Effects of nutrient supply,light availability and herbivory on the growth of heather and three competing grass species

Grasses are becoming more abundant in areas in NE Scotland which until recently were dominated by heather (Calluna vulgaris). However, it is not clear if grasses are aggressive competitors which are now able to outcompete the dwarf shrub due to changes in environmental factors (such as grazing pressure and increasing nutrient inputs), or just opportunistic invaders, occupying gaps in the canopy which occur when heather reaches the degenerate stage. Experiments in turves and in field plots were carried out in order to investigate the performance of three grass species, Nardus stricta, Deschampsia cespitosa and Deschampsia flexuosa growing in competition with heather. These three species were selected because they differ in their nutrient requirements, palatability to herbivores and tolerance of shading. The grasses were planted in heather canopies of different structure, either turves of heather of different height and age, or moorland plots with or without heavy grazing by sheep and deer. Fertiliser (NPK) was applied to half the experimental plants. The growth of the grass species and the heather in response to the fertiliser and grazing treatments was measured, together with the light levels penetrating the canopy and received by the grass plants.Results indicated that heather was likely to be outcompeted by grasses only when there are gaps in the canopy, resulting either from heavy grazing or from the heather being in the mature or degenerate phase. Fertiliser enhanced plant growth whereas fencing out herbivores led to strong competition for light as the heather canopy closed. It is concluded that grasses require gaps in the canopy to successfully invade heather moorland, or they tend to be shaded out. Thus better management of heather moorlands to maintain a dense canopy structure may help to preserve heather cover even under increasing nutrient inputs.     

Competition between heather and grasses on Scottish moorlands: Interacting effects of nutrient enrichment and grazing regime

Abstract. Considerable losses and degradation of heathlands (in moorlands and lowlands) have been reported across Europe, with Calluna vulgaris (heather) being replaced by other species, often grasses. Increasing atmospheric nitrogen deposition and overgrazing have been suggested as the driving factors behind this change. This possibility was investigated in a study of the interacting effects of nutrient inputs and grazing on heather and three grass species (Nardus stricta, Deschampsia cespitosa and D. flexuosa) in the field, on a moorland in northeastern Scotland. In addition, the interacting effects of increasing nutrients and Calluna canopy height on N. stricta and D. cespitosa were studied using turves in an outdoor experimental area. In the field, fencing had a larger effect than fertilizer on the growth of all species, except for N. stricta, the species most unpalatable to herbivores. Fencing led to an increase in the height of the Calluna canopy, which may reduce light availability for the grasses. In the turf experiment, the height of the Calluna canopy affected the diameter of the grass tussocks and percentage of green matter (i.e. live leaf material), with plants under the more closed Calluna canopies being smaller. This study suggests that the slow‐growing, evergreen Calluna is a more effective competitor than the faster growing grasses when it has a tall, intact canopy, even at increased levels of nutrient supply. However, overgrazing promotes gap formation in the Calluna canopy, providing an opportunity for grasses to take advantage of increased nutrients. Thus the conservation of heather moorlands requires an understanding of the grazing level which allows Calluna to maintain sufficient canopy structure to outcompete grasses for light.     

Importance of soil moisture and its interaction with competition and clipping for two montane meadow grasses

Meadow classification studies have demonstrated the importance of water table fluctuation patterns in determining plant community composition in the western United States. However, a mechanism causing an overall increase in Poa pratensis ssp. pratensis populations and local declines in Deschampsia cespitosa populations in western montane meadows over the past century has not been defined. In order to better understand plant species interactions in these often highly grazed systems, we observed aboveground responses of Poa and Deschampsia to changes in species composition, soil moisture gradients, and clipping in the field. As well, we conducted a factorial greenhouse experiment, varying plant density, water availability, and clipping. While Poa is adapted to dry meadows and Deschampsia to wet meadows, their ranges overlap in wet conditions where soil moisture averages 50% in the early growing season. Deschampsia appears to be excluded from dry meadows where Poa is prevalent and soil moisture is closer to 30% water content in the early growing season. Our greenhouse experiment revealed that Deschampsia’s competitive ability decreases, while Poa’s increases, at soil moistures of 19%. However in more mesic conditions (50% soil moisture), each species aboveground biomass, tillering, and inflorescence weight was adherent to soil moisture conditions, and species interactions were less important. Our early growing season clipping treatments significantly reduced biomass of both grasses, but did not appear to favor one species over the other. This work points to the importance of soil water content in determining the performance of each plant species and the level of species interactions in montane meadows.     

