Dry Extremes Research Articles

A 963-year reconstruction of summer (JJA) stream flow in the White River, Arkansas, USA, from tree-rings

An average of three baldcypress ( Taxodium distichum) ring-width chronologies was used to recon struct total summer (JJA) mean daily flow of the White River at Clarendon, Arkansas, for years 1023–1985. A quadratic transformation of the tree-ring data accounted for 68% (R2 adjusted for degrees of freedom lost) when regressed against total summer flow 1931–1985. The distribution of the quadratic reconstruction matched the gauged distribution much better than a simple linear model which only accounted for 62% of the variance. The model was validated by comparing regression estimates against independent data. Years with summer flow below the 25th percentile occur nonrandomly, i.e., they tend to cluster, in both the reconstructed and gauged data. Hydrologic regimes have apparently varied considerably in the past on annual to century time-scales, with extended dry and wet periods that exceeded anything in the modern record. The frequency of both wet and dry extremes has varied considerably over the last millennium. The eleventh through thirteenth cen turies were not analysed due to reduced replication, but the well-replicated fourteenth and twentieth centuries both have large numbers of extremes. The twentieth century appears to have more extreme low flows than the previous centuries and also to have a large number of high flows. The practical consequences for society of variation in extremes and persistence of low flows may be considerable. Climatic change or anthropogenic changes to the watershed (e.g., widespread upland clearing for agriculture and logging of bottomland forests) may be responsible for the change in hydrologic regime during the twentieth century.

Midtropospheric Circulation Patterns Associated with Extreme Dry and Wet Episodes over Equatorial Eastern Africa during the Northern Hemisphere Spring

The anomaly circulation patterns of contrasting wet and dry episodes over the equatorial eastern Africa (EEA) region are studied using the European Centre for Medium-Range Weather Forecasts analyses. The study focuses on the Northern Hemisphere spring (NHS). Examination of composite synoptic-scale features indicates that cyclonic circulation over southern Madagascar in the midtroposphere forces southerly flow through the Mozambique Channel. The southerlies turn eastward on approaching latitude 10°S giving broad westerlies in the latitude belt 10°S to the equator. These southerlies followed by westerlies were associated with wet spells over the EEA region. Dry spells exhibit stronger than normal easterly wind flow over the EEA region. These dry spells are associated with a cyclonic circulation with a center near the point 25°S, 25°E and an anticyclonic circulation over Madagascar that causes northerly winds to blow through the Mozambique Channel. Thus, the southerlies act as a trigger for active convection over EEA during the NHS by providing a surface leaning poleward with height over which the equatorial air mass rises.

Fire Regime Changes in La Michilía Biosphere Reserve, Durango, Mexico

Abstract: The ability of reserves to maintain natural ecosystem processes such as fire disturbance regimes is central to long‐term conservation. Fire‐scarred tree samples were used to reconstruct fire regimes at five study sites totaling approximately 230 ha in pine ( Pinus spp.) and oak (Quercus spp.) forests of La Michilía Biosphere Reserve on the dry east slope of the Sierra Madre Occidental, Durango, Mexico. Study sites covered a 20‐km environmental gradient of elevation, topography, and human land uses. Plant communities ranged from oak‐pine to mixed conifer forests. Fires were frequent at all sites prior to 1930, when large‐scale grazing of domestic livestock was initiated. Widespread fires have been excluded from four of the five sites since 1945, with an essentially uninterrupted regime of frequent fires continuing only in the reserve core. Xeric sites had many, smaller fires, whereas mesic sites had fewer but larger fires. On a reserve‐wide scale, a fire burned on at least one site nearly every year, usually in the dry spring or early summer season, but fire years were rarely synchronous among the sites. Fire occurrence was weakly related to the Southern Oscillation climate pattern; major reserve‐wide fire years almost never coincided with wet Southern Oscillation extremes but only occasionally matched dry extremes. Maintenance of the long‐term frequent‐fire regime in the reserve core is one indicator that the biosphere reserve model has been successful in conserving natural processes, but the protected area is small ( 7000 ha). Because of the key role of frequent‐fire regimes in regulating ecosystem structure and function, restoration of the ecological role of fire disturbance is a desirable conservation strategy.

