Megaherbivores play a central role in the evolution and functioning of ecosystems. In tropical forests elephant species are some of the few remaining megaherbivores. Through elephant foraging, nutrients that would be locked in leaves and stems, taking months or years to decay, are quickly liberated for use. In 10 experimental sites in Kibale National Park, Uganda, we set up 10 pairs of plots (4 × 4 m), each pair involved one treatment, elephant dung addition, and one control. After 1 y, we quantified growth (height and leaf number) and survival of young light-demanding (12) and shade-tolerant (19) plant species (439 stems in total). In general, the addition of elephant dung did not increase seedling growth, and it only increased the number of leaves in shade-tolerant plants with a large initial number of leaves. Researchers have speculated that the loss of elephants would shift the composition of African forests to slow-growing tree species. However, this is not supported by our finding that shows some slow-growing shade-tolerant plants grew more new leaves with additional nutrient input from elephant dung, a condition that would occur if elephant numbers increase.

Palmer amaranth (Amaranthus palmeri S. Wats. # AMAPA) residue was incorporated into soil to determine its allelopathic effects on the seedling growth of grain sorghum (Sorghum bicolor L. Moench.), cabbage (Brassica oleracea, var. capitata L.), carrot (Daucus carota L.), and onion (Allium cepa L.). Root and shoot growths were equally sensitive to the toxic effects of soil-incorporated Palmer amaranth. Growth of ‘Grand Slam’ cultivar of cabbage was 17 to 30% more sensitive than the growth of 'Sanibel’ cabbage. Growth of onion and carrot seedlings was less inhibited than either cabbage or grain sorghum. Growth of grain sorghum root was severely inhibited by 8000 and 16 000 ppm of Palmer amaranth in soil and was not affected by oven dryings other than lyophilization. Seedling growth was more severely inhibited by thyrsus and leaf tissues than by stem and root tissues of Palmer amaranth.

The effect of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] on the germination and growth of 14 plant species was investigated. A 10-3 M concentration of the herbicide did not reduce germination as much as 10% for corn (Zea mays L. ‘Pioneer 3369A’), English pea (Pisum sativum L. ‘Thomas Laxton’), sicklepod (Cassia obtusifolia L.), or wheat (Triticum aestivum L. ‘Coker 68–15′) but reduced germination of oat (Avena sativa L. ‘Coker 227′) to 0, lettuce (Lactuca sativa L. ‘Grand Rapids’) to 11%, and peanut (Arachis hypogaea L. ‘Florunner’) to 14%. A 10-6 M concentration did not affect the germination of any species tested. A 10-5 M concentration did not significantly decrease length or fresh weight of corn, morningglory, or wheat roots but had at least some adverse effect on all other species tested. At this concentration root length was less than 2/3 of the control for English pea, cucumber (Cucumis sativus L. ‘Ashley’), radish (Raphanus sativus L. ‘Scarlet Globe’), sicklepod, and soybean [Glycine max (L.) Merr. ‘Ransom’]. Metolachlor was supplied to germinating seeds of barley (Hordeum vulgare L. ‘Barsoy’), English pea, and corn either only in the shoot, seed, or root zone, or to all parts. One kg/ha applied in the shoot zone of barley and corn significantly reduced seedling height and dry weight but had no effect when applied in either the seed or root zone. Application in all zones was approximately twice as detrimental as when applied in only the shoot zone. Application of 4 kg/ha in only the shoot, seed, or root zones of English pea did not decrease seedling height or dry weight but application in all zones decreased these values about 98%. A 10-5 M concentration of metolachlor decreased leucine uptake by cucumber root tips but not its incorporation into protein. A 10-4 M concentration strongly reduced leucine uptake and slightly decreased leucine incorporation into protein.

Laboratory, greenhouse, and field studies were conducted to determine the allelopathic potential of sunflower (Helianthus annuus L.) cultivars to suppress weed germination and growth. Germination of wild mustard [Brassica kaber (DC.) L.C. Wheeler var. pinnatifida (Stokes) L.C. Wheeler] seeds at 25 C in undiluted aqueous extracts of sunflower leaf tissue was inhibited 75%, but was stimulated by up to 150% at 10- and 100-fold dilutions. Stem-tissue extracts at all concentrations stimulated wild mustard seed germination. The germination response of other weed species varied with the sunflower cultivar and concentration of tissue extract. In sand culture, leachates of dried sunflower leaf and stem tissue inhibited broadleaf - weed seedling growth, but had little or no effect on the growth of grass weeds. Sunflower root exudates inhibited seedling growth, but were less effective than leaf and stem tissue leachates. Germination of weed seeds was unaffected by root exudates. Over a 5 -yr period, weed density and percent ground cover increased less in field plots of sunflower than in control plots.

