Restoring savannas and tropical herbaceous ecosystems

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When people think of savannas, they think of the vast African landscapes where elephants, wildebeest, giraffes or lions used to roam. With the seasons, the vegetation – tall grasses more or less scattered with trees and shrubs – changes from green to yellow. These herbaceous ecosystems cover about 20% of the earth’s surface and are present throughout the world’s tropical belt, in Africa of course, but also in America and Asia. Recurrent fires or herbivores, themselves controlled by predators, keep these environments open by limiting the presence of trees and shrubs. Due to the complexity of savanna vegetation dynamics, the impacts of climate change and land use on savannas are highly uncertain. In addition, savannas are already largely threatened by human activities: conversion of land for agriculture, urbanization, etc. They are also threatened by the massive planting of trees, often implemented within the framework of carbon compensation. The natural resilience of these ecosystems to human-induced degradation is low and restoration actions are often necessary, but remain an ongoing challenge.

1. Tropical primary herbaceous ecosystems

Figure 1. The different terrestrial biomes, savannas and tropical herbaceous ecosystems are in light green (Grasslands, savannas and shrubland). [Source: WWF data, Terrestrial Ecoregions of the World dataset, March 2020). The map was produced by Leonardo Cancian. © All rights reserved]
Like primary tropical and equatorial forests (or ancient forests), primary or ancient* tropical herbaceous ecosystems are extremely species-rich ecosystems that took centuries to assemble [1]. These ecosystems, which include savannas and tropical grasslands, are commonly referred to as “savannas” and will therefore be used in the remainder of this paper.

Figure 2. Savannah landscapes in South Africa. A, Kruger Park; B, Pilanesberg Park. [Source: Photos © S. Le Stradic]
Highly diversified, savannas cover about 20% of the land surface and are present throughout the tropical belt (Figure 1). In our imagination, the term savanna evokes Africa, where it covers about 33.5% of the continent. They are associated with the continent’s emblematic megafauna: elephants, lions, zebras, buffaloes, giraffes and rhinos. Globally, savannas are thus present (Figures 1 to 4):

  • In Africa, in the sub-Saharan zone, in the greater East Africa region, in Central Africa and as far as South Africa (Figure 2) ;
  • in South and Central America (Figure 3), mainly in the centre of the continent, in a region locally known as Cerrado, but also in Venezuela and Colombia where the savannas are known as Llanos. They also exist in the middle of the Amazon rainforest and in Guyana ;
  • in northern Australia and southern New Guinea ;
  • in Asia, especially in India and China, where they are less well known and cover smaller areas.

Figure 3. Diversity of South American savanna landscapes. A, the “campo rupestre” of the Serra do Cip National National Park (Brazil); B, Cerrado in Brazil, Serra do Cipό National Park (Brazil); C, enclave of savannas in the middle of the Amazon rainforest, Parque Nacional dos Campos Amazônicos. [Source: Photos A & B, © S. Le Stradic; Photo C, © D. Borini].
We are talking about ecosystems found in tropical areas. Yet other herbaceous ecosystems share similar characteristics with savannahs, including frequent fires. This is the case of the pampas in southern Brazil, Uruguay and Argentina, located in the subtropics, or the oak savannas (Oak savannas) found in North America.

2. Organization of a savanna

Figure 4. Diagram of savanna organization with a continuous herbaceous layer with mostly C4 grasses, many forbs (non-woody herbaceous plants other than grasses), and the presence of more or less or less tree and shrub species. The two main disturbances of these vegetation are recurrent fires and herbivory, although herbivory pressure is not the same all over the world. [Source: © S. Le Stradig & E. Buisson]
The savannas present different physiognomies. Some are very open, with few or no shrub species; others, on the contrary, have a fairly closed canopy with many tree species (Figure 3). The common characteristic defining these ecosystems is the presence of a continuous herbaceous stratum, mainly composed of C4* grasses (see The path of carbon in photosynthesis) and non-woody herbaceous species (known as forbs*) (Figure 4).

2.1. Origin of savanna structure

While trees are often resistant* or resilient* to fire, species in the herbaceous stratum are generally shade intolerant [2]. This coexistence is quite unusual. It is the interaction of several processes related to water use, soil properties and disturbance regimes* -such as fire- which is at the origin of the vegetation structure of these ecosystems [2],[3]:

  • The climate (quantity of rainfall and length of the rainy season, among other things), since a minimum amount of rainfall is needed to allow tree to establish [4], [5], [6]. Tropical regions are characterized by a high seasonality with an marked dry and wet season [4], [6]. Many tropical areas are occupied by savannah -open ecosystems- [2], [5], [7], whereas one would expect to observe forests because of the amount of rainfall.
  • chronic disturbances, including fire and herbivory, limit woody cover and thus maintain savannas (Figure 5).
  • soil characteristics, such as composition and texture: sandy soil – with lower water retention – will tend to support more shrubs, while clay soil – with higher water retention – will have more herbaceous vegetation (see Figure 2).

savanna landscapes
Figure 5. Diversity of savanna landscapes in South Africa. A, herbivory example, a zebra in a South African savanna; B, fire stimulates the regrowth of vegetation, which is favorable to herbivores (white rhinoceros, South Africa). [Source: Photos © S. Le Stradic]
Tropical herbaceous ecosystems such as savannas are ancient ecosystems that have evolved over millions of years with fire and large herbivores. The origin of the C4 grasses at the end of the Oligocene and their dominance during the Miocene is considered evidence of the ancient origin of these ecosystems. The very great richness of these ecosystems and the high rate of endemism also testify of their antiquity. Some tropical herbaceous ecosystems contain species that are witnesses of a long evolutionary process [8].

2.2. Fire, a major player in savanna ecosystems

The distribution of savannas cannot therefore be predicted by climate alone. Thus, disturbances such as herbivory (especially that of “mega-herbivores”, such as elephants and ungulates) and fire play a major ecological and evolutionary role in savannas[2], [5], [9] (see Figures 2 to 5).

