Net ecosystem exchange and whole plant isoprenoid emissions by a mediterranean shrubland exposed to experimental climate change

We tested the effect of forecasted soil drought and warming climate conditions for the next decades on emission rates of isoprenoids by mediterranean shrublands. We measured isoprenoid emissions by whole dominant mediterranean woody plants (Erica multiflora L. and Globularia alypum L.) inhabiting the studied shrublands. Monoterpene emissions were detected in both species, but isoprene was emitted only by E. multiflora. Maximum emission rates were found during the hottest periods (except for G. alypum, in which they occurred in autumn), and minimum emission rates in winter in E. multiflora. Terpene emission rates ranged from 0.08 μg/(g dry wt h) in winter in E. multiflora to 8.8 μg/(g dry wt h) in G. alypum in autumn. In E. multiflora, the terpene emission rates decreased in response to soil drought only in summer, but increased in response to warming in spring and autumn. Isoprene emissions ranged from 0.1 μg/(g dry wt h) in spring to 4.4 μg/(g dry wt h) in summer. The effect of the treatments was only detected in autumn when soil drought and warming had a negative effect on isoprene emission rates. These data might improve our knowledge of isoprenoid emissions at the canopy level and in response to climate change, soil drought, or warming. Published in Russian in Fiziologiya Rastenii, 2009, Vol. 56, No. 1, pp. 35–45. The text was submitted by the authors in English.     

Contrasting growth changes in two dominant species of a Mediterranean shrubland submitted to experimental drought and warming

BACKGROUND AND AIMS: Climate projections predict drier and warmer conditions in the Mediterranean basin in the next decades. The possibility of such climatic changes modifying the growth of two Mediterranean species, Erica multiflora and Globularia alypum, which are common components of Mediterranean shrublands, was assessed. METHODS: A field experiment was performed from March 1999 to March 2002 to prolong the drought period and to increase the night-time temperature in a Mediterranean shrubland, where E. multiflora and G. alypum are the dominant species. Annual growth in stem diameter and length of both species was measured and annual stem biomass production was estimated for 1999, 2000 and 2001. Plant seasonal growth was also assessed. KEY RESULTS: On average, drought treatment reduced soil moisture 22 %, and warming increased temperature by 0.7-1.6 degrees C. Erica multiflora plants in the drought treatment showed a 46 % lower annual stem elongation than controls. The decrease in water availability also reduced by 31 % the annual stem diameter increment and by 43 % the annual stem elongation of G. alypum plants. New shoot growth of G. alypum was also strongly reduced. Allometrically estimated biomass production was decreased by drought in both species. Warming treatment produced contrasting effects on the growth patterns of these species. Warmer conditions increased, on average, the stem basal diameter growth of E. multiflora plants by 35 %, raising also their estimated stem biomass production. On the contrary, plants of G. alypum in the warming treatment showed a 14 % lower annual stem growth in basal diameter and shorter new shoots in spring compared with controls. CONCLUSIONS: The results indicate changes in the annual productivity of these Mediterranean shrubs under near future drier and warmer conditions. They also point to alterations in their competitive abilities, which could lead to changes in the species composition of these ecosystems in the long term.     

Drought advances spring growth phenology of the Mediterranean shrub Erica multiflora

Current climate projections predict drier and warmer conditions in the Mediterranean basin over the next century. While advanced spring growth due to warming has been described in the literature, few data are available on the effects of drought on phenology. Hence, the phenology and growth of two Mediterranean shrubs, Erica multiflora and Globularia alypum, was studied in a rainfall exclusion field experiment to simulate spring drought in a natural shrubland. We estimated the onset of growth in spring by monitoring the appearance of new stems, and the end of growth in summer by following the elongation of stems. Drought treatment caused earlier onset of the spring growing season in E. multiflora, whereas no advance was observed in G. alypum. However, growth cessation was not affected in E. multiflora. Drought reduced the growth of both shrubs, as reflected in less stem elongation. The results show that a drier climate might affect not only growth but also spring phenology of some Mediterranean species. We suggest that a reduction in the cooling effect of transpiration may have analogous effects to warming and might advance the start of growth in E. multiflora, a species whose phenology has been described as warming‐sensitive. The lengthening of the growing season resulting from advanced growth did not imply higher productivity, as growth was restricted by drought.     

