Growth and biomass allocation of shrub and grass seedlings in response to predicted changes in precipitation seasonality

Anthropogenic emissions contribute to an annual 0.5% increase in atmospheric CO₂. As global CO₂ levels increase, regional precipitation patterns will likely be altered. Our primary objective was to determine whether a reduction in summer precipitation or an increase in winter/spring precipitation, predicted by global climate change models, will favor the establishment of C₄ grasses or C₃ shrubs in southern savannas. Our secondary objective was to determine how defoliation and microsite light availability interact with altered precipitation regimes to influence grass and shrub seedling growth and biomass allocation patterns. Seedlings of 3 shrub species (Prosopis glandulosa var. glandulosa, Acacia berlandieri, and A. greggii var. wrightii) and 3 grass species (Aristida purpurea var. wrightii, Setaria texana, and Stipa leucotricha) were watered based on probable changes in precipitation in a CO₂ enriched atmosphere (0.6, 0.8, and 1.0 current ambient summer precipitation and 1.0, 1.15, and 1.30 current winter/spring precipitation). Seedlings were defoliated at 3 levels (non-defoliated, single defoliation, and repeated defoliation) within 2 levels of microsite light availability (100 and 50% ambient). Defoliation significantly reduced total shrub and grass seedling biomass. Reducing light availability decreased shrub seedling root:shoot ratio, but total biomass was not significantly affected. Grass seedling biomass and root:shoot ratio decreased when light availability was reduced. Changing the seasonality of precipitation by reducing summer rainfall or increasing winter/spring rainfall did not significantly influence growth or biomass allocation of grass and shrub seedlings in a semiarid savanna. Microsite variations in defoliation intensity and light availability influence seedling growth and biomass allocation more than changing seasonality of precipitation. Shrub and grass seedling establishment and growth on semiarid rangelands are already limited by summer precipitation, so a further reduction as proposed by climate change models will have a limited impact on seedling dynamics.     

Will increases in atmospheric CO2 affect regrowth following grazing in C4 grasses from tropical grasslands? A test with Sporobolus kentrophyllus

We grew a C₄ grass from the Serengeti ecosystem under ambient (370 ppm) and elevated (700 ppm) CO₂, and under clipped and unclipped conditions to test whether regrowth following grazing would be affected by elevated CO₂. Above-ground productivity was slightly decreased under elevated CO₂, and was similar between clipped and unclipped plants. Regrowth (clipping offtake) following clipping was similar in the two CO₂ treatments, and there was no CO₂ by clipping interaction on biomass, productivity, or leaf nutrient concentrations. Based on this evidence, we suggest that C₄ grasses from the Serengeti will show little direct response to future increases in atmospheric CO₂.     

What are the effects of substrate and grass removal on recruitment of <Emphasis Type="Italic">Acacia mellifera</Emphasis> seedlings in a semi-arid environment?

Acacia mellifera is one of the most important encroaching woody plants in southern African savannas. Previous studies found that this species encroaches far more readily on rocky areas than on sandy substrates, although it grows larger on sandy substrates. Rocky substrates are known to retain more water than sandy substrates, which may be of vital importance during recruitment in semi-arid and arid environments. A number of studies have also indicated that competition with grasses may reduce the recruitment and biomass of tree seedlings. We created an experiment in a semi-arid environment (mean annual rainfall = 388 mm) that tested for the effects of rockiness on A. mellifera recruitment. We also tested the hypothesis that grasses effectively compete with A. mellifera in this environment by simulating the effect of grazing by clipping grasses from half the plots in both the rocky and sandy treatments. Significantly more A. mellifera seedlings established in plots where grasses were clipped than in control plots. A. mellifera seedlings had greater biomass on sandy substrates than on rocky substrates. No significant interaction effects were found between substrate and grass clipping treatment for either seedling number or biomass. We conclude that A. mellifera seedlings are more likely to encroach in habitats with low grass density, although they may achieve greater biomass on sandy soils. Thus, it may be the lower grass density rather than rockiness, which increases the encroachment observed in naturally rocky habitats. These results are also consistent with our observations that adult A. mellifera trees are larger on sandy soils than on rocky soils.     

