Unshaded Plots Research Articles

The role of shade in influencing dung deposition and nutrient transfer by kangaroos (Macropus spp.) in an arid landscape

Where they occur at high densities, herbivores are likely to impact ecosystems by functioning as nutrient vectors that shift nutrients from areas where they graze to other locations. Such lateral shifts in nutrients are important to understand as they can alter the balance of species interactions and composition of ecological communities. In arid Australia, kangaroo (Macropus spp.) populations have irrupted due to suppression of their chief predator and their presence can have marked impacts on vegetation and soils in areas where they graze. Kangaroos are hypothesised to facilitate the transport of nutrients by ingesting organic material and depositing it in the form of faecal matter and urine under the shade of trees where they rest during the day. Here, we investigate the role that kangaroos and shade play as nutrient vectors and nutrient sinks, respectively. We compared kangaroo visitation and soil nutrients under experimental shades with unshaded areas in an arid landscape with little natural shade. Kangaroo visitation was greatest at shaded plots. Dung accumulation was greater at shaded plots than unshaded plots. Total carbon (TC) was greater in soils within shaded plots than control plots; however, there was no difference in TC between shaded plots and procedural control plots. There was no difference in total nitrogen and plant available phosphorus between shaded, control and procedural control plots. Our findings demonstrate that the presence of shade in an arid landscape influences the microhabitat choice of kangaroos, but only partially supports the hypothesis that nutrients concentrate within the soil under shade.

Solar Radiation Drives Potential Demographic Collapse in a Perennial Bunchgrass via Dramatically Reduced Seedling Establishment

Many perennial plants in semiarid rangelands have experienced population declines, and understanding the ecological and demographic processes behind these declines is important to slowing or reversing them. Although anthropogenic disturbances drive many declines, other sorts of environmental variability, such as the differences in solar radiation with aspect, may impact population success locally. We experimentally assessed the role of solar radiation in driving an apparent decline in a common perennial bunchgrass, bluebunch wheatgrass (Pseudoroegneria spicata), on south-facing slopes at a site in the Columbia Basin of North America. Across three separate experiments on south-facing slopes, we observed dramatically (5−36 ×) higher seedling success in plots shaded to approximate the solar radiation of north-facing slopes relative to adjacent open (unshaded) plots. When we applied the rates of seedling success from these experiments to demographic models from this site, we found that seedling success in unshaded plots was often too low to allow a stable population on south-facing slopes, but that seedling success in shaded plots was often high enough to allow a stable or increasing population on north-facing slopes. We originally hypothesized that the primary mechanism driving this effect was water stress caused by greater evaporation from hotter open plots. However, despite soils in open plots being consistently hotter than soils in shaded plots, we did not observe a clear pattern of greater soil moisture in shaded plots. Therefore, it appears that higher solar radiation or higher temperatures on south-facing slopes may be sufficient to dramatically reduce seedling survival, and that the higher density of bluebunch wheatgrass on north-facing slopes relative to south-facings slopes may be driven primarily by this low survival of seedlings. As climate warms, the reduced seedling survival that threatens our bluebunch wheatgrass population may be expected to threaten many other species of perennials in similar rangelands.

Effects of shade and deficit irrigation on maize growth and development in fixed and dynamic AgriVoltaic systems

Maize production is essential for global food security and represents a major supply in several value chains. However, the projected effects of climate change are likely to decrease drastically water availability for crops in many regions, affecting yield. AgriVoltaics (AV) systems are an innovative solution that may improve maize resilience in water-scarce regions mainly by protecting plants from excessive radiation and by reducing irrigation needs. However, shade from panels may also affect crop development and production. This study addresses the interplay between radiation transmission, crop development and irrigation needs of maize cropping in field conditions, by the description of crop development dynamics, distinguishing between fixed and dynamic panels. We showed that maize crop responded to both independent and combined stresses (shade and water deficit), with a significant decrease in leaf area index, total dry matter and grain yield. Concerning water use, we showed the potential of AV to reduce irrigation inputs (by up to 19–47% compared to unshaded plots) via reduced soil water depletion and reference evapotranspiration. The crop development was impacted by shade by increasing phyllochron and causing a generalized delay in phenology. At a finer temporal scale, we concluded that maize leaves react to shade by reducing stomatal conductance, net assimilation of CO2 and leaf temperature in a correlated way to radiation, opening the possibility to use this behavior to optimize water use and shading strategies. The spatial heterogeneities of radiation in fixed AV systems, compared to dynamic AV systems, were identified as a second-order effect at the plot level on leaf area index and phyllochron, compared to the effect of radiation reduction. Moreover, dynamic AV showed their ability to reduce the spatial heterogeneities in soil water depletion, showing the importance of controlled shade strategies in AV systems concerning water use.

