Notes on Quercus ilex L. in Liguria (NW Italy)

The climatic characteristics of the Liguria region have been outlined by means of raw data and derived indices (water balance related to potential evapotraspiration, Rivas-Martinez's index of mediterraneity and thermicity, De Martonne's index of dryness, and Emberger's pluviothermic quotient and index of summer dryness). Their interpretation suggests that Liguria is a boundary region between two different climatic areas: the Mediterranean and that of Central Europe.The distribution of Quercus ilex communities in Liguria have suffered the heavy consequences of human activity on the coastal belt of the region. There is, however, sufficient evidence to show that their presence is closely linked to climatic conditions. Quercus ilex communities are absent from the western coastal belt where the annual water balance is below –50 mm and high mean temperatures occur. They are also absent from marly limestone dominated areas where annual water balance is below 300 mm. Quercus ilex woods show a preference for water balance values between 0 and 600, with the better developed forests being found in central and eastern coastal Liguria.The Ligurian Quercus ilex woods are ascribed to Quercetum ilicis Br.-Bl. 1915. Given that the presence within their floristic composition of a group of species of Querco-Fagetea (more species in eastern stands, few in western ones) is an almost constant characteristic, the subass. fraxino-ostryetosum Mariotti 1984 can be retained for many of them. Quercus ilex is not present in the initial succession stages of Mediterranean Liguria vegetation. It can be found in later stages when the vegetation is more fully developed towards a maquis type structure. In this case it tends to become dominant and the community develops (if no fire or other human interference occurs) into a Quercus ilex forest. Frequently, this succession takes place under an old tree layer cover (generally Pinus sp.), a remanant of man's past management of the vegetation in the area.     

The efficiency of nitrogen retranslocation from leaf biomass in Quercus ilex ecosystems

Nitrogen retranslocation from senescing leaves represents a crucial adaptation by tree species towards a more efficient use of this nutrient. As a result, this part of the nitrogen cycle has received increasing attention in recent years. However, there remain strong discrepancies with respect to the factors responsible for interspecific differences in the efficiency of this process.In the present work the seasonal pattern of leaf growth and the movement of nitrogen in leaves have been studied in a series of Quercus ilex plots with different levels of rainfall and soil quality in central-western Spain, as well as in 20 other woody species typical of this area. The percentage of nitrogen retranslocated was estimated from the difference between the maximum mass of nitrogen stored in the leaf biomass and the amount of this nutrient returned annually to the soil through leaf fall. Q. ilex appears as one of the least efficient species in the Mediterranean region in the recovery of nitrogen from senescing leaves (29.7% of the maximum pool). Furthermore, the older leaves of Q. ilex do not show the cycles of nitrogen withdrawal during new flushes of shoot growth, such as occurs in Pinus spp. This suggests that older leaves in Q. ilex do not play an important role as nitrogen storage organs.     

Germination of Quercus ilex and Q. pubescens in a Q. ilex coppice

Quercus ilex L. (holm oak) coppices, widespread around the Mediterranean basin, are probably the result of 5 000 years of prolonged human disturbance of the original Quercus pubescens Willd. (downy oak) forests. Since disturbance has almost ceased in recent years, a question arises as to the development of these coppices: will the Q. pubescens forests return, or will Q. ilex remain the dominant species? To investigate the phenomenon, we analyzed the first stages, i.e. germination of the two species in holm oak coppices. Our experiments show that both species germinated better in coppices than in clearings or clear-cuts. Moreover, Q. pubescens appears to be slightly favored above Q. ilex and it is suggested auto-allelopathy is involved, at least partially inhibiting the germination of Q. ilex.     

Leaf flavonoid content in Quercus ilex L. resprouts and its seasonal variation

Here, we provide the first report on flavonoid content in holm oak (Quercus ilex L.) leaves, analyzed by HPLC–MS/MS. Flavanols and flavonols were the predominant groups, although proanthocyanidins and many soluble tannins had a relevant presence in all leaf samples. Seasonal variation of flavonoids was determined in extracts from Q. ilex leaves during resprouting after a forest fire in two Mediterranean forests. Similar seasonal trends were observed over 2 years during the two main stress seasons (winter and summer). The most abundant flavonoid was the flavanol epicatechin, which showed similar values during the two seasons. Hexosides of the flavonols, quercetin, kaempferol and rhamnetin showed considerably higher content in winter, especially at the lowest temperatures. These variations in both forests are discussed on the basis of the chlorophyll fluorescence results obtained. Anthocyanins were found practically absent in mature leaves. Nutrient or water availability differences between sites or seasons were not related to changes in leaf flavonol-hexoside content.     

