Howard Springs Research Articles

Documenting improvement in leaf area index estimates from MODIS using hemispherical photos for Australian savannas

This paper compares estimates of Leaf Area Index (LAI) obtained from the MODIS (Moderate Resolution Imaging Spectroradiometer) collections 4.8 (MC4) and 5.0 (MC5) with ground-based measurements taken along a 900km north–south transect through savanna in the Northern Territory, Australia. There was excellent agreement for both the magnitude and timing in the annual variation in LAI from MC5 and biometric estimates at Howard Springs, near Darwin, whereas MC4 overestimated LAI by 1–2m2m−2 for the first 200 days of the year. Estimates of LAI from MC5 were also compared with those obtained from the analysis of digital hemispherical photographs taken during the dry season (September 2008) based on algorithms that included random and clumped distribution of leaves. Linear regression of LAI from MC5 versus that using the clumping algorithm yielded a slope close to 1 (m=0.98). The regression based on a random distribution of leaves yielded a slope significantly different from 1 (m=1.37), with higher Mean Absolute Error (MAE) and bias compared to the clumped analysis. The intercept for either analysis was not significantly different from zero but inclusion of five additional sites that were visually bare or without green vegetation produced a statistically significant offset of +0.16m2m−2 by MC5. Overall, our results show considerable improvement of MC5 over MC4 LAI and good agreement between MC5 and ground-based LAI estimates from hemispherical photos incorporating clumping of leaves.

The effect of weaver ants Oecophylla smaragdina on the shoot borer Hypsipyla robusta on African mahoganies in Australia

1 African mahogany Khaya senegalensis is a high-value timber tree species widely grown in central Africa, south-east Asia and northern Australia. Pilot plantings show that the tree grows well in the wet-dry tropical areas of northern Australia, and the shoot borer Hypsipyla robusta (Lepidoptera: Pyralidae) is a potential pest of the tree. The weaver ant Oecophylla smaragdina is an efficient biocontrol agent in some horticulture crops. To investigate whether the ants control shoot borers, field experiments were conducted at two sites near Darwin, Australia from April 2006 to January 2009. 2 In the weaver ant treatments, the overall percentage of trees damaged by shoot borers was 0–2.7% at Berrimah Farm and 0–4.2% at Howard Springs, and the damaged trees were attacked once only. In the treatments without weaver ants, however, 9.9–52.1% trees were damaged at Berrimah Farm, and 6.3–64.6% at Howard Springs, and the damaged trees were generally attacked more than once. 3 At both sites, significantly fewer trees on each monitoring occasion were damaged in weaver ant treatments than in treatments without weaver ants. 4 The mean percentage of overall flushing shoots damaged by the pest at both sites was significantly lower in weaver ant treatments compared with treatments without weaver ants. 5 Fewer shoots were damaged per damaged tree in weaver ant treatments compared with treatments without weaver ants. 6 The data obtained suggest that weaver ants were effective biological control agents of the shoot borer, and that the ants can be used to manage the pest on African mahogany trees.

Weaver ants, Oecophylla smaragdina (Hymenoptera: Formicidae), as biocontrol agents on African mahogany trees, Khaya senegalensis (Sapindales: Meliaceae), in the Northern Territory of Australia

African mahogany, Khaya senegalensis, is a high quality timber tree that grows well in the wet-dry tropical areas of Australia. Most trees grown in the latter regions are branched at lower levels on the trunk (a symptom known as ‘low-branching’), which limits timber production per tree. Inferring that it may be caused by herbivorous insects, we sought to establish whether low-branching can be reduced by the presence of predatory weaver ants, Oecophylla smaragdina. Two field experiments on young African mahoganies were conducted at two sites in the Darwin area of Australia from 2006 to 2008. Each experiment had two treatments: trees with weaver ants and trees without the ants. Pest damage was found to be the most important correlate of the multiple branching, and weaver ants were effective in limiting the formation of multiple leaders. In 3-year-old mahoganies at Berrimah Farm, the percentage of trees with pest-caused multiple leaders was 4.8% in the weaver ant treatments (WAT) but 45.5% in the treatments without the ants (TWWA). In 2-year-old mahogany coppices at Howard Springs, no trees developed multiple leaders in WAT, but 22% developed such leaders in TWWA. The average pest damage on flushing shoots at both sites was significantly lower in WAT (1.8–2.5%) than in TWWA (22.9–24.9%).

