Annual Dicots Research Articles

Morphology and response of roots of pasture species to phosphorus and nitrogen nutrition

The root morphology of ten temperate pasture species (four annual grasses, four perennial grasses and two annual dicots) was compared and their responses to P and N deficiency were characterised. Root morphologies differed markedly; some species had relatively fine and extensive root systems (Vulpia spp., Holcus lanatus L. and Lolium rigidum Gaudin), whilst others had relatively thick and small root systems (Trifolium subterraneum L. and Phalaris aquatica L.). Most species increased the proportion of dry matter allocated to the root system at low P and N, compared with that at optimal nutrient supply. Most species also decreased root diameter and increased specific root length in response to P deficiency. Only some of the species responded to N deficiency in this way. Root morphology was important for the acquisition of P, a nutrient for which supply to the plant depends on root exploration of soil and on diffusion to the root surface. Species with fine, extensive root systems had low external P requirements for maximum growth and those with thick, small root systems generally had high external P requirements. These intrinsic root characteristics were more important determinants of P requirement than changes in root morphology in response to P deficiency. Species with different N requirements could not be distinguished clearly by their root morphological attributes or their response to N deficiency, presumably because mass flow is relatively more important for N supply to roots in soil.

Diets and foraging behaviour of red and eastern grey kangaroos in arid shrub land: is feeding behaviour involved in the range expansion of the eastern grey kangaroo into the arid zone?

Eastern grey kangaroos (Macropus giganteus) have expanded into arid areas usually the habitat of red kangaroos (Macropus rufus). Extra watering sites for domestic stock is the suggested reason. However, changes in vegetation also have occurred due to grazing from domestic stock. We investigated the foraging strategies of M. giganteus and M. rufus in arid rangeland to see if these are involved in the range changes. Foraging patterns were similar, with both species mostly foraging at night; total feeding times were the same. M. giganteus and M. rufus had differing diets, though there was considerable overlap of 81 - 87%. Both species were highly selective, having similar narrow dietary niche breaths. Differences in plant preferences occurred and though both species had a preference for grass that of M. giganteus was higher. A larger foregut in M. giganteus reinforces its focus on grass. M. rufus also selected dicot forbs and malvaceous sub-shrubs. Both kangaroos showed avoidance of abundant chenopod shrubs (saltbushes and bluebushes) and trees. It is likely that the changes to arid-zone vegetation, with a shift to grass and annual dicot forbs, has been important for the expansion of M. giganteus, possibly combined with the greater availability of water.

Direct and indirect effects of exotic annual grasses on species composition of a South Australian grassland

Abstract Invasion by Mediterranean annual grasses, such as Avena L. spp. and Bronms L. spp, is one of the major threats to temperate perennial grassland. This study investigated the effects of annual grasses and their litter on the species composition of a grassland near Burra, South Australia. The placement of annual grass litter on soil samples in the glasshouse decreased the establishment or growth of several exotic annual dicots. In the field the addition of annual grass litter slightly decreased the frequency of Danthonia Lam. & DC. tussocks. Furthermore, litter strongly reduced the species richness from 13 species in plots with no litter to nine species in plots with the highest litter level, mainly by decreasing the frequency of common exotic dicots. Native dicot frequency similarly appeared to be decreased by litter addition. In addition to the negative effects of their litter, annual grasses also directly competed with perennial grasses. The magnitude of the competitive effect varied systematically along a slope, suggesting that other factors such as soil properties may control competitive inter actions. The biomass of annual grasses also tended to increase with the addition of their own litter. This combination of positive and negative feedback mechanisms suggests that brief periods favourable for annual grasses, either through management changes or environmental conditions, can lead to persistent changes in the species composition of the system.