Leaf temperature effects on gas exchange in Quercus ilex L. growing under field conditions

ABSTRACT

Gas exchange temperature dependence in Quercus ilex shrubs growing in the Mediterranean maquis was analysed. The gas exchange trend was monitored during the year: photosynthetic activity (A _net) reached the highest average rates in early spring and autumn (12.5 µmol m^-2s^-1 was the absolute maximum A _net measured) and the lowest rates were monitored in the middle of June. There was a good correlation (r = 0.72) between A _net and g _s (A _net = 4.1246 ln g _s + 4316; P < 0.01), indicating that stomatal control of CO₂ diffusion plays an important role in controlling photosynthetic activity. Leaf temperature allowing the highest photosynthetic and stomatal conductance rates of Quercus ilex were in the range 17.5 – 29°C. A _net and g_s dropped below half its maximum value when leaf temperatures were below 11.5°C and above 35.7°C. Transpiration rates (E) were strongly related to leaf temperature; E increased as leaf temperature increased and the highest E rates were monitored in June, despite a 46% decrease in g _s. Leaf water loss from transpiration, during the drought period, could result in leaf water stress which would exacerbate heat effects on photosynthesis. During summer, the increase in leaf temperatures decreased g _s which in turn decreased A _net. Consequently, stomatal control in Quercus ilex may be considered as an adaptive strategy during drought.     

Modifications of the leaf surface structures of Quercus ilex L. in open, naturally CO2-enriched environments

Two Italian CO₂ springs allowed us to study the long-term effect of a 350–2600 μ mol mol^–1 increase in CO₂ concentrations on the surface structures of leaves of Quercus ilex L. Carbon dioxide increased the quantity of cuticular waxes, above an apparent threshold of 750 μ mol mol^–1 CO₂. Leaf wettability was not modified by CO₂ concentrations. Reduction in stomatal frequency was observable up to 750 μ mol mol^–1 CO₂, the slope being almost the same as that estimated for the increase in CO₂ concentration from pre-industrial times to the present. At higher concentrations, CO₂ seemed to exert no more impact on stomatal frequency.     

Annual and seasonal changes in fine root biomass of a Quercus ilex L. forest

The biomass, production and mortality of fine roots (roots with diameter <2.5 mm) were studied in a typical Mediterranean holm oak (Quercus ilex L.) forest in NE Spain using the minirhizotron methodology. A total of 1212 roots were monitored between June of 1994 and March of 1997. Mean annual fine root biomass in the holm oak forest of Prades was 71±8 g m^–2 yr^–1. Mean annual production for the period analysed was 260+11 g m^–2 yr^–1. Mortality was similar to production, with a mean value of 253±3 g m^–2 yr^–1. Seasonal fine root biomass presented a cyclic behaviour, with higher values in autumn and winter and lower in spring and summer. Production was highest in winter, and mortality in spring. In summer, production and mortality values were the lowest for the year. Production values in autumn and spring were very similar. The vertical distribution of fine root biomass decreased with increasing depth except for the top 10–20 cm, where values were lower than immediately below. Production and mortality values were similar between 10 and 50 cm depth. In the 0–10 cm and the 50–60 cm depth intervals, both production and mortality were lower.     

Implications of foliar terpene content and hydration on leaf flammability of Quercus ilex and Pinus halepensis

We investigated the implications of foliar hydration and terpene content on leaf flammability in two widely distributed forest species of the Mediterranean basin, Quercus ilex, which does not store terpenes, and Pinus halepensis, a terpene-storing species. The experiments were carried out in plants grown under different water regimes that generated a wide range of foliar hydration and terpene contents. We monitored the temperatures and time elapsed to reach the smoke, pyrolysis and flame phases. Smoke appeared much earlier (37 versus 101 s) and at lower temperatures (96 versus 139 degrees C) in Quercus ilex than in Pinus halepensis. Quercus ilex reached pyrolysis earlier than Pinus halepensis (278 versus 338 s) but at the same temperature (365-371 degrees C). There were no significant differences in time elapsed nor in temperature for flammability (386-422 s; 505-487 degrees C in both species). Quercus ilex had lower water hydration than Pinus halepensis (41 versus 100%) and the leaf content of terpenes in Quercus was three orders of magnitude lower. The results of this study show no differences in the flame phase between the two species and the absence of a significant relationship between temperature and elapsed time of the different flammability phases in relation to monoterpene content; thus indicating that the role of monoterpenes in flammability phases is smaller than that of the water content. This, however, does not exclude the effects of terpene content on plant combustibility and fire propagation once fires start.     

