Differences between understorey and canopy in herbivore community composition and leaf quality for two oak species in Missouri

1. From July 1994 to September 1995, at six censuses, the herbivore community associated with understorey (< 2.5 m height) and canopy (15–20 m) leaves of Quercus alba and Q. velutina was sampled in south-eastern Missouri, U.S.A. 2. Across all censuses, herbivore densities were not significantly different between canopy and understorey for Q. alba and Q. velutina, except in August 1994 when herbivore densities were 60% higher in the canopy on Q. alba. Little significant spatial variation in herbivore densities or community composition was found during the study years. 3. The herbivore community was diverse, consisting of 138 species of leaf-chewing insects. Species richness was significantly greater (by 5–20%) in the understorey than in the canopy for both tree species, and the relative abundance of the main families, different feeding guilds, and most common species differed significantly between understorey and canopy. 4. To determine the extent to which leaf quality explained the observed patterns, percentage nitrogen and protein binding capacity were measured in canopy and understorey leaves of Q. alba and Q. velutina. Per cent nitrogen was higher in canopy leaves for Q. velutina while protein binding capacity was higher in canopy leaves for Q. alba. 5. These results suggest that the herbivore community associated with these two species of Quercus comprises species that appear to respond individually to environmental and biological conditions encountered in the understorey and the canopy.     

Feeny revisited: condensed tannins as anti-herbivore defences in leaf-chewing herbivore communities of Quercus

Abstract. 1. Community level oak–tannin–insect patterns have been largely unexplored since Paul Feeny's ground‐breaking research. Two hypotheses were tested for Quercus velutina and Q. alba in the Missouri Ozarks: abundance and richness of leaf‐chewing herbivores are negatively correlated with foliar condensed tannin concentrations and variation in condensed tannin concentrations explains variation in herbivore community structure. 2. In 2001, foliar condensed tannins in the understorey and canopy of these two oak species were quantified simultaneously with censuses of herbivores in May, during leaf expansion, and in June and August, when leaves were fully expanded. Thirty‐eight of the 134 species encountered had densities sufficient to be analysed individually (n = 10). Of those, Acronicta increta (Noctuidae) and Attelabus sp. (Curculionidae), both oak specialists, were negatively correlated with condensed tannins in the canopy of Q. alba. One additional specialist, Chionodes pereyra (Gelechiidae), was marginally negatively correlated with condensed tannins in the understorey of Q. velutina. Understorey species richness of May Q. velutina herbivores was negatively correlated with condensed tannins, as were total canopy insect density and species richness of August herbivores on Q. alba. 3. Principal component analysis (PCA) of insect abundances indicated that understorey and canopy Q. velutina and Q. alba had different communities of leaf‐chewing insects. Furthermore, condensed tannin levels contributed significantly to variation in PCA scores for Q. velutina, explaining 25% of the total variation. 4. Overall, these results indicate that specialists were more likely than generalists both to correlate negatively with condensed tannins and to occur in lower tannin habitats; abundance and richness of both early and late season fauna correlated negatively with tannins; and species were more likely to correlate negatively with condensed tannins when feeding on Q. alba than on Q. velutina and when feeding in the canopy than in the understorey. Future studies of tannin–insect interactions should manipulate leaf quality in combination with manipulations of other factors that likely influence community structure.     

Stomatal activity within the crowns of tall deciduous trees under forest conditions

The variation in stomatal activity within the crowns ofAcer campestre, Carpinus betulus andQuercus cerris was measured by vapour exchange porometer on several summer days in an oak-hornbeam forest, in SW Slovakia, Czechoslovakia. Variation resulted from crown position in the forest stand and from leaf position within the canopy. The highest stomatal conductance was in sunlit sun leaves in the upper part of the canopy. Stomatal conductance decreased with increasing depth in the canopy. The steepest decrease was in the upper canopy, in the intermediate zone between fully sunlit and fully shaded leaves, and was caused by the decline in leaf irradiance and in stomatal density. In codominant trees, the conductance in shade leaves at the base of the crown was significantly lower than in the sun leaves at the top of the crown. In a dominant tree,Q. cerris, the differences in stomatal conductance were small and most frequently insignificant. Variation in incident light also determined the diurnal variation of stomatal conductance with respect to crown aspect. Differences between sun leaves on the east and west facing aspects of the overstory crown ofQ. cerris were demonstrated for several days.     