Rock Outcrop Plant Communities (Glades) in the Ozarks: A Synthesis

stress. Perennial grasses are largely confined to edges of openings adjacent to surrounding forest. The distinct vegetational zones of shallow soil are dominated by annual dicots, annual grasses, and perennial dicots, including some endemic species; these are called glades in the Southeast. In the Ozarks, glade areas are typically strongly sloping, so that soil erosion frequently exceeds weathering of bedrock. Soil removal rates vary greatly over short distances, producing a mosaic of soil depths and, thus, a mosaic of plant species with different depth requirements. Plants with different soil depth requirements are intermixed or distributed in small patches, rather than in distinct zones, so fire-supporting perennial grasses are interspersed among shallow soil herbs and patches of deeper soil that support cedar trees (Juniperus). Perennial grasses support periodic fires that kill most woody plants and promote continued grass abundance. Because shallow soil plants are interspersed among deeper soil plants in the glade mosaic, studies of vegetation of glades (the total vegetational mosaic) in the Ozarks find perennial grasses and cedars dominant, with shallow-soil plants, including endemics, as minor associates.

Root system topology and diameter distribution of species from habitats differing in inundation frequency

Summary We compared the root systems of seven halophytic species that occur at different elevations on a salt marsh, in order to (i) test the hypothesis that variations in root system architecture reflect adaptation to inundation frequency or nitrogen limitation, and (ii) verify the theoretically predicted relationships between root diameter, link magnitude and root topology. Diameters and lengths of individual laterals were determined along root axes, and branching patterns were quantified by calculating a topological index (TI). Chenopodiaceae (annual dicots) showed that with increasing elevation, the branch density and length of individual first‐order laterals tended to increase, so that the relative length of the main axes decreased. Root branching of the Chenopodiaceae at lower elevations was herringbone‐like, whereas species from higher elevations had smaller TIs because their branching patterns were more complex. The Gramineae, too, showed a tendency to increased length of individual laterals with increasing elevation. However, TI was not related to elevation, did not indicate a herringbone structure for all species, and was within the same range of that of the Chenopodiaceae. As root topology of the Chenopodiaceae is related to elevation, but that of the grasses is not, topology is not necessarily an important adaptive trait in all plant families that inhabit the salt marsh. Short first‐ and second‐order laterals may represent a more general architectural adaptation to frequent inundation, with longer first‐order laterals being favourable to competition for nutrients. Diameters at the root base tended to decrease if root branching was herringbone‐like (TI close to 1). Roots of first‐order laterals were approximately one‐third of the diameter of the main axes; second‐order laterals were approximately half the diameter of the first‐order laterals. These ratios illustrate the value of using the developmental segment‐ordering system in describing roots. The theoretically predicted relationship between root diameter and link magnitude was not present within individual orders of roots, whereas diameter did slowly increase with magnitude when combining different root orders. In the absence of a clear relationship between root diameter and link magnitude, the predicted high carbon costs associated with herringbone root systems disappear, whereas the advantage of minimized inter‐root competition remains. Consequently, herringbone root systems will be most efficient in terms of nutrients gained per carbon invested. However, dichotomous root systems offer a greater potential for exploring the soil, which contributes to the potential competitiveness of plants growing in nutrient limited habitats.

The Effect of Dairy Compost on Summer Annual Dicots Grown as Alternative Silages

Kenaf (Hibiscus cannibinus) var. “India”, lablab (Lablab purpureus) variety “Tecomate,” combine and iron-clay cowpeas (Vigna unguiculata), and sunflowers (Helianthus annuus) S682 and S573 were planted at the Stephenville Research and Extension Center in the spring seasons of 1998 and 1999. Dairy compost was incorporated into half the plots prior to seeding both years at 11.2 t DM ha−1. Average days to maturity and number of harvests per season varied from 66 to 84 d and 1 to 3 harvests, respectively. Average yields were highest for kenaf (13,762 kg DM ha−1 yr−1) followed by lablab (7,925 kg DM ha−1 yr−1). Acid detergent fiber and lignin DM concentrations were lowest in kenaf (average 26.9 and 3.57%, respectively) and cowpeas (average 25.9 and 4.39%, respectively). Crude protein and phosphorus (P) concentrations were high in the iron-clay cowpea (22.3 and 0.269, respectively), combine cowpea (18.3 and 0.276%, respectively), lablab (16.8 and 0.268%, respectively), and kenaf (14.9 and 0.204%, respectively) and relatively low in the sunflower (9.7 and 0.174%, respectively). Compost increased forage yields 21.6% and forage P concentration 18.8% following the second year’s application. Compost was applied to supply 130 kg P ha−1 yr−1; on average, forages removed only 34.2 kg P ha−1 yr−1. As a result, soil P was 2.4 times higher in plots with compost (26.9 ppm) by the end of the 2-yr trial. Kenaf had the highest P uptake, averaging 56.6 kg P ha−1 over 2 yr, accounting for 43% of the P applied.

