Changes In Rainfall Amounts Research Articles

Implications of Land Use and Land Cover Changes on Hydrological Regimes of the Malagarasi River, Tanzania

A study was conducted to investigate the impacts of land use and land cover changes on hydrological regimes of the Malagarasi River. Remote sensing techniques were used to inventory land use and land cover changes in the watershed for over 18 years. Hydrological data collected from 1975 to 2002 at Mbelagule, a gauging station far downstream before Lake Tanganyika were analyzed to reveal the alterations and trends in flows for the pre-1980 and post-1980 periods after a large influx of refugees in the catchment. Results revealed that between 1984 and 2002, there was a in settlement and cultivated area steadily increased from 42.91 km 2 to 51.01 km 2 , woodland area declined from 405.48km 2 to 398.91km 2 and wetland area declined from 36.35km 2 to 19.91km 2 . River flows were found to be highly variable within and between the years, and sensitive to land use and land cover changes. There was a shift in the peaking of the river post-1980 period, with high flows attained earlier in April instead of May in the pre-1980 period. The dry season (June to October) flows for the post- 1980 period declined progressively when compared to the pre- 1980. A trend analysis on annual rainfall did not indicate a significant change in amount of rainfall received in the area, likewise for the annual discharges, but a downward trend was detected in dry season flows. The flow duration curve analysis indicated a progressive decline in flows for the post-1980 period unlike the former. The study concludes that the modification of the land use and cover has resulted in changes in temporal distribution of runoff. The study highlights the need to consider the effects of land use and land cover changes on ecosystems and water resources for an informed decision on proper planning and management of the catchment.

Changes in rainfall amount and frequency do not affect the outcome of the interaction between the shrub Retama sphaerocarpa and its neighbouring grasses in two semiarid communities

We evaluated the net outcome of the interaction between the shrub Retama sphaerocarpa, our target plant, and different herbaceous neighbours in response to changes in the magnitude and frequency of rainfall events during three years. The experiment was conducted in natural and anthropogenic grasslands dominated by a perennial stress-tolerator and ruderal annual species, respectively. In spite of the neutral or positive effects of neighbours on water availability, neighbouring plants reduced the performance of Retama juveniles, suggesting competition for resources other than water. The negative effects of grasses on the photochemical efficiency of Retama juveniles decreased with higher water availabilities or heavier irrigation pulses, depending on the grassland studied; however, these effects did not extent to the survival and growth of Retama juveniles. Our findings show the prevalence of competitive interactions among the studied plants, regardless of the water availability and its temporal pattern. These results suggest that positive interactions may not prevail under harsher conditions when shade-intolerant species are involved. This study could be used to further refine our predictions of how plant–plant interactions will respond to changes in rainfall, either natural or increased by the ongoing climatic change, in ecosystems where grass–shrubs interactions are prevalent.

What does leaf wax δD from a mixed C3/C4 vegetation region tell us?

Hydrogen isotope values (δD) of sedimentary terrestrial leaf wax such as n-alkanes or n-acids have been used to map and understand past changes in rainfall amount in the tropics because δD of precipitation is commonly assumed as the first order controlling factor of leaf wax δD. Plant functional types and their photosynthetic pathways can also affect leaf wax δD but these biological effects are rarely taken into account in paleo studies relying on this rainfall proxy. To investigate how biological effects may influence δD values we here present a 37,000-year old record of δD and stable carbon isotopes (δ13C) measured on four n-alkanes (n-C27, n-C29, n-C31, n-C33) from a marine sediment core collected off the Zambezi River mouth. Our paleo δ13C records suggest that each individual n-alkanes had different C3/C4 proportional contributions. n-C29 was mostly derived from a C3 dicots (trees, shrubs and forbs) dominant vegetation throughout the entire record. In contrast, the longer chain n-C33 and n-C31 were mostly contributed by C4 grasses during the Glacial period but shifted to a mixture of C4 grasses and C3 dicots during the Holocene. Strong correlations between δD and δ13C values of n-C33 (correlation coefficient R2=0.75, n=58) and n-C31 (R2=0.48, n=58) suggest that their δD values were strongly influenced by changes in the relative contributions of C3/C4 plant types in contrast to n-C29 (R2=0.07, n=58). Within regions with variable C3/C4 input, we conclude that δD values of n-C29 are the most reliable and unbiased indicator for past changes in rainfall, and that δD and δ13C values of n-C31 and n-C33 are sensitive to C3/C4 vegetation changes. Our results demonstrate that a robust interpretation of palaeohydrological data using n-alkane δD requires additional knowledge of regional vegetation changes from which n-alkanes are synthesized, and that the combination of δD and δ13C values of multiple n-alkanes can help to differentiate biological effects from those related to the hydrological cycle.