Selected shade tree species improved cocoa yields in low-input agroforestry systems in Ghana

CONTEXTCocoa agroforestry systems differ in the diversity of shade tree species composition. Though cocoa benefits from shade, there is a lack of species-specific information on shade trees that enhance soil fertility and yield. OBJECTIVEWe examined how soil characteristics and cocoa yield were affected by eight commonly retained forest tree species, compared with unshaded control plots over a 3-year period. METHODSUsing 74 circular plots from 10 cocoa farms in the Western region of Ghana, we sampled soils from two random points within each plot. Soil nutrients at the beginning and end of the study were analyzed, and yield was expressed as number of harvested pods and dry weight of beans per hectare. RESULTS AND CONCLUSIONSLevels of soil K and Ca were below recommended values. Although soil available phosphorus (P) was higher in control plots than under shade trees, yield around shade trees were higher than on unshaded plots. Cocoa yield differences between shade tree species and control plots were significant only in the major crop season, but not in the minor crop season. Cocoa yields under Cedrela odorata, Khaya ivorensis, Terminalia superba and Milicia excelsa were significantly higher than on control plots. Hence, the inclusion of specific shade tree species in cocoa agroforestry systems is important to maintain high yields in cocoa systems with low inputs. SIGNIFICANCETo our knowledge, this study presents one of the first attempt to assess the impacts of specific shade tree species on soil characteristics and cocoa yield.

The Effect of Shading and Nutrient Addition on the Microphytobenthos, Macrofauna, and Biogeochemical Properties of Intertidal Flat Sediments

Proliferation of urban structures and mangrove forests in estuaries are altering the shading of intertidal sediments. Urbanisation also tends to increase nutrient loads in estuaries, which can have numerous direct and indirect effects on estuarine flora and fauna. Mangrove canopy shades the sediment and provides nutrients to the ecosystem via leaf litter. Microphytobenthos, macrobenthos, sediment erodibility and various biogeochemical properties of sediments have been shown to differ significantly between unshaded intertidal sediment and nearby sediment under a mangrove canopy. This study tested the effects of experimental manipulation of shading and addition of nutrients on the microphytobenthos, macrobenthos, sediment erodibility and selected biogeochemical properties of exposed mudflat next to the seaward edge of a mangrove forest. In the first of two experiments, shading of plots was increased by the addition of black Perspex roofs for comparison with unshaded plots and plots with clear Perspex roofs; the addition of nutrients was done in an orthogonal design. Clear roofs were originally intended as a procedural control, but due to sedimentation on their upper surface caused intermediate levels of shading. Sediment and benthos were sampled after two weeks. Nutrients were omitted in the second experiment, with plots sampled after two weeks or three months. The only effect of nutrients was a small negative effect on chlorophyll a and colloidal carbohydrate. Intermediate shading (clear roofs) consistently increased measures of microphytobenthos biomass (e.g. Fo and chlorophyll a) and biogeochemical properties associated with microphytobenthos such as colloidal carbohydrate. Substantial shading (black roofs) decreased measures of microphytobenthos biomass and microphytobenthos associated biogeochemical properties. Effects on the fauna were much smaller and inconsistent with previous studies, after 3 months assemblages were different under black roofs compared to clear roofs and control plots, with differences primarily driven by changes in the oligochaetes. Natural or anthropogenic changes in shading at larger spatial scales are likely therefore to directly and indirectly change microphytobenthos, sediment properties, macrofauna and hence ecosystem functions; but any flow-on effects to the fauna are difficult to predict without further experiments to understand the indirect and direct responses of fauna to changing microphytobenthos and properties of intertidal sediment.

