J. A. Pardos

Life histories of Mediterranean pines

The life history of Spanish pines and their relation to fire as the main disturbance factor in their ecosystems was analysed. The primary ecological attributes studied were the canopy seed bank (onset of cone production, percentage and persistence of serotinous cones), seed and cone morphology, sprouting and bark thickness. Four ecological groups were separated using multivariate cluster analysis and their life-history characteristics are discussed. Serotiny and early flowering in Pinus halepensis and P. pinaster reflect their evader strategy in relation to fire as this character is advantageous to survive frequent crown fires and to attain successful post-fire recruitment. Late flowering and absence of serotinous cones in P. nigra, P. sylvestris and P. uncinata indicate that their natural forest did not evolve under frequent crown fires. P. canariensis and P. pinea appeared in two single groups because of their sprouting capability and their seed size respectively. Intraspecific variation in P. pinaster was also analysed using the same criteria and high variability was found in its life history traits. A group of P. pinaster populations showed high levels of serotiny and thin bark as a possible adaptation to frequent stand-replacing crown fires. In contrast, a group of non- or weakly-serotinous populations seems to have evolved under a low-intensity fire regime where the best fitness corresponds to thick-barked individuals capable of surviving ground fires. Intermediate strategies were also evident in this species and were discussed in relation to the effect of different fire regimes caused by the understorey vegetation.

Water relations and gas exchange in Fagus sylvatica L. and Quercus petraea (Mattuschka) Liebl. in a mixed stand at their southern limit of distribution in Europe

Abstract Water status and gas exchange of beech (Fagus sylvatica L.) and sessile oak [Quercus petraea (Mattuschka) Liebl.] were studied in a mixed stand in the Montejo de la Sierra forest (central Iberian Peninsula), one of the southernmost locations of both species in Europe. Gas exchange and water potential were measured in leaves at different canopy levels over several days in two growing seasons. The daily variation pattern was established with the measurements of three selected dates per year, representative of the soil moisture content situations in early, mid- and late summer. A similar daily time course of leaf water potential was found for the two species. Nevertheless, beech showed a most noticeable decrease of water potential at midmorning and maintained lower leaf water potential than oak in the early afternoon. In 1994 the sessile oak saplings showed higher values of predawn water potential (Ψpd) than beech at the end of summer, when soil moisture content was lowest (20 cm depth). Beech showed a significantly lower net assimilation rate (A) than sessile oak for leaves under the same PPFD. Maximum net photosynthesis values (Amax) for beech and sessile oak on sunny leaves were 10.1±0.4 µmol m–2 s–1 and 17.8±1.7 µmol m–2 s–1 respectively, and those for water vapour stomatal conductance (gwv) were 265±31 mmol m–2 s–1 and 438±74 mmol m–2 s–1. Differences in A and gwv between the two species were maintained throughout the day on all measurement dates. No clear relationship was found between water status of saplings and stomata performance; there was only a negative correlation between Ψpd and gwvmid in beech. Nevertheless, a significant response to the air vapour pressure gradient between leaf and air was translated into stomata closure on an hourly basis, more intensively in beech.

Fire adaptations in the Canary Islands pine (Pinus canariensis)

A wide set of phenotypic characteristics related to life history were studied in mature stands of Pinus canariensis throughout its natural range of distribution in the Canary archipelago. Natural forests ranging from those located in xeric areas through to the sub-tropical cloud forests and high mountain stands were classified into eight ecological regions according to their main climatic features. The recent history of forest fires (covering the last 30 years) was taken into account using a categorical factor with three levels. The phenotypic variables studied included those related to seed dispersal (cone size, number of seed scales, seed and wing size and percentage of serotinous trees) and stem growth both on the breast height section (bark thickness, radial growth at various ages and sapwood and heartwood sizes) and on the entire stem (height growth related to age). The average percentage of serotinous trees present in the ecological regions studied varied from 3 to 35%. Average bark thickness in adult trees ranged from 22 to 49 mm and was found to be unrelated to age or diameter. Growth both in height and diameter was found to decline after an average of 25 years, although clear trends in relation to this could not be established across the ecological regions. A high correlation was found to exist between annual rainfall, fire frequency, serotiny and bark thickness at a regional level. Sapwood area per hectare proved to be a valuable indirect site-quality index for the objectives of this paper. Favourable sites (characterised by a high sapwood area per hectare) displayed the highest levels of both bark thickness and serotiny. These particular areas are those, which have suffered more frequent and intense fires over the last decades. The evolutionary implications of this trend and of other general traits of the species, such as vegetative resprouting, are discussed here in relation to the role of understorey vegetation in fire regimes, competition and volcanic history of the islands.

