Transmission of Podisus maculiventris tremulatory signals through plants

Males of the predaceous stink bug Podisus maculiventris (Say) (Heteroptera: Pentatomidae: Asopinae) emit low frequency tremulatory signals. Laser vibrometry was used to record and analyze naturally emitted signals, focusing on variation in signal velocity and frequency during transmission through plants (Phaseolus vulgaris L. and Plumbago auriculata Lam.) as a function of distance from the vibrational source. Signal velocity varied individually between 2 and 15 mm/s recorded on a plant close to the calling male and decreased by 0.3 to 1.5 dB/cm on bean and 0.3 to 0.9 dB/cm on plumbago. The dominant frequency of signals was variable at frequencies below 50 Hz. On bean frequencies centered around 10 Hz or 20 Hz were dominant for signals recorded at the source. Transmission through bean resulted in an increase in the 20 Hz peak relative to other frequencies in the signal. Variation of the dominant frequencies of signals transmitted through plumbago stems were more predictable, showing typical changes in amplitude relative to the distance from the source. The regular variation of the dominant frequency along the stem with linear increase of signal velocity at decreasing distance from the source may provide plant-dwelling insects with information about the distance to the calling individual.     

The Influence of Substrate on Male Responsiveness to the Female Calling Song in Nezara viridula

The female calling song (FCS) of the southern green stink bug Nezara viridula is composed of vibrational pulse trains that include either short pulses and a long pulse (FCS-1) or just short pulses (FCS-2). Their function in communication was studied by investigating male vibratory responses to natural and artificial signals on artificial and natural substrates. On a loudspeaker membrane, FCS-1 triggered in males from a Slovene and a French population significantly more courtship songs (MCrS) than FCS-2. Experiments with artificial signals showed that male responses are modulated by the duration of pulse trains and pulse repetition time. On a bean plant, males of both populations responded in the same way to the two types of female calling song pulse trains. Moreover, a laser vibrometer study of the transmission of different natural and artificial vibratory signals through the bean plant showed that the pulses of a high repetition rate are prolonged and fused at distances from the emitter. We conclude that female calling song pulse trains of different temporal structure have the same function in vibrational communication of the species. The temporal and spectral structures of the female calling songs of N. viridula are discussed in terms of effective transmission through plants.     

Vibrational directionality in the southern green stink bug, Nezara viridula (L.), is mediated by female song

We tested the hypothesis that male southern green stink bugs, Nezara viridula (L.), use substrate-borne songs to locate females. We recorded the responses of bugs on plants to the vibrations caused by a prerecorded female song and by an artificial sound. The female song caused males to walk, to respond with the calling and courtship songs and to approach the source of the song with characteristic search behaviour at junctions between branches on the plants. At a junction, a searching male stopped, stretched his legs and antennae and compared the vibratory signals on the two branches, with different combinations of legs and antennae. The males then left the junction and approached the source of the vibration. Males located the loudspeaker significantly more frequently in the presence than in the absence of vibratory stimuli on cyperus, Cyperus alternifolius L., and beans, Phaseolus vulgaris L. Vibrational directionality was also elicited by artificial pure tones whose spectral and temporal parameters were similar to those of natural female song. Females showed no reaction to vibratory stimulation and no vibrational directionality. We discuss possible mechanisms underlying vibrational directionality in the light of expected signal changes during transmission through plants. Copyright 1999 The Association for the Study of Animal Behaviour.     

Variation in Plant Substrates and its Consequences for Insect Vibrational Communication

Many insects and other arthropods communicate using plant‐borne vibrational signals. Vibration transmission along plant stems imposes a frequency filter on signals, and may cause signal degradation from reflected waves. Furthermore, different plant species and plant parts can differ in their transmission properties. This variability in the communication channel may constrain the reliability of signals, with important consequences for the evolution of vibrational communication systems, as well as for researchers studying signal variation at an individual, population, or species level. In this study we estimate the magnitude of substrate‐related variation in the mate advertisement signals of a treehopper (Hemiptera: Membracidae: Umbonia crassicornis). We used laser vibrometry to record the signals produced by 25 adult males on two different plant species, one host and one non‐host. We recorded male signals on two plants per species; within each plant, signals were recorded simultaneously at two distances. We measured three spectral characteristics (dominant frequency, relative amplitude of the signals’ high and low frequency components, frequency at the end of the signal) and two temporal characteristics (signal duration and click repetition rate). Spectral characteristics were influenced by the distance at which the signal was recorded, and this influence varied among plant species and individuals. Temporal characteristics were less influenced, although signal length was influenced by distance, an effect that varied among individual plants. Overall, the magnitude of the effects was small. Furthermore, there was significant within‐individual repeatability of almost all signal traits across different plant substrates. Signal characteristics were thus reliably associated with individuals, even when they signaled on different plants.     

