The mass of seeds contributes to both avoidance of disturbance and recovery from flood events Grime et al. The production of fewer, larger seeds may facilitate the formation of a seed bank so that seeds may lie dormant until conditions are suitable for germination and seedling growth. The production of many, small, well-dispersed, short-lived seeds enhances recolonization of disturbed sites following exposure Thompson et al. In addition, there is some evidence that seed size is positively related to establishment success where water availability is limited Leishman and Westoby Morphological-effect traits are those that influence the flow of water, transport of sediment, and stabilization of landforms on the basis of their architecture.
Geomorphic functions are the mechanisms by which the plant influences geomorphic process or form table 2. Plants influence force balances—and therefore flow and sediment transport processes—in rivers. At the individual plant scale, plant size generally controls the magnitude of the drag force.
Other measures of plant size, such as volume, biomass, height, or diameter, may also be correlated with the drag force Sand-Jensen , Whittaker et al.
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Plant size, specifically height, also controls whether the plant remains emergent, and therefore occupies the entire water column, or becomes submerged, and therefore occupies only a portion of the water column. Differences in submergence influence the velocity profile and the magnitude of mean and turbulent velocities, such that velocities are generally lower for emergent plants than for submerged ones, as well as the dominant direction of flow adjustments e.
The vertical component of velocity and vertical velocity fluctuations, negligible around emergent plants, are strong for submerged canopies, resulting in decreased suspended sediment deposition downstream from submerged patches, especially compared with the enhanced sediment deposition downstream from emergent plants Ortiz et al. As a result, a plant may transition from emergent to submerged, even when plant height exceeds flow depth. In some situations, the buoyancy of a plant, a function of the stem tissue density, will also influence its position within the water column Luhar and Nepf Plant pronation is often accompanied by streamlining that reduces frontal area and changes the shape of the plant, altering the drag coefficient.
These adjustments result in a decrease in drag. Vegetative drag and the relationship between drag and velocity are also determined by foliage characteristics Vogel When leaf area is as great as or greater than stem area, such as for woody seedlings, leaf area alone may predict the vegetation resistance Jalonen et al. Submerged foliated plants, when compared with bare stems, also decrease velocities within the vegetated zone and shift peak turbulence stresses up in the flow column Wilson et al. A plant's architecture , the three-dimensional organization of a plant's leaves, stems, and branches, also controls hydraulic and sediment transport properties around plants and patches of plants Lightbody and Nepf The vertical distribution of a plant's biomass is particularly important.
Jalonen and colleagues found that between model plants, those whose leaves were concentrated at the top had greater resistance than those that were more evenly distributed along the stem.
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Manners and colleagues , in contrast, showed that Populus seedlings whose foliage forms a distinct upper canopy had less of an impact on flow velocities—and therefore less resistance and topographic change Diehl et al. However, Populus and Tamarix seedlings also had different branching patterns: Tamarix is a shrubby plant with multiple stems growing from the ground, whereas the typical growth form for Populus is a single stem. These branching patterns influence the reconfiguration potential of a plant Whittaker et al. At the patch- to stand-scale, architectural details of the aboveground biomass become less important than the density, spacing, and distribution of plants Luhar and Nepf Greater plant densities generally decrease mean velocity, drag coefficients, and boundary shear stress within a patch Bennett et al.
Spacing and the distribution of plants affect the spatial variability of flow, altering flow resistance Sand-Jensen as a result of the erosional and depositional patterns. Small, densely spaced plants induce more uniform deposition than larger, sparsely spaced plants, which produce greater depositional variability and topographic heterogeneity Perignon et al. Belowground characteristics are also important for determining a plant's geomorphic function. Root architecture, including the maximum rooting depth, influences the cohesion of the channel or floodplain sediment and increases landform stability Pollen-Bankhead and Simon Such a geomorphic function is particularly important for systems that experience floods prior to the growing season, where the aboveground biomass may be absent or not sufficient to alter hydraulics, erosion, and deposition Gurnell Smaller diameter roots offer greater tensile strength per unit mass; Abernathy and Rutherford , and as a result, fine roots reduce erosion rates to a greater degree than large taproots De Baets et al.
