Diversification and phylogenetic correlation of functional traits for co-occurring understory species in the Chinese boreal forest

Functional traits impact species interactions, community composition, and ecosystem functioning. However, few studies have focused on the diversification and phylogenetic correlation of multiple functional traits over geological time. We conducted phylogenetic comparative analysis for boreal forest understory species in northeast China to examine the diversification and phylogenetic correlation in several functional traits: leaf area (LA), leaf carbon content (LCC), leaf dry matter content (LDMC), leaf nitrogen content (LNC), plant height (PH), and specific leaf area (SLA). Phylogenetic signals showed that there were very low levels of phylogenetic niche conservatism (PNC) in understory leaf-related traits and plant height, suggesting divergence of functional traits for the co-occurring understory species. The disparity through time analyses (DTT) indicated that trait disparities mainly originated during recent divergence events and there were no differences in the observed trait disparities compared to that expected under Brownian motion. Furthermore, we found both positive and negative phylogenetic correlations among the measured functional traits. The very low levels of PNC suggests that these functional traits diverged among co-occurring understory species, and that those species are distantly phylogenetically related. The phylogenetic correlations among traits maybe caused by both positively and negatively correlated adaptions which correspond to resource acquisition strategies. This study provides evidence that divergence in functional traits may reflect understory adaptations to boreal conditions.Funding provided by: National Natural Science Foundation of ChinaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001809Award Number: 31961133027Funding provided by: Natural Science Foundation from Science and Technology Department of Liaoning province*Crossref Funder Registry ID: Award Number: 2020-BS-021Funding provided by: National Natural Science Foundation of ChinaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001809Award Number: 31800376Funding provided by: National Natural Science Foundation of ChinaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001809Award Number: 42050410324Funding provided by: National Natural Science Foundation of ChinaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001809Award Number: 31971486Six functional traits (Table 1) which are closely associated with understory growth, survival, and light capture were measured (Meers et al., 2010; Refsland & Fraterrigo, 2017): including leaf area (LA), leaf carbon content (LCC), leaf dry matter content (LDMC), leaf nitrogen content (LNC), specific leaf area (SLA), and plant height (PH). These six functional traits were measured according to Pérez-Harguindeguy et al. (2013). In each sampled plot, we selected at least five reproductively mature individuals for each observed species. We sampled the relatively young, fully expanded and hardened leaves from adult plants. As understory plants grow between the forest canopy and the forest floor, light is usually the most limiting resource, affecting the establishment and growth of the understory plant species. For the species that typically grow partly in sunlight, the sampled leaves were taken from the parts that are most exposed to sunlight. Wherever possible, we avoided selecting leaves with obvious symptoms of pathogen or herbivore attack, or with a substantial covering of epiphylls. We put the leaf samples in sealed plastic bags, stored the bags in a portable cooler container, and transported them to the laboratory. To measure leaf area, we scanned leaves individually at 300 dpi using a Canon LiDE 120 series scanner coupled with a microcomputer. Then we used the scanned images to measure LA with ImageJ (Collins, 2007). The additional details on image processing can be found in the protocol proposed by Glozer (2008). After measuring LA, the leaf fresh weight was obtained using a electronic balance. Each leaf sample was subsequently placed in a paper envelope and dried at 80 ℃ for 48 h to determine the leaf dry weight with electronic balance. Other fresh leaf samples were dried at 80 ℃ for 48 h and ground until they would pass through a 2 mm sieve. LCC and LNC in ground leaf samples were measured using an automated elemental analyzer (EA1112 coupled with Delta-XP, Thermo Fisher Scientific K.K., Yokohama, Japan). PH was measured to the nearest cm from the top of the plant to the ground using a measuring tape