Lower Canopy Levels Research Articles

Exploring the influence of vegetated mid-channel bar on flow and turbulence in bifurcated channels: An experimental approach

The evolution of fluvial systems is greatly impacted by mid-channel bars, a typical morphodynamic process in natural rivers. Sometimes, the growth of vegetation over these bars complicates the morphological behaviour by interacting with the flow. It is therefore necessary to have a fundamental interpretation of the flow-turbulence structure around the mid-bar in presence of vegetation cover in order to understand braiding dynamics, still studies in this area are scarce. The present study investigates the process-form-vegetation-interaction through experimental investigation at a flume scale mid-channel bar model with different natural vegetation cover arrangements (paddy, leafy, and rigid stem). The flow-turbulence behaviour has been observed through the bifurcated channel using the three-dimensional Acoustic Doppler Velocimeter (ADV). Results showed that the longitudinal velocity component varies with the different vegetation cover, and it was highest with leafy vegetation (about 32%). Similarly, the Reynolds Stress and Turbulence Intensity were also observed to be higher in case of leafy vegetation. A unique pattern of flow-turbulence parameters was observed near the bar level, the lower canopy level, and the upper canopy level. Moreover, it was found that vegetation structure and its flexible nature influence both longitudinal velocity reduction and momentum transfer at and over the canopy, as well as the thickness of the shear layer region.

Light blocking film in a glasshouse impacts Capsicum annuum L. yield differentially across planting season.

High energy costs are a barrier to producing high-quality produce at protected cropping facilities. A potential solution to mitigate high energy costs is film technology, which blocks heat-producing radiation; however, the alteration of the light environment by these films may impact crop yield and quality. Previous studies have assessed the impact of ULR 80 [i.e., light-blocking film (LBF)] on crop yield and photosynthetically active radiation (PAR); however, an assessment of the spectral environment over different seasons is important to understand potential crop impacts through different developmental phases. In this study, two varieties (red and orange) of Capsicum annuum were grown across two crop cycles: one cycle with primary crop growth in the autumn (i.e., autumn experiment [AE]) and the other with primary crop growth in the summer (i.e., summer experiment [SE]). LBF reduced PAR (roof level: 26%-30%, plant canopy level: 8%-25%) and net radiation (36%-66%). LBF also reduced total diffuse PAR (AE: 8%, SE: 15%), but the diffuse fraction of PAR increased by 7% and 9% for AE and SE, respectively, potentially resulting in differential light penetration throughout the canopy across treatments. LBF reduced near-infrared radiation (700 nm-2,500 nm), including far-red (700 nm-780 nm) at mid- and lower-canopy levels. LBF significantly altered light quantity and quality, which determined the amount of time that the crop grew under light-limited (<12mol m-2 d-1) versus sufficient light conditions. In AE, crops were established and grown under light-limited conditions for 57% of the growing season, whereas in SE, crops were established and grown under sufficient light conditions for 66% of the growing season. Overall, LBF significantly reduced the yield in SE for both varieties (red: 29%; orange: 16%), but not in AE. The light changes in different seasons in response to LBF suggest that planting time is crucial for maximizing fruit yield when grown under a film that reduces light quantity. LBF may be unsuitable for year-round production of capsicum, and additional development of LBF is required for the film to be beneficial for saving energy during production and sustaining good crop yields in protected cropping.

Vertical Distribution of Arthropod Interactions Within Turfgrass.

Arthropod predators are abundant in turfgrass systems, and they play an important role in managing pests. Understanding the vertical distribution of predation is critical to developing cultural strategies that enhance and conserve predatory services. However, little is known on how the predation is vertically distributed within the turfgrass canopy. Thus, the objective of this study was to determine the vertical distribution of predation within the turfgrass canopy. Clay models were used to emulate the general appearance of Noctuidae caterpillars, to estimate the predatory activity. The choice and no-choice experiments were conducted by placing clay models at 2.54, 5.08, and 7.62 cm from the thatch surface and denoted as lower, intermediate, and upper levels, respectively, within turfgrass canopy. The predator-mediated impressions, paired mark, scratch, deep cut mark, deep distortion, prick, dent, stacked surface impression, scooped mark, granulation, and U-shaped mark, were identified on clay models. The incidence and severity of impressions were significantly greater on clay models placed at the lower canopy level than on those placed at the intermediate and upper canopy levels in the choice and no-choice experiments (P < 0.05). Thus, predators are more likely to find their prey at the soil level. This information can be used to refine management strategies, such as mowing height and insecticide use for effectively managing soil-borne and foliar-feeding arthropod pests and beneficial arthropods.

