Plants reach the top by preparing at the bottom

It takes work to live below the canopy of a forest. The older trees intercept most of the light, leaving little to live on. It means shade-tolerant plants have to invest carefully. A new study by Marcin Zadworny and colleagues shows that shade-intolerant plant are careful too, but in a different way.

Zadworny and colleagues looked at how plants worked with their roots to prepare them for life under the canopy. There was a simple idea. Shade-tolerant plants are prepared to stop under the canopy. In contrast, shade-intolerant plants need to be able to react fast if an opening appears overhead. They need to grow up to it before the gap gets filled. Does this need to adapt fast affect how plants prepare their roots? The team investigated the the relationship between root morphology and the hydraulic characteristics of several orders of fine roots (<2 mm) for species differing in shade tolerance (low, moderate and high).

They found that compared with shade-tolerant species, shade-intolerant species produced thinner absorptive roots with smaller xylem lumen diameters and underwent secondary development less frequently, suggesting that they had shorter life spans. Shade-tolerant species had greater root specific hydraulic conductance among these roots due to having larger diameter xylems, although these roots had a lower calculated critical tension for conduit collapse. In addition, shade-intolerant species exhibited greater variation in hydraulic conductance across different root growth rings in woody transport roots of the same root order as compared with shade-tolerant species.

They concluded: “This study supports previous ideas of plant growth strategies suggesting that species adapted to high light and resource-rich environments produce thinner roots with shorter life span (Walters and Reich, 1999), and that thinner roots are less reliant on mycorrhizal fungi, in contrast to thicker, extensively colonized roots. We also show that thinner roots of shade-intolerant species also had thinner xylem and lower hydraulic conductance, potentially limiting the risk of cavitation under low water conditions, which may occur with greater frequency under conditions of early succession where competition for water rather than light might be greater.”