Aerosols and their influence on radiation partitioning and savanna productivity in northern Australia

Aerosols have been shown to affect the quantity and quality of solar radiation on the Earth’s surface. Savanna regions are subject to frequent burning and release of aerosols that may impact on radiation components and possibly vegetation productivity in this region. Therefore, in this study, we have analyzed the optical properties of aerosols (aerosol optical depth (AOD) and Angstrom coefficient) from the Atmospheric Radiation Measurement site in Darwin for the periods from April 2002 to June 2005 as measured by a multifilter rotating shadowband radiometer. The influence of aerosols and their effect on surface shortwave incoming solar radiation and savanna productivity were examined for the dry season using sky radiation collection of radiometers and eddy covariance measurements from the Howard Springs flux site. Results indicated that aerosol concentrations in the region were relatively low compared to other savanna regions with the maximum monthly average AOD over the period being the greatest in October (0.29 ± 0.003 standard error at 500 nm). The highest monthly average Angstrom exponent was also found in October (1.38 ± 0.008). The relatively low aerosol concentration in this region can be attributed to the mixture of smoke aerosols with humidity haze and local circulations. Over a range of AODs from 0.1 to 0.4, we found a modest increase in the fraction of diffuse radiation to total radiation from 11% to 21%. This small increase in diffuse fraction did not affect the carbon flux significantly. However, because the current range of AOD in the region is relatively low, the region could be sensitive to increases in aerosols and diffuse fraction in the future.

An optimality‐based model of the dynamic feedbacks between natural vegetation and the water balance

The hypothesis that vegetation adapts optimally to its environment gives rise to a novel framework for modeling the interactions between vegetation dynamics and the catchment water balance that does not rely on prior knowledge about the vegetation at a particular site. We present a new model based on this framework that includes a multilayered physically based catchment water balance model and an ecophysiological gas exchange and photosynthesis model. The model uses optimization algorithms to find those static and dynamic vegetation properties that would maximize the net carbon profit under given environmental conditions. The model was tested at a savanna site near Howard Springs (Northern Territory, Australia) by comparing the modeled fluxes and vegetation properties with long‐term observations at the site. The results suggest that optimality may be a useful way of approaching the prediction and estimation of vegetation cover, rooting depth, and fluxes such as transpiration and CO2 assimilation in ungauged basins without model calibration.

The dendrochronology of Callitris intratropica in northern Australia: annual ring structure, chronology development and climate correlations

In this study we demonstrate the significant dendrochronological potential of Callitris intratropica, a native conifer distributed across much of the seasonal tropics of northern Australia. We developed two rigorously cross-dated chronologies from sites in the Northern Territory, Australia. The first chronology (1965–2004) was developed from plantation C. intratropica of known-age at Howard Springs and was heavily replicated both within and among trees to evaluate the quality of cross-dating for the species. The second chronology (1847–2006) was developed from trees growing naturally near Pine Creek and compared with long-term instrumental climate records to assess the potential for dendroclimatic reconstructions. Both chronologies had mean interseries correlations >0.65, the highest reported for a tropical tree species. The Howard Springs chronology demonstrated that C. intratropica produces anatomically distinct annual growth rings with the consistent ring-width variation within and among trees necessary for rigorous dendrochronological studies. The Pine Creek chronology exhibited statistically significant correlations with rainfall and the Palmer Drought Severity Index (PDSI; integrated measures of plant water stress), and to a lesser extent temperature. The strongest correlations between the climate data and tree-ring width indices were for early monsoon rainfall (October–December; Pearson’s r = 0.53) and late monsoon PDSI (May; Pearson’s r = 0.41). Our study demonstrates the significant potential of high-quality dendrochronological research on mainland Australia in general, and of C. intratropica, specifically, to reconstruct historical variation of the Australian monsoon system. In addition, C. intratropica may be able to provide novel insights into the dynamics of forests in the seasonal tropics of northern Australia and the role of disturbances, such as fire and cyclones, on these ecosystems.