The influence of summer and winter stress and water relationships on the distribution of Quercus ilex L.

Several aspects of plant-water relationships and canopy behaviour have been studied over several years in two experimental areas of Quercus ilex L. forests in the northeastern Iberian Peninsula. Water requirements, water status limits necessary for a positive carbon budget and the functional canopy behaviour in the face of abiotic stress were evaluated, in order to determine which factors influence the geographical range of these forests.The results showed that holm oak has a conservative water use, a low cuticular transpiration, a high capacity for osmotic adjustement and xerophytic characteristics in leaf morphology and canopy arrangement. More than 440 mm of annual rainfall are required for these forests to persist. Summer drought and winter cold are thus important abiotic factors limiting the distribution of Quercus ilex. In both cases, drought stress is involved.     

Factors affecting radial growth of Quercus ilex L. in a coppice stand in southern France

Quercus ilex woodlands in the south of France are characterized by low productivity. Several hypotheses have been put forward to provide an explanation for this phenomenon: (i) senescence of coppice stumps; (ii) limited availability of water and nutrient resources; (iii) intershoot and inter-plant competition. To test some of these hypotheses, growth response to rainfall distribution and nutrient supply (NPK), and the influence of edge effects after clear-cutting were studied in a 40-yr old stand.Curves of radial increment were compared with precipitation and umbrothermic patterns for the 6 years studied. Water availability apparently accounted for some inter-annual differences in growth. However, it was not mean annual rainfall, but rather rainfall occurring in late spring-summer (June, July) which played a large role in determining radial growth. Radial growth was also increased by nutrient supply. Its positive effect was strengthened by favourable rainfall in 1987. Edge-effects following a clear-cut were also found, both in the first year and in four succeeding years following the cut. This comprised in greater radial growth rate for all shoots, with reduced repression of smaller shoots by the larger ones. The effect decreased over a distance of 10 m with increasing distance from the cut field. This could be explained by increased availability of water, nutrient resources, and light.     

Variation in tissue element concentrations in Quercus ilex L. over a range of different soils

In order to study the variability in nutrient concentrations in four tissues of Q. ilex in relation to soil properties, we selected fifteen stands in both Quercus ilex forests and Q. ilex-Pinus halepensis mixed forests. These stands had developed on soils derived from eight different parent materials. Three soil groups were differentiated according to their chemical properties: calcareous soils, siliceous soils, and volcanic soils. Across sites, nutrient concentrations were generally less variable in current-year tissues than in older tissues. Nitrogen and potassium showed the lowest variability among sites, their concentrations in current-year leaves ranging from 1.17% to 1.39% for N and from 0.53% to 0.68% for K. There were few statistically significant correlations between tissue element concentrations, the most frequent being the antagonistic relationship between calcium and magnesium. Nitrogen concentration in current-year leaves was negatively correlated with soil chemical fertility (nitrogen, phosphorus and potassium). This may reflect a nutritional imbalance between nitrogen and other nutrients, some of which may be more limiting than nitrogen to Q. ilex growth in Catalonia forests. Negative correlations were also found between plant magnesium and soil calcium, and positive correlations between plant calcium and soil calcium.     

Seasonal and diurnal monitoring of leaf polyamine content in Quercus ilex L. resprouts after fire in relation to changes in irradiation and photosynthetic parameters

