Initial effects of light armored vehicle use on grassland vegetation at Fort Lewis, Washington.

Sustainable use of military training lands requires understanding and prediction of the effects of military vehicles on vegetation. We examined the initial impacts of an 8-wheeled, light armored vehicle (LAV) on grassland vegetation at Fort Lewis, Washington. The LAV drove replicate spiral paths at two starting velocities, 10.3 and 5.1 m s(-1). The disturbed width (width of ground impacted by the tires) increased as turning radius decreased, but was unaffected by vehicle velocity. An inverse-exponential model predicted disturbed width (r(2)=0.68) at all turning radii for both velocities combined. In low-velocity spirals, and for straight tracking (turning radius>40 m) and moderate turns (radius 20-40 m) in high-velocity spirals, all vegetation damage was imprint (plants flattened by wheels). During sharp (radius <20 m), high-velocity turns, most or all of the disturbed width was scraped free of surface vegetation and soil, which was piled to the outside of each tire track. Total plant cover (all species) was not affected by track curvature in low-velocity spirals, but decreased in the order straight tracking>moderate turns>sharp turns in high-velocity spirals. In low-velocity spirals, post-tracking cover of several plant growth forms (non-native species, perennial species, sod-forming grasses) was similar to pre-tracking cover, but in high-velocity spirals, post-tracking cover of these growth forms decreased in the order straight > or =moderate=sharp. Cover of native species and forbs decreased more in high- than in low-velocity spirals, but was unaffected by curvature. Pre- and post-tracking cover of annual species, bunchgrasses, and shrubs was < or =3%. The most severe vegetation damage caused by operation of wheeled LAVs on grasslands is associated with sharp, high-velocity turns.

Influence of pH and plant nutrient status on ion fluxes between tomato plants and simulated acid mists.

Diffusion along concentration gradients, ion exchange, active uptake, and physical damage are possible mechanisms causing fluxes of ions between plant tissues and acidified moisture on plant surfaces. The temporal patterns of ion fluxes during individual wetting events should vary in a predictable manner in relation to moisture pH, plant nutritional status, and the operational mechanism. Tomatoes (Lycopersicon esculentum Mill.) were grown hydroponically at relative nutrient addition rates (Ra ) of 7, 12, and 15% per day. At similar growth stages, the plants were exposed to HCl-acidified mists of pH 2-5, 4-0, or 5-6 for 4 h, or left unmisted as controls. Whole-plant throughfall was analysed for pH and ion concentrations after 15, 30, 60, 120, and 240 min. Surface chemical contamination was minimized by pre-rinsing with deionized water. Except for H+ during the first 15-30 min, all ions showed efflux for the duration of the mistings. Effluxes were low (< 150µequiv m-2 h-1 ), time-invariant, and unrelated to Ra at mist pH of 4.0 and 5.6. At pH 2.5, effluxes of K+ , Mg2+ , NO3 - , and PO4 3- increased over time, while Ca2+ efflux increased during the first hour, then remained constant. Maximum efflux (105 to 730 µequiv m-2 h-1 ) mostly occurred at pH 2.5 and Ra = 17 % d-1 , increasing in the order NH4 + = SO4 2- < NO3 - < PO4 3- < Ca2+ = Mg2+ < K+ < H+ . These results suggested that although ion diffusion and ion exchange contributed to observed effluxes, an additional mechanism, temporally accumulating physical damage to cuticles and cell membranes below a threshold pH between 4˙0 and 2˙5, was involved. Foliar concentrations of K, Ca, and Mg did not differ significantly among treatments, but ion effluxes into pH 2-5 mists removed 1-13 % of the whole-plant element contents, with the largest removal for Ca.