The combination of salts at the approximate ratio found in saline irrigation water, (NaCl 41.9 mM, CaCl2 20.9 mM, and MgCl2.6H2O 10.5 mM), with either 1 mM EPTC (S-ethyl dipropylthiocarbamate) or 0.1 mM CDEC (2-chloroallyl diethyldithiocarbamate) induced a greater reduction of root length, shoot length, and fresh weight of tomato (Lycopersicon esculentum Mill. ‘VF-315’) relative to either factor alone. The osmotic pressure of this salt solution is 0.40 MPa (mega Pascal), which is equivalent to 4.0 bar or ca. 4.0 atm. Similar results were found in lettuce (Lactuca sativa L. ‘Great Lakes–659’) using salts at 0.30 MPa (NaCl 31.4 mM, CaCl2 15.7 mM, and MgCl2.6H2O 7.3 mM) combined with either 0.75 mM EPTC or 0.2 mM CDEC. In tomato, the inhibition was additive for each parameter measured. In lettuce, however, the effect on fresh weight and shoot length was synergistic and the effect on root length was additive. Calcium chloride was the single salt component of the mixture, which when combined with either EPTC or CDEC, induced a significant reduction of root length, shoot length, and fresh weight in tomato relative to either factor alone. No additional reduction in growth was found in lettuce when each individual salt was combined with either EPTC or CDEC.

The allelopathic potential of wheat [Triticum aestivum (L.) ‘Doublecrop′] straw residue was evaluated on weed-seed germination and seedling growth. The inhibition of weed-seed germination and seedling growth was extract-, species-, and temperature-dependent. The extracts prepared by agitating and soaking caused greater inhibition than those obtained by leaching. The descending order of species susceptibility was ivyleaf morningglory [Ipomoea hederacea (L.) Jacq.], velvetleaf (Abutilon theophrasti Medic.), pitted morningglory (Ipomoea lacunosa L.), hemp sesbania [Sesbania exaltata (Raf.) Cory], sicklepod (Cassia obtusifolia L.), and Japanese barnyard millet [Echinochloa crus-galli var. frumetaceae (Roxb.) Link]. Incubation at 35 C caused the greatest inhibition of germination and growth.

Our previous study showed that smoke derived from alfalfa (Medicago sativa) caused different germination responses compared with that from prairie hay (Festuca hallii) and wheat straw (Triticum aestivum), but the mechanism remained unclear. In this study, we used Salad Bowl lettuce (Lactuca sativa) as a quick bioassay to trace the active compounds in each of these three smoke solutions. Column chromatography and high performance liquid chromatography (HPLC) were used to separate and identify active fractions. Seeds of four species from Fescue Prairie were primed for 24 h at room temperature in darkness using serial dilutions of separated active fractions, as well as karrikinolide (KAR1). After priming, seeds were dried at room temperature in darkness for 7 days and subsequently incubated at 10/0°C or 25/15°C in 12 h light–12 h dark or 24 h darkness for 49 days. KAR1 was in the smoke made from prairie hay, and wheat straw, but was absent in alfalfa smoke. Priming in KAR1 solutions increased germination of three native species. Priming in highly concentrated KAR1 reduced radicle length of Cirsium arvense, the only non-native species. Even though KAR1 has the potential to enhance regeneration of native species in the Fescue Prairie, KAR1 is not universally present in smoke derived from different plant materials. Unknown compound(s) in smoke derived from legumes remain to be identified.

