Abstract:
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.