Seasonal water relations of three broadleaved species (Fagus sylvatica L., Quercus petraea (Mattuschka) Liebl. and Quercus pyrenaica Willd.) in a mixed stand in the centre of the Iberian Peninsula.

Abstract Water relations were monitored in a natural mixed stand of beech ( Fagus sylvatica L.), sessile oak ( Quercus petraea (Mattuschka) Liebl.) and melojo oak ( Quercus pyrenaica Willd.) during a growing season. Soil moisture content and air temperature were recorded in an especially dry year with rainfall under 15 mm for 3 months during the middle of the growing season. Leaf samples of the three species were periodically taken at dawn and pressure-volume curves were performed with the transpirational method. Osmotic potentials, relative water content at turgor loss point, maximum bulk modulus of tissue elasticity and the dry weight/turgor weight ratio were obtained for the curves. Differences for osmotic potential at full turgor and at turgor loss point were significant between dates while considering the species altogether, with minimum values on 6 September, matching with the minimum soil moisture. Osmotic potential at full turgor decreases as soil drought increases, the extreme mean values (−1.32 MPa in spring and −2.31 MPa in summer) being reached by the melojo oak. Osmotic potential at turgor loss point follows a similar pattern of variation to the one at full turgor. Melojo oak and beech also show significant differences for the whole period. No apparent pattern is associated with relative water content at the turgor loss point and maximum bulk modulus of tissue elasticity, although there were significant differences for the growing season as a whole. The three species ranked according to decreasing values of the relative water content at the turgor loss point, in parallel to their increasing xerophytic character throughout the growing season. Beech has the highest value (87.22%) and melojo oak the lowest one (83.52%), sessile oak having an intermediate value (85.94%). The capacity of adaptation for living together may explain why the expected differences in the parameters between species and dates are not always significant. However, the data highlight the higher xerophytic condition of melojo oak and the more mesophytic character of beech despite the relict condition of the stand.

Effect of water stress conditioning on morphology, physiology and field performance of Pinus halepensis Mill. seedlings

Five-month-old Pinus halepensis Mill. seedlings were subjected to 4 irrigation treatments for 8 weeks. After the treatments, morphological and physiologicalattributes assessed included height, diameter, dry weight, water relations parameters, and determination of N, P, K, soluble carbohydrates, and starch concentrations. The remaining seedlings were field planted. Survival and height growth were recorded forfour growing seasons after planting. The mostsignificant effect of irrigation was onmorphology. Increased irrigation lead toseedlings with significantly higher height,root collar diameter and shoot and rootbiomass. Starch and soluble carbohydrateconcentrations were also affected byirrigation. However, parameters derived frompressure-volume curves were not significantlydifferent among treatments, nor were N, P, andK concentrations. There were no differencesamong treatments for survival in the field,which was highly related to summer rainfall. Incomparison, absolute and relative heightgrowth showed some significant but minordifferences among treatments.

Effects of thinning in a Pinus sylvestris L. stand on foliar water relations of Fagus sylvatica L. seedlings planted within the pinewood

Abstract. Planting of beech in old Scots pine plantations could facilitate the extension of adjoining small natural populations of beech in the centre of the Iberian Peninsula. To be successful the survival and growth of seedlings after outplanting must be compatible with microclimatic conditions within thinned pine woods. The present paper deals with water relations in beech seedlings following the variation of available radiation and water as a result of thinning. The seedlings were established under an old Scots pine plantation. After pine felling, four situations were generated in terms of radiation. Hemispherical photographs were taken, and global relative irradiance was calculated for every treatment. During two growing seasons predawn water potential (Ψpd) was measured and leaf water parameters were derived from P–V curves. The Ψpd diminished through the season in all four situations, and the highest values were found when pine density was lowest. Osmotic potential at turgor lost (Ψπo) was higher in early summer. A negative relationship was found between osmotic potential at full turgor (Ψπfull) and symplast solute content on a dry weight basis (Ns). Under the lowest irradiance (unthinned pine trees), Ψπfull and Ψπo were the highest and the bulk modulus of elasticity (εmax) the lowest; εmax decreased in response to the lowest Ψpd measured at the end of summer 1998. It was followed by a reduction in the symplastic relative water content and Ns, irrespective of the irradiance. Osmotic adjustment, as a drought tolerance mechanism, was limited under shade conditions (unthinned pine trees), restricting the acclimation of beech seedlings to drought. Therefore, the presence of overstory, necessary for a successful implantation, should not be extended for too many years because of the risk of negative effects on growth and survival, particularly after frequent dry summers.