Is there a relationship between herbaceous species richness and buffel grass (Cenchrus ciliaris)?

Abstract Cenchrus ciliaris L. (buffel grass) (Poaceae) is recognized as one of Australia's most serious environmental weeds. This introduced grass has been associated with loss of native species and alteration of fire regimes. However, it is also highly valued as a pasture species for arid and semiarid zones and its weed status is highly controversial. Quantitative studies are needed to determine its ecological effects. The relationship between C. ciliaris and herbaceous species richness was investigated in two studies at a range of scales up to 64 m² in open woodlands in the Dalrymple Shire, north‐eastern Queensland. In the first study, the herbaceous species composition of sites with and without C. ciliaris were compared. Cenchrus ciliaris‐dominated sites had fewer herbaceous species than non‐C. ciliaris sites at all scales investigated and this pattern was found for the major plant groups (perennial grasses, legumes and other forbs) present. In the second study, the relationship between varying levels of C. ciliaris biomass and species richness was investigated. The relationship between varying levels of a dominant native grass, Bothriochloa ewartiana (Domin) C.E. Hubb. (Poaceae), and species richness was also determined for comparison with the C. ciliaris biomass‐richness relationship. In this study, species richness was negatively associated with increasing C. ciliaris biomass at some scales and it appeared that C. ciliaris had a greater effect on richness than B. ewartiana. The negative association between C. ciliaris and species richness is consistent with the view that invasion by C. ciliaris poses a threat to biodiversity. However, the precise cause of the relationship has yet to be determined.     

Host genotype overrides endophyte infection effects on growth,physiology, and nutrient content of a native grass,Achnatherum sibiricum

The effect of infection by the fungal endophyte Neotyphodium, host genotype, and their interaction on growth and physiology, as well as photosynthesis, was investigated in the native grass Achnatherum sibiricum. We artificially inoculated the endophyte into mature tillers of endophyte-free A. sibiricum. Plants were clipped to 5 cm height after recording growth traits, and analyzed for total nonstructural carbohydrates (TNC %), the percentage of nitrogen (N %), and carbon (C %) in leaves before and after clipping. In our study, the prominent host genotype–endophyte infection interactions detected in A. sibiricum indicates that, for many growth and storage traits, endophyte infection can impact a little change. However, there is no overriding consistently positive effect of the endophyte on growth or storage in A. sibiricum before or after clipping. Our study showed that the interaction between endophyte and host grasses was highly contingent on plant genotypes. We found host genotype overrode fungal endophyte infection in influencing tiller number and photosynthetic properties of A. sibiricum before clipping. After clipping, host genotype accounted for more of the variation in regrowth and above-ground biomass of A. sibiricum than endophyte infection. Our study showed that host genotype affected the total nonstructural carbohydrates of A. sibiricum before and after clipping, whereas endophyte infection increased the carbon content after clipping. Genotype by infection interactions for plant height, leaf mass, total nonstructural carbohydrates, and photosynthetic characteristics indicated genotype-specific effects of endophytes on A. sibiricum physiology and photosynthetic capacity. The host genotype–endophyte infection interactions detected in A. sibiricum suggest that host genotype overrides fungal endophyte infection on growth, physiology, and nutrient content of this native grass. In contrast, endophyte effects did not appear to positively affect growth, physiology, or photosynthetic capacity before or after clipping.     

Clipping defoliation and nitrogen addition shift competition between a C3 grass (Leymus chinensis) and a C4 grass (Hemarthria altissima)

