Comparison of the growth traits of a commercial pioneer tree species,paper mulberry (Broussonetia papyrifera L. Vent.), with those of shade-tolerant tree species: investigation of the ecophysiological mechanisms underlying shade-intolerance

In rural areas of northern Laos, a commercially valuable pioneer tree species, paper mulberry (Broussonetia papyrifera L. Vent.) has been recommended for intercropping under plantations of commercial trees. However, less is understood about the growth of this pioneer tree species in the understorey and the mechanism underlying the shade intolerance. We measured growth characteristics for seedlings of paper mulberry under four light intensities. We compared the relative growth rates in aboveground biomass and standing leaf area (RGR_mass and RGR_leaf), light-capture efficiency, and seeding-level mass-based daily photosynthetic rates (A _mass) with those of field-grown seedlings of eight shade-tolerant species to identify factors potentially responsible for shade-intolerance. Most growth traits of the paper mulberry seedlings did not differ consistently from those of the shade tolerant species. The ecophysiological–architectural model software showed higher A _mass and RGR_mass capacity in paper mulberry than in shade-tolerant species. Despite their higher RGR_mass, paper mulberry seedlings had negative RGR_leaf under shaded conditions due to short leaf lifespan. The linear RGR_mass–RGR_leaf relationship for paper mulberry had a high RGR_mass intercept, indicating that a high RGR_mass was required to provide positive RGR_leaf. Progressive decreases in standing leaf area with time, and possibly photosynthesis, appear to be responsible for the shade-intolerance of paper mulberry. Although intercropping of paper mulberry has been suggested in the species’ native region, understorey cultivation of paper mulberry would only be possible with relatively open canopies.