Moderate Shade Research Articles

Geographic Distribution and Developmental Sites of Aedes albopictus (Diptera: Culicidae) During a Chikungunya Epidemic Event

Aedes albopictus is generally considered to have a low vectorial capacity because of its lack of host specificity. Nevertheless, it has been the sole vector of the Chikungunya virus in recent explosive epidemics on the islands of La Réunion and Mauritius. We report on investigations of the seasonal prevalence, container preferences, and geographic distribution of the species on La Réunion. Ae. albopictus showed strong ecological plasticity. In the warm wet season, small disposable containers were the principal urban breeding site, with 1939 positive containers in 750 houses. In the dry winter season, the species remained abundant throughout the island up to 800 m and was present to a maximum altitude of 1200 m. Natural containers were clearly important in this season, although productive sources were hard to find. The preferred natural developmental sites were bamboo stumps and rock holes, over 357 developmental sites observed in peri-urban and gully areas. Generalized logistic models indicated that the optimum sites contained clear water with high organic content and were situated in sites with moderate shade. Our data will provide input into the models of the epidemiology of the disease and design of vector control programs.

Spreading of the invasive Carpobrotus aff. acinaciformis in Mediterranean ecosystems: The advantage of performing in different light environments

AbstractQuestion: Do specific environmental conditions affect the performance and growth dynamics of one of the most invasive taxa (Carpobrotus aff. acinaciformis) on Mediterranean islands?Location: Four populations located on Mallorca, Spain.Methods: We monitored growth rates of main and lateral shoots of this stoloniferous plant for over two years (2002–2003), comparing two habitats (rocky coast vs. coastal dune) and two different light conditions (sun vs. shade). In one population of each habitat type, we estimated electron transport rate and the level of plant stress (maximal photochemical efficiency Fv/ Fm) by means of chlorophyll fluorescence.Results: Main shoots of Carpobrotus grew at similar rates at all sites, regardless habitat type. However, growth rate of lateral shoots was greater in shaded plants than in those exposed to sunlight. Its high phenotypic plasticity, expressed in different allocation patterns in sun and shade individuals, and its clonal growth which promotes the continuous search of available resources, contributed to a good growth and photochemical efficiency of Carpobrotus in the relatively moderate shade of the understories of Mediterranean shrublands and woodlands. Each main shoot of a Carpobrotus clone (which can have several dozens main shoots) grows ca. 40 cm per year, which explains its vigorous habitat colonization capacity.Conclusion: The highly plastic morphological response to different light regimes of this taxon contributes to a rapid colonization of heterogeneous coastal Mediterranean environments spreading well beyond the open sand dune systems where it has been often reported.

Interactive responses of Quercus suber L. seedlings to light and mild water stress: effects on morphology and gas exchange traits

• The combined effect of water stress and light on seedlings of forest species is a key factor to determine the best silvicultural and afforestation practices in the Mediterranean area. • The aims of this work was (1) to determine the optimal light level for the early development of cork oak seedlings under mild water stress and (2) to test if the combined effect of water stress and light followed the trade-off, the facilitation or the orthogonal hypothesis. • Shade reduced instantaneous photosynthetic rates and water use efficiency in cork oak. However, seedlings grown under moderate shade (15% of full sunlight) were capable to accumulate similar amount of biomass than those grown under more illuminated environments by increasing their specific leaf area. Absolute differences in net photosynthesis between light treatments were higher in well watered than in water stressed seedlings. However, the impact of both factors on overall growth was orthogonal. • We concluded that cork oak development is impaired under deep shade (5% of full sunlight) but it can be optimal under moderate shade (15% of full sunlight) even under moderate water stress. Implications of these patterns on regeneration, cultivation and afforestation of cork oak are discussed.

