Carbon and nitrogen partitioning in the biennial monocarp Arctium tomentosum Mill

Summary Growth and nitrogen partitioning were investigated in the biennial monocarp Arctium tomentosum in the field, in plants growing at natural light conditions, in plants in which approximately half the leaf area was removed and in plants growing under 20% of incident irradiation. Growth quantities were derived from splined cubic polynomial exponential functions fitted to dry matter, leaf area and nitrogen data.Main emphasis was made to understanding of the significance of carbohydrate and nitrogen storage of a large tuber during a 2-years' life cycle, especially the effect of storage on biomass and seed yield in the second season. Biomass partitioning favours growth of leaves in the first year rosette stage. Roots store carbohydrates at a constant rate and increase storage of carbohydrates and nitrogen when the leaves decay at the end of the first season. In the second season the reallocation of carbohydrates from storage is relatively small, but reallocation of nitrogen is very large. Carbohydrate storage just primes the growth of the first leaves in the early growing season, nitrogen storage contributes 20% to the total nitrogen requirement during the 2nd season. The efficiency of carbohydrate storage for conversion into new biomass is about 40%. Nitrogen is reallocated 3 times in the second year, namely from the tuber to rosette leaves and further to flower stem leaves and eventually into seeds. The harvest index for nitrogen is 0.73, whereas for biomass it is only 0.19.     

The Production and Distribution of Dry Matter in Maize after Flowering

An experiment in which different groups of leaf laminae wereremoved, or ears shaded, shortly after silking showed that mostof the dry-matter increase after flowering was produced by upperleaves. The top five, the middle four, and the bottom six laminaeaccounted, respectively, for 26 per cent, 42 per cent, and 32per cent of the leaf area duration (D) of the laminae afterflowering; the estimated contributions of the three groups todry-matter production by the laminae after flowering were about40 per cent, 35–50 per cent, and 5–25 per cent,respectively. The sheaths provided about one-fifth of the totalleaf area and probably contributed about one-fifth, and laminaefour-fifths, of the total dry matter produced after flowering.The contribution from photosynthesis by the ear was negligible,presumably because its surface area was only 2 per cent of thatof the leaves. Leaf efficiency (dry matter produced per unitarea) decreased greatly from the top to the base of the shoot.When laminae were removed, the grain received a larger fractionof the dry matter accumulated after flowering, less dry matterremained in the stem, and the photosynthetic efficiency of theremaining leaves was apparently increased. When alternate laminae were removed at the time of silking (half-defoliation)D was decreased by 40 per cent, and the subsequent productionof dry matter decreased nearly proportionately, so that netassimilation rate (E) was not affected but grain dry weightwas decreased by only 32 per cent. At the final harvest, thegrain of half-defoliated plants constituted 80 per cent of thedry matter accumulated after flowering, compared with 65 percent for intact plants. Stem weight decreased from two weeksafter flowering in half-defoliated plants, but remained nearlyconstant in intact plants. When pollination was prevented andno grains formed, E during the first month after flowering wasunaffected; the dry matter that would have passed into the grainaccumulated in the stem and husks, not in the leaves. The decrease in stem weight caused by defoliation suggests thatpreviously stored dry matter was moved to the grain. That suchmovement is possible was shown by keeping prematurely harvestedshoots in the dark for two weeks with their cut ends in water;the dry weight of the grain increased and that of the stem,laminae, husks, and core decreased. Nevertheless, dry-matterproduction after flowering was more than sufficient for graingrowth, and previous photosynthesis probably contributed littleto the grain.     