Mapping forest leaf area density from multiview terrestrial lidar

Abstract Terrestrial lidar data are known to be useful for estimating the three‐dimensional (3D) distribution of leaf area in forests. This type of product holds great potential for modelling canopy reflectance and light interception to study the links between structure and function. However, little is currently known about its potential and limits in dense forests. Higher leaf area density implies that more laser pulses emitted by the ground‐based instrument are intercepted in lower canopy levels, and the implications of such occlusion effects on radiative transfer simulations are unknown. Occlusion effects can be minimized by increasing the number of locations lidar data is acquired from; how many locations are required for a forest with a given structure? This paper aims to address these knowledge gaps. We acquired terrestrial lidar data using a very high density of scanning positions (5 m between positions) over four dense forest 60 m × 60 m plots along a structural gradient. Occlusion effects were quantified, and the 3D distribution of leaf area density was mapped using voxels (cubic volumes) for four different scan densities (one original and three downsampled). The voxel arrays were then input into a radiative transfer model to simulate bidirectional reflectance factors and vertical fraction of absorbed radiation. We found that the summation of leaf area estimates for all voxels within the plot provided leaf area index (LAI) values close to LAI values estimated using traditional methods at each site. Occluded areas occurred mostly at the top of bottom heavy canopies. Radiative transfer simulations suggest that modelling small scale (&lt;1 m) bidirectional reflectance factors (BRF) and light interception requires the highest scan position density used (5 m between scan positions), particularly at bottom heavy sites, and that 10 m between scan positions can be used for plot scale BRF simulations in forests with foliage density and vertical profiles similar to those tested here. This work establishes some initial guidelines for establishing terrestrial lidar survey protocols for mapping leaf area density in forests. The leaf area density voxel arrays derived are among the most accurate plot‐level 3D characterizations of foliage arrangement produced to date.

Effects of intra-genotypic variation, variance with height and time of season on BVOC emissions

Biogenic Volatile Organic Compounds (BVOCs) are trace gases other than CO2 and CH4 produced and emitted by the biosphere, where the amounts released depend on climatic factors such as temperature and solar irradiation. However, interpretation of leaf-level measurements is currently hampered by factors such as large within-genotypic variability, measurement height and time in the season. A campaign was performed between June and August in 2013 in Taastrup, Denmark to study these uncertainties. BVOC emissions were measured from leaves and needles at heights of 2 m, 5.5m and 12.5m in the canopy and for seven trees; four Norway spruces (Picea abies) of which two trees had a budburst approximately a week before the other two, two English oaks (Quercus robur) and one European beech (Fagus sylvatica). Differences in chemical composition and emission strength between June and August were observed between the different trees. English oak's main compound isoprene increased from 62-74%of the total emission in June to approximately 97% in August, which is linked to leaf development over the summer season. The total emission from all measured spruce trees decreased from July to August, but without a loss in the diversity of emitted compounds. The trees showed indications of drought stress as there was a period without precipitation lasting 21 days during the study. There were no differences in emission patterns within all of the measured Norway spruces. For measurement height, there was only a significant difference in emission pattern for European beech as the top of the canopy emitted 7-9 times more in relation to lower canopy levels. Our results suggest there was little within-genotype variability and the wide spacing between trees had an influence on the individual emission patterns. These results are important in order to understand the significance of within-genotypic variation, canopy height and seasonal development in relation to the emission patterns of the selected species. Furthermore, it will provide helpful insights for modelers who wish to improve their emission estimates. (Less)

Evaluating the Performance of High-Altitude Aerial Image-Based Digital Surface Models in Detecting Individual Tree Crowns in Mature Boreal Forests

Height models based on high-altitude aerial images provide a low-cost means of generating detailed 3D models of the forest canopy. In this study, the performance of these height models in the detection of individual trees was evaluated in a commercially managed boreal forest. Airborne digital stereo imagery (DSI) was captured from a flight altitude of 5 km with a ground sample distance of 50 cm and corresponds to regular national topographic airborne data capture programs operated in many countries. Tree tops were detected from smoothed canopy height models (CHM) using watershed segmentation. The relative amount of detected trees varied between 26% and 140%, and the RMSE of plot-level arithmetic mean height between 2.2 m and 3.1 m. Both the dominant tree species and the filter used for smoothing affected the results. Even though the spatial resolution of DSI-based CHM was sufficient, detecting individual trees from the data proved to be demanding because of the shading effect of the dominant trees and the limited amount of data from lower canopy levels and near the ground.

