Tree species richness modulates water supply in the local tree neighbourhood: evidence from woodδ¹³C signatures in a large-scale forest experiment

Abstract

Biodiversity is considered to mitigate the adverse effects of changing precipitation patterns. However, our understanding of how tree diversity at the local neighbourhood scale modulates the water use and leaf physiology of individual trees remains unclear. We made use of a large-scale tree diversity experiment in subtropical China to study eight tree species along an experimentally manipulated gradient of local neighbourhood tree species richness. Twig wood carbon isotope composition ( δ 13 C wood ) was used as an indicator for immediate leaf-level responses to water availability in relation to local neighbourhood conditions and a target tree's functional traits. Across species, a target tree's δ 13 C wood signatures decreased progressively with increasing neighbourhood species richness, with effects being strongest at high neighbourhood shading intensity. Moreover, the δ 13 C wood -shading relationship shifted from positive (thin-leaved species) or neutral (thick-leaved species) in conspecific to negative in heterospecific neighbourhoods, most likely owing to a lower interspecific competition for water and microclimate amelioration. This suggests that promoting tree species richness at the local neighbourhood scale may improve a tree's local water supply with potential effects for an optimized water-use efficiency of tree communities during drought. This assumption, however, requires validation by further studies that focus on mechanisms that regulate the water availability in mixtures.

Functionally dissimilar neighbours increase tree water use efficiency through enhancement of leaf phosphorus concentration

“…Our outcome represents the first and direct evidence that the neighbourhood complementarity effect increased foliar iWUE (decrease in foliar c i ) through the enhancement of foliar P concentrations and potentially the C assimilation rate. This finding, however, is in contrast with several studies reporting decreasing δ 13 C signatures of trees (increases in foliar c i ) and an improved water supply to focal trees in mixed‐species communities (Jansen et al, 2021; Metz et al, 2016). It should be noted that the response of a target species to neighbourhood diversity and trait dissimilarity may not be representative of the response of the community to plot diversity.…”

“…The ratio of carbon (C) uptake via assimilation per unit H 2 O loss from transpiration, or water‐use efficiency (WUE), is a key characteristic of ecosystem function that is central to the global cycles of water, energy and C. A few studies have shown that tree diversity enhances soil water availability and tree transpiration through complementarity effect, which arises from niche differentiation or facilitation, and decreases the WUE of individual trees and communities (Chaves et al, 2021; Guevara et al, 2010). The results, however, remain controversial (Grossiord et al, 2014; González de Andrés et al, 2018), and studies on the mechanisms that regulate WUE in forest mixtures are lacking (Jansen et al, 2021).…”

“…The results, however, remain controversial (Grossiord et al, 2014;González de Andrés et al, 2018), and studies on the mechanisms that regulate WUE in forest mixtures are lacking (Jansen et al, 2021).…”

“…NSR was calculated as the number of species in the neighbourhood containing the 8 neighbours closest to a focal tree.The average neighbour distance between a focal tree and their 8 neighbours was 95 cm. NHI is the average height difference between the neighbours and the focal tree, inversely weighted by spatial distance and is used to account for shading effects(Jansen et al, 2021) A negative NHI value indicates that neighbouring trees are on average smaller than the focal tree. More negative NHI values suggest that the focal trees experienced less neighbourhood shading.…”

The role of species interactions for forest resilience to drought

“…Ecophysiological and ecohydrological measurements in mixed forests, such as transpiration or sap flow, are still emerging, and the underlying processes in species mixtures remain elusive (Gebauer et al 2012;Forrester 2015;Lübbe et al 2016;Jansen et al 2021). In general, higher growth rates in mixtures are accompanied by an increased stand water consumption (Forrester et al 2010;Kunert et al 2012;Lübbe et al 2016;Torquato et al 2020), a feature which has also been observed for ecosystems with woody encroachment or invasion (Rascher et al 2011;Caldeira et al 2015;Haberstroh et al 2021).…”