• Human‐induced disturbances, including grazing and clipping, that cause defoliation are common in natural grasslands. Plant functional type differences in the ability to compensate for this tissue loss may influence interspecific competition.
• To explore the effects of different intensities of clipping and nitrogen (N) addition on compensatory growth and interspecific competition, we measured accumulated aboveground biomass (AGB), belowground biomass (BGB), tiller number, non‐structural carbohydrates concentrations and leaf gas exchange parameters in two locally co‐occurring species (the C₃ grass Leymus chinensis and the C₄ grass Hemarthria altissima) growing in monoculture and in mixture.
• For both grasses, the clipping treatment had significant impacts on the accumulated AGB, and the 40% clipping treatment had the largest effect. BGB gradually decreased with increasing defoliation intensity. Severe defoliation caused a significant increase in tiller number. Stored carbohydrates in the belowground biomass were mobilised and transported aboveground for the growth of new leaves to compensate for clipping‐induced injury. The net CO₂ assimilation rate (A) of the remaining leaves increased with clipping intensity and peaked under clipping intensities of 20% or 40%. Nitrogen addition, at a rate of 10 g·N·m^?2·year^?1, enhanced A of the remaining leaves and non‐structural carbohydrate concentrations, which benefited plant compensatory growth, especially for the C₃ grass. Under the mixed planting conditions, the clipping and N addition treatments lowered the competitive advantage of the C₄ grass.
• The results suggest that a combination of defoliation and N deposition have the potential to benefit the coexistence of C₃ and C₄ grasses.

     

The interaction of defoliation and nutrient uptake in Sporobolus kentrophyllus,a short-grass species from the serengeti plains

Summary Sporobolus kentrophyllus, a grazing-tolerant C₄ grass from the southeastern Serengeti Plains, was grown in solution culture to examine the effects of clipping on the uptake, preference and subsequent transport of varying nitrogen forms. Clipping reduced offtake mass, crown mass ane root mass, resulting in a 58% decline in plant mass. Proportional biomass allocation to roots decreased with clipping, while tillering rates increased. Clipping also increased the nitrogen concentrations of all tissues, and plant nitrogen uptake (nitrogen accumulated throughout the experiment per gram root). The ¹⁵N concentrations (% atom excess) of all tissues were higher in clipped compared with unclipped plants, and the average ¹⁵N uptake rate of clipped plants was twice that of unclipped plants. The relative ¹⁵N allocation to aboveground mass, a measure of canopy sink strength, was higher in clipped plants. Plants fed ¹⁵N-ammonium or ¹⁵N-nitrate during the ¹⁵N pulse experiment had greater ¹⁵N tissue concentrations compared with urea-fed plants, and ¹⁵N uptake rates were higher in ammonium-fed and nitrate-fed plants, compared with urea-fed plants. The relative magnitudes of these differences were higher when plants were clipped. Clipped plants had higher uptake rates for potassium, phosphorus and sodium, while differences between clipping treatments for calcium, iron, and magnesium were indistinguishable. Rapid uptake rates for species on the southeastern Serengeti plains, particularly during grazing periods, have important implications for nutrient cycling in this system.     

Belowground competitive suppression of seedling growth by grass in an African savanna

Coexistence of N₂-fixing legumes and non-legume trees with grasses in African savannas results in intense competition between these life-forms. We hypothesised that belowground competition might induce different nutritional constraints in N₂- versus non-N₂-fixing species. A field (Hluhluwe-imFolozi nature reserve, South Africa) competition experiment with two N₂-fixing legume species (Acacia burkei and Acacia karroo) and two non-N₂-fixing species (Schotia brachypetala and Spirostachys africana) both with clipped grass and without grass was established. Plants were supplied with no fertilizer, or generous amounts of fertilizer (200?kg?N?ha^?1, 100?kg P₂O₅ ha^?1, 7.1?kg K₂O ha^?1) supplied as either 28?C10 (N?CK), P or a combination of these fertilizers (NPK). Regularly clipped grass suppressed growth (by more than 90?%) of both N₂- and non-N₂-fixing seedlings equally. Biomass accumulation of seedlings grown with grass and the grasses themselves responded positively to NK and/or NPK, but not P, although P-fertilization did have effects on foliar [N] and ??¹⁵N values of trees and grasses showing that plants accessed the fertilizer. Tree ??¹⁵N values and foliar [N] were also modified by NPK, demonstrating access to fertilizer. However, the ameliorative effects of NPK on grass competition-induced biomass suppression were only partial. This may be due to ??non-resource competition?? (i.e. root gaps) imposed by dense grass roots. The fact that nutrients were able to partially ameliorate the effects of grass competition, however, indicates that such ??non-resource competition?? may be partially overcome by even more generous supply of fertilizers.     