Variation in radial growth responses to drought among species, site, and canopy strata

 Radial growth responses to drought were examined in the tree-ring records of six species growing within two locations of differing land-use history and soil moisture characteristics, and in overstory and understory canopy positions in northern Virginia. Tree species experienced differential ring-width reductions during or immediately following four severe drought periods occurring from 1930 to 1965 and were influenced by climatic variables including annual and summer temperatures, annual precipitation, and annual Palmer Drought Severity Index. Relative growth comparisons averaged across species before and after drought years indicated that understory trees on dry-mesic sites grew 11% faster after drought compared to pre-drought rates while mesic site trees in both canopy positions grew approximately 4% slower. Superposed epoch analysis indicated that Liriodendron tulipifera growing on mesic sites experienced greater ring-width reductions associated with drought than co-occurring, more drought-tolerant Quercus alba and Q. velutina. On dry-mesic sites, L. tulipifera also experienced greatly reduced growth as a result of drought but exhibited significant growth increases following individual drought events. Quercus alba was the only species that exhibited a consistent, significant ring-width decrease associated with all droughts on dry-mesic sites. In contrast, Pinus virginiana was least impacted by drought on dry-mesic sites but was much more impacted by drought on mesic sites, indicating a drought×site interaction for this species. Overstory Carya glabra and Q. alba experienced larger growth decreases during drought on dry-mesic versus mesic sites. Understory tree growth reductions did not differ between site types but were often significantly larger than overstory responses of the same species on mesic sites. Following drought, most trees exhibited growth reductions lasting 2–3 years, although several species experienced reductions lasting up to 6 years. The results of this study suggest that tree rings represent an important long-term proxy for leaf-level ecophysiological measurements of growth responses to drought periods. Received: 31 July 1996 / Accepted: 16 April 1997     

Correlations of understory herb distribution patterns with microhabitats under different tree species in a mixed mesophytic forest

Summary This study examines the role of canopy trees in the formation and maintenance of different herb microhabitats in a mixed mesophytic forest stand. Herb abundance and reproductive success were recorded in 54 circular plots under seven species of canopy trees and in 15 circular control plots>2 m from any tree. Soil moisture, soil nutrient levels, litter depth, and light intensity were measured in a subset of these plots. Ordination of plots by both herb relative abundance and by reproductive success of common species indicated that herb assemblages under most canopy tree species were similar to those away from trees. However, herb assemblages under Fagus grandifolia trees differed moderately from the others while plots under Quercus alba trees supported significantly different herb assemblages. Analyses of variance revealed that several herb species occurred at significantly closer mean distance to the base of Q. alba or Fagus trees or at higher densities under these tree species. Soils around Q. alba trees had significantly higher concentrations of calcium and sulfate ions, and higher pH than plots under other tree species and control plots. This correlated closely with Q. alba stemflow which had higher concentrations of calcium and sulfate ions and lower concentrations of hydrogen ions than stemflow from other trees at this site. The slightly lower soil pH near the base of Fagus trees may have been related to the high volumes of stemflow produced by this species. Stepwise regression showed significant correlations between abundances of five common herb species and soil nutrient patterns. Maintenance of spatial heterogeneity in forest floor resources by the presence of different species of canopy trees may therefore be important in the maintenance of diversity in these understory herb communities.     