Spreading of the invasive Carpobrotus aff. acinaciformis in Mediterranean ecosystems: The advantage of performing in different light environments

Question: Do specific environmental conditions affect the per formance and growth dynamics of one of the most invasive taxa (Carpobrotus aff. acinaciformis) on Mediterranean islands? Location: Four populations located on Mallorca, Spain. Methods: We monitored growth rates of main and lateral shoots of this stoloniferous plant for over two years (2002-2003) , comparing two habitats (rocky coast vs. coastal dune) and two different light conditions (sun vs. shade). In one population of each habitat type, we estimated electron transport rate and the level of plant stress (maximal photochemical efficiency Fv/ Fm) by means of chlorophyll fluorescence. Results: Main shoots of Carpobrotus grew at similar rates at all sites, regardless habitat type. However, growth rate of lateral shoots was greater in shaded plants than in those exposed to sunlight. Its high phenotypic plasticity, expressed in different allocation patterns in sun and shade individuals, and its clonal growth which promotes the continuous search of available resources, contributed to a good growth and photochemical efficiency of Carpobrotus in the relatively moderate shade of the understories of Mediterranean shrublands and woodlands. Each main shoot of a Carpobrotus clone (which can have several dozens main shoots) grows ca. 40 cm per year, which explains its vigorous habitat colonization capacity. Conclusion: The highly plastic morphological response to different light regimes of this taxon contributes to a rapid colonization of heterogeneous coastal Mediterranean environments spreading well beyond the open sand dune systems where it has been often reported.

Population ecology in a naturalsyneilesis palmata stand: II. vegetative growth and population structure

Vegetative growth and changes within populations ofSyneilesis palmata (THUNB>) MAX. Usannamul were monitored in two natural stands with different levels of incoming light. This perennial, shade-tolerant herb was measured for its plant size, petiole length, leaf breadth, and leaflet number over 11 years in a moderately shaded (MS) stand and over 7 years in a severely shaded (SS) stand. At the end of each growing season, a shoot primordium developed at the center of the root system. Correlation coefficients were mostly high between pairings of two properties - total weight, petiole length, petiole weight, leaf breadth, leaf blade weight, leaf area, specific leaf area, and leaflet number. Mean annual mortality of this species was higher in SS (22.97%) than in MS (8.85%), but great fluctuations were seen from year by year. Mortality was lowest for medium-sized plants. Regarding petiole length, differences in mean annual growth rates were conspicuous, i.e., 2.6% in MS and 8.4% in SS, while growth rates for leaf breadth were 3.1% in MS and 24.2% in SS. Changes in plant frequency within individual size classes roughly showed a normal distribution curve. However, the mode varied year by year, and such changes were more remarkable in SS than in MS. Under severe shading,S. palmata had a higher growth rate and but also greater mortality than did plants under moderate shade. Therefore, one can conclude that the population structure in SS was unstable over time.

Age-related change in canopy traits shifts conspecific facilitation to interference in a semi-arid shrubland

Shifts between facilitation and interference and their importance in shaping plant population and community dynamics have received wide recognition. Nevertheless, the causes and spatio-temporal scales of these shifts are poorly understood, yet strongly debated. This study tested the hypothesis that age-related changes in canopy structure shift the effect of a on their protege from facilitation to interference, using as a model system the interaction between the dwarf Sarcopoterium spinosum and conspecific new recruits, in the shrubland of the transition area between the Mediterranean and the semi-arid climatic zones of Israel. Foliation level (i.e. the percentage of canopy surface area covered with leaves), a measure of canopy structure, increased with age. Shading level was significantly and positively related to foliation level. Densities of new recruits in the shrubland showed a unimodal response to canopy structure and cover: the highest densities were associated with canopies presenting low and medium foliation levels (providing 71 and 82% shade, respectively), while high foliation levels (93% shade) and open spaces among canopies were characterized by very low densities. A related field experiment using shading nets revealed that seedling survival rates followed a similar unimodal pattern, with the highest survival (ca 60%) detected in moderate shade (70%), twice as much as in full sun, and the lowest survival (ca 10%) observed in extreme shade (90%). These results support the study hypothesis on age-dependent interactions. Thus, in a semi-arid shrubland ecosystem, the transition of the nurse shrub from young to old stage can shift facilitation to interference. Hence, the age structure of established populations determines a) the availability of suitable sites for seedling recruitment and b) the balance between facilitation versus interference effects on seedling establishment.