Local boundary-layer development over burnt and unburnt tropical savanna: an observational study

Fire scars have the ability to radically alter the surface energy budget within a tropical savanna by reducing surface albedo, increasing available energy for partitioning into sensible and latent heat fluxes and increasing substrate heat flux. These changes have the potential to alter boundary-layer conditions and ultimately feedback to local and regional climate. We measured radiative and energy fluxes over burnt and unburnt tropical savanna near Howard Springs, Darwin, Australia. At the burnt site a low to moderate intensity fire, ranging between 1,000 and 3,500kW m −1 , initially affected the land surface by removing all understorey vegetation, charring and blackening the ground surface, scorching the overstorey canopy and reducing the albedo. A reduction in latent heat fluxes to almost zero was seen immediately after the fire when the canopy was scorched. This was then followed by an increase in the sensible heat flux and a large increase in the ground heat flux over the burnt surface. Tethered balloon measurements showed that, despite the presence of pre-monsoonal rain events occurring during the measurement period, the lower boundary layer over the burnt site was up to 2 ◦ C warmer than that over the unburnt site. This increase in boundary-layer heating when applied to fire scars at the landscape scale can have the ability to form or alter local mesoscale circulations and ultimately create a feedback to regional heating and precipitation patterns that may affect larger-scale processes such as the Australian monsoon.

Savanna fires and their impact on net ecosystem productivity in North Australia

AbstractSavannas comprise a large area of the global land surface and are subject to frequent disturbance through fire. The role of fire as one of the primary natural carbon cycling mechanisms is a key issue in considering global change feedbacks. The savannas of Northern Australia burn regularly and we aimed to determine their annual net ecosystem productivity (NEP) and the impact of fire on productivity. We established a long‐term eddy covariance flux tower at Howard Springs, Australia and present here 5 years of data from 2001 to 2005. Fire has direct impacts through emissions but also has indirect effects through the loss of productivity due to reduced functional leaf area index and the carbon costs of rebuilding the canopy. The impact of fire on the canopy latent energy exchange was evident for 40 days while the canopy was rebuilt; however, the carbon balance took approximately 70 days to recover. The annual fire free NEP at Howard Springs was estimated at −4.3 t C ha−1 yr−1 with a range of −3.5 to −5.1 t C ha−1 yr−1 across years. We calculated the average annual indirect fire effect as +0.7 t C ha−1 yr−1 using a neural network model approach and estimated average emissions of fine and coarse fuels as +1.6 t C ha−1 yr‐1. This allowed us to calculate a net biome production of −2.0 t C ha−1 yr‐1. We then partitioned this remaining sink and suggest that most of this can be accounted for by woody increment (1.2 t C ha−1 yr‐1) and shrub encroachment (0.5 t C ha−1 yr‐1). Given the consistent sink at this site, even under an almost annual fire regime, there may be management options to increase carbon sequestration by reducing fire frequency.

Integrated pest management in mango orchards in the Northern Territory Australia, using the weaver ant, Oecophylla smaragdina , (Hymenoptera: Formicidae) as a key element

