Coarse root–shoot allometry of Pinus radiata modified by site conditions in the Western Cape province of South Africa

Abstract

The relationship between root and shoot growth and how it is modified by chronic or episodic drought stress is so far not well understood. Allometric partitioning theory (APT) supposes a constant root–shoot allometry. Optimal partitioning theory (OPT) assumes that plants’ root growth is enhanced under water limitation. However, recent studies show that fine and coarse roots react differently. This paper draws attention to the root–shoot allometry of adult Monterey pines (Pinus radiata D.Don) and its dependency on site conditions in South Africa. For assessment of the root–shoot-diameter relationship as an allometric relationship in general and for comparison with APT we used a sample of nine radiata pines from Jonkershoek and three maritime pines (Pinus pinaster Aiton) from Napier. In order to test for a site-dependency of the root–shoot allometry we sampled increment cores from stem and coarse roots of 48 radiata pines along a gradient from moist to dry sites in the Western Cape province. Tree ring analysis revealed an allometric relationship between root diameter (dr) and shoot diameter (ds) (ln(dr) = a + α dr,ds × ln(ds)). Despite strong variation of the allometric exponent α dr,ds we found a systematic deviation from 1.0 as would be predicted by APT. We also found α dr,ds to decrease with drought stress, which is contradictory to both APT and OPT. However, on sites with more pronounced drought stress we detected greater allometric factors a. We hypothesise that fine root growth, and also fine root mortality, is higher on dry sites. On these sites coarse roots seem to be less necessary for matter transport compared with moist and fertile sites. On the latter, fine roots are less ephemeral and require larger coarse roots for transport. We conclude that combined root shoot tree ring analyses have the potential for improving understanding and modelling ecosystems and better assessment of forest functions such as resource use efficiency, stand stability and belowground carbon storage.