Accumulation and Partitioning of Dry Matter in Taro [Colocasia esculenta (L.) Schott]

A field study was conducted as part of an ongoing effort tocollect data on patterns of leaf area development and dry matteraccumulation and partitioning among various plant parts duringgrowth and development of two taro cultivars. Plants were harvestedfor biomass about every 6 weeks during the growing season. Ateach harvest, plants were separated into various plant parts,and their dry matter content was determined. The first 80 dafter planting were characterized by low rates of dry matteraccumulation, with only leaves, petioles, and roots showingsubstantial growth. Afterwards, increases in total dry matterwere mainly the result of corm and sucker growth. Corm bulkingoccurred after the attainment of maximal leaf area indices.The absence of an optimal leaf area index for a longer periodof time may have prevented the realization of higher dry matteryields. The partitioning of dry matter to the corms of bothcultivars remained almost constant especially after 150 d afterplanting. This process was in contrast to the partitioning ofdry matter to the suckers, which increased significantly untilthe end of the growing cycle.Copyright 1995, 1999 Academic Press Taro, Colocasia sp., growth, dry matter partitioning     

Leaf development of the ostrich fern Matteuccia struthiopteris (L.) Todaro

The timing of emergence of the three different leaf types of Matteuccia struthiopteris is described from plants sampled over the course of a growing season. Vegetative leaves were first to appear, followed five weeks later by sporophylls and cataphylls. Leaf number and type, and total leaf dry weight per plant were assessed in weekly transects. Vegetative fronds contributed the most to total leaf dry weight, which increased during the first four weeks, and then remained constant for the remainder of the season. Cataphylls, although numerous by the end of the season, contributed little weight. Sporophylls occurred on the widest plants with the most vegetative leaves and greatest leaf weight, whereas cataphylls occurred on most plants except the smallest. Experimentally defoliated plants were re-examined in late summer. Following initial harvest, plants often produced a second smaller set of leaves. These were restricted to vegetative leaves and cataphylls. Ability to reissue leaves, especially vegetative fronds, declined very quickly after the first few weeks in the growing season. Defoliated plants draw on the extensive reservoir of developing leaves which are found on the rhizome, thus possibly diminishing the ability of the plant to withstand regular harvesting of the young fronds for food. Individual leaves were tagged and measured over the growing season. Non-linear regression curves fitted to the growth data for the three types of leaves indicate that growth was described best by a monomolecular growth curve for the vegetative and fertile fronds. Cataphyllar growth could be described equally well by either a monomolecular or a logistic function.     

Leaf development of the ostrich fern Matteuccia struthiopteris (L.) Todaro

The timing of emergence of the three different leaf types of Matteuccia struthiopteris is described from plants sampled over the course of a growing season. Vegetative leaves were first to appear, followed five weeks later by sporophylls and cataphylls. Leaf number and type, and total leaf dry weight per plant were assessed in weekly transects. Vegetative fronds contributed the most to total leaf dry weight, which increased during the first four weeks, and then remained constant for the remainder of the season. Cataphylls, although numerous by the end of the season, contributed little weight. Sporophylls occurred on the widest plants with the most vegetative leaves and greatest leaf weight, whereas cataphylls occurred on most plants except the smallest. Experimentally defoliated plants were re-examined in late summer. Following initial harvest, plants often produced a second smaller set of leaves. These were restricted to vegetative leaves and cataphylls. Ability to reissue leaves, especially vegetative fronds, declined very quickly after the first few weeks in the growing season. Defoliated plants draw on the extensive reservoir of developing leaves which are found on the rhizome, thus possibly diminishing the ability of the plant to withstand regular harvesting of the young fronds for food. Individual leaves were tagged and measured over the growing season. Non-linear regression curves fitted to the growth data for the three types of leaves indicate that growth was described best by a monomolecular growth curve for the vegetative and fertile fronds. Cataphyllar growth could be described equally well by either a monomolecular or a logistic function.     

Water relations of epiphytic and terrestrially-rooted strangler figs in a Venezuelan palm savanna