DETECTING SHAPE VARIATION IN OAK LEAF MORPHOLOGY: A COMPARISON OF ROTATIONAL-FIT METHODS

Rotational-fit methods were used to examine shape variation in oak leaf morphology by reference to a set of 14 landmarks for each leaf. Within-tree, between-tree, and between-species variations were examined. Generalized least-squares and generalized resistant-fit analyses revealed patterns of landmark variation that could be related to leaf architecture. The two species (Quercus palustris and Q. velutina) did not illustrate the same patterns of within-tree variability nor did they reveal similar degrees of between-tree variability. Incorporating an affine transformation algorithm resulted in little insight in some comparisons but suggested strong uniform shape change in other comparisons (especially between species). Resistant-fit mean square statistics were equally variable 1) in upper vs. lower crown samples within trees, 2) between trees within each species, and 3) between species. In addition, resistant-fit mean square statistics were found to be a poor measure of similarity, whether derived by comparison to a single reference object or by way of pairwise comparisons. Fundamental species differences in leaf shape are suggested by relationships among particular sets of landmarks, although overall shape differences cannot be explained fully by these analyses.     

Carbon dioxide assimilation of hardwood seedlings in relation to community dynamics in central illinois

Summary Field measurements of net assimilation and respiration for seedlings of four hardwood species were made periodically over a growing season with soil moisture tension maintained between 0 and 0.75 bar. Total net assimilation per day was significantly greater for Acer saccharum than either Quercus rubra or Quercus alba and for Quercus macrocarpa as compared with Q. rubra, when measurements were made under natural shade conditions and light intensity varied from 80 to 120 ft-c. Mean light compensation points determined under canopy shade were 50.3, 53.5, 87.2, and 102.5 ft-c., respectively, for Acer saccharum, Quercus macrocarpa, Q. rubra, and Q. alba. In a 0.04-hectare canopy opening, total net assimilation per day was not significantly different between Q. rubra, Q. alba, and A. saccharum but was significantly greater for Q. macrocarpa than Q. alba and A. saccharum. Relationships between photosynthetic efficiency and successional characteristics of these species are inferred from the data.     

Tannin assays in ecological studies: Lack of correlation between phenolics,proanthocyanidins and protein-precipitating constituents in mature foliage of six oak species

Summary There is no correlation between protein-precipitating capacity and either total phenolic or proanthocyanidin content of extracts of mature foliage from six species of oaks: Quercus alba (white oak), Q. bicolor (swamp white oak), Q. macrocarpa (bur oak), Q. palustris (pin oak), Q. rubra (red oak), and Q. velutina (black oak). It is argued that studies which probe the role of tannins in the selection and utilization of food by herbivores should include a protein-precipitation assay, since such an assay provides a measure of the property of tannins which is presumed to contribute to their utility as defensive compounds. A convenient modification of the bovine serum albumin (BSA) precipitation assay, which measures the amount of protein precipitated when a plant extract is added to a BSA solution, is described. Advantages of this procedure recommend its routine adoption in studies of the role of tannins in plant-herbivore interactions.     

Field patterns of leaf plasticity in adults of the long-lived evergreen Quercus coccifera

BACKGROUND AND AIMS: Quercus coccifera, as a long-lived sprouter, responds plastically to environmental variation. In this study, the role of foliar plasticity as a mechanism of habitat selection and modification within the canopy and across contrasted habitats was characterized. An examination was made of the differential contribution of inner and outer canopy layers to the crown plasticity expressed in the field by adult individuals and its dependence on environmental and genetic factors. METHODS: Within-crown variation in eight foliar traits was examined in nine populations dominated by Q. coccifera. The difference between mean trait values at the inner and outer canopy layers was used as a proxy for crown plasticity to light. Correlations between geographic distances, environmental differences (climatic and edaphic) and phenotypic divergence (means and plasticities) were assessed by partial Mantel tests. A subset of field measurements was compared with data from a previous common garden experiment. KEY RESULTS: Phenotypic adjustment of sun leaves contributed significantly to the field variation in crown plasticity. Plasticity in leaf angle, lobation, xanthophyll cycle pigments and beta-carotene content was expressed in sun and shade leaves concurrently and in opposite directions. Phenotypic plasticity was more strongly correlated with environmental variation than mean trait values. Populations of taller plants with larger, thinner (higher specific leaf area) and less spiny leaves exhibited greater plasticity. In these populations, the midday light environment was more uniform at the inner than at the outer canopy layers. Field and common garden data ranked populations in the same order of plasticity. CONCLUSIONS: The expression of leaf plasticity resulted in a phenotypic differentiation that suggests a mechanism of habitat selection through division of labour across canopy layers. Signs of plasticity-mediated habitat modification were found only in the most plastic populations. Intracanopy plasticity was sensitive to environmental variation but also exhibited a strong genetic component.     