Miocene paleotopography of the Central Alps

Reconstructing surface elevation, surface uplift, and relief histories is fundamental to understanding the growth of mountain ranges, to explore their topographic limits, and relate these to the interplay between geodynamic and Earth surface processes. Here, we aim to quantify Miocene paleoelevation of the Central European Alps through stable isotope paleoaltimetry. The novelty of our approach, which renders our analysis less sensitive to past climate change, is that stable isotope proxies of identical age are analyzed from both the high internal parts of the Alpine orogen and the adjacent foreland basin. Whereas the foreland basin was at or near sea level and traces the combined effects of upstream changes in rainfall amount and composition, the high Alpine site additionally records the effect of elevation on stable isotopes in precipitation. We compare hydrogen isotopic ratios (δD) in mica and chlorite that interacted with meteoric water along the Simplon detachment, a major normal fault that developed at high elevations, with meteoric water compositions deduced from carbonate-bearing paleosols of the North-Alpine foreland basin.δD values of muscovite (−126‰) and chlorite (−135‰) from the brittle hanging wall and of recrystallized muscovite (−108‰) and biotite (−140‰) from adjacent footwall mylonites provide unequivocal evidence for localized syntectonic meteoric water interaction along the Simplon detachment. Detailed 40Ar/39Ar and fission track geochronology constrains the timing of isotopic exchange to ca. 14.5Ma, when the footwall mylonites passed through the ductile to brittle transition. Age-equivalent oxygen isotope ratios (δ18O) measured within pedogenic carbonate from North-Alpine foreland paleosols that developed near Miocene sea level serve as our low-elevation point of reference. δ18O values in these paleosols, dated with ca. 100ka precision, vary between +19 and +25‰ (SMOW) with average values of +20‰ at ca. 14.5Ma. Using the relative differences between meteoric water compositions in the foreland basin and the high Alpine Simplon detachment, our isotope data are consistent with a mid-Miocene minimum average elevation difference of 2350 (+700/−500) m for the Simplon region. Our results indicate that Miocene Alpine elevations were at least comparable to those of today and are likely to have acted as an important barrier to Atlantic-derived moisture transport into central Europe and Eurasia since that time.

Short‐distance partial migration of Neotropical birds: a community‐level test of the foraging limitation hypothesis

Partial migration of tropical birds was long believed to be driven by variation in food abundance. Recent evidence from a partially‐migratory species suggests that in contrast, limited foraging opportunities at high elevations during severe wet season storms drives the most metabolically‐challenged individuals down to elevations where rainfall is lighter. Here, I test community‐level predictions of this hypothesis by examining the relationship between high‐elevation rainfall in the second half of the year and counts of migrant birds in lowland forest during late December each year from 1990–2009. I contrast results derived from analysis of all migrant species with both analyses of only the frugivorous migrants, and analyses of resident species. Counts of migrant species were on average positively associated with montane rainfall with differences of up to 72% in the numbers of birds counted in drier or wetter years. Frugivores and smaller birds responded more strongly to variation in rainfall compared to the broader migrant species pool. Interestingly, counts of resident species were also higher following wetter montane wet seasons. Results of analyses exploring the cause of resident responses were not consistent with climatic effects on breeding productivity or short‐term weather effects on detectability. Results were, however, consistent with cryptic down‐slope migration of individuals breeding at higher elevations augmenting lowland resident populations in wet years. These results suggest that changes in rainfall amount, storm intensity, and timing of severe weather events would lead to large increases in or losses of an important behaviour.