Solanum viarum, commonly known as tropical soda apple (TSA), is native to Brazil and Argentina but has become a harmful weed in many countries with tropical climates. This study was conducted to reassess the seed biology of TSA found in South Africa. Cold stratification (14 d), acid scarification (20% H2SO4 for 5 min), and sandpaper scarification (30 s) significantly improved percentage germination when compared to the control. The highest germination (99.5%) was achieved when seeds were germinated in 50% Hoagland's nutrient solution (HS). The lowest germination (66%) was recorded in the absence of phosphorus (P) under alternating light conditions. HS without nitrogen (N) completely inhibited seed germination of TSA under constant light conditions. These findings are useful in controlling TSA by amending the levels of N and P in soils. Seed germination of TSA was significantly enhanced by different concentrations of smoke-water and butenolide solution. Smoke-water dilution of 1:500 v/v and butenolide concentration of 10−8M showed the highest seedling vigor indices (6,688 and 6,666, respectively) in comparison to the control (1,251) and gibberellic acid (GA3) concentrations (< 5,327). These findings suggest that germination of seeds or seedbanks of TSA might be successfully stimulated using smoke solutions. Subsequently, patches of seedlings emerging after treatment can be mechanically uprooted to reduce the infestation of TSA. However, justifying this with field trials is essential.

We explored the allelopathic potential of kudzu as a function of its phenolics. Aqueous and methanol extracts of different kudzu organs (leaf, stem, root, and seed) were assayed for allelopathy with the use of lettuce and radish seeds. Both leaf and root extracts significantly inhibited all of the measured germination indices (total germination, speed of germination, and coefficient of the rate of germination) (all P < 0.01). When treated with leaf extract, the total germination of both species was ∼ 20% less than the control. Furthermore, the leaf extract significantly reduced the speed of germination to 38 and 53% that of the lettuce and radish controls, respectively. Lettuce and radish seeds soaked in leaf and root extracts for 24 h imbibed less water (∼ 30% for both species) than those soaked in distilled water (control), suggesting that a reduction of water imbibition might be one of the mechanisms of germination retardation. Stem and seed extracts affected neither the water uptake nor the germination indices of radish and lettuce seeds. Kudzu leaves and roots contain higher amounts of total phenolics (P = 0.001) and soluble phenolics (P = 0.005) than stems and seeds, consistent with the results of the germination bioassays. In agar plate bioassays, both litter and rhizosphere soil had phytotoxic effects on the radicle growth of radish (P = 0.003) and perennial ryegrass (P = 0.001) seedlings. Perennial ryegrass and cobbler's pegs seedlings grown on leaf and root leachate-amended soil gave ∼ 40% shorter roots and shoots and ∼ 50% less dry weight than those grown in leachate-free soil. Kudzu litter was incorporated and incubated in soil for 6 wk, at which point the soluble phenolics in the soil solution were at a level (> 150 ppm) considered to be allelopathic.

Ultraviolet-B (UVB, 280 to 320 nm) radiation levels reaching the earth's surface are increasing because of depletion of the stratospheric ozone layer. Adverse biological effects of this harmful radiation are a serious concern. The effects of UVB radiation on spotted chickweed, dandelion, downy brome, green foxtail, redstem filaree, and common catsear seed germination, shoot and radicle elongation, and seedling growth and development in a UVB-free environment after exposure to UVB were investigated in a greenhouse study. UVB radiation (4, 7, or 11 kJ m−2 d−1) did not influence seed germination of any species except downy brome. Postgermination UVB exposure (8 h d−1, for 10 d), however, inhibited radicle elongation in all species, except in spotted catsear. Shoot elongation was also inhibited in some species but to a lesser extent than radicle elongation. A significant inhibition by UVB treatment (11 kJ m−2 d−1) of root and shoot biomass, leaf area, and leaf number was observed in some species after transfer of UVB-treated seedlings to a UVB-free environment for 3 wk. The inhibition of postgermination shoot and radicle elongation by UVB radiation, the continuation of growth inhibition after transfer of the treated seedlings to a UVB-free environment, and the differential responses of weedy species could have significant implications for their ability to compete with each other and with associated crops in a UVB-enriched environment that is likely as the stratospheric ozone layer gets depleted.