How strong is intracanopy leaf plasticity in temperate deciduous trees?

Intracanopy plasticity in tree leaf form is a major determinant of whole-plant function and potentially of forest understory ecology. However, there exists little systematic information for the full extent of intracanopy plasticity, whether it is linked with height and exposure, or its variation across species. For arboretum-grown trees of six temperate deciduous species averaging 13-18 m in height, we quantified intracanopy plasticity for 11 leaf traits across three canopy locations (basal-interior, basal-exterior, and top). Plasticity was pronounced across the canopy, and maximum likelihood analyses indicated that plasticity was primarily linked with irradiance, regardless of height. Intracanopy plasticity (the quotient of values for top and basal-interior leaves) was often similar across species and statistically indistinguishable across species for several key traits. At canopy tops, the area of individual leaves was on average 0.5-0.6 times that at basal-interior, stomatal density 1.1-1.5 times higher, sapwood cross-sectional area up to 1.7 times higher, and leaf mass per area 1.5-2.2 times higher; guard cell and stomatal pore lengths were invariant across the canopy. Species differed in intracanopy plasticity for the mass of individual leaves, leaf margin dissection, ratio of leaf to sapwood areas, and stomatal pore area per leaf area; plasticity quotients ranged only up to ≈2. Across the six species, trait plasticities were uncorrelated and independent of the magnitude of the canopy gradient in irradiance or height and of the species' light requirements for regeneration. This convergence across species indicates general optimization or constraints in development, resulting in a bounded plasticity that improves canopy performance.     

Ontogenetic changes in leaf phenology of two co-occurring Mediterranean oaks differing in leaf life span

Large differences in leaf physiology and morphology between ontogenetic stages of a single woody species have often been observed. Far less attention, however, has been devoted to studying the ontogenetic changes observed in leaf phenology patterns, despite the relevance of leaf phenology in determining the leaf carbon balance and leaf and plant mortality. Leaf emergence patterns and leaf longevity were studied in the saplings and mature trees of the evergreen Quercus ilex and the deciduous Quercus faginea. Our aim here was to analyze and interpret the possible tree-age related differences in these leaf traits. Unlike the adults, in which only one flush of leaf growth was observed, several leaf cohorts were produced within each year in the saplings. Sapling leaves showed a lower mean duration than those of the adults. However, Q. faginea saplings exhibited large plasticity in leaf longevity, which was not seen in the case of Q. ilex. The differences in leaf emergence patterns and in leaf longevity between growth stages seemed to be related to differences in resource availability for leaf production and in leaf mass per unit area, respectively. We propose that the sequential leaf development in saplings may be an important mechanism enabling tree species to cope with resource limitation in the early stages of life.     

