Steven R. Radosevich

NEIGHBORHOOD APPROACH FOR QUANTIFYING INTERSPECIFIC COMPETITION IN COASTAL OREGON FORESTS

We examine use of the neighborhood approach for quantifying interspecific competition around Douglas-fir seedlings that were planted in early-successional forest vegetation. We manipulated woody and herbaceous vegetation in the neighborhoods of >685 trees on four sites in the Oregon Coast Range for five years. We sought the best neighborhood expression of interspecific competition by systematically examining the influence of various measures of woody-neighbor abundance, height, distance, and spatial arrangement in regression models that predict the annual basal area and height growth of Douglas-fir. We identified the optimum neighborhood height and radius for each year. Visual estimates of shrub species cover provided the best assessment of neighbor abundance. Stratifying the neighborhood by the height of shrubs relative to the height of the Douglas-fir improved regression models and quantified patterns of competitive asymmetry. Differences in the response of Douglas-fir height and basal area to neighborhood height suggested that neighbor influences on height growth were asymmetric, while effects on basal area growth were symmetric. Shrub neighbors whose crowns intermingled with that of the Douglas-fir had the greatest influence on Douglas-fir growth and defined the optimum neighborhood radius. The competitive influence of neighboring shrubs decreased rapidly as their distance increased beyond the optimum neighborhood radius. Accounting for neighbor spatial arrangement using two measures of angular dispersion did not improve the models. The best neighborhood dimensions for Douglas-fir seedlings changed with time, apparently with the dynamics of vertical structure in the plant community. The neighborhood approach was effective for (1) evaluating various approaches forest managers might use to quantify competing vegetation, (2) examining the dynamics of competitive effects, (3) quantifying asymmeteric competition, (4) determining the relative competitive effects of woody and herbaceous vegetation, and (5) documenting the differing response of tree height and basal area growth to neighboring plants.

Photomorphogenic processes in the agricultural environment

We review recent advances in our understanding of how photomorphogenic mechanisms influence processes of significance for agriculture, including weed seed germination, acclimation to solar UV‐B radiation, and plant interactions in canopies.

Plant invasions—process and patterns

Abstract Plant invasions that arise from human introductions of new species to a region, or from range expansion of some native plant species, can profoundly affect biodiversity and alter the structure and function of ecosystems. Although preventive strategies may be an effective way to limit plant invasions, they are difficult to achieve because adequate descriptions of biological and environmental characteristics are often lacking, and truly predictive models of invasive biology have been elusive. On the basis of a history of repeated plant introductions and the absence of a general predictive theory, it may be best to assume that plant invasions will continue. This paper explores ways to empirically study and to predict plant invasions through a study of the invasion process. Several approaches are explored, such as species demography, DNA analysis, and geographic information system reconstructions, to characterize source and satellite populations and factors that influence the spread of these populations.

Competition thresholds for the survival and growth of ponderosa pine seedlings associated with woody and herbaceous vegetation

Patterns of survival and stem-volume growth for planted seedlings of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) competing with various levels of woody and herbaceous vegetation were derived from three previous studies—one in Oregon and two in Montana. Negative hyperbolic curves of opposite concavity describe the relation between the abundance of woody or herbaceous vegetation and (1) the survival and (2) the stem volume of the pine seedlings. From these curves, two types of competition thresholds for managing forest vegetation are identified: (1) maximum-response threshold—a level of vegetation abundance where additional control measures will not yield an appreciable increase in tree performance; and (2) minimum-response threshold—a level of vegetation abundance that must be reached before additional control measures will yield an appreciable increase in tree performance. The maximum- and minimum-response thresholds for pine stem volume occurred at lower levels of vegetation abundance than those for pine survival. Thus, forest managers may need to consider ponderosa pine survival and stemvolume growth as separate objectives when managing woody and herbaceous vegetation in young plantations. Knowledge of maximum- and minimum-response thresholds also can be used to improve herbicide prescriptions.

COEVOLUTION OF AGROECOSYSTEMS AND WEED MANAGEMENT

coined by Ehrlich and Raven (1964) in their discussion of the evolution of plants and the insects that feed on plants. Coevolution may be described either as a pairwise process, wherein a trait of one species has evolved in response to a trait of another species, or as a more diffuse process, wherein a particular trait in one or more species has evolved in response to a trait or suite of traits in several other species (Futuyma and Slatkin 1983). Aspects of human behavior that determine particular farming practices may be looked at as coevolving with one or various components of the agroecosystem. For example, during early stages of agriculture, human social systems were tightly coupled with biological components of the agroecosystem on which they depended. Plant communities appeared in response to human disturbances, and new types of disturbances were created as humans attempted to manage the new plant communities. Feedback allowed coevolution of agroecosystems and weed-man-

Interspecific competition and herbicide injury influence 10-year responses of coastal Douglas-fir and associated vegetation to release treatments

Abstract Responses of competing vegetation and planted Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) were studied for 10 years after six herbicide and manual release treatments in the Washington and Oregon Coast Ranges. Research objectives were to quantify regional, long-term responses of vegetation to various levels of competition, light and soil water availability, and intensity versus importance of factors influencing Douglas-fir growth. Three treatments reduced shrub cover relative to the untreated check: triclopyr in year 1, glyphosate in years 1–5, and repeated control (via several herbicide applications) in years 1–10. Reductions in woody cover from glyphosate stimulated increases in herb cover in years 3 and 5, while repeated control reduced herb cover in years 1, 2 and 5. Through year 10, Douglas-fir survival (86–99%) varied little among treatments. Visual symptoms of herbicide injury to Douglas-fir from triclopyr (45% of trees) and glyphosate (17% of trees) were associated with 0.1 – 0.2 m reductions in first-year height. After adjusting for tree size, Douglas-fir growth in stem basal area 2 years after triclopyr was less than that of the untreated check, suggesting prolonged effects of herbicide injury. Because it sustained low levels of interspecific competition, caused minimal tree injury, and prevented overtopping cover from red alder (Alnus rubra Bong.), repeated control was the only treatment in which Douglas-fir size (9.8 m height and 21 cm basal diameter in year 10) significantly exceeded (P ≤ 0.02) that of the untreated check (7.8 m height and 12 cm diameter).