Arbuscular mycorrhizal fungi (AMF) are an important biotic factor that influences tropical ecological succession and differently affect the woody species belonging to different successional stages. However, little is known about the influence of AMF on growth and reproduction of herbaceous and shrubby species of early phases of tropical succession. Thus, we assessed the effect of AMF on the development of 27 heliophilous herbaceous and shrubby tropical species. Plants were grown in greenhouse, in low- and high-fertility soils, with or without AMF, for 100 d. Most species grown with AMF exhibited high root infection intensity (≈80%), irrespective of soil fertility. In the low-fertility soil, non-mycorrhizal plants exhibited about 88% less shoot dry mass (SDM) than mycorrhizal plants, and AMF were crucial for the survival of most species. Non-mycorrhizal plants also had lower relative growth rate (RGR), total leaf area (TLA), leaf area expansion (LAE) and total root length (TRL). Six species flowered in the low-fertility soil, and flowering increased with AMF in one plant species and four species only flowered when mycorrhizal. In the high-fertility soil, non-mycorrhizal plants exhibited about 13% less SDM than mycorrhizal plants and also exhibited lower TLA, LAE, and nutrient concentrations in shoots. On the other hand, no major changes were observed for RGR, TRL and root dry mass for most of the species. Sixteen plant species flowered in the high-fertility soil, but most had earlier (11) and more abundant (10) flowering when mycorrhizal. Thus, AMF have different influences on the survival, growth and flowering of herbaceous and shrubby tropical species, depending on soil fertility: in low-fertility soil, AMF especially affect the survival, growth and flowering, whereas in high-fertility soil, AMF mainly influence the shoot nutrient concentrations and flowering.

Fallen tree trunks (‘nurse logs’) are important recruitment sites for trees in temperate forest, however nurse log use is seldom reported in tropical forests. We predicted that logs should be important for the regeneration of small-seeded tropical pioneer species because surface leaf litter and competition with established vegetation reduces the establishment success of these species from soil seed banks. In a survey on Barro Colorado Island, Panama, we found that pioneer seedlings were present on logs in 40 of 95 recent treefall gaps. In gaps where seedlings were present on logs, seedling density was not significantly different from adjacent areas of soil. However, species composition was significantly different; logs were disproportionately colonized by smaller-seeded and wind-dispersed species. In growing-house experiments using 12 species, we found that wood substrate had little effect on seed germination. In contrast, seedling growth was 50% lower on decayed wood than soil. Furthermore, species growth rates on wood were not significantly correlated with growth rates in soil (df = 10, r = 0.48). If establishment on logs eventually leads to recruitment to the forest canopy, then logs may promote the maintenance of diversity by favouring a different group of species from those that recruit in soil.

Some large-seeded tree species have cotyledonary reserves that persist for months after seedling establishment. We carried out two screened growing-house experiments with seedlings of Gustavia superba (Lecythidaceae) to test hypotheses proposed to explain why cotyledons are retained. We grew seedlings from large and small seeds in sun and shade to determine if cotyledon reserves supplement photosynthetic carbon gain, and in a second experiment applied defoliation and shoot removal treatments to determine if reserves are allocated to resprout tissue. In each experiment we tracked cotyledonary resource use over time and measured the fraction of seedling biomass allocated to roots and shoots. We found no evidence that light environment, seed size or damage treatment affected the rate of cotyledon resource usage; 20% of the cotyledonary mass remained 9 wk after leaves were fully developed in both sun and shade and 25–30% of the cotyledonary mass remained 6 wk after leaf or shoot removal. Instead, cotyledon reserves appear to be slowly translocated to roots regardless of light environment or seedling damage. Once seedlings are established, lost tissue is replaced using reserves stored in roots; in high light, damaged seedlings had a lower root mass fraction (0.42) than undamaged ones (0.56) when considering the mass of tissue removed and resprout tissue combined. We conclude that cotyledon reserves are important for resprouting during early seedling emergence and establishment, but do not directly contribute to seedling growth or biomass recovery from herbivores at the post-establishment stage. Persistence of cotyledons may ultimately depend on the development of sufficient root mass for reserve reallocation.