Warming and drought alter soil phosphatase activity and soil P availability in a Mediterranean shrubland

We conducted a field experiment simulating the warming and drought in a Mediterranean shrubland dominated by Erica multiflora and Globularia alypum with the aim to simulate the next future climate conditions predicted by the IPCC and ecophysiological models. As P is frequently a limiting nutrient in Mediterranean ecosystems, we investigated the drought and warming effects on soil phosphatases activities, soil P contents and availability, litter and leaf P concentration, and the capacity of this community to maintain soil P reserves and retain this nutrient in the ecosystem. Warming treatment increased soil and air temperature (an average of 1°C) and drought treatment decreased soil water content in one of the seasons analysed (28% in autum 2004). Warming increased (68%) the activities of soil acid phosphatases in summer and alkaline phosphatase activity (22%) in spring 2004, and increased P concentrations in E. multiflora. Instead, warming decreased P concentrations in litterfall of this same species, E. multiflora, and soil HCO₃-extractable P_i (Olsen-P_i) in some seasons, decreasing total P soil concentration (37%) after 6 years of treatment. The drought treatment did not change soil phosphatase activities, nor available P_i. The effects of climate change on soil P dynamics in Mediterranean areas will thus be strongly dependent on whether the main variable involved in the local change is warming or drought. If warming is the main change without significant changes in water availability, the increases of biological activity can accelerate plant growth, P capture by plants and increase soil-phosphatase activity, altogether decreasing P contents in soil. If drought is the main change, a reduction in P demands by plants is expected, increasing P stocks in soils.     

Plant community dynamics in a calcareous grassland under climate change manipulations

This study investigates the effects of field manipulations of local climate to determine the potential impact of climate change on plant community dynamics in a calcareous grassland. The experimental site is located in a grassland at the Wytham estate, Oxfordshire, UK. The one hectare study area is within a 10 ha abandoned arable field on Jurassic corallian limestone. Two climate change scenarios were used: warmer winters with increased summer rainfall and warmer winters with summer drought. Plant cover and species richness were significantly increased in plots receiving supplemented summer rainfall, while the amount of litter was significantly reduced. Litter formation was significantly increased by winter warming and drought. The responses of the plant community to the climate manipulations were related to the life-history attributes of the dominant species. Seedling recruitment was limited by microsite availability, which also varied in the different climate manipulations. The results are discussed in terms of successional dynamics. They suggest that warmer winters may delay succession, as gap formation in the sward will provide sites for colonisation of annuals, thereby enabling their persistence in the sward. Under wetter conditions during summer, perennial grasses tend to close the sward, thereby inhibiting the establishment of later successional species.     

Development of vegetation patterns in early primary succession

Question: We investigated colonisation filters in early plant community development on a glacial outwash plain. We asked if these were related to seed limitation or to a lack of safe sites, if topographical heterogeneity affected species patchiness and how species life cycles influence successional trajectories. Location: An outwash plain (Skeiðarársandur) in southeast Iceland. Methods: We identified surface heterogeneity at two different scales, ca. 10–15 cm (larger stones and established plants) and ca. 50 m (shallow depressions representing dry river beds) at two study sites. We quantified species cover, flowering plant density, seed production, seed rain, seed bank density, seedling emergence and seedling survival from June 2005 to June 2007 for the whole plant community, and measured seed production for five species. Results: Mean vegetation cover was <2.5% at the sites. Low emergence rates and high seedling mortality were the two main recruitment filters. Only 1.4% of seedlings emerging in 2005 survived into the 2007 growing season. Topographical heterogeneity had little effect on plant colonisation. High annual variation was recorded, and the two study sites (ca. 2 km apart) differed in their colonisation success. Of the five species, establishment of Cerastium alpinum and Silene uniflora was most limited by lack of seeds, whereas establishment of Luzula spicata, Poa glauca and Rumex acetosella was most limited by safe sites. Conclusions: We conclude that colonisation processes and patterns in early primary succession on Skeiðarársandur were largely influenced by stochastic factors.     