Facilitative Effects of Aloe Shrubs on Grass Establishment,Growth, and Reproduction in Degraded Kenyan Rangelands: Implications for Restoration

Under the harsh environmental conditions present in severely overgrazed, semiarid rangelands, facilitator plants offer a promising tool for ecological restoration. This study investigated facilitative effects of Aloe secundiflora—a native drought‐tolerant, unpalatable, thorny shrub—on grass establishment in degraded rangelands in Kenya. We planted native perennial grass seeds adjacent to three neighbor treatments: transplanted mature aloe shrubs, piles of thorn branches that provided similar physical protection to aloes, and control treatments with no facilitator. We monitored grass performance for three growing seasons. During the first growing season, association with aloe shrubs significantly improved seedling survival and plant size of individual grasses, whereas grass survival in thorn treatments was intermediate between aloe and control treatments. At the population level, aloe neighbor treatments were associated with the greatest grass abundance and cover in all three seasons and reproductive output in the second season. Control treatments were associated with the poorest grass performance for all three variables. The findings indicate that planting aloes can improve the effectiveness of grass reseeding for rangeland restoration, exceeding the benefits gained from the more common strategy of using thorn branch piles. The utility of aloes in particular is further enhanced by the economic value of these plants; medicinal sap can be sustainably harvested from aloes planted for restoration.     

Competition by herbs as a limiting factor in shrub invasion in grassland: a test with different growth forms

Abstract. We tested the hypothesis that seedling establishment, the critical stage in the invasion of grassland by shrubs, is limited by competition with perennial grasses in seasonally wet/dry savannas. We placed seeds of two invasive exotic shrubs – Cryptostegia grandiflora, a woody vine, and Acacia nilotica, an arborescent legume – into pots with a wide range of existing above- and below-ground herbaceous biomass provided by either a tussock or a stoloniferous perennial grass. We also imposed different levels of watering frequency (5, 10 and 21 d), nutrient addition (+ and -) and grass clipping intensity (no clipping, clipped to 5 cm and clipped to 25 cm). There was no effect of any treatment on shrub seedling emergence or survival and all of the seedlings that emerged survived the 90-d growing period. Herbaceous competition also failed to have an effect on biomass accumulation in shrub seedlings. More frequent watering significantly increased above- and below-ground biomass accumulation for both shrub species and nutrient addition significantly increased Cryptostegia biomass accumulation. Based on these results, we question the proposition that reduction in competition by herbs via livestock grazing has been a significant factor in determining the rate or pattern of exotic shrub increase in the seasonally wet/dry tropics. We also question the suitability of the two-layer soil moisture hypothesis as a basis for management practices to control the ingress of woody species into grasslands and open savannas.     

Interactive effects of clipping and nutrient availability on the compensatory growth of a grass species

Resource availability is an important factor affecting the capacity of compensatory growth after grazing. We performed a greenhouse experiment with Poa bulbosa, a small perennial grass of the Mediterranean and Central Asian grasslands, to test the importance of nutrient availability for compensatory growth after clipping. We also compared the results with predictions of the limited resource model (LRM). Plants were grown at low and high fertilization levels and subjected to a clipping treatment. Contrary to the LMR, we found that in Poa plants compensatory growth occurred under the high fertilization level, while it did not occur under the low level. The LMR predicts a higher tolerance for grazing in the stressful environment. Our plants showed a significant decrease in their relative growth rates (RGR) after clipping. Although the plants allocated a 32–188% greater fraction of the mass to lamina growth after clipping, this greater allocation to the leaves did not fully compensate for the initial reduction in leaf area ratio (LAR). A sensitivity analysis showed for the clipped plants under the high fertilization treatment, that changes in leaf allocation (f _lam) enabled the plants to compensate for a part of the potential loss caused by defoliation. Probably, the increased biomass allocation comes largely from the bulbs. We conclude that the inconsistency of the LRM with our results originates in the lack of compensatory mechanisms in the model. To better understand how environmental conditions affect tolerance to herbivory, the effects of compensatory growth should be taken into account.     