Carbon assimilation, leaf area dynamics, and grain yield in contemporary earlier- and later-senescing maize hybrids

Maize breeding during the past 50 years has been associated with a delay of leaf senescence, but it is not clear whether this trait is likewise associated with higher grain yield in modern hybrids. Post-silking growth, leaf area dynamics, photosynthetic parameters and yield were compared in modern maize hybrids differing in canopy senescence rate. In the first two experiments, four hybrids were grown in the field at Balcarce, Argentina (37°45′ S, 58°18 W). In spite of differences in chlorophyll retention and photosynthesis of the ear leaf, post-silking growth and grain yield were very similar in all four hybrids while kernel N concentration was lower in the later-senescing hybrids. In a third experiment, a later-senescing (NK870) and an earlier-senescing (DK682) hybrid were grown to analyze the potential photosynthetic contribution of delayed leaf senescence. Leaf area and chlorophyll content were larger in NK870, especially at the lower canopy level (0.75m above the ground). However, hybrids did not differ for canopy light interception. Because photosynthetic photon flux density below 1m above the ground was less than 10% of incident radiation and photosynthesis quantum yield did not change during senescence, the potential photosynthetic output of lower leaves below 1m was very low. Lower leaves of NK870 had N concentrations higher than those needed to sustain photosynthesis at the light conditions below 1m. Therefore, we show that delayed senescence does not necessarily improve post-silking C accumulation because: (i) canopy light interception is not reduced by senescence except at very late stages of grain filling; (ii) contrasting hybrids show more pronounced senescence differences at canopy levels receiving less than 10% of incident radiation; (iii) delayed senescing hybrids present lower kernel N concentrations while extra N is retained in leaves exposed to a light limiting micro-environment. Delayed senescence at lower canopy levels may be unproductive, at least under non-stressing conditions.

Addressing the Effects of Canopy Structure on the Remote Sensing of Foliar Chemistry of a 3-Dimensional, Radiometrically Porous Surface

Airborne and spaceborne imaging spectroscopy applied to measuring foliar chemistry has received considerable attention in the literature. Typically, results are based on data measuring all the reflective components that make up a given pixel. This introduces confounding variables that cannot be easily modeled. Spectral unmixing methods yield estimates of the percentage endmember coverage in each pixel. This methodology fails to provide spectra representing variations in these specific components and thus is not as accurate for inferring chemistry. We report on the integration of airborne LiDAR data with high resolution imaging spectroscopy. We compared laboratory-based leaf-level pigment modeling with results from airborne data. In this comparison two airborne datasets were generated; one representing spectra composed of all reflective elements within a forested plot, and a second representing the top of the dominant/codominant canopy. Empirical modeling indicated that there is an influence on the spectral reflectance recorded over a defined area from the lower canopy levels. This influence did not, however, add to our understanding of forest biology and structure.

Habitat Use and Trophic Niche Overlap of Two Sympatric Colobines, Presbytis potenziani and Simias concolor, on Siberut Island, Indonesia