Marrow (Cucurbita pepo L.) seed quality was monitored during seed development and maturation in 2 years. Mass maturity (end of the seed-filling phase) was attained 61–63 d and 54 d after anthesis in 1989 and 1990, respectively, when seed moisture contents had declined to 40–48% (wet basis). Considerable dormancy was encountered during standard germination tests, but was overcome by decoating the seeds. The ability of dried, decoated seeds to germinate normally in standard tests reached near maximal values shortly after mass maturity; these values were more or less maintained in seeds from subsequent harvests. Maximum seed longevity in air-dry storage was detected in seeds harvested 24 d (1989) and 26–31 d (1990) after mass maturity. Seedling dry weights 15 d after sowing were greatest for seeds harvested 2–22 d (basal fruits) or 14 d (apical fruits) after mass maturity in 1989, and were positively correlated (P<0.01) with times from seedling emergence to seedling harvest. Consequently in the subsequent year the hypothesis that these differences in seedling dry weight were solely due to differences in times from sowing to emergence was tested (and confirmed). Seedling relative growth rates did not differ with seed harvest date (P>0.25) in 1990, but absolute seedling size did (P<0.005); seeds harvested 21–31 d after mass maturity had the greatest seedling weight and also growth rate (in absolute terms) at any one time after sowing. Decline in seed quality (when assessed by both potential seed longevity and seedling growth) was not detected until the final harvest interval in 1990 (85–90 d after anthesis, 31–36 d after mass maturity). These results for marrow contradict both aspects of the general hypothesis that seed quality is maximal at the end of the seed-filling phase and that viability and vigour begin to decline thereafter.

Germination and seedling growth of soybean (Glycine max L.) cv. Williams were examined on paper towels pre-moistened with a range of saline solutions (germination: 0–500 mMolal NaCl; seedling growth: 0–330 mMolal NaCl). The Na+, K+ and Ca2+ concentrations in the embryonic axis immediately before germination and in the seedling 3.5 d after germination were measured. Germination decreased at NaCl concentrations of 330 mMolal (81% germination) and above. At 420 mMolal NaCl, only 40% of seeds germinated, and at 500 mMolal NaCl there was no germination. Seedling growth rate decreased drastically with increasing salinity. At 220 mMolal NaCl, seedling growth rate had declined to 5% of the control, whereas at 330 mMolal NaCl seedling growth was almost zero 3–4 d after germination. Thus, soybean seeds were more tolerant of salinity in the germination than in the seedling phase. The results suggest that the greater tolerance of salinity during the germination phase might, in part, be the result of a lower sensitivity to high tissue Na+ concentrations. Germination (40%) was possible at a tissue Na+ concentration in the embryonic axis of 9.3 mg g FW-1, whereas seedling growth was completely inhibited at a tissue Na+ concentration of 6.1 mg g FW-1. Germination at higher tissue Na+ concentrations was associated with higher K+ and Ca2+ concentrations in the embryo axis, compared with growing seedlings, suggesting that these ions may protect the seeds in the pre-germination phase against salinity.

Saline soils contain multiple types of salt, each of which may exert a different effect on seed germination and seedling growth of plants. The effects of five types of salt on the initial growth of Haloxylon ammodendron, a shrub found on both saline and non-saline areas in deserts of China, were investigated. Seeds were incubated at 20°;C in the dark in a solution (0 to –5.1 MPa) of a salt (NaCl, MgCl2, CaCl2, Na2SO4 or MgSO4) or polyethylene glycol (PEG)-6000, or in a salt (NaCl or MgCl2) or PEG solution containing a low concentration of CaCl2. Seed germination, seedling growth and cation (Na+, Mg2+, Ca2+ and K+) contents of seedlings were examined. Each salt had a different effect on seed germination, seedling growth and influx and outflux of cations in the seedlings. In both NaCl and MgCl2 treatments, the addition of low concentrations of CaCl2 favoured seed germination and seedling growth, and reduced K+ outflux from seedlings, but caused no appreciable decrease in the influx of Na+ or Mg2+ into seedlings. Marked abnormalities in seedlings were found only in treatments with Mg2+ salts, but these effects were completely alleviated by a low concentration of Ca2+ (Ca2+/Mg2+ = 0.012). The different responses of the initial growth in H. ammodendron to different isotonic salt solutions were attributed to differences among salt components in membrane permeability, toxicity and effects on functions of the plasma membrane and/or the cell wall.