Morfofisiologia e valor nutritivo do capim-braquiária sob sombreamento natural e a sol pleno

O objetivo deste trabalho foi avaliar características morfofisiológicas e o valor nutritivo da Brachiaria decumbens, nas épocas chuvosas de 2003 e 2004, em um sistema silvipastoril, com faixas de 30 m de largura com essa gramínea, alternadas com faixas de 10 m com árvores de Acacia mangium, A. angustissima, Mimosa artemisiana, Leucaena leucocephala x L. diversifolia, Eucalyptus grandis. Foi adotado o delineamento de blocos ao acaso, em parcelas subsubdivididas (características morfofisiológicas) ou parcelas subdivididas (características do valor nutritivo), com quatro repetições. Em 2003, o sombreamento foi de 65% da radiação incidente fora do bosque e, em 2004, de 35%, depois da retirada de árvores de eucalipto. Sob sombreamento, a densidade de perfilhos, o índice de área foliar e a massa de forragem foram maiores no segundo ano. Os teores médios de fibra em detergente neutro (FDN) foram maiores a pleno sol que sob sombreamento, enquanto os valores médios de proteína bruta (PB) e a digestibilidade in vitro da matéria seca foram maiores à sombra. O sombreamento moderado aumenta os teores de PB, reduz os teores de FDN e incrementa a digestibilidade da forragem de gramíneas que crescem sob a copa das árvores.

Irradiance and oak seedling survival and growth in a heterogeneous environment

The Mediterranean region emerges highly heterogeneous at every spatio-temporal scale. Key species shape a mosaic of patches (landscape units) composed of different microhabitats. Within these ecosystems irradiance appears as one of the most changing factors affecting communities’ dynamic. However, the impact of irradiance on the species performance is still poorly known. In this study, we intended to explore whether irradiance heterogeneity influences early recruitment. We chose a patchy Mediterranean site. During 2001, 2002 and 2003 we marked naturally Quercus ilex emerged seedlings. We distinguished three landscape units ( Q. ilex woodlands, shrublands and afforestations) and six microhabitats (oak, broom, tall shrub, subshrub, pine and open). For each seedling, we measured (1) irradiance as global site factor (GSF); (2) acorn presence; (3) acorn depth; (4) burial layer; (5) distance to the nearest co-specific adult. As estimates of seedling performance, we measured seedling survival and height. Estimates of seedling performance were higher in 2001 and 2002 than in 2003. Oak or pine microhabitats achieved higher survival rates than brooms or open areas whilst under oaks, brooms or in open areas seedling height was lower than under tall shrubs. Irradiance significantly affected seedling survival and growth. The lower the irradiance, the higher the survival. Maximal growth was found at intermediate irradiance, indicating that, in this system, moderate shade is crucial for Q. ilex seedling recruitment. Some microhabitats provide intermediate irradiance that combines positive effects on survival and growth. As a final point, dispersers’ activity creates a heterogeneous distribution of acorn deposition filtering out part of the total irradiance present in the study site.

Growth of Ramin (Gonystylus bancanus (Miq) Kurz.) Plantation on Various Peat Swamp Forests in Indonesia

Ramin ( Gonystylus bancanus (Miq) Kurz.) is a trade name of timber produced from a group of Gonystylus species. It is a well-known species because of its high timber quality and value. Ramin natural population has been decreasing sharply since the last two decades due to over exploitation, and nowadays leads to extinction. The objective of this research was to analyze the growth of ramin plantation on various sites. Data were collected from February to April 2005 through field survey on five sites of peat-swamp forest areas in four provinces, Riau, Jambi, West and Central Kalimantan. The result showed that ramin annual early growth varied across sites. It grew better on deep peat soil (3 - 4 m in depth) and under moderate shading (55 - 60%) during early growth. The highest early height growth (52.27 cm/year) occurred at Sei Bakau, West Kalimantan. Whereas, it highest early diameter growth (0.73 cm/year) was found at Rokan Hilir, Riau. The characteristics of site are very important for a successful growth of ramin plantation, including peat depth and level of shading. Annual early growth of ramin as along in line with increasing of the plant age, namely a four- times increase of plant age (from 1.6 - 2 years to 6.5 - 7.3 years), the annual early height and diameter growth decreased until they reached a half and one third of the starting values. Since the growth of ramin is very slow, it is necessary to find and develop technologies to accelerate growth rate of the ramin plants. Application of mycorrhizal fungal inoculum is a promising technology, but their role in promoting ramin growth and the needed cost must be tested and analyzed under various conditions of the peat-swamp soils in Indonesia.