Antarctic macrolichen modifies microclimate and facilitates vascular plants in the maritime Antarctica – a reply to Casanova‐Katny et al. (2014)

In a current article in the Journal of Vegetation Science, Casanova‐Katny et al. addressed a comment about an article by Molina‐Montenegro et al., which demonstrated the climate modification induced by the macrolichen Usnea antarctica and its role as facilitator. They provided useful corrections concerning species identification and pointed out several issues that, in their view, weakened our study. They indicated that the role of U. antarctica as a facilitative species in the maritime Antarctica is merely philosophical and has no ecological relevance. In this commentary, we argue why these critiques are unsubstantial, and provide evidence that the macrolichen can modify the microclimate, ameliorating the harsh conditions prevailing in Antarctica, establishing positive interactions and eventually facilitating vascular species. Thus, the macrolichen U. antarctica would act as a ‘nurse species’, playing a key role in structuring the maritime Antarctic plant community.     

Tufted Hairgrass (Deschampsia caespitosa) Exhibits a Lower Photosynthetic Plasticity than Antarctic Hairgrass (D. Antarctica)

The aim of our work was to assess photosynthetic plasticity of two hairgrass species with different ecological origins (a temperate zone species, Deschampsia caespitosa (L.) Beauv. and an Antarctic species, D. antarctica) and to consider how the anticipated climate change may affect vitality of these plants. Measurements of chlorophyll fluorescence showed that the photosystem Ⅱ (PSII) quantum efficiency of D. caespitosa decreased during 4 d of incubation at 4℃ but it remained stable in D. antarctica. The fluorescence half-rise times were almost always lower in D. caespitosa than in D. antarctica,irrespective of the incubation temperature. These results indicate that the photosynthetic apparatus of D. caespitosa has poorer performance in these conditions. D. caespitosa reached the maximum photosynthesis rate at a higher temperature than D. antarctica although the values obtained at 8℃ were similar in both species. The photosynthetic water-use efficiency (photosynthesis-to-transpiration ratio, P/E) emerges as an important factor demonstrating presence of mechanisms which facilitate functioning of a plant in non-optimal conditions. Comparison of the P/E values, which were higher in D. antarctica than in D. caespitosa at low and medium temperatures, confirms a high degree of adjustability of the photosynthetic apparatus in D. antarctica and unveils the lack of such a feature in D. caespitosa.     

Growth and reproduction of Antarctic vascular plants in response to warming and UV radiation reductions in the field

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_BE) 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_BE by about 82%), or absorbed both UV-B and UV-A (reducing UV-B_BE and UV-A_BE 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. Received: 4 August 1998 / Accepted: 1 December 1998     

Current status of the Antarctic herb tundra formation in the Central Argentine Islands

Changes in the higher plant populations of the Argentine Islands over the last four to five decades have been central to developing an understanding of the likely biological responses to the globally exceptional rates of regional climate change, in particular warming, experienced along the western Antarctic Peninsula over the same period. In this study, we reassessed local populations and distribution of the two indigenous flowering plants on two islands in this archipelago, the grass Deschampsia antarctica and the pearlwort Colobanthus quitensis , in order to compare with previous partial and detailed surveys carried out by the British Antarctic Survey between 1963 and 1990. Our major finding was that the strong trend of recent increase in population size documented in 1990 has not continued, with current population sizes of both higher plants now being slightly lower than but still comparable with those recorded in the last survey in 1990. We discuss reasons underlying this, including possible limits imposed by the suitability of available habitat, and a recent plateauing of the local climate warming trend in comparison with that seen before the 1990 survey, with no significant short-term warming apparent in annual or seasonal meteorological data since 1990.     