The acoustic behaviour of the bushcricket Tettigonia cantans II. Transmission of airborne-sound and vibration signals in the biotope

The airborne-sound and the vibratory signals produced by stridulating $\text{Tettigonia cantans}$ males, and the transmission of these signals in the natural biotope were investigated.The song of $\text{T. cantans}$ is composed of repeated uniform syllables with a rate of ca. 30/sec. Intensity approaches 100 dB SPL, 10 cm away from the animal. The spectrum shows three dominant frequency ranges around 8, 16 and 32 kHz.Airborne transmission of the song in such vegetation layers as are found in the biotopes of $\text{T. cantans}$ shows an excess attenuation which increases with frequency. The relative intensities of the frequency components of the song vary as a result of the kind of vegetation, the positions of emitter and receiver, and the separation distance. These relative differences in intensity may be useful during the phonotactic approach to conspecific partners, providing a measure of the distance from the sound source.Stridulating males also produce vibratory signals in the plants they sit on. The spectrum of these signals includes frequencies up to 8 kHz, the first dominant frequency of the song: low frequency components are induced in the plants via the legs and abdomen of the animal. The vibratory signals are transmitted mainly in the form of bending waves. Near the animal, amplitude modulation corresponds to that of the song. At greater distances, reflections and frequency-dependent propagation velocities, cause distortions of this time pattern. Transmission depends greatly on the mechanical properties of the particular plant, attenuation values of 20–50 dB/m being found. Nevertheless, in most cases, vibratory signals may be perceived up to 1.5 – 2 metres away from a stridulating male.     

Temporal processing of vibratory communication signals at the level of ascending interneurons in Nezara viridula (Hemiptera: Pentatomidae)

During mating, males and females of N. viridula (Heteroptera: Pentatomidae) produce sex- and species-specific calling and courtship substrate-borne vibratory signals, grouped into songs. Recognition and localization of these signals are fundamental for successful mating. The recognition is mainly based on the temporal pattern, i.e. the amplitude modulation, while the frequency spectrum of the signals usually only plays a minor role. We examined the temporal selectivity for vibratory signals in four types of ascending vibratory interneurons in N. viridula. Using intracellular recording and labelling technique, we analyzed the neurons' responses to 30 pulse duration/interval duration (PD/ID) combinations. Two response arrays were created for each neuron type, showing the intensity of the responses either as time-averaged spike counts or as peak instantaneous spike rates. The mean spike rate response arrays showed preference of the neurons for short PDs (below 600 ms) and no selectivity towards interval duration; while the peak spike rate response arrays exhibited either short PD/long ID selectivity or no selectivity at all. The long PD/short ID combinations elicited the weakest responses in all neurons tested. No response arrays showed the receiver preference for either constant period or duty cycle. The vibratory song pattern selectivity matched the PD of N. viridula male vibratory signals, thus pointing to temporal filtering for the conspecific vibratory signals already at level of the ascending interneurons. In some neurons the responses elicited by the vibratory stimuli were followed by distinct, regular oscillations of the membrane potential. The distance between the oscillation peaks matched the temporal structure of the male calling song, indicating a possible resonance based mechanism for signal recognition.     

Palomena prasina (Hemiptera: Pentatomidae) vibratory signals and their tuning with plant substrates

Palomena prasina is interesting for the study of vibrational communication within the Pentatomid subfamily Pentatominae, because its host range is limited to woody plants, unlike the better known Nezara viridula, whose vibrational communication is commonly used as a model for the whole family. The vibrational repertoire of P. prasina was described several decades ago and is redescribed in this paper using modern methods for non-contact vibration recording. Additionally, we hypothesized that this species has retained the capacity for signal frequency variation necessary for tuning to resonance properties of various host plants of Pentatominae, but if the signals are emited in the absence of mechanical feedback, they are tuned more specifically to their native acoustic environment — woody plants. By recording live bugs signalling on different substrates and comparing spectral properties of their signals among substrates, we found that there is a match between the signals emitted on a woody branch and those emitted on a non-resonant surface, while spectral properties of signals emitted on herbaceous plants differ. Our findings provide evidence in support of the signal tuning hypothesis and shed further light on the crucial role of substrate in vibrational communication of insects.     

