Tree-grass Dynamics Research Articles

The distribution and drivers of tree cover in savannas and forests across India

The distribution of forest and savanna biomes and the role of resources (climate and soil) and disturbances (fire and herbivory) in determining tree-grass dynamics remains elusive and variable across geographies. This is especially problematic in Indian savannas which have been historically misclassified as degraded forests and are targeted for tree-planting. Here, we examine biome distribution and determinants through the lens of tree cover across India. Our analyses reveal four distinct zones of differing tree cover, with intermediate zones containing savanna vegetation. Rainfall seasonality determines maximum possible tree cover non-linearly. Once rainfall seasonality is factored out, soil sand fraction and topography partially explain residual variation of tree cover. High domestic livestock herbivory and other anthropogenic pressures reduce tree cover. Lastly, lack of detectable fires precludes robust conclusions about the relationship between fire and tree cover. By considering these environmental drivers in restoration planning, we can improve upon simplistic tree planting initiatives that may be detrimental to Indian savannas.

Spatio-temporal modelling of tree-grass dynamics in humid savannas: Interplay between nonlocal competition and nonlocal facilitation

For about twenty years, the question about the essential factors promoting the long-lasting coexistence of trees and grasses in humid savannas is at the center of several mathematical works, by the construction of deterministic and/or stochastic mathematical models. A closely related topic is coexistence of open savanna and forest patches at landscape scales, which raises the challenge of accounting for contrasted spatial patterns under similar climate conditions through fire mediated tree-grass interaction models. In this work, we propose and study a deterministic spatio-temporal fire-mediated tree-grass interactions model. The model is based on two nonlocal reaction-diffusion equations with kernels of intra and inter-specific interactions, corresponding to woody and grassy biomasses. A novelty in this paper is the consideration of a kernel-based nonlocal facilitation of trees by other trees to promote growth of seedlings/shrubs and, indirectly, limit fire propagation and its impact. We also take into account a kernel-based nonlocal competition of trees on grasses for light availability and nutrients. A qualitative analysis of the model is carried out and it reveals several ecological thresholds that shape the overall dynamics of the system. Depending on these thresholds, monostability of the forest, grassland or savanna space-homogeneous stationary state and multistabilities (i.e. involving more that one space-homogeneous stationary state) are proven possible. Thanks to the nonlocal biomasses interactions, our model accounts for the occurrence of space inhomogeneous solutions, including a possibly periodic spatial structuring sometimes observed in the humid savanna zone. Specifically, linear stability analyses, performed in the vicinity of space-homogeneous stationary states, provides conditions for the appearance of space inhomogeneous solutions including spatially periodic or aperiodic ones. Finally, numerical simulations are presented to illustrate our theoretical results. Notably, we verify that the computed spatial wavelengths were in good agreement with the predictions from the theoretical analysis.

Rooting depth varies differentially in trees and grasses as a function of mean annual rainfall in an African savanna.

A significant fraction of the terrestrial biosphere comprises biomes containing tree-grass mixtures. Forecasting vegetation dynamics in these environments requires a thorough understanding of how trees and grasses use and compete for key belowground resources. There is disagreement about the extent to which tree-grass vertical root separation occurs in these ecosystems, how this overlap varies across large-scale environmental gradients, and what these rooting differences imply for water resource availability and tree-grass competition and coexistence. To assess the extent of tree-grass rooting overlap and how tree and grass rooting patterns vary across resource gradients, we examined landscape-level patterns of tree and grass functional rooting depth along a mean annual precipitation (MAP) gradient extending from~450 to~750mmyear-1 in Kruger National Park, South Africa. We used stable isotopes from soil and stem water to make inferences about relative differences in rooting depth between these two functional groups. We found clear differences in rooting depth between grasses and trees across the MAP gradient, with grasses generally exhibiting shallower rooting profiles than trees. We also found that trees tended to become more shallow-rooted as a function of MAP, to the point that trees and grasses largely overlapped in terms of rooting depth at the wettest sites. Our results reconcile previously conflicting evidence for rooting overlap in this system, and have important implications for understanding tree-grass dynamics under altered precipitation scenarios.

