Surrounding species diversity improves subtropical seedlings’ carbon dynamics

Abstract

Increasing biodiversity has been linked to higher primary productivity in terrestrial ecosystems. However, the underlying ecophysiological mechanisms remain poorly understood. We investigated the effects of surrounding species richness (monoculture, two‐ and four‐species mixtures) on the ecophysiology of Lithocarpus glaber seedlings in experimental plots in subtropical China. A natural rain event isotopically labelled both the water uptaken by the L. glaber seedlings and the carbon in new photoassimilates through changes of photosynthetic discrimination. We followed the labelled carbon (C) and oxygen (O) in the plant–soil–atmosphere continuum. We measured gas‐exchange variables (C assimilation, transpiration and above‐ and belowground respiration) and δ13C in leaf biomass, phloem, soil microbial biomass, leaf‐ and soil‐respired CO 2 as well as δ18O in leaf and xylem water. The 13C signal in phloem and respired CO 2 in L. glaber in monoculture lagged behind those in species mixture, showing a slower transport of new photoassimilates to and through the phloem in monoculture. Furthermore, leaf‐water 18O enrichment above the xylem water in L. glaber increased after the rain in lower diversity plots suggesting a lower ability to compensate for increased transpiration. Lithocarpus glaber in monoculture showed higher C assimilation rate and water‐use efficiency. However, these increased C resources did not translate in higher growth of L. glaber in monoculture suggesting the existence of larger nongrowth‐related C sinks in monoculture. These ecophysiological responses of L. glaber, in agreement with current understanding of phloem transport are consistent with a stronger competition for water resources in monoculture than in species mixtures. Therefore, increasing species diversity in the close vicinity of the studied plants appears to alleviate physiological stress induced by water competition and to counterbalance the negative effects of interspecific competition on assimilation rates for L. glaber by allowing a higher fraction of the C assimilated to be allocated to growth in species mixture than in monoculture.

Drought impacts on tree phloem: from cell-level responses to ecological significance

“…Competitive or facilitative interactions with other trees are expected to influence the changes in phloem transport (Salmon et al 2018) and allocation under drought based on optimal partitioning models of biomass allocation (Franklin et al 2012). However, experimental data on carbon allocation under both competition and drought stress are extremely rare (for a rare example of such study see Hommel et al 2016).…”

Hydraulic diversity stabilizes productivity in a large-scale subtropical tree biodiversity experiment

“…Conversely, water savers can reduce this risk but may face carbon starvation under prolonged droughts 28 even though starvation is less ubiquitous than cavitation 47 . These contrasting stomatal control strategies themselves may induce strong inter-annual changes in tree growth while also determining the water availability in mixed-stands through soil water partitioning between co-existing species 48,49 . In tree neighbourhoods comprising species with different stomatal control strategies, water spenders may benefit from soil water left by their water-saving neighbours during drought, while water savers may capitalize on improved soil water conditions after a drought due to their potentially faster drought recovery 48 .…”