Mirrored genome size distributions in monocot and dicot plants.

The variation in genome size and basic chromosome number was analyzed in the wide range of angiosperm plants. A divergence of monocots vs. dicots (eudicots) genome size distributions was revealed. A similar divergence was found for annual vs. perennial dicots. The divergence of monocots vs. dicots genome size distributions holds at different taxonomic levels and is more pronounced for species with larger genomes. Using nested analysis of variance, it was shown that putative constraints on genome size variation are not only stronger in dicots as compared to monocots but in the former they start to operate already at the family level, whereas in the latter they do so only at the order level. At the same time, variation in basic chromosome number is constrained at the order level in both groups. Higher basic chromosome numbers were found in perennial plants as compared to the annual ones, which can be explained by their need for a higher genetic recombination as compensation for the longer life-cycles. A negative correlation was found between genome size and basic chromosome number, which can be explained as a trade-off between different recombination mechanisms.

Using upper boundary constraints to quantify competitive response of desert annuals

Using a target‐neighborhood approach with six annual dicot species in the Negev Desert of Israel, we tested whether neighborhood biomass constrained the upper limit of plant performance and if the slope of the upper boundary was correlated to species trait means, such as relative growth rate (RGR) and seed mass. Target individuals were measured in early spring and then collected at the onset of the dry season along with all naturally occurring neighbors within a 5‐cm radius of the target. Using a minimum of 50 samples for each of the six target species, we found no significant relationships between target performance and either density or aboveground biomass of neighbors, when including all target individuals in the regressions. However, aboveground neighbor biomass did influence the maximum potential size of targets. When regressions were restricted to the largest target plant within classes of neighbor biomass, significant negative relationships were found for all six species, with neighbor biomass explaining 55 to 94% of the variance in maximum target biomass. The slopes of the regressions were used as an index of competitive response. The correlations between competitive response and estimates of species traits were not significant with the possible exception that species with lower RGRmax may be better response competitors (i.e. were less sensitive to competition). These findings indicate the usefulness of the boundary regression technique for describing competitive interactions among neighbors, particularly in low productivity environments.

Floristic diversity, size, and vertical distribution of the weed seedbank in ridge and conventional tillage systems

Abstract The floristic diversity and the vertical distribution of the weed seedbank were studied in ridge tillage (RT) and conventional tillage (CT) systems in clay and clay loam soils. Viable seedbank populations were monitored during 3 yr using germination in a greenhouse. Ridge-tilled fields had a larger soil seedbank (2,992 seeds m−2) than moldboard-plowed fields (1,481 seeds m−2) in the top 15 cm. This result can be explained by the larger perennial seedbank of RT fields at both the 0- to 5-cm and 5- to 15-cm depths. Annual dicot seeds were more abundant in clay soils than in clay loams at the two soil depths. Annual grass seeds were more abundant under CT than under RT in clay soils at the two sampled depths. In clay loams, the density of annual grass seeds in RT fields was six times greater than in CT fields in the top 5 cm of soil and two times greater at the 5- to 15-cm depth. The vertical distribution of total seeds in soil did not differ between tillage systems. The top 5 cm of the 15-cm soil c...