Spatially explicit modelling of tree–grass interactions in fire-prone savannas: A partial differential equations framework

Long term tree–grass dynamics is an important issue in many places around the world. Especially, if we intend to take into account environmental and human perturbations, like fire events, herbivory, etc. In this paper, we complete and extend temporal models developed in recent papers by Yatat et al. (2014, 2017) and Tchuinte Tamen et al. (2014), using diffusion operators, into spatio-temporal models, in order to study the long term dynamics of a mosaic of forest and grassland in a humid context of Central Africa. We take into account local biomass diffusion, as well as local tree–grass competition for nutrients and light. For this model, we carry out a qualitative analysis that highlights some ecological thresholds shaping the dynamics of the system. In addition, we show that monostable and bistable travelling wave solutions may exist. We illustrate some of our theoretical results with numerical simulations, and, also explore the use of impulsive fires on the dynamics of a mosaic of forest and grassland in a humid savanna. We found that depending on fire-return time as well as difference in diffusion potential of woody and herbaceous vegetation, fire events are able to greatly slow down or even stop the progression of forest in humid regions.

Managing the human component of fire regimes: lessons from Africa.

Human impacts on fire regimes accumulated slowly with the evolution of modern humans able to ignite fires and manipulate landscapes. Today, myriad voices aim to influence fire in grassy ecosystems to different ends, and this is complicated by a colonial past focused on suppressing fire and preventing human ignitions. Here, I review available evidence on the impacts of people on various fire characteristics such as the number and size of fires, fire intensity, fire frequency and seasonality of fire in African grassy ecosystems, with the intention of focusing the debate and identifying areas of uncertainty. Humans alter seasonal patterns of fire in grassy systems but tend to decrease total fire emissions: livestock have replaced fire as the dominant consumer in many parts of Africa, and fragmented landscapes reduce area burned. Humans alter the season and time of day when fires occur, with important implications for fire intensity, tree-grass dynamics and greenhouse gas (GHG) emissions. Late season fires are more common when fire is banned or illegal: these later fires are far more intense but emit fewer GHGs. The types of fires which preserve human livelihoods and biodiversity are not always aligned with the goal of reducing GHG concentrations. Current fire management challenges therefore involve balancing the needs of a large rural population against national and global perspectives on the desirability of different types of fire, but this cannot happen unless the interests of all parties are equally represented. In the future, Africa is expected to urbanize and land use to intensify, which will imply different trajectories for the continent's fire regimes.This article is part of the themed issue 'The interaction of fire and mankind.

A minimalistic model of tree–grass interactions using impulsive differential equations and non-linear feedback functions of grass biomass onto fire-induced tree mortality

Since savannas are important ecosystems around the world, their long term dynamics is an important issue, in particular when perturbations, like fires, occur more or less often. In a previous paper, we developed and studied a tree–grass model that take into account fires as pulse events using impulsive differential equations. In this work, we propose to improve this impulsive model by considering the impact of pulse fire on tree biomass by means of combination of two nonlinear functions of grass and tree biomasses respectively. By considering two impact functions, our model yields more complex dynamics, allowing for the possibility of various bistabilities and periodic solutions, in either grassland or savanna states in the ecosystem. Our mathematical analysis allows extensive and realistic description of savannas ecosystems, than previous modelling approaches. We also highlight several threshold parameters that summarize all possible dynamics, as well as three main parameters of bifurcations in the tree–grass dynamics: the fire period, the tree–grass facilitation/competition parameter, and the fire intensity. Using an appropriate nonstandard numerical scheme, we provide numerical simulations to discuss some ecologically interesting cases that our model is able to exhibit along a rainfall gradient, observable in Central Africa.