Mango is the most important commercial fruit crop in the Northern Territory, Australia. Growers currently rely on insecticides to control insect pests of mango, resulting in environmental and social problems. To reduce dependency on insecticides, a suitable IPM programme is needed. Previous work showed that weaver ants can control the major mango insect pests, but they protect soft scales, damage fruits by their formic acid and annoy operatives during fruit harvest. Further research addressing these constraints showed that certain chemicals can reduce soft scale numbers without seriously affecting weaver ant populations, the isolation of ant colonies reduces fruit damage by formic acid, and water spray reduces the ant activity during harvest. A field experiment with two treatments, weaver ants plus soft chemicals (WPS) and weaver ants plus chemical insecticides (CI), was conducted at Howard Springs in the Northern Territory. In two out of the three years, the yield of first class fruit was 20% higher in WPS than in CI. This was mostly explained by lower insect pest damage, lower incidence of mango scab disease and lower infestation of lenticels in WPS. Trees in WPS produced significantly more fruits than in CI in 2002. Overall, trees in WPS resulted in a profit of A$14.50/tree per year, but trees in CI produced only A$8.38/tree per year. During harvest, farmers experienced only minor problems with ant disturbance. An IPM model using weaver ants as a key element is discussed with respect to ‘organic’ production.

The utility of the eddy covariance techniques as a tool in carbon accounting: tropical savanna as a case study

Global concern over rising atmospheric CO2 concentrations has led to a proliferation of studies of carbon cycling in terrestrial ecosystems. Associated with this has been widespread adoption of the eddy covariance method to provide direct estimates of mass and energy exchange between vegetation surfaces and the atmosphere. With the eddy covariance method, fast-response instruments (10–20Hz) are typically mounted above plant canopies and the fluxes are calculated by correlating turbulent fluctuations in vertical velocity with fluctuations in various scalars such as CO2, water vapour and temperature. These techniques allow the direct and non-destructive measurement of the net exchange of CO2 owing to uptake via photosynthesis and loss owing to respiration, evapotranspiration and sensible heat. Eddy covariance measurements have a high temporal resolution, with fluxes typically calculated at 30-min intervals and can provide daily, monthly or annual estimates of carbon uptake or loss from ecosystems. Such measurements provide a bridge between ‘bottom-up’ (e.g. leaf, soil and whole plant measures of carbon fluxes) and ‘top-down’ approaches (e.g. satellite remote sensing, air sampling networks, inverse numerical methods) to understanding carbon cycling. Eddy covariance data also provide key measurements to calibrate and validate canopy- and regional-scale carbon balance models. Limitations of the method include high establishment costs, site requirements of flat and relatively uniform vegetation and problems estimating fluxes accurately at low wind speeds. Advantages include spatial averaging over 10–100ha and near-continuous measurements. The utility of the method is illustrated in current flux studies at ideal sites in northern Australia. Flux measurements spanning 3years have been made at a mesic savanna site (Howard Springs, Northern Territory) and semi-arid savanna (Virginia Park, northern Queensland). Patterns of CO2 and water vapour exchange at diurnal, seasonal and annual scales are detailed. Carbon dynamics at these sites are significantly different and reflect differences in climate and land management (impacts of frequent fire and grazing). Such studies illustrate the utility of the eddy covariance method and its potential to provide accurate data for carbon accounting purposes. If full carbon accounting is implemented, for ideal sites, the eddy covariance method provides annual estimates of carbon sink strength accurate to within 10%. The impact of land-use change on carbon sink strength could be monitored on a long-term basis and provide a valuable validation tool if carbon trading schemes were implemented.

Fire impacts on surface heat, moisture and carbon fluxes from a tropical savanna in northern Australia

Savannas form a large fraction of the total tropical vegetation and are extremely fire prone. We measured radiative, energy and carbon exchanges over unburned and burned (both before and after low and moderate intensity fires) open forest savanna at Howard Springs, Darwin, Australia. Fire affected the radiative balance immediately following fire through the consumption of the grass-dominated understorey and blackening of the surface. Albedo was halved following fire of both intensities (from 0.12 to 0.07 and from 0.11 to 0.06 for the moderate and low intensity sites, respectively), but the recovery of albedo was dependent on the initial fire intensity. The low intensity fire caused little canopy damage with little impact on the surface energy balance and only a slight increase in Bowen ratio. However the moderate fire resulted in a comprehensive canopy scorch and almost complete leaf drop in the weeks following fire. The shutdown of most leaves within the canopy reduced transpiration and altered energy partitioning. Leaf death and shedding also resulted in a cessation of ecosystem carbon uptake and the savanna turned from a sink to a source of carbon to the atmosphere because of the continued ecosystem respiration. Post-fire, the Bowen ratio increased greatly due to large increases in sensible heat fluxes. These changes in surface energy exchange following fire, when applied at the landscape scale, may have impacts on climate through local changes in circulation patterns and changes in regional heating, precipitation and monsoon circulation.