In the herbaceous stratum, grasses are a major biofuel. Their presence favours fires, limiting the growth of trees and shrubs. The life cycle of grasses, and herbaceous species in general, is well adapted to fire: they are able to regrow and reproduce very quickly following a fire (Figure 6). Their presence in these ecosystems therefore depends on the recurrence of fires.

Figure 6. Plant flowers shortly after a fire. A, Gomphrena lanigera Pohl ex Moq (Amaranthaceae); B, Bulbostylis paradoxa (Spreng.) Lindm. (Cyperaceae). Serra do Cipó, Brazil. [Source: Photos © S. Le Stradic]
Moreover, the fire resistance of tree species found in savannas is not the same as that of species found in forests [7], [10] :

  • Savannah species exhibit traits related to fire resistance, in particular thick bark, such as Curatella americana L. (Dilleniaceae), Caryocar brasiliense (Caryocraceae), Handroanthus ochraceus (Bignoniaceae), Pterodon emarginatus (Fabaceae), Bowdichia virgilioides (Fabaceae), Annona crassiflora (Annonaceae) found in Cerrado and Llanos ;
  • Forest species have much thinner bark, making them vulnerable to fire such as Copaifera langsdorffii (Fabaceae), Swartzia flaemingii (Fabaceae), Hymenaea coubaril (Fabaceae), Cariniana estrellensis (Lecythidaceae), Xylopia aromatica (Annonaceae).

In tropical regions, many savannas would be forests without the presence of these disturbances [11], as the amount of rainfall is sufficient to allow forests to occur. In these regions, forests and savannas are then considered as alternative biome states* [5], [7]. Environmental conditions allow the presence of either forest or savannah, and the presence of either state will therefore be mainly defined by the occurrence of disturbances, their intensity and frequency.

3. Biodiversity and conservation issues

Savannas represent an exceptional heritage. They are ecosystems extremely rich in biodiversity and particularly in endemic species. As an example, the Brazilian savanna called Cerrado, which extends over two million km², contains more than 12,000 plant species. It is the most species-rich savanna in the world (see Focus The Cerrado Biome).

African savannas, in addition to their rich vegetation, are home to an emblematic megafauna. Elephants, zebras, giraffes, lions and cheetahs are better adapted to these open ecosystems. The savannas are also part of our cultural heritage and history. For example, some studies have shown that humans have an innate preference for open landscapes, which may stem from our long evolutionary history in the East African savannas. These environments are often considered to be one of the driving forces of human evolution.

However, human activities, even if they are not at the origin, have, over time, profoundly impacted the fire and herbivory regimes and indeed the distribution of the savannas [12]. Today many people still live at the heart of these ecosystems and depend on the services they provide, such as water quality control, grazing opportunities and the presence of game.

Despite the cultural and natural heritage they represent, tropical herbaceous ecosystems are largely threatened by the conversion of large areas for agriculture, by fire suppression policies, and by the planting of trees for commercial purposes or sometimes in the context of carbon offsetting. The Cerrado, for example, register a much higher conversion rate to agricultural land than the conversion rates recorded in the Amazonian forest [13], [14].

4. Savanna resilience

Figure 7. Prescribed fire experiments in Cerrado, Serra do Tombador, Brasil. A, Vegetation before fire; B, recently burned plot; and C, few weeks after fire. [Source : A & B © Juliana Teixeira ; C, © Alessandra Fidelis, Laboratório de Ecologia da Vegetação – LEVeg, Rio Claro].
Savannas are extremely resilient to natural disturbances: the maintenance of open physionomies even depends on the presence of these disturbances [15]. In Africa, the presence of large herbivores helps to maintain open environments, along with regular fires that ensure regeneration of the herbaceous cover. In other regions of the world, fire alone is mainly responsible for maintaining open environments.

Although fires in savannas are originally natural (lightning during thunderstorms), fire regimes have long been influenced by human activities (at least 300,000 years in Africa). Human evolution is linked to an increase in burned areas [12]. The substantial impact of human activities on fire regimes, however, seems to be more recent and may date back about 4000 years. The introduction of pastoralism has been accompanied by a decrease in the surface area of burnt land: livestock grazing has reduced the amount of biofuel available [12].

The availability of nutrients in the form of ash, the presence of underground buds, the protection of buds under thick bark, or the presence of large underground organs for nutrient storage allow vegetation, whether woody or herbaceous, to grow back very quickly after a fire [5] (Figure 7).

On the other hand, in recent decades, human activities have greatly altered fire regimes, frequency and intensity, increasing the overall area burned, and changing the size of burned areas, although regional differences can be observed [12]. Many governments (e.g. Brazil, Botswana, Zimbabwe, South Africa) have established fire prevention and suppression programmes: all fires should be systematically fought and extinguished where possible.

The degradation and conversion of savannas can be very rapid and often not very reversible. Degradation processes are variable, from the introduction of invasive exotic species, the introduction of livestock, the exclusion of fires, the densification of tree cover, and the planting of trees in open environments. The level of degradation obviously depends on the duration and intensity of the degradation. When disturbances are more intense, or in the case of major environmental changes such as afforestation, conversion to agricultural land or mining activities, the changes in vegetation and soil are so great that natural resilience is reduced or even non-existent [15].

Very often, the threshold of degradation beyond which spontaneous recovery of the savanna is impossible (or will take a very long time) is quickly exceeded:

  • community assembly processes are slow and dependent on many interactions;
  • seed dispersal is often limited.

Active restoration actions are essential in these cases.

5. Restoration of tropical herbaceous ecosystems

Figure 8. In case of major degradations, tropical herbaceous ecosystems are not resilient and restoration programs are then necessary. Democratic Republic of Congo. [Source: © Soizig Le Stradic].
As these ecosystems are the result of several million years of evolution and complex ecological interactions, their restoration remains a real challenge today.

