Susanne Schwinning

Mechanisms determining the degree of size asymmetry in competition among plants

Abstract When plants are competing, larger individuals often obtain a disproportionate share of the contested resources and suppress the growth of their smaller neighbors, a phenomenon called size-asymmetric competition. We review what is known about the mechanisms that give rise to and modify the degree of size asymmetry in competition among plants, and attempt to clarify some of the confusion in the literature on size asymmetry. We broadly distinguish between mechanisms determined primarily by characteristics of contested resource from those that are influenced by the growth and behavior of the plants themselves. To generate size asymmetric resource competition, a resource must be “pre-emptable.” Because of its directionality, light is the primary, but perhaps not the only, example of a pre-emptable resource. The available data suggest that competition for mineral nutrients is often size symmetric (i.e., contested resources are divided in proportion to competitor sizes), but the potential role of patchily and/or episodically supplied nutrients in causing size asymmetry is largely unexplored. Virtually nothing is known about the size symmetry of competition for water. Plasticity in morphology and physiology acts to reduce the degree of size asymmetry in competition. We argue that an allometric perspective on growth, allocation, resource uptake, and resource utilization can help us understand and quantify the mechanisms through which plants compete.

Assessing the Response of Terrestrial Ecosystems to Potential Changes in Precipitation

Abstract Changes in Earths surface temperatures caused by anthropogenic emissions of greenhouse gases are expected to affect global and regional precipitation regimes. Interactions between changing precipitation regimes and other aspects of global change are likely to affect natural and managed terrestrial ecosystems as well as human society. Although much recent research has focused on assessing the responses of terrestrial ecosystems to rising carbon dioxide or temperature, relatively little research has focused on understanding how ecosystems respond to changes in precipitation regimes. Here we review predicted changes in global and regional precipitation regimes, outline the consequences of precipitation change for natural ecosystems and human activities, and discuss approaches to improving understanding of ecosystem responses to changing precipitation. Further, we introduce the Precipitation and Ecosystem Change Research Network (PrecipNet), a new interdisciplinary research network assembled to encourage and foster communication and collaboration across research groups with common interests in the impacts of global change on precipitation regimes, ecosystem structure and function, and the human enterprise.

Hierarchy of responses to resource pulses in arid and semi-arid ecosystems

In arid/semi-arid ecosystems, biological resources, such as water, soil nutrients, and plant biomass, typically go through periods of high and low abundance. Short periods of high resource abundance are usually triggered by rainfall events, which, despite of the overall scarcity of rain, can saturate the resource demand of some biological processes for a time. This review develops the idea that there exists a hierarchy of soil moisture pulse events with a corresponding hierarchy of ecological responses, such that small pulses only trigger a small number of relatively minor ecological events, and larger pulses trigger a more inclusive set and some larger ecological events. This framework hinges on the observation that many biological state changes, where organisms transition from a state of lower to higher physiological activity, require a minimal triggering event size. Response thresholds are often determined by the ability of organisms to utilize soil moisture pulses of different infiltration depth or duration. For example, brief, shallow pulses can only affect surface dwelling organisms with fast response times and high tolerance for low resource levels, such as some species of the soil micro-fauna and -flora, while it takes more water and deeper infiltration to affect the physiology, growth or reproduction of higher plants. This review first discusses how precipitation, climate and site factors translate into soil moisture pulses of varying magnitude and duration. Next, the idea of the response hierarchy for ecosystem processes is developed, followed by an exploration of the possible evolutionary background for the existence of response thresholds to resource pulses. The review concludes with an outlook on global change: does the hierarchical view of precipitation effects in ecosystems provide new perspectives on the future of arid/semiarid lands?

