Maternal and direct effects of elevated CO2 on seed provisioning, germination and seedling growth in Bromus erectus

Elevated CO₂ can affect plant fitness not only through its effects on seed production but also by altering the quality of seeds and therefore germination and seedling performance. We collected seeds from mother plants of Bromus erectus grown in field plots at ambient and elevated CO₂ (m-CO₂, maternal CO₂) and germinated them in the greenhouse in a reciprocal design under ambient and elevated CO₂ (o-CO₂, offspring CO₂). This design allowed us to examine both the direct effects of elevated CO₂ on germination and seedling growth and the indirect (maternal) effects via altered seed quality. Elevated m-CO₂ significantly increased seed mass and increased the C:N ratio of seeds from field-grown plants. Percentage and rate of germination were not affected by the m-CO₂ or o-CO₂ treatments. Similarly, elevated m-CO₂ had no significant effect on seedling size as estimated by the total leaf length. When differences in seed mass were adjusted by using seed mass as a covariate in ANOVA, a negative effect of m-CO₂ on seedling size appeared which increased with increasing seed mass (significant covariate×m-CO₂ interaction). This may indicate that the advantage of increased seed mass at elevated m-CO₂ was offset by the reduced concentration of nitrogen (and possibly other nutrients) in these seeds. In contrast to m-CO₂, elevated o-CO₂ greatly increased seedling size, and this stimulatory effect of elevated o-CO₂ was found to increase with increasing seed mass (significant covariate×o-CO₂ interaction). Taken together, these results suggest that in B. erectus transgenerational effects of elevated CO₂ are relatively small. However, other factors (genetic and environmental) that contribute to variation in seed provisioning can critically influence the responsiveness of seedlings to elevated CO₂. Received: 10 May 1999 / Accepted: 6 January 2000     

Eco‐evolutionary responses of Bromus tectorum to climate change: implications for biological invasions

How plant populations, communities, and ecosystems respond to climate change is a critical focus in ecology today. The responses of introduced species may be especially rapid. Current models that incorporate temperature and precipitation suggest that future Bromus tectorum invasion risk is low for the Colorado Plateau. With a field warming experiment at two sites in southeastern Utah, we tested this prediction over 4 years, measuring B. tectorum phenology, biomass, and reproduction. In a complimentary greenhouse study, we assessed whether changes in field B. tectorum biomass and reproductive output influence offspring performance. We found that following a wet winter and early spring, the timing of spring growth initiation, flowering, and summer senescence all advanced in warmed plots at both field sites and the shift in phenology was progressively larger with greater warming. Earlier green‐up and development was associated with increases in B. tectorum biomass and reproductive output, likely due early spring growth, when soil moisture was not limiting, and a lengthened growing season. Seeds collected from plants grown in warmed plots had higher biomass and germination rates and lower mortality than seeds from ambient plots. However, in the following two dry years, we observed no differences in phenology between warmed and ambient plots. In addition, warming had a generally negative effect on B. tectorum biomass and reproduction in dry years and this negative effect was significant in the plots that received the highest warming treatment. In contrast to models that predict negative responses of B. tectorum to warmer climate on the Colorado Plateau, the effects of warming were more nuanced, relied on background climate, and differed between the two field sites. Our results highlight the importance of considering the interacting effects of temperature, precipitation, and site‐specific characteristics such as soil texture, on plant demography and have direct implications for B. tectorum invasion dynamics on the Colorado Plateau.     

Shrub persistence and increased grass mortality in response to drought in dryland systems