Field study of beech (t Fagus sylvatica L.) and melojo oak (t Quercus pyrenaica Willd) leaf litter decomposition in the centre of the Iberian Peninsula

The effect of the canopy on leaf decomposition of beech (t Fagus sylvatica) and melojo oak (t Quercus pyrenaica) was studied during a period of 660 days in a mountain forest of central Spain; response of leaves to leaching was also studied to determine the effects of rainfall after leaf fall. Beech leaves lost 5.8% of their weight by leaching, whereas melojo oak leaves lost 13%. Under both types of canopy, beech leaves showed almost no difference in their decomposition patterns, with decay constants of 0.31 and 0.32 respectively. Melojo oak leaves showed quite a different behaviour under both canopies; decay constant was 0.47 under t Quercus pyrenaica and 0.77 under beech canopy. Total immobilization of nitrogen was less in the melojo oak forest. Effects of summer dryness were sharper in the melojo oak forest, where decomposition stopped during the summer. This delay in the decomposition might have been due to the lower canopy density in the melojo oak forest. The decomposition patterns of the leaves of both species, under the canopy of the other species, suggests what might happen in mixed stands. Thus, the presence of melojo oaks in beech forest would increase decay and decrease nitrogen immobilization. The presence of beech trees in melojo oak forests would improve microclimatic conditions and increase decay, whereas beech litter on the soil would immobilizate more nitrogen.

Heartwood and sapwood development and its relationship to growth and environment in Pinus canariensis Chr.Sm ex DC

Abstract Pinus canariensis yields a coloured resinous heartwood which has been widely used for centuries in the Canary Islands. Nowadays, although it reaches a high commercial value, very little is known about the influence of environmental, silvicultural and genetic factors on heartwood development in this species. Thus, the influence of environment and stem growth factors on heartwood and sapwood development was investigated in Pinus canariensis 20–200 years old. Samples were taken in natural stands from La Palma and Tenerife islands. The number of growth rings, diameter under bark and radius to the 25th ring were the growth factors most strongly related to heartwood radius. Best prediction (r2 = 0.91) for average heartwood radius was obtained with a function of number of growth rings and radius to the 25th ring. Environmental parameters (annual water surplus and influence of Trade Winds) were found to be related more to sapwood width and diameter growth rates than to heartwood width. Stand density, estimated as the number of trees per hectare, was negatively correlated with heartwood and sapwood width, radius to the 25th ring, and average diameter growth rate. It is concluded that good site conditions improve both heartwood and sapwood development in Pinus canariensis.

Xylem cavitation, leaf growth and leaf water potential in Eucalyptus globulus clones under well-watered and drought conditions

Leaf growth, predawn leaf water potential (Ψpd), evapotranspiration, stem maximum permeability, and its percentage loss of hydraulic conductivity (PLC) were measured in rooted cuttings of selected clones of Eucalyptus globulus Labill. subjected to well-watered and drought conditions. Drought significantly reduced evapotranspiration, leaf growth and maximum permeability. E. globulus clones lost up to 70% of conductivity at values of Ψpd less negative than -1 MPa. PLC values higher than 85% could not be measured without causing leaf shedding. The coefficient related to the slope of the vulnerability curves ranged from 1.52-2.23. The lowest value was measured in the most drought-resistant clone, as estimated from field trials. Plants from this clone displayed higher drought-induced reductions in maximum permeability than plants from other clones, had significantly smaller leaves and maintained higher values of predawn leaf water potential as soil water content (SWC) declined.

Vertical heartwood variation patterns and prediction of heartwood volume in Pinus canariensis Sm.

Heartwood and sapwood distribution along the stem were assayed in 26 Canary Island pines growing along the natural range of the species at the Canary Archipelago. Trees were sampled at breast height and approximately every 2 m up to the lowest section without heartwood. Heartwood and sapwood thickness, together with age and radial growth traits were measured in each section. Heartwood tapered towards the top of the tree and disappeared at a certain height. Sapwood distribution was more variable among individuals; sapwood width was almost constant in some individuals but tapered slightly or with a curvilinear variation in others. Two general patterns of heartwood and sapwood variation were defined, one characterised by a constant decrease in heartwood percentage and subsequently a slight sapwood variation along the stem (uniform pattern) and another one with a peak in heartwood percentage at a height between 4 and 8 m (irregular pattern). The relative crown height to total tree height was significantly higher in the second group, averaging 49% versus 32% in the uniform group. A mechanical‐physiological hypothesis is given to explain this result in the light of similar results in other species. A model including heartwood radius at breast and tree height as predicting variables was able to explain 90% of heartwood radius at a given height in the stem. A similar model with sapwood width at breast height explained 80% of sapwood width variation along the stem. Heartwood volume was predicted by means of log‐linear regression models including heartwood diameter at breast height and tree height ðr 2 ¼ 97:5%Þ. Sapwood volume was better predicted through stem diameter at breast height and tree height ðr 2 ¼ 92:7%Þ.