Consistent responses to moisture stress despite diverse growth forms within mountain fynbos communities

Abstract

Understanding climate change impacts on the Cape Floristic Region requires improved knowledge of plant physiological responses to the environment. Studies examining physiological responses of mountain fynbos have consisted of campaign-based measurements, capturing snapshots of plant water relations and photosynthesis. We examine conclusions drawn from prior studies by tracking in situ physiological responses of three species, representing dominant growth forms (proteoid, ericoid, restioid), over 2 years using miniature continuous sap flow technology, long-term observations of leaf/culm water potential and gas exchange, and xylem vulnerability to embolism. We observed considerable inter-specific variation in the timing and extent of seasonal declines in productivity. Shallow-rooted Erica monsoniana exhibited steep within-season declines in sap flow and water potentials, and pronounced inter-annual variability in total daily sap flux (Js). Protea repens showed steady reductions in Js across both years, despite deeper roots and less negative water potentials. Cannomois congesta—a shallow-rooted restioid—was least negatively impacted. Following rehydrating rain at the end of summer, gas exchange recovery was lower in the drier year compared with the normal year, but did not differ between species. Loss of function in the drier year was partially accounted for by loss of xylem transport capacity in Erica and Cannomois, but not Protea. Hitherto unseen water use patterns, including inter-annual variability of gas exchange associated with contrasting water uptake properties, reveal that species use different mechanisms to cope with summer dry periods. Revealing physiological responses of key growth forms enhances predictions of plant function within mountain fynbos under future conditions.

Assessing vulnerability to embolism and hydraulic safety margins in reed‐like Restionaceae

“…Minimum water potentials for Cannomois acuminata (now C. parviflora) were measured as >À1.9 MPa between October to May at the same field site used for this study (Miller et al 1984;von Willert et al 1989). Similarly, a multi-year field ecophysiological investigation of the shallow-rooted C. congesta found that Ψ min never fell below À2 MPa, even in a particularly dry year, typically remaining >À1.2 MPa throughout the year (Skelton et al 2023). Gas exchange and sap flow were also maintained in these culms throughout the year, at relatively low levels (Skelton et al 2013;Skelton et al 2023), most likely sustained by the uptake of the regular dew and cloud moisture events during rain-free periods (Skelton et al 2023).…”

“…Here we demonstrate that the optical and pneumatic methods can successfully generate robust estimates of vulnerability to embolism for restios, that will enable future examination of trait-environment relationships in this and other morphologically similar graminoid groups. Doing so allows us to demonstrate that restio occupancy of dry environments involves more than the evolution of highly resistant xylem, suggesting that other aspects of water relations are key to understanding trait-environment relationships in this diverse and ecologically important group (West et al 2012;Skelton et al 2023).…”

“…Our study supports this by demonstrating sigmoidal vulnerability curves, with positive safety margins for these species, which we consider a considerable advance in our ability to accurately measure vulnerability curves for these species. Furthermore, Cannomois are evergreen perennials, retaining functional culms all year round (Linder 2018;Skelton et al 2023). They do not have sterile culms, which may follow more acquisitive strategies, retaining only fertile culms which are more conservative (Ehmig et al 2019).…”

“…The ability to tolerate flooding stress has been well linked to the presence of aerenchymatous roots in the restios (Huber & Linder 2012), however less attention has been paid to the adaptations and trade-offs associated with coping with drought-related stress in this group, and the extent to which this relates to fundamental and realized niches. Interestingly, restios in mesic mountain fynbos have been shown to be remarkably drought resistant (West et al 2012), maintaining high water potentials and low rates of transpiration over the dry summer (Skelton et al 2023) despite being shallow rooted (West et al 2012;Skelton et al 2023); a phenomenon that has been attributed to their ability to utilize cloud (Marloth 1903(Marloth , 1905Nagel 1956) and dew moisture (Skelton et al 2023) during rain-free periods. However, the broad environmental range, and considerable lability in root architecture (Ehmig & Linder 2020) of the restios, demand that traits relating to drought stress are more widely investigated in this group.…”

In situ characterisation of whole‐plant stomatal responses to VPD using leaf optical dendrometry

“…Furthermore, leaf-level gas exchange typically involves single-point measurements taken at infrequent intervals (several days) and is thus not suitable for continuous and long-term monitoring in the field. At the whole-plant level, sap flow is the most commonly applied method for continuous monitoring of transpiration, from which stomatal response to VPD can be inferred (Goulden & Field, 1994;Hogg & Hurdle, 1997;Motzer et al, 2005;Skelton et al, 2013Skelton et al, , 2023. However, there are several issues that limit the resolution of sap flow data for assessing the dynamic regulation of stomatal conductance.…”