Below‐ground determinants and ecological implications of shrub species' degree of isohydry in subtropical pine plantations

Abstract

The degree of plant iso/anisohydry is a popular framework for characterising species-specific drought responses. However, we know little about associations between below-ground and above-ground hydraulic traits as well as the broader ecological implications of this framework.For 24 understory shrub species in seasonally dry subtropical coniferous plantations, we investigated contributions of the degree of isohydry to species' resource economy strategies, abundance, and importance value, and quantified the hydraulic conductance (K h ) of aboveground and below-ground organs, magnitude of deep water acquisition (WA deep ), shallow absorptive root traits (diameter, specific root length, tissue density), and resource-use efficiencies (A max , maximum photosynthesis rate; PNUE, photosynthetic nitrogen-use efficiency).The extreme isohydric understory species had lower wood density (a proxy for higher growth rates) because their higher WA deep and whole-plant K h allowed higher A max and PNUE, and thus did not necessarily show lower abundance and importance values. Although species' K h was coordinated with their water foraging capacity in shallow soil, the more acquisitive deep roots were more crucial than shallow roots in shaping species' extreme isohydric behaviour.Our results provide new insights into the mechanisms through which below-ground hydraulic traits, especially those of deep roots, determine species' degree of isohydry and economic strategies.

Unlocking Drought-Induced Tree Mortality: Physiological Mechanisms to Modeling

“…Intuitively, species with deeper rooting systems have a greater chance to capitalize on relatively stable water resources such as deep soil water or groundwater, so there will be less risk of experiencing negative water potential during protracted drought stress. This suggests that the rooting depth trait can be informative in predicting species-specific mortality vulnerability, especially for co-occurring species or individuals showing contrasting drought response ( Johnson et al, 2018b ; Li et al, 2018b ; Jiang et al, 2020 ). Interestingly, in a study conducted by Wu et al (2018) , the authors noted that the legacy effects of drought, as represented by the offsets between observed growth and predicted growth, persisted longer in deep-rooted trees compared with shallow-rooted functional types such as grasses or shrubs, indicating that deeper roots may compromise drought resilience, therefore questioning the role of deeper roots in facilitating overall fitness.…”

Opportunities, challenges and pitfalls in characterizing plant water‐use strategies

“…This limitation is particularly striking considering that the highly dynamic below-ground environment plays a crucial role in mediating plant water stress responses (Brunner et al, 2015;Phillips et al, 2016). Indeed, plant water-use strategies can shift within a species due to variation in rooting depth, root hydraulics and soil biotic interactions (Jiang et al, 2020;Kannenberg & Phillips, 2017;Matheny, Mirfenderesgi, et al, 2017). Inferences regarding plant water sourcing and below-ground behaviour are often limited by current model representations that treat the rhizosphere as a one-dimensional profile, with no lateral interaction or competition (Warren et al, 2014).…”

Experimental and conceptual approaches to root water transport

“…L p is however measured without normalization by the root size (eg. Jiang et al 2020 ). Finally, Tsuda et al showed a good correspondence between L o (plus the total plant conductance, K plant , and leaf conductance, K leaf ) as measured with an HPFM and the same parameters, but measured by another technique, the evaporative method (Tsuda and Tyree 2000 ).…”