Shading enhances plant species richness and diversity on an extensive green roof

Green roofs can promote biodiversity in urban areas. The extent to which green roofs stimulate plant diversity can depend on roof characteristics such as roof age, substrate depth and shading. We exploratively studied the vegetation on a Dutch green roof in 50 permanent plots (1 m2) over eight years (2012–2019) following roof construction. Plots were situated either on low substrate depth (6 cm light-weight extensive substrate) or high substrate depth (6 cm light-weight extensive substrate topped with 14 cm native soil) and differed in the amount of shading received from a higher building floor. Increased substrate depth and shading additively increased plant species richness and plant diversity, with high shaded plots supporting on average 6.4 more plant species than low unshaded plots. Shading likely acts via reducing drought stress, whereas increasing substrate depth with native soil may also enhance plant diversity via addition of nutrients and native seeds. The vegetation composition on the roof was dynamic and changed over the years. Sedum acre was initially dominant but disappeared within the first years, whereas Sedum kamtschaticum increased and became dominant in the last years. Trifolium arvense was the most abundant forb species and was especially dominant three years after roof construction. We conclude that increased substrate depth and shading can promote plant species richness and diversity and recommend that both aspects are considered when green roofs are designed. Shading can be achieved by a stepped building architecture and by placing structures on the roof itself, such as solar panels on standards.

Emergence and control of gorse seedlings after the 2017 Port Hills fire

An experiment on the Port Hills, Canterbury, after mature gorse was burnt in the fires of February 2017, showed an oversown Italian ryegrass mix out-competed the rapidly germinating gorse seedlings. The shaded gorse seedling population reached a peak of 680 plants/m2 in June, declining to ~450 plants/m2 in October compared with >600 plants/m2 in the unshaded plots. As soil moisture dropped in summer, the gorse seedling population decreased to 10 plants/m2 by March 2018, compared with 73 plants/m2 in the unshaded plots. Gorse seedlings that had been shaded by Italian ryegrass had shorter roots and lower dry weights than those grown without competition. The oversown mix was more successful on the south than north-facing slope where more bare ground enabled patches of gorse to re-establish. The oversowing of Italian ryegrass was shown to be a viable option to control gorse particularly after an unplanned burn that removed the fences and water supply.

An Experimental Evaluation of Dock Shading Impacts on Salt Marsh Vegetation in a New England Estuary

Docks constructed over salt marsh can reduce vegetation production and associated ecosystem services. In Massachusetts, there is a 1:1 height-to-width ratio (H:W) dock design guideline to reduce such impacts, but this guideline’s efficacy is largely untested. To evaluate dock height effects on underlying marsh vegetation and light availability, we deployed 1.2-m-wide experimental docks set at three different heights (low (0.5:1 H:W), intermediate (1:1 H:W), and high (1.5:1 H:W)) in the high and low marsh zones in an estuary in Massachusetts, USA. We measured temperature, light, vegetation community composition, and stem characteristics under the docks and in unshaded control plots over three consecutive growing seasons. Temperature and light were lower under all docks compared with controls; both increased with dock height. Maximum stem height and nitrogen content decreased with available light. In the Spartina patens-dominated high marsh, stem density and biomass were significantly lower than controls under low and intermediate but not high docks. Spartina alterniflora, the dominant low marsh vegetation, expanded into the high marsh zone under docks. S. alterniflora aboveground biomass significantly differed among all treatments in the low marsh, while stem density was significantly reduced for low and intermediate docks relative to controls. Permit conditions and guidelines based on dock height can reduce dock impacts, but under the current guideline of 1:1 H:W, docks will still cause significant adverse impacts to vegetation. Such impacts may interfere with self-maintenance processes (by decreasing sediment capture) and make these marshes less resilient to other stressors (e.g., climate change).

Seasonal microbial and nutrient responses during a 5-year reduction in the daily temperature range of soil in a Chihuahuan Desert ecosystem.