Elevated CO 2 changes the moderate shade tolerance of yellow birch seedlings

To demonstrate the existence of light thresholds in plant growth and to examine the effects of elevated CO 2 on the shade tolerance of a tree species an experiment consisting of a completely randomized design for a total of 96 yellow birch ( Betula alleghaniensis Britton) seedlings was conducted with 3 light levels (2.9% 7.7% 26.1% of full sunlight) × 2CO 2 levels (350 and 700±10 ppm) with 4 replications in a phytotron. The study proved that thresholds exist and they vary in different plant organs. In ambient CO 2 the thresholds were 13.3% 18.7% 15.0% 15.2% and 15.6% of full sunlight for stem leaf root total plant biomass and the averaged value respectively. In 700 ppm CO 2 the corresponding thresholds were 16.7% 21.3% 18.1% 21.7% and 19.5% for stem leaf root total plant biomass and the averaged value respectively. The lowest threshold in the stem is an indicator of the minimal light intensity for regular growth for seedlings of this species. Below this threshold light-stressful growth occurs. The result of a paired t-test indicated that the thresholds in elevated CO 2 were significantly higher than in ambient CO 2. This suggests that yellow birch will lose its moderate shade tolerance evolutionally becoming a shade-intolerant species and that it may become more difficult to naturally regenerate in the future.

Photosynthetic responses to understory shade and elevated carbon dioxide concentration in four northern hardwood tree species

Seedling responses to elevated atmospheric CO(2) concentration ([CO(2)]) and solar irradiance were measured over two growing seasons in shade-tolerant Acer saccharum Marsh. and Fagus grandifolia J.F. Ehrh. and shade-intolerant Prunus serotina, a J.F. Ehrh. and Betula papyrifera Marsh. Seedlings were exposed to a factorial combination of [CO2] (ambient and elevated (658 micromol mol-1)) and understory shade (deep and moderate) in open-top chambers placed in a forest understory. The elevated [CO(2)] treatment increased mean light-saturated net photosynthetic rate by 63% in the shade-tolerant species and 67% in the shade-intolerant species. However, when measured at the elevated [CO(2)], long-term enhancement of photosynthesis was 10% lower than the instantaneous enhancement seen in ambient-[CO(2)]-grown plants (P < 0.021). Overall, growth light environment affected long-term photosynthetic enhancement by elevated [CO(2)]: as the growth irradiance increased, proportional enhancement due to elevated [CO(2)] decreased from 97% for plants grown in deep shade to 47% for plants grown in moderate shade. Results suggest that in N-limited northern temperate forests, trees grown in deep shade may display greater photosynthetic gains from a CO(2)-enriched atmosphere than trees growing in more moderate shade, because of greater downregulation in the latter environment. If photosynthetic gains by deep-shade-grown plants in response to elevated [CO(2)] translate into improved growth and survival of shade-intolerant species, it could alter the future composition and dynamics of successional forest communities.

