M. Maestro-Martínez

Leaf morphology and leaf chemical composition in three Quercus (Fagaceae) species along a rainfall gradient in NE Spain

Abstract Leaf features were examined in three Quercus species (Q. coccifera, Q. ilex and Q. faginea) along a steep rainfall gradient in NE Spain. The analyzed leaf traits were area, thickness, density, specific mass, leaf concentration of nitrogen, phosphorous, lignin, cellulose and hemicellulose, both on a dry weight basis (Nw, Pw, Lw, Cw, Hw) and on an area basis (Na, Pa, La, Ca, Ha). These traits were regressed against annual precipitation and correlated with each other, revealing different response patterns in the three species. Q. faginea, a deciduous tree, did not show any significant correlation with rainfall. In Q. coccifera, an evergreen shrub, Nw, Na, Lw, La and Ca increased with higher annual rainfall, while Hw decreased. In Q. ilex, an evergreen tree, leaf area, Pw and Lw increased with precipitation, whereas specific leaf mass, thickness and Ha showed the reverse response. Correlations between the leaf features revealed that specific mass variation in Q. faginea and Q. coccifera could be explained by changes in leaf density, while in Q. ilex specific leaf mass was correlated with thickness. Specific leaf mass in the three species appeared positively correlated with all the chemical components on a leaf area basis except with lignin in Q. ilex and with P in Q. ilex and Q. faginea. In these two tree species Pw showed a negative correlation with specific leaf mass. It is suggested that each species has a different mechanism to cope with water shortage which is to a great extent related to its structure as a whole, and to its habit.

Leaf morphology, leaf chemical composition and stem xylem characteristics in two Pistacia (Anacardiaceae) species along a climatic gradient

Summary Leaf and stem xylem features in Pistacia lentiscus (an evergreen shrub) and P. terebinthus (a deciduous shrub) were analysed along an Atlantic-Mediterranean gradient, which is characterised by a variation of rainfall and winter temperature. Leaf features were foliage area of 3-year-old shoots (FA), leaflet area (LA), specific mass (SLM), thickness (LTh), density (LD) and chemical composition on a dry weight basis (N, P, cellulose Cel, hemicellulose Hcel, and lignin Lig). Stem traits referred to xylem anatomy: mean vessel diameter of the 25 largest vessels (MxD), vessel element length (VEL), number of vessels per xylem cross section area unit (VD) and the ratio xylem/foliage surface of shoots (HV). Comparisons of traits have been made both at an interspecific and at an intraspecific level, this latter by analysing how these traits respond to variations of precipitation and winter temperature. P. terebinthus showed higher FA, LA, MxD, Nand Cel, but lower SLM, LTh, Hcel, VD and HV than P. lentiscus. Both species showed opposite responses of LTh to the reduction of mean minimum January temperature, decreasing in the former and increasing in the latter. In P. lentiscus VD increased and VEL decreased when winter temperature became colder, while YD decreased and N increased towards the arid part of the gradient. It is suggested that leaf habit plays an important role in determining the function of species and their response to the climatic gradient.

Does the Gradualness of Leaf Shedding Govern Nutrient Resorption from Senescing Leaves in Mediterranean Woody Plants

The resorption of nutrients from senescing leaves is a key component of the nutrient conservation strategy of plants. Despite its relevance, the regulation of the efficiency of this process is poorly understood. The aim of this work was to test the hypothesis that species that shed leaves gradually along the year are less efficient reabsorbing nutrients from senescing leaves than species that shed leaves in a short period. N-, P-, and K-resorption-efficiencies were measured in 11 Mediterranean species and regressed against an index of the gradualness of leaf shedding. Additionally, the bivariate relations among leaf nutrient content before senescence, nutrient content in senesced leaves, pool of nutrients reabsorbed during senescence, and nutrient resorption efficiency, were examined. K-resorption-efficiency was markedly lower in species with protracted leaf-shedding, in agreement with the initial hypothesis. This pattern was less significant for N- and P-resorption-efficiencies. When leaf nutrient content before senescence was high, the amount of nutrients reabsorbed and the amount of nutrients in senesced leaves were high. Consequently, nutrient resorption efficiency was unaffected by the leaf nutrient status before senescence. It is concluded that the leaf shedding pattern per se influences nutrient resorption in Mediterranean perennials, irrespective of additional environmental controls. Furthermore, it is suggested that plants differing in nutrient status do not exhibit different nutrient resorption efficiencies because the nutrient content of leaves before senescence affects the components of resorption efficiency in countervailing ways.

Acorn production is linked to secondary growth but not to declining carbohydrate concentrations in current-year shoots of two oak species

In trees, reproduction constitutes an important resource investment which may compete with growth for resources. However, detailed analyses on how growth and fruit production interact at the shoot level are scarce. Primary canopy growth depends on the development of current-year shoots and their secondary growth might also influence the number and size of fruits supported by them. We hypothesise that an enhanced thickening of current-year shoots is linked positively to acorn production in oaks. We analysed the effect of acorn production on shoot growth of two co-occurring Mediterranean oak species with contrasting leaf habit (Quercus ilex, Quercus faginea). Length and cross-sectional area of current-year shoots, apical bud mass, number of leaves and acorns, xylem and conductive area, number of vessels of acorn-bearing and non-bearing shoots were measured in summer and autumn. Nitrogen and carbohydrates analyses were also performed in stems and leaves of both shoot types. Stem cross-sectional area increased in acorn-bearing shoots when compared with non-bearing shoots for both species and such surplus secondary growth was observed since summer. In bearing shoots, the total transversal area occupied by vessels decreased significantly from basal to apical positions along the stem as did the xylem area and the number of vessels. Leaves of bearing shoots showed lower nitrogen concentration than those of non-bearing shoots. Carbohydrate concentrations did not differ in stems and leaves as a function of the presence of acorns. Such results suggest that carbohydrates may preferentially be allocated towards reproductive shoots, possibly through enhanced secondary growth, satisfying all their carbon demands for growth and reproduction. Our findings indicate that acorn production in the two studied oaks depends on shoot secondary growth.

Phosphorus Accretion in Old Leaves of a Mediterranean Shrub Growing at a Phosphorus-Rich Site

Studies dealing with changes in the plant internal nutrient cycling in response to natural, long-term P-fertility gradients are scarce. In this short report, we show some evidence on how leaf P dynamics can be drastically altered when plants typical from nutrient-poor sites grow in long-term P-enriched soils. The study was conducted in two natural populations of the Mediterranean evergreen shrub Pistacia lentiscus L.: one in a P-poor site and the other in a P-rich site. Soil texture and N, P, and organic matter contents were measured at each site. Leaf N and P concentrations were also measured in current-year, 1-year-old, and 2-year-old leaves, and in the senesced leaves. In the P-poor site, leaf P and N decreased as the leaves aged. This occurs because of nutrient reabsorption to other plant organs and/or dilution of nutrients by carbon compounds. In contrast, the leaves from the P-rich site acummulated P (but not N) during leaf lifespan. Consequently, P concentration in senesced leaves was very high in the P-rich site. These results show that, in long-lived perennials living in the field, long-term P enrichment can switch the normal process of P resorption during leaf aging to P accretion in the leaf. P accumulation in the leaves, which are periodically shed, might constitute a simple P excretion mechanism for plants typical from P-poor soils.