A first step is to take into account the natural disturbances caused by fires and large herbivores. Savanna restoration techniques therefore include the reintroduction of natural disturbances such as the use of prescribed burns, grazing management, reintroduction of herbivores, but also the elimination of invasive species.

In the case of major degradation (Figure 8), it must also be possible to restore the geomorphology and properties of the soil and then reintroduce native species, which can be complicated for most herbaceous species whose ecology and biology are poorly documented. For example, some species :

  • produce few or no seeds, or do not reproduce regularly;
  • depend on fire for sexual reproduction or have a dormancy that must be broken [16].

Although restoration of the herbaceous stratum is fundamental to restore the ecological processes of these ecosystems, it is often unsuccessful, often limited by :

  • colonization and competition with invasive species;
  • the fact that seeds of indigenous herbaceous species are not available and/or their propagation is not efficient [15].

6. Afforestation

Another problem with the restoration of savannas is that they are often misregarded as degraded forests. Their biodiversity and the importance of the (ecosystem) services they provide to societies are not always perceived by the general public and decision-makers, including government entities that might implement conservation programmes.

Often erroneously, savannas are considered degraded forests, which complicates perceptions about these ecosystems and justifies fire exclusion programs.

Thus many savannah “so-called restoration” projects involve planting trees. Mass tree planting is not an appropriate restoration technique for savannas, especially because herbaceous species, especially C4 grasses,cannot grow in shaded environments. Spontaneous regeneration of the herbaceous stratum is thus largely compromised, if not impossible, in the presence of significant tree cover.

As a result, the return to fire regimes close to natural ones, the presence of herbivores and the resulting processes will also be compromised in the absence of herbaceous biomass (food source for herbivores and fuel). Restoring the herbaceous stratum must be a priority to restore these ecosystems.

Launched in 2011, the “Bonn Challenge [17]” aims to restore 150 million hectares of degraded and deforested land by 2020 and 350 million hectares by 2030. In this context, several initiatives have emerged to promote reforestation and large-scale tree planting as a solution to climate change; trees absorb during their growth and conserve a large part of the CO2 emitted by human activities. Several initiatives have highlighted potential areas for forest restoration [18], [19]. In some cases, tree planting can have a practical objective such as combating the harmful effects of desertification and blocking the advance of the desert (see The Great Green Wall: a hope for greening the Sahel?). In this specific case, trees provides essential ecosystem services because it helps combat desertification and because the local population is highly dependent on woody biodiversity. While the existence of such projects can be welcomed when it comes to restoring degraded forests, or to meet practical objectives of combating desertification, the risk of afforesting areas where savannas and other tropical grasslands previously existed must be denounced.

Several studies have already pointed out the risk associated with tree planting in “open” ecosystems (even though tree cover can be quite high), such as savannas where the herbaceous layer is primordial and dominated by shade-intolerant species [1], [5].

Figure 9. Mechanisms of degradation of tropical forests, savannas and primary grasslands. The degradation of these ecosystems can be due to changes in fire regimes, tree planting and deforestation. The degradation of these species-rich ecosystems is accompanied by losses in animal and plant biodiversity, but also in terms of ecosystem services such as the recharge of groundwater. The degraded or transformed ecosystem is sometimes wrongly called savanna or sometimes derived savanna. [Source: diagram © S. Le Stradig & E. Buisson, inspired by references [20, 23], all rights reserved]
Here we must be clear: planting trees in such ecosystems is not restoration, and orienting public policy in this direction is an even more serious mistake when the trees chosen are of exotic origin and only one tree species is planted over large areas. [20] It is also important to remember that most programs promoting large-scale tree planting are developed on the basis of current climatic conditions. However, due to changing climatic conditions, it is to be expected that many places that are now favourable for tree growth will not be so in a few years time. It is therefore important to take into account potential climate change when implementing restoration projects.

It is very important to distinguish between naturally occurring savannas, which are species-rich and should be conserved, and degraded forests, sometimes erroneously called “savannas” or derived savannas, which are species-poor and can be restored to forest [21].

Moreover, if the major argument for planting trees is that it is one of the best solutions to fight climate change, it is also important to remember that in the savannas the majority of the biomass is underground, representing about 70% of the total biomass. The carbon stocks located underground are therefore far from negligible, particularly well protected from fires, and stable.

In addition to the loss of plant and animal biodiversity associated with the afforestation of these open ecosystems [22], the damage is also measurable at the economic level, especially for certain populations whose activities rely on savannas for grazing, game supply or the security of certain ecosystem services, including water supply and quality. In addition to the loss of biodiversity, habitats for many animal species and the degradation of many ecosystem services, advocating the mass planting of trees in fire-prone ecosystems increases the risk of mass fires and ultimately the loss of stored carbon (Figure 9).

7. Savannas & climate change: contrasting projections

Already greatly affected by anthropogenic land use changes, savannas are expected to be profoundly altered by climate change and rising CO2 concentrations in the atmosphere. [23] Indeed, the extent of the savannas depends on the amount of precipitation and the seasonality of the climate. For the future, contrasting projections are given by the different modelling approaches. [24] Generally speaking, climate change is tending towards a reduction in the extent of the savannahs on a global scale, according to two different mechanisms:

  • In areas where reduced rainfall is predicted, there is a risk of desertification. Because the two types of vegetation that structure savannas – trees C3 and grasses C4 – respond differently to the same environmental controls, [25] savannas responses to drought may therefore be different from those of forests and grasslands [26]. Moreover, increased drought can limit the presence of trees while promoting fires in ecosystems, such as forests, that are not adapted to fire.
  • In other regions, an increase in rainfall is expected, which would lead to a densification of tree and shrub cover (scrambling) to the detriment of the savannah. This process is already underway due to the increase in atmospheric CO2. [27] In some regions, a reduction in herbaceous vegetation can have serious consequences on the local economy if it depends on livestock farming and grazing, for example.

For example, climate change will lead to widespread erosion of differences between ecological communities in the Cerrado biome [28], and this will be one of the main drivers of loss of biodiversity and ecosystem services.

