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.     

Regeneration by sprouting of holm-oak (Quercus ilex) stands exploited by selection thinning

Holm-oak forests of the Montseny Massif have traditionally been exploited by selection thinning, which consists of a partial removal of standing trees and biomass. Cutting percentages within each size class indicate that this management technique represents a compromise between forest improvement and harvesting.Sprouting is the mean regeneration process of these holm-oak stands. The mean number of living sprouts per stool is considerably higher than in other coppiced hardwood species, and decreases along the cutting-cycle according to a negative exponential function. Mean number of surviving sprouts per stool is significantly correlated with regenerative age and stool size. Death of sprouts throughout the cutting cycle is compensated by growth of surviving sprouts. Thus, the weight of living sprouts per stool increases linearly with time, and also with the basal area coppiced.Sprouts developing within the first fifteen years of regeneration belong to a single cohort that appears during the first year after cutting. After that period of time, new sprouts appear from dormant buds, so that after 30 years of regeneration a wide range of cohorts of different ages are present.     

Processus dynamiques de reconstitution dans la série du Quercus ilex en Corse

The use of permanent plots in representative sites of various phytosociologically defined stages of the Quercus ilex series in NW Corsica allows us to analyse the natural dynamical process occurring in the development sequence. The regular floristic analysis of stands and transects (mainly using a linear cover measure) during 5 yr not only shows variations of composition and structure but also results in a diachronic understanding of the temporal relations between the stages. The study of forest plots (structure, number and development of seedlings) enables us to generalize the principle of ponctual and cyclic self-transformation of climatic and subclimatic stages.

Avec la collaboration de Mme D. Chiaverini pour l'illustration. II nous est agréable de remercier ici l'A.P.E.E.M. (Association pour l'Etude Ecologique du Maquis) pour la possibilité qui nous a été donnée d'utiliser les installation du laboratoire de Pirio, ce qui a grandement facilité les campagnes successives de terrain.     

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.     

Organic matter distribution and fluxes within a holm oak (Quercus ilex L.) stand in the Etna volcano

Located at 1100 m above sea level, on the western site of the Etna volcano, the ecophysiology of the Mount Minardo holm oak coppice has been investigated for more then twenty years. In this stand, now 31 years old, the above ground biomass amounts to 15000 g m^-2 of organic material, including leaves and perennial woody material. During these 31 years, the mean annual production has been around 775 g m^-2. The yearly mean litterfall amounts to 310 g m^-2, including 200 g m^-2 of leaves, mostly two years old. The soil surface is covered by a litter layer amounting to 3150 g m^-2. Each year, following Jenny's decomposition rate and field measurements, 290 g m^-2 of the litter turns into CO₂, or becomes incorporated in the soil organic matter.     

Effect of cleaning and thinning on height growth and girth increment in holm oak coppices (Quercus ilex L.)

This article studies the influence of cleaning or thinning intensity and of coppice age at the time of silvicultural treatment, on the growth of shoots remaining on the stool, in Quercus ilex coppices.Two experimental designs were used to answer these questions. The first studied the effect of two cleaning intensities on 4, 8, 15, 20, and 25 year old coppices. The second studied the effect of 4 thinning intensities (26%, 42%, 58%, or 78% removal of the initial basal area) on 43 and 57 year old coppices. In both cases, control plots undergoing no silvicultural treatment were maintained for each age class.The results, 4 and 5 years after silvicultural treatment, concerned girth increment and height growth of coppice shoots.Cleaning or thinning consistently had a positive influence on girth increment. In young coppices, less than 25 years old, mean annual girth increment was approximately 6 mm in the controls and double that under heavy cleaning. In older coppices, girth increment was lower in controls (3 mm per year) but the effect of thinning was very positive and, under very heavy thinning, growth was more than 11 mm per year.Cleaning and thinning often modify height growth. This effect was found to be either positive or negative and no general trend could be defined which was valid for all the age classes.Silvicultural treatments also had a positive effect on the appearance and growth of epicormic shoots on the holm oak trunks.The results were also analysed at the stool level. The number or percentage of shoots removed had a positive effect on the individual growth of selected shoots.     