High daily temperature range of soil (DTRsoil) negatively affects soil microbial biomass and activity, but its interaction with seasonal soil moisture in regulating ecosystem function remains unclear. For our 5-year field study in the Chihuahuan Desert, we suspended shade cloth 15 cm above the soil surface to reduce daytime temperature and increase nighttime soil temperature compared to unshaded plots, thereby reducing DTRsoil (by 5 ºC at 0.2 cm depth) without altering mean temperatures. Microbial biomass production was primarily regulated by seasonal precipitation with the magnitude of the response dependent on DTRsoil. Reduced DTRsoil more consistently increased microbial biomass nitrogen (MBN; +38%) than microbial biomass carbon (MBC) with treatment responses being similar in spring and summer. Soil respiration depended primarily on soil moisture with responses to reduced DTRsoil evident only in wetter summer soils (+53%) and not in dry spring soils. Reduced DTRsoil had no effect on concentrations of dissolved organic C, soil organic matter (SOM), nor soil inorganic N (extractable NO3 (-)-N + NH4 (+)-N). Higher MBN without changes in soil inorganic N suggests faster N cycling rates or alternate sources of N. If N cycling rates increased without a change to external N inputs (atmospheric N deposition or N fixation), then productivity in this desert system, which is N-poor and low in SOM, could be negatively impacted with continued decreases in daily temperature range. Thus, the future N balance in arid ecosystems, under conditions of lower DTR, seems linked to future precipitation regimes through N deposition and regulation of soil heat load dynamics.

Disturbance, Resources, and Exotic Plant Invasion: Gap Size Effects in a Redwood Forest

Abstract Fluctuations in plant resource availability are hypothesized to promote exotic plant invasion by allowing propagules already present in an area a chance to successfully compete for unused resources. To examine the relationship between resource enrichment and exotic species invasion, we used selective logging canopy gaps over a range of sizes (56 m2 to >1500 m2) in a redwood forest (Santa Cruz County, CA) as a surrogate for disturbance intensity and level of pulsed resource enrichment. Measurements of abiotic conditions in gaps ca. 10 yr after logging suggest light is the primary difference in current resource availability, though a pulse of light and nutrients likely occurred at the time of gap formation. Exotic species richness and relative cover increased significantly as gap size increased. In a separate manipulative experiment, we compared understory plant composition between artificially shaded and unshaded plots in 2.5-year-old logging gaps. Shaded plots had a lower proportion of exotic spe...

Experimental assessment of the effects of shade on an intertidal kelp: Do phytoplankton blooms inhibit growth of open coast macroalgae?

Benthic primary producer abundance and satellite-derived chlorophyll concentrations are strongly negatively correlated along the Oregon and Northern California coast, suggesting an antagonistic interaction. Direct field observations of interannual and among-site changes in abundance of low intertidal macrophytes also suggest a negative interaction. Several years (2001–2006) of quantification of surf-zone chlorophyll concentrations and light reaching the intertidal benthos suggested that severe light attenuation from dense phytoplankton blooms is an important mechanism underlying this inverse phytoplankton-macrophyte relationship. From early June to September of 2004 we quantified the response of the intertidal kelp Saccharina sessile to experimentally manipulated light regimes that mimicked the attenuation during blooms. Shading frames were installed in the low intertidal zone to manipulate the light levels available to the benthos at two sites of contrasting long-term differences in average phytoplankton abundance. Treatments included shaded and unshaded plots that were 0.25 m2 in area. Although the magnitude of the effect was context-dependent, shading strongly decreased growth rates and abundance of S. sessile. These results are comparable to results in estuarine studies that have demonstrated adverse effects on benthic macrophytes as a result of eutrophication and subsequent light limitation and are the first demonstration of phytoplankton-induced light limitation for energetic open coasts. In these systems where annual benthic production can exceed the pelagic production and where perennial macrophytes such as kelp and surf grasses are important habitat modifiers, large-scale reduction of macrophytes via phytoplankton shading could lead to profound modifications of coastal ecosystem dynamics.