Drought and warming induced changes in P and K concentration and accumulation in plant biomass and soil in a Mediterranean shrubland

A field experiment involving drought and warming manipulation was conducted over a 6-year period in a Mediterranean shrubland to simulate the climate conditions projected by IPCC models for the coming decades (20% decreased soil moisture and 1°C warming). We investigated P and K concentration and accumulation in the leaves and stems of the dominant species, and in soil. Drought decreased P concentration in Globularia alypum leaves (21%) and in Erica multiflora stems (30%) and decreased K concentration in the leaves of both species (20% and 29%, respectively). The general decrease of P and K concentration in drought plots was due to the reduction of soil water content, soil and root phosphatase activity and photosynthetic capacity that decreased plant uptake capacity. Warming increased P concentration in Erica multiflora leaves (42%), but decreased it in the stems and leaf litter of Erica multiflora and the leaf litter (33%) of Globularia alypum, thereby demonstrating that warming improved the P retranslocation and allocation from stem to leaves. These results correlate with the increase in photosynthetic capacity and growth of these two dominant shrub species in warming plots. Drought and warming had no significant effects on biomass P accumulation in the period 1999–2005, but drought increased K accumulation in aboveground biomass (10 kg ha⁻¹) in Globularia alypum due to the increase in K concentration in stems. The stoichiometric changes produced by the different responses of the nutrients led to changes in the P/K concentration ratio in Erica multiflora leaves, stems and litter, and in Globularia alypum stems and litter. This may have implications for the nutritional value of these plant species and plant–herbivore relationships. The effects of climate change on P and K concentrations and contents in Mediterranean ecosystems will differ depending on whether the main component of change is drought or warming.     

Soil Solution Chemistry and Element Fluxes in Three European Heathlands and Their Responses to Warming and Drought

Soil water chemistry and element budgets were studied at three northwestern European Calluna vulgaris heathland sites in Denmark (DK), The Netherlands (NL), and Wales (UK). Responses to experimental nighttime warming and early summer drought were followed during a two-year period. Soil solution chemistry measured below the organic soil layer and below the rooting zone and water fluxes estimated with hydrological models were combined to calculate element budgets. Remarkably high N leaching was observed at the NL heath with 18 and 6.4 kg N ha^–1 year^–1 of NO₃–N and NH₄–N leached from the control plots, respectively, indicating that this site is nitrogen saturated. Increased soil temperature of +0.5°C in the heated plots almost doubled the concentrations and losses of NO₃–N and DON at this site. Temperature also increased mobilization of N in the O horizon at the UK and DK heaths in the first year, but, because of high retention of N in the vegetation or mineral soil, there were no significant effects of warming on seepage water NO₃–N and NH₄–N. Retention of P was high at all three sites. In several cases, drought increased concentrations of elements momentarily, but element fluxes decreased because of a lower flux of water. Seepage water DOC and DON was highly significantly correlated at the UK site where losses of N were low, whereas losses of C and N were uncoupled at the NL site where atmospheric N input was greatest. Based on N budgets, calculations of the net change in the C sink or source strength in response to warming suggest no change or an increase in the C sink strength during these early years.     

Emulating nurse plants to restore oak forests

Several forested areas of Mexico are dominated by oaks (Quercus spp.), but these forests have suffered strong changes in land use throughout the last century and need to be restored. Most of these areas, however, are within seasonally dry ecosystems and heavy losses of oak seedlings occur in the dry season. Nurse plants that ameliorate extreme environmental conditions have been proved to enhance the success of reforestation practices in these ecosystems. Nevertheless, at several sites in Mexico the density of putative nurse plants is too low to consider this practice as a viable restoration strategy. This study explores the possibility of emulating the effects of nurse plants by means of artificial-shade structures. The study areas were located at the ecological park Flor del Bosque (State of Puebla, México). At the beginning of the rainy season, seedlings of Quercus castanea and Quercus laurina were transplanted beneath and outside artificial-shade structures at two deforested areas of the park. We monitored seedling mortality during 22 weeks, until the middle of dry season; each week we recorded if individual seedlings were killed by abiotic stress or predation. We also measured chlorophyll fluorescence on seedlings to assess whether the shade structures improve their physiological performance. Comparisons of mortality rates indicated that, at both experimental sites, oak seedlings were less affected by drought and predation beneath the shade structures than outside them. Indeed, seedlings beneath these structures showed higher physiological performances. This suggests that artificial-shade structures can emulate the effects of nurse plants at deforested sites, and that this strategy could help to improve the recovery of oak forests.     