A comparison of leaf physiology and anatomy of Quercus (section Erythrobalanus-Fagaceae) species in different light environments

Physiological and anatomical attributes of leaves were examined of three species of Quercus section Erythrobalanus. All three species occur in moist temperate deciduous forests of eastern North America. Seedlings of each species were grown in different light conditions for comparison. The attributes measured were net photosynthesis, stomatal conductivity, blade and cuticle thickness, stomatal density, thickness of upper and lower epidermis, and thickness of palisade mesophyll. The results generally demonstrate the close association between anatomical adaptations and efficiency of physiological processes; they also elucidate the distribution patterns of the three Quercus species across the forest topography in southern New England. The most drought-tolerant and light-demanding species, Q. velutina (Lam.), exhibited the greatest leaf anatomical plasticity, the highest net photosynthesis in the different light conditions, and the lowest stomatal area per unit area of leaf. The most drought-intolerant species, Q. rubra (L.), exhibited the least leaf anatomical plasticity, the lowest net photosynthesis in the different light conditions, and the highest stomatal area per unit area of leaf. Quercus coccinea (Muenchh.) usually exhibited values that were intermediate between Q. rubra and Q. velutina.     

Differential effects of host plant hybridization on herbivore community structure and grazing pressure on forest canopies

Interspecific hybridization in plants is known to have ecological effects on associated organisms. We examined the differences in insect herbivore community structure and grazing pressure on tree canopy leaves among natural hybrids and their parental oak species. We measured leaf traits, herbivore community structure, and grazing pressure on leaves of two oak species, Quercus crispula and Q. dentata, and their hybrids. The concentration of nitrogen in canopy leaves was greater in hybrids and in Q. dentata than in Q. crispula. The concentration of total phenolics was lower in hybrids than in Q. crispula. The concentration of condensed tannin was greater in hybrids than in Q. crispula. Relative herbivore abundance and species richness were greater on oak hybrids than on either parental species; herbivore species diversity and composition on hybrids were close to those on Q. crispula. Herbivore grazing pressure was lower on hybrids and Q. dentata than on Q. crispula. There was a negative correlation between herbivore grazing pressure and leaf nitrogen, suggesting that interspecific variation among oak taxa in herbivore pressure may be explained by leaf nitrogen; variation in herbivore community structure among oak taxa is likely to be controlled by polygenic leaf traits. Differing responses of (1) herbivore community structure and (2) herbivore grazing pressure to host plant hybridization may play important roles in regulating herbivore biodiversity in cool‐temperate forest canopies.     

The effects of host plant phenology on the demography and population dynamics of the leaf-mining moth, Cameraria hamadryadella (Lepidoptera: Gracillariidae)

Abstract.

• 1 We examined the effects of variation in the timing of spring leaf production and autumn leaf fall on the survival, mortality and abundance of Cameraria hamadryadella on Quercus alba and Q.macrocarpa.
• 2 We monitored and manipulated the timing of foliation on field and potted Q.alba trees and observed the abundance of C.hamadryadella on those trees. We also monitored and manipulated the timing of leaf fall on Q.alba and Q.macrocarpa trees in the field and observed its effects on survival, mortality and abundance of C.hamadryadella.
• 3 Variation in the timing of spring leaf production has no effect on C. hamudryadella abundance. However, a warm winter and spring in 1991 led to accelerated development and the imposition of a facultative third generation in one out of ten years of observation.
• 4 In 1989, leaves fell relatively early and leaf fall in the autumn accounted for more than 50% of the mortality of C.hamudryadella. in 1990 and 1991 leaves fell relatively late and leaf fall induced mortality was substantially reduced and overwinter survival was markedly increased.
• 5 The abundance of C.hamadryadella remained constant in the spring and summer of 1990 following the previous autumn's relatively early leaf fall, but increased by 10-fold in the spring of 1991 following the relatively late leaf fall of autumn 1990. The abundance of C.hamadryadella also increased 4-fold between the summer of 1991 and the spring of 1992 after another autumn of relatively late leaf fall. We attribute these increases in abundance in part to reduced mortality because of later leaf fall.
• 6 Variation in the timing of autumn leaf fall may be responsible for initiating outbreaks of C.hamadryadella.

