Are ecophysiological adaptive traits decoupled from leaf economics traits in wetlands?

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2019 The Authors. Functional Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society. Institute of Environmental Sciences (CML), Leiden University, Leiden, The Netherlands


| INTRODUC TI ON
Wetland ecosystems include a wide variety of fresh and saltwater habitats (e.g., marshes, peatlands, mangroves, rivers, lakes, intertidal mudflats and rice paddies) that are distinguished from terrestrial habitats by a different hydrological regime (Ramsar Convention Secretariat, 2013). This causes wetland ecosystems to have unique features in terms of oxygen availability, nutrient cycles, soil pH and redox potential. These deviating environmental conditions strongly affect the survival and functioning of wetland plants. In response, wetland plants have developed a suite of adaptive traits, including tolerance and escape traits, to waterlogging or inundation and other conditions characteristic of wetlands (DeLaune & Pezeshki, 2001;Jackson & Armstrong, 1999;Pezeshki & DeLaune, 2012). These traits are strongly related to wetland plant performance, sometimes even vital to their survival. Previous studies on these adaptive traits have commonly focused only on one or a few species at the individual level, which makes these adaptive traits hard to incorporate into trait-based wetland ecology. In contrast, leaf economics spectrum (LES) traits such as leaf nitrogen (leaf N), leaf phosphorus (leaf P), specific leaf area (SLA) and photosynthetic rate (A mass or A area ) have received more attention, but do not include those traits that are considered vital to the survival of plants under wetland conditions in ecophysiological studies (van Bodegom, de Kanter, Bakker, & Aerts, 2005;Visser, Colmer, Blom, & Voesenek, 2000;Voesenek & Bailey-Serres, 2015).
Moreover, the functional importance of most traits is contextspecific (Baastrup-Spohr, Sand-Jensen, Nicolajsen, & Bruun, 2015;Shipley et al., 2016;Wright & Sutton-Grier, 2012). This context may well differ for wetland ecosystems compared to terrestrial ecosystems, because trait selection is strongly driven by environmental factors DeLaune & Pezeshki, 2001). A recent review paper (Moor et al., 2017) carefully reviewed both wetland adaptive traits and LES traits as well as their effect on ecosystem functioning, and the authors suggested not to simply employ the LES/plant economics spectrum (PES) to understand wetland ecosystems, since they vary widely in site conditions (bogs, peatland, marsh etc.). The study called for the inclusion of LES/PES and adaptive traits to get a better understanding of wetland ecology. To move towards this goal, we need to understand how these two groups of traits, if taken as the two major trait axes, position in relation to each other. In other words, it is important to disentangle the different roles that wetland adaptive traits and LES traits play in plant survival and resource utilization, respectively, their relationships being orthogonal (reflecting a decoupling) or coordinated (reflecting coupling through synergies or trade-offs), and the consequent effects on ecosystem functioning.
Phytohormones such as ethylene, gibberellin and abscisic acid also play important roles in changing cellular and organ structure that alleviate the oxygen deficiency (Bailey-Serres & Voesenek, 2008;Vartapetian & Jackson, 1997). Most of these primarily ecophysiological studies on wetland plants, though, are limited to an experiment-based assessment of one individual trait for a few species at a time. Unfortunately, it is rather difficult to scale up results from such detailed studies to the impacts of different plants and communities on wetland ecosystem functioning. Therefore, we need to integrate these ecophysiological traits into a more general ecological framework (Figure 1a).
There is some circumstantial evidence that wetland adaptive traits may be orthogonal to (i.e., independent of or decoupled from) LES/PES: wetland adaptive traits are the premise of plant existence in wetlands since they are vital to the survival of plants under hazardous anaerobic conditions. Based on that premise, one may expect trait selection processes in wetlands to be strong. At the same time, while LES traits are principally constrained by nutrient availability (Maire et al., 2015), wetland habitats span a wide fertility gradient from very infertile bogs to very fertile floodplains/ marshes at a global scale. This provides the conditions to allow for a full range of leaf N if wetland adaptive traits are orthogonal to LES/ PES ( Figure 1b). However, if trade-offs between the two axes predominate, one would expect only a subset of LES/PES would remain available for wetlands ( Figure 1c). The wide variety of growth strategies in wetlands, from conservative strategies associated with, for example, bogs to acquisitive strategies in highly productive systems such as reed lands, suggests that wetland plants can sufficiently develop adaptive traits to cope with multiple and varied wetland conditions. This pattern also suggests an orthogonal relationship between adaptive traits and LES/PES traits.
In this paper, we present an exploratory analysis to quantify the relationships between wetland adaptive traits and LES/PES traits.
We hypothesize that adaptive traits are principally decoupled from LES/PES traits in wetlands, assuming that these adaptive traits are not costly to have. Consequently, we predict that we will see a wide range of LES/PES in wetland plants. Using published and unpublished data, we assess the relationship between wetland adaptive traits and LES/PES traits. Then, we illustrate how wetland adaptive traits and LES/PES traits together impact wetland ecosystem functioning.
While the lack of integration of wetland adaptive traits into more generic trait-based approaches has formed a barrier to the direct employment of trait-based methods to wetland ecosystems to date, we propose that a more comprehensive understanding of wetland ecology can be obtained through the quantification of the relationships between the two suites of traits. This will also allow us to make better-informed decisions with respect to one of the standard dilemmas in trait-based community ecology: the choice of measuring traits for ease of measurements and low cost versus functional/mechanistic importance (Lavorel & Garnier, 2002;Wright et al., 2010).

