Showing papers by "Steven I. Higgins published in 2014"

Savanna Vegetation-Fire-Climate Relationships Differ Among Continents

Abstract: Ecologists have long sought to understand the factors controlling the structure of savanna vegetation. Using data from 2154 sites in savannas across Africa, Australia, and South America, we found that increasing moisture availability drives increases in fire and tree basal area, whereas fire reduces tree basal area. However, among continents, the magnitude of these effects varied substantially, so that a single model cannot adequately represent savanna woody biomass across these regions. Historical and environmental differences drive the regional variation in the functional relationships between woody vegetation, fire, and climate. These same differences will determine the regional responses of vegetation to future climates, with implications for global carbon stocks.

The influence of interspecific interactions on species range expansion rates

Abstract: Ongoing and predicted global change makes understanding and predicting species' range shifts an urgent scientific priority. Here, we provide a synthetic perspective on the so far poorly understood effects of interspecific interactions on range expansion rates. We present theoretical foundations for how interspecific interactions may modulate range expansion rates, consider examples from empirical studies of biological invasions and natural range expansions as well as process-based simulations, and discuss how interspecific interactions can be more broadly represented in process-based, spatiotemporally explicit range forecasts. Theory tells us that interspecific interactions affect expansion rates via alteration of local population growth rates and spatial displacement rates, but also via effects on other demographic parameters. The best empirical evidence for interspecific effects on expansion rates comes from studies of biological invasions. Notably, invasion studies indicate that competitive dominance and release from specialized enemies can enhance expansion rates. Studies of natural range expansions especially point to the potential for competition from resident species to reduce expansion rates. Overall, it is clear that interspecific interactions may have important consequences for range dynamics, but also that their effects have received too little attention to robustly generalize on their importance. We then discuss how interspecific interactions effects can be more widely incorporated in dynamic modeling of range expansions. Importantly, models must describe spatiotemporal variation in both local population dynamics and dispersal. Finally, we derive the following guidelines for when it is particularly important to explicitly represent interspecific interactions in dynamic range expansion forecasts: if most interacting species show correlated spatial or temporal trends in their effects on the target species, if the number of interacting species is low, and if the abundance of one or more strongly interacting species is not closely linked to the abundance of the target species.

Using dynamic vegetation models to simulate plant range shifts

Abstract: Dynamic vegetation models (DVMs) follow a process-based approach to simulate plant population demography, and have been used to address questions about disturbances, plant succession, community composition, and provisioning of ecosystem services under climate change scenarios. Despite their potential, they have seldom been used for studying species range dynamics explicitly. In this perspective paper, we make the case that DVMs should be used to this end and can improve our understanding of the factors that influence species range expansions and contractions. We review the benefits of using process-based, dynamic models, emphasizing how DVMs can be applied specifically to questions about species range dynamics. Subsequently, we provide a critical evaluation of some of the limitations and trade-offs associated with DVMs, and we use those to guide our discussions about future model development. This includes a discussion on which processes are lacking, specifically a mechanistic representation of dispersal, inclusion of the seedling stage, trait variability, and a dynamic representation of reproduction. We also discuss upscaling techniques that offer promising solutions for being able to run these models efficiently over large spatial extents. Our aim is to provide directions for future research efforts and to illustrate the value of the DVM approach.

Invasive plants have broader physiological niches

Abstract: Invasive species cost the global economy billions of dollars each year, but ecologists have struggled to predict the risk of an introduced species naturalizing and invading. Although carefully designed experiments are needed to fully elucidate what makes some species invasive, much can be learned from unintentional experiments involving the introduction of species beyond their native ranges. Here, we assess invasion risk by linking a physiologically based species distribution model with data on the invasive success of 749 Australian acacia and eucalypt tree species that have, over more than a century, been introduced around the world. The model correctly predicts 92% of occurrences observed outside of Australia from an independent dataset. We found that invasiveness is positively associated with the projection of physiological niche volume in geographic space, thereby illustrating that species tolerant of a broader range of environmental conditions are more likely to be invasive. Species achieve this broader tolerance in different ways, meaning that the traits that define invasive success are context-specific. Hence, our study reconciles studies that have failed to identify the traits that define invasive success with the urgent and pragmatic need to predict invasive success.