     

Growth patterns of early versus late successional multipurpose trees of the western Himalaya

In the mid-western Himalaya (altitude 1350 m, rainfall 1100 mm), multipurpose trees found as escapees in agricultural fields or naturally growing in the forests, play an important role in providing fuel, fooder and small timber to the farmers. Shoot elogation, and tree architecture of 4 year old trees of Grewia optiva, Robinia pseudoacacia and Celtis australis (early successionals), and Quercus leucotrichophora, Q. glauca and Ilex odorata (late successionals), were analyzed. All the late successional species showed a proleptic type of bud and branch production, while the early successional trees made growth through syllepsis. The shoot elongation differed significantly (P <0.05) with the crown position, and ranged from 11 to 30 cm in different species. Early successional species tended to maintain a comparatively narrow crown and showed a significantly (P <0.05) higher ramification ratio, and multilayered canopy. The late successionals, in contrast, showed a wide crown with monolayered canopy, adapted to the weak light intensity. There was only one flush of leaves in Q. leucotrichophora and Q. glauca while in the rest of the species there were two distinct flush periods. The results are important for the management of agroforestry trees.     

Within- and among-tree variation in leaf morphology of <Emphasis Type="Italic">Quercus petraea</Emphasis> (Matt.) Liebl. natural populations

Variation in leaf morphology of Quercus petraea in response to several ecological conditions has been studied extensively, although not explicitly in the context of within- and among-tree variation. This study examined leaf morphology and anatomy of Q. petraea, growing in five natural Italian populations adapted to different ecological environments, to understand the pattern of within- and among-tree variation in this species. We used an ANOVA model with both crossed and nested effects. All levels contributed significant components of variation. Within-tree variation due to branch position was large, particularly in thickness and productivity (40%). For 19 of 32 variables, the variation among trees was surprisingly lower than the within-tree variation explained by branch position. Trends in leaf morphology and anatomy with branch position exhibited the sun-shade dichotomy. Patterns of crown plasticity showed lower values in the two xeric populations. Results suggest the need for taxonomic studies to consider variation as a quantitative attribute of individual trees.     

Spatial and temporal variation in islands of fertility in the Sonoran Desert

In many arid and semi-arid ecosystems, canopy trees and shrubs have a strong positive influence on soil moisture and nutrient availability, creating islands of fertility where organic matter and nutrients are high relative to areas outside the canopy. Previous studies of canopy effects on soil processes have rarely considered how landscape context may modulate these effects. We measured the effects of velvet mesquite trees (Prosopis velutina) on soil moisture and the biogeochemistry of nitrogen at different positions along a topographic gradient from upland desert to riparian zone in the Sonoran Desert of central Arizona. We also examined how landscape position and patterns of precipitation interact to determine the influence of P. velutina on soil moisture, N availability assessed using ion exchange resins, net N mineralization and net nitrification, and microbial biomass C and N. P. velutina clearly created islands of fertility with higher soil organic matter, net N mineralization and net nitrification rates, and microbial biomass under mesquite canopies. These effects were consistent across the landscape and showed little temporal variability. Magnitude and direction of effect of mesquite on soil moisture changed with landscape position, from positive in the upland to negative in the terrace, but only when soil moisture was >4%. Resin N showed responses to mesquite that depended on precipitation and topographic position, with highest values during wet seasons and under mesquite on terraces. We suggest changes in proximity of P. velutina to groundwater lead to shifts in biogeochemical processes and species interactions with change in landscape position along a topographic gradient.     