Factors affecting the species composition of arable field boundary vegetation

Summary 1. In recent decades the botanical diversity of arable field boundaries has declined drastically. To determine the most important factors related to the species composition of arable field boundaries, the vegetation composition of 105 herbaceous boundaries, 1‐m wide, in the central and eastern Netherlands was surveyed. Biomass samples of the boundary were taken at 0–33, 34–66 and 67–100 cm from the adjacent arable field. 2. Farmers were interviewed with respect to boundary management and land use on the adjacent arable field. The two data sets containing the botanical data and the environmental variables were linked and analysed by multivariate analysis and analysis of variance. 3. The nitrogen and phosphorus inputs to the crop were closely correlated to another environmental variable, crop rotation. Crop rotations dominated by maize cultivation received significantly higher nutrient inputs than the other rotations. 4. Nitrogen, phosphorus and crop rotation were strongly correlated with the composition of the boundary vegetation. Species richness of the boundary vegetation was negatively related to nitrogen and phosphorus inputs to the crop, while total boundary biomass was negatively related to nitrogen inputs only. Furthermore, there was a significant relationship between the partitioning of the boundary vegetation between different functional groups and both nitrogen inputs and crop rotation on the adjacent arable field. No significant relationships were found between the vegetation composition and either herbicide use in the boundary or boundary management. This may have been due to the descriptive approach of the study: little variation in boundary management as well as herbicide use in the boundary was observed in the study area. 5. The boundary vegetation was characterized by a peak in biomass production in the zone near the arable field. In this zone the perennial arable weed Elymus repens, as well as annual dicot species, were significantly more abundant, while perennial dicots and monocots were significantly less abundant compared with the zones further from the arable field. 6. Efforts to protect field boundary vegetation need to focus on a reduction or cessation of fertilizer applications in the outer metre(s) of crops that require high nutrient inputs. The determination of the exact nature of the relationship between the vegetation composition of arable field boundaries and the intuitively important management and herbicide use in the boundary requires an experimental approach.

Competitive attributes ofA. sativa, T. aestivum, andH. vulgareare conserved in no-till cropping systems

The robustness of competitive attributes of cereals such as rapid and uniform seedling emergence, tillering, early biomass accumulation and canopy closure, and height advantage over weeds have not yet been tested under environmental conditions typical of no-till (NT) cropping systems. Our objective was to evaluate the effects or NT practices on growth and productivity ofAvena sativa, Triticum aestivum, Hordeum vulgare, and associated weeds. The experiment was conducted on a Kamouraska clay at La Pocatière, QC, in 1994, 1995, and 1996.Avena sativa, T. aestivum, andH. vulgarewere grown under tilled and NT practices. Cereal growth parameters were measured six (1994) or seven (1995) times between planting and the 11th week after planting but only once in 1996. Grain yields and yield components were determined at crop maturity.Avena sativaandH. vulgarepopulations were little affected by tillage, whereasT. aestivumpopulations were reduced by 16 to 20% in NT systems. Growth in height in NT systems was either similar or greater than in tilled systems in all three cereals. Cereal leaf area index (LAI) and biomass accumulation was also comparable between tillage systems, except forT. aestivumLAI in 1994, which was greater in tilled plots on two sampling dates. Response of annual dicots to tillage was inconsistent in all crops. Annual monocots dominated in some but not all NT systems. Perennial dicots dominated in NT systems, whereas perennial monocots were more abundant in tilled systems in all three cereals.Avena sativaandT. aestivumyields in NT plots were comparable or greater than in tilled plots, in spite of having either lower test weights (A. sativa) or lower 1,000-grain weights (T. aestivum). NTT. aestivumproductivity was maintained in spite of reduced plant establishment.Hordeum vulgareyields were also similar across tillage systems, except in 1995, when yields in tilled plots were greater than in NT plots. The height advantage observed for NTH. vulgaredid not result in improved yields. All three cereals, and particularlyA. sativa, appeared well suited to NT systems, despite the pressure provided by different weed groups, compared to tilled systems. However, results suggest that NT production of cereals could benefit from improved attention to perennial dicot control and crop seedling establishment, particularly forT. aestivum.

