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Aim Little is known about the distribution of temperate forest shrubs in eastern North America during the Last Glacial Maximum (LGM), and how far they subsequently migrated to occupy previously glaciated landscapes. We assessed whether populations of Dirca palustris L. persisted near the glacial margin during the LGM. Such populations might explain how the species colonized northern latitudes, despite scant evidence for contemporary long-distance seed dispersal.Location Eastern North America, Mexico and California.Methods We conducted ecological niche modelling (ENM) and sequenced five non-coding regions of the chloroplast genome from 390 plants among 104 populations representing the four species of Dirca. We used a Bayesian approach to identify relationships among individuals and to estimate timing of lineage splits. The spatial genetic structure of D. palustris was evaluated qualitatively and by statistical methods, including Monmonier's algorithm and analysis of molecular variance (AMOVA).Results Ecological niche modelling indicated suitable climate at the LGM in the Gulf Coastal Plain, but also north to the Ozark Plateaus Province (northern Arkansas), Interior Low Plateaus and Appalachian Plateaus Provinces (portions of Tennessee and Kentucky), and the Piedmont and Coastal Plain Provinces of the Carolinas and Virginia. Sequence variation was extensive and indicated deep lineage splits within D. palustris. Numerous haplotypes, many private, were identified at middle latitudes (c. 35-40°N), Monmonier's algorithm identified multiple genetic discontinuities within this region, and northern and southern regions differed by AMOVA.Main conclusions Ecological niche modelling and genetic analyses indicate that populations likely existed within several hundred km of the Laurentide ice sheet at the LGM. Deep Pleistocene diversification, along with weak geographical fidelity of haplotypes, probably resulted from repeated isolation, differentiation, and secondary contact during Quaternary glacial cycles. The absence of many northern haplotypes south of 35°N supports the hypothesis that temperate understorey shrubs accompanied temperate trees of eastern North America in populations near the glacial margin at the LGM.

Although overhead mist revolutionized the propagation industry, it does suffer from potential drawbacks that include the application of large volumes of water, potentially unsanitary conditions, irregular misting coverage, and leaching of foliar nutrients. We explored the feasibility of submist as an alternative as it might avoid these problems by applying water exclusively from below the cutting, which is inserted basally into an enclosed rooting chamber. We propagated cuttings of korean lilac (Syringa pubescens ssp. patula) and inkberry (Ilex glabra) using both overhead mist and submist to compare effectiveness of the systems. Cuttings of korean lilac were wounded and dipped basally into 8000 mg·L−1 of the potassium salt of indole-3-butyric acid (K-IBA), and those in the overhead mist systems were inserted into coarse perlite. Cuttings of inkberry were wounded and treated with 5000 mg·L−1 K-IBA, and those in the overhead mist systems were inserted into 50:50 peat:perlite (by vol). Cuttings of korean lilac in the submist systems produced more than twice as many roots as cuttings in the overhead mist systems, with roots more than 2.6 times the length. Similarly, cuttings of inkberry in the submist systems produced more than three times the root counts and root lengths as cuttings in the overhead mist systems. For korean lilac, root dry weights averaged 58 mg for cuttings in the submist system, compared with only 18 mg among cuttings receiving overhead mist. Likewise, root dry weights averaged 70 and 7 mg for cuttings of inkberry propagated by submist and overhead mist, respectively. Rooted cuttings of korean lilac transplanted well into a soilless substrate, where they more than tripled their root biomass to 218 mg (vs. 59 mg for cuttings transplanted from overhead mist). We did not evaluate transplant performance of inkberry. Our results show that submist systems might merit consideration for the propagation of woody plants by leafy stem cuttings.

Overhead mist (OM) facilitates the propagation of stem cuttings by preventing transpirational water loss. However, drawbacks to OM include the application of large volumes of water, potentially unsanitary conditions, irregular misting coverage, and leaching of foliar nutrients. We explored three alternatives to OM that might avoid these problems by applying moisture below, rather than overhead. These included 1) a submist (SM) aeroponic system configured to provide intermittent mist only to the rooting zone, 2) a subirrigation (SI) system that provided water via capillary action through perlite from a reservoir maintained below the base of each cutting, and 3) a subfog (SF) aeroponic system that was configured to provide constant fog only to the rooting zone. To initiate each system, we wetted perlite or filled reservoirs using either water or quarter-strength Hoagland solution. Stem cuttings of ‘Wizard Mix’ coleus (Plectranthus scutellarioides) were propagated in the systems for 21 days. Cuttings in the SM system produced more than three times as many roots as cuttings in the OM system, with roots more than six times the length. Root dry weights averaged 28 mg for cuttings in the SM system, compared with only 3.5 mg among cuttings receiving OM. The SF and SI systems produced results broadly comparable to the OM. Fertilizer did not consistently improve rooting measures across the systems. Although we observed few fine roots on cuttings rooted using SM, they transplanted well into a soilless substrate and quickly produced new root growth. The SM system used less than 1/5 the water used by the SI system, and less than 1/50 the water used by the SF system. In comparison, a single OM nozzle operating for 10 seconds released about one-third of the total water lost through transpiration from each SM system over the entire experiment. Our results show that SM systems merit further evaluation for propagation of plants by stem cuttings.

We propagated manchurian lilac (Syringa pubescens subsp. patula ‘Miss Kim’) vegetatively from stem cuttings using overhead mist, submist, and combination propagation systems. Cuttings were collected when terminal buds were already set, after the period of tender growth that is optimal for lilac propagation. Net photosynthesis (Pn) was recorded to assess whether differences in rooting could be attributed to differences in photosynthetic activity of cuttings within each system. The propagation environment differed significantly among systems, with vapor pressure deficit (VPD) substantially greater for submist systems than for overhead mist or combination systems, and root zones warmer in submist and combination systems than in overhead mist. Pn of cuttings did not differ among systems and was initially low, but increased about when the first root primordia were visible. Rooting percentages were 90% among cuttings in the combination system, with cuttings in overhead mist and submist rooting at lower, but similar, percentages (68% and 62%, respectively). Cuttings in the combination and submist systems produced significantly more and longer roots than those in the overhead mist system, and retained nearly all of their leaves. Overall, the use of systems that provide intermittent mist to the basal end of each cutting was effective for propagating manchurian lilac. Our results demonstrate that cuttings in submist alone experience a much greater VPD than those in overhead mist, but may nonetheless root at comparable percentages and produce superior measures of root system quality. Combination systems show promise for rooting of species like manchurian lilac, because cuttings rooted at high percentages and with consistent root system quality, despite having been collected after the optimal spring period for lilac propagation.