Co-ordination between xylem anatomy, plant architecture and leaf functional traits in response to abiotic and biotic drivers in a nurse cushion plant

Abstract

Background and aims: Plants in dry Mediterranean mountains experience a double climatic stress: at low elevations, high temperatures coincide with water shortage during summer, while at high elevations temperature decreases and water availability increases. Cushion plants often act as nurses by improving the microclimate underneath their canopies, hosting beneficiary species that may reciprocally modify their benefactors' microenvironment. We assess how the nurse cushion plant Arenaria tetraquetra subsp. amabilis adjusts its hydraulic system to face these complex abiotic and biotic constraints.

Methods: We evaluated intra-specific variation and co-ordination of stem xylem anatomy, leaf functional traits and plant architecture in response to elevation, aspect and the presence of beneficiary species in four A. tetraquetra subsp. amabilis populations in the Sierra Nevada mountains, southern Spain.

Key results: Xylem anatomical and plant architectural traits were the most responsive to environmental conditions, showing the highest mutual co-ordination. Cushions were more compact and had smaller, more isolated conductive vessels in the southern than in the northern aspect, which allow minimization of the negative impacts of more intense drought. Only vessel size, leaf mass per area and terminal branch length varied with elevation. Nurse cushions co-ordinated plant architecture and xylem traits, having higher canopy compactness, fewer leaves per branch and fewer, more isolated vessels than non-nurse cushions, which reflects the negative effects of beneficiary plants on nurse water status. In non-nurse cushions, plant architecture co-ordinated with leaf traits instead. The interacting effects of aspect and elevation on xylem traits showed that stress due to frost at high elevation constrained xylem anatomy in the north, whereas stress due to drought had a parallel effect in the south.

Conclusions: Trait co-ordination was weaker under more demanding environmental conditions, which agrees with the hypothesis that trait independence allows plants to better optimize different functions, probably entailing higher adjustment potential against future environmental changes.

Keywords: Arenaria tetraquetra subsp. amabilis; Mediterranean high mountains; drought stress; intra-specific trait variation; leaf functional traits; nurse cushion plant; plant architecture; quantitative wood anatomy; trait co-ordination.

Fig. 1.

(A) Location of the study populations in the Sierra Nevada mountains (southern Spain) with x- and y-axes displaying longitude and latitude in degrees (0.2° equals approx. 22.2 km). The location of the study area in the Iberian Peninsula is indicated by a star. (B) Population on the southern aspect and at high elevation (2830 m a.s.l., 37°01′N, 03°18′W). (C) Arenaria tetraquetra subsp. amabilis nurse cushion with some individuals of Lotus corniculatus subsp. glacialis (yellow and orange flowers) growing within its canopy. (D) View of A. tetraquetra terminal branches. (E) Microsection of the root collar of an A. tetraquetra individual growing in the south–high population. Vessels are identified as larger white circles with thick pink walls. The whole microsection comprises a surface of 0.5 mm².

Fig. 2.

Plant architecture (A–D), leaf functional traits (E and F) and xylem anatomical traits (G–L) of Arenaria tetraquetra subsp. amabilis in the Sierra Nevada mountains (Spain). Mean ± s.e. per site (N, north; S, south), elevation (L, low elevation; H, high elevation), population (NL, north low; NH, north high, SL, south low; SH, south high) and facilitation role (F, nurse cushion; NF, non-nurse cushion) are shown. See trait abbreviations in Table 1.

Fig. 3.

Correlations (i.e. normalized vectors, represented by arrows) between the variables of each sub-set (plant architecture, leaf functional and xylem anatomical traits) and the MFA site scores on axes (dimensions; Dim) 1 and 2. Brownish, green and pink colours correspond to architectural, leaf functional and xylem anatomical traits, respectively. Only significant correlations at P < 0.0001 are shown. The percentage of the total variance represented by each axis is indicated. See trait abbreviations in Table 1.