Needle length in pines as a key trait regulating hydraulic resistance

In conifers, leaf length exhibits remarkable variation across and within species, even within the same individual. Leaves are often shorter in drier sites and at the tops of taller trees. Several hypotheses have been proposed to explain this shortening, but a clear causal framework is lacking. We hypothesize that conifer needles should exhibit a low rate of tip-to-base conduit widening leading to higher hydraulic resistance in long needles, explaining adaptive leaf shortening.

We sampled needles from 22 Pinus species and one Sequoia sempervirens across a range of environmental conditions. We conducted a detailed intraspecific analysis on four Pinus species by measuring tracheid diameter along the needle, and an interspecific comparison by measuring tracheid diameter at the needle base across all species. In both analyses, we fitted tracheid diameter against distance from the needle tip and calculated the slope (b) of tip-to-base tracheid widening.

A low mean intraspecific widening slope (b = 0.12) was found, indicating that tracheid diameter increases only slightly from tip to base. This low widening rate cannot fully compensate for the increase in hydraulic resistance, which therefore increases with needle length. The interspecific slope of mean tracheid diameter at the needle base vs. needle length (0.25) was higher than the intraspecific mean, suggesting that longer-needled species may have wider conduits at the needle apex, offsetting needle length-imposed resistance.

Our findings suggest that shorter needles should reduce hydraulic resistance under dry conditions or with height growth, maintaining leaf-specific conductance. We offer a novel explanation for the commonly observed pattern of needle shortening, interpreting it as an adaptive response rather than a physiological limitation.