Climbing plants beat trees to soil nutrient patches

Charles Darwin reasoned that because climbing plants are freed from the need to be mechanically self-supporting, their stems can remain thin, elongate quickly, and efficiently colonize and display leaves in well-illuminated areas where trellises are available1. Herein, I report that this tremendous exploratory capacity also applies below-ground — roots of woody climbers (i.e., lianas) consistently beat tree roots to fertilized soil patches, apparently because lianas do not invest in thick roots. This claim is based on results of a greenhouse experiment in which individual seedlings (N = 5 individuals per species) of four liana and four tree species were planted in the centers of 60 x 15 cm rectangular sand-filled boxes. In the direction of a usually covered Plexiglas end wall, a nutrient gradient was established by adding increasing amounts of slow-release fertilizer in four 6 cm-wide vertical bands; no nutrients were added in the other direction. Entire plants were harvested by section when their first root reached the end wall. Roots from all four liana species reached the highly fertilized end of the planting box faster than all tree roots (Figure 1A; for statistical results, see the Supplementary Information). A Vitis rotundifolia root arrived after just 67 days, a Campsis radicans root after 84 days, another Vitis root after 91 days, and then a Wisteria sinensis root after 94 days, and the fastest root of Gelsemium sempervirens grew the 24 cm to the end wall in 149 days. In contrast to the liana species, the fastest tree roots reached the end wall in 235 days for Magnolia grandiflora, 253 days for Quercus hemisphaerica, 263 days for Nyssa sylvatica, and 272 days for Liquidambar styraciflua. This capacity to explore soil rapidly may help explain why lianas are such potent below-ground competitors2 and why their removal substantially increases tree growth rates3,4. Liana roots colonize nutrient-rich patches more rapidly than tree roots, which likely provides a competitive advantage in heterogeneous soils.