Charles F. Musil

How substantial are ultraviolet-B supplementation inaccuracies in experimental square-wave delivery systems?

Square-wave (SQW) ultraviolet-B (UV-B: 280–315 nm) radiation supplementation systems continue to be used in outdoor experimental locations due to the economically restrictive installation and maintenance costs, and technical expertise required to effectively operate more advanced modulated (MOD) delivery systems. However, continued yet contentious criticisms of SQW delivery systems risk creating prejudices as to the validity of plant responses measured in these with potentially negative repercussions on future UV-B experimentation. Consequently, we quantified the magnitude of UV-B supplementation inaccuracies in our typical outdoor step-wise SQW delivery system using 7-year records of computer-modeled and instrument-measured solar UV-B irradiances and synchronous measurements of total solar (300–3000 nm) radiation and daily sunshine duration. Both broad- and narrow-band instrument measurements confirmed that our step-wise SQW delivery system rendered larger total daily supplemental UV-B exposures (time-integrated UV-B irradiances) than a MOD delivery system on only substantially overcast days (25% or less daily sunshine duration). These larger supplemental UV-B exposures were augmented with increased magnitude of the added artificial UV-B supplement. However, their ranges did not exceed those in a MOD delivery system by more than 10% for added UV-B supplements of realistic magnitude (30% or less above background), except on virtually completely overcast days (5% or less daily sunshine duration). Also, our step-wise SQW delivery system rendered higher photon flux ratios of UV-B/total solar radiation than a MOD delivery system on only substantially overcast days, the ranges of which were also augmented with increased magnitude of the added artificial UV-B supplement. However, these features were restricted to high solar angles, since with reduced solar angle these higher photon flux ratios also included progressively less overcast days. Nevertheless, ranges of photon flux ratio increases were well below reported thresholds inhibiting to plant growth at all solar angles for the added artificial UV-B supplements of realistic magnitude, except on virtually completely overcast days. Results point to an under-estimation of clear-sky UV-B irradiance by the computer-encoded semi-empirical model commonly utilized to predict background and supplemental UV-B irradiances for SQW delivery systems. They confirm the superiority of MOD delivery systems in providing more realistic conditions of UV-B increases but likewise demonstrate little justification on balance for branding results derived from all field-based SQW delivery systems as exaggerated where sensible irradiation protocols and realistic UV-B supplements are applied. This is the final, accepted and revised manuscript of this article. Use alternative location to go to the published article. Requires subscription. (Less)

Experimental climate warming decreases photosynthetic efficiency of lichens in an arid South African ecosystem

Elevated temperatures and diminished precipitation amounts accompanying climate warming in arid ecosystems are expected to have adverse effects on the photosynthesis of lichen species sensitive to elevated temperature and/or water limitation. This premise was tested by artificially elevating temperatures (increase 2.1–3.8°C) and reducing the amounts of fog and dew precipitation (decrease 30.1–31.9%), in an approximation of future climate warming scenarios, using transparent hexagonal open-top warming chambers placed around natural populations of four lichen species (Xanthoparmelia austroafricana, X. hyporhytida , Xanthoparmelia. sp., Xanthomaculina hottentotta) at a dry inland site and two lichen species (Teloschistes capensis and Ramalina sp.) at a humid coastal site in the arid South African Succulent Karoo Biome. Effective photosynthetic quantum yields (

Root niche partitioning between shallow rooted succulents in a South African semi desert: implications for diversity

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The threat of alien invasive grasses to lowland Cape floral diversity: an empirical appraisal of the effectiveness of practical control strategies

) were measured hourly throughout the day at monthly intervals in pre-hydrated lichens present in the open-top warming chambers and in controls which comprised demarcated plots of equivalent open-top warming chamber dimensions constructed from 5-cm-diameter mesh steel fencing. The cumulative effects of the elevated temperatures and diminished precipitation amounts in the open-top warming chambers resulted in significant decreases in lichen

Non rainfall moisture interception by dwarf succulents and their relative abundance in an inland arid South African ecosystem

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Differential interception and evaporation of fog, dew and water vapour and elemental accumulation by lichens explain their relative abundance in a coastal desert

. The decreases were more pronounced in lichens from the dry inland site (decline 34.1–46.1%) than in those from the humid coastal site (decline 11.3–13.7%), most frequent and prominent in lichens at both sites during the dry summer season, and generally of greatest magnitude at or after the solar noon in all seasons. Based on these results, we conclude that climate warming interacting with reduced precipitation will negatively affect carbon balances in endemic lichens by increasing desiccation damage and reducing photosynthetic activity time, leading to increased incidences of mortality.