The aim of this study was to investigate whether there was a relationship between growth of sunflower seedlings at 15°C in the dark and activities of enzymes involved in scavenging of reactive oxygen species (ROS), especially superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR), or in production of free radicals, namely lipoxygenase (LOX). Untreated control seeds were compared with seeds exposed to accelerated ageing (5 d at 45°C and 100% relative humidity), osmopriming (7 d at 15°C with a polyethylene glycol (PEG) solution at –2 MPa) and accelerated ageing followed by priming. Accelerated ageing decreased seed germinability and slowed down hypocotyl growth, whereas priming resulted in an increase in germination rate and enhanced seedling development. Osmopriming of aged seeds almost completely restored the initial rate of germination and seedling growth. The activity of all the enzymes studied increased during seed germination and seedling development, except that of SOD. Seed imbibition or radicle protrusion were related mainly with an increase in CAT activity and, to a lesser extent, in GR activity. Increase of LOX activity was clearly associated with the onset of hypocotyl elongation. However, in all cases, malondialdehyde measurements did not reveal intense lipid peroxidation. Priming induced a marked stimulation of CAT and GR during seed imbibition or very early during seedling development, as compared to the control seedlings and particularly to the seedlings generated by aged seeds. Hydrogen peroxide (H2O2) contents of seeds and seedlings were closely correlated to the activities of CAT and GR and to the kinetics of seedling development. The results obtained establish a clear relationship between sunflower seed vigour and ROS scavenging.

The endangered alpine plant Aconitum heterophyllum was conserved under ex situ conditions (greenhouse and open) of Palampur by overcoming seed dormancy and plant establishment through hot water treatment (40–60 °C for 30–120 s) of seeds. High seed germination (88 and 76%) was obtained when seeds were treated with hot water at 45 and 50 °C for 90 s. Correlation studies also revealed that treatment enhanced vegetative growth and reproductive yield of the plants. The total alkaloid content of the roots and rhizomes of the plants growing under ex situ conditions was lower than the ones collected from the natural habitat in the 1-year-old plants. However, in the 2-year-old plants, it was almost at par with the ones collected from nature. The life cycle of the plants growing under ex situ conditions also did not vary largely from the plants growing under in situ conditions. Therefore, the present study indicates the successful adaptation of A. heterophyllum plants in conditions other than their natural habitat and hence its potential for sustainable commercialization.

Twelve native woody species were studied to investigate the influences of soil fertility and root morphology on colonization by arbuscular mycorrhizal (AM) fungi during seedling establishment and growth. Seedlings were grown in soils of low and high natural fertility, uninoculated or inoculated with AM fungi, under greenhouse conditions. The mycorrhizal root colonization and plant responsiveness were higher among early successional species than late successional ones. Among early successional species, in both soils, mycorrhizal colonization provided significant increase in total dry mass, growth rates of shoot and root, root length, density of root tissues, root surface area and P concentration and content in the shoot. Early successional species grown with AM fungi displayed significant decreases in carbon allocation to roots, specific root length and the length and incidence of root hairs. Mycorrhizal colonization did not affect the root morphology of the late successional species in either soil. The growth of these woody species was influenced by differences in soil fertility. There was positive correlation between the degree of plant responses to AM inoculation with the percentage of root colonized by AM fungi. In both soils, plant responsiveness and mycorrhizal root colonization correlated positively to root-hair incidence and root-hair length and correlated negatively to fine-root diameter. The results suggest that during the establishment of seedlings, the large responses to the inoculation and colonization of roots by AM fungi are related to both the successional status and root morphological plasticity of the host plant, regardless of soil fertility.

The relationships between arbuscular mycorrhizal fungi and root morphological characteristics were studied under greenhouse conditions of 78 tropical native woody species and 47 seedling species collected in the field. Seedlings of native woody pioneer and early secondary species that generally exhibited fine roots with a dense cover of long root hairs showed higher mycorrhizal response and root mycorrhizal colonization than late-secondary and climax species with coarse roots with a sparse cover of short root hairs. Root-hair length and incidence decreased with the progression among the successional groups while fine-root diameter increased, both in the greenhouse and in the field. The mycorrhizal response was highly correlated to root mycorrhizal colonization in the greenhouse and in the field. These parameters were inversely correlated with the seed mass and fine-root diameter, but directly correlated with root-hair incidence, both in the greenhouse and in the field. Mycorrhizal response and root mycorrhizal colonization were also directly correlated with the root-hair length and root/shoot ratio of uninoculated plants. The seedling mycorrhizal status of the early successional woody species suggests that the root traits of these fast-growing species can be more receptive to attraction, infection and colonization by arbuscular mycorrhizas than root traits of late-successional species.