Nutrient content in Quercus ilex canopies: Seasonal and spatial variation within a catchment

Spatial and temporal changes in canopy nutrient content were studied in 1988 and 1989 in a Mediterranean Quercus ilex ssp. ilex L. forest in north-eastern Spain. Sampling was conducted in parallel at two sites which represent endpoints along a slope gradient within a small catchment (ridge top at 975 m and valley bottom at 700 m). Deeper soils resulted in significantly higher N and P concentrations, and N content on a leaf area basis at the valley bottom site. In contrast, K concentration in leaves was significantly higher at the ridge top site, where soil K concentration was also higher. At both sites, N and P content on a leaf area basis was highest at the top of the canopy, where leaf area is highest. N resorption efficiency decreased from top to bottom of the canopy. Results suggested a minor role of shaded leaves as nutrient storage sites. Lower P resorption efficiency was found at the ridge top site. Seasonal changes of P and N concentration on a leaf area basis suggest P replenishment, and to a lesser degree N, during periods of lower growth activity due to low temperatures, but coinciding with higher water availability (autumn-early spring period). Thus, N and P resorption from the remaining foliage in the canopy took place, and to a larger degree at the valley bottom site, coinciding with a slightly higher leaf area index and productivity at this site.     

Radial distribution of Ni in stemwood of Quercus ilex L. trees grown on serpentine and sandy loam (umbric leptosol) soils of NE-Portugal

Concentrations of Ni were determined in xylem and phloem of Quercus ilex trees growing on serpentine and sandy loam soils of northeast Portugal. Radial distribution patterns of Ni in stemwood were compared to variations in annual growth increments of the trees.Concentrations of Ni in xylem and phloem were higher in Q. ilex from serpentine soil, when compared with Q. ilex of a sandy loam soil.The radial distributions of Ni showed large variations among the trees, although they all grew in the same area within a short distance from each other. These differences can be caused by small-scale spatial variations in the soil. Therefore, the use of such radial Ni distributions for a retrospective biomonitoring of the Ni pollution of larger areas appears questionable.     

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.     

Morphological adaptions of Quercus ilex leaves in the Castelporziano forest

The annual course of vegetative growth of the most representative species of a Quercus ilex L. forest in Castelporziano (Latium, Italy) was studied through periodical analysis of selected ecophysiological leaf indexes, for the period 1987–1990. The results demonstrate that the local climate facilitates continuous vegetative activity of the sclerophyllous species without a latent phase. The variable morphology of sun and shade leaves of Q. ilex illustrates the plant's response to environmental stress.     

Productivity and nutrient uptake in a holm oak (Quercus ilex L.) stand and during regeneration after clearcut

The effect of clearcutting on biomass production and nutrient consumption was examined during the two years in a clearcut and a mature Quercus ilex L. coppice. The comparison was limited to the leaf compartment and to the annual shoots, which were the only comparable compartments in the two situations. At the same time, the dynamics of the nutrient pool (N, P, K, Ca, Mg), monthly uptake and recycling from the foliage, were studied.The second year after cutting, the leaf production was roughly the same at the two sites. However, the photosynthates appear to be used differently: in the young stand the perennial wood production was greater than in the mature stand, where the maintenance costs are higher. This is corroborated by the patterns of nutrient use.     

Effect of water stress on monoterpene emissions from young potted holm oak (Quercus ilex L.) trees

