Understorey phylogenetic diversity in thermophilous deciduous forests: overstorey species identity can matter more than species richness

Abstract

Background: Understorey vegetation is a key biodiversity component of forest ecosystems. Previous studies examined its relations with the overstorey mainly in terms of taxonomic diversity, composition or productivity. So far, none focused on the phylogenetic aspect, which represents the deepest component of diversity in a community. Here, we explored the relations between overstorey species richness and identity and the phylogenetic structure of the understorey vegetation. As a model system we used a network of 36 plots in thermophilous deciduous forests of central Italy that are part of a European project on forest biodiversity and functions. The plots, characterized by similar site conditions, represent a gradient of overstorey richness from 1 to 4-species mixtures, with variable composition. After surveying the understorey in each plot, measures of phylogenetic diversity and structure (Phylogenetic Diversity, PD; Net Relatedness Index, NRI; and Nearest Taxon Index, NTI) were calculated from a fully resolved seed-plant phylogeny obtained from a ITS-5.8S nrDNA dataset including original sequences from local plant material. Results: The resulting understorey phylogenetic tree allowed reliable estimation of PD, NRTI and NTI. Phylogenetic diversity was dependent on species richness and the strength of this relationship did not change along the gradient of overstorey tree species richness. Net Relatedness and Nearest Taxon indices were both mostly positive but nonsignificant, indicating random phylogenetic structure of the understorey. The presence of Quercus petraea, usually found in more favorable site conditions, was associated with a significant phylogenetic clustering at the species level. Conclusions: Overstorey species richness was not related to understorey phylogenetic structure, while species identity (Q. petraea) was associated with an increase in evolutionary relatedness in this forest layer, possibly due to competitive exclusion in favorable site conditions. Further studies should examine whether and how understory phylogenetic structure is affected by variables such as small scale variations in site conditions and presence of a shrub layer, both present but not accounted for in our model system. A better understanding of the role of these variables and their interaction may contribute to fill the current gap between the theoretical field of phylogenetic ecology and forest management.

Understorey changes after an extreme drought event are modulated by overstorey tree species mixtures in thermophilous deciduous forests

“…Shifts occurred in the same direction in monospecific and mixed stands, though only in the latter these were significant due to cover variations and to, a lesser extent, species turnover. Changes were of different magnitude also depending on the focal tree species present in the overstorey, being smaller and not significant in the plots with the two more southern and drought-adapted species (either Q. ilex or Q. cerris) than in those with one of the three more mesophilous species (O. carpinifolia, C. sativa and Q. petraea; Coppi et al, 2019). While suggesting a possible tree species identity effect, this finding supports higher tolerance to water stress in understorey species of lowland mediterranean and sub-mediterranean woodlands, compared with those of more mesic forests at more northern latitudes or higher altitudes, as a result of the multiple drought adaptive traits of Mediterranean forest plants (Nardini et al, 2014).…”

“…mixtures). For the purpose of this work, the only 5-species mixture plot was pooled together with the 4-species plots, as in Coppi et al (2019). Each species richness level was replicated with different species combinations of these trees, to separate species diversity from composition effects; the 27 species combinations represented in our design and the frequency of each species in the four richness levels are given in Supplementary Table 1.…”

Effects of thinning on functional and phylogenetic diversity of regenerated woody plants in plantations over time

“…In addition, FDiv had a significant relationship with soil phosphorus content in middle‐aged LPs, which suggested that environmental factors have a certain degree of influence on functional diversity (Spasojevic & Suding, 2012), and the effects varied with the age of LPs. The possible reason is that the provenance of the regenerated woody plants was different, making the species of regenerated plants different with the age of LPs (Lu et al, 2014; Coppi et al, 2019; Sabatini et al, 2014).…”

“…Increasing the plant diversity of monoculture plantations is beneficial for promoting ecological functions and realizing the sustainable development of plantations (Zhang et al, 2021). In particular, understorey vegetation is an important component in the monoculture plantation ecosystem (Coppi et al, 2019). Studies have shown that understorey plants represent up to 80% of the plant species richness (Gilliam, 2007), and are critical for maintaining the functions of the plantation ecosystem (Chastain et al, 2006; Wayman & North, 2007).…”

Tree diversity and identity modulate the growth response of thermophilous deciduous forests to climate warming

“…Quercus petraea is a widely distributed species in central Europe, with several scattered populations in the Mediterranean region (Bruschi et al 2003), where it behaves as a mesophilous tree (Coppi et al 2019). The radial growth of this species proved to be affected by climate, with special reference to the spring air temperature and precipitation (Lebourgeois et al 2004).…”