TOLERANCE TO SHADE, DROUGHT, AND WATERLOGGING OF TEMPERATE NORTHERN HEMISPHERE TREES AND SHRUBS

Lack of information on ecological characteristics of species across different continents hinders development of general world-scale quantitative vegetation dynamic models. We constructed common scales of shade, drought, and waterlogging tolerance for 806 North American, European/West Asian, and East Asian temperate shrubs and trees representing about 40% of the extant natural Northern Hemisphere species pool. These scales were used to test the hypotheses that shade tolerance is negatively related to drought and waterlogging tolerances, and that these correlations vary among continents and plant functional types. We observed significant negative correlations among shade and drought tolerance rankings for all data pooled, and separately for every continent and plant functional type, except for evergreen angiosperms. Another significant trade-off was found for drought and waterlogging tolerance for all continents, and for evergreen and deciduous angiosperms, but not for gymnosperms. For all data pooled, for Europe and East Asia, and for evergreen and deciduous angiosperms, shade tolerance was also negatively associated with waterlogging tolerance. Quantile regressions revealed that the negative relationship between shade and drought tolerance was significant for species growing in deep to moderate shade and that the negative relationship between shade and waterlogging tolerance was significant for species growing in moderate shade to high light, explaining why all relationships between different tolerances were negative according to general regression analyses. Phylogenetic signal in the tolerance to any one of the three environmental factors studied was significant but low, with only 21-24% of cladogram nodes exhibiting significant conservatism. The inverse relationships between different tolerances were significant in phylogenetically independent analyses both for the overall pool of species and for two multispecies genera (Pinus and Quercus) for which reliable molecular phylogenies were available. Only 2.6-10.3% of the species were relatively tolerant to two environmental stresses simultaneously (tolerance value � 3), and only three species were tolerant to all three stresses, supporting the existence of functional trade-offs in adjusting to multiple environmental limitations. These trade-offs represent a constraint for niche differentiation, reducing the diversity of plant responses to the many combinations of irradiance and water supply that are found in natural ecosystems.

Salinity tolerance of 'Valencia' orange trees on rootstocks with contrasting salt tolerance is not improved by moderate shade

The effects of shading in combination with salinity treatments were studied in citrus trees on two rootstocks with contrasting salt tolerance to determine if shading could reduce the negative effects of salinity stress. Well-nourished 2-year-old 'Valencia' orange trees grafted on Cleopatra mandarin (Cleo, relatively salt tolerant) or Carrizo citrange (Carr, relatively salt sensitive), were grown either under a 50% shade cloth or left unshaded in full sunlight. Half the trees received no salinity treatment and half were salinized with 50 mM Cl- during two 9 week salinity periods in the spring and autumn interrupted by an 11 week rainy period. The shade treatment reduced midday leaf temperature and leaf-to-air vapour pressure deficit regardless of salinity treatments. In non-salinized trees, shade increased midday CO2 assimilation rate (A(CO2)) and stomatal conductance, but had no effect on leaf transpiration (E(lf)). Shade also increased leaf chlorophyll and photosynthetic water use efficiency (A(CO2)/E(lf)) in leaves on both rootstocks and increased total plant dry weight in Cleo. The salinity treatment reduced leaf growth and leaf gas exchange parameters. Shade decreased Cl- concentrations in leaves of salinized Carr trees, but had no effect on leaf or root Cl- of trees on Cleo. There were no significant differences in leaf gas exchange parameters of shaded and unshaded salinized plants but the growth reduction from salinity stress was actually greater for shaded than for unshaded trees. Shaded trees on both rootstocks had higher leaf Na+ than unshaded trees after the first salinity period, and this shade-induced elevated leaf Na+ persisted after the second salinity period in trees on Carr. Thus, shading did not alleviate the negative effects of salinity on growth and Na+ accumulation.

Tradeoff between shade adaptation and mitigation of photoinhibition in leaves of Quercus mongolica and Acer mono acclimated to deep shade