Sand as a medium for transmission of vibratory signals of prey in antlions Euroleon nostras (Neuroptera: Myrmeleontidae)

Abstract.  European pit-building antlions ( Euroleon nostras / Geoffroy in Fourcroy/) detect their prey by sensing the vibrations that prey generate during locomotory activity. The behavioural reactions and some of the physical properties of substrate vibrations in sand are measured to observe signal transmission through the substrate. The frequency range of the signals of four arthropod species ( Tenebrio molitor , Pyrrhocoris apterus , Formica sp. and Trachelipus rathkei ) is 0.1–4.5 kHz and acceleration values are in the range 400 μm s⁻² to 1.5 mm s⁻². Substrate particle size and the frequency of prey signals both influence the propagation properties of vibratory signals. The damping coefficient at a frequency 300 Hz varies from 0.26 to 2.61 dB cm⁻¹ and is inversely proportional to the size of the sand particle. The damping coefficient is positively correlated with the frequency of the pulses. Vibrations in finer sand are attenuated more strongly than in coarser sand and, consequently, an antlion detects its prey only at a short distance. The reaction distance is defined as the distance of the prey from the centre of the pit when the antlion begins tossing sand as a reaction to the presence of prey. The mean reaction distance is 3.3 cm in the finest sand (particle size ≤ 0.23 mm) and 12.3 cm in coarser sand (particle size 1–1.54 mm). The most convenient sands for prey detection are considered to be medium particle-sized sands.     

Vascular tissue in the stem and roots of woody plants can conduct light

The role of vascular tissue in conducting light was analysed in 21 species of woody plants. Vessels, fibres (both xylem and phloem fibres) and tracheids in woody plants are shown to conduct light efficiently along the axial direction of both stems and roots, via their lumina (vessels) or cell walls (fibres and tracheids). Other components, such as sieve tubes and parenchyma cells, are not efficient axial light conductors. Investigation of the spectral properties of the conducted light indicated that far-red light was conducted most efficiently by vascular tissue. Light gradients in the axial direction were also investigated and revealed that conducted light leaked out of the light-conducting structures to the surrounding living tissues. These properties of the conducted light suggest a close relationship with metabolic activities mediated by phytochromes. The results therefore indicate not only that signals from the external light environment can enter the interior of stems above ground and are conducted by vascular tissue towards roots under ground, but also that the light conducted probably contributes directly to photomorphogenic activities within them.     

Transmission of stridulatory signals of the burrower bugs, Scaptocoris castanea and Scaptocoris carvalhoi (Heteroptera: Cydnidae) through the soil and soybean

Abstract.  Males and females of the burrower bug species Scaptocoris castanea Perty and Scaptocoris carvalhoi Becker emit stridulatory signals when on the roots of soybean. The substrate-borne components of the signal can be recorded on the plant but not on the surrounding soil surface. The stridulatory apparatus is composed of the tergal plectrum (lima) and the stridulitrum (stridulatory vein) on the underside of the hind wings. The male plectrum has one ridge and the female lima has 13 ridges. Stridulitra of different species differ in the length and in the number of teeth. Rubbing of plectrum (lima) ridges over the stridulitrum in one or both directions produces pulse trains. The velocity of signals that are recorded less than 0.5 cm from the bug is below 0.013 mm s⁻¹ on the soil and below 0.066 mm s⁻¹ on the leaf surface. Broadband spectra have a dominant frequency of less than 1 kHz and subdominant peaks extending up to 7 kHz. The dominant frequency of the stridulatory signal transmitted through a plant decreases together with the proportion of its higher frequency spectral components. Signals are attenuated for 3–9 dB cm⁻¹ when transmitted through the soil or soybean leaf and for approximately 1 dB cm⁻¹ when transmitted through soybean stem.     

Evidence for the transmission of information through electric potentials in injured avocado trees