Tufted Hairgrass （Deschampsia caespitosa） Exhibits a Lower Photosynthetic Plasticity than Antarctic Hairgrass （D. antarctica）

The aim of our work was to assess photosynthetic plasticity of two hairgrass species with different ecological origins （a temperate zone species, Deschampsia caespitosa （L.） Beauv. and an Antarctic species, D. antarctica） and to consider how the anticipated climate change may affect vitality of these plants. Measurements of chlorophyll fluorescence showed that the photosystem Ⅱ （PSⅡ） quantum efficiency of D. caespitosa decreased during 4 d of incubation at 4℃ but it remained stable in D. antarctica. The fluorescence half-rise times were almost always lower in D. caespitosa than in D. antarctica, irrespective of the incubation temperature. These results indicate that the photosynthetic apparatus of D. caespitosa has poorer performance in these conditions. D. caespitosa reached the maximum photosynthesis rate at a higher temperature than D. antarctica although the values obtained at 8 ℃ were similar in both species. The photosynthetic water-use efficiency （photosynthesis-to-transpiration ratio, PIE） emerges as an important factor demonstrating presence of mechanisms which facilitate functioning of a plant in non-optimal conditions. Comparison of the PiE values, which were higher in D. antarctica than in D. caespitosa at low and medium temperatures, confirms a high degree of adjustability of the photosynthetic apparatus in D. antarctica and unveils the lack of such a feature in D. caespitosa.     

Karyological studies in <Emphasis Type="Italic">Deschampsia antarctica</Emphasis> Desv. (Poaceae)

Deschampsia antarctica Desv. (Poaceae), known as Antarctic hairgrass, is the only grass species and one of the two vascular plant species native to Antarctica and it is a valuable source for gene discovery associated with freezing-tolerance. In this work the karyotype of D. antarctica collected near Jubany Antarctic Base was reported for the first time. This species presents a chromosome number of 2n = 2x = 26, with karyotypic formula 10m + 6sm + 8st + 2t. The nucleolar organizer region is located in the short arm in one of the submetacentric pairs forming a terminal satellite. Aneusomaty, a phenomenon already reported in the genus, was detected during this study. The cytogenetic information, together with the recent phylogenetic data will be useful for breeding strategies in agronomically valuable crops.     

水分胁迫下发草(Deschampsia caespitosa)叶片脯氨酸及其代谢产物变化

以发草(Deschampsia caespitosa)为供试材料,通过盆栽模拟水分胁迫,研究重度干旱、中度干旱、轻度干旱、植物正常需水量(对照)、轻度水涝、中度水涝、重度水涝处理下发草叶片脯氨酸(Pro)积累状况及其代谢途径中底物、中间产物和关键酶的动态变化,以期从脯氨酸代谢途径对发草抗旱/涝机理进行初步探讨。结果显示,干旱和水涝胁迫前期发草叶片Pro含量显著升高,谷氨酸(Glu)和鸟氨酸(Orn)含量显著下降,Δ¹-吡咯琳-5-羧酸合成酶(P5CS)活性、鸟氨酸转氨酶(δ-OAT)活性、Δ¹-吡咯琳-5-羧酸还原酶(P5CR)活性均显著增强,而脯氨酸脱氢酶(ProDH)活性显著降低,表明干旱和水涝胁迫前期发草叶片通过脯氨酸合成代谢的加强和分解代谢的抑制共同积累脯氨酸,以缓解干旱和水涝胁迫产生的危害,Glu途径和Orn途径协同作用于叶片脯氨酸合成代谢过程。中度、轻度干旱和轻度水涝处理21 d后Pro含量趋于稳定,持续21 d的重度干旱处理和持续28 d的重度水涝处理时发草死亡,共同显示了发草对水涝和干旱具有较强的耐受性。结论为高寒沼泽湿地旱涝"共耐性"植物的研究提供理论基础,同时为利用发草开展退化高寒沼泽湿地植被恢复提供科学依据。     

Photosynthetic and respiratory acclimation and growth response of Antarctic vascular plants to contrasting temperature regimes