We investigated susceptibility to photoinhibition in leaves acclimated to different light regimes in intermediately shade-tolerant Japanese oak (Quercus mongolica Fisch. ex Turcz. var. crispula (Blume) Ohashi) and shade-tolerant Japanese maple (Acer mono Maxim. var. glabrum (Lév. et Van't.) Hara), to elucidate adaptability to gap formation in leaves differing in shade acclimation. We hypothesized that there is a tradeoff between shade adaptation and capacity to mitigate photoinhibition associated with leaf morphology. We simultaneously measured chlorophyll fluorescence and gas exchange in seedlings that had been grown in full sunlight (open), 10% of full sun (moderate shade) and 5% of full sun (deep shade). Shade-tolerant A. mono adapted to deep shade through changes in leaf morphology, lowering its leaf mass per area (LMA), but Q. mongolica showed little change in LMA between moderate and deep shade. Photochemical quenching (qP) did not differ between species in full sunlight and moderate shade; however, in deep shade, qP of Q. mongolica was higher than that of A. mono, suggesting that Q. mongolica grown in deep shade is less susceptible to photoinhibition at gap formation. This is consistent with the finding that chronic photoinhibition 3 days after the transfer to full sunlight, indicated by the decrease in maximum photochemical efficiency, Fv/Fm, at predawn, was less in deep-shade-grown Q. mongolica than in deep shade-grown A. mono. In deep shade, the electron transport rate (ETR) of Q. mongolica was higher than that of A. mono, whereas thermal energy dissipation through photosystem II antennae, indicated by non-photochemical quenching, was lower in Q. mongolica than in A. mono. In deep shade, the greater ETR capacity in Q. mongolica in association with higher LMA and higher leaf N content could contribute to maintaining high qP and mitigating photoinhibition. These results indicate that, by maintaining a high electron transport capacity even in deep shade, the gap-dependent and intermediate-shade-tolerant Q. mongolica trades improved shade adaptation for higher growth potential when a gap event occurs.

Interactive effects of shade and irrigation on the performance of seedlings of three Mediterranean Quercus species

Shade and irrigation are frequently used to increase the success of Mediterranean Quercus spp. plantations. However, there is controversy about the combined effects of these treatments on plant performance. We assessed the effects of two irradiances (full sunlight and moderate shade) and two summer watering regimes (high (daily) and low (alternate days)) on leaf and whole-plant traits of 1-year-old seedlings of Quercus coccifera, Q. ilex subsp. ballota and Q. faginea grown outdoors for 8.5 months. Leaf traits included measures of morphology, nitrogen concentration, gas exchange and photochemical efficiency, and measures of whole-plant traits included biomass allocation patterns, growth phenology, across-summer leaf area change and relative growth rate (RGR). Moderate shade reduced leaf mass per area, increased photochemical efficiency, maximum carbon assimilation rate (Amax) and allocation to leaves, and prolonged the growing period in one or more of the species. Daily watering in summer increased Amax of Q. ilex and prolonged the growing period of Q. ilex and Q. faginea. Both treatments tended to increase RGR. The effect of shade was greater in the low-watering regime than in the high-watering regime for two of the 15 studied traits, with treatment effects being independent for the remaining 13 traits. Leaf nitrogen and the ability to maintain leaf area after the arid period, rather than biomass allocation traits, explained the variation in seedling RGR. Trait responsiveness to the treatments was low and similar among species and between study scales, being unexpectedly low in Q. faginea leaves. This may be because selective pressures on leaf plasticity act differently in deciduous and evergreen species. We conclude that moderate shade and daily summer watering enhance the performance of Mediterranean Quercus seedlings through species-specific mechanisms.

Diameter growth performance varies with species functional-group and habitat characteristics in subtropical rainforests

We examined tree diameter growth in 20 plots subjected to various disturbance intensities (natural, low, moderate and intensive logging) in a bid to understand the general tree growth responses in relation to habitat characteristics in subtropical rainforests of north-eastern NSW, Australia. Species-specific regeneration strategy, maximum size and level of shade tolerance were used to classify species into five groups: emergent and shade tolerant main canopy (group 1), shade tolerant mid canopy (2), shade tolerant understoreys (3), moderate shade tolerant (4) and shade intolerant (5) tree species. Data series for trees ≥10 cm diameter at 1.3 m above the ground level (dbh) providing observations spanning over 36 years were used in multilevel regression analyses. The results showed that spatial and temporal effects in tree growth at the stand level are a combination of the differences between species functional-group compositions and environmental gradients. High growth responses were observed in the shade intolerant species while increasing level of shade tolerance and decreasing maximum size decreased trees growth rates. Tree growth increased with altitude on a large scale across regions, and with disturbance intensity on a small scale at the plot (stand) level. Increase in northness (south through flat to north facing sites) increased growth in species group 1 for trees <67 cm dbh, but beyond this dbh threshold the opposite was true. These showed that saplings of species group 1 may require increased illumination to reach the forest canopy, but once in the canopy, low soil water availability may be limiting to tree growth in the north facing sites. Decrease in northness was associated with increased growth in species group 2 indicating that reduced illumination and improved soil moisture in the south facing sites were conducive for maximum growth in this species group. Maximum growth potential in species groups 4 and 5 increased with decrease in eastness, suggesting that the increased afternoon solar radiation and temperature were conducive for high growth rates in these species. Although topographic gradient may determine the spatial and temporal variations in tree growth where growth appeared to increase from the crest down the slope into the creek/gully, its effects on soil fertility and water availability, and interactions between these and other factors may make it difficult to discern clear growth patterns.