Plasticity in bundle sheath extensions of heterobaric leaves

? Premise of the study: Leaf venation is linked to physiological performance, playing a critical role in ecosystem function. Despite the importance of leaf venation, associated bundle sheath extensions (BSEs) remain largely unstudied. Here, we quantify plasticity in the spacing of BSEs over irradiance and precipitation gradients. Because physiological function(s) of BSEs remain uncertain, we additionally explored a link between BSEs and water use efficiency (WUE). ? Methods: We sampled leaves of heterobaric trees along intracrown irradiance gradients in natural environments and growth chambers and correlated BSE spacing to incident irradiance. Additionally, we sampled leaves along a precipitation gradient and correlated BSE spacing to precipitation and bulk δ(13)C, a proxy for intrinsic WUE. BSE spacing was quantified using a novel semiautomatic method on fresh leaf tissue. ? Key results: With increased irradiance or decreased precipitation, Liquidambar styraciflua decreased BSE spacing, while Acer saccharum showed little variation in BSE spacing. Two additional species, Quercus robur and Platanus occidentalis, decreased BSE spacing with increased irradiance in growth chambers. BSE spacing correlated with bulk δ(13)C, a proxy for WUE in L. styraciflua, Q. robur, and P. occidentalis leaves but not in leaves of A. saccharum. ? Conclusions: We demonstrated that BSE spacing is plastic with respect to irradiance or precipitation and independent from veins, indicating BSE involvement in leaf adaptation to a microenvironment. Plasticity in BSE spacing was correlated with WUE only in some species, not supporting a function in water relations. We discuss a possible link between BSE plasticity and life history, particularly canopy position.     

Leaf plasticity in response to light of three evergreen species of the Mediterranean maquis

Morphological, anatomical, biochemical and physiological traits of sun and shade leaves of adult Quercus ilex, Phillyrea latifolia and Pistacia lentiscus shrub species co-occurring in the Mediterranean maquis at Castelporziano (Latium) were studied. Fully expanded sun leaves had 47% (mean of the three species) greater leaf mass area (LMA) and 31% lower specific leaf area (SLA) than shade leaves. Palisade parenchyma thickness contributed on an average 42% to the total leaf thickness, spongy layer 43%, upper epidermal cells 5%, and upper cuticle thickness 3%. Stomatal size was greater in sun (25.5 μm) than in shade leaves (23.6 μm). Total chlorophyll content per fresh mass was 71% greater in shade than in sun leaves, and nitrogen content was the highest in sun (13.7 mg g⁻¹) than in shade leaves (11.8 mg g⁻¹). Difference of net photosynthetic rates (P _N) between sun and shade leaves was 97% (mean of the three species). The plasticity index (sensu Valladares et al., New Phytol 148:79–91, 2000a) was the highest for physiological leaf traits (0.86) than for morphological, anatomical and biochemical ones. Q. ilex had the highest plasticity index of morphological, anatomical and physiological leaf traits (0.37, 0.28 and 0.71, respectively) that might explain its wider ecological distribution. The higher leaf plasticity of Q. ilex might be advantageous in response to varying environmental conditions, including global change.     

VARIATION IN THE NUTRIENT CONTENT OF LEAVES OF QUERCUS ALBA,QUERCUS COCCINEA,AND PINUS RIGIDA IN THE BROOKHAVEN FOREST FROM BUD-BREAK TO ABSCISSION

Leaves of Quercus coccinea, Q. alba, and Pinus rigida were collected at six dates during the growing season and analyzed for N, P, K, Ca, Mg, Fe, S, and Na. Leaf weights per unit of leaf area (or length) were determined for the same period. Quercus coccinea and Q. alba leaves increased in weight per unit area by about 30 % and 50 %, respectively. First-year pine leaves increased in weight per unit length by about 65 %. During the second year the weight of pine leaves changed little. Two broad patterns in the nutrient content of leaves were apparent when nutrient content was expressed on the basis of leaf area rather than leaf weight. N, P, and K concentrations increased to a peak in mid- or late summer and declined abruptly just prior to abscission. Concentrations of other elements tended to rise slowly throughout the life of the leaves in all three species. The differences among nutrients and among species support the hypothesis that differential partitioning of the nutrient pool occurs as a result of evolutionary adaptation. The changes in weight of leaves per unit area and in nutrient content during the growing season are important for studies of net primary production and in appraisals of the cycling of nutrients. Least distortion of nutrient relationships occurs when area or length of leaf is used as the basis for expression of nutrient content.