We investigated the effects of a short period of water stress on monoterpene emissions from Quercus ilex, a common oak species of the Mediterranean vegetation and a strong emitter of monoterpenes. The experiment was carried out on two young saplings with a branch enclosure system under semi-controlled conditions. Under unstressed conditions, small qualitative (cis--ocimene, trans--ocimene, -caryophyllene and 1,8-cineol) and large quantitative (as much as 40% for the main compounds emitted) differences were observed between the two apparently similar trees. Nevertheless these differences did not affect the short-and long-term responses to temperature and water stress. Daily courses of emissions and gas exchanges were similar before and after the stress. During the most severe stress, emissions were reduced by a factor of two orders of magnitude and the log-linear relationship between emissions and temperature no longer existed. Photosynthesis and transpiration rates decreased as soon as the soil started to dry, whereas monoterpene emissions slightly increased for few days and then dropped when the daily CO₂ balance approached zero. We concluded that under water stress monoterpene emissions were highly limited by monoterpene synthesis resulting from a lack of carbon substrate and/or ATP. After rewatering, both emissions and gas exchanges recovered immediately, but to a level lower than the pre-stress level. These results have many implications for monoterpene emission modelling in the Mediterranean area, since the dry period generally extends from May to August. If our results are confirmed by field experiments, water stress could lead to a large overestimation of the emissions under summer conditions, when the algorithms based on light and temperature would give high emission rates.     

Bacteria isolated from the rocks supporting prehistoricpaintings in two shelters from Sierra de Cazorla,Jaen, Spain

The Sierra de Cazorla, Jaen, Spain, is characterizedby the presence of a number of shelters or smallcavities with important late prehistoric paintings,the so-called schematic rock art painting. Themicrobial population of the rocks supporting paintingsfrom two shelters was investigated. Bacteria isolatedfrom Cueva del Encajero triplicated the number ofthose isolated from Cueva de la Graja, as correspondedto a higher environmental humidity. However, thebacteria isolated from Cueva de la Graja weremetabolically more active and able to use a highernumber of carbon sources than those of Cueva delEncajero. As demonstrated by cluster analysis themicrobial communities from both shelters are related,in spite of the different ecological conditions andgeographical distribution.     

Post-fire, seasonal and annual dynamics of the ectomycorrhizal community in a Quercus ilex L. forest over a 3-year period

Two study plots, burned and control, were established in autumn 1998 in a Quercus ilex forest located in northern Spain, part of which had been affected by a low intensity fire in 1994. Soil samples for ectomycorrhizae (ECM) were taken over a 3-year period in each study plot in spring, summer, autumn and winter. ECM morphotypes were identified and the relative abundance of each morphotype in each soil sample calculated, along with species richness, Shannon diversity index and percentage of mycorrhization in each soil sample. The relative abundance of certain ECM morphotypes differed between burned and control plots, and the percentage of mycorrhizal tips was significantly lower in the burned than in the control plot. Nevertheless, there were no significant differences in the diversity, species richness or species composition of the ECM community in the burned and control plots. The dominant ECM morphotypes in both stands were Cenococcum geophilum and several thelephoroid fungi. Sphaerosporella brunnea and Pisolithus tinctorius thrived especially in the burned plot, whereas three ectomycorrhizal morphotypes assigned to the genus Hebeloma were especially abundant in the control plot. There was no significant variation in the relative abundance of the ECM morphotypes between seasons, but ECM community species richness was highest in autumn and lowest in summer. The percentage of mycorrhizal tips reached a maximum in winter, with its minimum in autumn. Collection of samples over the 3-year period also enabled us to detect a significant increase in percentage of ECM colonisation in the burned stand over time.     

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.     

Quercus ilex facing water stress: a functional equilibrium hypothesis

A boundary line analysis of the scatterplot relating pre-dawn leaf water potential to pre-dawn minus minimum leaf water potential was applied to study the efficiency of the Mediterranean evergreen oak species to buffer the effects of variability in water resources. The results are discussed in term of stomatal regulation and changes in soil-to-leaf hydraulic conductance of water flow, both induced by changes in leaf water potential. For this purpose, we used data for leaf water potential measured in three stands of Quercus ilex across a soil water availability gradient in Southern France, and two others obtained in California and Arizona for Quercus turbinella and Portugal for Quercus coccifera. A classification of plant responses from mesic to xeric sites is proposed.     

