Needle moisture saturation
The fractional moisture of the needles varies within a relatively narrow range. For the survey period beginning in October 2019, the value on average hovered above 0.8 (Fig. 1a). October is generally taken as the beginning of the local rainy season, which lasts until April. There was no precipitation in October 2019 (Fig. 1b), but over a year from October 2019 to September 2020, the cumulative rainfall was 345 mm, which was 32% above the historic average of 262.6 mm. The rainfall in Fig. 1b is based on the NWS record rather than ClimateWNA, which as of this study has not posted data beyond 2019. The rainfall pattern during that year was episodic. After several large rainstorms in November and December of 2019, the fractional moisture was as high as 0.92. The weather was unusually warm and dry at the beginning of 2020. The total precipitation of April 2020 was reasonable, but it all came at the beginning of the month. Based on NWS records, a hot spell of several days went up to 32 °C, and the fractional moisture dropped to 0.76 near the end of April 2020.
The onset of onshore stratus clouds and ground level fog in the marine boundary layer arrived in early May 2020. Despite the lack of rainfall and warmer temperatures, the fractional moisture creeped up through the summer. Fluctuations to an extent are tied to hot spells. For example, in June 2020, the temperature went as high as 34 °C in the middle of the month. The dashed line represents the average fractional moisture of 0.85 from October 2019 through September 2020. After the unusual wet year, the rainy season beginning in October 2020 was extremely dry, and the rainfall was well below average. Nonetheless, the fractional moisture remained within the neighborhood of 0.8. April 2021 was a dry month, but it already received periodic FLCC, which became consistent and heavy in May 2021. The temperature did not go above 24 °C in May and June of 2021. This condition is reflected in the measurements. The fractional moisture dropped to only 0.81 in April 2021 through June 2021. The values were better than the corresponding months in 2020.
SEM
A scanning electron micrograph of one-year-old P. torreyana needle surfaces indicates that there are only remnants of epicuticular wax, exposing the underlying epidermis. On both the abaxial or curved (Fig. 2a) and adaxial or flat (Fig. 2b) surfaces, one can identify ridge lines with spines spaced sparsely apart and more distinctly, stomata with Florin rings of epicuticular wax, especially on the adaxial surface. The spacing in between stomatal lines is not uniform. The quantity of vestigial epicuticular wax is uneven; there are more residues on the adaxial surface, where there is epicuticular wax between stomata and between some stomatal lines. The uneven surface leads to variability of droplet measurements.
Foliar uptake by fractional immersion
Following the works of Leyton and coworkers (1963, 1968), we made an attempt to measure foliar uptake by immersing various lengths of needles, beginning from the distal end. The time varied from 18 min to 24 h. The short 18 min was chosen to be so brief that the uptake would be well under the absorption capacity of the needle based on droplet measurements, thus limiting the extent of probable vascular transport. This possibility is clearly exhibited by needles immersed to one-fourth of its length (Fig. 3). Within 18 min, the uptake quantity was hardly measurable. However, by 1 h immersion, it would appear that this section near the tip could uptake water much more quickly, as compared with the one-half immersion to full-length submersion cases. This apparent uptake is accentuated when the quantity is normalized by the immersion length as in Fig. 3. It is apparent that after water is absorbed near the distal end, it can be transported to the proximal part of the needle. This vascular transport becomes less important as more of the needle is immersed, as with the half and three-quarter immersion cases. When the entire needle is submerged, the 24 h weight gain corresponds to a saturated needle. The uptake within 18 min appeared high, but it could be an unusually low measurement at 1 h. It is evident that fractional immersion is not a good method to measure spatial variations.
