Understanding the sensitivity of tundra vegetation to climate warming is critical to forecasting future biodiversity and vegetation feedbacks to climate. In situ warming experiments accelerate climate change on a small scale to forecast responses of local plant communities. Limitations of this approach include the apparent site-specificity of results and uncertainty about the power of short-term studies to anticipate longer term change. We address these issues with a synthesis of 61 experimental warming studies, of up to 20 years duration, in tundra sites worldwide. The response of plant groups to warming often differed with ambient summer temperature, soil moisture and experimental duration. Shrubs increased with warming only where ambient temperature was high, whereas graminoids increased primarily in the coldest study sites. Linear increases in effect size over time were frequently observed. There was little indication of saturating or accelerating effects, as would be predicted if negative or positive vegetation feedbacks were common. These results indicate that tundra vegetation exhibits strong regional variation in response to warming, and that in vulnerable regions, cumulative effects of long-term warming on tundra vegetation -and associated ecosystem consequences -have the potential to be much greater than we have observed to date.

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Along the west coast of the Antarctic Peninsula springtime ozone depletion events can lead to a two-fold increase in biologically effective UV-B radiation (UV-B) and summer air temperatures have risen ≈1.5°C during the past 50 years. We manipulated levels of UV radiation and temperature around Colobanthus quitensis (a cushion-forming plant, Caryophyllaceae) and Deschampsia antarctica (a tussock grass) along the Peninsula near Palmer Station for two field seasons. Ambient levels of UV were manipulated by placing filters that either transmitted UV (filter control), absorbed UV-B (reducing diurnal levels of UV-B by about 82%), or absorbed both UV-B and UV-A (reducing UV-B and UV-A by about 88 and 78%, respectively) on frames over naturally growing plants from November to March. Half the filters of each material completely surrounded the frames and raised diurnal and diel air temperatures around plants by an average of 2.3°C and 1.3°C, respectively. Reducing UV or warming had no effect on leaf concentrations of soluble UV-B absorbing compounds, UV-B absorbing surface waxes or chlorophylls. Warming had few effects on growth of either species over the first season. However, over the second field season warming improved growth of C. quitensis, leading to a 50% increase in leaf production (P < 0.10), a 26% increase in shoot production, and a 6% increase in foliar cover. In contrast, warming reduced growth of D. antarctica, leading to a 20% decline in leaf length, a 17% decline in leaf production (P < 0.10), and a 5% decline in foliar cover. Warming improved sexual reproduction in both species, primarily through faster development of reproductive structures and greater production of heavier seeds. Over the second field season, the percentage of reproductive structures that had reached the most developed (seed) stage in C. quitensis and D. antarctica was 20% and 15% higher, respectively, under warming. Capsules of C. quitensis produced 45% more seeds under warming and these seeds were 11% heavier. Growth of D. antarctica was improved when UV was reduced and these effects appeared to be cumulative over field seasons. Over the second season, tillers produced 55% more leaves and these leaves were 32% longer when UV-B was reduced. Tillers produced 137% more leaves that were 67% longer when both UV-B and UV-A were reduced. The effects of UV reduction were not as pronounced on C. quitensis, although over the second season cushions tended to be 17% larger and produce 21% more branches when UV-B was reduced, and tended to be 27% larger and produce 38% more branches when both UV-B and UV-A were reduced (P < 0.10). Few interactions were found between UV reduction and warming, although in the absence of warming, reducing UV led to slower development of reproductive structures in both species. The effects of warming and UV reduction were species specific and were often cumulative over the two field seasons, emphasizing the importance of long-term field manipulations in predicting the impacts of climate change.

The ultraviolet-B radiation (UV-B, 300 nm) screening effectiveness of foliage of a diverse group of plants was examined by measuring epidermal transmittance and depth of penetration of UV-B with a fiberoptic microprobe. In addition, the concentration of UV-B-absorbing compounds and various anatomical characteristics were measured to assess whether they were useful predictors of UV-B screening. Sun foliage of naturally growing individuals of seven species were sampled in each of six life forms comprising two evergreen groups (gymnosperms and angiosperms) and four deciduous angiosperm groups (trees, shrubs and vines, herbaceous dicotyledons, and grasses). There was significant life-form variation in epidermal transmittance and depth of penetration of UV-B, concentration of UV-B-absorbing compounds (leaf-area basis), epidermal (including cuticle and hypodermis) thickness, and specific leaf area. Values of these parameters tended to be related to leaf longevity, with the most notable differences apparent between evergreen and deciduous species. The mean epidermal transmittance and depth of penetration of UV-B in foliage averaged 4% and 32 μm in evergreens, compared to 28% and 75 μm in deciduous species. These values are conservative estimates since the microprobe was oriented in foliage such that much of the side- and backscattered UV-B was ignored. The strongest predictors of epidermal transmittance and depth of penetration were epidermal thickness and the concentration of absorbing compounds, which averaged 32 μm and 1.50 A cm in evergreens, but only 19 μm and 0.99 A cm in deciduous foliage. However, the variation found in these relationships implies that additional factors warrant consideration in assessing UV-B-screening effectiveness. The relatively ineffective screening of UV-B by foliage of many deciduous plants suggests they may be more responsive to enhanced UV-B than evergreen species.

▪ Abstract  Ozone depletion by anthropogenic gases has increased the atmospheric transmission of solar ultraviolet-B radiation (UV-B, 280–315 nm). Our understanding of the consequencences of enhanced UV-B levels on primary producers has grown dramatically over the past 20 years, but it has been hampered by how realistically experimental UV-B exposures mimic ozone-depletion scenarios. Overcoming these shortcomings will require sophisticated and creative approaches. Biological weighting functions and solar spectral irradiance estimates are critical in evaluating effects and require more attention. Whereas UV screening compounds in terrestrial and aquatic producers commonly increase with UV-B exposure, the implications, while potentially far reaching, are unclear. Photosynthesis is more sensitive to UV-B in phytoplankton than in terrestrial plants, probably owing to less effective screening in phytoplankton. Productivity of terrestrial plants is usually unaffected by enhanced UV-B, although reduced growth has been observed and may increase in magnitude over successive years. Aquatic productivity is often compromised by short-term exposures to enhanced UV-B, and long-term assessments are complicated by the dynamic nature of aquatic systems and by nonlinear responses. Recent work examining UV-B effects on multiple trophic levels suggests that outcomes will be diverse and difficult to predict. Such effects may lead to feedbacks on primary producers.