Herbivory effects on saplings are influenced by nutrients and grass competition in a humid South African savanna

Woody plant encroachment is a common consequence of disturbance in savannas. Grazers and browsers interfere with sapling establishment dynamics by direct consumption of plant tissue, changing soil nutrient status (through fertilization and trampling) and grass competition. Studies evaluating the effects of herbivory on sapling establishment have mostly been extrapolated from single species. In a controlled field experiment, we studied the effects of clipping (simulating grazing and browsing), nutrients, grass competition, and their interactive effects on sapling survival and growth of four dominant humid and four dominant mesic savanna species. We conducted this experiment in a humid South African savanna. We found no effects on sapling survival by the treatments provided. However, clipped saplings of all species increased their investment in relative growth rate of stem length (RGR_L). Clipping had a greater negative impact on relative growth rate of more humid than mesic species in terms of stem diameter (RGR_D), total dry biomass and proportion of leaf biomass. Nutrients had a positive effect on the RGR_L and sapling biomass of three mesic species. Positive effects of nutrients on RGR_L of one humid and two mesic species were observed in their clipped saplings only. Grass competition had a strong negative impact on all growth parameters measured. Clipped saplings of one humid and two mesic species had lower RGR_L with grass competition whereas intact saplings showed no significant response. After clipping, humid savanna species were more vulnerable to grass competition than mesic species, with reduced ability to use nutrients. In conclusion, herbivory increases sapling vulnerability to grass competition, with humid species being more susceptible than mesic species, indicating that woody-plant control strategies are more likely to be effective in humid savannas.     

Short‐Term and Long‐Term Effects of Soil Ripping,Seeding, and Fertilization on the Restoration of a Tropical Rangeland

Rangeland degradation is a serious problem in semiarid Africa. Extensive areas of bare, compacted, nutrient‐poor soils limit the productivity and biodiversity of many areas. We conducted a set of restoration experiments in which all eight combinations of soil tilling, fertilization, and seeding with native perennial grasses were carried out in replicated plots. After 6 months, little aboveground biomass was produced in plots without tilling, regardless of seeding or fertilization. Tilling alone tripled plant biomass, mostly of herbaceous forbs and annual grasses. Perennial grasses were essentially limited to plots that were both tilled and seeded. The addition of fertilizer had no significant additional effects. After 7 years, vegetation had declined, but there were still large differences among treatments. After 10 years, one tilled (and seeded) plot had reverted to bare ground, but the other tilled plots still had substantial vegetation. Only one seeded grass (Cenchrus ciliaris) was still a contributor to total cover after 10 years. We suggest that restoration efforts on these soils be directed first to breaking up the surface crust, and second to the addition of desirable seed. A simple ripping trial inspired by this experiment showed considerable promise as a low‐cost restoration technique.     

Post‐fire resprouting responses of native and exotic grasses from Cumberland Plain Woodland (Sydney,Australia) under elevated carbon dioxide

This study investigated the effect of elevated CO₂ on the post‐fire resprouting response of a grassland system of perennial grass species of Cumberland Plain Woodland. Plants were grown in mixtures in natural soil in mesocosms, each containing three exotic grasses (Nassella neesiana, Chloris gayana, Eragrostis curvula) and three native grasses (Themeda australis, Microlaena stipoides, Chloris ventricosa) under elevated (700 ppm) and ambient (385 ppm) CO₂ conditions. Resprouting response after fire at the community‐ and species‐level was assessed. There was no difference in community‐level biomass between CO₂ treatments; however, exotic species made up a larger proportion of the community biomass under all treatments. There were species‐level responses to elevated CO₂ but no significant interactions found between CO₂ and burning or plant status. Two exotic grasses (N. neesiana and E. curvula, a C₃ and a C₄ species respectively), and one native grass (M. stipoides, a C₃ species) significantly increased in biomass, and a native C₄ grass (C. ventricosa) significantly decreased in biomass under elevated CO₂. These results suggest that although overall productivity of this community may not change with increases in CO₂ and fire frequency, the community composition may alter due to differential species responses.     