Foliar uptake by droplet absorption
Droplet measurements provide a better characterization of probable spatial distributions of wettability and foliar uptake. As hinted in Fig. 2, there is large variability because of the heterogeneity of the uneven surfaces. All droplets have different contact angles at the two edges of a 2D image, but unlike the advancing and receding angles on an inclined plane, the present differences are not as drastic. Within this study, the maximum difference of the two droplet contact angles on the abaxial surface was 7.3° with a mean of 0.9° and on the adaxial surface, a maximum of 8.1° with a mean of 1°. In the analyses, the average of the two edge readings was taken as the contact angle of a given droplet. Droplet placement was guided by a scale. On the abaxial surface, the maximum placement error in absolute value was 2.9 mm with a mean error of 0.07 mm, and on the adaxial surface, the maximum error was 2.1 mm with a mean of 0.06 mm. Hence, in the regressions of contact angle and uptake rate versus location, the x-axis location error was taken to be negligible. Standard error bars are not based on repeated sampling as the needle properties cannot be ensured to remain identical after each use. Instead, needles from adjacent fascicles were used, and hence the error bars are an indication of the variability of similar needles. The results in Fig. 4 revealed that while there are variations in contact angles and water uptake rates, the spatial gradients are relatively small, especially that of the uptake flux.
The needle samples in Fig. 4 were from the same branch. The one-year and two-year samples were about 10 and 22 months old respectively. Figs. 4a and b show adaxial measurements, while Figs. 4c and d are from the abaxial surface. The scattered plots are accompanied by their respective ordinary least square regression lines and 95% confidence bands. Comparing the contact angles, which are all under 65°, both surfaces of the two-year old needles have lower values. However, the spatial gradients have slight differences. From an ordinary least squares, both slopes of the abaxial surface are statistically significant (p < 0.05, Table 1), even though the coefficients of determination, r2, are quite low (0.44 and 0.72, respectively), implying other physical factors may also play a role in the spatial gradient.
The droplet absorption fluxes in Figs. 4b & d, with the exception of the year-2 adaxial surface, are lower toward the needle tip, where the contact angles are larger. In Fig. 4b, the two-year old needles also have a noticeably higher absorption flux. It is logical to associate a more wettable surface with a lower contact angle and a correspondingly higher water uptake rate. Nonetheless, this pattern is not definitive; the functional dependence is not precise on these uneven surfaces. With an ordinary least squares calculation, only the regression of the year-1 adaxial uptake flux is statistically significant (p = 0.034, Table 1) and has a relatively low r2. The year-2 adaxial uptake clearly demonstrated no spatial dependence (p = 0.91). In Fig. 4d, the abaxial uptake fluxes of both the one-year and two-year old needles appear to be the same, but because of the variability of the data, the statistical significance is not strong (two-tailed t-test of the slopes, p = 0.05). Clearly, the spatial variation of the uptake flux does not strictly follow that of the contact angle.
Because of the large data scatter, a weighted least square calculation was also applied to reduce the weights of outliers (Table 1). The regressions of the contact angle become significant except for the year-2 adaxial surface (p = 0.051), while the values of r2 remain relatively low. With the uptake flux, three of the slopes are significant with the exception of the year-2 adaxial surface (p = 0.33). These three slopes have a reasonable coefficient of determination, but considering the small gradient, O(10-4) mg cm-2 s-1 per cm change, the physical functional dependence of the spatial variation along the needle is not large.
A Tukey test, which does a comparison disregarding spatial distribution, was also applied to the data set. In terms of the contact angles, the surfaces are different (p < 0.05), except for the pairing between year-2 adaxial and year-1 abaxial. These 2 surfaces also turned out to have similar slopes of linear regression in Table 1. With respect to the uptake flux, the year-2 adaxial surface differs from the other surfaces, which among themselves are similar. Considering the variability of the data and the varying spatial gradients of the contact angle within the data set, it is probable that there are no fundamental differences in the needle surface. Observations, including the year-2 adaxial measurements, were results of uneven weathering of the needle surfaces.