Tree–grass interaction dynamics and pulsed fires: Mathematical and numerical studies

Savannas are dynamical systems where grasses and trees can either dominate or coexist. Fires are known to be central in the functioning of the savanna biome although their characteristics are expected to vary along the rainfall gradients as observed in Sub-Saharan Africa. In this paper, we model the tree–grass dynamics using impulsive differential equations that consider fires as discrete events. This framework allows us to carry out a comprehensive qualitative mathematical analysis that revealed more diverse possible outcomes than the analogous continuous model. We investigated local and global properties of the equilibria and we showed that various states exist for the physiognomy of vegetation. Though several shifts between vegetation states appeared to be determined by fire periodicity, we showed that direct shading of grasses by trees is also an influential process embodied in the model by a competition parameter leading to bifurcations. Relying on a suitable nonstandard simulation scheme, we carried out numerical simulations in reference to three main climatic zones as observable in Central Africa.

Defoliation by pastoralists affects savanna tree seedling dynamics by limiting the facilitative role of canopy cover.

Recurrent tree defoliation by pastoralists, akin to herbivory, can negatively affect plant reproduction and population dynamics. However, our understanding of the indirect role of defoliation in seedling recruitment and tree-grass dynamics in tropical savanna is limited. In West African savanna, Fulani pastoralists frequently defoliate several fodder tree species to feed livestock in the dry season. We investigated the direct and indirect effects of recurrent defoliation of African mahogany (Khaya senegalensis) by Fulani people on seedling (< 2 cm basal diameter) and sapling dynamics in West Africa using four years of demographic data on seedling and sapling density, growth, and survival, coupled with fruit production and microhabitat data over the same time period. Tree canopy cover facilitated seedlings but had negative effects on sapling growth possibly via intraspecific competition with adult plants. Interspecific competition with grasses strongly reduced seedling survival but had a weak effect on sapling growth. Fire reduced seedling survival and weakly reduced growth of seedlings and saplings, but did not affect sapling survival. These results indicate that the effect of fire on seedlings and saplings is distinct, a mechanism suitable for an episodic recruitment of seedlings into the sapling stage and consistent with predictions from the demographic bottleneck model. Defoliation affected seedling density and sapling growth through changes in canopy cover, but had no effect on seedling growth and sapling survival. In the moist region, sapling density was higher in sites with low-intensity defoliation, indicating that defoliation may strengthen the tree recruitment bottleneck. Our study suggests that large-scale defoliation can alter the facilitative role of nurse trees on seedling dynamics and tree-sapling competition. Given that tree defoliation by local people is a widespread activity throughout savanna-forest systems in West Africa, it has the potential to affect tree-grass coexistence. Incorporating the influence of large tree defoliation into existing models of savanna dynamics can further our understanding of tree-grass coexistence and improve management. A rotating harvest system, which allows seedlings to recruit episodically, or a patchwork harvest, which maintains some nursery trees in the mosaic, could help sustain seedling recruitment and minimize the indirect effects of harvest.

Transpiration dynamics support resource partitioning in African savanna trees and grasses

It is still far from clear whether and to what extent trees and grasses partition soil moisture in tropical savannas. A major reason for this is that we don't know how snapshot data on rooting differences translate into ecologically relevant patterns of water use at seasonal scales. We used stable isotopes in soil and stem water to quantify functional rooting profiles in grasses and two tree species in a South African savanna. Concurrently, we measured tree sap‐flow velocity, grass canopy temperature (a transpiration correlate), and soil moisture content at multiple depths over the course of a growing season. We used lasso regression to identify the dominant soil moisture layers driving daily variation in tree and grass water‐use metrics while controlling for weather variables. We found clear rooting depth differences between grasses (shallow) and trees (deep) from the isotopic data, and these patterns were strongly supported by the water‐use data, which showed that grasses and trees predominantly responded to soil moisture availability at 5 and 40 cm depth, respectively. Our results provide a rare example of mechanistic support for the resource partitioning hypothesis in savannas, with important implications for our understanding of tree–grass dynamics under altered precipitation regimes.