Increasing atmospheric CO2 overrides the historical legacy of multiple stable biome states in Africa

Abstract: Summary The dominant vegetation over much of the global land surface is not predetermined by contemporary climate, but also influenced by past environmental conditions. This confounds attempts to predict current and future biome distributions, because even a perfect model would project multiple possible biomes without knowledge of the historical vegetation state. Here we compare the distribution of tree- and grass-dominated biomes across Africa simulated using a dynamic global vegetation model (DGVM). We explicitly evaluate where and under what conditions multiple stable biome states are possible for current and projected future climates. Our simulation results show that multiple stable biomes states are possible for vast areas of tropical and subtropical Africa under current conditions. Widespread loss of the potential for multiple stable biomes states is projected in the 21st Century, driven by increasing atmospheric CO2. Many sites where currently both tree-dominated and grass-dominated biomes are possible become deterministically tree-dominated. Regions with multiple stable biome states are widespread and require consideration when attempting to predict future vegetation changes. Testing for behaviour characteristic of systems with multiple stable equilibria, such as hysteresis and dependence on historical conditions, and the resulting uncertainty in simulated vegetation, will lead to improved projections of global change impacts.

How to tell a shrub from a tree: A life‐history perspective from a South African savanna

Abstract: The ecological differences between 'shrubs' and 'trees' are surprisingly poorly understood and clear ecological definitions of these two constructs do not exist. It is not clear whether a shrub is simply a small tree or whether shrubs represent a distinct life-history strategy. This question is of special interest in African savan- nas, where shrubs and trees often co-dominate, but are often treated uniformly as 'woody plants' even though the tree to shrub ratio is an important determinant of ecosystem functioning. In this study we use data from a long-term fire experiment, together with a trait-based approach to test (i) if woody species usually classified as shrubs or trees in African savanna differ in key traits related to disturbance and resource use; and (ii) if these differences justify the interpretation of the two growth forms as distinct life-history strategies. We measured for 22 of the most common woody plant species of a South African savanna 27 plant traits related to plant architecture, life-history, leaf characteristics, photosynthesis and resprouting capacity. Furthermore we evaluated their performance during a long-term fire experiment. We found that woody plants authors call (i) shrubs; (ii) shrubs sometimes small trees; and (3) trees responded differently to long-term fire treatments. We additionally found significant differences in architecture, diameter-height-allometry, foliage density, resprouting vigour after fire, minimum fruiting height and foliar δ 13 C between these three woody plant types. We interpret these findings as evidence for at least two different life-history-strategies: an avoidance/adaptation strategy for shrubs (early repro- duction + adaptation to minor disturbance) and an escape strategy for trees (promoted investment in height growth + delayed reproduction).

Contrasting architecture of key African and Australian savanna tree taxa drives intercontinental structural divergence

Abstract: Aim We examined differences in the architecture of African and Australian savanna trees. We sought to attribute variation in tree architecture to current environments, wood density and phylogeny, and thereby elucidate the relative importance of biogeographic idiosyncrasies versus current factors in underpinning architectural differences. Location Africa and Australia. Methods We compiled canopy diameters and stem diameters from 4867 trees of 97 species and heights and stem diameters from 10,786 trees of 155 species from a range of African and Australian savanna ecosystems and climates. Using Bayesian methods we first estimated continental-scale savanna tree allometries, ignoring species differences. We then examined continental differences in species-specific allometries accounting for trait covariation using a phylogeny of our study species. Environmental variables and wood density data were included as covariates, allowing us to assess the potential underpinning of regional differences in tree allometries by differences in current environments and traits. Results Substantial allometric differences exist between Australian and African savanna trees. Australian trees are on average 6 m taller at 20 cm diameter, with a 53% smaller canopy area than African trees. However, this extreme continental-scale variation is driven by the architecture of only a few taxa in this study – Vachellia and Senegalia versus Eucalyptus and Corymbia – rather than systematic differences between species, wood density and environment. These same genera often dominate the woody strata of South African and Australian savannas, respectively. Main conclusions Stark differences in the architecture of African and Australian savanna tree taxa are not a product of environmental differences and are not consistent across species. Rather, the most likely explanation is the different evolutionary histories of African and Australian savannas, which share no woody species. We consider that these architectural differences are likely to impact regional patterns of woody biomass accumulation.

