Plant Biomass Allocation across a Precipitation Gradient: An Approach to Seasonally Dry Tropical Forest at Yucatán, Mexico

It has been assumed that plant biomass partitioning to stems and roots at the ecosystem level follows a single strategy according to which the stem biomass scales isometrically with root biomass, a hypothesis known as 'isometric scaling'. In this study, we examined an alternative theory used for plants: plant biomass is allocated preferentially to the plant organ that harvests the limiting growth resource, a theory known as the 'balanced growth hypothesis'. Our objective was to test these two alternative hypotheses across a water availability gradient. We quantified the stem and root biomass in a seasonally dry tropical forest (SDTF) in three regions of the Yucatán peninsula along a precipitation gradient. Reduced major axis analysis showed that the slopes of the relationship between stem and root biomass across the study regions were statistically similar and significantly different from 1.0 (common slope = 2.5), which contrasts with the 'isometric scaling' hypothesis. The allometric coefficient was different between regions along the precipitation gradient, which showed that plant biomass allocation to stems is higher in high than in low water availability regions where biomass is allocated in greater proportions to roots. The stem:root ratio increases following the low to high water availability gradient. Our results showed that plant biomass allocation in the SDTF follows a simple allometric strategy in which greater plant biomass is allocated to stems irrespective of water availability, suggesting to the forest level that plant biomass allocation strategy is invariant across the water availability gradient.