According to ecological theory, the concept of niche differentiation is essential to our understanding of how sympatric species can limit competition over resources. We here examine ecological niche differentiation in 2 sympatric Asian langurs, Presbytis potenziani and Simias concolor, both endemic to the Mentawai Islands off the west coast of Sumatra. We collected data (home range size, canopy use, activity budgets, diet, and niche breadth and overlap) using GPS and scan/focal animal sampling methods on 2 groups of Presbytis potenziani and 3 groups of Simias concolor living in a mixed primary rain forest in northern Siberut. Results show that home ranges of the 2 species overlapped completely and that the home range size of Presbytis potenziani was ca. 4 times larger than that of Simias concolor. Lower canopy levels ( 20 m. Apart from foraging and other activities, there was little difference in overall activity budgets of the 2 species. Regarding diet, although 60% of all food species examined were used by both langur species, they shared only 3 of the 10 most commonly eaten species. Presbytis potenziani fed more selectively on fruits, whereas Simias concolor fed predominantly on leaves. Levin’s niche breadth indices revealed that the diet of Simias concolor is more general (0.34) than that of Presbytis potenziani (0.22). Based on a Pianka index of 0.32, we conclude that there is a relatively small food niche overlap between the 2 colobine species and that diet represents an important mechanism enabling their coexistence.

Biodiversity hanging by a thread: the importance of fungal litter-trapping systems in tropical rainforests

The exceptionally high species richness of arthropods in tropical rainforests hinges on the complexity of the forest itself: that is, on features such as the high plant diversity, the layered nature of the canopy and the abundance and the diversity of epiphytes and litter. We here report on one important, but almost completely neglected, piece of this complex jigsaw-the intricate network of rhizomorph-forming fungi that ramify through the vegetation of the lower canopy and intercept falling leaf litter. We show that this litter-trapping network is abundant and intercepts substantial amounts of litter (257.3 kg ha(-1)): this exceeds the amount of material recorded in any other rainforest litter-trapping system. Experimental removal of this fungal network resulted in a dramatic reduction in both the abundance (decreased by 70.2 ± 4.1%) and morphospecies richness (decreased by 57.4 ± 5.1%) of arthropods. Since the lower canopy levels can contain the highest densities of arthropods, the proportion of the rainforest fauna dependent on the fungal networks is likely to be substantial. Fungal litter-trapping systems are therefore a crucial component of habitat complexity, providing a vital resource that contributes significantly to rainforest biodiversity.

Sunburn incidence of apples is affected by rootstocks and fruit position within the canopy but not by fruit position on the cluster

Authors investigated sunburn incidence of apples on the combinations of three different growth inducing rootstocks (M.9,MM.106 and seedling) and five varieties (‘Smoothee’, ‘Golden Reinders’, ‘Granny Smith’, ‘Gloster’ and ‘Jonagold Jonica’). Symptoms were classified as sunburn browning, sunburn necrosis and photooxidative sunburn. The frequency of symptoms was recorded at various parts of the canopy (N, E,W, S, and lower canopy, upper canopy) and on the cluster (terminal, lateral). Cultivar susceptibility varied between 0.30 and 5.65% on M.9 rootstock, ‘Granny Smith’ seemed to be the most susceptible cultivar whereas relatively low percentage of damaged fruit was observed for ‘Gloster’. On MM.106 and seedling rootstocks, damage level was significantly lower than on M.9. Remarkable differences were not observed in the share of the three sunburn types between cultivars. The most common symptom observed was sunburn browning. Far less fruit was affected by sunburn necrosis and photooxidative sunburn. Photooxidative sunburn symptoms were not found on ‘Granny Smith’ and ‘Gloster’ fruits on MM.106 rootstock. Latter cultivar did not show sunburn necrosis symptoms either. With increasing growing vigor of the rootstocks the share of sunburn browning increased. Fruits with sunburn symptoms were found in a great majority on theW quadrant of the trees. This was true for all cultivars. Remarkable differences in the location within the canopy of affected fruits between the three types of sunburn were not observed. Specific distribution of sunburned fruit was observed along the vertical axis of the canopy, too. Most of the damaged fruit were found in the upper canopy. This is particularly true for trees on vigorous stocks such as MM.106 and seedling. On M.9 rootstock, depending on cultivars 5.9 to 38.9% of sunburned fruit was located in the lower canopy. Most common symptom in the lower canopy was the sunburn browning, however symptoms of sunburn necrosis were not found at lower canopy level. Low rate of photooxidative sunburn was observed such lower canopy conditions. Sunburn incidence was very similar on king or side fruit. Significant differences were not found in the share of each sunburn types between fruit positions on the cluster. This was not influenced by rootstocks either.