Electrical excitability and signaling, frequently associated with rapid responses to environmental stimuli, have been documented in both animals and higher plants. The presence of electrical potentials (EPs), such as action potentials (APs) and variation potentials (VPs), in plant cells suggests that plants make use of ion channels to transmit information over long distances. The reason why plants have developed pathways for electrical signal transmission is most probably the necessity to respond rapidly, for example, to environmental stress factors.We examined the nature and specific characteristics of the electrical response to wounding in the woody plant Persea americana (avocado). Under field conditions, wounds can be the result of insect activity, strong winds or handling injury during fruit harvest. Evidence for extracellular EP signaling in avocado trees after mechanical injury was expressed in the form of variation potentials. For tipping and pruning, signal velocities of 8.7 and 20.9 cm/s, respectively, were calculated, based on data measured with Ag/AgCl microelectrodes inserted at different positions of the trunk. EP signal intensity decreased with increasing distance between the tipping and pruning point and the electrode. Recovery time to pre-tipping or pre-pruning EP values was also affected by the distance and signal intensity from the tipping or pruning point to the specific electrode position. Real time detection of remote EP signaling can provide an efficient tool for the early detection of insect attacks, strong wind damage or handling injury during fruit harvest.Our results indicate that electrical signaling in avocado, resulting from microenvironment modifications, can be quantitatively related to the intensity and duration of the stimuli, as well as to the distance between the stimuli site and the location of EP detection. These results may be indicative of the existence of a specific kind of proto-nervous system in plants.     

Prey detection in antlions: propagation of vibrational signals deep into the sand

Trap‐building antlion larvae detect their prey according to the substrate vibrations produced during movement of the prey on the sand surface. Although most studies are devoted to surface vibrational waves, in the present study, we determine the role of vibrations travelling through deeper sand layers. A behavioural experiment confirms that vibrational stimuli from prey insects on the surface of the sand stimulate the antlions buried in deeper sand layers to move towards the surface. Sand depth and particle size both have a strong effect on signal transmission. The damping coefficient (α₁₀) varies from 0.49 dB to 3.30 dB cm^?1 and depends on frequency (in the range from 100 to 300 Hz), particle size (from finest to coarse sand) and distance from the source of the vibrations. The deeper the sand, the narrower the frequency range of the signal becomes. Sand is a filter for higher frequencies. The smaller the sand particles, the more intense the filtering becomes. Fine sand with a mean sand particle size of 360 μm is a more efficient filter than coarse sand; consequently, high frequencies (> 2.5 kHz) are eliminated at a depth of 3 cm. Mean frequency depends on both depth and particle size. However, low frequency signals still propagate at a certain distance, which is biologically important in prey detection. Although the most efficient signal propagation appears to occur in coarse sand, it contains overly large particles that are inconvenient for relatively small antlion larvae. Predators seek a compromise between fine and coarse sand choosing medium sand.     

海南岛霸王岭热带季雨林树木的死亡率

树木死亡是调节群落组成和结构的一种方式, 在森林生态系统动态中发挥着重要作用。该文在对海南岛霸王岭热带季雨林群落大量调查和树种功能群划分研究的基础上, 探索了热带季雨林群落及其不同功能群树木的死亡率及其随径级和环境条件的变化规律。结果表明: 海南岛霸王岭热带季雨林群落内树木死亡率的变化范围为3.42%-18.71%, 平均值为7.60%。按照功能群比较, 乔木死亡率显著高于灌木, 落叶树种死亡率高于常绿树种, 但具刺树种死亡率显著低于无刺树种。按照树木径级划分, 5-30 cm径级范围的死亡率均超过10%, 最高值出现在5-10 cm径级范围, 在相同径级范围内比较时, 落叶树种在 胸径(DBH ) < 5 cm时的死亡率显著高于常绿树种, 而在其他径级范围内, 二者之间无显著差异, 具刺和无刺树种的死亡率在所有径级范围内均无显著差异。乔木死亡率与群落距河流距离具有显著的相关性, 距离河流越远死亡率越高; 落叶树种和无刺树种则与坡位存在显著的相关性, 坡上部的死亡率显著高于坡中部。热带季雨林内树木的死亡率与其所处的群落生境有关, 较干旱的生境是导致树木死亡率较高的重要原因。     

The regulation of cell wall extensibility during shade avoidance: a study using two contrasting ecotypes of Stellaria longipes

Shade avoidance in plants involves rapid shoot elongation to grow toward the light. Cell wall-modifying mechanisms are vital regulatory points for control of these elongation responses. Two protein families involved in cell wall modification are expansins and xyloglucan endotransglucosylase/hydrolases. We used an alpine and a prairie ecotype of Stellaria longipes differing in their response to shade to study the regulation of cell wall extensibility in response to low red to far-red ratio (R/FR), an early neighbor detection signal, and dense canopy shade (green shade: low R/FR, blue, and total light intensity). Alpine plants were nonresponsive to low R/FR, while prairie plants elongated rapidly. These responses reflect adaptation to the dense vegetation of the prairie habitat, unlike the alpine plants, which almost never encounter shade. Under green shade, both ecotypes rapidly elongate, showing that alpine plants can react only to a deep shade treatment. Xyloglucan endotransglucosylase/hydrolase activity was strongly regulated by green shade and low blue light conditions but not by low R/FR. Expansin activity, expressed as acid-induced extension, correlated with growth responses to all light changes. Expansin genes cloned from the internodes of the two ecotypes showed differential regulation in response to the light manipulations. This regulation was ecotype and light signal specific and correlated with the growth responses. Our results imply that elongation responses to shade require the regulation of cell wall extensibility via the control of expansin gene expression. Ecotypic differences demonstrate how responses to environmental stimuli are differently regulated to survive a particular habitat.     