Correlation of the local absorption flux with contact angle
To explore the functional dependence of the local absorption flux versus contact angle, all the data in Fig. 4 with additional data from young shoots is plotted in Fig. 5. Overall, the decrease in uptake flux at higher contact angles can be described by a linear regression. The slope is -4.3 x 10-4 mg cm-2 s-1 per degree change using ordinary least squares (r2 = 0.45, p = 4 x 10-8), and with a weighted least squares to discount outliers, the adjusted slope is -4.5 x 10-4 mg cm-2 s-1 per degree change (r2 = 0.94, p = 2 x 10-16).
The contact angles of the droplets on matured needles are predominately below 65°. At the low end (< 45°) and the high end (~ 60°), there is some consistency in that a lower contact angle corresponds with a higher absorption flux. However, in the range roughly from 50° to 55°, the variability of the absorption flux is extremely high. This aspect is what leads to the variability of the spatial gradients in Fig. 4. The contact angles of droplets on young shoots are higher, between 60° and 80°, but there are exceptions, as shown by the hydrophobic droplet.
Comparison with other plants
To address how the adaptation of the Torrey pine may compare with other species, several plants with large, broad, evergreen foliage were chosen. They are Heteromeles arbutifolia (toyon), Malosma laurina (laurel sumac), and Rhus integrifolia (lemonadeberry) from maritime chaparral and Eriodictyon crassifolium (thick-leaf yerba santa) from coastal sage scrub. They are common where P. torrey inhabits.
Detached leaf samples were left drying on a benchtop under ambient conditions. The drying is taken as evaporative. The initial fractional moisture of the leaf samples were not the same (Table 2). Nonetheless, the values show that all these plants, including the pine, have moisture content within a comparable range. How well the leaves of each plant withhold water is shown in Fig. 6. The leaves lose moisture via evaporation relatively quickly in the beginning, but moisture loss levels off when the moisture content drops to ambient relative humidity. E. crassifolium loses moisture most rapidly, with a precipitous drop in weight within a few days (Fig. 6a), which corresponds with a high evaporative flux (Fig. 6b). M. laurina also loses moisture relatively quickly, while H. arbutifolia has a more steady drop. R. integrifolia retains its moisture almost as well as P. torreyana for the first five days, after which it starts to lose moisture more quickly in an unsteady pattern. In contrast, P. torreyana never exhibits a steep drop in weight or sharp rise in evaporative flux. From the beginning, the needles hold a steady evaporative flux and lose moisture very gradually.
The adaxial surface contact angles and water uptake of the four leaves are compared with the pine in Table 2. The properties of E. crassifolium stand out. With its tomentose surface, a droplet placed carefully on its surface demonstrates a Cassie-Baxter state with hydrophobic contact angles (Dorrer and Rühe 2007). The droplet uptake flux is not absolute zero because as the droplet becomes small enough via evaporation, it may slip through the hairs and impale the surface, transitioning to the Wenzel state. The E. crassifolium leaf is extremely absorbent if water is in direct contact with its surface, consistent with how it loses moisture the fastest in Fig. 6a.
It is rather unexpected that the leaf surfaces of the three chaparral plants are hydrophilic and capable of foliar uptake, especially R. integrifolia, which is thick and leathery. The difference is small, but H. arbutifolia, which has a larger contact angle, also has a lower water uptake flux. Even though these leaves are capable of foliar uptake, their uptake fluxes are lower than the matured needles of P. torreyana. Specifically, the uptake flux by the adaxial surfaces of E. crassifolium, M. laurina, H. arbutifolia, and R. integrifolia are only, respectively, 0.05, 0.38, 0.48, and 0.6 of the rate of the year-1 adaxial surface of P. torreyana. The average contact angle of the young shoots is higher (Table 2) and similar to that of M. laurina, but the young shoots have a higher uptake flux, which is about half of the matured needle. The surface under the base sheath tends to have a slightly higher contact angle than the exposed needle surface, with a corresponding smaller decrease in the uptake flux.