Movement of Seeds by the Burrowing Activity of Mole-Voles on Disturbed Soil Mounds in the Spanish Pyrenees

Three species of Microtus (voles) living in Pyrenean subalpine pastures brought seeds to the surface on vole mounds. This activity was studied to learn about the voles' influence on seed composition and vegetation gap colonization. Twenty-two plant species germinated from seed in the soil mounds. The plant composition of pastures was dominated by graminoids-hemicryptophytes. Nearly half of the seeds that occurred in mounds are of annual dicots able to quickly colonize the gaps and complete their vegetative cycle before the entry of perennial plants. Mound revegetation occurs chiefly by means of vegetative reproduction strategies. Nevertheless germination of seed brought to the surface can be important at the early stages of colonization after soil disturbance. Thus, such disturbances increase short-term plant diversity and patch heterogeneity.

Relative influence of crop rotation, tillage, and weed management on weed associations in spring barley cropping systems

Generalizations concerning the effects of management practices on weed community dynamics often lack robustness, most likely because of the concomitant effects of agronomic and environmental factors. However, such generalizations, when valid, provide useful grounds for predictions and are thus desirable. This study attempted to evaluate the relative importance of crop rotation, tillage, and weed management as factors affecting weed communities and tested the hypothesis of an association between management practices and weeds from certain life cycle groups. Principal component analysis (PCA) of weed density data from a 4-yr field study conducted on a Kamouraska clay and a Saint-André gravelly sandy loam at La Pocatière QC, Canada, identified groups of weed species, while an analysis of variance (ANOVA) of PCA scores associated these groups with management factors. A multivariate analysis of variance (MANOVA) of regression coefficients describing time courses of density for each species confirmed treatment effects. Species segregated roughly according to life cycles. Interactions among weed management intensity, tillage, and crop rotation mostly explained species dominance in the various cropping systems. A first group of species, mostly annual dicots, largely dominated in minimum weed management treatments; their relative importance in each rotation varied with their level of susceptibility to postemergence herbicides. A second group included annuals and perennials, whose commonality seemed to be their tolerance to herbicides; these species also had a particular affinity for chisel and no-till treatments. A third group was formed by perennial species, each with a different response to tillage. The tenuous correspondence between commonly used classification schemes and management factors suggests that other aspects of weed biology (e.g., seed size, dispersal, production, germination requirements, and seedbank longevity) should be considered when trying to explain and predict the presence and dominance of certain weed species with regard to management practices.

Seed bank strategies in Patagonian semi-arid grasslands in relation to their management and conservation

We addressed the following questions: (a) May different seed bank functional types (SBFT), defined on the basis of singularities of their persistence in soil, be identified in the Festuca pallescens grasslands? (b) Do different above-ground vegetation states modify the expression of SBFT singularities? (c) Is the potential for vegetation recovery from the soil seed bank associated with the characteristics of their SBFT? To address these questions we examined the sizes of immediately and non-immediately-germinable seed banks (IGSB and NIGSB, respectively) in four different vegetation states of the steppe of F. pallescens resulting from the combined effects of topography and grazing. The results show that four lifeforms (perennial grasses, perennial graminoid non-grasses, annual dicots and annual grasses) display three characteristic SBFT. These are: (1) a transient type constituted by perennial grasses (SBFT1), (2) an intermediate type represented by annuals (SBFT2), and (3) a persistent type characteristic of one perennial graminoid non-grass (SBFT3). Other lifeforms (perennial dicots and biennials) could not be assigned to any of these types or characterized into a definite one. Seed bank functional types do not change among vegetation states in terms of the relative proportions of IGSB and NIGSB, although differences in the absolute and the relative size of each SBFT could be found. Species with persistent or mixed persistent/non-persistent seed bank types (SBFT2, SBFT3, etc.) colonize habitats disturbed by grazing with more success than those with non-persistent seed banks (SBFT1). Species with persistent seed banks are over-represented in the soil seed bank in relation to their contribution to above-ground cover. Management strategies for conservation in these grasslands could conveniently be targeted to increase the seed rain of perennial grasses as well as to the creation of suitable micro-environmental conditions to enhance their establishment.