Climate change and variability in Sub-Saharan Africa: a review of current and future trends and impacts on agriculture and food security

Sub-Saharan Africa has been portrayed as the most vulnerable region to the impacts of global climate change because of its reliance on agriculture which is highly sensitive to weather and climate variables such as temperature, precipitation, and light and extreme events and low capacity for adaptation. This article reviews evidence on the scope and nature of the climate change challenge; and assesses the impact of climate change on agriculture and food security in Sub-Saharan Africa. From the review, it is apparent that the climate in Africa is already exhibiting significant changes, evident by changes in average temperature, change in amount of rainfall and patterns and the prevalence of frequency and intensity of weather extremes. The review also revealed that although uncertainties exist with regards to the magnitude of impacts, climate will negatively affect agricultural production in Sub-Saharan Africa. Specifically, as result of current and expected climate change, the area suitable for agriculture, the length of growing seasons and yield potential, particularly along the margins of semi-arid and arid areas, are expected to decrease. These impacts will affect all components of food security: food availability, food accessibility, food utilisation and food stability and hence increase the risk of hunger in the region. The review thus confirms the general consensus that Sub-Saharan Africa is the most vulnerable region to climate change. It suggests that, policymakers and development agencies should focus on formulating and implementing policies and programmes that promote farm level adaptation strategies currently being practiced by farmers across the region.

Application of flow variability analysis to identify impacts of agricultural land-use change on the River Axe, southwest England

Flow variability analysis based on the annual number and duration of pulses above flow thresholds has been used to identify potential impacts of agricultural land-use change on the River Axe catchment (288.5 km2). The analysis shows significant trend and step changes in runoff response which coincide both with changes in land use and management and with changes in rainfall amount and intensity. The effects of climatic variability are taken into account by regression analysis; residuals from regression continue to show a trend which is ascribed to land-use change. Seasonal analysis indicates that the winter season has the greatest change in runoff response but little change in rainfall. In contrast, the summer season shows little change in runoff response but altered rainfall frequency. The success of flow variability methods in identifying trends and the sources of trends in the runoff response on the Axe catchment indicates the potential of the methods for more general application for the development of flood risk management policies based on land-use and management.

Holocene variability of the East Asian summer monsoon from Chinese cave records: a re-assessment

Oxygen isotope records from stalagmites in caves in southern China, interpreted as proxy rainfall records reflecting the intensity of the East Asian summer monsoon, indicate gradual monsoon weakening for the last ~9000 years, as also documented for the Indian monsoon. Coupled with high-precision dating, the speleothem proxy records have been used to test monsoon links with orbital forcing, solar changes, iceberg discharges in the North Atlantic, ocean currents and atmospheric methane. However, these `benchmark' cave records do not match other published, dated E Asian proxy rainfall records (specifically here, independently calibrated rainfall records from loess/palaeosol magnetic properties, and cave oxygen isotope intercomparisons), which show variable E Asian monsoon intensity through the entire Holocene. The strong correlation of the cave records with the extraregional Indian monsoon record yet their mismatch with these other dated Chinese rainfall records might be reconciled if the speleothem isotope variations reflect not changes in Holocene rainfall amount but in rainfall source. Declining Holocene influence of isotopically lighter, Indian monsoon-sourced moisture over China would have resulted in increasing proportions of isotopically heavier rainfall, sourced from the more oceanic E Asian monsoon. Individual speleothems may thus regionally record Holocene changes in Indian monsoon intensity and isotopic influence. Conversely, the other Chinese proxy records described here reflect changes in rainfall amount, and thus in E Asian summer monsoon intensity. For the Holocene, the E Asian and the Indian monsoon responses to orbital forcing are likely to have differed, specifically due to E Asian internal feedbacks and the seasonal contrasts between the two monsoon systems.

Impact of changes in rainfall amounts predicted by climate-change models on decomposition in a deciduous forest

Climate-change models predict a more intense hydrological cycle, with both increased and decreased amounts of rainfall in areas covered with temperate deciduous forests. These changes could alter rates of litter decomposition, with consequences for rates of nutrient cycling in the forest ecosystem. To examine impacts of predicted changes in precipitation on the rate of decay of canopy leaves, we placed litterbags in replicated, fenced 14 m 2 low-rainfall and high-rainfall plots located under individual rainout shelters. Unfenced, open plots served as an ambient treatment. Litter in the high-rainfall and ambient plots decayed 50% and 78% faster, respectively, than litter in the low-rainfall plots. Litter in the ambient plots disappeared 20% faster than in the high-rainfall treatment, perhaps via greater leaching during heavy rainfall events. Ambient rainfall during the experiment was similar in total amount to the high-rainfall treatment, but was more variable in intensity and timing. We used litterbags of different mesh sizes to examine whether changes in rainfall might alter the impacts of major categories of the fauna on litter decay. However, we found no consistent evidence that excluding arthropods of different sizes affected litter decay rate within any of the three rainfall treatments. This research reveals that changes in rainfall predicted to occur with global climate change will likely strongly alter rates of litter decay in deciduous forests.