However, deeper roots can add mechanical support over greater depths Docker and Hubble Plant height, leaf area, and root architecture or depth appear in both lists of ecological-response table 1 and morphological-effect traits table 2. Stem tissue density and flexural rigidity are related, but the relationship differs between growth forms i.
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Because of plant strategy trade-offs, some ecological traits are highly related to other ecological traits and may be correlated with morphological ones. In addition, physiological constraints on vascular plant geometry, including the efficient transport of water and nutrients and structural soundness, dictate that many plant traits covary with size West et al. Plant height is an important component of a plant's competitive ability and not only governs access to photosynthetically active radiation from the sun but also provides an indication of the extent of the root system, which is related to nutrient and water acquisition Stromberg Plant height also influences hydraulics and geomorphic processes.
Tall plants generally have greater biomass, typically concentrated higher up on the plant, in order to capture light. Larger-diameter stems and denser stem tissue are necessary to support their height. More rigid, taller, larger-stemmed plants have a lower likelihood of submergence, with associated effects on hydraulic and sediment transport conditions.
Plant height is also related to the total frontal area, or biomass, of a plant, and plant size scales to the density of individual plants in resource-limited settings Enquist et al. Thus, height strongly influences the drag, vertical distribution of velocity, and shear stress at the plant to patch scale figure 1. Leaf area, important for the water and carbon balance of a plant, directly influences the frontal area and the streamlining potential especially, because a plant with larger leaves tends to have larger total leaf area Westoby and Wright Leaf size covaries with numerous other traits.
Stem tissue density, specifically for woody species, decreases as leaf area increases Pickup et al. Stromberg and Merritt found that leaf length and wood density were inversely related along Arizona streams and that plants with lower wood density had affinities to wetter habitats and were less tolerant of drought stress.
In addition, smaller leaves typically grow on narrow, frequently branched twigs, whereas larger leaves are supported by thicker twigs that branch less Westoby and Wright Thus, leaf area also indirectly influences the geomorphic functions of a plant, including the stem's flexibility and its branching structure, further contributing to the pronation and streamlining potential of the plant. Root architecture, including the maximum root depth, determines a plant's ability to draw alluvial water from deep water tables and in turn influences stream-bank and bar stability Pollen-Bankhead and Simon As was previously mentioned, the size of the root system scales to the aboveground biomass, thus also indirectly influencing the hydraulic and sediment transport conditions.
The strategies available for plants to directly resist mechanical disturbance from floods have an influence on geomorphic processes. As we discussed above, plants may either tolerate or avoid high flow velocities and other flood-associated stresses Puijalon et al. Stem tissue density is related to stem flexibility Niklas Plants that avoid high flow velocities often do so with flexible stems and high streamlining potential that results in a reduction of drag on the plant. Tolerant plants have large, rigid stems and a tall stature and experience larger drag forces.
Eleven ecological and morphological plant trait values were collected from individual plants, comprising 34 species and 8 life-history stages for the 3 dominant woody species Tamarix ramosissima , Salix exigua , and Populus fremontii , for a total of 39 species and life-history observations. Our sampling scheme targeted individuals three per species, on average that best represented the growing conditions in Dinosaur National Monument and are therefore not representative of the full range of trait values possible for a species.
Riparian vegetation and fluvial geomorphology
Because our sample sizes are small and our study is limited to two rivers, we consider our data set and associated analyses to represent a pilot study illustrating the efficacy of using plant response and effect traits in modeling riparian systems, thereby illustrating an approach with strong potential for broader applicability. Measured traits were used to create two sets of guilds ecological and morphological from a hierarchical cluster analysis based on a Gower dissimilarity index figures 2 and 3 ; Stromberg and Merritt At this significance level, species clustered into groupings that were consistent with our professional understanding of their ecological and morphological functions.