Comparison of Abscission Spray Application Practices for Mechanical Harvesting of Oranges

Abstract Two air-carrier sprayers were used to investigate the feasibility of applying abscission materials from one side of the tree in a two-row bedded orange grove. One-sided (bed side) and two-sided (both bed and ditch side) applications at 2.4 and 4.8 km/h were compared using each sprayer. Tank mixture contained active ingredient (a.i.) 5-chloro-3-methyl-4-nitro-1-H-pyrazole (CMN-P) at about 350 mg/L a.i., a surfactant, a drift retardant, and a fluorescent tracer. Leaf samples were collected from different locations on the bed and ditch sides, and deposition was determined by fluorometry. The efficacy of abscission application was assessed in terms of fruit detachment force (FDF) and percent fruit removal (PFR) by trunk shakers. With both sprayers, one-sided applications resulted in comparable depositions to two-sided applications at the far side of the lower canopy level. This result implies that one-sided applications might provide acceptable spray coverage in lightly foliated canopies. Abscission sprays at one-half or full a.i. (526 or 1052 g/ha) rate reduced fruit detachment force significantly. The best results were obtained with two-sided applications at lower ground speed (highest volume/a.i. rate). The air-curtain sprayer at full a.i. rate and the air-blast sprayer at one-half or full a.i. rate resulted in a significant increase in percent fruit removal.

Light spectral composition in a tropical forest: measurements and model.

I present a simple model that simulates vertical variations in the light spectrum within a forest canopy. The model considers only the vertical, downward transmission of light. The light in each canopy level was assumed to consist of non-intercepted radiation and radiation intercepted within the level and transmitted. The spectrum of non-intercepted light in each canopy level is the same as that of incident light above the canopy (input parameter), whereas the spectrum of transmitted light depends on leaf area index (LAI) and the mean transmission spectrum of leaves. The model was tested in a forest and provided adequate predictions of measured values. Stronger deviations were produced in the near infrared (NIR) waveband in lower canopy levels. Multiple regression between LAI, as the dependent variable, and spectral characteristics (Blue, Green, Red and NIR intensities) had an r(2) of 0.926. As a complement to other methods, I suggest light spectrum analysis as a non-destructive technique for estimating LAI in forest canopies.

The survival and saprophytic competitive ability of the Botrytis spp. antagonist Ulocladium atrum in lily canopies

In 1995, conidia of Ulocladium atrum were applied to a canopy of green lily (Lillium spp.) leaves in order to investigate its survival, colonisation of artificially induced necrotic leaf tissues and competitive ability against Botrytis spp. and naturally occurring saprophytes. U. atrum conidia density cm-2 at the top and middle canopy levels was not significantly different following application of the antagonist with a propane powered backpack sprayer. In repeat experiments, conidia density on leaves at the lower canopy level was 18% to 20% of that deposited onto leaves at the top of the lily canopy. There was a significant (P < 0.001) linear decline of U. atrum conidia over time and after 21 days conidia density had declined by up to 73%. Germination of U. atrum on green leaves in the field reached a maximum of 81%, seven days after antagonist application. Conidial viability, measured as germination potential, declined slightly (100% to 88%) after seven days exposure to field conditions but there were no further changes in the germination potential even after 21 days of field exposure. The germination potential was not affected by canopy level. The ability of surviving U. atrum conidia to colonise necrotic tissues, artificially induced with paraquat, was measured. U. atrum colonisation was consistently highest on necrotic leaves at the top level of the canopy and consistently lower on leaves from the bottom canopy level. Necrotic leaf colonisation by U. atrum decreased over time from 51% (necrosis induced immediately after antagonist application) to 21% when necrosis was induced 21 days after antagonist application. A significant (P < 0.001) linear relationship (R2 = 0.713) between colonisation of necrotic tissues and conidia density prior to induction of necrosis was detected. When necrosis was induced immediately after antagonist application, U. atrum outcompeted commonly occurring saprophytic Alternaria spp. and Cladosporium spp. The ability of U. atrum to significantly reduce colonisation by Alternaria spp. was maintained for up to 21 days. Botrytis spp. did not occur in these field experiments. It was concluded that U. atrum had the ability to survive and persist in the phyllosphere for up to 21 days in the field and provided further evidence that U. atrum has the necessary survival characteristics to be a successful biological control agent of Botrytis spp.

