Uptake Of Rainwater Research Articles

Influence of polycyclic aromatic hydrocarbons on water storage capacity of two lichens species

Abstract The wide variability in functional traits that enable the cosmopolitan distribution of lichens often includes the water storage capacity, S, of their thallus. Lichen S in forest canopies can be large enough to intercept and evaporate significant amounts of rainwater, contributing to the runoff-reduction ecosystem services provided by urban forests; however, S is likely influenced by the presence of air pollutants (polycyclic aromatic hydrocarbons, PAHs) in urban areas. PAHs, being both chemically hydrophobic and damaging to lichen thalli, are expected to reduce lichens’ S and, thereby, limit their contribution to hydrologic ecoservices of urban forests. Hence, the relationship between PAH accumulation and rainwater uptake was examined for two lichen species, common in urban forests around the world – Platismatia glauca and Pseudevernia furfuracea. Samples were collected from an area of low air pollution and another area in a highly urbanized city centre with high air pollution exposure (Kraków, Poland). Lichen S was determined using laboratory-simulated rainfall. PAH bioaccumulation differed between species and among the samples from clean and polluted environments. After exposure to polluted air, the concentration of PAHs was higher in P. glauca than P. furfuracea. Samples from the non-urban setting, however, showed no differences between the two species. In the case of P. glauca, S decreased from 35.8% in samples from clean environment to 8.3% after six months of exposure in the urban setting. The respective S values for P. furfuracea were 25.4% and 12.4%. Results strongly suggest that PAH exposure reduces S in both lichen species. The obtained results are important both in ecohydrology and microclimatology and are part of the research on the condition of urban forests.

Rainwater Use Process of Caragana intermedia in Semi-Arid Zone, Tibetan Plateau

Summer rain is changeable in the semi-arid climate, and is the limiting factor to the survival and growth of sand-fixing vegetation, especially in Tibetan Plateau. Caragana intermedia is the dominant sand-binding shrub on sand dune in the northeast Tibetan Plateau. Stable hydrogen and oxygen isotope of xylem water, soil water, and groundwater were examined to study rainwater uptake process by C. intermedia in mid-summer. Soil water content was monitored continuously before and after a 13.9 mm rain. Contribution of each water source was analysed by MixSIAR model. The results showed surface soil water (0-10 cm) was replenished by rainwater through increasing soil water content and reducing its stable isotope. Caragana intermedia mainly used surface soil water (0-10 cm) replenished by rainwater one day after the rain, accounting for about 60% of its total water source. As water availability decreased in upper soil layers, it used soil water at deeper levels (10-150 cm), the same water source as one day before the rain. The rapid and efficient uptake of rainwater by C. intermedia reflected its response to summer rain, which is an adaptive strategy to the semi-arid environment. Therefore, C. intermedia could survive on sand dune by shallow soil water replenished from rainwater in Tibetan Plateau.

Responses of two desert shrubs to simulated rainfall pulses in an arid environment, northwestern China

Desert environments are characterized by limited and highly variable rainfall, which is an intermittent source of water critical to the evolution of the structure and functioning of desert ecosystems. The present study was the first to assess the effects of different amounts of rainfall received through discrete rainfall events and of their seasonality on two little-studied desert shrubs, namely Reaumurica soongorica and Nitria sphaerocarpa, in an arid inland river basin in north-western China. Pulse rainfall events were simulated at the following magnitudes (amount of water in millimeters per rainfall event): 0, 5, 10, 15, 25, and 40 mm on two days in 2016, 24th June (early growing season) and 4th August (middle growing season), respectively. The rainfall effects were measured in terms of the following parameters: plant uptake of rainwater by plants as assessed by δ18O in the water within plant tissue, pre-dawn plant water potential, and the rates of photosynthesis and stomatal conductance. N. sphaerocarpa shows faster response to simulated pulse rainfall events than R. soongorica, and the response of both species was quicker in the middle- than in early- growing season. Water uptake increased non-linearly with the amounts of rainfall, reaching a plateau at about 25 mm in R. soongorica and 20 mm in N. sphaerocarpa. The accumulative carbon gain relative to the controlled experiment increased with the amounts of rainfall, the increase being greater in the middle growing season. Our results clearly show that both the seasonality and the amounts of rainfall in discrete rainfall events took great impacts on response of plant ecophysiological activities and such responses are species dependent. The challenge ahead is to understand the consequences of long-term variability in rainfall for the physiology of desert plants and species dynamics in desert ecosystems.

Direct uptake of canopy rainwater causes turgor-driven growth spurts in the mangrove Avicennia marina.