Stem biomechanics of the giant moss Dendroligotrichum dendroides s.l. and its significance for growth form diversity in mosses

Abstract

The giant moss Dendroligotrichum dendroides s.l. grows as self-supporting plants up to 40 cm in height in forest habitats in Chile and New Zealand. This moss represents one of the tallest self-supporting bryophytes. Biomechanical tests indicate that the stems can develop a high degree of stiffness (Young’s modulus) via a dense hypodermal sterome that is comparable with that of woody stems of vascular plants. A comparison with mechanical properties of other terrestrial and aquatic mosses indicates that different moss growth and life forms can produce very different mechanical architectures. Values of stem stiffness can vary between different growth forms of mosses to a comparable extent to that observed among diverse growth forms of vascular plants. Plants varying profoundly in overall size, development, and phylogenetic position nevertheless appear to develop comparable mechanical adaptations and growth forms in response to certain environmental conditions.     

Range of corixid sound signals in the biotope

ABSTRACT. The propagation of sound in the frequency band (2–12 kHz) used by the Corixidae was measured in two shallow natural ponds. At distances of more than 1 m from the shore the water was at least O.4 m deep. The first pond was eutrophic and contained no plants. The spread of sound into the open water obeyed approximately the geometric attenuation of the sound pressure level (SPL); a loss of 6dB for each doubling of the distance from the point sound source. Near the shore the attenuation was considerably greater, especially for low frequencies; for a 2 kHz signal the damping of the SPL was c. 40–50 dB/m.
The second pond had dense plant growth, and the sound attenuation depended strongly on the photosynthetic activity of the waterplants. Measurements in winter, with an overcast sky, revealed only a slight damping effect of the plants for a 10kHz test signal. During intense sunlight in summer, however, in addition to the geometric attenuation the damping effect of the plants over a distance of O.5 m was 50 dB for a 2 kHz signal and 80 dB for 10 kHz. This effect was due to gas bubbles produced during intense photosynthesis.
Song A of Corixa dentipes Thms. (Heteroptera) males elicits usually a response by male conspecifics. The threshold SPL for this response was measured to be c. 40 dB lower than the SPL at a distance of O.1 m from a stridulating animal. From the measurements of sound propagation it follows, therefore, that the effective range of Song A in the most favourable case is at least 10m, though in a pond overgrown with plants it can be less than O.4 m.     

Internal axial light conduction in the stems and roots of herbaceous plants

In order to reveal any roles played by stems and roots of herbaceous plants in responding to the surrounding light environment, the optical properties of the stem and root tissues of 18 herbaceous species were investigated. It was found that light was able to penetrate through to the interior of the stem and was then conducted towards the roots. Light conduction was carried out within the internodes and across the nodes of the stem, and then in the roots from the tap root to lateral roots. Light conduction in both the stem and root occurred in the vascular tissue, usually with fibres and vessels serving as the most efficient axial light conductors. The pith and cortex in many cases were also involved in axial light conduction. Investigation of the spectral properties of the conducted light made it clear that only the spectral region between 710 nm and 940 nm (i.e. far-red and near infra-red light) was the most efficiently conducted in both the stem and the root. It was also found that there were light gradients in the axial direction of the stem or root, and the light intensity generally exhibited a linear attenuation in accord with the distance of conduction. These results revealed that tissues of the stem and root are bathed in an internal light environment enriched in far-red light, which may be involved in phytochrome-mediated metabolic activities. Thus, it appears that light signals from above-ground directly contribute to the regulation of the growth and development of underground roots via an internal light-conducting system from the stem to the roots.     