The potential impact of climate change on the Australian wool industry by 2030

By 2030, climate change is likely to have implications for the Australian wool industry, principally through effects on forage and water resources, land carrying capacity and sustainability, animal health, and competition with other sectors, in particular cropping. The nature and scale of these impacts will vary between the wool growing regions, depending on the manifestation of the climate change. The growth and quality of pasture and fodder crops may be affected by changes in rainfall amounts and variability as well as higher CO 2 concentrations. Water resources in many regions are projected to decrease and become more variable. Animal health is expected to be adversely affected by rising temperatures and a greater incidence and range of pests and diseases. There is likely to be greater stress on the landscape principally brought about by rainfall deficits and increased climatic variability. There is also a strong possibility of increased competition for water and land resources from other agricultural activities, particularly cropping and meat production. The combination of these effects is likely to have an impact on both wool production and quality, with reduced productivity in marginal areas, possibly increased productivity in higher rainfall regions, increases in vegetable fault and dust contamination and changes in mean fibre diameter and staple strength. National and international markets could also be affected, with reductions in demand for apparel wool fibre in response to a more temperate climate. International production and supply markets might also shift, with the wetter wool growing areas of both New Zealand and China potentially being advantaged by climate change, and the drier wool regions of these countries being disadvantaged. A preliminary qualitative scenario analysis suggests that although the wool industry will be significantly affected by climate change, as a whole it is likely to be relatively robust to it. Early adaptation, for example through efforts to produce low emission grazing systems, more sustainable management especially in the rangelands, and improved management of the effects of climate variation, could significantly reduce the downsides of climate change impacts.

Modeling response of soil erosion and runoff to changes in precipitation and cover

Global climate has changed over the past century. Precipitation amounts and intensities are increasing. In this study we investigated the response of seven soil erosion models to a few basic precipitation and vegetation related parameters using common data from one humid and one semi-arid watershed. Perturbations were made to inputs for rainfall intensities and amounts, and to ground surface cover and canopy cover. Principal results were that: soil erosion is likely to be more affected than runoff by changes in rainfall and cover, though both are likely to be significantly impacted; percent erosion and runoff will likely change more for each percent change in rainfall intensity and amount than to each percent change in either canopy or ground cover; changes in rainfall amount associated with changes in storm rainfall intensity will likely have a greater impact on runoff and erosion than simply changes in rainfall amount alone; changes in ground cover have a much greater impact on both runoff and erosion than changes in canopy cover alone. The results do not imply that future changes in rainfall will dominate over changes in land use, since land use changes can often be drastic. Given the types of precipitation changes that have occurred over the last century, and the expectations regarding changes over the next century, the results of this study suggest that there is a significant potential for climate change to increase global soil erosion rates unless offsetting conservation measures are taken.

Increased rainfall variability and reduced rainfall amount decreases soil CO2flux in a grassland ecosystem

