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.

We assessed whether exposure to solar ultraviolet-B radiation (UV-B) affects the mass loss of Larrea tridentata (creosotebush) litter in the Sonoran Desert of central Arizona. We placed three types of litter (leaves, twigs, or a natural mixture of leaves, twigs, and seeds) in bags constructed of UV-B-transmitting or UV-Babsorbing filter material that allowed either 85% (near-ambient UV-B treatment) or 15% (reduced UV-B treatment) of the biologically effective solar UV-B to reach litter inside the bags. Bags were placed outdoors for 4-5 months during the winter at two sites: a balcony or on the soil surface of the desert. Mass loss of leaf litter was greater under near-ambient UV-B than reduced UV-B at both sites: 21 (near-ambient) vs. 18% (reduced) on the balcony, and 18 vs. 14% at the desert site. Mass loss of twig litter was also greater under near-ambient UV-B at the desert site. Mass loss of the natural mixture of litter was also greater when exposed to near-ambient UV-B on the balcony, and tended to be greater at the desert site. We estimate that about 14-22% of the total mass loss of leaf litter during our 4-5 month experiments was attributable to solar UV-B exposure. Leaf litter exposed to near-ambient UV-B had lower concentrations of lignin, and fats and lipids, and slightly higher concentrations of holocellulose. The greater mass loss of litter under near-ambient UV-B appeared mainly attributable to loss of lignin, although losses of fats and lipids were also appreciable. A primary reason for greater mass loss of litter under solar UV-B appeared to be photodegradation, particularly of lignin.

The photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica was examined by measuring whole‐canopy CO2 gas exchange and chlorophyll (Chl) a fluorescence of plants growing near Palmer Station along the Antarctic Peninsula. Both species had negligible midday net photosynthetic rates (Pn) on warm, usually sunny, days (canopy air temperature [Tc]> 20°C), but had relatively high Pn on cool days (Tc<10°C). Laboratory measurements of light and temperature responses of Pn showed that high temperature, not visible irradiance, was responsible for depressions in Pn on warm sunny days. The optimal leaf temperatures (Tl) for Pn in C. quitensis and D. antarctica were 14 and 10°C, respectively. Both species had substantial positive Pn at 0°C Tl, which were 28 (C. quitensis) and 32% (D. antarctica) of their maximal Pn, and we estimate that their low‐temperature compensation points occurred at −2°C Tl (C. quitensis) and −3°C (D. antarctica). Because of the strong warming trend along the peninsula over recent decades and predictions that this will continue, we were particularly interested in the mechanisms responsible for their negligible rates of Pn on warm days and their unusually low high‐temperature compensation points (i.e., 26°C in C. quitensis and 22°C in D. antarctica). Low Pn at supraoptimal temperature (25°C) appeared to be largely due to high rates of temperature‐enhanced respiration. However, there was also evidence for direct impairment of the photosynthetic apparatus at supraoptimal temperature, based on Chl fluorescence and Pn/intercellular CO2 concentration (ci) response curve analyses. The breakpoint or critical temperature (Tcr) of minimal fluorescence (Fo) was ≈42°C in both species, which was well above the temperatures where reductions in Pn were evident, indicating that thylakoid membranes were structurally intact at supraoptimal temperatures for Pn. The optimal Tl for photochemical quenching (qp) and the quantum yield of photosystem II (PSII) electron transfer (φPSII) were 9 and 7°C in C. quitensis and D. antarctica, respectively. Supraoptimal temperatures resulted in lower qp and greater non‐photochemical quenching (qNP), but had little effect on Fo, maximal fluorescence (Fm) or the ratio of variable to maximal fluorescence (Fv/Fm) in both species. In addition, carboxylation efficiencies or initial slopes of their Pn/ci response were lower at supraoptimal temperatures, suggesting reduced activity of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco). Although continued warming along the peninsula will increase the frequency of supraoptimal temperatures, Tc at our field site averaged 4.3°C and was below the temperature optima for Pn in these species for the majority of diurnal periods (86%) during the growing season, suggesting that continued warming will usually improve their rates of Pn.