8. Messages to remember

  • Restoration of degraded forests, including replanting trees, is essential, but tree planting should not compromise the conservation and restoration of other ecosystems.
  • Ecological restoration should restore degraded ecosystems, but not destroy natural ecosystems.
  • Since the restoration of savannas remains difficult, environmental policies should give priority to their conservation.
  • Valuing biodiversity and recognizing the ecosystem services that savannas provide is a first step to improve their conservation, in parallel with the management of natural areas. This implies the use of prescribed burns, the presence of natural fires and/or herbivory by indigenous megafauna.

This article is a modified version of the ‘regard’ R90 by Soizig Le Stradic and Elise Buisson, published by the Société Française d’Ecologie et Evolution (SFE2) and posted on its website in February 2020.


Notes and References

Cover image. [Source : Photo © S. Le Stradig]

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[2] Pausas, J.G., & Bond, W.J. 2020. Alternative Biome States in Terrestrial Ecosystems. Trends in Plant Science. doi: 10.1016/d.tplants.2019.11.003.

[3] Hoffmann, W.A. 1998. Fire and population dynamics of woody plants in a neotropical savanna: matrix model projections. Ecology 80: 1354-1369.

[4] Sankaran, M., Hanan, N.P., Scholes, R.J., Ratnam, J., Augustine, D.J., Cade, B.S., Gignoux, J., Higgins, S.I., Le Roux, X., Ludwig, F., Ardo, J., Banyikwa, F., Bronn, A., Bucini, G., Caylor, K.K., Coughenour, M.B., Diouf, A., Ekaya, W., Feral, C.J., February, E.C., Frost, P.G.H., Hiernaux, P., Hrabar, H., Metzger, K.L., Prins, H.H.T., Ringrose, S., Sea, W., Tews, J., Worden, J., & Zambatis, N. 2005. Determinants of woody cover in African savannas. Nature 438: 846-849.

[5] Staver, A.C., Archibald, S., & Levin, S.A. 2011. The global extent and determinants of savanna and forest as alternative biome states. Science 334: 230-232.

[6] Lehmann, C.E.R., Anderson, T.M., Sankaran, M., Higgins, S.I., Archibald, S., Hoffmann, W.A., Hanan, N.P., Williams, R.J., Fensham, R.J., Felfili, J., Hutley, L.B., Ratnam, J., San Jose, J., Montes, R., Franklin, D., Russell-Smith, J., Ryan, C.M., Durigan, G., Hiernaux, P., Haidar, R., Bowman, D.M.J.S., & Bond, W.J. 2014. Savanna vegetation-fire-climate relationships differ among continents. Science 343: 548-552.

[7] Dantas, V. de L., Hirota, M., Oliveira, R.S., & Pausas, J.G. 2016. Disturbance maintains alternative biome states (M. Rejmanek, Ed.). Ecology Letters 19: 12-19.

[8] Vasconcelos, T.N.C., Alcantara, S., Andrino, C.O., Forest, F., Reginato, M., Simon, M.F., & Pirani, J.R. 2020. Fast diversification through a mosaic of evolutionary histories characterizes the endemic flora of ancient Neotropical mountains. Proceedings of the Royal Society B: Biological Sciences 287: 20192933.

[9] Simon, M.F., Grether, R., Queiroz, L.P. De, Skema, C., Pennington, R.T., & Hughes, C.E. 2009. Recent assembly of the Cerrado, a neotropical plant diversity hotspot, by in situ evolution of adaptations to fire. Proceedings of the National Academy of Sciences of the United States of America 106: 20359-20364.

[10] Charles-Dominique, T., Beckett, H., Midgley, G.F., & Bond, W.J. 2015. Bud protection: a key trait for species sorting in a forest-savanna mosaic. New phytologist. doi: 10.1111/nph.13406; Charles-Dominique, T., Midgley, G.F., & Bond, W.J. 2017. Fire frequency filters species by bark traits in a savanna-forest mosaic (S. Scheiner, Ed.). Journal of Vegetation Science 28: 728-735.

[11] Bond, W.J., Woodward, F.I., & Midgley, G.F. 2004. The global distribution of ecosystems in a world without fire. New Phytologist 165: 525-538.

[12] Archibald, S., Lehmann, C.E.R., Gomez-Dans, J.L., & Bradstock, R.A. 2013. Defining pyromes and global syndromes of fire regimes. Proceedings of the National Academy of Sciences 110: 6442-6447.

[13] Beuchle, R., Grecchi, R.C., Shimabukuro, Y.E., Seliger, R., Eva, H.D., Sano, E., & Achard, F. 2015. Land cover changes in the Brazilian Cerrado and Caatinga biomes from 1990 to 2010 based on a systematic remote sensing sampling approach. Applied Geography 58: 116-127.

[14] Overbeck, G.E., Müller, S.C., Fidelis, A., Pfadenhauer, J. Pillar, V.D., Blanco, C.C., Boldrini, I.I., Both, R., & Forneck, E. (2007). Brazil’s neglected biome: The South Brazilian Campos. Perspectives in Plant Ecology, Evolution and Systematics. 9. 101-116. 10.1016/j.ppees.2007.07.005.

[15] Buisson, E., Le Stradic, S., Silveira, F.A.O., Durigan, G., Overbeck, G.E., Fidelis, A., Fernandes, G.W., Bond, W.J., Hermann, J., Mahy, G., Alvarado, S.T., Zaloumis, N.P., & Veldman, J.W. 2019. Resilience and restoration of tropical and subtropical grasslands, savannas, and grassy woodlands. Biological Reviews 94: 590-609.

[16] Dayrell, R.L.C., Garcia, Q.S., Negreiros, D., Baskin, C.C., Baskin, J.M., & Silveira, F.A.O. 2017. Phylogeny strongly drives seed dormancy and quality in a climatically buffered hotspot for plant endemism. Annals of Botany 119: 267-277.