Phenotypic variation in seedlings of a "keystone" tree species (Quercus douglasii): the interactive effects of acorn source and competitive environment.

Blue oak (Quercus douglasii) is a deciduous tree species endemic to California that currently exhibits poor seedling survival to sapling age classes. We used common garden techniques to examine how genetic variation at regional and local scales affected phenotypic expression in traits affecting oak seedling growth and survival. Between-population variation was examined for seedlings grown from acorns collected from a northern, mesic population and a southern, xeric population. Within-population variation was examined by comparing seedlings from different maternal families within the mesic population. Acorns were planted into neighborhoods of an annual dicot (Erodium botrys), an annual grass (Bromus diandrus), and a perennial bunchgrass (Nassella pulchra). By varying the species composition of herbaceous neighborhoods into which acorns were planted, the interactive effects of competition and acorn germplasm source on phenotypic expression could also be examined. Potential maternal effects, expressed as variation in acorn size, were assessed by weighing each acorn before planting. Probability of seedling emergence increased significantly with acorn size in the xeric population but not in the mesic population. Similarly, the effect of acorn size on seedling leaf area, stem weight, and root weight was also population-dependent. At a within-population level, acorn size effects on seedling traits varied significantly among maternal families. In addition to acorn size effects, rates of oak seedling emergence were also dependent on an interaction of population source and competitive environment. Interactions between maternal family and competitive environment in the expression of seedling leaf characters suggest the possibility of genetic variation for plasticity in traits such as specific leaf area. Using carbon isotope discrimination (Δ) as an index of relative water-use efficiency (WUE), higher water use efficiency was indicated for oak seedlings grown in the annual plant neighborhoods compared to seedlings grown in the bunchgrass neighborhood. This trend may represent an adaptive plastic response because, compared to the bunchgrass neighborhood, soil water depletion was more rapid within annual plant neighborhoods.

Effects of kangaroo rat exclusion on vegetation structure and plant species diversity in the Chihuahuan Desert.

Long-term (1977-90) experimental exclusion of three species of kangaroo rats from study plots in the Chihuahuan Desert resulted in significant increases in abundance of a tall annual grass (Aristida adscensionis) and a perennial bunch grass (Eragrostis lehmanniana). This change in the vegetative cover affected use of these plots by several other rodent species and by foraging birds. The mechanism producing this change probably involves a combination of decreased soil disturbance and reduced predation on large-sized seeds when kangaroo rats are absent. Species diversity of summer annual dicots was greater on plots where kangaroo rats were present, as predicted by keystone predator models. However, it is not clear whether this was caused directly by activities of the kangaroo rats or indirectly as a consequence of the increase in grass cover. No experimental effect on species diversity of winter annual dicots was detected. Our study site was located in a natural transition between desert scrub and grassland, where abiotic conditions and the effects of organisms may be particularly influential in determining the structure and composition of vegetation. Under these conditions kangaroo rats have a dramatic effect on plant cover and species composition.

Impact of Agronomic Practices on Weed Communities: Tillage Systems

Adverse changes in weed communities are a limiting factor for the adoption of conservation tillage practices. Predictions of an increased association of annual and perennial grasses, perennial dicot weeds, wind-disseminated species, and volunteer crops as weeds, and decreased association of annual dicot weeds in reduced-tillage systems were tested. Field experiments involving zero-, minimum-, and conventional-tillage systems were conducted in Saskatchewan from 1986 to 1990 at Ituna and Waldron, and from 1986 to 1988 at Tadmore. Weed community composition was analyzed for years 1988 to 1990 by canonical discriminant analysis. An increased association of perennial and annual grasses with zero tillage did not generally occur. Wind-dispersed species and volunteer crops were associated with reduced tillage and summer annual dicots with conventional tillage, but exceptions occurred. Species responded differently among sites or within a site over time. Within the time frame of this research, changes in weed communities were influenced more by location and year than by tillage systems, indicating fluctuational rather than directional or consistent changes in community composition.

