These conditions are representative of extreme climatic events that are predicted to become more common and are expected to have different outcomes on plant community function and productivity than more gradual climatic change , such as increasing invasion by lowering a community’s biotic resistance . This long-term field experiment provides important insights into the strength and nature of priority effects, showing they can be important even in systems with high annual variability, and can have different roles prior, during, and after the severe drought. Our study aimed to test priority effects in as natural a setting as possible, and so we relied on natural colonization throughout the site rather than explicitly controlling the order of arrival through seed addition, as is most common ; as such we are relying on our knowledge of species movement throughout site but cannot verify similar propagule pressure among plots. We initially observed strong priority effects between all three functional groups, both in maintaining the highest cover when seeded alone as well as suppressing the establishment of other recruiting species. However, the benefit of being planted alone weakened over the course of the multi-year drought; in most cases, species composition became the same regardless of seeding treatment by the third drought year. Similarly, vertical farming supplies recruitment of unplanted species began during the drought, also showing a weakening of priority effects, but with the notable exception of native perennial grasses continuing to suppress noxious weeds.
The large compositional shift strongly suggests that drought served as a catalyst to reset the communities. Drought often disrupts communities by reducing cover, lowering seed production, increasing thatch, and impacting the microbial community , and can make the community more susceptible to invasion if the current resident species cannot recover as quickly as recruiting species can establish . Both exotic annuals and native grasses are negatively impacted by drought , but in our experiment the annual noxious weeds were most affected. After the drought, we began to observe differences among each functional group in whether priority effects had truly disappeared or may have only not been observable due to low statistical power during the drought years. Priority effects among the two exotic annual grass groups completely disappeared and they reached co-existence, for each group’s cover was the same when planted alone, with each other, and where each recruited into the other’s priority plots. The loss of priority effects between the annuals may be attributed to a reduction in seed production during the multi-year drought, as decreased neighborhood propagule pressure can increase susceptibility to invasion , and as well as the reduction in decomposition since thatch build up can hinder germination . Both exotic groups, however, were able to suppress recruitment of the native perennials after the drought, as expected with their higher propagule pressure and competitive seedling dynamics, although not as successfully as before the drought.
Priority effects persisted in the system when natives were involved, with the strongest effects observed between the native perennial and noxious annual grasses, which share the later season-phenological niche. Each group continued to reduce cover of the other when seeded together and limited the other from recruiting into their monotypic seeded priority plots. However, while the role of phenology affected priority for both groups, the additional aspect of niche pre-emption of long-lived individuals resulted in stronger priority effects for the native perennials, which continuously kept noxious weed cover below 5%. The perennial deep-root system of the natives can limit soil moisture availability by using it early in the season and competing for residual moisture into the dry season. These results follow with the idea of limiting similarity, in that a species cannot invade if the niche is already occupied and that priority effects are stronger among species of similar functional groups . These principles are employed in restoration , and perennials are often planted to suppress grassland weeds . The ability of the native perennial grasses to consistently suppress noxious weed recruitment despite the drought was striking when compared to their contrasting lack of resistance to the naturalized exotic annuals. The sharp increase in naturalized exotics after the natives decreased in cover during the drought isn’t surprising, since native biomass is an important factor in preventing invasion , and niche pre-emption of perennials is dependent on individuals continuing in the same space overtime.
Naturalized exotic recruitment in the native monotypic priority plots could have negatively impacted the adult natives , which may explain why by the final year native cover was the same where planted alone and with the naturalized exotics. Regardless, native perennial cover increased to high levels when seeded, indicating that once individuals are established, they can persist among exotics, although they remain strongly seed limited . For natives, the importance of seedlings being given an extra year of priority before potential natural colonization of exotic annuals arose only during post-drought recovery, eight years into the experiment. In native priority plots, giving two years of priority resulted in greater cover than those given one year, as well as resulted in significant observable priority effects over being seeded with the noxious weeds for most of the post-drought recovery. Root establishment the first year is crucial to native seedling survival , and an extra year without competition for light and resources may have led to greater and deeper root biomass with greater below ground competitive ability . More root surface area to access available water likely leads to faster recovery post-drought . A quicker recovery post-disturbance would limit open opportunities for invasion . From a restoration standpoint, this suggests that an extra year of weed control may have long-lasting benefits to project success and add resiliency to future drought disturbance. Whether the changes we observed in the community composition would have still occurred without the drought is unknown and we cannot be certain about whether the strong priority effects would have lasted longer in less extreme conditions. Similarly, it is not clear if some of the weaker priority effects in the last couple years of monitoring are due to a weakening of priority effects over time, or variation in the strength of priority effects. Other long-term studies found that priority effects maintained lasting effects on the identity of the species present, but the communities converged in trait and functional diversity . A nine-year experiment on native vernal pool species showed strong initial priority effects between the seeded species, but could not comment on long-term dynamics, as an extreme wet year followed by a drought year led to dominance of a non-seeded exotic species across all plots .Our study has implications for management of annual grasses, which are increasingly common across many systems , Fynbos in South Africa . Late-season noxious weeds are invading naturalized exoticdominated rangelands, forming extensive monocultures of poor forage quality and causing economic and ecological harm . Our study suggests targeting noxious weed management during drought years as well as prioritizing native restoration alongthe invasion front . However, our results also confirm the necessity of weed control in the beginning of restoration as well as the need to manage for naturalize exotics after drought events. Overall, our study adds to the evidence that giving priority to native species can enhance their establishment , as well as suppress future invasions, vertical weed grow particularly when the natives and invasives share similar functional roles or phenological niches .Johnsongrass reaches 6 to 8 feet in height, with wide open, purplebrown panicles 4 to 20 inches long .