Air temperatures have risen over the past 50 yr along the Antarctic Peninsula, and it is unclear what impact this is having on Antarctic plants. We examined the growth response of the Antarctic vascular plants Colobanthus quitensis (Caryophyllaceae) and Deschampsia antarctica (Poaceae) to temperature and also assessed their ability for thermal acclimation, in terms of whole-canopy net photosynthesis (P(n)) and dark respiration (R(d)), by growing plants for 90 d under three contrasting temperature regimes: 7°C day/7°C night, 12°C day/7°C night, and 20°C day/7°C night (18 h/6 h). These daytime temperatures represent suboptimal (7°C), near-optimal (12°C), and supraoptimal (20°C) temperatures for P(n) based on field measurements at the collection site near Palmer Station along the west coast of the Antarctic Peninsula. Plants of both species grown at a daytime temperature of 20°C had greater RGR (relative growth rate) and produced 2.2-3.3 times as much total biomass as plants grown at daytime temperatures of 12° or 7°C. Plants grown at 20°C also produced 2.0-4.1 times as many leaves, 3.4-5.5 times as much total leaf area, and had 1.5-1.6 times the LAR (leaf area ratio; leaf area:total biomass) and 1.1-1.4 times the LMR (leaf mass ratio; leaf mass:total biomass) of plants grown at 12° or 7°C. Greater RGR and biomass production at 20°C appeared primarily due to greater biomass allocation to leaf production in these plants. Rates of P(n) (leaf-area basis), when measured at their respective daytime growth temperatures, were highest in plants grown at 12°C, and rates of plants grown at 20°C were only 58 (C. quitensis) or 64% (D. antarctica) of the rates in plants grown at 12°C. Thus, lower P(n) per leaf area in plants grown at 20°C was more than offset by much greater leaf-area production. Rates of whole-canopy P(n) (per plant), when measured at their respective daytime growth temperatures, were highest in plants grown at 20°C, and appeared well correlated with differences in RGR and total biomass among treatments. Colobanthus quitensis exhibited only a slight ability for relative acclimation of P(n) (leaf-area basis) as the optimal temperature for P(n) increased from 8.4° to 10.3° to 11.5°C as daytime growth temperatures increased from 7° to 12° to 20°C. There was no evidence for relative acclimation of P(n) in D. antarctica, as plants grown at all three temperature regimes had a similar optimal temperature (10°C) for P(n). There was no evidence for absolute acclimation of P(n) in either species, as rates of P(n) in plants grown at a daytime temperature of 12°C were higher than those of plants grown at daytime temperatures of 7° or 20°C, when measured at their respective growth temperatures. The poor ability for photosynthetic acclimation in these species may be associated with the relatively stable maritime temperature regime during the growing season along the Peninsula. In contrast to P(n), both species exhibited full acclimation of R(d), and rates of R(d) on a leaf-area basis were similar among treatments when measured at their respective daytime growth temperature. Our results suggest that in the absence of interspecific competition, continued warming along the Peninsula will lead to improved vegetative growth of these species due to (1) greater biomass allocation to leaf-area production (as opposed to improved rates of P(n) per leaf area) and (2) their ability to acclimate R(d), such that respiratory losses per leaf area do not increase under higher temperature regimes.     

Nitrate nutrition ofDeschampsia flexuosa (L.) Trin. in relation to nitrogen deposition in Sweden

Summary Current and maximally induced nitrate reductase activity (NRA), total-N, nitrate, K, P, Ca, Mg, Mo and sucrose in leaves ofDeschampsia flexuosa was measured three times during the vegetation period in forests along a deposition gradient (150 km) in south Sweden, in north Sweden where the nitrogen deposition is considerably lower, and at heavily N-fertilized plots. In addition, the interaction between nitrogen nutrition and light was studied along transects from clearings into forest in both south and north Sweden. Plants from sites with high nitrogen deposition had elevated current NRA compared to plants from less polluted sites, indicating high levels of available soil nitrate at the former. Current NRA and total N concentration in grass from sites with high deposition resembled those found at heavily N-fertilized plots. Under such circumstances, the ratio current NRA: maximally induced NRA as well as the concentration of nitrate was high, while the concentration of sucrose was low. This suggests that the grass at these sites was already utilizing a large portion of its capacity to assimilate nitrate. Light was found to play an important role in the assimilation of nitrate; leaf concentration of sucrose was found to be negatively correlated with both nitrate and total N. Consequently, grass growing under dense canopies in south Sweden is not able to dilute N by increasing growth. The diminished capacity of the grass to assimilate nitrate will increase leaching losses of N from forests approaching N saturation.