Leaf length and hydraulic resistance: crucial factors for conifers dealing with dry conditions

Hydraulic resistance plays a critical role in determining leaf morphology in response to environmental conditions. In the leaves of angiosperms, tip-to-base xylem conduit widening has proven to be quite high, scaling with path length with a power of ~0.40. Rapid widening concentrates hydraulic resistance towards the apical end, allowing to produce long leaves while maintaining virtually constant total leaf path resistance regardless of leaf length. Conversely, conifer leaves tend to be shorter, especially in drier and colder environments and in taller individuals (e.g. Sequoia), suggesting a different hydraulic strategy. We hypothesize a much lower conduit widening ratio in conifers, compared to broadleaves, leading to increased hydraulic resistance with leaf length, that could be, therefore, the main parameter that regulates hydraulic resistance in conifer leaves. To test this hypothesis, we sampled needles of various Pinus species, from different continents and environments. Cross-sections of needles were analyzed to measure mean tracheid diameter at various distances from the tip, and the exponent of the fitting power law was calculated. Preliminary results showed a conduit widening slope of 0.12, significantly lower than that observed in broadleaf species. This suggests that in dry conditions, the presence of shorter needles results in reduced total leaf resistance, representing the habitus with the best fitness, because it optimizes water supply and enhances resistance and resilience to water shortage, particularly in taller trees. These findings shed light on the functional anatomy of conifer needles, providing insight into their acclimatization responses to environmental challenges. This study offers a novel perspective on the significance of shorter needle length observed in conifers in dry environments and in tallest trees (i.e. redwoods), as an adaptive response to lower water availability.