A WATER BALANCE MODEL FOR A SUBARCTIC SEDGE FEN AND ITS APPLICATION TO CLIMATIC CHANGE

A model to calculate the water balance of a hummocky sedge fen in the northern Hudson Bay Lowland is presented. The model develops the potential latent heat flux (evaporation) as a function of net radiation and atmospheric temperature. It is about equally sensitive to a 2% change in net radiation and a 1°C change in temperature. The modelled potential evaporation agrees well with the Priestley–Taylor formulation of evaporation under conditions of a non-limiting water supply. The actual evaporative heat flux is modelled by expressing actual/potential evaporation as a function of potential accumulated water deficit. Model evaporation agrees well with energy balance calculations using 7 years of measured data including wet and dry extremes. Water deficit is defined as the depth of water below reservoir capacity. Modelled water table changes concur with measurements taken over a 4 year period. When net radiation, temperature and precipitation measurements are available the water balance can be projected to longer time periods. Over a 30 year interval (1965–1994) the water balance of the sedge fen showed the following. During the growing season, there was an increase in precipitation, no change in temperature and a decrease in net radiation, evapotranspiration and water deficit. There was also a decrease in winter snow depths. The fen was brought back to reservoir capacity during final snowmelt every year but one. Summer rainfall was the most important single factor affecting the water balance and the ratio actual/potential evaporation emerged as a linear function of rainfall amount. A 2 × CO2 climate warming scenario with an annual temperature increase of 4°C and no precipitation change indicates lesser snow amounts and a shorter snow cover period. A greater summer water deficit, triggered mainly by greater evaporation during the month of May, is partially alleviated by lesser evaporation magnitudes in July. The greater water deficit would be counterbalanced by a 23% increase in summer rainfall. On average, the fen's water reservoir would still be recharged after winter snowmelt but the ground would remain at reservoir capacity for a shorter time. The warming scenario with a 10% decline in summer rainfall would create a large increase in the longevity and severity of the water deficit and this would be particularly evident during drier years. The carbon budget and peat accumulation and breakdown rates are strongly affected by changes in the water balance. Some evidence implies that greater water deficits lead to an increase in net carbon emissions. This implies that the sedge peatland could lose biomass under such conditions. An example is given where increased water deficit results in large decreases in local wetland streamflow.

Monsoon Forests in Northwestern Australia I. Vegetation Classification and the Environmental Control of Tree Species

An indirect gradient analysis was performed on woody species basal area data collected from 144 monsoon forests situated throughout northewestern Australia. In order to eliminate edge effects each monsoon forest was differentiated from surrounding vegetation by inspection of sorted lists of tree species recorded from quadrats $(10 \times 20 m)$ placed on a transect that traversed each forest boundary. On each transect the monsoon forest quadrats were amalgamated to yield a single averaged sample of tree species basal area and environmental characteristics to represent each fo the 144 sampled forests. Surface soil physical and chemical characteristics were related to compositional patterns of tree species abundance data in one-, two-and three-dimensional Hybrid Multi-dimensional Scaling ordination solutions. The unidimensional ordination recovered a major environmental gradient where dry fertile soils, often on rocky slopes occur at one extreme and infertile waterlogged soils charcterize the other. Mean basal area of the fifty species with the highest importance values showed a continuously varying sequence of unimodal curves over the unidimensional ordination axis, although individual species distribution and abundance showed considerable variation on the gradient. Forest biomass, as measured by mean canopy height and mean total basal area, increased linearly over the gradient. With the exception of the dry extreme, stem density was relatively uniform across the gradient. Both the wet and dry extremes of the gradient have lower species richness per $200 m^2$ and fewer species with peaks of abundance. There appears to be a relatively sharp environmental and physiognomic discontinuity along the gradient as indicated by surface soil moisture content, and the cover of ferns and palm regeneration. This beak is consistent with a primary dichotomy of wet and dry forest formations as derived by other floristic and structural classifications of this vegetation. Of the common tree species (149), 53% of the dry forest species (125) only occurred in that community, while 29% of the wet forest species (83) were restricted to the wet forest.

Diets of Black-stripped wallabies in New South Wales

In both the wet and dry extremes of its remnant range in northern New South Wales, the black-striped wallaby, Macropus dorsalis, eats predominantly monocotyledonous plants, selecting leaves and, in some seasons, seedheads. The wallabies eat a wide range of monocot species, taking these from the habitat in which they shelter in the daytime as well as from that in which they forage at night. This study suggests that reserves for the conservation of the species need to include nocturnal foraging as well as diurnal sheltering habitats.

Winter Rainfall Over the Interior of South Africa During Extreme Dry Years

It is hypothesised that during extreme dry years rainfall over the interior of southern Africa will increase due to an equatorward shift of the circumpolar westerlies. Examination of extreme wet an...

Influences of pH, Temperature, and Moisture on Gaseous Tritium Uptake in Surface Soils

In South Carolina surface soils, the uptake of gaseous tritium (T(2), HT, or both) showed a broad optimal temperature response from about 20 to 50 degrees C, with the highest rates at 35 to 45 degrees C. The optimal pH was in the range of 4 to 7. Uptake rates declined at the wet and dry extremes in soil moisture content. Inhibition seen upon the addition of hydrogen or carbon monoxide to the soil atmosphere suggested that hydrogenase may be responsible for T(2)-HT uptake in soil. During the period of most rapid recovery in a 36-day incubation of reinoculated, sterilized soil, T(2)-HT uptake rates doubled every 2 to 4 days. Thus, T(2)-HT uptake appears to be biologically mediated. Soil uptake of T(2)-HT was not severely limited by pH, temperature, or moisture in the soils tested. Thus, rapid exchange of gaseous tritium into soil water must be expected and accounted for in modeling the isotope distributions around nuclear facilities.