Impact of an introduced ant on native rain forest invertebrates: Pheidole megacephala in monsoonal Australia.

Pheidole megacephala is an exotic ant species that has severely affected native invertebrate biodiversity throughout the tropics. Its impacts have been documented extensively in relatively depauperate invertebrate communities, but not in species-rich habitats such as tropical rain forests. Here we describe the local distribution of P. megacephala and its impacts on native invertebrate assemblages in and around a rain forest patch at Howard Springs, in Australia's monsoonal tropics. P. megacephala was found to be confined to a single area of approximately 25 ha, with its distribution centered on drainage lines and the rain forest. Significant but weak correlations were found between its abundance and vegetative canopy cover (positive) and distance from the rain forest (negative). In the most heavily infested area within the rain forest, the abundance of P. megacephala was 37-110 times that of total native ant abundance found within uninfested plots, as measured by pitfall traps. The abundance and richness of native ants and other invertebrates were significantly reduced in litter samples, pitfall catches and foliage beats where P. megacephala was present, inversely relative to the abundance of P. megacephala. Only two individuals of a single native ant species were found within the most infested plot, with native ant richness being reduced to about half in the least infested plot. The most persistent functional groups of native ants in infested plots were Cryptic species, which forage primarily within soil and leaf litter, and Opportunists, which exhibit highly generalised foraging behaviour. The highest abundance of P. megacephala corresponded with a 42-85% decrease in the abundance of other native invertebrates. Insect larvae were totally absent from foliage beats collected at the most heavily infested plot. P. megacephala was found overall to be expanding its range, averaging 12 m range expansion in the dry season and contracting 7 m in the wet season. It is able to spread into surrounding savanna habitats by occupying relatively sheltered microsites, such as beneath logs and at the bases of trees. However, it is unlikely to attain high population densities in open savanna habitats because of its relative intolerance of desiccation, and the prevalence of behaviourally dominant native ant species. Howard Springs is currently the only rain forest patch in monsoonal Australia known to be infested by P. megacephala, but clearly this ant is a serious potential threat to the region's rain forest invertebrate fauna.

How many queens are there in mature colonies of the green ant, Oecophylla smaragdina (Fabricius)?

AbstractIn an ongoing study of the green ant, Oecophylla smaragdina (Fabricius), to control insect pests, we surveyed newly established ant colonies in a 3‐year‐old mango block in March 1997 at Howard Springs, Northern Territory. We also surveyed 12 established ant colonies from May to July 1997 in Darwin, and six of them were surveyed again in February 1998. the aims were to determine patterns of initial colonisation by dealate queens of O. smaragdina and to locate the queen ant nest in established colonies of the ant. the number of dealate queens in new communal nests varied from one to 15. In a total of 73 nests, 71% of nests contained more than one dealate queen and 12% of nests contained segments or corpses of dealate queens, which were either parasitised by a fungus or killed by predators. This confirms that pleometrosis during the initial establishment of a colony is very common in O. smaragdina, and suggests that natural enemies affect the success of initial colonisation. of 12 established green ant colonies, 10 colonies with two to six queens were all in one nest, and only two colonies had one queen. In those colonies with multiple queens, we found that each queen was almost equally attractive to the major workers. Contrary to previous reports, these data suggest that pleometrosis in O. smaragdina during the initial period of colony establishment can lead to polygyny in mature colonies. This is the first time that polygyny has been documented in mature colonies of O. smaragdina.