AbstractPredicted climate changes in the US Central Plains include altered precipitation regimes with increased occurrence of growing season droughts and higher frequencies of extreme rainfall events. Changes in the amounts and timing of rainfall events will likely affect ecosystem processes, including those that control C cycling and storage. Soil carbon dioxide (CO2) flux is an important component of C cycling in terrestrial ecosystems, and is strongly influenced by climate. While many studies have assessed the influence of soil water content on soil CO2flux, few have included experimental manipulation of rainfall amounts in intact ecosystems, and we know of no studies that have explicitly addressed the influence of the timing of rainfall events. In order to determine the responses of soil CO2flux to altered rainfall timing and amounts, we manipulated rainfall inputs to plots of native tallgrass prairie (Konza Prairie, Kansas, USA) over four growing seasons (1998–2001). Specifically, we altered the amounts and/or timing of growing season rainfall in a factorial combination that included two levels of rainfall amount (100% or 70% of naturally occurring rainfall quantity) and two temporal patterns of rain events (ambient timing or a 50% increase in length of dry intervals between events). The size of individual rain events in the altered timing treatment was adjusted so that the quantity of total growing season rainfall in the ambient and altered timing treatments was the same (i.e. fewer, but larger rainfall events characterized the altered timing treatment). Seasonal mean soil CO2flux decreased by 8% under reduced rainfall amounts, by 13% under altered rainfall timing, and by 20% when both were combined (P<0.01). These changes in soil CO2flux were consistent with observed changes in plant productivity, which was also reduced by both reduced rainfall quantity and altered rainfall timing. Soil CO2flux was related to both soil temperature and soil water content in regression analyses; together they explained as much as 64% of the variability in CO2flux across dates under ambient rainfall timing, but only 38–48% of the variability under altered rainfall timing, suggesting that other factors (e.g. substrate availability, plant or microbial stress) may limit CO2flux under a climate regime that includes fewer, larger rainfall events. An analysis of the temperature sensitivity of soil CO2flux indicated that temperature had a reduced effect (lower correlation and lowerQ10values) under the reduced quantity and altered timing treatments. Recognition that changes in the timing of rainfall events may be as, or more, important than changes in rainfall amount in affecting soil CO2flux and other components of the carbon cycle highlights the complex nature of ecosystem responses to climate change in North American grasslands.

Application of MAGIC to Lake Redó (Central Pyrenees): an assessment of the effects of possible climate driven changes in atmospheric precipitation, base cation deposition, and weathering rates on lake water chemistry

The process-oriented catchment-scale model MAGIC was used to simulate water chemistry at Lake Redó, a high mountain lake in the Central Pyrenees, Spain. Data on lakewater and atmospheric deposition chemistry for the period 1984-1998 were used to calibrate the model, which was then used to reconstruct past and to provide forecasts for three hypothetical future scenarios of deposition. Forecast scenarios considered several combinations of changes in S and N deposition due to abatement strategies, and in base cation deposition due to climate-induced changes in air-mass trajectories from northern Africa. Scenario 1 assumed constant deposition of base cations at the present level plus the expected decrease in S and N deposition resulting from reduced emissions; scenario 2 (best case) assumed an increase in base cation deposition plus the same decrease in S and N deposition as in scenario 1; scenario 3 (worst case) assumed a decrease in base cation deposition plus no decrease in S and N deposition. The hindcast indicated that during the past 140-year period changes in lake water chemistry have been significant for a remote mountain catchment, although no substantial acidification has occurred. In this regard Lake Redó can be described as a "non-sensitive lake" maintaining a reference condition. The forecasts indicated changes that do not affect this status, but the trends, even if slight, were different between scenarios. A slight decline in the surface water ANC is predicted by Scenario 3. The N budget indicates an unusually low retention in the catchment, which may result in enhanced sensitivity to further increased N deposition. Some of the discrepancy between modelled and measured Ca2+ in lake water during 1984-98 could be explained by changes in rainfall amounts and by increased weathering rates due to increases in air temperature.

Site and temporal variation of soil respiration in European beech, Norway spruce, and Scots pine forests