Nitrogen:phosphorous supply ratio and allometry in five alpine plant species

In terrestrial ecosystems, atmospheric nitrogen (N) deposition has greatly increased N availability relative to other elements, particularly phosphorus (P). Alterations in the availability of N relative to P can affect plant growth rate and functional traits, as well as resource allocation to above‐ versus belowground biomass (M_A and M_B). Biomass allocation among individual plants is broadly size‐dependent, and this can often be described as an allometric relationship between M_A and M_B, as represented by the equation , or log M_A = logα + βlog M_B. Here, we investigated whether the scaling exponent or regression slope may be affected by the N:P supply ratio. We hypothesized that the regression slope between M_A and M_B should be steeper under a high N:P supply ratio due to P limitation, and shallower under a low N:P supply ratio due to N limitation. To test these hypotheses, we experimentally altered the levels of N, P, and the N:P supply ratio (from 1.7:1 to 135:1) provided to five alpine species representing two functional groups (grasses and composite forbs) under greenhouse conditions; we then measured the effects of these treatments on plant morphology and tissue content (SLA, leaf area, and leaf and root N/P concentrations) and on the scaling relationship between M_A and M_B. Unbalanced N:P supply ratios generally negatively affected plant biomass, leaf area, and tissue nutrient concentration in both grasses and composite forbs. High N:P ratios increased tissue N:P ratios in both functional groups, but more in the two composite forbs than in the grasses. The positive regression slopes between log M_A and log M_B exhibited by plants raised under a N:P supply ratio of 135:1 were significantly steeper than those observed under the N:P ratio of 1.7:1 and 15:1. Synthesis: Plant biomass allocation is highly plastic in response to variation in the N:P supply ratio. Studies of resource allocation of individual plants should focus on the effects of nutrient ratios as well as the availability of individual elements. The two forb species were more sensitive than grasses to unbalanced N:P supplies. To evaluate the adaptive significance of this plasticity, the effects of unbalanced N:P supply ratio on individual lifetime fitness must be measured.     

Effects of moisture,nitrogen, grass competition and simulated browsing on the survival and growth of Acacia karroo seedlings

The effects of irrigation, nitrogen fertilization, grass competition and clipping were investigated for one growing season at the research farm of the University of Fort Hare in the Eastern Cape Province of South Africa. The aim of the experiment was to assess the short‐term performance of Acacia karroo seedlings under different environmental conditions and the implications of such factors on the long‐term recruitment of plant species in savanna rangelands. There were no significant treatment effects on the survival of A. karroo seedlings. Using stem length and basal diameter as growth parameters, it was observed that irrigation enhanced both variables, while nitrogen fertilization did not have any significant effects. Clipping, grass competition and their interaction greatly suppressed the growth of the seedlings. Clipping increased the mean stem length when they were irrigated and fertilized. Control and fertilized plants had the highest stem length in the absence of grass competition, while grass competition combined with clipping resulted in the lowest stem length in both irrigated and nonirrigated plants. It was concluded that in the presence of grass competition, controlled browsing could be a viable solution to the problem of bush encroachment in savanna rangelands.     

A simulation model of Bouteloua gracilis biomass dynamics on the North American shortgrass prairie

Summary A grassland primary producer model for simulating intraseasonal biomass dynamics as a function of temperature, moisture, light, and nitrogen was developed for Bouteloua gracilis (H.B.K.) Lag., the dominant C₄ grass of the North American shortgrass prairie. Plant state variables included young and mature leaves, crowns, and roots from three depth categories while simulated processes included spring regrowth, photosynthesis, respiration, photosynthate allocation, death, and litterfall. Sensitivity analyses revealed the model was most sensitive to changes in photosynthesis and photosynthate allocation and least sensitive to changes in initial values of state variables, leaf dark respiration rates, and rate of spring regrowth.An abiotic submodel driven by observed weather data was used in conjunction with the primary producer model to simulate plant biomass dynamics under a variety of conditions including untreated controls (C), nitrogen fertilization (F), irrigation (I), and irrigation plus fertilization (IF). Model predictions of life shoot biomass (B _s) and annual aboveground net primary production (NPP _A) followed the same trends as field measurements with B _sand NPP _Aof IF>I>F>C. Failure of the model to accurately predict measured declines in peak B _sand NPP _Aafter several years of irrigation may have been caused by failure to account for growth lags following water stress, inadequate simulation of interspecific competition, or failure to simulate response to some mineral nutrients which had become limiting after several years of this treatment. A simulated annual carbon budget for plants in the four treatments suggests that from 61% (IF) to 80% (C) of the net carbon fixed above ground is ultimately translocated and utilized below ground.     