The piercing-sucking herbivores Lygus hesperus and Nezara viridula induce volatile emissions in plants

Plant volatiles induced by herbivory are often used as olfactory cues by foraging herbivores and their natural enemies, and thus have potential for control of agricultural pests. Compared to chewing insects and mites, little is known about plant volatile production following herbivory by insects with piercing-sucking mouthparts. Here, we studied factors (insect life stage, gender, the role of salivary glands, and type of bioassay used for volatile induction) that influence the induction of plant volatiles by two agriculturally important hemipterans, Lygus hesperus and Nezara viridula. Feeding on intact cotton by virgin females of L. hesperus induced 2.6-fold greater volatile response compared to that induced by mated females, possibly due to increased feeding activity by virgin females. This plant volatile response was associated with elicitors present in the insect's salivary glands as well as to the degree of mechanical injury. Feeding injury by N. viridula females also increased volatile emissions in intact maize by approximately 2-fold compared to control plants. Maize seedlings injured by N. viridula emitted higher amounts of the monoterpene linalool, the sesquiterpenes (E)-beta-caryophyllene, alpha-trans-bergamotene, and (E,E)-beta-farnesene, and the homoterpene (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene, but not amounts of green leaf volatiles, compared to uninjured plants. Emissions from intact maize injured by adult males were lower than those emitted by adult females of the same age and did not differ from those emitted by uninjured plants. Similarly, feeding by virgin female N. viridula followed by excision led to 64% higher quantities of volatiles compared to untreated plants. Volatile emission in excised plants, however, was considerably greater than in intact plants, suggesting that careful consideration must be given to bioassay design in studies of herbivore-induced plant volatiles. Salivary gland extracts of N. viridula led to sesquiterpene emissions approximately 2.5-fold higher than for controls, although no significant differences were observed for green leaf volatiles, monoterpenes, and homoterpenes. These results indicate that L. hesperus and female N. viridula feeding induce volatile production in plants, and that volatile production is affected by gender and life stage of the bug. Although oviposition and mechanical injury by stylets may increase release of volatiles, elicitors from salivary glands of L. hesperus and N. viridula also seem to play a role in the emission of plant volatiles.     

Feeding preference and movement of Nezara viridula and Euschistus serous (Hemiptera: Pentatomidae) on individual cotton plants

Experiments were conducted in an environmental growth chamber to determine the movement and feeding preferences of Nezara viridula (L.) and Euschistus serous (Say) on individual cotton plants. Fifth instars were caged by species on a single cotton plant (FM 9063 B2F) containing four discrete boll sizes ranging from 1.1 to 3.0 cm in diameter over a period of 5 d per replication. Two digital video cameras were simultaneously focused on each of the four bolls per plant to visually confirm stink bug resting and movement. During the study, a total of 4,080 h of video footage was recorded and analyzed. Results showed that N. viridula and E. serous did not prefer the exact same boll sizes. In a trial with eight stink bugs per plant, N. viridula spent more time on the three larger boll classes, 1.6-2.0, 2.1-2.5, and 2.6-3.0 cm. In a separate trial with one stink bug per plant, N. viridula spent more time on the larger boll classes while E. serous exhibited the strongest preference for 1.1-1.5 and 2.1-2.5 cm bolls. N. viridula moved more often than E. serous and both species moved more often during photophase compared with scotophase. Regardless of species or number of bugs released, bolls in the smallest boll size class fell off the plant about 3 d after the bugs were released. These results confirm that scouts who are estimating stink bug damage should select bolls in the 2.1-2.5 cm diameter boll size class.     

Response of winter chemical defense in Alaska paper birch and green alder to manipulation of plant carbon/nutrient balance

Summary Plant carbon/nutrient balance has been implicated as an important factor in plant defensive chemistry and palatability to herbivores. We tested this hypothesis by fertilizing juvenile growth form Alaska paper birch and green alder with N, P and N-plus-P in a balanced 2x2 factorial experiment. Additionally, we shaded unfertilized plants of both species. Fertilization with N and N-plus-P increased growth of Alaska paper birch, reduced the concentration of papyriferic acid in internodes and increased the palatability of birch twigs to snowshoe hares. Shading decreased birch growth, decreased the concentration of papyriferic acid in internodes and increased twig palatability. These results indicate that the defensive chemistry and palatability of winter-dormant juvenile Alaska paper birch are sensitive to soil fertility and shade. Conversely the defensive chemistry and palatability of green alder twigs to snowshoe hares were not significantly affected by soil fertility or shade. The greater sensitivity of Alaska paper birch defensive chemistry and palatability to snowshoe hares in comparison to green alder is in agreement with the hypothesis that early successional woody plants that are adapted to high resource availability are more plastic in their chemical responses to the physical environment than are species from less favorable environments.