SHADING FACILITATES SESSILE INVERTEBRATE DOMINANCE IN THE ROCKY SUBTIDAL GULF OF MAINE

Dramatic shifts in community composition occur between vertical and horizontal rocky surfaces in subtidal environments worldwide, yet the forces mediating this transition are poorly understood. Vertical rock walls are often covered by lush, diverse communities of sessile suspension-feeding invertebrates, while adjacent horizontal substrates are dominated by algae, or corals in the tropics. Multiple factors, including light, sedimentation, water flow, and predation have been proposed to explain this pattern, but experimental tests of these hypotheses are lacking. We manipulated light level and predation to test whether variation in these mechanisms could be responsible for the shift in composition of sessile communities between vertical and horizontal surfaces in the rocky subtidal Gulf of Maine. Shaded horizontally oriented granite plots were dominated by invertebrates (e.g., ascidians, barnacles, bryozoans) after 25 months. Unshaded plots were dominated by macroalgae, which was virtually absent in shaded plots. Exclusion of grazers with cages had no effect on percent cover of invertebrates or algae. Preferential settlement of invertebrate larvae to shaded plots, due to larval behaviors such as negative phototaxis, did not seem to play a role. Shading likely affects post-settlement mortality of invertebrates by alleviating competition for space with algae, although greater abundance of micropredators in algal-dominated communities could also be important. Communities on shaded plots lacked many taxa present on natural wall communities, likely due to greater disturbance on horizontal substrates and/or lack of sufficient time for colonization of these taxa. These results suggest that light plays a key role in controlling the structure, composition, and function of shallow subtidal communities.

Effects of three years of soil warming and shading on the rate of soil respiration: substrate availability and not thermal acclimation mediates observed response

AbstractIn a number of recent field studies, the positive response of soil respiration to warming has been shown to decline over time. The two main differing hypotheses proposed to explain these results are: (1) soil microbial respiration acclimates to the increased temperature, and (2) substrate availability within the soil decreases with warming so reducing the rate of soil respiration. To investigate the relative merits of these two hypotheses, soil samples (both intact cores and sieved samples) from a 3‐year grassland soil‐warming and shading experiment were incubated for 4 weeks at three different temperatures under constant laboratory conditions. We tested the hypothesis that sieving the soils would reduce differences in substrate availability between warmed and control plot samples and would therefore result in similar respiration rates if microbial activity had not acclimated to soil warming. In addition, to further test the effect of substrate availability, we compared the respiration rates of soils taken from shaded and unshaded plots. Both soil warming and shading significantly reduced respiration rates in the intact cores, especially under higher incubation temperatures. However, sieving the soil greatly reduced these differences suggesting that substrate availability, and not microbial acclimation to the higher temperatures, played the dominant role in determining the response of heterotrophic soil respiration to warming. The effect of shading appeared to be mediated by reduced plant productivity affecting substrate availability within the soil and hence microbial activity. Given the lack of evidence for thermal acclimation of microbial respiration, there remains the potential for prolonged carbon losses from soils in response to warming.

The effects of in situ shading on the growth of a seagrass, Syringodium isoetifolium

The effect of light reduction on a tropical seagrass, Syringodium isoetifolium, was examined over a period of six months (July 2001 to December 2001) in relation to leaf elongation rates, shoot densities and chlorophyll content of the leaf blades. The experiment was carried out at Poste Lafayette, Mauritius, in winter (July to September) and summer (October to December) in the same seagrass meadow. In both seasons three plots were shaded with Sarlon cloth (75% light cut-off), with three unshaded plots as controls. The growth rate for both winter and summer was higher in the shaded plots than in the control. Leaf elongation rates were higher in shaded plots in summer than in the shaded plots in winter. Blade chlorophyll was also higher in the shaded plots than in unshaded plots both in winter and in summer. There was no significant difference in the shoot density between plots. These results are consistent with the suggestion that high irradiance brings about a reduction in seagrass growth. The results also show that light is one of the factors that control the vertical growth response of S. isoetifolium as well as its photoreceptors.