Differential efficiency of photosynthetic oxygen evolution in flashing light in triazine-resistant and triazine-susceptible biotypes of Senecio vulgaris L.

Abstract Photosynthetic oxygen evolution in response to flashing light was studied in triazine-susceptible and triazine-resistant biotypes of Senecio vulgaris L. Studies were conducted to determine if the modification of the herbicide-binding site which confers s -triazine resistance also affects the oxygen-evolving system. Oxygen evolution was measured using a Joliot-type oxygen-specific electrode on broken, stroma-free chloroplasts of both biotypes. We observed abnormal patterns of oxygen evolution in resistant chloroplasts. The S′ 1 → S 2 transition is slower while the S 2 decay is faster. The S′ 2 → S 3 transition, in contrast, is slightly faster in resistant chloroplasts, while the decay of the S 3 state is the same as in susceptible chloroplasts. These altered kinetics may be due to altered Q → B (B − ) electron flow in resistant chloroplasts. These results are also consistent with the hypothesis that back-reactions from the reducing (acceptor) side of Photosystem II to the oxidizing (donor) side occur with greater frequency in resistant than susceptible chloroplasts. These events are responsible for lower oxygen yield and increased ‘misses’ and ‘double hits,’ resulting in abnormal yield patterns and lower quantum yield of CO 2 fixation in resistant chloroplasts compared to the susceptible ones.

Influence of red alder competition on cambial phenology and latewood formation in Douglas-fir

SUMMARY To better understand the influence of competition on wood formation and wood quality in Douglas-fir ( Pseudotsuga menziesii [Mirb.] Franco), patterns of cambial growth and latewood production were examined for one growing season in 15-year-old plantations with similar densities but differing Douglas-fir/red alder ( Alnus rubra Bong.) ratios. The treatments consisted of plots having different proportions of Douglas-fir vs. red alder, different red alder planting dates, and one of two total planting densities. Cambial growth was tracked using the pinning method. Cambial activity in most trees began between May 12 and May 23, and ended between August 27 and September 10. Mean date of transition to latewood was July 6. In the treatment with the highest mean red alder basal area, Douglas-fir trees began radial growth later and ended earlier in the year than those in pure Douglas-fir stands. There was no evident effect of competi tion from red alder on the duration of cambial activity in treatments with intermediate to low red alder basal areas. In all treatments, the duration of radial growth was shorter in smaller-diameter trees. Early transition to latewood production was also associated with higher red alder basal area. Percent latewood was unaffected by treatment, but it was dependent on the date of a treeʼs transition to latewood production.

Concerns a Weed Scientist Might Have About Herbicide-Tolerant Crops: A Revisitation'

Over a decade ago, the Weed Science Society of America sponsored a symposium on the then emerging technology of herbicide-tolerant crops (HTCs). The symposium and subsequent proceedings addressed potential benefits and concerns about that new technology to control weeds. Technological, biological, and ethical questions were addressed that were likely to emerge from the widespread adoption of HTCs. It was suggested at that time that if such questions were answered, HTC development would proceed on a more solid foundation, whereas continued uncertainty and criticism would probably result if the questions were not answered. We now review developments in HTC technology. Questions and concerns posed one decade ago are still pertinent, but current knowledge is still insufficient to address them. Adoption of HTC has risen dramatically since their commercial introduction, but there is still no evidence of associated production cost reductions or enhanced yields. Additional index words: Biotechnology, herbicide-tolerant crops, weeds. Abbreviations: HTC, herbicide-tolerant crops.

Landscape Factors Influencing the Abundance and Dominance of the Invasive Plant Potentilla Recta

Abstract Little is known about the relative importance of environmental, biotic, historical, and spatial factors that influence invasive plant abundance, dominance, and distribution across landscapes. We identified factors that influence the abundance and dominance of Potentilla recta L. (sulfur cinquefoil) in bunchgrass grasslands of northeastern Oregon to better understand the conditions under which it becomes a major component of plant communities. We estimated P. recta stem density and dominance from field measurements across the landscape and used classification and regression tree analyses to assess the importance of environmental, biotic, spatial, and historical factors in explaining P. recta presence, stem density, and dominance. Plots were sampled within a systematic grid with 250-m spacing within our 6.5-km2 study landscape. At each sample point we recorded P. recta presence, stem density, and dominance as well as 11 biological, environmental, spatial, and historical variables. P. recta was widely distributed, with stem densities in occupied plots averaging 5.8 stems • m−2 and dominance values ranging from 1% to 52%. Percent cover of bare ground was the most important variable to predict the presence of P. recta, though the model fit was poor, likely because the entire study area is suitable for P. recta establishment. A strong relationship between P. recta dominance and habitat type (r2  =  67.5%) was found, with dominance greatest in old fields on relatively flat slopes (mean dominance of 34.1%). Dominance estimates were ≤ 1% in plots located in forest, shrub, and grassland habitats. Factors that make old fields susceptible to dominance remain unknown, though microsite conditions that increase P. recta seedling survival rates and limited native propagule availability due to previous cultivation may be involved. Since old fields are found throughout the region, are highly susceptible to P. recta invasion, and represent a source of seeds, containment and restoration activities should focus on these areas.