Stems and leaves are bright to deep green; leaves have a prominent white midvein and may reach 1 inch or more in width and 24 or more inches in length. Johnsongrass can reproduce via seed through self- or cross-fertilization and will reproduce vegetatively via a robust rhizome network. Johnsongrass establishes well in disturbed areas, rangelands, pastures, abandoned fields, and canals, as well as in virtually any cropping system. It thrives in moist environments, but due to its extensive rhizome system it can persist in drier topsoils if they are above high water tables. Johnsongrass may not be a significant threat to recovered areas with sufficient established vegetation, so avoiding disturbance in these areas is critical to preventing new or repeat infestations. Infestations usually begin in the margins of affected areas, but livestock and machinery may inadvertently spread seed to the interiors of fields, orchards, pastures, and other areas. Johnsongrass is an alternate host to Maize Dwarf Mosaic Virus , which affects corn , oats , millets such as pearl millet and foxtail millet , and sorghum . It also readily hybridizes with grain, forage, or sudangrass sorghums, which can reduce harvest quality due to contaminated seed.Since johnsongrass builds a substantial rhizome system in its first year, it can be very difficult to control once it is well established. Rhizome sprouts typically emerge early in the season and quickly reach full height, and they can easily outgrow and overtop desired plants. In addition, the extensive rhizome network can allow a single plant to be very competitive for water and nutrients over a substantial area. Significant crop yield reductions can be expected with acute johnsongrass infestation. For example, cotton can see a 20% yield reduction with as few as two tillers per square foot within rows . Corn without adequate control of johnsongrass can see yields reduced by up to 2 to 3 tons per acre . Additionally, disturbed portions of non-crop areas can quickly become dominated by thick stands of johnsongrass if management is not practiced.Johnsongrass can grow from seed or from overwintering rhizomes. Seed can germinate within a year and can remain viable for up to 6 years in the soil. Seedlings may form new rhizomes as early as the 5- to 6-leaf stage, about a month after emergence; buds of the new rhizomes may sprout the following year. Rhizome sprouting typically occurs in the early spring when daytime temperatures average above 60°F, but seed will germinate later, when daytime temperatures are about 70° to 75°F. Date ranges for rhizome and seedling emergence vary by region, but early to late March for warmer areas and late March to mid April for cooler areas are good approximations. Sprouts from new rhizomes grow more rapidly than from seed, but both grow quickly, and their development is essentially identical. Tillers usually appear at the 6-leaf stage, about a month after emergence. Flowering usually occurs 2 months from emergence and in California typically lasts from May to October, with each panicle producing up to 400 seed . The rhizome network later expands during seed ripening; individual plants can produce 200 to 300 feet of new rhizome growth per year . Above ground structures and older rhizomes die off over winter, but new rhizome growth persists and forms new sprouts the following spring. Because it can propagate via self-crossing and rhizomes, a single plant, if left undisturbed, can cause a significant infestation of up to 180 square feet in 2 years . Johnsongrass seedlings can resemble young corn seedlings, but because their seed remain attached, johnsongrass is easily discernable if plucked. The first leaf is usually parallel to the ground; early leaves have a smooth surface with no discernable midrib and smooth edges. A white midvein may begin to show at the leaf base of seedlings. Seedling collars are usually pale green to whitish, and sheaths may begin to take on a red-purple tinge as the plant matures.In California, johnsongrass can be as much as 8 feet tall at maturity . Stems grow from the crown, are erect and unjointed at the nodes, and may have a red-purple base or tinted internode. Nodes may have some fine hairs, but internodes are usually smooth. Prop roots may form near the base of stems . The leaves are rolled in the bud, generally emerge flat, and have a prominent midrib, especially at the base . Mature leaves are hairless to nearly hairless and may have slightly rough edges, with a prominent fringed ligule up to 0.2 inch long or more . Sparse hairs may be present on the leaf or sheath near the collar. Inflorescences form as large, open panicles that are pyramidal or conical in shape. Spikelets can range from pale green or gray to golden to purple-brown. They may remain on the panicle or shatter into pairs or trios that consist of a lower fertile spikelet and one or two sterile upper spikelets.