An organization of plants from the field-study area into ecological guilds. The guilds are shown as unique symbols: circles for herbaceous guilds, plus signs for woody ones. The species names associated with species symbols are shown in supplemental table S1. Height and stem tissue density were used to identify ecological guilds in the clustering analysis; flexural rigidity and plant architecture were used to identify morphological guilds. The online version contains color. An organization of plants from the field-study area into morphological guilds. Guilds are shown as unique symbols: circles for herbaceous guilds, plus signs for woody ones Symphoricarpos occidentalis , a woody plant within the rigid branching herb guild, is an exception.
Plant height and the traits that co-vary with height root depth, flexural rigidity, frontal area, and plant density describe the variability along the first axis. Species names associated with species symbols are shown in supplemental table S1. The online version for color. We designed our trait list to quantify, with a broad spread of values, a suite of morphological and ecological traits that could be measured in the field or laboratory or found in the literature.
The traits chosen were the minimum number necessary to identify the range of flow and sediment-transport impacts, as well as the range of life-history strategies for plants in a semiarid riparian environment. Morphological traits included height, maximum plant density, frontal area, flexural rigidity, plant architecture, and root depth. Ecological traits included growth form woody or herbaceous , height, diameter, stem tissue density, specific leaf area, seed weight, and root depth see supplemental material.
For the gradient represented by our data, plant height and general habitat type i. As such, plant height generally increases along the gradient of water availability figure 2 b. Stem tissue density and height also show a general trend, in which shorter plants have smaller densities than taller ones figure 2 c.
ncof.co.uk/map19.php Greater variability in stem tissue density, a potential measure of drought tolerance, indicates that tall xeric plants may also have other strategies that allow growth in drier areas of the channel. For example, the xeric late-seral shrub guild has deep roots and a small specific leaf area figure 2. We also infer a trend in the tolerance or resistance of plants to fluvial disturbance on the basis of trait averages within ecological guilds.
Average stem tissue density increases with height, although not consistently. The flexural rigidity of ecological guilds, a more robust measure of stem flexibility, does have a strong relationship with plant size and moisture availability, as does frontal area not shown; figure 2 c.
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Plant architecture, although variable within most ecological guilds, does tend to favor single-stemmed morphologies for smaller, herbaceous plants and multistemmed morphologies for larger, woody plants figure 2 c. Single-stemmed plants have greater potential for a more streamlined, drag-reducing profile in the flow than do multistemmed ones. As such, plants that tolerate drier conditions are taller and more rigid, resulting in both greater exposure to and ability to withstand fluvial disturbance, whereas hydric plants are short and flexible, with a greater potential to streamline in the flow in a manner that minimizes their exposure to fluvial disturbance.
The cluster analysis identified nine morphological guilds figure 3. Most of the variability among guilds is described by plant size attributes and the traits that covary with size root depth, frontal area, plant density, and flexural rigidity; figure 3 a. Plant architecture further differentiates guilds depending on their branching structure.
In addition, species clustered according to growth form five herbaceous and four woody , even though we did not include this variable in the cluster analysis. As such, a key finding is that growth form, an ecologically relevant trait, is related to important morphological properties. Guild analyses based separately on morphological and ecological guilds suggest significant overlap in ecological and morphological properties.
Morphological traits are grouped in a significant way within ecological guilds. In other words, the range of flexural rigidity values for the species within an ecological guild does not overlap those species values within another ecological guild. Overlap in species membership in ecological and morphological guilds further supports these observations. Sixty-nine percent of the 39 observations overlap with at least one other species in both types of guilds, and the two groupings have a Cohen's kappa value of 0.
As such, our data set and the accompanying analyses confirm a strong link between response and effect traits. Because ecological guilds are composed of plants that are morphologically similar, the geomorphic impact of plants may show a spatial signature as a result of the distribution of abiotic stressors. Of course, these abiotic stressors are also important for geomorphic processes.
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