The Big-headed Ant Pheidole megacephala: a new threat to monsoonal northwestern Australia

The Big-headed Ant Pheidole megacephala is a major threat to native invertebrate assemblages and to agricultural production world-wide. This paper reviews its known biology including its foraging ecology, colony founding and dispersal behaviour. A case study is presented to illustrate its potential conservation significance for northern Australia. At Howard Springs Nature Park in the Darwin region of the Northern Territory, an infestation of P. megacephala was found to cover 25 ha and is continuing to spread, with its distribution centred on a rainforest patch. The abundance of P. megacephala within the rainforest was 37?110 times that of total native ant abundance at uninfested sites. Only two individuals of a single native ant species were found in the highest abundance of P. megacephala and abundance of other invertebrates was only 15% of natural levels. Pheidole megacephala is a serious potential threat to native biodiversity in monsoonal Australia. Successful eradication on a large scale is a realistic option and control methods are discussed, including chemicals and fire.

Lead shot ingestion and lead poisoning of magpie geese anseranas semipalmata foraging in a Northern Australian hunting reserve

Sediments at Howards Springs Hunting Reserve in tropical Northern Territory have accumulated high densities of lead shot (330 000 shot ha −1), most of which is at depths (<20 cm) accessible to foraging magpie geese Anseranas semipalmata. Mean shot density is approximately 9% of density of sedge Eleocharis dulcis tubers, a favoured dry-season food which overlaps with shot in size. Large quantities of grit are available on the fringes of the swamp, but densities of the larger, shot-sized items (> 2 mm) that appear to be preferentially ingested or retained in gizzards are sometimes absent in areas where magpie geese forage. Lead shot are frequently ingested. Over two years 21·4% of a sample of mobile birds ( n = 103 ) had ingested shot in their gizzards. In 1989, 45·2% (n = 65) showed lead concentrations in their livers, indicating lead exposure. However, few birds had accumulated high concentrations of lead in wingbones, suggesting that repeated sublethal exposures are uncommon. We suggest that the combination of high shot ingestion rates and limited recurrence of exposure implies high mortalities from lead toxicosis. This conclusion is supported by the presence of substantial numbers of incapacitated lead-poisoned birds at Howard Springs and other wetlands in the Darwin region. If criteria developed in the USA were applied, these observations would justify immediate prohibition of the use of lead shot at Howard Springs. We discuss steps to inhibit exposure of waterbirds to existing lead shot deposits following a conversion to steel shot.

Some Manchester Chroniclers

MANCHESTER, like most of our provincial cities, is undergoing a transformation as its grimy Victorian buildings crumble beneath the demolition squads. In their place rise faceless blocks of offices and shops, whose glass and concrete soon lose their early gleam and smoothness and become streaked and shabby. They do not mature with the same charm as the brickwork and the carved and moulded stonework of the buildings they replace, for in spite of its reputation for ugliness and the accepted view of a drab city of umbrella‐carrying citizens and shawl‐clad and clog‐footed millworkers, there was an aura of romance about the older Manchester, something that prompted a man who was not a native of the city, Howard Spring, to write:

CHROMOSOMES OF CRUCIFERAE I. DESCURAINIA

As UNDERSTOOD by Schulz (1924), Descurainia Webb & Berthelot is a natural assemblage of 43 species of annual and biennial cruciferous plants. Dething (1939) revised North American species of genus. Interpreting species broadly on morphologic, ecologic, and geographic evidence, he regards the subspecies as product of selective action of a particular environment and, accordingly, recognized for Descurainia North America 9 native and I adventive species. (Fernald [1940] claims that Detling, at least some instances, incorrectly used term subspecies in sense of long-established term varietas.) Southwestern United States is distributional center of native representatives of genus. Dr. LeRov E. Detling sent seeds collected from wild plants of four species and ten subspecies of Descurainia. Plants from those seeds were grown Botanical Gardens of University of Michigan. Chromosomes of plants were counted at metaphase leaf smears prepared by method of Baldwin (1939). CHROMOSOMES AND THE PLANTS.-The chromosomes of Descurainia are uniformly small. The chromosome numbers form, with exception of one report, a regular polyploid series founded on 7. Previous workers have published chromosome numbers for four species of genus. Two of those species are South American: D. Cumtngiana (Fisch & Meyer) Prantl, with 14 somatic (Manton, 1932), 7 gametic (Jaretzky, 1932) chromosomes; D. myriophylla (Willd.) Fries, with 14 and, some cells, 28 somatic chromosomes (Manton, 1932). The North American representatives of Descurainia for which chromosome numbers are known are given below as interpreted systematically by Detling. Data on geographic occurrence are taken from that author. Detling's herbarlum numbers and places of collection are put brackets. D. Sophia (L.) Webb, type species, is an annual or biennial plant of Old World and is now distributed throughout United States, Canada, and Alaska. Manton (1932) reported for species 28 and, some cells, 56 somatic chromosomes. Jaretzky (1932) found 14 gametic chromosomes, Mayor (1934) emphasized difficulty of counting chromosomes of this plant and provisionally reported 10 gametic, 20 somatic chromosomes for species, designated by him Sisymbrium Sophia L. The present writers determined a 2n-number of 28 for species [IDetling 2217, Mountain Home, Idaho] (fig. 1). D. Richardsonii (Sweet) Schulz is biennial andoccurs, for most part, mountains of western Unlted States; one subspecies is northern plains. Two collections of subspecies vis' Received for publication July 17, 1940. Papers from Department of Botany of University of Michigan, No. 723. cosa (Rydb.) Detl. [Detling 2257, Targhee Pass, Idaho; Detling 2213, Hilgard, Oregon] were found dulring this study to have 14 somatic chromosomes (fig. 2), subspecies procera (Greene) Detl. [Detling 2263, Howard Springs, Targhee Pass, Idaho) to have a 2n-number of 28 (fig. 3), and subspecies incisa (Engelm.) Detl. [Detling 2380, Flagstaff, Arizona], a 2n-number of 42 (fig. 4). It is clear that these subspecies are separated by polyploid differences. Consideratlon of ranges given by Detling shows that diploid viscosa and hexaploid incisa occupy geographic areas of approximately same magnitude, latter subspecies being more southern its occurrence but extending into range of viscosa; and that, of these three subspecies, tetraploid procera has most restricted range and occurs almost entirely within region where both incisa and viscosa are found. D. obtusa (Greene) Schulz is an aggregate of strict, coarse, biennial plants mostly mountains or on high plateaus of southwestern United States and northern Mexico. The present authors found that subspecies typica Detl. [Detling 2381 Williams, Arizona] has 2n=14 (fig. 5), and that subspecies brevisiliqua Detl. [Detling 2375, Gonzales, New Mexico, type collectionDetling 2374, Thoreau, New Mexico] has 2n=42 (fig. 6). The diploid typica has a greater geographic range than hexaploid brevisiliqua. Most of range of diploid is south of that of hexaploid. D. pinnata (Walt.) Britt. is a group of annual plants. Evolution within this species seems to have followed two distinct lines, giving rise to one complex of subspecies hot arid regions of Southwest and another complex more northern cooler and frequently moister regions of Rocky Mountains, Pacific Northwest and Canada (Detling, 1939). The two complexes intergrade geographically and morphologicallv. Sometimes northern association is segregated as D. brachycarpa (Richards) Schulz. The subspecies ha-ltctorum (Cockerell) Detl. includes both glandular and non-glandular plants; plants differ chromosome number. Cockerell (1901) gave specific rank to non-glandular one. The authors found glandular plant [Detling 2291, Boulder, Wyoming ; Detling 2300, Red Desert, Wyoming] to have 42 somatic chromosomes (fig. 7), non-glandular one [Detling 2360, Mesa Verde, Colorado] to have 28 somatic chromosomes (fig. 8). Likewise, subspecies glabra (Woot. & Standl.) Detl. [Detling 2387, Mojave Desert, California] was discovered to have a 2n-number of 28 (fig. 9). Subspecies Menziesii (DC.) Detl. was investigated by Manton (1932), who gave plant specific status and reported for it 28 somatlc chromosomes. The present authors counted chromosomes of three other subspecies: subspecies intermedia (Rydb.) Detl.