气候变暖对领春木沿海拔分布潜在影响的实验模拟

在神农架山地不同海拔高度进行领春木(Euptelea pleiospermum)种子萌发实验和萌发后实生幼苗的存活状况研究,模拟气候变化对领春木分布的潜在影响.结果显示:(1)林窗内种子萌发率显著高于林下,林下的幼苗全部死亡;(2)在领春木的分布上限(2090 m)和以上区域种子萌发率高,且幼苗死亡率低,在领春木的分布下限(900 m)和以下区域种子萌发率低,且除620 m阳坡林下以外,幼苗均全部死亡;(3)种源对领春木种子萌发率和幼苗死亡率均无显著影响;(4)冬季播种的种子比春季播种种子萌发率低,萌发后幼苗死亡率高.研究结果表明,珍稀植物领春木具备向高海拔地区迁移的潜能,但向高海拔的迁移扩散可能会受到一定的限制;在低海拔区域,领春木幼苗更新受到威胁,但一些小生境可以缓冲气候变暖带来的威胁.     

Implications of climate warming on seedling emergence and mortality of African savanna woody plants

Seedling emergence from scarified seeds and mortality of different seedling cohorts of five African savanna woody species (Acacia polyacantha, A. sieberana, Bauhinia thonningii, Dichrostachys cinerea and Ziziphus abyssinica) were studied under field conditions at a site in central Zambia. The study was conducted over a 4-year period, from 2003 to 2007. The objectives of the study were to determine climate factors that significantly influence seedling emergence rate and mortality in order to assess likely responses of the studied species to a warmer climate. Mean seedling emergence rate was 12% in D. cinerea and Z. abyssinica, 17% in B. thonningii, 47% in A. poyacantha and 62% in A. sieberana. Climate factors did not significantly affect seedling emergence in A. sieberana while temperature significantly influenced seedling emergence rate in the other species. Under a 1° warmer climate, seedling emergence rate was predicted to decline in A. polyacantha, B. thonningii and Z. abysssinica but is likely to increase slightly in D. cinerea. Time of seedling emergence during the wet season did not appear to affect seedling survival. Temperature also significantly influenced seedling mortality in all the studied species such that under a warmer climate, mortality was predicted to increase in A. sieberana and D. cinerea but decrease in A. polyacantha, B. thonningii and Z. abyssinica. As the studied species exhibited differential optimum temperature conditions for seedling emergence and seedling survival, they are likely to respond to climate warming in different but predictable ways. The results of the study are useful to forest management and development of climate change adaptation strategies in southern Africa.     

Recruitment limitations in Primula vulgaris in a fragmented landscape

Plant recruitment is limited by dispersal, if seeds cannot arrive at potential recruitment sites, and by establishment, due to a low availability of safe sites for recruitment. Seed-sowing experiments, scarcely applied along gradients of landscape alteration, are very useful to assess these limitations. Habitat loss and fragmentation may foster recruitment limitations by affecting all the processes from seed dispersal to seedling establishment. In this study, we perform a seed-sowing experiment to disentangle the importance of dispersal and establishment limitations in different stages of recruitment of the perennial herb Primula vulgaris in fragmented forests of the Cantabrian Range (Northwestern Spain). We evaluated the influence of ecological gradients resulting from habitat loss and fragmentation (modifications of habitat amount at the landscape and microhabitat scales, changes in the species’ population size, changes in seed predation and seedling herbivory) on seedling emergence, survival and early growth. We found strong evidence of dispersal limitation, as seedling emergence was very low in experimental replicates where no seeds were added. This limitation was independent of landscape alterations, as we found no relation with any of the ecological gradients studied. Establishment limitations at the germination phase were also unrelated to ecological gradients, probably because these limitations are more related to fine-scale environmental gradients. However, further monitoring revealed that seedling survival after summer and winter periods and seedling growth were conditioned by landscape alteration, as we found effects of habitat amount at the landscape and microhabitat scales, of presence of populations of P. vulgaris and of seedling herbivory. These effects were complex and sometimes opposite to what can be expected for adult plants, revealing the presence of different requirements between life stages.