Seasonal variations in free putrescine, spermidine and spermine content, gas-exchange and chlorophyll fluorescence parameters were followed during winter and summer on leaves of a similar age from undisturbed holm oak trees (control, C) and resprouts (R) originated after fire. We observed a general trend of putrescine content decrease with increasing irradiance. Putrescine content decreased markedly from winter to summer, especially in R, which were located on a site with much higher irradiation. Daily summer variations in putrescine showed a decline at midday from morning values, and they were also more accentuated in R. Measurement of gas-exchange and chlorophyll fluorescence parameters showed marked differences between C and R under their respective light conditions. R showed higher values of PSII quantum yield (Φ_PSII), photochemical quenching (qP) and intrinsic efficiency of open PSII centres () The Φ_PSII/PPFD response curve showed that under the same irradiance, Φ_PSII was enhanced in R and mainly under high light conditions. In spite of increasing irradiance from winter to summer, and especially in burned areas, the mentioned chlorophyll fluorescence parameters were maintained indicating the adaptation of the photosynthetic apparatus. Results derived from A/C _i and A/PPFD response curves showed enhanced photosynthetic capacity and lower non-stomatal limitation of photosynthesis in R during summer stress. The contribution of putrescine decline in the photoadaptation of the photosynthetic apparatus of species growing in natural forest habitats is considered.     

Assessment of stress conditions in Quercus ilex L. leaves by O-J-I-P chlorophyll a fluorescence analysis

This reasearch, carried out within the MON.I.TO. programme (Intensive Monitoring of Forests in Tuscany, central Italy), was aimed at defining the stress conditions of two Quercus ilex woods at Colognole (CL) and Cala Violina (CV). In vivo chlorophyll fluorescence was measured for a whole year (1997 - 1998) using a portable 'continuous fluorescence' fluorimeter. The fluorescence parameters were calculated using the values at the steps 0-J-I-P of the polyphasic rising transient (this analysis is called JIP-test). Measurements were performed monthly, pre-dawn and midday. The findings revealed a condition of stress (drought and high temperatures) in the summer at CV during the central hours of the day, and a winter cold stress condition at CL observed in the pre-dawn measurements. At CV, summer stress was combined with processes of biologically regulated de-excitation; at CL, the winter stress appeared to be linked to structural damage affecting the photosynthetic apparatus. Results are discussed in relation to the existing literature on stress affecting Mediterranean vegetation, and to the findings reported in previous investigations in the same study areas.     

Evaluation of actual evapotranspiration of a Quercus ilex L. stand by the Bowen Ratio-Energy Budget method

Actual evapotranspiration from a closed-canopy Quercus ilex L. stand has been estimated by applying the Bowen Ratio-Energy Budget method. Daily water loss was 3.5 mm day^–1, with a peak rate near 0.6 mm hour^–1. The phenomenon of thermal inversion, quite common in mediterranean climates, seemed to play a significant role in reducing evapotranspiration, by promoting dew formation and delaying the establishment of fluxes of latent and sensible heat away from the canopy. Dew, which may form over many hours in the night, appears to be a major sink of available energy in the early morning and may represent a useful water source for stressed foliage. The alternating processes of condensation and evaporation may have a beneficial effect on the closed stand micro-environment.     

Different responses to drought and freeze stress of Quercus ilex L. growing along a latitudinal gradient

The vulnerability to drought and freeze stress was measured in young plants of Quercus ilex L. growing in the field in two natural sites within the Italian distribution area of this species, i.e. Sicily (Southern Mediterranean Basin) and Venezia Giulia (Northeastern Italy), respectively. In particular, the resistance strategies adopted by Q. ilex to withstand the two stresses were estimated in terms of seasonal and/or diurnal changes in leaf conductance to water vapour (g_L), water potential (_L) and relative water content (RWC) as well as of xylem embolism in the stem and root hydraulic conductance (K_RL). Sicilian (SI) plants showed to reduce water loss by stomatal closure (g_L decreased) in summer, thus maintaining average RWCs at 88–90%. Moreover, SI plants showed considerable resistance to xylem cavitation in the stem (the loss of hydraulic conductance, PLC, was less than 12% throughout the year) and to maintain the hydraulic conductance of their roots (K_RL), constantly high even in summer. Plants growing in Venezia Giulia (VG plants), on the contrary, underwent leaf dehydration in the winter due to freeze stress so that RWC measured in April was still 78% on a diurnal basis. This was apparently due to consistent xylem embolism in the stem. In fact, PLC was as high as 40% between November and March. Only in the summer was PLC similar to that recorded in SI plants. Moreover, K_RL of VG plants decreased in November from about 1.5 to 0.8×10^–4 kg s^–1 m^–2 MPa^–1, i.e. about 50%, and in February K_RL dropped further to 0.4×10^–4 kg s^–1 m^–2 MPa^–1. On the basis of the above, we conclude that: (a) Q. ilex was more sensitive to freeze than to drought stress so that freeze stress can be considered as a factor limiting the distribution area of this species; (b) drought and freeze stress were faced by Q. ilex adopting two different resistance strategies, i.e. drought avoidance based on water saving in Sicily and freeze tolerance in Venezia Giulia.     

