Grime’s strategies (Competitor, Stress tolerator, Ruderal; CSR) represent viable trait combinations with which species deal with environmental conditions. CSR strategies are broadly used to understand plant adaptation to the environment, yet their plastic responses have received little attention. A globally-calibrated tool (StrateFy) estimates CSR strategies using specific leaf area (SLA), leaf dry matter content (LDMC) and leaf area (LA) data, but these three traits can hardly characterise whole-plant responses to the environment individually. CSR strategies reflect trade-offs among growth, survival and reproduction, at both leaf and whole-plant levels, thus integrating several functions. We hypothesised that CSR strategies and the three constituent traits would show independent plasticity patterns, and that CSR strategies would be more likely to show adaptive responses, i.e., to fit expected functional responses to environmental gradients. We compared phenotypic plasticity to drought in single traits (SLA, LDMC and LA) with the integrated plasticity of the resulting CSR strategy. The study species was the invasive plant Mesembryanthemum crystallinum, which is distributed in arid and semiarid Chile. We found that trait plasticity was rather idiosyncratic and contrary to what would be expected from a functional adjustment to drought: LDMC did not change (expected response: increase) and SLA increased (expected response: decrease). Conversely, plastic responses of CSR strategy and LA were consistent with functionally adaptive responses to drought in all populations: S-strategy increased, while C-strategy and LA decreased. We advocate the use of Grime’s CSR theory as an integrative approach to further our understanding of adaptive plasticity in plants.

Mesembryanthemum crystallinum is an annual succulent native to South Africa that has invaded (semi)arid regions worldwide (Adams et al. 1998). In Chile, M. crystallinum is distributed in coastal areas between 24°S and 34°S, from the Atacama Desert to Mediterranean regions (Fuentes et al. 2013, Madrigal-González et al. 2013). We assessed phenotypic plasticity to drought in M. crystallinum in a greenhouse experiment with two watering treatments (moist vs drought). Seeds were collected in ten coastal populations distributed between 29.10°S and 30.56°S (Supporting information). We sowed ca. 400 seeds from each population (20 seeds from 20 mother plants) in 1-L pots filled with a mixture (2:1) of sand and field-collected soil. Soil was collected in a site close to the centre of the distribution range of M. crystallinum in Chile (El Romeral; see Supplementary material); specifically, we collected soil from microsites free from M. crystallinum, where soil salinity is not high enough to trigger responses to stress conditions (León et al. 2011, De La Cruz et al. 2023). We thinned the seedling groups to ensure growth synchrony across populations, which showed similar germination percentages (χ²_(9,400) = 7.09, p = 0.628; arcsine-transformed data). Two paired seedlings from each mother plant were assigned to each water treatment 10-12 weeks later. Plants in moist and drought treatments were watered with 140 ml of tap water twice a week and 70 ml of tap water once a week, respectively. This watering schedule is equivalent to 29.2% (50 mm) and 25.4% (14 mm) of mean annual precipitation in the southernmost and second-to-northernmost populations, respectively, which corresponds to the precipitation reported for the last four years (Supporting information; www.ceazamet.cl). Initial N was 800 plants (10 populations × 2 treatments × 20 mother plants × 2 replicates) distributed across 16 blocks. A total of 61 plants died during the experiment. After three months growing in the watering treatments (i.e., plants totalling six months in the greenhouse), and when plants produced the first flower buds, we harvested three leaves per individual. We chose leaves that had recently transitioned to maturity and located in the mid of a side shoot. These leaves were scanned and weighed to obtain leaf area (LA, mm²) and fresh leaf weight (mg). Leaves were dried at 70 °C for 72 h and dry weight was recorded (mg). Subsequently we estimated specific leaf area (SLA, leaf area/leaf dry mass; mm² mg^-1) and leaf dry matter content (LDMC, 100*leaf dry weight/leaf fresh weight; %), following Pérez-Harguindeguy et al. (2013). We estimated the relative contribution (%) of competitiveness (C), stress tolerance (S) and ruderality (R) for all individuals using StrateFy (Pierce et al. 2017).