On the potential vegetation feedbacks that enhance phosphorus availability – insights from a process-based model linking geological and ecological timescales

Abstract: In old and heavily weathered soils, the availability of P might be so small that the primary production of plants is limited. However, plants have evolved several mechanisms to actively take up P from the soil or mine it to overcome this limitation. These mechanisms involve the active uptake of P mediated by mycorrhiza, biotic de-occlusion through root clusters, and the biotic enhancement of weathering through root exudation. The objective of this paper is to investigate how and where these processes contribute to alleviate P limitation on primary productivity. To do so, we propose a process-based model accounting for the major processes of the carbon, water, and P cycles including chemical weathering at the global scale. Implementing P limitation on biomass synthesis allows the assessment of the efficiencies of biomass production across different ecosystems. We use simulation experiments to assess the relative importance of the different uptake mechanisms to alleviate P limitation on biomass production. We find that active P uptake is an essential mechanism for sustaining P availability on long timescales, whereas biotic de-occlusion might serve as a buffer on timescales shorter than 10 000 yr. Although active P uptake is essential for reducing P losses by leaching, humid lowland soils reach P limitation after around 100 000 yr of soil evolution. Given the generalized modelling framework, our model results compare reasonably with observed or independently estimated patterns and ranges of P concentrations in soils and vegetation. Furthermore, our simulations suggest that P limitation might be an important driver of biomass production efficiency (the fraction of the gross primary productivity used for biomass growth), and that vegetation on old soils has a smaller biomass production rate when P becomes limiting. With this study, we provide a theoretical basis for investigating the responses of terrestrial ecosystems to P availability linking geological and ecological timescales under different environmental settings.

Soil water retention curves for the major soil types of the Kruger National Park

Abstract: Soil water potential is crucial to plant transpiration and thus to carbon cycling and biosphere–atmosphere interactions, yet it is difficult to measure in the field. Volumetric and gravimetric water contents are easy and cheap to measure in the field, but can be a poor proxy of plant-available water. Soil water content can be transformed to water potential using soil moisture retention curves. We provide empirically derived soil moisture retention curves for seven soil types in the Kruger National Park, South Africa. Site-specific curves produced excellent estimates of soil water potential from soil water content values. Curves from soils derived from the same geological substrate were similar, potentially allowing for the use of one curve for basalt soils and another for granite soils. It is anticipated that this dataset will help hydrologists and ecophysiologists understand water dynamics, carbon cycling and biosphere–atmosphere interactions under current and changing climatic conditions in the region.

Progress in DGVMs: a comment on "Impacts of trait variation through observed trait–climate relationships on performance of an Earth system model: a conceptual analysis" by Verheijen et al. (2013)

Abstract: Dynamic global vegetation models (DGVMs) are now central elements in Earth system models, and our ability to understand past and anticipate future changes in the Earth system is intimately linked to the quality of DGVMs (Prentice et al., 2007). There are many ways in which DGVMs need improvement and there are many exciting initiatives under way. In a recent manuscript, Verheijen et al. (2013) describe one pathway. To provide a context for their work, they compare their approach to other initiatives. In this contribution we wish to point out ways in which Verheijen and colleagues misrepresented the aDGVM2 (which they incorrectly call the aDGVM, which is in fact a different model published by Scheiter and Higgins, 2009) as presented in Scheiter et al. (2013). While the aim of this piece is primarily to set the record straight, we additionally point out similarities and differences between the approach described by Verheijen et al. (2013) and that described by Scheiter et al. (2013). Verheijen et al. (2013) motivate their study by stating in reference to the Jena Diversity-DGVM (JeDi-DGVM) (Pavlick et al., 2013) and aDGVM2 (Scheiter et al., 2013) that “none of the approaches so far tried to maximally include trait variation based on observational trait data and capture multiple sources of this variation by relating trait data to environmental variables”. Although we appreciate that this statement was designed to illustrate the uniqueness of Verheijen et al. (2013) and the statistical approach they adopt, it does have the side-effect of suggesting that these two papers ignored variations in traits and the relationships between traits and the environment. We would like to point out that Fig. 5 of our paper plots the positions of modelled individuals in multivariate trait space and relates the axes of this trait space to environmental variables. In the same paragraph, the authors go on to suggest that DGVM modellers need to apply assembly theory to understand and model relationships between traits and the environment better, when they state that “such relationships between environmental conditions and traits can potentially be understood via ecological assembly theory”. This is exactly what we propose in Scheiter et al. (2013), where the introduction explicitly proposes that DGVM modelling could benefit from two branches of community ecology, namely coexistence theory and community assembly theory. Moreover, the title of Scheiter et al. (2013) includes the words “learning from community ecology”. Our impression from reading Pavlick et al. (2013) is that the traits that JeDi-DGVM predicts at a site are, as is the case with aDGVM2, a function of how environmental attributes select for trait combinations. This is an interpretation that Verheijen et al. (2013) appear, in apparent contradiction to their statement we cite above, to share in their discussion, when they state that “some DGVMs also implement the concept of environmental filtering, like the JeDi-DGVM (Pavlick et al., 2013)”.