Nocturnal transpiration due to incomplete closure of stomata or high vapour pressure deficits can create and maintain high water potential gradients between the soil and leaves and thus limit water movement to other parts of the root system. In addition,rehydration of roots in dry soil layers may be slower where there are significant constraints to water movement, including embolisms, small-diameter and curved xylem vessels with extensive branching , high frequency of pits and endplate membranes , and the presence of heartwood, latewood, and rays . Lateral water movement may also be limited by the high resistances that form at the stem base–root junction . Vaccinium corymbosum has been reported to be a species that does not effectively distribute water laterally . Using a split-root water application, Abbott and Gough found that dyes did not move laterally from one stem to another and observed root mortality in the unwatered root container. The main objective of this study was to quantify patterns of nocturnal internal hydraulic redistribution and conductances among the first seven orders of V. corymbosum roots under severe drought conditions and its implications for root tissue rehydration. Vaccinium corymbosum was selected due to its unique root anatomy and morphology and its reported limited ability to redistribute water. It was hypothesized that root tissue rehydration at night would be delayed,collection pot especially in the distal root orders because of their greater hydraulic constraints .The V. corymbosum root system is characterized as being very efficient in terms of biomass allocation for the production of a large amount of root surface area.
Its highly branched root system is composed of very fine roots that usually proliferate in the top 20–30 cm of the soil . Water and nutrient uptake presumably occurs mostly in the first three root orders, which correspond to the mycorrhizal and non-woody, ephemeral section of the root system . These non-woody roots have very few and small-diameter vessels. Such root anatomical characteristics may lead to a high cavitation vulnerability of the distal roots during severe drought, especially when water is available to only a fraction of the root system such as might occur with drip irrigation or when there are only a few deeper roots. Plants may cope with drought under conditions of heterogeneous soil moisture conditions by internally redistributing water from roots in moist soil to those in dry soil during the night, such as in grape , but to a much more limited extent in blueberry. In the present work, patterns of root water potentials measured throughout the night revealed how water redistributes internally within the root system of Vaccinium. Vaccinium rehydrated the distal first five root orders in dry soil from 60% to 70% in a period of 11 h when well irrigated in another portion of the root system. No other work has previously quantified the rate or magnitude of change of tissue rehydration over the night as water moved through different branch orders in the root system. Root rehydration at night was influenced by several factors, including root hydraulic constraints, duration of the nocturnal period, water availability in the wet-side, and water potential gradients among roots. The overall final rehydration achieved by the distal root orders in dry soil served as an indicator of how efficiently or inefficiently water was hydraulically redistributed through the root system.
As suspected, the distal, finest root orders experienced the lowest water potentials at the onset of the nocturnal period, followed by medium and coarse roots. Although water was readily available to roots in the wet soil and transpiration was minimal, it took the whole night-time period of 12 h for the distal finest roots under dry soil conditions to reach the same water potentials as fine roots in wet soil. Even though roots in dry soil equilibrated with roots in wet soil, the equilibrium point reached before sunrise was still approximately –1.2 MPa, indicating that the tissues were not fully rehydrated, i.e. not fully in equilibrium with the soil water potential in the wettest portion of the rooted soil. Even with an additional hour and a half of no transpiration, water potentials for all root orders were predicted to range between –0.6 MPa to –0.7 MPa . Therefore, even with the additional time, roots in dry soil would not be predicted to reach values greater than –0.5 MPa, suggesting that the duration of the nocturnal period was not sufficient for roots in dry soil to be fully rehydrated. The main factors influencing water transport were the hydraulic properties of the conductive system. In very fine roots, internal water movement was probably delayed by either very high hydraulic resistances due to small diameter vessels in these roots or by additional resistances caused by the occurrence of xylem embolisms associated with severe water stress conditions . With the single-branch model, it was possible to estimate the water potential of each of the seven root orders over the night and to identify those orders with the highest hydraulic resistance. As expected, the magnitude of hydraulic resistance per individual root was highest in 1st-order roots due to these roots having the fewest number of vessels and the smallest vessel diameters.