Intraindividual variations of volatile terpene contents in Pinus caribaea needles and its possible relationship to Atta laevigata herbivory

Volatile terpene content variations in Pinus caribaea needles are examined in relation to season, dry hour, and canopy level. According to the results, the concentration of s-pinene was lower and that of sabinene and caryophyllene higher in the dry season than in the rainy season. s-Pinene, camphene, and s-phellandrene were more abundant in needles located at the lower canopy level, and limonene, α-ocimene, and the mixture of myrcene+α-phellandrene were more abundant at the upper canopy level. Camphene, sabinene, myrcene+α-phellandrene, and s-cadinene+γ-cadinene were more abundant in the afternoon, with lower concentrations at night. These results indicate that intraindividual variations in the contents of volatile terpenes in Pinus caribaea needles are significant even in regard to interindividual differences. The foraging pattern of the leafcutting ant Atta laevigata, as observed in a Caribbean pine plantation, could be, in part, related to these intraindividual variations.

Effect of Three Adjuvants on Coverage, Persistence, and Efficacy of Ground-Applied Chlorpyrifos for Suppression of Cotton Aphid (Homoptera: Aphididae)

Effects of adjuvants on chlorpyrifos for suppression of cotton aphid, Aphis gossypii (Glover), in cotton, Gossypium hirsutum (L.), were evaluated using four parameters: percentage cotton aphid mortality, spray droplet coverage (percentage area covered) on water-sensitive cards, spray droplet penetration (average spot diameter frequency, distributions) into the cotton canopy, and the rate of disappearance (decay) of chlorpyrifos residues from cotton leaf surfaces. All factors were measured on cotton averaging 171.4 cm in height at three canopy levels (from terminal): upper, 0 cm; middle, 45.7 cm; lower, 76.2 cm. Mortality, of aphids in the upper canopy level (89%) was higher than middle (66%) and lower (57%) levels. Spray coverage, measured as mean percentage area of water-sensitive cards covered by spots, decreased at each lower canopy level as did mean total number of spots per square centimeter. Chlorpyrifos alone generally had a higher proportion of spots per square centimeter in the middle and lower canopy levels relative to the upper level, compared with the adjuvant combinations. No differences in spot diameter frequency distributions were detected among treatments within a canopy level or within a treatment among canopy levels. Initial mean chlorpyrifos deposits on cotton leaf surfaces were highest at the upper canopy level (678 ng/cm2) and decreased by approximately 50% at each descending canopy level. Rate of disappearance of chlorpyrifos from cotton leaf surfaces was significantly reduced at the middle and lower canopy levels compared with the upper level. Adjuvants did not significantly affect chlorpyrifos efficacy, deposition characteristics, or rate of disappearance of chlorpyrifos. Canopy level was important for all three measured criteria.

Penetration of Oak Canopies by a Commercial Preparation of Bacillus thuringiensis Applied by Air

The volume of spray deposit throughout an oak canopy was measured at a single-leaf resolution. Profiles of deposit (international units per cm2) are compared for upper and lower-canopy levels and positions within these levels. Droplet sizes and densities are shown for upper- and lower-canopy levels. Artificial targets were used to measure the deposit at ground level, the deposit on a horizontal profile at midcrown, and the deposit over a vertical profile from the canopy top to the ground. No significant differences were found between the levels or positionsin the canopy ( P > 0.05). However, highly significant variation was shown among sample trees ( P < 0.001). The distribution of dose among individual leaves was shown to be highly variable with approximately a lognormal distribution.

The Frequency Distribution of Eggs and Larvae of Anaphe venata Butler (Lepidoptera: Notodontidae) on Triplochiton scleroxylon K. Schum

The spatial distribution of the eggs and larvae of Anaphe venata Butler within canopies of its host, Triplochiton scleroxylon K. Schum situated at Araromi Village northeast of Ibadan was determined. Sampling branches of T. scleroxylon once in a spiral from base of canopy upwards was sufficient in determining the distribution of eggs and larvae of A. venata . Although the differences in the mean count of egg masses and larval groups in the lower, middle and upper canopy levels were not significant, the highest percentage of egg masses and larvae occurred on the tips of branches in the lower canopy. Sampling studies on the developmental stages of A. venata may therefore be done at the lower canopy level for best precision. Eggs and larvae are aggregated while the larval dispersion on the host suggest photo-positivity. The use of mounted, motorised elevators is proposed in order to enhance efficiency of sampling and avoid destructive sampling.