AbstractGlobal warming and changes in rainfall amount and distribution may affect soil respiration as a major carbon flux between the biosphere and the atmosphere. The objectives of this study were to investigate the site to site and interannual variation in soil respiration of six temperate forest sites. Soil respiration was measured using closed chambers over 2 years under mature beech, spruce and pine stands at both Solling and Unterlüß, Germany, which have distinct climates and soils. Cumulative annual CO2 fluxes varied from 4.9 to 5.4 Mg C ha−1 yr−1 at Solling with silty soils and from 4.0 to 5.9 Mg C ha−1 yr−1 at Unterlüß with sandy soils. With one exception soil respiration rates were not significantly different among the six forest sites (site to site variation) and between the years within the same forest site (interannual variation). Only the respiration rate in the spruce stand at Unterlüß was significant lower than the beech stand at Unterlüß in both years. Soil respiration rates of the sandy sites at Unterlüß were limited by soil moisture during the rather dry and warm summer 1999 while soil respiration at the silty Solling site tended to increase. We found a threshold of −80 kPa at 10 cm depth below which soil respiration decreased with increasing drought. Subsequent wetting of sandy soils revealed high CO2 effluxes in the stands at Unterlüß. However, dry periods were infrequent, and our results suggest that temporal variation in soil moisture generally had little effect on annual soil respiration rates. Soil temperature at 5 cm and 10 cm depth explained 83% of the temporal variation in soil respiration using the Arrhenius function. The correlations were weaker using temperature at 0 cm (r2 = 0.63) and 2.5 cm depth (r2 = 0.81). Mean Q10 values for the range from 5 to 15 °C increased asymptotically with soil depth from 1.87 at 0 cm to 3.46 at 10 cm depth, indicating a large uncertainty in the prediction of the temperature dependency of soil respiration. Comparing the fitted Arrhenius curves for same tree species from Solling and Unterlüß revealed higher soil respiration rates for the stands at Solling than in the respective stands at Unterlüß at the same temperature. A significant positive correlation across all sites between predicted soil respiration rates at 10 °C and total phosphorus content and C‐to‐N ratio of the upper mineral soil indicate a possible effect of nutrients on soil respiration.

Diatom-inferred salinity and carbonate oxygen isotopes in Holocene waterbodies of the western Sahara and Sahel (Africa)

Thirteen Holocene palaeolakes in the western Sahara and Sahel have provided diatom records, with carbonate oxygen isotope profiles available from eight of them. Most of these palaeolakes were groundwater-fed. Lake water chemistry is reconstructed using diatom transfer functions. Lake water salinity and 18O records are assembled with some isotopic and chemical groundwater data to better understand the response of the hydrological systems to climate changes over the past 15,000 yr. Data are in general agreement with climate simulations using coupled atmosphere–ocean–vegetation models which show a mid-Holocene wetting over the whole of northwest Africa, and a rapid drying by 6–4 ka. The lake record also shows that at many sites the major lake infilling lags the end of the Younger Dryas by 1–2 ka. Regional differences also appear in the timing of the lake hydrological optimum: ca. 10.5–8.5 and 7.5–4.5 ka in the northern Sahara, 10–8.5 ka in the Aı̈r–Ténéré, 10–5.7 or 4.5 in the Sahel, and 7.5 ka in Lake Chad. The whole of the Holocene is punctuated by short-term drying events. Changes in water isotopic composition through time are partly explained by changes in rainfall amount and air humidity. During the wet Holocene period however, the very low δ values in the southern Sahara also imply changes in the moisture transport pattern or rainfall mechanisms. Data suggest an apparent decrease in 18O content of precipitation along the monsoon flow, in contrast with modern patterns. Changes in water availability and quality have driven population migrations in and out of the Sahara-Sahel, but relationships between climate and cultures are complex. Short-term dry events might have driven inventive adaptations. In the Sahara, drying at 5–4.5 ka coincides with both the collapse of the classical Neolithic civilization and the settlement of new cultures.

Effects of prolonged soil drought on CH4 oxidation in a temperate spruce forest

Our objective was to determine potential impacts of changes in rainfall amount and distribution on soil CH4 oxidation in a temperate forest ecosystem. We constructed a roof below the canopy of a 65‐year‐old Norway spruce forest (Picea abies (L.) Karst.) and simulated two climate change scenarios: (1) an extensively prolonged summer drought of 172 days followed by a rewetting period of 19 days in 1993 and (2) a less intensive summer drought of 108 days followed by a rewetting period of 33 days in 1994. CH4 oxidation, soil matric potential, and soil temperature were measured hourly to daily over a 2‐year period. The results showed that annual CH4 oxidation in the drought experiment increased by 102% for the climate change scenario 1 and by 41% for the climate change scenario 2, compared to those of the ambient plot (1.33 kg CH4 ha−1 in 1993 and 1.65 kg CH4 ha−1 in 1994). We tested the relationships between CH4 oxidation rates, water‐filled pore space (WFPS), soil matric potential, gas diffusivity, and soil temperature. Temporal variability in the CH4 oxidation rates corresponded most closely to soil matric potential. Employing soil matric potential and soil temperature, we developed a nonlinear model for estimating CH4 oxidation rates. Modeled results were in strong agreement with the measured CH4 oxidation for the ambient (r2 = 0.80) and drought plots (r2 = 0.89) over two experimental years, suggesting that soil matric potential is a highly reliable parameter for modeling CH4 oxidation rate.

