Eco-evolutionary context modifies a destructive plant invader's response to climate
No abstract available.
New Phytologist↗
Geology topics
Toby Matthew Maxwell
Publications and source records attributed to Toby Matthew Maxwell.
Loss of ecosystem resistance to invasion after wildfire facilitated by a pulse of soil nitrogen availability: Experimental evidence in the burned sagebrush steppe
The impacts of climate shifts on ecosystems dominated by long-lived perennials are likely most pronounced during community reassembly after disturbances such as fire and confounded by interactions between disturbances and plant community composition. A 30-year-experimental hydroclimate manipulation of multiple sagebrush steppe communities was completely consumed by a 2019 wildfire, providing an opportunity to evaluate effects of precipitation deficit on ecosystem recovery. Ambient precipitation was doubled for 23 years via irrigation in winter or summer in grassland or shrub–steppe communities until 2016. Plots that had received irrigation thus experienced drought for three years preceding and continuing after the fire. These landscapes are vulnerable to invasion by exotic annuals such as Bromus tectorum L. (cheatgrass) that promote wildfire occurrence, which favors even greater invasion levels. Thus, we asked whether patterns of invasion after the compound disturbance of drought and fire related to the long-term pre-fire climate and plant community structure. Established theory led to the prediction that plant communities developed under wetter climates would have greater resistance to invasion. The most resistant plots were the most arid (that is, never irrigated control plots with no drought) which had the least pre-fire canopy cover of shrubs and nitrogen-fixing forbs and greater proportional cover of perennial bunchgrasses. Plots that developed under winter irrigation had greater cover of shrubs and N-fixing forbs, corresponding to pulses of plant available soil nitrogen that were 8.2-fold greater than pre-fire levels, compared to a 0.20-fold post-fire reduction in soil nitrogen observed in the ambient plots. Nitrogen pulses and invasion were most evident in the inter-canopy bare-soil patches (‘interspaces’) and were least evident where perennial grasses were most abundant. Long-term hydroclimate altered pre-fire plant community composition in ways that affected post-fire resistance to invasion such that the combined effects of fire and water deficit led to greater than expected invasion in wetter regions that are conventionally considered resistant to invasion and resilient to wildfire.
Ecosystems↗
Local adaptation to climate has facilitated the global invasion of cheatgrass
Local adaptation may facilitate range expansion during invasions, but the mechanisms promoting destructive invasions remain unclear. Cheatgrass ( Bromus tectorum ), native to Eurasia and Africa, has invaded globally, with particularly severe impacts in western North America. We aimed to identify mechanisms and consequences of local adaptation in the North American cheatgrass invasion. We sequenced 307 range-wide genotypes and conducted controlled experiments. We found that diverse lineages invaded North America, where long-distance gene flow is common. Nearly half of North American cheatgrass is comprised of a mosaic of ~19 locally adapted near clonal genotypes, each seemingly very successful in a different part of its range. Additionally, ancestry- and phenotype- environment clines in the native range predicted those in the invaded range, indicating pre-adapted genotypes colonized different regions. Common gardens showed directional selection on flowering time that reversed between warm and cold sites, potentially maintaining clines. In the Great Basin, genomic predictions of strong local adaptation identified sites where cheatgrass is most dominant. Our results indicate that multiple introductions and ongoing migration within the invaded range likely fueled pre-adaptation and subsequent dominance of cheatgrass in western North America. Understanding how environment and gene flow shape invasive adaptation is critical for managing ongoing invasions.
Nature Communications↗
Invasion of perennial sagebrush steppe by shallow-rooted exotic cheatgrass reduces stable forms of soil carbon in a warmer but not cooler ecoregion
Soil organic carbon ('SOC') in drylands comprises nearly a third of the global SOC pool and has relatively rapid turnover and thus is a key driver of variability in the global carbon cycle. SOC is also a sensitive indicator of longer-term directional change and disturbance-responses of ecosystem C storage. Biome-scale disruption of the dryland carbon cycle by exotic annual grass invasions (mainly Bromus tectorum, 'Cheatgrass') threatens carbon storage and corresponding benefits to soil hydrology and nutrient retention. Past studies on cheatgrass impacts mainly focused on total C, and of the few that evaluated SOC, none compared the very different fractions of SOC, such as relatively unstable particulate organic carbon (POC) or relatively stable, mineral-associated organic carbon (MAOC). We measured SOC and its POC and MAOC constituents in the surface soils of sites that had sagebrush canopies but differed in whether their understories had been invaded by cheatgrass or not, in both warm and relatively colder ecoregions of the western USA. MAOC stocks were 36.1% less in the 0–10 cm depth and 46.1% less in the 10–20 cm depth in the cheatgrass-invaded stands compared to the uninvaded stands of the warmer Colorado Plateau, but not in the cooler and more carbon-rich Wyoming Basin ecoregion. In plots where cheatgrass increased SOC, it was via unstable POC. These findings indicate that cheatgrass effects on the distribution of soil carbon among POC and MAOC fractions may vary among ecoregions, and that cheatgrass can reduce forms of carbon that are otherwise considered stable and 'secure', i.e. sequestered.
Environmental Research Communications↗
Local adaptation to climate facilitates a global invasion
Local adaptation may facilitate range expansion during invasions, but the mechanisms underlying successful invasions remain unclear. Cheatgrass ( Bromus tectorum ), native to Eurasia and Africa, has invaded globally, with severe impacts in western North America. We aimed to identify mechanisms and consequences of local adaptation in the North American cheatgrass invasion. We sequenced 307 range-wide genotypes and conducted controlled experiments. We found that diverse lineages invaded North America, where long-distance gene flow is common. Nearly half of North American cheatgrass comprises a mosaic of ∼19 locally adapted, near-clonal genotypes, each seemingly very successful in a different part of North America. Additionally, ancestry, phenotype, and allele frequency-environment clines in the native range predicted those in the invaded range, indicating pre-adapted genotypes colonized different regions. Common gardens showed directional selection on flowering time that reversed between warm and cold sites, potentially maintaining clines. In the USA Great Basin, genomic predictions of strong local adaptation identified sites where cheatgrass is most dominant. Our results indicate that multiple introductions and migration within the invaded range fueled local adaptation and success of cheatgrass in western North America. Understanding how environment and gene flow shape adaptation and invasion is critical for managing ongoing invasions.
BioRxiv↗
Annual grass invasions and wildfire deplete ecosystem carbon storage by >50% to resistant base levels
Ecological disturbance can affect carbon storage and stability and is a key consideration for managing lands to preserve or increase ecosystem carbon to ameliorate the global greenhouse gas problem. Dryland soils are massive carbon reservoirs that are increasingly impacted by species invasions and altered fire regimes, including the exotic-grass-fire cycle in the extensive sagebrush steppe of North America. Direct measurement of total carbon in 1174 samples from landscapes of this region that differed in invasion and wildfire history revealed that their impacts depleted soil carbon by 42–49%, primarily in deep horizons, which could amount to 17.1–20.0 Tg carbon lost across the ~400,000 ha affected annually. Disturbance effects on soil carbon stocks were not synergistic, suggesting that soil carbon was lowered to a floor—i.e. a resistant base-level—beneath which further loss was unlikely. Restoration and maintenance of resilient dryland shrublands/rangelands could stabilize soil carbon at magnitudes relevant to the global carbon cycle.
Nature Communications, Earth & Environment↗