Elevated CO2 alters deployment of roots in "small" growth containers.

Previously we examined how limited rooting space and nutrient supply influenced plant growth under elevated atmospheric CO2 concentrations (McConnaughay et al. 1993). We demonstrated that plant growth enhancement under elevated CO2 was influenced more by the concentration of nutrients added to growth containers than to either the total nutrient content per pot or amount or the dimensions of available rooting space. To gain insight into how elevated CO2 atmospheres affect how plants utilize available belowground space when rooting space and nutrient supply are limited we measured the deployment of roots within pots through time. Contrary to aboveground responses, patterns of belowground deployment were most strongly influenced by elevated CO2 in pots of different volume and shape. Further, elevated CO2 conditions interacted differently with limited belowground space for the two species we studied,Abutilon theophrasti, a C3 dicot with a deep taproot, andSetaria faberii, a C4 monocot with a shallow fibrous root system. ForSetaria, elevated CO2 increased the size of the largest region of low root density at the pot surface in larger rooting volumes independent of nutrient content, thereby decreasing their efficiency of deployment. ForAbutilon, plants responded to elevated CO2 concentrations by equalizing the pattern of deployment in all the pots. Nutrient concentration, and not pot size or shape, greatly influenced the density of root growth. Root densities forAbutilon andSetaria were similar to those observed in field conditions, for annual dicots and monocots respectively, suggesting that studies using pots may successfully mimic natural conditions.

Path Analysis of Local Adaptation in Two Ecotypes of the Annual Plant Diodia teres Walt. (Rubiaceae)

Reciprocal transplant experiments were conducted to measure local adaptation in morphologically distinct ecotypes of the weedy annual dicot poorjoe (Diodia teres). Local ecotypes were generally favored in comparisons of survival, growth, and seed production. Pathcoefficient analysis was used to estimate coefficients of a model of causal relations among fitness components (nge-specific sizes and seed production) and to examine how differences in growth between ecotypes over a succession of phases lead to selection against introduced ecotypes at the level of seed production. The path-coefficient analysis revealed large differences between ecotypes in performance over many intervals and in many relations between fitness components. However, differences did not occur at all growth stages, and introduced ecotypes were superior in several comparisons. Path-coefficient analysis offers insights into the comparative adaptation of ecotypes beyond those possible from analyses that do not account for causal relations among fitness components. By focusing attention on life-history stages at which fitness differences arise between ecotypes, it can facilitate investigations of ecophysiological and other mechanisms underlying adaptive differences among ecotypes.

Comparative Functional Morphology of the Anchorage Systems of Annual Dicots

Studies of root anchorage have shown that the mechanical function of roots is restricted to strengthened basal areas, whereas delicate distal roots have a role largely in absorption. Analysis further shows that to most economically withstand the upward grazing forces to which procumbent and climbing plants are subjected a fibrous root system is optimal, while tall free-standing plants need to invest in a further stiff element such as a tap-root to resist toppling. Relative investment in the fibrous roots of procumbent and climbing plants should fall with plant size but investment in the tap-root of upright plants should be size independent. These predictions were tested in three surveys in which the aboveand below-ground morphology of several species of full-grown annual dicots were examined and the relative dry mass investment in the shoot, tap-root, fibrous roots and absorption roots were measured. Below-ground morphology varied with above-ground morphology both between and within species and with plant size as predicted: free-standing plants had tap-root systems containing a relatively constant proportion (3-O/6 of total dry mass) in the tap-root and 3-5% in the fibrous roots; procumbent plants had fibrous root systems containing 15-12% of the total dry mass, larger plants having relatively smaller root systems. Multi-stemmed and rosette species had root systems of intermediate morphology. There was no relationship between above-ground form and the investment in absorption root. Mechanics has helped us explain below-ground differences in the morphology of annual dicots and may help explain growth responses to the environment.