The newly-germinated seedling stage is critical for determining species distributions and migration. Most tree species experience the highest mortality rates during their first year frequently attributed to water stress, yet there remains a paucity of first-year species-specific seedling research in regards to vascular development and plant-water relations. There are numerous aspects that are unknown such as: (1) tracheid size distributions, (2) contributions of primary and secondary xylem to theoretical conductivities, and (3) anatomical and functional trait parameters associated with embolism resistance (i.e. P50) such as thickness-to-span ratios and pit characteristics, as previous work on mature woody tissues has found correlations between xylem anatomy and both embolism resistance and hydraulic conductivity.
In this study, we measured anatomical and functional traits of Pseudotsuga menziesii, Larix occidentalis, and Pinus ponderosa seedlings at five different time periods over the first 10 weeks after seeding to evaluate potential maximum hydraulic capabilities and resistance to embolism. Five individuals per species per time period were imaged utilizing confocal laser scanning microscopy (CLSM) and analyzed with ImageJ.
Based on anatomical measurements, we hypothesized that protoxylem, metaxylem, and secondary xylem represent the order of increasing resistance to embolism, which was later confirmed by a microCT experiment.
During the initial 10 weeks, all species experienced increases in theoretical embolism resistance, as interpreted from thickness-to-span ratios and torus-to-pit aperture overlaps. Seedlings concomitantly experienced changes in maximum theoretical conductivities as tracheid-size distributions shifted over the course of the experiment.
P. ponderosa tracheids exhibited the widest distributions and the largest diameters whereas P. menziesii exhibited the narrowest distributions and lowest mean diameters, especially at the later time periods. In early stages, L. occidentalis and P. menziesii exhibit similar functional characteristics, but by week 10, the two species had diverged.
By week 10, based on key functional traits, L. occidentalis, P. ponderosa and P. menziesii appeared to be the least to most embolism resistant. However, considerable changes in anatomy over the initial 10-week period for all species indicate that seedling age at the onset of in situ limited water availability will likely have a significant influence on survivability, especially due to lack of external mitigating factors, such as deep roots and large capacitive stores. This may in turn have a substantial influence on changing species distributions and local species abundance in light of changing precipitation regimes.