Initial success of native grasses is contingent on multiple interactions among exotic grass competition, temporal priority, rainfall and site effects.

Ecological communities are increasingly being recognized as the products of contemporary drivers and historical legacies that are both biotic and abiotic. In an attempt to unravel multiple layers of ecological contingency, we manipulated (i) competition with exotic annual grasses, (ii) the timing of this competition (temporal priority in arrival/seeding times) and (iii) watering (simulated rainfall) in a restoration-style planting of native perennial grasses. In addition, we replicated this experiment simultaneously at three sites in north-central California. Native perennial grasses had 73-99 % less cover when planted with exotic annuals than when planted alone, but this reduction was greatly ameliorated by planting the natives 2 weeks prior to the exotics. In a drought year, irrigation significantly reduced benefits of early planting so that these benefits resembled those observed in a non-drought year. There were significant differences across the three sites (site effects and interactions) in (i) overall native cover, (ii) the response of natives to competition, (iii) the strength of the temporal priority effect and (iv) the degree to which supplemental watering reduced priority effects. These results reveal the strong multi-layered contingency that underlies even relatively simple communities

Boxplots comparing <i>M</i>. <i>spicatum</i> RGR in the concurrent and priority treatments across shade levels.

<p>Box edges mark the 1st and 3^rd quartile, and the median is shown with a dark line. Whiskers extend to a maximum of 1.5 x interquartile range outward, and values beyond this are indicated by circles. Diamonds show means of each competition treatment*shade level combination. “NS” indicates non-significant differences in pairwise t-tests between competition treatments.</p

Boxplots comparing RGR of <i>M</i>. <i>spicatum</i> and <i>E</i>. <i>nuttallii</i> planted concurrently across shade levels.

<p>Box edges mark the 1st and 3^rd quartile, and the median is shown with a dark line. Whiskers extend to a maximum of 1.5 x interquartile range outward, and values beyond this are indicated by circles. Diamonds show the mean of each shade level*species combination. Asterisks indicate significant differences (p < 0.05) in pairwise t-tests between species, and “NS” indicates non-significant differences.</p

ANOVA statistics.

<p>ANOVA statistics.</p

Mean values and ranges of key nutrient levels of water and soil samples in the experimental channels.

<p>*Ammonium was not detected (mdl = 0.05 mg/L) in any samples.</p><p>**Three samples had orthophosphate levels below detection limits (0.05 mg/L)</p><p>Mean values and ranges of key nutrient levels of water and soil samples in the experimental channels.</p

Experimental Tests of Priority Effects and Light Availability on Relative Performance of <i>Myriophyllum spicatum</i> and <i>Elodea nuttallii</i> Propagules in Artificial Stream Channels

<div><p>Submersed macrophytes have important ecological functions in many streams, but fostering growth of beneficial native species while suppressing weedy invasives may be challenging. Two approaches commonly used in management of terrestrial plant communities may be useful in this context: (1) altering resource availability and (2) establishing desirable species before weeds can invade (priority effects). However, these approaches are rarely used in aquatic systems, despite widespread need for sustainable solutions to aquatic weed problems. In artificial stream channels in California, USA, I conducted experiments with asexual propagules of non-native invasive <i>Myriophyllum spicatum</i> (Eurasian watermilfoil) and native <i>Elodea nuttallii</i> (western waterweed) to address the questions: (1) How does light availability affect relative performance of the two species?; (2) Does planting the native earlier than the invasive decrease survival or growth rate of the invasive?; and (3) Do light level and priority effects interact? The relative performance between <i>E</i>. <i>nuttallii</i> and <i>M</i>. <i>spicatum</i> had an interesting and unexpected pattern: <i>M</i>. <i>spicatum</i> had higher growth rates than <i>E</i>. <i>nuttallii</i> in the zero and medium shade levels, but had similar performance in the low and high shade levels. This pattern is most likely the result of <i>E</i>. <i>nutallii’s</i> sensitivity to both very low and very high light, and <i>M</i>. <i>spicatum’s</i> sensitivity to very low light only. Native priority did not significantly affect growth rate or survival of <i>M</i>. <i>spicatum</i>, possibly because of unexpectedly poor growth of the <i>E</i>. <i>nuttallii</i> planted early. This study suggests that altering light levels could be effective in reducing growth of an invasive macrophyte, and for changing the competitive balance between a native and a non-native species in the establishment phase. Further investigations into the use of priority effects and resource alteration for submersed macrophyte management are warranted, given their mixed results in other (limited) studies.</p></div

Correlation of <i>M</i>. <i>spicatum</i> and <i>E</i>. <i>nuttallii</i> RGR in the concurrent treatment.