Water use patterns of two species of strangler fig, Ficus pertusa and F. trigonata, growing in a Venezuelan palm savanna were contrasted in terms of growth phase (epiphyte and tree) and season (dry and wet). The study was motivated by the question of how C3 hemiepiphytes accommodate the marked change in rooting environment associated with a life history of epiphytic establishment followed by substantial root development in the soil. During the dry season, stomatal opening in epiphytic plants occurred only during the early morning, maximum stomatal conductances were 5 to 10-fold lower, and midday leaf water potentials were 0.5–0.8 MPa higher (less negative) than in conspecific trees. Watering epiphytes of F. pertusa during the dry season led to stomatal conductances comparable to those exhibited by conspecific trees, but midday leaf water potentials were unchanged. During the rainy season, epiphytes had lower stomatal conductances than conspecific trees, but leaf water potentials were similar between the two growth phases. There were no differences in ¹³C between the two growth phases for leaves produced in either season. Substrate water availability differed between growth phases; tree roots extended down to the permanent water table, while roots of epiphytic plants were restricted to material accumulated behind the persistent leaf bases of their host palm tree, Copernicia tectorum. Epiphytic substrate moisture contents were variable during both seasons, indicating both the availability of some moisture during the dry season and the possibility of intermittent depletion during the rainy season. Epiphytic strangler figs appear to rely on a combination of strong stomatal control, maintenance of high leaf water potentials, and perhaps some degree of stem water storage to cope with the fluctuating water regime of the epiphytic environment.     

A role for young leaves in vernalization of cauliflower: I. Analysis of leaf development during curd induction

Growth analysis techniques are used to test the hypothesis that chilling induces curd (flower) initiation in the cauliflower ( Brassica oleracea Botrytis L. cv. Perfection) through inhibiting leaf growth, thereby increasing the availability of growth factors to the stem apex and enabling differentiation of the curd. Effects of chilling on leaf growth and curd induction are compared in juvenile and mature, vegetative plants. Chilling at 5°C reduced dry matter accumulation in the total leaf complement by ca 60% in juvenile plants and 40% in mature plants, compared to control plants growth at 20°C. Juvenile plants showed slower rates of leaf initiation than mature plants. Leaf initiation was retarded by chilling in both plant types with the most marked effect seen in the juvenile plants. This was consistent with dry matter availability to the stem apex limiting differentiation more severely in juvenile plants than in mature plants. The rate of dry matter accumulation in existing leaves, however, was faster in juvenile plants than in mature plants at 20°C. Plants that were juvenile during chilling produced an average of 43 leaves below the curd whereas those that were mature produced 25.
Dry matter accumulation in younger leaves was more markedly inhibited by chilling than in older leaves. Chilling also reduced the rate at which enlarging leaves became positionally more remote from the stem apex. Possible roles for such leaves in regulating apical development are considered.     

Partitioning of Dry Matter in Tanier (Xanthosoma spp.) Irrigated with Fractions of Evapotranspiration

The accumulation and partitioning of dry matter was determinedin tanier plants irrigated with fractions of the water lostthrough evapotranspiration (WLET) in an effort to establishgrowth analysis data from which a tanier growth (simulationmodel could be developed. The irrigation regimes were basedon Class A pan factors ranging from 0·33 to 1·32with increments of 0·33. Tanier plants were planted inthe field and harvested for biomass production about every 6weeks during the growing season. At each harvest, plants wereseparated into various plant parts, and their dry matter contentwas determined. The first 90 d after planting (DAP) were characterizedby low rates of dry matter accumulation, with only leaves andpetioles showing substantial growth. A grand growth period followedin which leaves, petioles, and roots rapidly accumulated drymatter until 278 DAP. During this period, plants that received0·99 and 1·32 WLET exhibited similar total drymatter content, and this was significantly greater than in plantssupplied with 0·33 and 0·66 WLET. Cormel dry mattercontent peaked at 29% of total plant dry matter by 322 DAP inplants replenished with 1·32 WLET. Partitioning of drymatter to cormels in other treatments was significantly reduced.Partitioning of dry matter to corms increased linearly throughoutthe growing season in all treatments. Dry matter partitioningto suckers and the number of suckers formed from plants replenishedwith 0·33 and 0·66 WLET was greater than in themore irrigated treatments.Copyright 1994, 1999 Academic Press Tanier, Xanthosoma spp., growth, dry matter partition, irrigation, evapotranspiration     

