Effects of belowground grazing by collembola on growth,mycorrhizal infection,and P uptake of Geranium robertianum

We hypothesized that the grazing of vesicular-arbuscular mycorrhizal (VAM) hyphae by soil animals could be responsible for the lack of a direct relationship between mycorrhizal infection intensity and nutrient uptake under field conditions. To test this hypothesis, we determined the effect of a range of densities of the collembola, Folsomia candida, on growth, VAM infection, and P uptake in Geranium robertianum, a common forest herb, under greenhouse conditions. Total and aboveground growth were greater at low collembola density than either at higher collembola density or without collembola. These differences were greater when the plants were grown in a high organic content soil mix than when grown in sand. Root mass was not affected by collembola density. In the soil mix, root length decreased with increasing collembola density, but not in the sand. The percent of root length infected with VAM was lower at any collembola density than when collembola were absent. Total infected root length decreased linearly with increasing collembola density. Few significant differences in P uptake or tissue concentration were found. Thus, plant growth (but not P uptake) may be stimulated at low collembola density and inhibited at high. We discuss mechanisms which may be responsible for this non-linear response, and the implications of the pattern of response to studies of plant competition, nutrient turnover, and revegetation.     

Fruit-discharge-type in Geranium (Geraniaceae): its use in classification and its evolutionary implications

Three main types of seed-discharge in Geranium are made the basis of its division into subgenera: Geranium subgenus Geranium , with a ballistic expulsion of the seed from the mericarp, termed 'seed-ejection'; Geranium subgenus Robertium , with forcible discharge of the mericarp with the seed in it, separately from the awn, termed 'carpel-projection', and Geranium subgenus Erodioideae , with the seed-containing mericarp being thrown off with the attached awn, which becomes helically coiled, called the 'Erodium-type'. Variants of the seed-ejecting type permit the division of Geranium subgenus Geranium into three sections. Other criteria are used to divide Geranium subgenus Robertium into eight sections and Geranium subgenus Erodioideae into two. Species are fully enumerated except for Geranium section Geranium , which comprises the bulk of the genus, and for which some tentative subgroups are given in an Appendix. Diversity of fruit-type in Geranium is greatest in the Mediterranean Region. Characters of the fruit in other genera of Geraniaceae are surveyed. Geographical distributions, chromosome numbers, pollen morphology and phytochemistry are reviewed. It is suggested that Erodium-type fruit discharge, shared with the four remaining genera of the family, is primitive, and that carpel-projection and seed-ejection arose from it separately, the latter probably more than once. The very large, mainly perennial, Geranium subgenus Geranium is contrasted with Geranium subgenus Robertium , half of which is hapaxanthic, and which occupies marginal habitats and shows greater morphological and chromosomal variation despite its being only one tenth the size. Geranium subgenus Erodioideae is smaller still and probably relictual.     

Composting One Invasive Species to Control Another

The need for research and development of effective approaches to weed control continues to increase globally. Adaptive protocols using diverse control methods are often required in ecological restoration as recruitment of native species is highly site‐specific, species‐specific, and experimental. The use of composted weed refuse to control other weeds may be a practical option; yet, the option is not well studied due to the accompanied risk of introducing weed propagules to areas where weed control is desired. Here, we tested the effectiveness of different physical control techniques including the use of mulch made by composting weed refuse on‐site. English ivy (Hedera helix), a non‐native, invasive species in the Pacific northwestern United States, was removed from a heavily invaded site, shredded, and composted. The mulch was compared with other methods of suppressing herb Robert (Geranium robertianum), another invasive species on‐site. Five treatments were tested: flame‐weeding, hand‐pulling, mulching, hand‐pulling followed by mulching, and flame‐weeding followed by mulching. The mulch and pull/mulch treatments were the most effective, reducing G. robertianum cover by 92 and 86% of pre‐treatment levels, respectively, and suppressing G. robertianum 2.9 and 1.6 times more than the control, respectively. The mechanism behind the effectiveness of the mulch is uncertain, but may be related to weed seed burial or the allelopathic potential of the mulch. Composting one invasive species to use as mulch to control another can be effective and merits trial elsewhere.     

Complex effects of grazing treatment on an annual in a species-poor grassland community

Abstract. The effects of grazing upon the establishment, survival, growth and reproduction of a grassland annual Geranium dissectum growing in a sward dominated by grasses were examined in a replicated grazing experiment with sheep. Seeds were sown in both summer and autumn, and grazing was controlled to produce two levels of grazing in winter, two levels in spring, and two in summer, combined in a 2 x 2 x 2 factorial experimental design. Higher intensities of grazing in the period immediately before emergence benefitted plant establishment, but subsequent survival showed many interactions between factors, demonstrating that under certain conditions and at certain times grazing was detrimental. It is suggested that the frequency of G. dissectum in the grassland was low because the heavy grazing conditions that foster seedling emergence also jeopardize subsequent survival. This may also be why productive grassland communities in general contain few palatable dicots.