Abstract:

Aims Plant functional traits reflect the resource trade-offs and allocation strategy between different organs and functions in the process of plant adapting to environmental changes. Branches and leaves are the key components of tree canopy, and understanding the relationship of leaf-stem allometry is important for revealing adaptive strategies of desert plants with changing environmental constraints. In this study, our objective is to explore the scaling relationships between leaf display efficiency and current-year twig stem configuration, and trade-off strategy shift along groundwater gradients in extremely arid region.
Methods Leaf number, area, mass and stem length, diameter, volume, mass of current-year twigs were measured for Populus euphratica with 30 trees within three different groundwater depths in Tarim basin, Xinjiang, China. The stem length, stem slender ratios and stem volume of current-year twigs were used as the proxy of stem configuration traits. Density of leaf number (leaf number per stem length), leaf area ratio (total leaf area per stem mass) and leaf/stem mass ratio (total leaf mass per stem mass) were used as the proxies of leaf display efficiency. The standardized major axis (SMA) regression was used to examine the scaling relationship between stem configuration traits and leaf display efficiency within current-year twigs across groundwater gradients.
Important findings Stem diameter, leaf display efficiency, specific leaf area, individual leaf mass and area, all decreased with the increase of groundwater depths (GWD). In contrast, stem length, stem slender ratio and leaf number per twig increased with GWD. There was significant difference in stem and leaf functional traits across groundwater gradients. The density of leaf number, leaf area ratio and leaf/stem mass ratio as the proxies of leaf display efficiency were all significantly negatively correlated with stem length, stem slender ratio and stem volume of the current-year twigs. These results suggest that leaf display efficiency decreases with stem configuration variation of current-year twigs, which may reflect the trade-off of twig-leaf size, water conduction and mechanical support. The allometric exponents (slope) of stem configuration and leaf display efficiency decreased with the increase in GWD, likely because the leaf area or mass per unit stem investment decreased with GWD. It may reflect that the desert woody plants tend to adopt a conservative adaptive strategy of high consumption and low benefit with habitat deterioration. When Populus euphratica responds to environmental stress, it tends to have a large number of small leaves on long twigs, or tends to have relatively few large leaves on short twigs, which reflects the strategy of resource utilization and trade-off mechanism of twig-leaf size. Our results demonstrate that GWD is a key factor for regulating the leaf-stem allometric relationship, and low leaf display efficiency is an adaptive strategy for P. euphratica to cope with the worsening arid desert environment.

Key words: Populus euphratica, stem configuration, leaf display efficiency, groundwater depth, trade-off