| LITER ATURE RE VIE W ON THE REL ATI ON S HIPS B E T WEEN WE TL AND ADAP TIVE TR AITS AND LE S/PE S TR AITS
Some trade-offs among wetland adaptive traits and nutrient uptake have been described. In general, wetland plants may experience more nutrient stress than other plants under similar conditions of nutrient availability, because some adaptation to oxygen or redox stress result in a reduced adaptation to nutrient stress (Silvertown, Araya, & Gowing, 2015). In turn, this is likely to negatively affect leaf nutrient contents, which are part of LES/PES. For instance, decreasing root respiration and increasing aerenchyma leave less energy and active root biomass, respectively, for the active uptake of nutrients (van der Werf, Kooijman, Welschen, & Lambers, 1988). A root barrier that retards oxygen leakage may also reduce the efficiency of nutrient uptake (Colmer, 2003b), although studies suggest that symplastic aquaporin activity can prevent this effect (Rubinigg, Stulen, Elzenga, & Colmer, 2002). In some cases, cortical aerenchyma also inhibits nutrient transport (Hu, Henry, Brown, & Lynch, 2014). In the case of SLA, such a relationship is rather complex as SLA may be seen as part of LES/PES and other plant strategy axes, such as the size axis (Wright et al., 2010), and it may also relate to wetland plant's adaptation to water stress. For example, community mean SLA increased with flooding, suggesting that SLA contributed to the plant's waterlogging tolerance (Violle et al., 2011). Also, Mommer, Wolters-Arts, Andersen, Visser, and Pedersen et al. (2007) found, across nine species, that the internal oxygen partial pressure, the trait that enhances waterlogging tolerance in plants, was positively F I G U R E 1 A summary of most commonly studied wetland adaptive traits and leaf economics spectrum (LES)/plant economics spectrum (PES) traits (a); the relationships between these two suites of traits determine wetland plant adaptive and competitive strategies, and wetland ecological functioning. If wetland adaptive traits are orthogonal to LES/PES, even if environmental filtering to a specific setting of the water regime selects a subset of adaptive traits, almost a full range of LES/PES trait values would still be visible among wetland species (b). If trade-offs are predominant, environmental filtering of wetland conditions selects a subset of adaptive traits, and consequently, only a corresponding subset of LES/PES remains (c) Trait-based methods in wetland ecology correlated to SLA and negatively correlated to leaf thickness and cuticle thickness (while plasticity in these traits was not). Another extensive meta-analysis, comparing tens of species, suggested that the link between tolerance to oxygen stress and SLA response was significant but rather weak (Douma, Bardin, Bartholomeus, & van Bodegom, 2012).
While the examples above suggest some coordination for individual trait sets, when analysing tolerance towards waterlogging (presumably related to wetland adaptive traits) versus shade or drought (as related to LES/PES traits), a decoupling seems to prevail.
A study of 806 shrubs/trees across continents suggested that correlations among shade, drought and waterlogging tolerance indices were significant but very weak (Hallik, Niinemets, & Wright, 2009;Niinemets & Valladares, 2006). This suggests that oxygen stress-related traits (waterlogging tolerance) might be decoupled from leaf economics traits (shade tolerance). Also, the fact that environmental drivers of the LES/PES traits are different from those driving wetland adaptive traits suggests that some orthogonality may occur among these sets of traits.
Given the partially contradictory evidence listed in our qualitative literature review and since none of the above studies specifically tested the relationships of different trait axes, we provide an exploratory quantitative analysis in the next section.