Modelling Fires as Pulses in Tree/Grass Interactions: Study of Long-term Impact in Forest-Savanna Dynamical Systems

It is usually admitted that fires play an important role in tree-grass interactions in savanna ecosystems. In this talk, we present a model of tree-grass dynamics using impulsive differential equations, considering fires as discrete events [1]. This framework allows us to carry out a comprehensive qualitative mathematical analysis that revealed more possible outcomes than the analogous continuous model [2]. We investigated local and global properties of the equilibria and show that various states may co-exist. Though fire periodicity may drive the system to different and abrupt shifts between vegetation, we also show that direct shading of grasses by trees, through a facilitation/competition parameter, is an influential process too, leading to bifurcations. Finally, using a suitable numerical approach [3], we carried out numerical simulations related to three main climatic zones, observable in Central Africa, to illustrate our theoretical results.

Competition between trees and grasses for both soil water and mineral nitrogen in dry savannas

The co-existence of trees and grasses in savannas in general can be the result of processes involving competition for resources (e.g. water and nutrients) or differential response to disturbances such as fire, animals and human activities; or a combination of both broad mechanisms. In moist savannas, the tree–grass coexistence is mainly attributed to of disturbances, while in dry savannas, limiting resources are considered the principal mechanism of co-existence.Virtually all theoretical explorations of tree–grass dynamics in dry savannas consider only competition for soil water. Here we investigate whether coexistence could result from a balanced competition for two resources, namely soil water and mineral nitrogen. We introduce a simple dynamical resource-competition model for trees and grasses. We consider two alternative hypotheses: (1) trees are the superior competitors for nitrogen while grasses are superior competitors for water, and (2) vice-versa. We study the model properties under the two hypotheses and test each hypothesis against data from 132 dry savannas in Africa using Kendall's test of independence. We find that Hypothesis 1 gets much more support than Hypothesis 2, and more support than the null hypothesis that neither is operative. We further consider gradients of rainfall and nitrogen availability and find that the Hypothesis 1 model reproduces the observed patterns in nature. We do not consider our results to definitively show that tree–grass coexistence in dry savannas is due to balanced competition for water and nitrogen, but show that this mechanism is a possibility, which cannot be a priori excluded and should thus be considered along with the more traditional explanations.

Humanizing savanna models: integrating natural factors and anthropogenic disturbance regimes to determine tree–grass dynamics in savannas

Explaining co-dominance of grasses and trees in savannas has confounded ecologists and biogeographers for decades. One reason is the complex nature of savanna human ecology; co-dominance can be caused by edaphic conditions, biotic factors, and/or human practices. New models consider savannas to be in disequilibrium and emphasize tree life histories, recognizing that events create opportunities for tree establishment. While much emphasis has been placed on how disturbances such as fire suppress trees, there has been less focus on how multiple disturbances interact to alter tree establishment. This study examines the impacts of hoe-farming, grazing, and burning on tree establishment and growth. It combines results from a 4-year field experiment with those from a long-term natural experiment. The study finds that the subtle changes in soil compaction and permeability as well as grass cover that result from hoe-farming counter the negative effects of fire on tree establishment and growth, especially on clay soils.

Herbivore–vegetation feedbacks can expand the range of savanna persistence: insights from a simple theoretical model

Theoretical models of tree–grass coexistence in savannas have focused primarily on the role of resource availability and fire. It is clear that herbivores heavily impact vegetation structure in many savannas, but their role in driving tree–grass coexistence and the stability of the savanna state has received less attention. Theoretical models of tree–grass dynamics tend to treat herbivory as a constant rather than a dynamic variable, yet herbivores respond dynamically to changes in vegetation structure in addition to modifying it. In particular, many savannas host two distinct herbivore guilds, grazers and browsers, both of which have the potential to exert profound effects on tree/grass balance. For example, grazers may indirectly favor tree recruitment by suppressing the destructive effects of fire, and browsers may facilitate the expansion of grassland by reducing the competitive dominance of trees. We use a simple theoretical model to explore the role of grazer and browser dynamics on savanna vegetation structure and stability across fire and resource availability gradients. Our model suggests that herbivores may expand the range of conditions under which trees and grasses are able to stably coexist, as well as having positive reciprocal effects on their own niche spaces. In addition, we suggest that given reasonable assumptions, indirect mutualisms can arise in savannas between functional groups of herbivores because of the interplay of consumption and ecosystem feedbacks.