However, it was found that in a root branch composed of seven root orders, 3rd-, 4th-, and 5th-orders , exhibited the highest overall hydraulic resistances within the root branch . Thus, when many resistances are arranged in parallel , the total resistance added to the system was not highest in the 1st- and 2nd-order roots but in the medium root orders, which had fewer individuals within the root branch . In the case of root orders greater than five, although the number of these roots was small, numerous and wider vessels helped to compensate for the limited length and number of roots . Therefore, the high hydraulic resistances exhibited by 3rd-, 4th-, and 5th-order roots may contribute considerably to delayed rehydration of the finest root orders. Interestingly, these roots represented the transition from the more permanent roots with secondary development to the more ephemeral roots without secondary development . The possibility of intermediate-order roots serving as hydraulic controllers has important implications for the function of the whole root system. The observed pattern of hydraulic resistances in Vaccinium roots is consistent with the segmentation concept proposed by Zimmermann for above-ground hydraulic architecture. Similar to the occurrence of embolism within stem junctions, which may cause the sacrifice of minor branches and leaves during severe water stress conditions, hydraulic failure in 5th- or 4th-order roots may lead to the sacrifice of the lower root orders under drought conditions, but the maintenance of higher order roots. In summary, it has been found that under severe water stress conditions the root system of V. corymbosum did not fully redistribute water from roots in wet soil to roots in dry soil. This was mainly attributed to anatomical constraints on water movement and because of the severe degree of water stress of roots in dry soil. Root orders with the highest hydraulic resistances corresponded to the lowest orders of the permanent root system , indicating the possible location of a hydraulic safety control in the root system of this species.Human-altered landscapes are expanding globally and are often associated with declining natural habitat, non-native species, fragmentation, and transformations in structure, inputs, climate, and connectivity. These changes collectively have resulted in shifts in both spatial distributions and species diversity across many taxa including birds, mammals, reptiles, amphibians, invertebrates, and plants. One common driver of global change is urbanization, which in the extreme is associated with a reduction in biodiversity compared to habitats in their more natural state. However,10 plastic plant pots in moderately urbanized areas, the effects of urban impacts on species distribution and diversity can vary greatly and depends on region, type of change, and taxonomic group, among other factors.
Documenting the effects of urbanization compared to natural communities has proven problematic, making predictions of community change associated with urbanization difficult. Human-altered landscapes are often associated with many non-native species which add to species diversity but also can obscure changes in community dynamics. Thus, to assess accurately the complex impacts of land use change on ecological communities, one must look beyond species richness to investigate ecological processes themselves. Ecological processes are the links between organisms in a functioning ecosystem, and are critical in understanding how altered biodiversity can lead to changes in ecosystem functioning. Global environmental change has been found to have a wide variety of impacts on ecological processes in different systems. Pollinator-plant relationships in particular are found to be particularly vulnerable to land use change, resulting in decreases in interaction strength and frequency. Pollination services are crucial ecosystem processes in natural systems, but also in agricultural and urban areas. Bees provide the majority of animal-mediated pollination services on which it is estimated 87.5% of flowering plants depend. The value of pollination in agriculture is estimated at $200 billion worldwide, largely due to many foods that are essential for food security and a healthy human diet, including numerous fruits, vegetables, and nuts that require bee pollination. As urban areas expand, there has been increasing interest in urban agriculture to ensure food security and access to healthy foods for growing populations, and these systems also depend on pollination. For example, Kollin estimated that the economic value of urban fruit trees to be worth $10 million annually in San Jose, California. Despite the important role of pollinators and concerns about bee declines, there remain many uncertainties regarding the impact of land use change on pollinators. Urbanization has resulted in more interfaces with both natural and agricultural landscapes, creating new transitional zones of peri-urbanization. While there has been extensive pollinator research in agricultural and natural systems, less attention has focused on pollination in neighboring urban areas and how the changing landscape has impacted pollination. In addition, very few studies of urban areas have looked beyond changes in bee diversity to understand explicitly the effect of urbanization on pollinator-plant interactions. Here, we investigate the effect of land use change on pollinatorplant ecosystem processes. We make use of a ‘‘natural experimental design’’ in which urban, agricultural, and natural areas intersect. Bees visit flowers for both pollen and nectar resources, and floral visitation is a commonly used as an index of pollination services. However, depending on the flower, certain bee groups are much more effective pollinators than others. Thus, while visitation is important, it alone does not definitively indicate whether pollination services were received by the plant. When pollen is limited by other factors, consequences for plant fitness can include failure to set seed, production of smaller fruits, and even complete lack of reproduction. By looking at rates of bee visitation and comparing this with other measures of plant fitness, such as seed set, we can develop a more complete understanding of how shifts in bee distributions between areas that differ in land use are impacting pollination services. To study the impact of changing land use on pollinator-plant interactions, we focus on bee pollination of a widespread plant, yellow starthistle , a common weed found in natural, agricultural, and urban habitats. Using standardized observations of floral visitation and seed set measurements of yellow starthistle, we test the hypotheses that increasing urbanization decreases 1) rates of bee visitation, 2) viable seed set, and 3) the efficiency of pollination . In addition to contributing to a better understanding of how change in landscape use, particularly urbanization, affects pollination-plant interactions, the study illustrates the importance of use of neighboring lands for pollination services.Yellow starthistle has composite flowers, which are aggregations of anywhere from 20–80 florets. At each site, 12 yellow star thistle buds were randomly selected from different plants and covered with a mesh bag. Yellow starthistle blooming cycles have been described in detail in other publications. We selected buds at stage BU-4, when buds had no yellow petals exposed, but had well-developed straw-colored spines. When in full flowering, 10 bags were opened for a 4 hour period from 10 am to 2 pm, while 2 were kept closed as controls to verify that self-pollination was not occurring. At the opening and re-closing of the bags, the number of florets that had their stigmas extended were counted. Later, when flowers were fully mature , seed heads were collected, and later dissected in the lab.