A Climate Change Scenario for Carbon Dioxide and Dissolved Organic Carbon Fluxes from a Temperate Forest Soil Drought and Rewetting Effects

Our objective was to assess the effect of changes in rainfall amount and distribution on CO2 emissions and dissolved organic C (DOC) leaching. We manipulated soil moisture, using a roof constructed below the canopy of a 65‐yr‐old Norway spruce plantation [Picea abies (L.) Karst.] at Solling, Germany. We simulated two scenarios: a prolonged summer drought of 172 d followed by a rewetting period of 19 d and a shorter summer drought of 108 d followed by a rewetting period of 33 d. Soil CO2 emission, DOC, soil matric potential, and soil temperature were monitored in situ for 2 yr. On an annual basis no significant influence of the droughts on DOC leaching rates below the rhizosphere was observed. Although not significantly, the droughts tended to reduce soil respiration. Rewetting increased CO2 emissions in the first 30 d by 48% (P < 0.08) in 1993 and 144% (P < 0.01) in 1994. The CO2 flush during rewetting was highest at high soil temperatures and strongly affected the annual soil respiration rate. The annual emission rate from the drought plot was not affected by the drought and rewetting treatments in 1993 (2981 kg C ha−1 yr−1), but increased by 51% (P < 0.05) to 4813 kg C ha−1 yr−1 in 1994. Our results suggest that reduction of rainfall or changes in rainfall distribution due to climate change will affect soil CO2 emissions and possibly C storage in temperate forest ecosystems.

Coral Reefs and Environmental Change: Adaptation to What?

SYNOPSIS. The present concern about future climate change and sea-level rise due to the enhanced greenhouse effect is put in the context of past changes. Best estimates of future changes are detailed, with an explanation of methods and uncertainties. Considerable progress is being made in regard to estimates of future sealevel rise and its regional variation, and towards predicting likely changes in the behaviour of the El Nino-Southern Oscillation (ENSO) and tropical cyclones. Changes in rainfall amounts and intensity, and in extremes of surface temperature are other critical climatic variables for coral reefs. Impacts on coral reefs will result from a combination of stresses arising from several aspects of global change, including stresses due to sea-level rise, extreme temperatures, human damage (from mining, dredging, fishing and tourism), and changes in salinity and pollutant concentrations (nutrients, pesticides, herbicides and particulates), and in ocean currents, ENSO, and storm damage. These may be exacerbated by any reduction in calcification rates of corals due to changes in ocean chemistry. In view of ongoing uncertainties regarding future rates of change, especially at the local scale, impact and adaptation assessments cannot provide unequivocal answers, but rather must be couched in terms of probabilities and risk. Reef communities which are presently under stress are likely to be particularly vulnerable. Both autonomous and managed (or planned) adaptations should be considered.

Predicting watershed acidification under alternate rainfall conditions

The effect of alternate rainfall scenarios on acidification of a forested watershed subjected to chronic acidic deposition was assessed using the model of acidification of groundwater in catchments (MAGIC). The model was calibrated at the Panola Mountain Research Watershed, near Atlanta, Georgia, U.S.A. using measured soil properties, wet and dry deposition, and modeled hydrologic routing. Model forecast simulations were evaluated to compare alternate temporal averaging of rainfall inputs and variations in rainfall amount and seasonal distribution. Soil water alkalinity was predicted to decrease to substantially lower concentrations under lower rainfall compared with current or higher rainfall conditions. Soil water alkalinity was also predicted to decrease to lower levels when the majority of rainfall occurred during the growing season compared with other rainfall distributions. Changes in rainfall distribution that result in decreases in net soil water flux will temporarily delay acidification. Ultimately, however, decreased soil water flux will result in larger increases in soil-adsorbed sulfur and soil-water sulfate concentrations and decreases in alkalinity when compared to higher water flux conditions. Potential climate change resulting in significant changes in rainfall amounts, seasonal distribution of rainfall, or evapotranspiration will change net soil water flux and, consequently, will affect the dynamics of the acidification response to continued sulfate loading.