Greenhouse experiment
We performed a common garden experiment in a greenhouse with a subset of
15 populations (Fig. 2, Table S1). The experiment spanned almost the
entire geographical and environmental native range, and included three
non-native populations to increase the breadth of source environmental
conditions (Appendix S2). We pooled all the collected seeds at the
population level. We sowed 2,728 seeds (180-200 per population) and
obtained 1485 seedlings in individual pots after 25 days. Seedlings were
then exposed to treatments with two levels of water supply crossed with
three levels of light availability (one block with six treatment
combinations). We used 18 seedlings per treatment combination for each
population (except for BG, RO and TW populations with, respectively, 14,
10 and 8 seedlings per treatment combination; Table S1). The treatments
were chosen to compare their effects with those of two source
environmental drivers: Aridity (related to water availability) and
Vegetation cover (related to light availability). These treatments also
represent parameters likely affected by climate and land-use change. For
the water treatments, half of the plants were watered every three days
(“wet” treatment), and the other half every nine days (“dry”
treatment), by flooding the supporting trays until soil was soaked with
water. Each water treatment level was divided into three light levels:
1) 100% light, 2) 64% light and 3) 33% light (Appendix S2). Watering
and light levels were designed to span a wide environmental range,
characteristic of cosmopolitan plants.
To collect trait data in the greenhouse, we measured plant leaves,
flowering status and inflorescences 2.5 months after the onset of
treatments in the same way as in field populations. To account for
possible maternal effects, usually more manifest in early life stages
(Roach & Wulff 1987), control leaf measurements were also taken 1 month
after the onset of treatments. At the end of the experiment, the longest
healthy leaf was collected from each of 10 individuals per population
and treatment combination. Leaves were scanned to estimate leaf area,
oven-dried (60ºC), and weighed to calculate SLA. Root:shoot ratio (RSR)
was also calculated in the individuals used for SLA measurements, but
only for eight populations (Table S1) and excluding the intermediate
light treatment due to logistical constraints. To measure RSR, the
remaining leaves and the roots were collected, roots were washed, and
both leaves and roots were oven-dried.