Erinea formation on Quercus ilex leaves: anatomical, physiological and chemical responses of leaf trichomes against mite attack

Structures on the surfaces of leaves, such as dense layers of non-glandular trichomes, strongly affect phylloplane mite activities. On the other hand the feeding of eriophyoid mites on leaf surfaces can cause hyperplasia of leaf trichomes (erinea formation). In many cases the hyperplasia is accompanied by the accumulation of pigments within trichome cells, causing an impressive red-brown colouration of the erineum. There is no information, however, on the structure of these pigments as well as on the chemical alterations in the phenolic content of plant trichomes in response to mite attack. Erinea formation on the abaxial surface of Quercus ilex leaves upon Aceria ilicis (Acari: Eriophyoidea) attack provides an excellent model on this topic. Differences in the structure and chemical composition of isolated trichomes derived either from healthy (normal trichomes) or mite attacked (hypertrophic trichomes) leaves were examined. Carbon investment was comparable between the two different trichome types, but the cell walls of the hypertrophic trichomes appeared thinner and did not contain microcrystalline cellulose. Observations under the fluorescence microscope showed that the emitted fluorescence was different between the two trichome types, indicating a different composition in fluorescencing phenolic compounds. The chemical analyses confirmed that hypertrophic trichomes contained higher concentrations of the feeding deterrents proanthocyanidin B3 and catechin, as well as of quercetin-3-O-glucoside, but lower concentrations of acylated flavonoid glycosides, than the normal ones. The results showed that the structural and functional changes in leaf trichomes upon mite attack may be an effort of the leaf to compensate the damage caused by the pest.     

Polyphenol deposition in leaf hairs of Olea europaea (Oleaceae) and Quercus ilex (Fagaceae)

The subcellular localization (cytoplasm, vacuoles, cell walls) of polyphenol compounds during the development of the multicellular nonglandular leaf hairs of Olea europaea (scales) and Quercus ilex (stellates), was investigated. Hairs of all developmental stages were treated with specific inducers of polyphenol fluorescence, and the bright yellow-green fluorescence of individual hairs was monitored with epifluorescence microscopy. During the early ontogenetic stages, bright fluorescence was emitted from the cytoplasm of the cells composing the multicellular shield of the scales of O. europaea. Transmission electron micrographs of the same stages showed that these cells possessed poor vacuolation and thin cell walls. The nucleus of these cells may be protected against ultraviolet-B radiation damage. The progressive vacuolation that occurred during maturation was followed by a shifting of the bright green-yellow fluorescence from the perinuclear region and the cytoplasm to the cell walls. The same trends were observed during the development of the nonglandular stellate hairs of Quercus ilex, in which maturation was also accompanied by a considerable secondary thickening of the cell walls. Despite the differences in morphology, high concentrations of polyphenol compounds are initially located mainly in the cytoplasm of the developing nonglandular hairs, and their deposition on the cell walls takes place during the secondary cell wall thickening. These structural changes during the development of the leaf hairs make them a very effective barrier against abiotic (uv-B radiation) and probably biotic (pathogenic) stresses.     

Evidence of the Photosynthetic Origin of Monoterpenes Emitted by Quercus ilex L. Leaves by 13C Labeling

The carbon of the four main monoterpenes emitted by Quercus ilex L. leaves was completely labeled with 13C after a 20-min feeding with 99% 13CO2. This labeling time course is comparable with the labeling time course of isoprene, the terpenoid emitted by other Quercus species and synthesized in leaf chloroplasts. It is also comparable with that of phosphoglyceric acid. Our experiment therefore provides evidence that monoterpenes emitted by Q. ilex are formed from photosynthesis intermediates and may share the same synthetic pathway with isoprene. By analyzing the rate and the distribution of labeling in the different fragments, we looked for evidence of differential carbon labeling in the [alpha]-pinene emitted. However, the labeling pattern was quite uniform in the different fragments, suggesting that the carbon skeleton of the emitted monoterpenes comes from a unique carbon source.