Some factors affecting the distribution of organic arsenical silvicides in the forest ecosystem Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/df65vb328

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  • The initial distribution of organic arsenical silvicides was measured in the trees, litter and soil of four forest types in the Pacific Northwest following a standard, precommercial chemical thinnning. Factors which influenced the persistence, movement or fate of these compounds were noted and used to evaluate differences in the generalized patterns of distribution after four different treatments. The treatments included two different organic arsenicals, monosodium methanearsonate (MSMA) and dimethylarsine oxide (cacodylic acid), applied during fall or spring seasons. The project was part of a cooperative study on the safety of organic arsenicals as thinning agents initiated by the U S Forest Service in 197O The choice of chemical and season of treatment both influenced the pattern of distribution and the absolute concentration of arsenic in the tree. Concentrations of arsenic following spring treatments were about 20 ppm greater than after fall treatments, and increased more strikingly toward the extremities of the branches. Patterns of distribution for cacodylic acid were similar for the two treatment seasons, but varied for MSMA treatments. Trees treated with MSMA in the fall had relatively low, uniform concentrations throughout the crown MSMA spring treatments resulted in higher average concentrations and steeper gradients. This response was similar in pattern to both cacodylic acid treatments. The intensity of treatment on a given site also affected the overall arsenic concentration. The factors that controlled within tree concentration gradients also affected the rate of arsenic deposition in litter and soil. Highest stem-tip values were associated with highest litter concentrations. Litter values normally ranged from 20 ppm to 30 ppm arsenic after treatment. Apparent additions of arsenic to the soil were close to the minimum detection limit. They were also somewhat erratic, due to low readings in reference to substantial inherent variability in background soil arsenic. Arsenic tied up in the wood of treated trees was not obviously exposed to significant leaching or degradation processes prior to decomposition. The different patterns of arsenic distribution between treatments substantiates and expands on the basic hypothesis of mechanisms that control arsenic distribution in conifers. Distribution of arsenic within trees is apparently related to an interaction between the chemicals and the season-dependent transport mechanisms in the tree. Mobility of arsenic in conifers seems to be regulated by the direction and degree of phloem transport at the time of application if the transport system is not immobilized by treatment. Chemicals that do not depend on temperature for phytotoxicity (e.g. cacodylic acid) probably concentrate at the ends of growing tips because they immobilize the phloem transport system but not the xylem. Strongly temperature-dependent chemicals (eg. MSMA) apparently do not immobilize phloem transport during cool weather and result in equilibration of the chemical throughout various sink areas in the tree In contrast during warm seasons these chemicals would become more phytotoxic and immobilize the phloem, limiting basipetal movement Thus basipetal transport of temperature-sensitive toxicants in conifers is likely to be restricted to both cool weather and season of basipetal movement, In this context, fall treatments of MSMA can be employed to maximize concentration in roots, and minimize exposure to above-. ground organisms. Most plant samples in this study were analyzed for total arsenic content using a graphite tube adaptation on an atomic absorption spectrophotometer, The procedure developed was faster and more economical but less precise, than standard arsenic determination procedures. This analytical technique permitted the sampling intensity necessary for variable biological material in ecosystem studies.
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