| E XPLOR ATI ON OF THE REL ATI ON S HIPS B E T WEEN WE TL AND ADAP TIVE TR AITS AND LE S/PE S TR AITS
To quantitatively explore the so far rather anecdotal and possibly  Figure 1b). This is also supported by evolutionary evidence: aquatic species have evolved at least 200 times from terrestrial species (Cook, 1999).
Another type of adaptive traits relates to the tolerance, rather than avoidance or escape, of stressful conditions in wetlands. As a key stress tolerance characteristic of wetland plants, iron tolerance has been long considered as the cause for differential survival, growth and distribution among wetland plants (Snowden & Wheeler, 1993). Iron reduction along with manganese reduction takes place in the redox sequence after the depletion of nitrate, and produces phytotoxic ferrous iron. The physiological mechanisms behind iron tolerance are probably a combination of oxidation of the rhizosphere (partly contributed by ROL) and a true tolerance for Fe 2+ . Due to a lack of quantitative traits expressing these true iron tolerance mechanisms, we used the iron tolerance index proposed by Snowden and Wheeler (1993) as a proxy trait. In that study, an iron tolerance experiment was set up for 44 British fen species seedlings, cultivated under in 10% Rorison solution containing reduced iron (as ferrous sulphate). The iron tolerance index was estimated based on the impact of iron on the relative growth rate (RGR) in comparison with the RGR in a control group (Snowden & Wheeler, 1993). To test how iron tolerance relates to LES/PES traits, we derived SLA of the corresponding species (with the exception of Oryza sativa which was not available) from the LEDA database (Kleyer et al., 2008). A linear regression between the iron tolerance index and SLA showed that the iron tolerance index decreased strongly and significantly with an increasing SLA (r 2 = 0.237, Figure 4).
This pattern may indicate a true trade-off between iron tolerance trait and LES/PES traits. We hypothesize that tolerance-in contrast to avoidance or escape traits-may be costly and hence induce coupling with LES traits. It will require further experimental work to test this hypothesis more fully with other traits and in other systems.
Such experimental evaluation should consider other LES traits than SLA in relation to tolerance, given that SLA may also directly play a role in wetland adaptation (as discussed in Section 2).
The three exploratory investigations presented here suggest that both potentially coupled and decoupled relationships exist be-

| SC ALING FROM WE TL AND PL ANT TR AITS TO ECOSYS TEM FUN C TI ONING
Considering the importance of wetland ecosystems to humans, with regard to ecosystem services including water quantity and quality regulation and habitat provisioning for water birds and fish (Doherty The relationships between root porosity and leaf N. The data are from measurements from a glasshouse experiment (van Bodegom et al., 2008) and field measurements of three habitats: fen, marsh and forested/shrub wetlands (P. M. van Bodegom, unpublished data, Supporting Information Figure S3a-c in Appendix S1) In addition to affecting the functioning of wetlands, wetland adaptive traits may also affect the community structure of wetlands in a complicated way. ROL relates to oxygen leaking from roots into the soil, which results in microaerophilic conditions in the rhizosphere (van Bodegom & Scholten, 2001). This allows detoxification of several potentially toxic compounds such as S 2− and Fe 2+ .
The microaerophilic conditions induced by ROL do not only favour growth of the plant species that have ROL, but also facilitate the growth of less-adapted species that would not survive under purely anoxic soil conditions (Schat, 1984). As a consequence, the facilitation of these less-adapted species leads to a competition with the adapted species and a higher turnover of species than would have occurred otherwise (Grootjans, Ernst, & Stuyfzand, 1998).
Radial oxygen loss also contributes to community composition in a more direct way, through its coupling of the nitrification and denitrification processes. Compared to cases in which ROL is absent, the increased availability of soil oxygen in communities with ROL induces nitrification. The produced nitrate diffuses into the anoxic bulk soil and is denitrified, and hence leads to increased nitrogen losses and decreased nutrient availability in wetland ecosystems (Adema, Van de Koppel, Meijer, & Grootjans, 2005;Reddy, Patrick, & Lindau, 1989). Low nutrient availability makes it harder for competitors to invade, as many grow less effectively in such an environment. As a consequence, the community of stress-tolerating plant species that grow less quickly at high nutrient levels may remain more stable (Adema & Grootjans, 2003).
The relationship between specific leaf area (SLA) and iron tolerance (linear regression, adjusted r 2 = 0.237, p < 0.001, n = 43). SLA data were from the LEDA database (Kleyer et al., 2008), and iron tolerance data were estimated by Snowden and Wheeler (1993)