THE EFFECT OF GIBBERELLIC ACID ON LEAF AREA AND DRY-MATTER PRODUCTION IN MAJESTIC POTATO

When gibberellic acid (50 p.p.m. in aqueous solution) was sprayed twice or six times at weekly intervals on potato plants (var. Majestic) with a low or high nitrogen supply it did not affect rate of leaf production on the main axis, but caused earlier senescence of leaves, especially with the more frequent spraying, and inhibited leaf production and growth on laterals of the high-nitrogen plants at nodes 10 and 11 but not at other nodes. This central region of the stem appears to have a low growth potential, probably because it lies midway between two zones of active growth, viz. the basal branches and the younger leaves on the main stem. Competition between these is increased by gibberellic acid. Gibberellic acid increased leaf area even when lack of nitrogen was restricting growth but this did not produce extra dry matter. Tuber weight was increased more in high-nitrogen plants by two sprayings than by six sprayings. The net assimilation rate of low-nitrogen plants was halved by spraying but was not changed in high-nitrogen plants where the value was similar to that of low-nitrogen control plants. The high-nitrogen plants had absorbed nearly all the available nitrogen between the second and third harvests, but plants treated with gibberellic acid, nevertheless, had more total dry weight and tuber dry weight than the controls. The nitrogen content of the leaves expressed on an area basis was lower in sprayed plants and, with continued spraying, fell at the third harvest to equal that of low-nitrogen plants. Evidently, the effect of gibberellic acid depended on the interaction between the rate of application and the nitrogen supply, but further work is necessary to define the conditions that give the maximal effect on dry-matter production.     

Improved growth and water use efficiency of cherry saplings under reduced light intensity

Cherry (Prunus avium L.) saplings were grown under natural sunlight (controls) or moderate shading (up to 30%, depending on the incident light intensity and the hour of the day). Reduced light intensity increased the dry mass of each of the plant components studied. Consequently, the total dry mass of shaded plants was significantly greater than that of controls at the end of the growing season. However, the diurnal trend in the level of photosynthesis (per unit of leaf area) of shaded plants was similar to the controls in August, but lower in September. As the growing season proceeded, reduced photosynthetic rates, thinner mesophyll and larger specific leaf area in the shaded plants indicated that leaf development had adapted to shaded conditions throughout the growing season. It is suggested that increased growth of shaded plants was caused by a higher initial relative growth rate and a greater whole-plant photosynthesis. Shading consistently reduced transpiration over the season, therefore improving water use efficiency of shaded leaves. Our results suggest that a moderate reduction in light intensity can be a useful method for improving growth and saving water in hot and dry environments.     

<Emphasis Type="SmallCaps">P</Emphasis>lastic responses to temporal variation in moisture availability: consequences for water use efficiency and plant performance

The ability to appropriately modify physiological and morphological traits in response to temporal variation should increase fitness. We used recombinant hybrid plants generated by crossing taxa in the Piriqueta caroliniana complex to assess the effects of individual leaf traits and trait plasticities on growth in a temporally variable environment. Recombinant hybrids were used to provide a wide range of trait expression and to allow an assessment of the independent effects of individual traits across a range of genetic backgrounds. Hybrid genotypes were replicated through vegetative propagation and planted in common gardens at Archbold Biological Station in Venus, Florida, where they were monitored for growth, leaf morphological characters, and integrated water use efficiency (WUE) (C isotope ratio; δ¹³C) for two successive seasons. Under wet conditions only leaf area had significant effects on plant growth, but as conditions became drier, growth rates were greatest in plants with narrow leaves and higher trichome densities. Plants with higher WUE exhibited increased growth during the dry season but not during the wet season. WUE during the dry season was increased for plants with smaller, narrower leaves that had higher trichome densities and increased reflectance. Examination of alternative path models revealed that during the dry season leaf traits had significant effects on plant growth only through their direct effects on WUE, as estimated from δ¹³C. Over the entire growing season, plants with a greater ability to produce smaller and narrower leaves with higher trichome densities in response to reduced water availability had the greatest growth rate. These findings suggest that plants making appropriate changes to leaf morphology as conditions became dry had increased WUE, and that the ability to adjust leaf phenotypes in response to environmental variation is a mechanism by which plants increase fitness.     