Seasonal changes in depth of water uptake for encroaching trees Juniperus virginiana and Pinus ponderosa and two dominant C4 grasses in a semiarid grassland

We used the natural abundance of stable isotopic ratios of hydrogen and oxygen in soil (0.05-3 m depth), plant xylem and precipitation to determine the seasonal changes in sources of soil water uptake by two native encroaching woody species (Pinus ponderosa P. & C. Lawson, Juniperus virginiana L.), and two C(4) grasses (Schizachyrium scoparium (Michx.) Nash, Panicum virgatum L.), in the semiarid Sandhills grasslands of Nebraska. Grass species extracted most of their water from the upper soil profile (0.05-0.5 m). Soil water uptake from below 0.5 m depth increased under drought, but appeared to be minimal in relation to the total water use of these species. The grasses senesced in late August in response to drought conditions. In contrast to grasses, P. ponderosa and J. virginiana trees exhibited significant plasticity in sources of water uptake. In winter, tree species extracted a large fraction of their soil water from below 0.9 m depth. In spring when shallow soil water was available, tree species used water from the upper soil profile (0.05-0.5 m) and relied little on water from below 0.5 m depth. During the growing season (May-August) significant differences between the patterns of tree species water uptake emerged. Pinus ponderosa acquired a large fraction of its water from the 0.05-0.5 and 0.5-0.9 m soil profiles. Compared with P. ponderosa, J. virginiana acquired water from the 0.05-0.5 m profile during the early growing season but the amount extracted from this profile progressively declined between May and August and was mirrored by a progressive increase in the fraction taken up from 0.5-0.9 m depth, showing plasticity in tracking the general increase in soil water content within the 0.5-0.9 m profile, and being less responsive to growing season precipitation events. In September, soil water content declined to its minimum, and both tree species shifted soil water uptake to below 0.9 m. Tree transpiration rates (E) and water potentials (Psi) indicated that deep water sources did not maintain E which sharply declined in September, but played an important role in the recovery of tree Psi. Differences in sources of water uptake among these species and their ecological implications on tree-grass dynamics and soil water in semiarid environments are discussed.

Tree–grass coexistence in the Brazilian cerrado: demographic consequences of environmental instability

AbstractAim Tropical savanna ecosystems are uniquely characterized by the co‐dominance of both trees and grasses. An operational understanding of the ecological processes involved in maintaining this condition is essential for understanding both the functioning of savanna systems as well as their potential response to environmental change. A simple model is presented to explore the potential for a demographic mechanism of long‐term tree persistence and temporal physiognomic stability in the Brazilian cerrado.Location The model is developed based on data from the humid cerrado of Brazil.Methods In contrast to many existing models of tree–grass dynamics a model is presented which is based on data from the humid cerrado of Brazil, which is both qualitatively and quantitatively different from many of the more arid savannas of the palaeotropics. The model focuses on the dynamics of a synthetic tree population, with particular attention given to reproduction, seedling establishment and fire effects; with separate sub‐models for grass production, fire and rainfall.Results The model successfully predicts coexistence across the full range of observed vegetation physiognomies, but only under limited conditions. Under coexistence conditions, the dynamics of the tree population are characterized by long periods of gradual decline, punctuated by occasional bursts of growth. However, in agreement with earlier studies, the model consistently over‐predicts domination by the tree component. Fire is identified as an overriding factor in determining model behaviour, and the response of reproduction and sapling recruitment to variance in the frequency of fire ignition is identified to be of potential importance in the functioning of the Brazilian cerrado. The key dynamics of the model which promote tree–grass coexistence are consistent with a number of established determinants of ecological resilience in savanna systems.Main conclusions The model identifies the importance of the effective exploitation of rare opportunities for favourable recruitment (e.g. exclusion from fire) by the tree population, in promoting coexistence within a predominantly adverse environment. Support is provided for an alternative demographic mechanism of tree–grass coexistence in the cerrado (the storage effect), which is not based on the limiting assumption of niche partitioning through differences in rooting depth. The results are consistent with those presented by recent modelling work based on the more arid savannas of southern Africa. The model presented here differs in the emphasis given to particular environmental and life‐history attributes which are critical in determining the tree–grass balance, but provides further general support for the potential role of demographic mechanisms (such as the storage effect) in determining the structure of tropical savannas. Despite having clear limitations, models can serve as valuable heuristic tools to aid the integration and exploration of existing data sets as well as our present understanding of key ecological processes.

