The response of functional traits to falling water levels
Declining water levels in alpine wetlands pose a great threat to plant growth, And can even lead to the degradation of wetland ecosystems (Bai et al. 2013; Wu et al. 2017). Plants adapt to environmental stresses by regulating their own trait characteristics, forming an adaptive strategy to spatial resources (Yin et al. 2018; Reich, 2014). Based on photosynthetic trait clustering analysis, wetland plants were classified into hydrophytes (Carex muliensis, Equisetum ramosissimum and Caltha scaposa) and mesophytes (Pedicularis longiflora var. tubiformis and Juncus allioides). We found that hydrophytes had greater morphological characteristics (e.g. higher LDMC and SLA) compared to mesophytes, but that morphological characteristics decreased with decreasing water level gradients (Figs. 3a, b, c, d, e, 5a). This change was mainly attributed to differences in water use by two types of plants with different levels of water demand, hydrophytes are themselves adapted to grow in environments with higher water content, and when water levels drop and plants are exposed to drought stress, hydrophytes shorten morphological traits such as leaf area by limiting cell division and growth rate to meet their growth requirements (Ma et al. 2019; Khanday et al. 2017; Liu et al. 2018; Wang et al. 2015). In addition, when water levels drop, hydrophytes have suitable water requirements and plants improve their resource competition and expand their ecological niche by increasing trait inputs such as leaf height (Craine et al. 2001; Robroek et al. 2017). Interestingly, the leaf thickness of hydrophytes did not decrease as traits such as leaf area decreased with the water level gradient but increased. One explanation is that hydrophytes conserve intracellular water by increasing leaf thickness to increase the distance and resistance of water molecules inside the cell to diffuse outside the cell (Li Qun et al. 2018; Lei Lei et al. 2018). Another explanation is that increasing leaf thickness will reduce light transmission, weakening other physiological processes including photosynthesis and reducing water consumption (Gonzalez-Paleo et al. 2018).
The leaf is the main organ for photosynthesis, and photosynthetic capacity is mainly reflected by photosynthetic properties (chlorophyll fluorescence parameters and photosynthetic gas exchange parameters). In general, chlorophyll fluorescence parameters are a better indicator of the absorption, transfer, and dissipation of light energy in plants than photosynthetic gas exchange parameters (Lin et al. 1992). Our results found that Fv/Fm was higher for Carex muliensis, Equisetum ramosissimum and Caltha scaposa (hydrophytes) than for Pedicularis longiflora var. tubiformis and Juncus allioides (mesophytes) at WT10 water levels, while Juncus allioides was lower than the three hydrophytes at WT-50 water levels. This indicates that the photosynthetic capacity of the hydrophytes was stronger than that of the mesophytes under high water conditions and vice versa. It was also found that the ETRmax and qP of mesophytes (e.g. Pedicularis longiflora var. tubiformis and Juncus allioides) were significantly higher than those of hydrophytes (e.g. Equisetum ramosissimum and Caltha scaposa). Which could be attributed to the electron allocation pattern of the photosynthetic dark reaction process, as the light energy absorbed by hydrophytes was inhibited in the photochemical reaction process under stress conditions, in order to reduce the damage caused by excessive light radiation (Feng et al. 2002), plants release the part of the absorbed light energy that is not used in the PSII reaction center as thermal dissipation (Bai et al. 2011). However, thermal dissipation causes a decrease in ETRmax, which ultimately affects the photosynthetic rate of the plant (Zhang et al. 2005; Ma et al. 2020). In conclusion, in alpine wetland ecosystems with low temperatures and short growing seasons, plants need to increase photosynthetic rates to fix the required organic carbon in a short period of time. Declining water tables will inevitably affect photosynthesis in hydrophytes, resulting in reduced carbon sequestration (Guo et al. 2018; Duan et al. 2019).
The stoichiometric characteristics are closely related to nutrient storage and reflect the regulation of physiological mechanisms during the growth of the organism (Sardans et al. 2012; Güsewell et al. 2004). In particular, N content indicates the photosynthetic capacity of the plant, the P content is related to leaf longevity, etc., while N/P reflects the degree of plant limitation by N or P elements (Santiago et al. 2004; Takahashi et al. 2008). In this study, the TN and TP contents of hydrophytes (Carex muliensis, Equisetum ramosissimum and Caltha scaposa) were smaller than those of mesophytes (Juncus allioides) (Figs. 3j, k, 5c). This result can be explained by the relationship between plant growth and nutrient uptake, in the context of declining water levels in wetlands, on the one hand, hydrophytes are weakened by water stress to absorb N and P elements during cell reproduction and division (Zhao et al. 2014; Tang et al. 2020). On the other hand, P elements are involved in the reverse fusion of ATP and ADP resulting in a decrease in their content (Kasurinen et al. 2006; Taub et al. 2008). Our study also found that the N/P of both groups was less than 14, indicating that alpine wetland plants are N-limited rather than P-limited. This is supported by the results of long-term experimental observations (Vitousek et al. 1997) showing that plants are N-limited in the early stages of primary succession and P-limited in the later stages. This may be due to differences in the amount and form of the two elements in the soil, as prolonged inundation of alpine wetlands reduces the availability of nitrogen by inhibiting the rate of nitrogen mineralization, whereas phosphorus is usually present as soluble phosphate in water and is readily leached for plant uptake (Fonseca et al. 2000). In addition, hydrophytes (Carex muliensis and Caltha scaposa) were found to have higher N/P than mesophytes (Pedicularis longiflora var. tubiformis and Juncus allioides) (Figs. 3l and 5c), suggesting that hydrophytes are better able than mesophytes to adapt to stress through rapid adjustment of elemental ratios under changing water level conditions.