Mangrove forests depend on a dense structure of sufficiently large trees to fulfil their essential functions as providers of food and wood for animals and people, CO2 sinks and protection from storms. Growth of these forests is known to be dependent on the salinity of soil water, but the influence of foliar uptake of rainwater as a freshwater source, additional to soil water, has hardly been investigated. Under field conditions in Australia, stem diameter variation, sap flow and stem water potential of the grey mangrove (Avicennia marina (Forssk.) Vierh.) were simultaneously measured during alternating dry and rainy periods. We found that sap flow in A. marina was reversed, from canopy to roots, during and shortly after rainfall events. Simultaneously, stem diameters rapidly increased with growth rates up to 70 μm h-1, which is about 25-75 times the normal growth rate reported in temperate trees. A mechanistic tree model was applied to provide evidence that A. marina trees take up water through their leaves, and that this water contributes to turgor-driven stem growth. Our results indicate that direct uptake of freshwater by the canopy during rainfall supports mangrove tree growth and serve as a call to consider this water uptake pathway if we aspire to correctly assess influences of changing rainfall patterns on mangrove tree growth.

Robust moisture reference year methodology for hygrothermal simulations

Hygrothermal models allow designers to evaluate the hygrothermal performance of building envelopes. A key question in hygrothermal modeling however remains in the proper selection of representative exterior moisture conditions, i.e. a moisture reference year. A Climatic Index combining wind-driven rain load and potential evaporation is developed allowing considering the effect of climatic variability on the performance of the building envelope. The proposed Climatic Index combines thus both the wetting load and evaporation potential. This index is suitable for the evaluation of the level of moisture damage risk of wall assemblies where typical moisture problems are mainly caused by rainwater uptake or ingress.The Climatic Index is determined for four cities in Switzerland. The hygrothermal performance of the assemblies is simulated and evaluated using a hygrothermal indicator, called the RHT Index. A clear correlation between Climatic Index and RHT Index is found. However, the moisture reference year selected based solely on a 10% level criterion of the Climatic Index may not correspond to the year with the most significant moisture damage risk. Therefore, a new procedure is proposed which combines a first selection of three years around the 10% level criterion, followed by a careful comparison of these years based on hygrothermal simulations and selection of the year with the largest RHT Index as moisture reference year. The combination of Climatic Index and RHT Index from hygrothermal simulations allows for the selection of moisture reference years that are around the 10% level, but excluding the ambiguity.

High-resolution isotope measurements resolve rapid ecohydrological dynamics at the soil-plant interface.

Plants rely primarily on rainfall infiltrating their root zones - a supply that is inherently variable, and fluctuations are predicted to increase on most of the Earth's surface. Yet, interrelationships between water availability and plant use on short timescales are difficult to quantify and remain poorly understood. To overcome previous methodological limitations, we coupled high-resolution in situ observations of stable isotopes in soil and transpiration water. We applied the approach along with Bayesian mixing modeling to track the fate of (2) H-labeled rain pulses following drought through soil and plants of deciduous tree ecosystems. We resolve how rainwater infiltrates the root zones in a nonequilibrium process and show that tree species differ in their ability to quickly acquire the newly available source. Sessile oak (Quercus petraea) adjusted root uptake to vertical water availability patterns under drought, but readjustment toward the rewetted topsoil was delayed. By contrast, European beech (Fagus sylvatica) readily utilized water from all soil depths independent of water depletion, enabling faster uptake of rainwater. Our results demonstrate that species-specific plasticity and responses to water supply fluctuations on short timescales can now be identified and must be considered to predict vegetation functional dynamics and water cycling under current and future climatic conditions.

The water relations of two evergreen tree species in a karst savanna

The ecohydrology of karst has not received much attention, despite the disproportionally large contribution of karst aquifers to freshwater supplies. Karst savannas, like many savannas elsewhere, are encroached by woody plants, with possibly negative consequences on aquifer recharge. However, the role of savanna tree species in hydrological processes remains unclear, not least because the location and water absorption zones of tree roots in the spatially complex subsurface strata are unknown. This study examined the water sources and water relations of two savanna trees, Quercus fusiformis (Small) and Juniperus ashei (Buchholz) in the karst region of the eastern Edwards Plateau, Texas (USA). Stable isotope analysis of stem water revealed that both species took up evaporatively enriched water during the warm season, suggesting a relatively shallow water source in the epikarst, the transition zone between soil and bedrock. Q. fusiformis had consistently higher predawn water potentials than J. ashei during drought, and thus was probably deeper-rooted and less capable of maintaining gas exchange at low water potentials. Although the water potential of both species recovered after drought-breaking spring and summer rain events, associated shifts in stem water isotope ratios did not indicate significant uptake of rainwater from the shallow soil. A hypothesis is developed to explain this phenomenon invoking a piston-flow mechanism that pushes water stored in macropores into the active root zones of the trees. Epikarst structure varied greatly with parent material and topography, and had strong effects on seasonal fluctuations in plant water status. The study suggests that tree species of the Edwards Plateau do not commonly reduce aquifer recharge by tapping directly into perched water tables, but more likely by reducing water storage in the epikarst. A more general conclusion is that models of savanna water relations based on Walter's two-layer model may not apply unequivocally to karst savannas.