Dissipation of the Herbicide Clopyralid in an Allophanic Soil: Laboratory and Field Studies

Soil dissipation of the herbicide clopyralid (3,6‐dichloropicolinic acid) was measured in laboratory incubations and in field plots under different management regimes. In laboratory studies, soil was spiked with commercial grade liquid formulation of clopyralid (Versatill®, 300 g a.i. L− 1 soluble concentrate) @ 0.8 µg a.i. g− 1 dry soil and the soil water content was maintained at 60% of water holding capacity of the soil. Treatments included incubation at 10°C, 20°C, 30°C, day/night cycles (25/15°C) and sterilized soil (20°C). Furthermore, a field study was conducted at the Waikato Research Orchard near Hamilton, New Zealand starting in November 2000 to measure dissipation rates of clopyralid under differing agricultural situations. The management regimes were: permanent pasture, permanent pasture shielded from direct sunlight, bare ground, and bare ground shielded from direct sunlight. Clopyralid was sprayed in dilute solution @ 600 g a.i. ha− 1 on to field plots. Herbicide residue concentrations in soil samples taken at regular intervals after application were determined by gas chromatograph with electron capture detector. The laboratory experiments showed that dissipation rate of clopyralid was markedly faster in non‐sterilized soil (20°C), with a half‐life (t1/2) of 7.3 d, than in sterilized soil (20°C) with t1/2 of 57.8 d, demonstrating the importance of micro‐organisms in the breakdown process. Higher temperatures led to more rapid dissipation of clopyralid (t1/2, 4.1 d at 30°C vs 46.2 d at 10°C). Dissipation was also faster in the day/night (25/15°C) treatment (t1/2, 5.4 d), which could be partly due to activation of soil microbes by temperature fluctuations. In the field experiment, decomposition of clopyralid was much slower in the shaded plots under pasture (t1/2, 71.5 d) and bare ground (t1/2, 23.9 d) than in the unshaded pasture (t1/2, 5.0 d) and bare ground plots (t1/2, 12.9 d). These studies suggest that environmental factors such as temperature, soil water content, shading, and different management practices would have considerable influence on rate of clopyralid dissipation.

LIFE-HISTORY DIFFERENTIATION AND THE MAINTENANCE OF MONOECY AND DIOECY IN SAGITTARIA LATIFOLIA (ALISMATACEAE)

The existence of monoecious and dioecious populations within plant species is rare. This limits opportunities to investigate the ecological mechanisms responsible for the evolution and maintenance of these contrasting sexual systems. In Sagittaria latifolia, an aquatic flowering plant, monoecious and dioecious populations exist in close geographic proximity but occupy distinct wetland habitats differing in the relative importance of disturbance and competition, respectively. Life-history theory predicts contrasting evolutionary responses to these environmental conditions. We propose that the maintenance of monoecy and dioecy in S. latifolia is governed by ecological selection of divergent life-history strategies in contrasting habitats. Here we evaluate this hypothesis by comparing components of growth and reproduction between monoecious and dioecious populations under four conditions: natural populations, a uniform glasshouse environment, a common garden in which monoecious and dioecious populations and their F1 progeny were compared, and a transplant experiment using shaded and unshaded plots in a freshwater marsh. Plants from dioecious populations were larger in size and produced heavier corms in comparison with monoecious populations. Monoecious populations flowered earlier and produced more flowers, clonal ramets, and corms than dioecious populations. The life-history differences between the sexual systems were shown to have a quantitative genetic basis, with F1 progeny generally exhibiting intermediate trait values. Survival was highest for each sexual system in field plots that most closely resembled the habitats in which monoecious (unshaded) and dioecious (shaded) populations grow. These results demonstrate that monoecious and dioecious populations exhibit contrasting patterns of investment in traits involved with growth and reproduction. Selection for divergent life histories between monoecious and dioecious populations of S. latifolia appears to be the principal mechanism maintaining the integrity of the two sexual systems in areas of geographic overlap.

The Effects of Dock Shading on the Seagrass Halodule wrightii in Perdido Bay, Alabama

Aboveground and belowground biomass, density, blade length, and chlorophyll content of seagrass growing directly under docks were compared with adjacent unshaded sites. The amount of light reduction due to dock shading was measured using quantum spherical irradiance meters. Seagrasses were present under docks; they were shaded such that light levels were 19% and 16% of surface irradiance, at shallow and deep sites, respectively. Shoot density was 40–47% lower in shaded plots than in unshaded plots; total biomass was 30–33% lower in shaded plots than in unshaded plots. In contrast, blade length and chlorophyll content were increased at shaded plots relative to unshaded plots. Seagrasses were not found under docks at light levels less than 14% of surface irradiance. These results are in agreement with published in situ compensation irradiance estimates of 15–18% for Halodule wrightii in Texas coastal waters. Shading effects were most apparent between the hours of 1000 and 1500; therefore, the light received during the early morning and late afternoon hours may be critical for plants growing under docks. The north-south orientation of the docks in this study is likely an important factor contributing to the continued survival and growth of the seagrasses.

