Elisa Carrari

Diversity of secondary woody species in relation to species richness and cover of dominant trees in thermophilous deciduous forests

ABSTRACT One of the prominent compositional features of thermophilous deciduous forests of southern Europe is their richness in secondary woody species (AWS). To date, no studies have focused on the diversity and contribution to the ecosystem functions and socio-economic benefits provided by these species. Here, we first characterized species richness and diversity for three main functional groups of AWS, sporadic trees, shrubs and vines, in 53 representative stands of central Italy. Secondly, we explored variations in AWS diversity and composition along a four-level gradient of species richness of dominant trees under comparable site conditions. The 40 AWS taxa recorded showed a remarkable taxonomic singularity at both the genus and the family rank. Dominant tree species richness had no significant effect on AWS species richness and composition, while Shannon diversity of AWS, especially shrubs, was associated with cover of the dominant trees. Each AWS possessed, on average, ca. five ability traits over a total of 13 considered that contributed to important ecosystem functions or expected socio-economic benefits such as wood and food production, conservation of genetic resources (wild crop relatives) and ornamentals. This highlights the key role of AWS, and in particular of sporadic trees, for enhancing multifunctionality of thermophilous deciduous forests.

Ozone-induced stomatal sluggishness changes stomatal parameters of Jarvis-type model in white birch and deciduous oak

Stomatal ozone flux is closely related to ozone injury to plants. Jarvis-type multiplicative model has been recommended for estimating stomatal ozone flux in forest trees. Ozone can change stomatal conductance by both stomatal closure and less efficient stomatal control (stomatal sluggishness). However, current Jarvis-type models do not account for these ozone effects on stomatal conductance in forest trees. We examined seasonal course of stomatal conductance in two common deciduous tree species native to northern Japan (white birch: Betula platyphylla var. japonica; deciduous oak: Quercus mongolica var. crispula) grown under free-air ozone exposure. We innovatively considered stomatal sluggishness in the Jarvis-type model using a simple parameter, s, relating to cumulative ozone uptake (defined as POD: phytotoxic ozone dose). We found that ozone decreased stomatal conductance of white birch leaves after full expansion (-28%). However, such a reduction of stomatal conductance by ozone fell in late summer (-10%). At the same time, ozone reduced stomatal sensitivity of white birch to VPD and increased stomatal conductance under low light conditions. In contrast, in deciduous oak, ozone did not clearly change the model parameters. The consideration of both ozone-induced stomatal closure and stomatal sluggishness improved the model performance to estimate stomatal conductance and to explain the dose-response relationship on ozone-induced decline of photosynthesis of white birch. Our results indicate that ozone effects on stomatal conductance (i.e. stomatal closure and stomatal sluggishness) are crucial for modelling studies to determine stomatal response in deciduous trees, especially in species sensitive to ozone.

Responses of serpentine plants to pine invasion: Vegetation diversity and nickel accumulation in species with contrasting adaptive strategies

Introduction of non-native trees is one of the major threats to ecosystem integrity and biodiversity. Stands of maritime pine (Pinus pinaster Ait.) introduced decades ago represent a threat to the specialized plant communities of serpentine outcrops in Italy. This study investigates the effects of such invasions at the community and species level, based on vegetation sampling in three selected sites with comparable environmental conditions. Pine cover caused a decrease of α-diversity by lowering the species evenness of the community, though species richness was not negatively affected. Compositional changes between the two habitats were significant but not clearly associated with a decrease in taxonomic distinctness in the pine stands. As many as nine indicator species were found in the open vegetation, along with the obligate endemics Odontarrhena bertolonii and Armeria denticulata. Both of them declined in the pine stands. Here, an increase in the phytoavailable nickel fraction was associated with a decrease in total nickel concentration in the soil, via mobilization of the metal caused by lowering of pH induced by the conifer litter. The nickel-hyperaccumulator O. bertolonii was able to maintain high metal concentrations in the shoots despite a decrease in root concentration, resulting in a higher shoot/root ratio in the pine stands (~20). Conversely, shoot/root ratio in the non-accumulator Plantago holosteum was <1 and not affected by the conifer, as well as its abundance in this anthropogenic habitat. Contrasting responses of the two species were likely due to their different sensitivity to modified light and soil conditions, whereas stability of shoot nickel-concentration in O. bertolonii did not support increased predation by natural enemies as one of the causes for its decline under the conifer. Progressive thinning of these stands is advocated to limit soil nickel mobilization and to restore a unique ecosystem with its endemic metallophytes.

