For the past ten years, experiments to choose a professional relationship anywhere between grams

_op and g_maximum, or the g_op ? g_max ratio, have yielded broadly consistent results (Dow et al. 2014; Franks et al. 2014; McElwain et al. 2016b); however, these studies have been taxonomically limited and rarely included both measured g_op and calculated theoretical g_max parameters from the same leaves. The aim of this study was to advance our current understanding of the nature of the relationship between g_op and g_max across multiple species and biomes to determine whether such relationships are widely conserved. More simply put, we asked whether theoretical g_max, which is calculated from stomatal anatomy according to the diffusion equation (eq. ; Parlange and Waggoner 1970; Franks and Beerling 2009), is a good predictor of g_op measured in the field.

We explored the relationship between g_op and g_max by measuring g_op in a wide range of woody angiosperm species in natural ecosystems and then calculating g_max from the same leaves on which the g_op measurements were taken to establish the nature of the relationship at biological and ecological levels. Therefore, we tested the relationship across many species, plant growth habits (trees and shrubs), habitats (open canopy and understory subcanopy), and biomes (boreal forest, temperate rain forest, tropical rain forest, and tropical seasonal [moist] forest). If we can establish consistency in the nature of the relationship between g_op and g_max, this would be valuable for historical herbarium studies and deep-time fossil studies because it would allow estimation of physiological stomatal conductance from observations of anatomical stomatal traits. It would also have an important application for climate and Earth system models in which g_op can be estimated from the stomatal traits and, in turn, open up the possibility of studying vegetation feedbacks on the hydrologic cycle.

Biome and you will Species Options

For this study, we used a published field data set of stomatal conductance measurements of C_{step 3} woody angiosperm species from seven biomes called STraits (Murray et al. 2019). We chose the following four out of the seven biomes included in the STraits data set for our current study on the basis that they spanned wide geographic, climatic, and species ranges and are the least well represented in the literature: boreal forest, temperate rain forest, tropical seasonal (moist) forest, and tropical rain forest. We selected 74 species from a total 136 species included in the STraits data set across these biomes (Murray et al. 2019; Table 1). Based on the APG IV system of flowering plant classification (APG et al. 2016), our study covers 35 woody angiosperm families and 16 orders, all of which are from the Eudicot clade, which includes the Rosid and Asterid clades. Phylogenetic coverage in this study excludes the basal angiosperm Magnoliids, the Chloranthales, gymnosperms, monocots, and ferns (APG et al. 2016). One species, Sambucus racemosa, occurred in both the boreal forest and the temperate rain forest and was therefore counted as two separate species occurrences, resulting in 75 separate species analyzed (Table 1).

Note. BF = boreal forest, TRF = temperate rain forest, TF = tropical rain forest, TSF = tropical seasonal (moist) forest, g_op = operational stomatal conductance, pa_max = maximum stomatal pore area, g_max = theoretical maximum stomatal conductance, leaves (n) = number of leaves used for the calculation of mean g_max, and g_op (n) = number of g_op measurements on the same leaves for calculation of mean g_op (mean g_op values are weighted mean values). All errors are standard deviation.

Leaf-Level Working Stomatal Conductance Data

The term “operational stomatal conductance” (g_op) used here refers to stomatal conductance as it performs under natural field conditions, following the definition of McElwain et al. (2016b). The g_op data used in this study are taken from the published STraits data set of Murray et al. (2019). In Murray et al. (2019), stomatal conductance measurements were obtained by the author using an SC-1 steady-state leaf porometer (Decagon Devices, Pullman, WA) over the course of three summer growing seasons between 2013 and 2015, when atmospheric CO₂ concentrations ranged from 396.5 to 400.8 ppm. Measurements were made on the abaxial surface of sun leaves located at the canopy edge or, in the case of naturally occurring understory shrub species, on the abaxial surface of leaves exposed to sun flecks. Mean species g_op was calculated from an average total of 12 g_op measurements per species (i.e., a single g_op measurement taken from one leaf of each of three individuals on three or four consecutive days). This yielded a total 854 measurements on 243 individual leaves (Table 1). Measurements were taken between 0830 and 1400 hours at each site under ambient environmental conditions to capture natural day-to-day variability in photosynthetically active radiation (PAR), temperature, and vapor pressure deficit (VPD), a modification of the variance protocol described in McElwain et al. (2016b). Detailed methods are available yubo mobile in Murray et al. (2019).