Economic and hydraulic divergences underpin ecological differentiation in the Bromeliaceae

Abstract

Leaf economic and hydraulic theories have rarely been applied to the ecological differentiation of speciose herbaceous plant radiations. The role of character trait divergences and network reorganization in the differentiation of the functional types in the megadiverse Neotropical Bromeliaceae was explored by quantifying a range of leaf economic and hydraulic traits in 50 diverse species. Functional types, which are defined by combinations of C 3 or Crassulacean acid metabolism (CAM) photosynthesis, terrestrial or epiphytic habits, and nonspecialized, tank-forming or atmospheric morphologies, segregated clearly in trait space. Most classical leaf economic relationships were supported, but they were weakened by the presence of succulence. Functional types differed in trait-network architecture, suggesting that rewiring of trait-networks caused by innovations in habit and photosynthetic pathway is an important aspect of ecological differentiation. The hydraulic data supported the coupling of leaf hydraulics and gas exchange, but not the hydraulic safety versus efficiency hypothesis, and hinted at an important role for the extra-xylary compartment in the control of bromeliad leaf hydraulics. Overall, our findings highlight the fundamental importance of structure-function relationships in the generation and maintenance of ecological diversity.

Hydraulics link leaf shape and environmental niche in terrestrial bromeliads

“…. K leaf,max values were comparable with those reported for related Pitcairnia and terrestrial Bromelioideae species in Males and Griffiths (). Statistical comparison of hydraulic traits among species was not possible due to the low number of species representative of each leaf shape.…”

“…Since the stomata are exclusively abaxial, if K leaf,max is limited by the venation architecture associated with these leaf shapes, abaxial water‐storage tissue could provide an important reservoir to buffer against rapid changes in leaf water status following any sudden increase in evaporative demand. Meanwhile, the combination of low cellular drought tolerance with the use of stored water to buffer chlorenchyma water potential during drought is a key characteristic of bromeliad ecophysiology (Nowak & Martin , Males & Griffiths , in review). Internal water remobilization could be important in partially mitigating the impact of leaf shape and venation architecture on hydraulic limitations of gas exchange in the succulent terrestrial Bromelioideae.…”

Linking water relations and hydraulics with photosynthesis

“…storage within a tissue is often described as the water content per dry mass (g g À1 ), and is clearly related with sapwood density and capacitance in stems (Stratton et al, 2000;Scholz et al, 2011;Pratt and Jacobsen, 2017). The role and mechanisms of water storage have been extensively described in succulent species (Lamont and Lamont, 2000;Males and Griffiths, 2017), although its contribution to the water transport buffering is doubted (Males, 2017). In stems, stored water in the xylem can significantly contribute to the total amount of daily transpired water (Scholz et al, 2011).…”

Functional types in the Bromeliaceae: relationships with drought‐resistance traits and bioclimatic distributions

“…Other traits in epiphytic bromeliads that are likely to enhance the conservatism of water use and facilitate drought avoidance include low stomatal conductance and leaf hydraulic conductance (North et al . , ; Males & Griffiths, ). We hypothesised that this complex of drought avoidance traits could reduce the need for drought tolerance mechanisms in tank epiphytes, which was supported by the less negative values of π o in tank epiphytes than in terrestrials.…”

“…How this distinctive drought resistance syndrome of tank epiphytes relates to their heteroblastic development from tank-less juvenile forms should be investigated further, particularly given the importance of drought-related juvenile mortality in bromeliad demography (Hietz, Ausserer & Schindler 2002;Winkler, H€ ulber & Hietz 2005). Other traits in epiphytic bromeliads that are likely to enhance the conservatism of water use and facilitate drought avoidance include low stomatal conductance and leaf hydraulic conductance (North et al 2013(North et al , 2015Males & Griffiths, 2017). We hypothesised that this complex of drought avoidance traits could reduce the need for drought tolerance mechanisms in tank epiphytes, which was supported by the less negative values of p o in tank epiphytes than in terrestrials.…”