<p>Thin, solid line is 1:1 line. Most points fall to the upper left of this line, indicating a higher RGR for <i>M</i>. <i>spicatum</i> over <i>E</i>. <i>nuttallii</i>. The Pearson correlation coefficient (r) and p-values for each shade level are as follows (DF = 10 for all tests): zero: r = 0.54, p = 0.07; low: r = 0.80, p = 0.002; medium: r = 0.61, p = 0.04; high: r = 0.14, p = 0.68.</p

Initial success of native grasses is contingent on multiple interactions among exotic grass competition, temporal priority, rainfall and site effects.

Ecological communities are increasingly being recognized as the products of contemporary drivers and historical legacies that are both biotic and abiotic. In an attempt to unravel multiple layers of ecological contingency, we manipulated (i) competition with exotic annual grasses, (ii) the timing of this competition (temporal priority in arrival/seeding times) and (iii) watering (simulated rainfall) in a restoration-style planting of native perennial grasses. In addition, we replicated this experiment simultaneously at three sites in north-central California. Native perennial grasses had 73-99 % less cover when planted with exotic annuals than when planted alone, but this reduction was greatly ameliorated by planting the natives 2 weeks prior to the exotics. In a drought year, irrigation significantly reduced benefits of early planting so that these benefits resembled those observed in a non-drought year. There were significant differences across the three sites (site effects and interactions) in (i) overall native cover, (ii) the response of natives to competition, (iii) the strength of the temporal priority effect and (iv) the degree to which supplemental watering reduced priority effects. These results reveal the strong multi-layered contingency that underlies even relatively simple communities

Plant communities in harsh sites are less invaded: a summary of observations and proposed explanations.

Plant communities in abiotically stressful, or 'harsh', habitats have been reported to be less invaded by non-native species than those in more moderate habitats. Here, we synthesize descriptive and experimental evidence for low levels of invasion in habitats characterized by a variety of environmental stressors: low nitrogen; low phosphorus; saline, sodic or alkaline soils; serpentine soils; low soil moisture; shallow/rocky soils; temporary inundation; high shade; high elevation; and high latitude. We then discuss major categories of hypotheses to explain this pattern: the propagule limitation mechanism suggests invasion of harsh sites is limited by relatively low arrival rates of propagules compared with more moderate habitats, while invasion resistance mechanisms suggest that harsh habitats are inherently less invasible due to stressful abiotic conditions and/or increased effects of biotic resistance from resident organisms. Both propagule limitation and invasion resistance may simultaneously contribute to low invadedness of harsh sites, but the management implications of these mechanisms differ. If propagule limitation is more important, managers should focus on reducing the likelihood of propagule introductions. If invasion resistance mechanisms are in play, managers should focus on restoring or maintaining harsh conditions at a site to reduce invasibility

Initial success of native grasses is contingent on multiple interactions among exotic grass competition, temporal priority, rainfall and site effects

Ecological communities are increasingly being recognized as the products of contemporary drivers and historical legacies that are both biotic and abiotic. In an attempt to unravel multiple layers of ecological contingency, we manipulated (i) competition with exotic annual grasses, (ii) the timing of this competition (temporal priority in arrival/seeding times) and (iii) watering (simulated rainfall) in a restoration-style planting of native perennial grasses. In addition, we replicated this experiment simultaneously at three sites in north-central California. Native perennial grasses had 73–99 % less cover when planted with exotic annuals than when planted alone, but this reduction was greatly ameliorated by planting the natives 2 weeks prior to the exotics. In a drought year, irrigation significantly reduced benefits of early planting so that these benefits resembled those observed in a non-drought year. There were significant differences across the three sites (site effects and interactions) in (i) overall native cover, (ii) the response of natives to competition, (iii) the strength of the temporal priority effect and (iv) the degree to which supplemental watering reduced priority effects. These results reveal the strong multi-layered contingency that underlies even relatively simple communities