[17] The Bonn Challenge is a global effort to restore 150 million hectares of deforested and degraded land worldwide by 2020, and 350 million hectares by 2030. It was launched in 2011 by the German government and IUCN and endorsed and extended by the New York Declaration on Forests at the 2014 UN climate summit.

[18] Bastin, J.-F., Finegold, Y., Garcia, C., Mollicone, D., Rezende, M., Routh, D., Zohner, C.M., & Crowther, T.W. 2019. The global tree restoration potential. Science 365: 76-79.

[19] Forest: sustaining forests for people and planent; World Resources Institute.

[20] Veldman J.W., Aleman J.C., Alvarado S.T., Anderson T.M., Archibald S. et al. 2019. Comment on “The global tree restoration potential.” Science 366, Issue 6463, eaay7976; DOI: 10.1126/science.aay7976

[21] Meli, P., Holl, K.D., Rey Benayas, J.M., Jones, H.P., Jones, P.C., Montoya, D., & Moreno Mateos, D. 2017. A global review of past land use, climate, and active vs. passive restoration effects on forest recovery (S. Joseph, Ed.). PLOS ONE 12.

[22] Abreu, R.C.R., Hoffmann, W.A., Vasconcelos, H.L., Pilon, N.A., Rossatto, D.R., & Durigan, G. 2017. The biodiversity cost of carbon sequestration in tropical savannah. Science Advances 3: e1701284.

[23] Osborne C.P., Charles-Dominique T., Stevens N., Bond W.J., Midgley G. & Lehmann C.E.R. (2018) Human impacts in African savannas are mediated by plant functional traits. New Phytologist 220:10-24.

[24] Moncrieff, G.R., Scheiter, S., Langan L., Trabucco, A. & Higgins, S.I. 2016. The future distribution of the savannah biome: model-based and biogeographic contingency. Phil. Trans. R. Soc. B37120150311

[25] With the increase of CO2 concentration in the atmosphere, C3 plants could reach photosynthetic activities approaching those of C4 plants (see The path of carbon in photosynthesis).

[26] Sankaran M. (2019) Droughts and the ecological future of tropical savanna vegetation. J. Ecol. 107:1531-1549.

[27] O’Connor, T.G., Puttick, J.R. & M Hoffman, M.T. (2014) Bush encroachment in southern Africa: changes and causes, African Journal of Range & Forage Science, 31:2, 67-88, DOI: 10.2989/10220119.2014.939996

[28] Hidasi-Neto, J., Joner, D. C., Resende, F., Monteiro, L. D., Faleiro, F. V., Loyola, R. D., & Cianciaruso, M. V. (2019). Climate change will drive mammal species loss and biotic homogenization in the Cerrado Biodiversity Hotspot. Perspectives in Ecology and Conservation. doi:10.1016/j.pecon.2019.02.001


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To cite this article: LE STRADIC Soizig, BUISSON Elise (July 24, 2020), Restoring savannas and tropical herbaceous ecosystems, Encyclopedia of the Environment, Accessed November 7, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/life/restoring-savannas-and-tropical-herbaceous-ecosystems/.

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恢复热带稀树草原和热带草地生态系统

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  当人们想到热带稀树草原时,脑海里会出现大象、牛羚、长颈鹿或狮子在广阔非洲大地上漫游。高大的草丛中或密或稀地散布着乔木和灌木,随着季节的更替,植被在绿色到黄色见变换。这类草地生态系统占据了地球表面20%的土地,分布在地球的热带地区,除了非洲,还有美洲和亚洲。由于经常发生大火,以及食草动物的存在(其数量也受到捕食者的控制),限制了乔木和灌木的扩张,从而维持了环境的开放性。由于热带稀树草原植被状况极为复杂,气候变化和土地利用对其影响具有高度不确定性。此外,热带稀树草原已经在很大程度上受到人类活动的影响:如农业用地开垦、城市化占用等。在碳补偿的框架下实施的植树工程也影响了热带稀树草原。这类生态系统因人类活动导致退化,其自然恢复力很低,必须开展恢复行动,而且是一个持久的挑战。

1. 热带原生草地生态系统

环境百科全书-热带稀树草原-陆地生物区系
图1. 不同陆地生物群系,其中稀树草原和热带草地生态系统(草地、稀树草原和灌丛)用浅绿色表示。[资料来源:世界野生动物基金会(WWF)数据集和世界地球生态区(TEW)数据集,2020年3月)。地图由列昂纳多·坎西安制作。©版权所有]

  就像原生热带森林赤道森林(又称为原始森林) 一样,原生或原始热带草地生态系统是经过几个世纪才形成的物种极其丰富的生态系统[1]。这些原始或古老的草地生态系统包括稀树草原和热带草地,它们通常被统称为“萨王纳草原”(savannas),本文在后面的部分也统一使用“萨王纳”指代。

环境百科全书-热带稀树草原-南非萨王纳景观
图2.南非萨王纳景观。A:克鲁格公园(Kruger Park);B:皮兰斯伯格公园(Pilanesberg Park)。[照片来源: ©索·勒斯特拉迪奇]

  萨王纳的类型高度多样化,面积约占陆地表面的20%,在整个热带地区都有分布(图1) 。一提到萨王纳这个词,就会让人联想非洲,确实如此,它占据了整个非洲大陆面积的33.5%,养育了那里极具象征性的巨型动物群,包括大象、狮子、斑马 、野牛、长颈鹿和犀牛。全球的萨王纳分布如下(图1~4) :