Summer rain pulse size and rainwater uptake by three dominant desert plants in a desertified grassland ecosystem in northwestern China

To examine the different effects of rain pulse size on uptake of summer rains by three dominant desert plants in field conditions of desertified grasslands on the Ordos Plateau of northwestern China, we studied relationships between precipitation event size and rainwater uptake using stable isotopes of hydrogen in plant and soil water. Four natural precipitation events that represented precipitation sizes of 5.3, 8.3, 13.3, and 65.3 mm in the summer were chosen for the experiment. The perennial grass Stipa bungeana, the shrub Artemisia ordosia, and the herb Cynanchum komarovii – the dominant species in the communities – were compared for their use of summer rains with different pulse sizes based on the changes in the hydrogen isotope ratios (dD) of their stem water 7 days following each natural rain event. We found that S. bungeana and C. komarovii took advantage of shallow water sources derived from small ( 65 mm) rain events, and C. komarovii relied primarily on rain events of intermediate (10 –20 mm) size. These different responses to rain pulses among species suggested that more frequent small rain events will promote the dominance of S. bungeana and C. komarovii, medium-sized events will facilitate development of C. komarovii, and large events will advance A. ordosia in this community. The rainwater utilization patterns of the three species would allow the coexistence of S. bungeana and A. ordosia or the coexistence of A. ordosia and C. komorovii in various successional serals following the disturbances. With an increase in variability of summer rain pulse size as predicted by climate change models, we expect that the structure of this community will undergo significant change in the future. Altered precipitation regimes, especially in combination with anthropogenic-related disturbances such as over-grazing, are likely to accelerate rates of degradation in northwestern China.

Dominant cold desert plants do not partition warm season precipitation by event size.

We conducted experiments to examine the quantitative relationships between rainfall event size and rainwater uptake and use by four common native plant species of the Colorado Plateau, including two perennial grasses, Hilaria jamesii (C(4)) and Oryzopsis hymenoides (C(3)), and two shrubs, Ceratoides lanata (C(3)), and Gutierrezia sarothrae (C(3)). Specifically, we tested the hypothesis that grasses use small rainfall events more efficiently than shrubs and lose this advantage when events are large. Rainfall events between 2 and 20 mm were simulated in spring and summer by applying pulses of deuterium-labeled irrigation water. Afterwards, pulse water fractions in stems and the rates of leaf gas exchange were monitored for 9 days. Cumulative pulse water uptake over this interval (estimated by integrating the product of pulse fraction in stem water and daytime transpiration rate over time) was approximately linearly related to the amount of pulse water added to the ground in all four species. Across species, consistently more pulse water was taken up in summer than in spring. Relative to their leaf areas, the two grass species took up more pulse water than the two shrub species, across all event sizes and in both seasons, thus refuting the initial hypothesis. In spring, pulse water uptake did not significantly increase photosynthetic rates and in summer, pulse water uptake had similar, but relatively small effects on the photosynthetic rates of the three C(3) plants, and a larger effect on the C(4) plant H. jamesii. Based on these data, we introduce an alternative hypothesis for the responses of plant functional types to rainfall events of different sizes, building on cost-benefit considerations for active physiological responses to sudden, unpredictable changes in water availability.

Assessing the influence of shrubs and their interspaces on enhancing infiltration in an arid Australian shrubland.

In assessments of rangeland condition, the fraction of the soil surface lying beneath the canopy of a plant has been widely adopted as a measure of soil protection afforded by the plant, and of the size of the area of altered micro-environment associated with it. In this local environment, various factors produce more permeable soils that allow faster uptake of rainwater than occurs through soils in the open interspaces between plants, or in degraded rangeland where plant canopy cover has been diminished. However, the nature and location of the transition from 'shrub' to 'interspace' hydrologic behaviour has remained illdefined, and the appropriateness of this two-phase subdivision of shrublands remains to be established. This paper reports on a study designed to begin this process by assessing the size of the 'zone of influence' exerted by bluebush shrubs (Maireana spp.) on water infiltration rates in their vicinity, using a test site at Fowlers Gap, north of Broken Hill, NSW. Using radial transects of infiltrometer data extending from close to the stem into the open interspace, it is shown that infiltration rates do not change abruptly at the canopy margin. Rather, enhanced water uptake persists for some distance into the interspace. According to present results, the plant canopy only covers the central one-third of this larger zone of influence, and so provides an inadequate measure of it. Measures of the persistence in time and lateral extent of the zone of influence may provide more meaningful and more sensitive measures of rangeland condition in the arid and semi-arid shrublands.