Desenvolvimento de mudas de Aroeira (Schinus terebinthifolius) e sombreiro (Clitoria fairchildiana) sob condições de sombreamento

Objetivou-se com este trabalho avaliar o desenvolvimento das mudas de Schinus terebinthifolius e Clitoria fairchildiana sob condições de sombreamento e pleno sol. As mudas foram cultivadas sob sombrites de 70% e 50% de luz e sob luz plena. O experimento foi conduzido em DBC com 4 repetições de 20 mudas. Foram avaliados a altura do caule e o diâmetro do colo a cada vinte e um dias. Ao final do experimento foram realizadas as avaliações de peso seco total (PST-g), área foliar (AF-dm²), razão de peso foliar (RPF -g/g); e peso específico de folha (PEF -g/ dm²). As sementes de aroeira do campo e do sombreiro apresentaram 70% e 85% de germinação, respectivamente. S.terebinthifolius tolera bem um sombreamento moderado (70% de luz), sendo seu cultivo a pleno sol o mais indicado, pois é quando as mudas atingem menor altura (32,93 cm), diâmetro médio de 7,15 mm, maior PST (74,66 g); menor PEF (0,025); as mudas de C.fairchildiana sob condições de pleno sol não se desenvolveram satisfatoriamente, embora a altura não tenha variado entre os níveis de luz (16,39 cm), apresentaram menor diâmetro (5,32 mm), menor PST (26,22 g), menor AF (32,98 dm²) e maior PEF (0,4).

Shade tolerance, photoinhibition sensitivity and phenotypic plasticity of Ilex aquifolium in continental Mediterranean sites

Shade tolerance, plastic phenotypic response to light and sensitivity to photoinhibition were studied in holly (Ilex aquifolium L.) seedlings transported from the field to a greenhouse and in adult trees in the field. All plants were growing in, or originated from, continental Mediterranean sites in central Spain. Seedlings tolerated moderate but not deep shade. Mortality was high and growth reduced in 1% sunlight. Survival was maximal in 12% sunlight and minimal in full sunlight, although the relative growth rate of the seedlings surviving in high light was similar to that of plants in moderate shade. Maximum photochemical efficiency at predawn was significantly lower in sun plants than in shade plants in the field, revealing chronic photoinhibition that was most pronounced in winter. Plasticity in response to available light varied according to the variable studied, being low for photosynthetic capacity and stomatal conductance, and high for specific leaf area, root:shoot ratio and leaf area ratio, particularly in seedlings. Differences in water relations and hydraulic features between sun and shade plants in the field were marginal. High water potential at the turgor loss point of field-grown plants suggested that holly is sensitive to drought during both the seedling and the adult stage. Low relative growth rates in both high and low light with low physiological plasticity in response to light indicate the existence of a stress-tolerance mechanism. We conclude that holly is a facultative understory plant in areas of oceanic and relatively mild climate, but an obligate understory plant in dry continental areas such as the study site. The impact of abandonment of traditional management practices and climate change on these Mediterranean populations is discussed.