Canopy structure within a Quercus ilex forested watershed: variations due to location,phenological development,and water availability

Spatial and temporal changes in canopy structure were studied in 1988 and 1989 in a Mediterranean Quercus ilex forest in north-eastern Spain. Due to differences in precipitation patterns the 1989 growing season was drier than the 1988 growing season. Sampling was conducted in parallel at two sites which represent endpoints along a slope gradient within a watershed (ridge top at 975 m, and valley bottom at 700 m). At both sites, similar inter-annual changes in canopy structure were observed in response to differences in water availability. Samples harvested in the upper 50 cm of the canopy during 1989 exhibited a decrease in both average leaf size and the ratio of young to old leaf and stem biomass relative to samples obtained in 1988. At the whole canopy level, a decrease in leaf production efficiency and an increase in the stem to leaf biomass ratio was observed in 1989. Temporal changes in canopy leaf area index (LAI) were not statistically significant. Average LAI values of Q. ilex at the two sites were not significantly different despite differences in tree stature and density (4.6 m² m^–2 at the ridge top, and 5.3 m² m^–2 at the valley bottom). Vertical distribution of leaves and stems within the canopy was very similar at the two locations, with more than 60% of the total LAI in the uppermost metre of the canopy. The possible significance of such an LAI distribution on the canopy carbon budget is discussed.     

Importance of microhabitat and acorn burial on Quercus ilex early recruitment: non-additive effects on multiple demographic processes

This study investigates the effect of microhabitat and seed burial on the main demographic processes operating during the early recruitment of Quercus ilex, such as postdispersal seed predation, seed germination, and seedling emergence, survival and growth. The effect of burial was positive over all the processes analysed in this study, since predation rate was lower (63.6% vs. 88%), whereas germination (53.1% vs. 21.8%) and emergence (32.0% vs. 5.5%) were higher for buried acorns. The quality of some microhabitats remained similar throughout the stages and processes studied. Thus, afforestation provided especially suitable microhabitats for oak establishment, since seed predation was lower, while germination, emergence and seedling survival were higher, than in any other microhabitat. By contrast, the quality of some microhabitats, such as open sites and Holm oaks, differed between recruitment stages. Acorns in open sites escaped predation and germinated easily, but most seedlings died due to summer drought. Similarly, although acorns under Holm oaks can germinate and survive drought, they cannot survive to postdispersal predators. This uncoupling results in a post-dispersal change in the spatial distribution of Q. ilex recruits. Furthermore, there were significant interactions between burial and microhabitat for some demographic processes. The recruitment was in afforestations high irrespective of burial, suggesting that burial is not as beneficial in high-quality habitats as it is in lower-quality ones. An accurate understanding of plant recruitment requires the determination not only of the direct effects of limiting factors but also the potential interactions occurring between them.     

Ecological significance of seed desiccation sensitivity in Quercus ilex

Background and Aims

Several widespread tree species of temperate forests, such as species of the genus Quercus, produce recalcitrant (desiccation-sensitive) seeds. However, the ecological significance of seed desiccation sensitivity in temperate regions is largely unknown. Do seeds of such species suffer from drying during the period when they remain on the soil, between shedding in autumn and the return of conditions required for germination in spring?

Methods

To test this hypothesis, the Mediterranean holm oak (Quercus ilex) forest was used as a model system. The relationships between the climate in winter, the characteristics of microhabitats, acorn morphological traits, and the water status and viability of seeds after winter were then investigated in 42 woodlands sampled over the entire French distribution of the species.

Key Results

The percentages of germination and normal seedling development were tightly linked to the water content of seeds after the winter period, revealing that in situ desiccation is a major cause of mortality. The homogeneity of seed response to drying suggests that neither intraspecific genetic variation nor environmental conditions had a significant impact on the level of desiccation sensitivity of seeds. In contrast, the water and viability status of seeds at the time of collection were dramatically influenced by cumulative rainfall and maximum temperatures during winter. A significant effect of shade and of the type of soil cover was also evidenced.

Conclusions

The findings establish that seed desiccation sensitivity is a key functional trait which may influence the success of recruitment in temperate recalcitrant seed species. Considering that most models of climate change predict changes in rainfall and temperature in the Mediterranean basin, the present work could help foresee changes in the distribution of Q. ilex and other oak species, and hence plant community alterations.