Scaling relationship between leaf mass and leaf area: A case study using six alpine Rhododendron species in the Eastern Tibetan Plateau

Exploring the adaptation strategies of plants in alpine regions could provide reliable suggestions for plant conservation. As an irreplaceable organ, leaf functional traits and correlation have the closest contact with environment, which is the main index to measure survival fitness of plants. Rhododendron, the typical broad-leaved shrubs, are an important constituent of alpine ecosystems, especially in the Tibetan Plateau and the surrounding regions. The scaling relationship of leaf mass (LM) and leaf area (LA) of natural shrubs is still lacking. In this study, we measured LA, leaf fresh mass (LFM), leaf dry mass (LDM), leaf dry matter content (LDMC), leaf water content (LWC), and specific leaf area (SLA) of six Rhododendron spp. (Rhododendron maculiferum, Rhododendron capitatum, Rhododendron oreodoxa, Rhododendron przewalskii, Rhododendron watsonii, Rhododendron rufum) to study the leaf traits and scaling relationship between LM and LA. Our results showed that: the leaf traits varied significantly in species. R. rufum belongs to slow investment-return, R. przewalskii and R. capitatum belong to quick investment-return. The scaling relationship showed that, the scaling exponent (α) for R. maculiferum and R. capitatum were significantly greater than 1.0, while the α for R. oreodoxa was less than 1.0, which meant that the increase of leaf dry mass did not keep pace with the increase of leaf fresh mass in some species. The scaling exponent of LFM vs. LA were all greater than unity at six species. The LDM vs. LA scaling relationship was similar to LFM vs. LA to a large extent. Only R. oreodoxa showed a scaling exponent that was equal to one. For all species, the coefficients of determination (i.e. R2) for LFM vs. LA were always greater than that of LDM vs. LA. Thus, it might be more meaningful to describe the relationship between LM and LA using LFM vs. LA. Our results suggest that the six species locate in different ends on the leaf economic spectrum. With the increase of leaf dry mass, the fresh mass does not keep pace. Six alpine Rhododendron species incomplete support diminishing returns law. In future studies, the influence of various environmental pressures on the scaling relationship between leaf mass and leaf area relationships should not be neglected when analyzing the relationships of foliar biomass allometry allocation patterns. Our study highlighted that the diminishing returns for alpine plants, which provided a new perspective to understand the adaptation of alpine evergreen plants to harsh habitats.