Water levels affect the coupling of functional traits
In alpine wetland ecosystems where water is the main limiting factor, the difference in survival strategies between hydrophytes and mesophytes along a water table gradient lies in the way water is used. Water conservation by both types of plants can be achieved through strategies that increase water use efficiency and reduce water loss (Markesteijn et al. 2009). Plants improve their adaptability to different water levels by adjusting the way traits such as leaf morphology, photosynthetic and stoichiometric ratios are distributed (Niinemets., 2001; Liu et al. 2015).
In this study, LH, LA, SLA, LDMC, TN, and TP of hydrophytes were positively correlated with the water table gradient, while LA, SLA, TN, and TP/TP of mesophytes were negatively correlated with the water table gradient (Fig. 4a, b). This is a result of trait adjustment of different water-demanding plants in response to changes in the water table to improve plant survival (Gao et al. 2016; Reich et al. 1998). During the decline of the water table in wetlands, it is water stress for hydrophytes, while the opposite is true for mesophytes. To reduce water loss and conserve water, on the one hand, hydrophytes reduce water consumption by photosynthesis and transpiration by reducing leaf area and specific leaf area, and increase the distance and resistance of water diffusion from the interior to the surface of the leaf by increasing leaf thickness (Maharjan et al. 2011; Geng et al. 2012). On the other hand, by limiting the rate of cell division, the plant weakens its water demand during growth (Garnier et al. 2004). Furthermore, stepwise regression analysis showed that the functional traits of hydrophytes were related to the water table gradient (WT Hygrophyte = 18.988-2.755XLA-15.040XTN, R2 = 0.972, P < 0.01) and mesophytes as (WT Mesophyte = -28.272+6.388XLA+14.795XTN+5.414XSLA, R2 = 0.905, P < 0.01). Interestingly, although the two types of plants with different levels of water demand showed opposite traits in response to the water table gradient, both contained LA and TN. This result suggests that both LA and TN traits are sensitive to changes in the water table gradient and can be used as primary traits to indirectly predict the response of alpine wetland plants to water table decline.
Inter-trait adaptation strategies and trade-offs
Plants have evolved over time to adapt to their environment not only through single traits but more importantly, through the coordination of traits (Osnas et al. 2013; Jin et al. 2019; Shipley et al. 2011). This combination of traits, selected by natural selection, is the most competitive trade-off between plants along a particular ecological strategy axis (Wright et al. 2007; Westoby et al. 2002; Soliveres et al. 2014). Studying the trade-offs between plant functional traits can help to understand the adaptation strategies of alpine wetland plants under different water level gradients and explore the mechanisms underlying ecological niche differentiation.
In functional ecology, the two ends of a trait combination represent two different adaptive strategies for plants, namely the trade-off between rapid resource acquisition and efficient resource storage (Garnier et al. 2001; Wei et al. 2021). Generally, plants that adopt rapid growth have greater water transport efficiency and carbon sequestration capacity but are less adapted to drought stress (Arredondo et al. 2003). In this study, the number and degree of correlations between functional traits were higher in hydrophytes than in mesophytes (Fig. 6a, b). This suggests that hydrophytes are more sensitive to wetland water level decline than mesophytes and have more efficient trade-offs between traits, which is supported by a previous study (Cao et al. 2017). During wetland water level decline, plants will ensure their own survival and reproduction by coordinating and sacrificing some traits based on the total amount of water and nutrient resources available in the soil, especially during water stress (Moor et al. 2017). Hydrophytes under water stress adopt a conservative growth strategy to minimize water dissipation and improve resource use efficiency (Catford et al. 2014). On the one hand, plants use more photosynthetic products for dry matter accumulation (e.g. higher LDMC) to increase their competitive advantage (Garnier et al. 2004; Hikosaka et al. 2009). On the other hand, reduced leaf area and specific leaf area increase the distance and resistance to water dissipation and improve water use efficiency (Thomas et al. 2017). It is worth noting that a reduction in morphological traits weakens the photosynthetic capacity and nutrient uptake of plants, resulting in a reduction in LH, LA, Fv/Fm, ETRmax, TN, and TP (Fig. 3a, b, f, g, j, k) and an increase in LT, LDMC, and NPQ (Fig. 3c, d, i), and vice versa. It can be seen that both hydrophytes and mesophytes form an optimal combination of traits in response to water table decline through mutual regulation and trade-offs between traits. However, hydrophytes are more efficient at coordinating traits for water table decline than mesophytes.