Should we see urban trees as effective solutions to reduce increasing ozone levels in cities

Outdoor air pollution is considered as the most serious environmental problem for human health, associated with some million deaths worldwide per year. Cities have to cope with the challenges due to poor air quality impacting human health and citizen well-being. According to an analysis in the framework of this study, the annual mean concentrations of tropospheric ozone (O3) have been increasing by on average 0.16 ppb year-1 in cities across the globe over the time period 1995-2014. Green urban infrastructure can improve air quality by removing O3. To efficiently reduce O3 in cities, it is important to define suitable urban forest management, including proper species selection, with focus on the removal ability of O3 and other air pollutants, biogenic emission rates, allergenic effects and maintenance requirements. This study reanalyzes the literature to i) quantify O3 removal by urban vegetation categorized into trees/shrubs and green roofs; ii) rank 95 urban plant species based on the ability to maximize air quality and minimize disservices, and iii) provide novel insights on the management of urban green spaces to maximize urban air quality. Trees showed higher O3 removal capacity (3.4 g m-2 year-1 on average) than green roofs (2.9 g m-2 year-1 as average removal rate), with lower installation and maintenance costs (around 10 times). To overcome present gaps and uncertainties, a novel Species-specific Air Quality Index (S-AQI) of suitability to air quality improvement is proposed for tree/shrub species. We recommend city planners to select species with an S-AQI>8, i.e. with high O3 removal capacity, O3-tolerant, resistant to pests and diseases, tolerant to drought and non-allergenic (e.g. Acer sp., Carpinus sp., Larix decidua, Prunus sp.). Green roofs can be used to supplement urban trees in improving air quality in cities. Urban vegetation, as a cost-effective and nature-based approach, aids in meeting clean air standards and should be taken into account by policy-makers.

Ozone risk assessment is affected by nutrient availability: Evidence from a simulation experiment under free air controlled exposure (FACE)

Assessing ozone (O3) risk to vegetation is crucial for informing policy making. Soil nitrogen (N) and phosphorus (P) availability could change stomatal conductance which is the main driver of O3 uptake into a leaf. In addition, the availability of N and P could influence photosynthesis and growth. We thus postulated that the sensitivity of plants to O3 may be changed by the levels of N and P in the soil. In this study, a sensitive poplar clone (Oxford) was subject to two N levels (N0, 0 kg N ha-1; N80, 80 kg N ha-1), three P levels (P0, 0 kg P ha-1; P40, 40 kg P ha-1; P80, 80 kg P ha-1) and three levels of O3 exposure (ambient concentration, AA; 1.5 × AA; 2.0 × AA) for a whole growing season in an O3 free air controlled exposure (FACE) facility. Flux-based (POD0 to 6) and exposure-based (W126 and AOT40) dose-response relationships were fitted and critical levels (CLs) were estimated for a 5% decrease of total annual biomass. It was found that N and P availability modified the dose-response relationships of biomass responses to O3. Overall, the N supply decreased the O3 CLs i.e. increased the sensitivity of poplar to O3. Phosphorus alleviated the O3-caused biomass loss and increased the CL. However, such mitigation effects of P were found only in low N and not in high N conditions. In each nutritional treatment, similar performance was found between flux-based and exposure-based indices. However, the flux-based approach was superior, as compared to exposure indices, to explain the biomass reduction when all nutritional treatments were pooled together. The best O3 metric for risk assessments was POD4, with 4.6 mmol m-2 POD4 as a suitable CL for Oxford poplars grown under various soil N and P conditions.

Protecting the photosynthetic performance of snap bean under free air ozone exposure

Tropospheric ozone (O3) is a major air pollutant and causes serious injury to vegetation. To protect sensitive plants from O3 damage, several agrochemicals have been assessed, including cytokinin (e.g., kinetin, KIN) and ethylenediurea (EDU) with cytokinin-like activity. In higher plant, leaves are primarily injured by O3 and protective agrochemicals are often applied by leaf spraying. To our knowledge, the mitigating abilities of EDU and KIN have not been compared directly in a realistic setup. In the present research, impacts of elevated O3 (2× ambient O3, 24hr per day, for 8days) on an O3 sensitive line (S156) of snap bean (Phaseolus vulgaris), which is often used for biomonitoring O3 pollution, were studied in a free air controlled exposure system. The day before starting the O3 exposure, plants were sprayed with a solution of EDU (300ppm), KIN (1mmol/L) or distilled water, to compare their protective abilities. The results demonstrated that 2× ambient O3 inhibited net photosynthetic rate and stomatal conductance, increased the minimal fluorescence yield of the dark-adapted state, decreased the maximal quantum yield of PSII photochemistry, and led to visible injury. KIN and EDU alleviated the reduction of the photosynthetic performance, and visible injury under O3 fumigation. The plants sprayed with EDU showed greater ability to mitigate the O3 damage than those sprayed with KIN. Chlorophyll fluorescence imaging may have detected more precisely the differences in O3 response across the leaf than the conventional fluorometer.