Establishing Native Grasses in a Big Sagebrush–Dominated Site: An Intermediate Restoration Step

AbstractMany semiarid rangelands in the Great Basin, U.S.A., are shifting dominance to woody species as a consequence of land degradation including intense livestock grazing and fire suppression. Whereas past rehabilitation efforts in Big sagebrush (Artemisia tridentata) steppes removed the shrub and added introduced forage grasses to successfully shift communities from shrublands to grasslands, current consensus is that native species should be included in restoration projects and that retention of some woody plants is desirable. We examined the potential for interseeding grasses into dense shrub communities as a precursor to thinning shrubs and releasing grasses from shrub interference. We compared seedling establishment of the native grass, Bluebunch wheatgrass (Pseudoroegneria spicata), with that of the Eurasia grass, Crested wheatgrass (Agropyron desertorum), in denseAr. tridentatastands. Shrubs may play an important role as nurse plants for seedling establishment (reduced solar radiation, “island of fertility” effect) but result in highly contrasting light environments and root interference for seedlings. In experimental plots, we examined effects ofAr. tridentatashade levels (0, 40, 70, and 90% reduction of solar radiation) and initial root exclusion (present/absent) on the establishment and growth ofP. spicataandAg. desertorumseedlings. With this design we evaluated the interference effects ofAr. tridentataon the two grasses and identified the most beneficial microsites for grass restoration inAr. tridentata–dominated communities. We predicted seedling survival and growth to be greater under moderate shade (40% reduction) and limited root competition than under no or strong shade conditions (0 and 90%) and unrestricted root interactions. Fifty to 85% of theP. spicataandAg. desertorumseedlings survived the dry summer months of 1995 and 1996 and the intervening winter. Neither shading nor root exclusion fromAr. tridentataaffected final seedling survival of either species. Seedling biomass of both grass species was negatively affected by initial root interactions withAr. tridentata.However, the analysis of seedling biomass variability (coefficient of variation) indicated that in all shade and root‐exclusion treatments, some seedlings of both species developed to large individuals to survive inAr. tridentata–dominated rangelands. Thus, the use of interseeding techniques shows promise for restoring herbaceous species in denseAr. tridentatastands and should be given further consideration when shrub retention is an important consideration.

Implications of opposite phyllotaxis for light interception efficiency of Mediterranean woody plants

Opposite leaves lead to a greater leaf overlapping than leaves spirally arranged along a shoot, decreasing light interception efficiency (Ea, fraction of the light reaching the plant actually intercepted by the leaves) of the crown. However, Ea results from a whole suite of morphological traits. The interplay between phyllotaxis, crown architecture, leaf morphology and Ea was explored in 12 woody species from Mediterranean-type ecosystems, where the abundance of woody species with opposite phyllotaxis is unusually high. The three-dimensional model Y-plant was used to estimate Ea in unbranched, vertical shoots of each species encompassing the natural morphological variation found from moderate shade to open light environments. Ea exhibited significant interspecific differences, ranging from 0.25 in Daphne gnidium to 0.75 in Cistus ladanifer, Olea europaea and Salvia officinalis, decreasing with leaf inclination angle and leaf area ratio (LAR), and increasing with internode-to-leaf-length ratio and supporting biomass. Species with spiral vs. opposite phyllotaxis did not differ in their mean Ea. However, the former had higher Ea than the latter at short internode lengths. The natural range of variation in internode length had a larger effect on Ea than the natural range of leaf elevation angle. Principal component analysis segregated species with opposite phyllotaxis from those with spiral leaves because of their greater self-shading for high sun elevation angles (>45°); they were in turn distributed in two groups, one with high Ea, large investment in supporting biomass and long internodes, and another with low Ea and large LAR. Species with spiral phyllotaxis all had intermediate or low Ea and steep leaf elevation angles. Species with opposite phyllotaxis can compensate their less efficient leaf arrangement by decreasing leaf elevation angle and increasing internode length, but they may experience a real phylogenetic constraint for light interception when biomass allocation to supporting tissues (internodes and petioles) becomes very costly. This constraint could be involved in the shade intolerance of woody Mediterranean species exhibiting opposite phyllotaxis.