Canopy development of a determinate and an indeterminate cultivar of Vicia faba L. under contrasting plant distributions and densities

Field experiments were conducted in 1987 and 1988 to quantify differences in canopy formation between an indeterminate and a determinate genotype of Vicia faba L., grown at two plant densities and three spatial distributions. The number of stems per unit area produced by determinate plants was related to the growth rate before flowering. Leaf production per stem per unit of thermal time was similar in both plant types, but twice as many leaves per stem were produced by the indeterminate cultivar. The indeterminate cultivar produced fewer and smaller leaves in the warmer and drier weather of 1988 than in 1987. The determinate genotype produced similar sizes and numbers of leaves in both years, but fewer tillers developed in 1988 than in 1987. Accordingly, leaf mass per unit ground area was greater in 1987 than in 1988 in both genotypes. Except during early flowering, relationships between leaf mass and leaf area were constant, with higher specific leaf areas in the determinate than the indeterminate genotype. Shoot dry matter partitioning into leaves was identical in both years for indeterminate plants, but differed in determinate ones.
It is concluded that canopy development is regulated through individual leaf weight and leaf number per stem in non-tillering indeterminate, and by stem numbers per unit area in tillering determinate plants.     

Differences in the structure, growth and survival of Parasenecio yatabei ramets with contrasting water relations on the slope of a stream bank

Parasenecio yatabei (Asteraceae), a summer-green perennial herb, is widely distributed on sloping mountain stream banks in cool-temperate zone forests of Japan. We investigated the growth pattern, leaf longevity and leaf water relations of vegetatively independent plants (ramets) growing in two contrasting soil water conditions, that is, upper and lower stream banks (U ramets and L ramets, respectively). The objective of the present study was to clarify the physiological and morphological responses of the ramets to soil water conditions. Dry matter allocation to subterranean parts was higher in U ramets than in L ramets. The U ramet leaves survived for approximately 2 months longer than L ramet leaves. The ratio of subterranean part to aerial part dry matter was greater in U ramets than L ramets. Leaf mass per leaf area (LMA) tended to be greater in U ramets than L ramets throughout the growing season. The leaf bulk modulus of elasticity at full hydration was significantly higher in U ramets. Thus, ramet growth patterns and morphological traits varied with changing soil water conditions. The greater longevity of U ramet leaves may play a role in compensating for the reduced annual net carbon gain caused by lower photosynthetic activity. U ramets growing in environments with less water availability achieved high water-use efficiency by a high passive water absorption capacity via a progressed root system and high productivity via longer leaf longevity with higher LMA and elasticity. Therefore, P. yatabei growing along mountain streams could have the ability to colonize the upper bank through higher survivorship based on these traits.     

Effects of growth regulators, CCC and B9, on some potato varieties

The effects of CCC and B 9 on the growth habit of potato differed between varieties. CCC diminished stem lengths and dry weight more than Bo because CCC was applied early when shoots emerged from the soil, but B 9 was applied about 3 weeks later when several leaves had formed. In some varieties lateral stem growth was increased by treatment and in others decreased. There was an inverse relation between main stem and lateral stem growth so that varieties with vigorous main stem growth had poor lateral growth and vice-versa. Treatment with the growth-regulators diminished leaf dry weight of main stem leaves less than leaf area, but the degree of magnitudes of the changes depended on the variety. Both regulators lessened net assimilation rate. Net assimilation rate and dry matter per unit area of leaf were inversely related, possibly because accumulation of substances in leaves decreases photosynthesis. Stolon dry weight was positively correlated with main stem leaf area. There was a direct relation between stem length and tuber dry weight, suggesting that tuber initiation occurs at different stem lengths in different varieties. Tubering was earliest in Epicure and latest in King Edward. Epicure had the greatest tuber weight and smallest stem length.     

