How Does Pedogenesis Drive Plant Diversity?
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TREE-1676; No. of Pages 10 Opinion How does pedogenesis drive plant diversity? 1 2 3 1 Etienne Laliberte´ , James B. Grace , Michael A. Huston , Hans Lambers , 1 4 5 Franc¸ois P. Teste , Benjamin L. Turner , and David A. Wardle 1 School of Plant Biology, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia 2 US Geological Survey, National Wetlands Research Center, 700 Cajundome Boulevard, Lafayette, LO 70506, USA 3 Department of Biology, Texas State University, San Marcos, TX 78666, USA 4 Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa, Ancon, Republic of Panama 5 Department of Forest Ecology and Management, Faculty of Forestry, Swedish University of Agricultural Sciences, SE901-83 Umea˚ , Sweden Some of the most species-rich plant communities occur Glossary on ancient, strongly weathered soils, whereas those on recently developed soils tend to be less diverse. Mecha- Base saturation: proportion of negatively charged soil exchange sites 2+ 2+ + occupied by base cations [i.e., calcium (Ca ), magnesium (Mg ), K , and nisms underlying this well-known pattern, however, + sodium (Na )]. remain unresolved. Here, we present a conceptual mod- Horizon: soil layer, parallel to the soil surface, with physical, chemical, or el describing alternative mechanisms by which pedo- biological characteristics that are distinct from the layers above or below it, and usually differentiated based on color, texture, and organic matter. genesis (the process of soil formation) might drive plant Humus: relatively stable soil organic matter that does not retain structural diversity. We suggest that long-term soil chronose- characteristics of the plant or animal tissues from which it originates. quences offer great, yet largely untapped, potential as Indicator variable: observed variable that serves as an indirect measure of a quantity of causal interest in a structural equation model [6]. ‘natural experiments’ to determine edaphic controls Kaolinite: aluminosilicate clay mineral with low cation exchange capacity, over plant diversity. Finally, we discuss how our con- common in old, strongly weathered soils. ceptual model can be evaluated quantitatively using Mycorrhizal network: network of mycorrhizal fungi that connect plants together via hyphal links [39]. structural equation modeling to advance multivariate Natural experiment: field study where variation in a driving factor (e.g., soil theories about the determinants of local plant diversity. nutrient availability or time since disturbance) is controlled via careful site This should help us to understand broader-scale diver- selection, rather than by imposed experimental treatments. Nutrient stoichiometry: the relative abundance of plant elements in the envi- sity patterns, such as the latitudinal gradient of plant ronment or in biomass. In particular, the N:P ratio is often emphasized, because diversity. they are the two nutrients that most often limit terrestrial plant productivity. More broadly, ecological stoichiometry is the balance of multiple chemical substances that affect, and are affected by, organisms, and that influence Links between plant diversity and pedogenesis ecological interactions and processes [56]. One of the most striking global ecological patterns is the Oxisol: an order in Soil Taxonomy, the US soil classification system. Oxisols are increase in plant diversity from the poles to the equator [1]. defined as soils with an oxic horizon [i.e., containing <10% weatherable miner- als in the fine sand fraction (50–200 mm) and a low cation-exchange capacity] or This increase in diversity at lower latitudes coincides with a kandic horizon (clay enriched and low cation-exchange capacity) with >40% a global gradient in soil development, whereby lower- clay in the surface soil. Oxisols represent soils that have undergone extreme latitude soils tend to be older and more strongly weathered weathering, are low in nutrients, and occur predominantly in stable regions of the lowland tropics. (see Glossary) compared with higher-latitude soils [2,3] Parent material: original material from which a soil is derived. (Box 1). This raises the question: is plant diversity linked to Pedogenesis: the process of soil formation, generally leading to the formation of soil horizons. The five major soil-forming factors are: parent material, organ- pedogenesis and, if so, what mechanisms are involved? isms, topography, climate, and time [57]. Identifying causal links between pedogenesis and plant Plant–soil feedback: process by which an individual plant modifies its local diversity across broad latitudinal gradients is problematic, abiotic or biotic soil properties, which in turn influences its own performance