  • 非洲的萨王纳主要分布于撒哈拉以南地区、东非地区、中非,甚至远至南非 (图2);
  • 南美洲和中美洲(图3)萨王纳主要分布于大陆的中部;巴西的萨王纳分布地区被当地人称为“塞拉多”(Cerrado),委内瑞拉和哥伦比亚的萨王纳则被称为“亚诺斯”(Llanos)。在亚马孙雨林的中部和圭亚那也有分布;
  • 澳大利亚的萨王纳主要分布在北部和新几内亚南部;
  • 在亚洲,萨王纳主要分布于印度和中国,但是面积小,不太为人所知。
环境百科全书-热带稀树草原-南美多样的萨王纳景观
图3. 南美多样的萨王纳景观。A:巴西塞拉多西普国家公园 (Parc National de la Serra do Cipό) 的“岩石草原”;B:巴西塞拉多西普国家公园的“塞拉多”热带稀树草原;C:亚马孙坎波斯国家公园(Parque Nacional dos Campos Amazônicos),点缀在亚马孙雨林里的热带草原。 [图片来源: A和B由©索·勒斯特拉迪奇提供,C由©德·博里尼惠赐]。

  我们谈论的是热带地区的生态系统。但是,其他草地生态系统也与萨王纳有着相似的特征。其中一点就是火灾频繁发生,比如位于亚热带的巴西南部、乌拉圭和阿根廷的潘帕斯草原,以及北美的橡树稀树草原(橡树萨王纳)。

2. 萨王纳的结构

环境百科全书-热带稀树草原-萨王纳的组成示意图
图4. 萨王纳的组成示意图,具有主要由C4草本植物和杂类草(非禾本科的非木质草本植物)构成的连续草本层,间或分布着乔木和灌木物种。反复的火烧和动物采食是两个主要干扰因子,世界各地萨王纳的动物捕食压力有很大的差异。[图片来源:©索·勒斯特拉迪奇和埃·比松]

  萨王纳的地貌多种多样,有些非常开阔,灌木稀少或不存在;而另一些恰恰相反,数目种类多,形成了相对封闭的树冠层(图3)。这些生态系统的共同特征是存在一个主要由C4*草本植物(详见光合作用的碳代谢途径)和和非木质草本植物(又称杂类草)组成连续的草本层

2.1. 萨王纳结构的起源

  树木通常能抵御火烧,或者在火灾发生后有一定复原力,但是草本层中的树种往往不耐荫[2]。因此,树木和草本植物在萨王纳中共存就显得不同寻常,这是与水资源、土壤性质和干扰机制(如火灾)相关的诸多过程相互作用的结果,也是这些生态系统植被结构的起源[2][3]

  • 气候条件(尤其是降雨量和雨季的时长) ,因为树木稳定生长必须满足其最小降雨量需求[4][5][6]。热带地区气候的特点是显著的季节性,干湿季节分明[4][6]。许多热带地区分布着大量的萨王纳——开阔的生态系统,单凭降雨量判断,这类地区应该有大片森林[2][5][7]
  • 长期干扰,包括火灾和食草动物采食抑制了树木覆盖度的扩大,使萨王纳得以维持(图5) 。
  • 土壤性质,比如土壤成分和质地会影响植被类型:在持水性能较弱的沙质土壤可以生长较多的灌木,而在持水能力较强的黏质土壤上更多生长的是草本植物(见图2)。
环境百科全书-热带稀树草原-多样的南非萨王纳
南非萨王纳的多样性。A:食草动物的典型例子——南非萨王纳的斑马;B:火灾促进草本植被的再生,有利于食草动物的生存(图中是南非的白犀牛)。[图片来源:©勒·斯特拉迪奇]

  萨王纳等热带草地生态系统是一种古老的生态系统,在火灾和大型食草动物共存的情况下经历了数百万年的演变。C4草本植物起源于渐新世末期,到中新世逐渐占据了主导地位,证明这类生态系统有久远的历史,其物种丰富度和高比例的特有种也是有力的证据。一些热带草地生态系统还生长着一些见证了漫长进化历程的特殊物种[8]

2.2. 火——塑造萨王纳生态系统的主要力量

  仅凭气候特征不能准确判断萨王纳的分布。食草动物(特别是“巨型食草动物”,如大象和有蹄类动物)和火灾干扰在萨王纳的生态和进化中发挥着主要作用[2][5][9](见图2~5)。

  萨王纳生态系统草本层中的草类作为一种主要生物燃料促进了火灾的发生,同时又作为遮蔽物抑制了乔木和灌木的生长。草类和一般草本物种的生命周期适应了火灾,能够在火灾后非常迅速地再生和繁殖(图6)。因此,它们在萨王纳生态系统中的存在有赖于火灾反复出现。

环境百科全书-热带稀树草原-火烧不久后开花的植物
图6. 巴西塞拉多波西萨王纳中火烧不久后开花的植物。A:一种苋科千日红属植物(Gomphrena lanigera Pohl ex Moq);B:一种莎草科球柱草属植物(Bulbostylis paradoxa (Spreng.) Lindm.)。[图片来源: ©勒·斯特拉迪奇]

  此外,萨王纳树种与森林树种的耐火性不同[7][10]

  • 萨王纳的树木表现出一些耐火的相关特征,特别是厚的树皮,如在塞拉多和亚诺斯生长的美洲锡叶树(Curatella americana,五桠果科)、巴西油桃木(Caryocar brasiliense,油桃木科)、风铃木属乔木(Handroanthus ochraceus,紫葳科)、翼齿豆属乔木(Pterodon emarginatus,豆科)、鲍迪豆属乔木(Bowdichia virgilioides,豆科)和番荔枝属乔木(Annona crassiflora,番荔枝科);
  • 森林乔木的树皮要薄得多,易受火烧伤害,如柯柏胶树(Copaifera langsdorffii,豆科)、铁木豆属乔木(Swartzia flaemingii,豆科)、孪叶豆(Hymenaea coubaril,豆科)、翅玉蕊属乔木(Cariniana estrellensis,玉蕊科)、芳香木瓣树属乔木(Xylopia aromatica,番荔枝科)。

  在热带地区,若没有火灾等因素的干扰,许多萨王纳的降雨量足以形成森林[11]。在这些地区,森林和萨王纳是生物群落的两种状态[5][7],其环境条件既可能出现森林,也可能出现萨王纳,实际出现哪一种植被主要取决于干扰因素,特别是干扰的强度和频率。