Loss of viability of Dematophora necatrix in solarized soils

To study the relationship between temperature regimes and loss of viability of Dematophora necatrixin soil, two field experiments were conducted to determine the effectiveness of soil solarization on reducing the population of D. necatrix colonizing avocado root segments buried at a depth of 15‐60 cm. Increase of maximum hourly temperatures attributable to soil solarization reached, depending on depth, 6.7‐4.6 C in unshaded areas and 3.9‐ 1.5C for shaded areas in the first experiment (starting in early June, 1995). The better environmental conditions in the second experiment (starting by mid-July, 1995) led to higher temperature increases (8.6‐5.6 C, depending on depth) when solarization was conducted in unshaded areas. One, 4, 5 and 6 weeks of solarization were required to eliminate the viability of D. necatrix at 15, 30, 45 and 60 cm depths in the first experiment, whereas only 8, 10, 15 and 22 days of solarization were needed for the loss of viability of D. necatrix at the same depths in the second experiment. In shaded areas, however, soil solarization attained significant effectiveness at 15 cm depth. Regression analyses of fungal viability (ln-transformed data) over accumulated temperature‐time showed best fits when the minimum threshold temperature was 30 C. Although eradication of D. necatrixin soil can be achieved down to 60 cm depth in solarized plots, and at 15 cm depth in unsolarized unshaded plots, the accumulation of temperature‐time appeared less effective in reducing inoculum viability in the latter.

Low Light Stress Influences lower Abscission and Yield of Six Bell Pepper Cultivars

High temperatures during flowering frequently limit yields of some bell pepper cultivars in New York fields. Previous research has shown that subjecting the plants to low light at flowering can have similar effects. To determine if cultivar differences in flower abscission and yield could be accentuated by such a shade stress, field plots of six cultivars were subjected to 1 week of low light during flowering. Shade cloth tunnels were erected over the plant rows in two experiments, reducing incident light by 80%. Nondestructive abscission counts were taken at the start, and 7 days after the end of a 7-day shade period. Mature green fruit were harvested periodically. Low light stress resulted in 68% and 86% abscission at the first three fruiting nodes in 1992 and 1994, respectively. Cultivars showed differential abscission in unshaded plots, and after shade, producing a significant cultivar: shade interaction. `Ace' showed least abscission and maintained yields with shading; `Camelot' lost nearly all flowers and buds with low light stress, and was reduced by 75% and 91% in marketable yield in 1992 and 1994, respectively. Results indicate that shade stress accentuates abscission susceptibility in bell pepper cultivars. Pepper lines selected for low light tolerance may show promise in resisting flower abscission at high temperature.

Ramet size equalisation in a clonal plant, Phragmites australis.

The influence of shading from older generations of dead culms (standing litter) on density, growth rate and development of size structure at the ramet level was investigated in a pure stand of Phragmites australis by experimental neutral shading of plots after removal of standing litter. Initial differences in height distribution between autumn and spring cohorts disappeared in the course of shoot growth. The Gini coefficients of shoot heights and estimated shoot weights indicated that the size structure of the shoots became more equal with increasing mean size in both shaded and unshaded plots. Relative growth rate for height (RHGR) and weight of individual shoots was negatively related to shoot size during the early and presumably storage-dependent growth period, suggesting a strong support for growth of smaller shoots. No etiolation was indicated by mean or maximum height in shaded and unshaded plots, or by the relationship between shoot height and weight. Mean shoot density was significantly lower in shaded than in unshaded plots in one of two shade treatment years. A regression model indicated a small but significant effect of shoot density on the approximately linear relationship between RHGR and the logarithm of height. The growth rate of small shoots was slightly larger at low than at high shoot density. Therefore, it is suggested that the shade from standing litter in P. australis stands can decrease shoot natality in the spring cohort, and thereby increase the support to fewer small shoots.