Recovery of leaf area through accelerated shoot ontogeny in thrips-damaged cotton seedlings

BACKGROUND AND AIMS: Leaf area of cotton seedlings (Gossypium hirsutum) can be reduced by as much as 50 % by early season thrips infestations, but it is well documented that plants can regain the difference in leaf area once infestation ceases. The processes involved in the recovery have not been identified. Hypotheses include enhancement of the photosynthetic rate of the damaged leaves, more efficient leaf construction (i.e. more leaf area per unit of dry matter invested in new leaves), and more branching. METHODS: This 2-year field study examined these hypotheses and found that thrips-affected plants recovered from a 30 % reduction in total leaf area. During the recovery period, repeated measurements of gas exchange, leaf morphology and individual leaf areas at all nodes were made to assess their contribution to the recovery. KEY RESULTS: Recovery was not achieved through the previously proposed mechanisms. The pattern of nodal development indicated that the duration of leaf expansion of the smaller deformed leaves was shorter than that of control leaves, possibly because they had fewer cells. The production and expansion of healthy upper node leaves in thrips-affected plants could, therefore, begin sooner, about 1-2.5 nodes in advance of control plants. The proposed process of recovery was evident but weaker in the second year where thrips numbers were higher. CONCLUSIONS: It is concluded that thrips-affected plants overcame the leaf area disparity through an accelerated ontogeny of main stem leaves. By completing the expansion of smaller but normally functioning lower node leaves earlier, resources were made available to the unfolding of larger upper node leaves in advance of control plants. The generality of this mode of plant resistance in pest damage remains to be determined.     

Population distributions of plant size and light environment of giant ragweed (Ambrosia trifida L.) at three densities

Summary Plots in a naturally occurring population of giant ragweed (Ambrosia trifida L.) near Ames, Iowa, USA were left unthinned (high density,=693 plants/m²) or were thinned in early June 1989 to create low and medium densities of 10 and 50 plants/m². Size and light environment of individual plants were measured at monthly intervals from June to September. By September, low density plants had 15 times greater biomass/plant and 30 times greater leaf area/plant than high density plants, although biomass and leaf area per unit land area decreased with decreasing density. Plants at high density allocated more biomass to stem growth, but plants at medium and low density had successively higher leaf area ratios, higher potential photosynthetic rates, higher allocation to leaves, and higher growth rates. Average light on leaves decreased with increasing density and also decreased over the growing season in the low and medium densities. The distribution of light environments of individual plants was non-normal and skewed to the left in most months, in contrast to the rightwards skew of distributions of plant size parameters. Inequality in the distributions, as measured by coefficient of variation and Gini coefficients, increased over most of the growing season. There was little effect of density on inequality of stem diameter, height, or estimated dry weight, but inequality in reproductive output greatly increased with density. There was greater inequality in number of staminate flowers produced than in number of pistillate flowers and seeds produced. Path analysis indicated that early plant size was the most important predictor of final plant size and reproductive output; photosynthesis, conductance, and light environment were also significantly correlated with size and reproduction but usually were of minor importance. Variation in growth rate apparently increased inequality in plant size at low density, whereas belowground competition and death of smaller plants may have limited increases in inequality at high density.     

Architecture and growth of an annual plant <Emphasis Type="Italic">Chenopodium album</Emphasis> in different light climates

Light climates strongly influence plant architecture and mass allocation. Using the metamer concept, we quantitatively described branching architecture and growth of Chenopodium album plants grown solitarily or in a dense stand. Metamer is a unit of plant construction that is composed of an internode and the upper node with a leaf and a subtended axillary bud. The number of metamers on the main-axis stem increased with plant growth, but did not differ between solitary and dense-stand plants. Solitary plants had shorter thicker internodes with branches larger in size and number than the plant in the dense stand. Leaf area on the main stem was not different. Larger leaf area in solitary plants was due to a larger number of leaves on branches. Leaf mass per area (LMA) was higher in solitary plants. It did not significantly differ between the main axis and branches in solitary plants, whereas in the dense stand it was smaller on branches. Dry mass was allocated most to leaves in solitary plants and to stems in the dense stand in vegetative growth. Reproductive allocation was not significantly different. Branch/main stem mass ratio was higher in solitary than dense-stand plants, and leaf/stem mass ratio higher in branches than in the main axis. Nitrogen use efficiency (NUE) (dry mass growth per unit N uptake) was higher and light use efficiency (LUE) (dry mass growth per unit light interception) was lower in the plant grown solitarily than in the dense stand.     