3. 生物多样性和保护问题

  萨王纳是一种特殊的遗产,有着极其丰富的生物多样性,特别是有丰富的地方特有物种。例如,被称为塞拉多的巴西萨王纳,其面积超过200万平方公里,有超过12 000种植物,是世界上物种最丰富的萨王纳(见《焦点:塞拉多生物群系》)。

  非洲的萨王纳除了具有多样的植被类型外,还是代表性巨型动物的家园,大象、斑马、长颈鹿、狮子和猎豹非常这些开阔的生态系统。此外,萨王纳也是人类文化遗产和历史的一部分。例如,一些研究表明人类天生偏爱开阔的景观,这可能源于人类在东非萨王纳的长期演化过程,这些环境通常被认为是人类进化的驱动力之一。

  然而,人类活动即使不是影响的源头,但随着时间的推移也深刻地改变了火灾和动物的采食模式,进而影响了萨王纳的分布[12]。目前许多人仍然生活在这些生态系统的中心,并依赖其提供的生态系统服务,如保持水质清洁、提供放牧场所和狩猎机会。

  热带草地生态系统是文化和自然遗产,但是当前它们受到严峻的威胁,包括大面积土地转为农用地控制火灾的政策以及为商业目的或为碳补偿进行的植树造林。例如,亚马孙地区塞拉多草地转变为农业用地的比例比该地区森林转化为农业用地的比例要高得多[13][14]

4. 萨王纳的恢复力

环境百科全书-热带稀树草原-火烧实验
图7. 在巴西塞拉多汤姆巴多尔(Serra do Tombador)的塞拉多进行的火灾实验。A:火灾前的植被;B:刚刚烧过的样地;C:火灾几周后的情景[图片来源:A和B来自©朱莉安娜·特谢拉;C由©亚历山德拉·菲德里斯提供,里奥克拉罗(Rio Claro)的素食者生态实验室-LEVeg,]。  萨王纳对自然干扰有极强的复原力,甚至依赖这些干扰维持开阔的特征。[15]。在非洲,大型食草动物的存在维持了开阔的环境,周期性火灾确保了植被再生。而在世界其他地区,维持开阔环境的主要原因是火灾。
虽然萨王纳发生火灾火烧最初都是天然原因(如雷暴时的闪电),但后来火灾发生的模式长期受到了人类活动的影响(在非洲至少有30万年的历史)。人类的演化导致火灾面积增加[12]。然而,人类活动对火灾产生实质性影响的时间相对较短,大致可以追溯到4000年前。畜牧业的发展导致火灾面积减少,因为放牧减少了地表生物燃料的数量[12]

  灰烬形式的养分、地下芽的存在、厚树皮对芽的保护以及植物有用于储存养分的大型地下器官,这些条件使植被(包括木本以及草本植物)在火灾之后能迅速恢复生长[5](图7)。

  另一方面,近几十年来,人类活动极大地改变了火灾的模式,包括频率和强度。尽管区域之间存在差异,但是总体上火灾面积增加,同时每次过火区域的大小也发生了变化[12]。许多地方的政府(如巴西、博茨瓦纳、津巴布韦、南非)制定了防火和灭火方案,力图尽可能处理和扑灭所有发生的火灾。

  萨王纳的退化和转化可能发生得非常迅速,而且往往不可逆。退化过程多种多样,如外来物种的引入、家畜放牧、排除火灾、树木植被密集化,以及在开放环境植树造林。退化程度取决于退化的持续时间和强度。如果干扰强度大,或者发生重大环境变化,如植树造林、农田改造或者采矿等,植被和土壤的性质就会发生大幅变化,萨王纳的自然恢复能力会降低,甚至完全消失[15]

  由于以下原因,上述退化过程通常会很快超过退化阈值,使得萨王纳无法再实现自发恢复(或者需要非常长的时间):

  • 成熟群落的形成依赖于许多过程的相互作用,非常缓慢;
  • 恢复需要的种子传播范围往往有限。

  在这些情况下,采取积极的恢复行动至关重要。

5. 恢复热带草地植物生态系统

环境百科全书-热带稀树草原-刚果退化的热带草本生态系统恢复力
图8. 刚果民主共和国严重退化的热带草地生态系统恢复力,需要进行人工恢复。[图片来源:©索·勒斯特拉迪奇]。

  由于萨王纳生态系统是几百万年进化和复杂生态相互作用的结果,因此如何恢复至今仍然是一个巨大的挑战。

  第一步是要考虑火灾和大型食草动物的自然干扰,这是萨王纳形成和维持的必要条件。因此,萨王纳恢复技术要包括重新引入自然干扰,如计划烧除、放牧管理、重新引入食草动物等,此外还要清除入侵物种。

  在退化严重的情况下(图8),还需要尽可能恢复地形地貌和土壤性质,然后重新引入本地物种。当然,这对于大多数缺乏生态学和生物学特征记录的草本物种来说比较困难。例如,某些物种:

  • 很少产生种子甚至没有种子,或繁殖期不固定;
  • 依赖火烧才能有性繁殖,或需要打破休眠状态[16]

  虽然重建草本层是恢复此类生态系统生态过程的基础,但往往难以成功,通常受到以下方面的制约:

  • 入侵物种的定殖和竞争;
  • 无法获得本地草本植物的种子,或者种子的繁殖效率低下[15]

6. 植树造林

  恢复萨王纳的另一个问题是它们经常被误认为是退化的森林。公众和决策者,甚至包括可能实施保护项目的政府部门,并不总能意识到萨王纳的生物多样性及其提供的(生态系统)服务的重要性。

  萨王纳通常被错误地认为是退化的森林,这使人们对此类生态系统的看法复杂化,并为烧除措施提供了理由。

  因为上述误解,许多所谓“恢复萨王纳”的项目都包括植树。大规模植树并不是恢复萨王纳合理技术,因为草本植物不能在荫蔽环境中生长,尤其是C4草本植物。如果出现大量树木的荫蔽,草本层的自发再生即使不是完全不可能,也会受到强烈的抑制。