A theoretical framework of ecological phase transitions for characterizing tree-grass dynamics.

This paper describes a theoretical framework of ecological phase transitions for modeling tree-grass dynamics and analyzing the shifts or phase transitions from one vegetation structure to another in the southern Texas landscape. This framework implements the integration of percolation theory, fractal geometry and phase transition theory as a method for modeling the spatial patterns of tree-grass dynamics, and nonlinear Markov non-equilibrium thermodynamic stability theory as a method for characterizing temporal tree-grass dynamics and phase transition. An historical sequence of aerial photographs at a Prosopis-thornscrub savanna parkland site in southern Texas was used to determine the parameters of the models. The preliminary analytical result accords well with current understanding and field survey of vegetation dynamics in the southern Texas landscape. The potential of such approaches and other relevant theories such as self-organized criticality and synergetics to vegetation dynamics is also discussed.

Some aspects of tree-grass dynamics in Queensland's grazing lands

This review examines the interactions between grasses and trees that are relevant to Queensland's grazing lands. Soils and climate determine the potential amount of woody vegetation within an area and clearing, fire and grazing management can modify that potential. In general, the presence of non-leguminous trees reduces the potential grass production beneath their canopy and within woodland patches. Some non-native leguminous trees (e.g. Indian siris, Albizia lebbeck) enhance production of grasses beneath their canopies, whereas the widespread native leguminous trees, mulga (Acacia aneura) and brigalow (A. harpophylla) have not been reported to enhance grass production in the same way. At the patch scale, pasture production beneath woodlands with moderate to high tree basal areas is generally less than in open patches under the same soil and climatic conditions. At a landscape level, tree density, rainfall amount and distribution, and soil type modify pasture production within forest/woodland/shrubland systems. Grasses can reduce tree seedling survival but have little impact on mature woody plants, apart from providing fuel to carry a fire. Modelling studies have been used to examine some aspects of tree-grass production. Firstly, the relationship between grass production and an increasing amount of trees can vary from linear decrease, to exponential decrease to initial stimulation followed by a decrease, depending solely on the relative strengths of stimulatory and competitive effects of trees on grasses. Secondly, simulated pasture production within woodlands shows that the pasture production may be up to 50% higher in paddocks that have high variability in the distribution of those trees compared with areas where trees are uniformly distributed. This is due to the non-linear (negative exponential) relationship between pasture production and tree density that is commonly observed within Queensland. Lastly, simulation studies show that total aboveground production (trees and pasture) of mulga woodland increases as the number of trees per hectare increases. The pasture production response to tree clearing or tree planting depends on tree species, rainfall, soil type, climatic history and post-clearing management including fire and grazing, and will change with time since clearing. The greatest relative increase in pasture production following the removal of woody vegetation occurs: when the initial tree basal area is highest; where rainfall is evenly distributed though the year and on fertile soils with a low water holding capacity.