The role of seed limitation and resource availability in the recruitment of native perennial grasses and exotics in a South Australian grassland

Abstract We investigated what factors lead to invasion of exotics or re‐colonization of native perennial grasses in the South Australian mid‐north grasslands. We manipulated 160 experimental quadrats by clipping, irrigation and seed addition and assessed recruitment by exotics in an area dominated by perennial grasses and perennial grass recruitment in an area dominated by exotics. Treatment effects differed with season for exotics: their biomass increased with irrigation in autumn and seed addition in winter. However, in both periods other factors, probably soil properties, also had a strong effect. We detected no perennial grass seedlings in the quadrats over 1 year, possibly due to unsuitable environmental conditions or persistent high competition levels. Under controlled conditions the presence of the invasive annual Avena barbata had a strong negative effect on the recruitment of the native perennial Austrodanthonia caespitosa at any moisture and nutrient availability. Avena also germinated faster and more frequently than Austrodanthonia, especially at low soil moisture. During an imposed drought Austrodanthonia seedlings survived longer in the absence of Avena. The results suggest that annual exotics are highly responsive to resources and can quickly invade areas, while the re‐colonization of invaded areas by native grasses requires a complex (and less likely) rainfall regime.     

Seed maturation time influences the germination requirements of perennial grasses in desert climate of Arabian Gulf

Qatar has a dry, subtropical desert climate, with minimum annual rainfall and intensely hot and humid summers. Using indigenous grass, those adapted to local conditions have the potential to be used for fodder and can also be used for restoration or rehabilitation of degraded rangelands. Chloris virgata, Coelachyrum brevifolium and Cenchrus ciliaris bloom twice a year from April to May (summer) and September to October (winter) under the nursery condition. Therefore, it is important to understand, how seeds produced in different seasons affect the dormancy as well as germination of these species. Seeds of C. virgata, C. brevifolium and C. ciliaris, three desert grasses, were collected from the plants growing on Shahniya nursery in two different seasons, summer (May) and winter (October). The seeds collected in May (summer) were stored up to winter. However seeds collected in October (winter) were immediately used for experiment. We compared the germination potential of seeds that matured in different season at different alternating temperatures at 15/25, 20/30 and 25/35 °C. Lower temperatures correspond to the dark period, while higher temperatures reflect the light period. Seeds collected in summer season (old seeds) were heavier as compared to seeds collected in winter season (new seeds). Winter seeds of C. virgata seem to be dormant, while summer seeds, germinated well in all the tested temperature regimes. However, C. ciliaris seeds showed opposite trends.     

An invasive grass shows colonization advantages over native grasses under conditions of low resource availability

Increased or fluctuating resources may facilitate opportunities for invasive exotic plants to dominate. This hypothesis does not, however, explain how invasive species succeed in regions characterized by low resource conditions or how these species persist in the lulls between high resource periods. We compare the growth of three co-occurring C₄ perennial bunchgrasses under low resource conditions: an exotic grass, Eragrostis curvula (African lovegrass) and two native grasses, Themeda triandra and Eragrostis sororia. We grew each species over 12?weeks under low nutrients and three low water regimes differentiated by timing: continuous, pulsed, and mixed treatments (switched from continuous to pulsed and back to continuous). Over time, we measured germination rates, time to germination (first and second generations), height, root biomass, vegetative biomass, and reproductive biomass. Contrary to our expectations that the pulsed watering regime would favor the invader, water-supply treatments had little significant effect on plant growth. We did find inherent advantages in a suite of early colonization traits that likely favor African lovegrass over the natives including faster germination speed, earlier flowering times, faster growth rates and from 2?weeks onward it was taller. African lovegrass also showed similar growth allocation strategies to the native grasses in terms of biomass levels belowground, but produced more vegetative biomass than kangaroo grass. Overall our results suggest that even under low resource conditions invasive plant species like African lovegrass can grow similarly to native grasses, and for some key colonization traits, like germination rate, perform better than natives.