and that of other plants. because many factors co-vary with latitude. However, the Resource partitioning: occurs when two (or more) co-occurring individuals use global pattern of increasing plant diversity with pedogen- different resources (or different forms of the same resource), thereby reducing esis is reproduced at local scales along well-defined soil-age competition between them. Soil chronosequence: a series of soils that are derived from similar parent gradients from around the world [4,5] (Box 1). We propose material but differ in time since initiation of pedogenesis (Box 2). Variation in that these ‘soil chronosequences’ (Box 2) offer considerable other soil-forming factors (i.e., climate, topography, parent material, and organ- potential for evaluating multiple hypotheses for this in- isms) is minimized along a soil chronosequence [50,51]. In most soil chron- osequences, sites have not experienced constant climate and vegetation crease in plant diversity with soil age. throughout their pedogenic development, but current climate, parent material, This article has three sections. First, we present a set of and topography are usually well constrained. hypotheses about how pedogenesis might drive local plant Structural equation modeling (SEM): framework where a (generally multivari- ate) hypothetical causal model is first specified and then evaluated quantita- species diversity (Figure 1). Second, we discuss how these tively against data [6]. Weathering: physical, chemical, and biological processes through which less Corresponding author: Laliberte´, E. ([email protected]). stable (e.g., primary) minerals are converted to more stable minerals. In relatively young soils, weathering is the main source of rock-derived nutrients 0169-5347/$ – see front matter (e.g., P). ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tree.2013.02.008 Trends in Ecology & Evolution xx (2012) 1–10 1 TREE-1676; No. of Pages 10 Opinion Trends in Ecology & Evolution xxx xxxx, Vol. xxx, No. x Box 1. Long-term pedogenesis and local plant species diversity Although there is a wide diversity of soils in both tropical and deep, infertile Oxisols [62]. In addition, species-rich fynbos vegetation –1 temperate regions, the most strongly weathered soils in the world of South Africa (up to 114 species 0.1 ha ) [63] also occurs on ancient, (Oxisols) occur almost exclusively in the tropics, and underlie vast nutrient-impoverished soils. Similarly, some southwestern Australia areas of hyperdiverse lowland tropical forest. Conversely, many soils kwongan shrublands show high local species richness (up to 121 –1 at higher latitudes have been rejuvenated following recent glacia- species 0.1 ha ) [64] on heavily leached, infertile sandy soils with tions. The best known and most extreme cases of high local (i.e., extremely low P availability [65]. alpha) plant diversity on strongly weathered soils come from tropical Although these examples show that some of the most species-rich rainforests [11] (Figure I). For example, lowland tropical rainforest in plant communities on Earth occur on strongly weathered, infertile –1 Yasunı´ National Park (Ecuador) can contain up to 644 tree species ha soils, such broad-scale comparisons do not enable strong causal [58]. Soils in the Yasunı´ region are strongly weathered, very acidic, inference between pedogenesis and plant diversity, because different kaolinitic, have high exchangeable aluminum (Al) concentrations and regions can have very different geological and evolutionary histories very low base saturation [59]. Another example of hyperdiverse (e.g., recent glaciations at higher latitudes). Soil chronosequences, by tropical rainforest on strongly weathered soils in the Asian tropics is contrast, enable comparisons between ecosystems within much the Lambir Hills National Park (Sarawak, Malaysia), where up to 610 smaller regions, where factors such as climate and parent material –1 tree species ha can be found, with the greatest diversity on infertile are controlled [50,51]. The available data reveal that total plant Ultisols [60]. species richness usually increases with soil age across soil chron- High alpha diversity on strongly weathered soils is not, however, osequences spanning boreal, temperate, subtropical, and Mediterra- restricted to tropical rainforests (Figure I). Some areas of the Brazilian nean climates [4,5,24] (Figure II). Together, these global and smaller- cerrado moist savannah biome show relatively high plant alpha scale patterns motivate the search for underlying mechanisms that –1 diversity of up to 120 tree or shrub species ha [61]; cerrado soils are can explain the greater plant diversity on older soils. (A) (B) (C) (D) (E) (F) (G) TRENDS in Ecology & Evolution Figure I. Examples of species-rich plant