Variation of leaf functional traits at different developmental stages
Different leaf functional traits characterize different functional properties (Liu and Ma, 2015). LA reflects the light capture capacity of the plants (Xiao et al., 2004). LWC reflects the active degree of leaf metabolism to some extent (Cornelissen et al., 2003). LT reflects the ability of leaves to acquire resources and preserve water (Bei et al., 2011). SLA reflects the ability to acquire and use nutrients (Xiao et al., 2004). LNC and LPC reflect the level of N and P metabolism in plants (Hu et al., 2014). Leaf functional traits vary at different stages of growth and development, which have specific characteristics at different times (He et al., 2013). Yang et al. (2016) studied Populus euphratica and Yao et al. (2013) studied mono maple (Acer mono Maxim.) and found that the leaf functional traits were different with respect to the growth characteristics at different developmental stages. In our study, the leaf functional traits of D. cercidifolius var. longipes also varied with the developmental stages. LA and LT increased, while LWC, SLA, LNC, and LPC decreased with the increase in plant development, which indicated that LA and LT were positively correlated with the developmental stages, while LWC and SLA were negatively correlated with the developmental stages.
The variation in the traits was closely related to the physiological characteristics at different stages of growth and development. With an increase in age, plants increase their LA so as to assimilate and produce more organic substances to meet their needs for growth and development. An increase in LA enhances transpiration, so plants increase LT to increase the distance and resistance of water transport from inside to the surface of the leaf (Ackerly et al., 2002) so as to reduce water loss (Ding et al., 2014). The increase in LT also promotes the formation of more ducts, cellulose, and lignin, which form a strong support frame and satisfy the needs of the plant to develop from seedlings to adults (Roderick et al., 1999). LWC is at a high level at the seedling and sapling stage due to considerable growth and a high metabolic rate. At the adult plant stage, as the transpiration area increases and other physiological activities such as flowering and reproduction are underway, LWC decreases in order to increase water use efficiency (Pérez-Harguindeguy et al., 2013). The decrease in SLA with the increase in development may be due to the need for more skeletal structures in large leaves (Duan et al., 2017). At the seedling stage, due to limited resources, it is difficult for the plant to support the large scale and long return cycle of construction in the leaves per unit area; as a result, the LA is small, and the SLA is high. At the adult stage, it is more difficult for the plant to transport water to the top of the stem. The water potential at the top of an adult plant is usually lower than that of seedlings, which leads to lower SLA and LWC of the top leaves of the adult plants so as to avoid dysregulation of cell turgor pressure (Liu et al., 2006; Li et al., 2013).
The N and P contents of plant leaves often show large variations at different stages (Zheng et al., 2014). The LNC and LPC of plant leaves gradually decrease with an increase in leaf age (Hu et al., 2014). The LNC and LPC also decreased with the development of D. cercidifolius var. longipes. This may be due to the fact that the metabolism in leaves is at a high level at the seedling stage; in addition, growth requires high amounts of protein and nucleic acid, which increases the N and P concentrations. By contrast, in saplings and adult plants, most of the nutrients acquired by the plant are transported to other organs to meet the reproduction requirement; as a result, the N and P contents in the leaves of the adult plants decrease (Poorter et al., 2009).
Relationship Between Trait Syndrome And Developmental Stages
In recent years, biologists have considered the leaf functional traits as the consequence of the coordinated collaboration of the components in a complex developmental system (Niinemets et al., 2007), which ultimately embodies varied growth strategies and life history strategies (Niinemets, 2002). Plant leaf traits do not work alone (Kenzo et al., 2015). Once all individual traits show a difference at different developmental stages, the trait syndrome naturally differs among different stages. According to the plant economic spectrum theory, the variation in most plant functional traits is in a covariant or trade-off relationship, which can be classified as the single axis of "fast return–slow return" (Li et al., 2017; Hu et al., 2014). The fast return end is mainly characterized by higher LWC, SLA, LNC, and LPC. The slow return end embodies higher LT and LA. Plant growth and development are generally at the expense of metabolic decline (Wu et al., 2002); hence, compared with seedlings, adult plants have a slow return strategy (Xie et al., 2003), the manifestation of which is that the SLA, LNC, and LPC of seedlings are higher than those of adult plants (Wright et al., 2004; Reich et al., 2014; Díaz et al., 2016).
The PCA in our study showed that the correlation coefficient between the first component and leaf traits LA, LT, SLA, LNC, and LPC was the highest, while the correlation coefficient between the second principal component and LWC was the highest. The PCA results of Disanthus cercidifolius var. longipes plants at different developmental stages showed that LWC was the dominant trait in the trait syndrome at the seedling stage. At the sapling stage, LPC, LT and LA were the dominant traits. At the adult plant stage, LT and LA were the dominant traits in the trait syndrome. With an increase in the plant developmental stage, the trait syndrome of D. cercidifolius var. longipes changed along the principal component axis 1 toward the direction with a negative value, which meant that LT and LA increased, while LWC, SLA, LNC, and LPC decreased, indicating that the plants changed from a fast return strategy (resource acquisition) to a slow return strategy (resource conservation). The results of our study support the conclusion that the growth and development of individual plants have a regulatory effect on the economic spectrum of the species (Sendall et al., 2013).
Disanthus cercidifolius var. longipes plants have different economic strategies at different developmental stages, and proper culture and protection measures should be taken according to their requirement at different developmental stages. Seedlings of D. cercidifolius var. longipes require high LWC. It has been reported that LWC and the photosynthetic rate of the leaves of D. cercidifolius var. longipes seedlings increase with increased shading (Mason et al., 2013), indicating that a humid and shady environment is suitable for the growth of seedlings. Therefore, during relocation of the seedlings, shading is needed to ensure and promote their growth. At the sapling and adult plant stages, LT and LA gradually increase, and plants need strong light for photosynthesis. Therefore, for local or relocated saplings and adult plants, the surrounding trees should be properly trimmed and the debris removed to guarantee the light required by D. cercidifolius var. longipes plants.