  在缺乏草本层生物质(食草动物的食物来源和火灾的燃料)的情况下,很难恢复接近自然状态的火灾模式、食草动物群落以及它们承担的生态过程。恢复草本层是恢复此类生态系统的首要任务。

  “波恩挑战[17]”于2011年启动,目标是到2020年恢复1.5亿公顷退化土地和被砍伐的森林,到2030年恢复3.5亿公顷退化土地和被砍伐的森林。在这一背景下,出现了几项促进再造林和大规模植树的倡议,作为应对气候变化的举措。因为树木在生长过程中会吸收和储存人类活动排放的大部分二氧化碳。若干项倡议指明了可开展森林恢复的潜在区域[18][19]。有时,植树有非常务实的目标,如防治荒漠化的有害影响和阻止沙漠推进(详见绿色长城:绿化萨赫勒的希望?)。在这种特殊情况下,树木提供了不可或缺的生态系统服务功能,一方面有助于防治荒漠化,另一方面当地居民高度依赖木本植物的生物多样性。虽然在恢复退化森林或实现防治荒漠化的具体目标时,这类项目的可能受到支持,但在以前的萨王纳草原以及其他热带草原地区造林带来的风险必须受到谴责。

  几项研究已经表明,在“开放”的生态系统中植树存在风险(即使该生态系统的树冠比较高)。萨王纳就是一个典型案例,其草本层以原始、不耐荫物种为主[1][5]

环境百科全书-热带稀树草原-退化机制
图9. 热带森林、热带稀树大草原和原始草原的退化机制。这些生态系统的退化可能是由于火灾模式、植树和森林砍伐发生变化。这些物种丰富的生态系统退化时,往往伴随着动植物生物多样性的流失以及生态系统服务功能的流失,例如地下水的补给。退化或转化的生态系统有时被误认为是萨王纳或者退化的萨王纳。[图片来源: ©索·勒斯特拉迪奇和埃·比松,©受参考文献[2023]启发,保留所有权]

  这里我们必须明确指出:在萨王纳这样的生态系统中植树并不是恢复,特别是外来树种大面积种植的政策方向是尤为更严重的错误[20]。同时需要谨记,大多数大规模植树项目是基于当前的气候条件而提出。然而,由于气候条件的变化,预计许多现在有利于树木生长的区域几年之后可能就不适合其生长。因此,在执行恢复项目时必须考虑到潜在的气候变化。

  区分自然形成的萨王纳和退化的森林非常重要,天然萨王纳物种丰富,应当得到保护;而退化的森林有时被误以为是萨王纳或衍生的萨王纳,但是其中物种匮乏并且可以恢复成森林[21]

  此外,如果植树的主要理由是这是应对气候变化的最佳解决方案之一,那么同样需要重视的一点是萨王纳的大部分生物量地下,约占总生物量的70%。因此,位于地下的碳储量绝对不可忽视,特别是因为其免受火灾影响且能保持稳定碳汇。

  在这类开阔生态系统植树造林,除了导致植物和动物生物多样性流失[22],还会造成多方面的经济损失。特别是对于特定人群,他们的放牧、狩猎等活动有赖于萨王纳,同时依赖萨王纳的供水和保证供水质量等生态系统服务功能。除了生物多样性和许多动物栖息地丧失,很多生态系统服务功能退化等损失外,在容易起火的生态系统中广泛植树还会增加大型林火的风险,最终导致碳储存损失(图9)。

7. 萨王纳和气候变化:相反的预测

  萨王纳已经受到人为土地利用的严重影响,预计还会因气候变化和大气中二氧化碳浓度上升而发生显著变化[23]。事实上,萨王纳的分布范围取决于降水量和气候的季节性。对于未来的预测,不同的模型给出了截然不同的结果[24]。总体而言,气候变化会通过两种不同的机制减少全球萨王纳的分布范围:

  • 降水量预计会减少的地区,萨王纳面临沙漠化的风险。因为构成萨王纳的两种植被——C3树木和C4草本植物——对相同的环境因子有不同的反应[25],因此,萨王纳对干旱的反应可能不同于森林和草原[26]。此外,森林等不适应火灾的生态系统中,干旱的加剧会影响树木的生存,同时促进火灾的发生。
  • 在其他降水量预测会增加的地区,乔木和灌木覆盖更加浓密(竞争干扰),危害萨王纳。由于大气中二氧化碳浓度增加,这一过程已经开始了[27]。在一些依赖畜牧业的地区,草本植物的减少可能会对当地经济造成重创。

  例如,气候变化将导致塞拉多生态群落的严重同质化[28],这是生物多样性丧失和生态系统服务功能受损的主要驱动因素之一。

8. 重要信息

  • 恢复退化的森林——包括重新种植树木——很重要,但是植树不应损害其他生态系统的保护和恢复。
  • 生态恢复应恢复退化的生态系统,而不是破坏自然生态系统。
  • 由于恢复萨王纳仍然很困难,因此在环境政策方面应该优先保护。
  • 重视生物多样性和认识到萨王纳提供的生态系统服务功能是改善其保护、优化自然区域管理的第一步,这意味着需要运用计划烧除、允许自然火灾存在、引入本土巨型食草动物等措施。

  本文是勒·斯特拉迪奇和依·布韦松的“尊重”R90的修改版本,由法国生态与进化协会(SFE2)出版,并于2020年2月发布在其网站上。

 


参考资料及说明

封面图:[图片来源: ©索·勒斯特拉迪奇]

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To cite this article: LE STRADIC Soizig, BUISSON Elise (March 12, 2024), 恢复热带稀树草原和热带草地生态系统, Encyclopedia of the Environment, Accessed November 7, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/zh/vivant-zh/restoring-savannas-and-tropical-herbaceous-ecosystems/.

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