Stratospheric ozone depletion by anthropogenic chlorofluoro-propanoids such as ferulic acid may constrain cell expansion carbons has lead to increases in ultraviolet-B radiation (UV-B;and leaf elongation. In both species, HPLC analysis revealed 280-320 nm) along the Antarctic Peninsula during the austral that ferulic and p-coumaric acid were major components of both insoluble and soluble phenylpropanoids. Although there spring. We manipulated UV-B levels around plants of Antarctic hair grass (Deschampsia antarctica; Poaceae) and Antarc-were no significant differences in concentrations between UV-B treatments, concentrations of insoluble ferulic acid in D. tic pearlwort (Colobanthus quitensis; Caryophyllaceae) for one field season near Palmer Station along the west coast of antarctica tended to be higher under ambient and near-ambithe Antarctic Peninsula. Treatments involved placing frames ent UV-B than under reduced UV-B (P =0.17). We also over naturally growing plants that either (1) held filters that examined bulk-leaf concentrations of soluble (methanol exabsorbed most biologically effective radiation (UV-B BE ; 're-tractable) UV-B-absorbing compounds and found that concenduced UV-B', 22% of ambient UV-B BE levels), (2) held filters trations were higher in plants exposed to near-ambient and ambient UV-B than in plants exposed to reduced UV-B. We that transmitted most UV-B BE ('near-ambient UV-B', 87% of also assessed the UV-B-screening effectiveness of leaves that ambient UV-B BE levels), or (3) lacked filters ('ambient UV-B'). Leaves on D. antarctica exposed to near-ambient and had developed on plants at the field site with a fiber-optic microprobe. Leaf epidermal transmittance of 300-nm UV-B ambient UV-B were 16-17% shorter than those exposed to reduced UV-B, and this was associated with shorter epidermal was 4.0 and 0.6% for D. antarctica and C. quitensis, respectively, which is low compared to grasses and herbaceous cells at the leaf base and tip. Leaves on C. quitensis exposed to near-ambient and ambient UV-B tended to be shorter dicotyledonous plants found in more temperate climates. While the leaves of Antarctic vascular plants are relatively (P=0.18) and epidermal cells at the leaf base tended to be smaller than those under reduced UV-B (PB0.10). In order effective at screening UV-B, levels of UV-B in Antarctica are to further explain reductions in leaf length, we examined leaf sufficient to reduce leaf epidermal cell size and leaf elongation in these species, although the mechanisms for these reductions concentrations of insoluble (cell-wall bound) phenylremain unclear. propanoids, since it has been proposed that wall-bound phenyl-

We passively warmed tundra on the Antarctic Peninsula over four growing seasons and assessed its effect on dry mass and C and N stocks associated with the vascular plants Colobanthus quitensis (a cushion-forming forb) and Deschampsia antarctica (a tussock grass), and mosses. Temperature treatments involved a warmed treatment that raised diurnal and diel canopy air temperatures by 2.3 and 1.3 1C, respectively, and a nearambient temperature treatment that raised diurnal and diel temperatures by 0.2 1C. These two different temperature regimes were achieved by wrapping filters around the frames to different extents and were nested within three UV treatments that filtered different solar UV wavebands. The experiment also included an ambient control treatment (unfiltered frames), and supplemental water and fertilizer treatments (applied to unfiltered frames). After four growing seasons, we collected cores of each vascular plant species and assessed the mass and C and N content of the aboveground current-year biomass, the litter layer (which included nongreen live stems), and the organic soil horizon (which included roots). The thin nature of the organic soil horizon allowed us to sample this complete horizon and estimate near-total ecosystem C and N stocks. A comparison of the warmed and near-ambient temperature treatments found that warming led to greater aboveground biomass of C. quitensis, and more C in the aboveground biomass of both vascular plant species. Warming resulted in lower N concentrations of the aboveground biomass of both species. The water use efficiency of both species was greater under warming, based on their higher d 13 C values. The mass of the litter layer under C. quitensis was greater under warming, and this layer contained more C and N and had a higher C : N ratio. The mass of the organic soil horizon under both species was greater under warming, and this horizon also contained more C and N. Warming also changed the species composition of the plant community -cover of C. quitensis increased while that of mosses declined. Warming resulted in the input of biomass into the system that had greater C : N ratios (and was likely more recalcitrant to decomposition) because (1) warming increased the C : N ratio of the biomass produced by both vascular plant species, (2) these inputs increased with warming because of greater biomass production, and (3) increases in C. quitensis cover led to greater biomass inputs by this species and its biomass had a greater C : N ratio than D. antarctica. Water or fertilizer supplements had few effects on aboveground biomass or C and N concentrations or pools, consistent with the relatively wet maritime climate and high soil nutrient levels of this system. Total C pools in the aboveground biomass, litter, and organic soil horizon were greater under warming. Warmed plots contained from 272 to 319 g m À2 more C than plots under near-ambient temperatures, corresponding to a 23-34% increase in ecosystem C.Correspondence: Thomas A. Day, tel. 1 1 480 965 8165, fax 1 1 480 965...