands through the Perennialization of Lots by Implementing Perennial Forage Exotic Grass

Argentina is the country with the highest proportion of arid and semi-arid ecosystems in Latin America.In the rangelands of Southwestern Buenos Aires(Patagones Department),there is a clear advancement of the agricultural frontier to the detriment of the native forest in this region.Due to rainfall variation and seed acquisition,Thinopyrum ponticum is cultivated as a forage perennial crop in this region.Our objective was to evaluate the performance of T.ponticum as a facilitating crop for the medium-term rehabilitation of natural grasslands in semi-arid areas.The working hypotheses were that:1)native perennial grass cover increases over the years and 2)diversity and specific richness of the vegetation are enhanced by the duration of Tall Wheatgrass implantation.Data were collected from commercial plots where T.ponticum was shown:recent implantation(5–8 years,RI);medium implantation(13–15 years,MI);and old implantation(20–22 years,OI).Thirty-four species were identified and classified into seven functional groups:Annual grasses,annual herbs,perennial herbs,exotic perennial herbs,perennial forage grasses,exotic perennial forage grasses,perennial forage exotic grasses,and nonforage perennial grasses.Thinopyrum ponticum’s total cover was between RI and OI.Total cover,species richness,and Shannon-Weaver diversity index showed no differences among treatments.Perennial forage grasses exhibited higher cover values in sites with greater implantation age and annual grasses showed the opposite response.Our results indicated that T.ponticum does not invade the sampled plots and enhances the colonization of the planted plots by perennial forage native species.However,even though the herbaceous cover had been recovered,the woody layer which could provide environmental services and specific values for conservation was not.