Population Dynamic Consequences of Competitive Symmetry in Annual Plants

Asymmetric competition is a form of resource division among plants, in which large plants greatly suppress the growth of smaller neighbors. In annual plants, small size differences between seedlings at the onset of competition are magnified into large differences in seed-set by asymmetric competition. We formulate a novel neighborhood model, which reflects this seedling size effect as modified by the type of competitive symmetry. In the model, competition type is represented by a single, biologically meaningful parameter. We implement the model in a population growth model for two species, one at low density (the invader), and one at high density (the resident). The species are the same, except for their seedling biomass distributions. Under these conditions, we find that asymmetric competition always favors invasion by the species with lager average seedling size, but impairs invasion by the other species. Based on this invasibility criterion, we conclude that asymmetric competition always favors competitive exclusion in our model. However, by modifying some of the model assumptions, we suggest scenarios in which asymmetric competition may promote coexistence

Periodic oscillations in an ideal-free predator-prey distribution.

We simulated the habitat selection behavior in a three population predator-prey system with a mid-level predator that is also prey. There were two habitats, one of which was a relative refuge from predation. Individuals in the model moved to wherever they could improve their fitness, as if subject to the rules of the ideal-free distribution. However, the three populations could generally not achieve three simultaneous ideal-free distributions. Instead, individuals shifted back and forth between the habitats. Such oscillations were stabilized in three ways: 1) increase in the protection provided by the refuge; 2) increase in intraspecific competition among the prey; 3) the presence of a threshold in fitness difference, below which individuals would not change habitats. In the presence of a threshold, population distributions became stable without having achieved a simultaneous ideal-free distribution.

Effects of Nitrogen Deposition on an Arid Grassland in the Colorado Plateau Cold Desert

Abstract Historically, ecosystems in the southwestern United States derived much of their nitrogen (N) from N-fixation in biological soil crusts. Today, these regions have highly reduced crust cover, and atmospheric deposition may be the dominant source of N. This study investigates the effects of increased nitrogen deposition on nitrogen uptake, photosynthesis, and growth of the two main forage grasses on the Colorado Plateau, galleta (Hilaria jamesii [Torr.] Benth.) and Indian ricegrass (Oryzopsis hymenoides, [Roemer & J.S. Schultes] Ricker ex Piper). Plots were fertilized for 2 years with 0, 10, 20, and 40 kg nitrogen ha−1 annually, up to 4× the estimated current annual deposition rate, in 2 applications per year (spring and summer). Half-plots were fertilized with either (NH4)2SO4 in KCl solution or with KNO3 solution to determine possible differences in the effects of NH4+ and NO3− in this system. Neither grass increased leaf photosynthesis or tiller size due to supplemental N. Galleta also did not increase tiller density, while estimated live tiller density in Indian ricegrass increased up to 50% in the second year. Nitrogen applications accelerated the onset of water stress in both species presumably through stimulating ecosystem transpiration. Nitrogen form did not significantly affect any aspect of grass physiological performance or growth. However, leaf nitrogen in NH4+-fertilized plants was significantly more isotopically enriched than in NO3−-fertilized plants, suggesting that both species incorporated NH4-N only after it had been enriched by soil turnover. Seedlings of Russian Thistle (Salsola iberica, Sennen & Pau), a noxious annual invasive weed on western rangelands, grew rapidly in the first summer on plots with 40 kg nitrogen ha−1 per annum, and more so on plots fertilized with NO3− than with NH4+. The study suggests that changes in the timing and amount of nitrogen input may alter community composition through facilitating the invasions of summer-active noxious weeds.

Woody Plant Encroachment: Causes and Consequences

Woody vegetation in grasslands and savannas has increased worldwide over the past 100–200 years. This phenomenon of “woody plant encroachment” (WPE) has been documented to occur at different times but at comparable rates in rangelands of the Americas, Australia, and southern Africa. The objectives of this chapter are to review (1) the process of WPE and its causes, (2) consequences for ecosystem function and the provision of services, and (3) the effectiveness of management interventions aimed at reducing woody cover. Explanations for WPE require consideration of multiple interacting drivers and constraints and their variation through time at a given site. Mean annual precipitation sets an upper limit to woody plant cover, but local patterns of disturbance (fire, browsing) and soil properties (texture, depth) prevent the realization of this potential. In the absence of these constraints, seasonality, interannual variation, and intensity of precipitation events determine the rate and extent of woody plant expansion. Although probably not a triggering factor, rising atmospheric CO2 levels may have favored C3 woody plant growth. WPE coincided with the global intensification of livestock grazing that by reducing fine fuels, hence fire frequency and intensity, facilitated WPE. From a conservation perspective, WPE threatens the maintenance of grassland and savanna ecosystems and its endemic biodiversity. Traditional management goals aimed at restoring forage and livestock production after WPE have broadened to support a more diverse portfolio of ecosystem services. Accordingly, we focus on how WPE and management actions aimed at reducing woody plant cover influence carbon sequestration, water yield, and biodiversity, and discuss the trade-offs involved when balancing competing management objectives.