沙地云杉苗期生长与干物质生产关系的研究

本文用不同_的模型定量地研究了沙地云杉苗期的生长规律、季节动态及不同生长时期干物质在各器官之间的分配规律。1)1年生幼苗一直保持较高的生长速度,根生长尤为迅速。5年生幼苗在接近生长上限时,增长越来越慢。2)根、茎、叶干物质的生产符合理查德模型;根、茎、叶干重与全株干重之百分比表现出不同的变化趋势,反映了于物质在各器官中的分配规律。3)不同年度各器官的干重变化反映了由于自疏造成的叶的脱落和部分枝的脱落情况。4)各模型的相关系数几乎都达到了极显著的水平。     

Effects of simulated winter browsing on mountain birch foliar chemistry and on the performance of insect herbivores

Winter browsing by mammalian herbivores is known to induce a variety of morphological and physiological changes in plants. Browsing has been suggested to decrease the carbohydrate reserves in woody plants, which might lead to reduced tannin production in leaves during the following summer, and consequently, to increased herbivore damage on leaves. We conducted a clipping experiment with mature mountain birch trees and measured the effects of clipping on birch growth, leaf chemistry and toughness, as well as on the performance of insect herbivores. Leaves grew larger and heavier per unit area in the clipped ramets and had a higher content of proteins than leaves in the control trees. Clipping treatment did not affect the total content of sugars in the leaves (mg g^?1), suggesting that a moderate level of clipping did not significantly reduce the carbohydrate pools of fully‐grown mountain birch trees. Furthermore, the contents of proanthocyanidins (condensed tannins) and gallotannins were slightly higher in the leaves of clipped ramets, contrary to the hypothesis of reduced tannin production. The effects of clipping treatment on leaf and shoot growth and on foliar chemistry were mainly restricted to the clipped ramets, without spreading to untreated ramets within the same tree individual. The effects of clipping on leaf characters varied during the growing season; for instance, leaf toughness in clipped ramets was higher than toughness in control trees and ramets only when leaves were mature. Accordingly, clipping had inconsistent effects on insect herbivores feeding at different times of the growing season. The generally small impact of clipping on herbivore performance suggests that the low intensity of natural browsing at the study area, simulated by our clipping treatment, does not have strong consequences for the population dynamics of insect herbivores on mountain birch via enhanced population growth caused by browsing‐induced changes in food quality.     

Dry season watering alters the significance of climate factors influencing phenology and growth of saplings of savanna woody species in central Zambia,southern Africa

Although tree growth in southern African savannas is correlated with rainfall in the wet season, some studies have shown that tree growth is controlled more by rainfall in the dry season. If more rainfall occurred in the dry season in future climates, it would affect the growth of savanna trees, especially saplings that have shallower roots which limit access to subsoil water during the dry season when leaf flush and shoot extension occur. Recent paleobotanical evidence has revealed that there was relatively more precipitation in the dry season in eastern Africa in the Eocene than under the current climate. Saplings therefore can be expected to respond more to water addition during the dry season than mature trees that have more stored water and deeper roots that access subsoil water. Accordingly, I hypothesized that irrigation in the dry season should (i) advance the onset of the growing season, (ii) increase growth rates and (iii) alter the growth responses of saplings to climate factors. To test these hypotheses saplings of five savanna woody species were irrigated during the hot‐dry season at a site in central Zambia and their monthly and annual growth rates compared to those of conspecifics growing under control conditions. Although the responses among the species were variable, all irrigated saplings had significantly higher monthly and annual growth rates than control plants. In addition, dry season watering significantly altered the climatic determinants of sapling growth by either strengthening the role of the same climatic factors that were important under control conditions or displacing them altogether. In conclusion, more precipitation during the hot‐dry season is likely to have significant positive effects on sapling growth and consequently reduce the sapling‐tree transition periods and promote future tree population recruitment in some southern African savanna tree species.