Droughts in the southwest United States have led to major forest and grassland die‐off events in recent decades, suggesting plant community and ecosystem shifts are imminent as native perennial grass populations are replaced by shrub‐ and invasive plant‐dominated systems. These patterns are similar to those observed in arid and semiarid systems around the globe, but our ability to predict which species will experience increased drought‐induced mortality in response to climate change remains limited. We investigated meteorological drought‐induced mortality of nine dominant plant species in the Colorado Plateau Desert by experimentally imposing a year‐round 35% precipitation reduction for eight continuous years. We distributed experimental plots across numerous plant, soil, and parent material types, resulting in 40 distinct sites across a 4,500 km² region of the Colorado Plateau Desert. For all 8 years, we tracked c. 400 individual plants and evaluated mortality responses to treatments within and across species, and through time. We also examined the influence of abiotic and biotic site factors in driving mortality responses. Overall, high mortality trends were driven by dominant grass species, including Achnatherum hymenoides, Pleuraphis jamesii, and Sporobolus cryptandrus. Responses varied widely from year to year and dominant shrub species were generally resistant to meteorological drought, likely due to their ability to access deeper soil water. Importantly, mortality increased in the presence of invasive species regardless of treatment, and native plant die‐off occurred even under ambient conditions, suggesting that recent climate changes are already negatively impacting dominant species in these systems. Results from this long‐term drought experiment suggest major shifts in community composition and, as a result, ecosystem function. Patterns also show that, across multiple soil and plant community types, native perennial grass species may be replaced by shrubs and invasive annuals in the Colorado Plateau Desert.     

Using phenological niche separation to improve management in a Northern Glaciated Plains grassland

Many of the remaining patches of untilled (native) prairie in the Northern Glaciated Plains of North America are heavily invaded by the cool‐season grasses, Bromus inermis and Poa pratensis. However, the native vegetation in these patches contains many warm‐season species. This difference in phenology can be used to benefit restoration. We conducted an experiment to examine the efficacy of restoration treatments (mowing and prescribed fire) applied early in the growing season for consecutive years to decrease cool‐season invasive plant biomass without impacting the native warm‐season species. Our treatments were successful at significantly decreasing invasive cool‐season plant biomass and increasing native warm‐season plant biomass. No differences between treatments (mowing and prescribed fire) were found. Results suggest that incorporating differences in phenology between target and nontarget species into management may increase restoration success.     

Determinants of plant community composition of remnant biancane badlands: a hierarchical approach to quantify species‐environment relationships

Question: Which environmental variables best explain patterns in the vegetation of biancane badlands? What is the role of spatial scales in structuring the vegetation of biancane badlands within the agricultural matrix? Location: Five biancane badlands in Central Italy (Tuscany). Methods: An object‐oriented approach on high‐resolution multispectral images was used to classify physiognomic vegetation types in five biancane badlands. Within each badland, data on vascular plant species abundance were collected using a stratified random design. Variation partitioning based on partial redundancy analysis was used to evaluate the contribution of three sets of environmental predictors, recorded at the spatial scales of plot, patch and biancane badland in explaining patterns in plant community composition. Results: Environmental variables included in the final model – electrical conductivity and carbon/nitrogen ratio (plot scale), shape index (patch scale) and area (biancane badland scale) – accounted for 15.5% of the total variation in plant community composition. Soil characteristics measured at the plot level explained the majority of variation. In the smallest badlands, Bromus erectus perennial grasslands were absent, while annual grasslands, linked with harsh soil conditions (i.e. high soil salinity), were not affected by either the surface area of biancane badlands or by the soil nitrogen availability. Conclusions: The identification of the major predictors of patterns in remnant vegetation requires conducting investigations at multiple spatial scale. Management strategies should operate at different spatial scale, preventing any further reduction in the size of existing badlands and relying on habitat‐ instead of area‐focused conservation practices.     

克隆整合对无芒雀麦在异质性盐分环境中存活和生长的影响

无芒雀麦是浑善达克沙地植物群落中占优势的多年生根茎禾草.研究了克隆整合特性对无芒雀麦在异质性盐分环境中存活和生长的影响.结果表明,克隆整合显著提高了无芒雀麦分株在高盐环境中的存活能力,耗-益分析表明无芒雀麦在高盐斑块中分株的生物量、分株数、根茎节数和根茎总长显著受益于克隆整合,而与之相连的非盐分斑块中的分株却没有产生显著的损耗.因而,克隆整合特性是无芒雀麦对异质性环境形成的重要适应对策,它使无芒雀麦能够扩展到不适合植物生长的高盐分斑块中,从而增加了无芒雀麦在浑善达克沙地中的存活和生长,提高了其在半干旱沙化地区的适合度.