Plant competition, temporal niches and implications for productivity and adaptability to climate change in water‐limited environments

Summary We review plant competition in water-limited environments with focus on temporal niche dynamics and examine implications for diversity–productivity relationships and the response of ecosystem productivity to changes in water availability. The main theses under examination are that (i) plant functional types (PFTs) have distinct resource pulse use and coexist through mechanism of temporal resource use complementarity; and (ii) species of same PFT (functionally redundant species) coexist through distinct recruitment niches. In water-limited systems, opportunities for plant resource uptake and growth fluctuate through time, dependent on precipitation patterns. Species differ in the sensitivities of germination, seedling mortality and adult productivity to pulses of water availability, and this generates opportunity for temporal niche diversification. We illustrate this in two case studies. Case study I. Savannas: This is an example of niche separation between two distinct plant functional types (PFTs), trees and grasses. Several models suggest that the two PFTs have complementary resource pulse use, which regulates their abundances, but other models suggest that tree abundance is regulated by the narrow recruitment niche of trees. Overly restrictive recruitment niches can cause a mismatch between resource availability, PFT composition and ecosystem productivity. Case study II. The tropical dry forest: Here, we examine niche separation between closely related species of same PFT. These species commonly have distinct temporal recruitment niches based on differences in seed and seedling traits. A diversification of recruitment niches may be necessary for sympatric speciation and has the effect of broadening of the recruitment ‘portfolio’ of a phylogenetic lineage and PFT. Synthesis: Functional diversity, characterized by differences in adult resource use, optimizes ecosystem function in a pulsed resource environment only if PFT abundances are regulated by adult resource use. Regulation through recruitment niches tends to uncouple plant productivity from resource availability. However, we hypothesize that a diversification of recruitment niches within PFTs may help alleviate recruitment limitations and help communities attain a PFT composition that optimizes resource use and permits adaptation to climate change.

Accelerated development in Johnsongrass seedlings (Sorghum halepense) suppresses the growth of native grasses through size-asymmetric competition

Invasive plant species often dominate native species in competition, augmenting other potential advantages such as release from natural enemies. Resource pre-emption may be a particularly important mechanism for establishing dominance over competitors of the same functional type. We hypothesized that competitive success of an exotic grass against native grasses is mediated by establishing an early size advantage. We tested this prediction among four perennial C4 warm-season grasses: the exotic weed Johnsongrass (Sorghum halepense), big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparius) and switchgrass (Panicum virgatum). We predicted that a) the competitive effect of Johnsongrass on target species would be proportional to their initial biomass difference, b) competitive effect and response would be negatively correlated and c) soil fertility would have little effect on competitive relationships. In a greenhouse, plants of the four species were grown from seed either alone or with one Johnsongrass neighbor at two fertilizer levels and periodically harvested. The first two hypotheses were supported: The seedling biomass of single plants at first harvest (50 days after seeding) ranked the same way as the competitive effect of Johnsongrass on target species: Johnsongrass < big bluestem < little bluestem/switchgrass, while Johnsongrass responded more strongly to competition from Johnsongrass than from native species. At final harvest, native plants growing with Johnsongrass attained between 2–5% of their single-plant non-root biomass, while Johnsongrass growing with native species attained 89% of single-plant non-root biomass. Fertilization enhanced Johnsongrass’ competitive effects on native species, but added little to the already severe competitive suppression. Accelerated early growth of Johnsongrass seedlings relative to native seedlings appeared to enable subsequent resource pre-emption. Size-asymmetric competition and resource-pre-emption may be a critical mechanism by which exotic invasive species displace functionally similar native species and alter the functional dynamics of native communities.