Control of Aphids in Cotton in Mississippi, 1985

Abstract A small plot field study was conducted to measure the relative efficacy of several insecticides for control of aphids on cotton in the Delta region of MS. Since the species of aphids was not determined, we assumed that the population was predominantly composed of Aphis gossypii. The study was conducted on a grower farmer where aphids were found at measurable severs. According to the grower the field had previously been treated several times with pyrethroid insecticides. Plot size was 4 rows × 50 ft. Applications were applied with a John Deere high clearance sprayer equipped with 2 TX-4 nozzles/row that delivered a spray volume of ca. 3.0 gpa. Each treatment was replicated 4 times in a randomized complete block design. Pretreatment observations indicated that the aphid population was between 10-20 aphids/upper leaf. Posttreatment observations were made by randomly collecting 10 leaves from the upper, mid, and lower canopy levels in each plot at 2, 7, and 14 days after application. Each collected leaf was classified as having (1) 0 aphids, (2) 1-5 aphids, (3) 6-15 ;iphids, (4) 16-25 aphids, or (5) 25+ aphids. For purposes of comparing the various treatments, the data were converted to estimated number of aphids/leaf by multiplying the mid-point of each classification by the number of observations for that classification. Data were subjected to AOV and means were separated by Duncan’s multiple range test.

Patch Dynamics and Stability of Some California Kelp Communities

This paper considers three concepts of stability as they relate to the dynamics of distinctive patch types of algal canopy guilds in southern and central California kelp communities: (1) persistence of a patch through more than one generation of the dominant species, which was evaluated by using life tables and observations of patch borders; (2) inertia or the resistance of different patches to invasion or disturbance, which was evaluated by artificially enhancing gametophytes by transplanting sporogenic material, by removing canopy, and by evaluating some important disturbance processes; and (3) resilience or recoverability of a patch following a perturbation sufficient to allow invasion of different species, which was studied by defining some of the mechanisms of successful invasion or succession. By working in distinct habitats in southern (Pt. Loma and Santa Catalina Island) and central (Pt. Piedras Blancas) California, we could evaluate different types of physical stresses as they related to these stability concepts. Taller perennial canopy guilds were dominant competitors for light, but were more susceptible to physical wave stress. Dominance hierarchies in the competition for light appeared to be reversed in areas exposed to increasing wave stress. The main causes of mortality at Pt. Loma were entanglement with storm—dislodged Macrocystis plants and, in some areas, sea urchin grazing. Mortality in central California was due to winter storms. In most cases, distinct patches resisted invasion for &gt;10 yr. The mechanisms of resistance involved (1) competition for light and, possibly, nutrients, and (2) limits to spore dispersal. When succession occurred, it was often mediated by many factors, including seasonality of spore production, which coincided with winter storm—related mortalities; mechanisms of kelp dispersal, which were most effective via drifting plants and fragments of fertile material held against the substrate by invertebrates; and survivorship of gametophytes and small sporophytes, which was influenced by local scour and grazing. Appropriate spatial scales, stability, and succession studies in these kelp communities were determined by the size of the disturbed area, which varied from the free space resulting from detachment of single plants to the free space resulting from catastrophies such as overgrazing or unusual storms. Temporal scales were influenced by seasonality of disturbance and algal reproductive condition and aperiodic episodes of cool, nutrient—rich water advected into the patch. There appeared to be conflicting morphological adaptations of the canopy guilds: exploitation of light was enhanced at higher canopy levels, whereas the lower canopy levels were better adapted to tolerate stress from wave surge. The adaptations of the algae appeared to form four distinct groups of tactics: (1) ruderals or plants, such as Nereocystis and Desmarestia, with opportunistic life histories; (2) kelps, such as Macrocystis, adapted to exploitative competition for light and nutrients; (3) kelps (Eisenia, Dictyoneurum) adapted to physical stress such as wave surge; and (4) those algae, such as corallines and Agarum, adapted to heavy grazing. Within any given area, the relative patch stability was determined by biological relationships; between areas, the patch stability patterns were attributable to physical differences.