<p><strong>Abstract.</strong> Coexisting plant species in a karst ecosystem may use diverse strategies of trade off between carbon gain and water loss to adopt to the low soil nutrient and low water availability conditions. An understanding of the impact of CO₂ diffusion and maximum carboxylase activity of Rubisco (<i>V</i>_cmax) on the light-saturated net photosynthesis (<i>A</i>) and intrinsic water use efficiency (iWUE) can provide insight into physiological strategies

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... the water–carbon regulation of coexisting plant species used in adaptation to karst environments at the leaf scale. We selected 63 dominant species (across 6 life forms) in a subtropical karst primary forest in southwestern China, measured their CO₂ response curves, and calculated the corresponding stomatal conductance to CO₂ (<i>g</i>_s), mesophyll conductance to CO₂ (<i>g</i>_m), and <i>V</i>_cmax. The results showed that <i>g</i>_s and <i>g</i>_m varied about 7.6- and 34.5-fold, respectively, and that <i>g</i>_s was positively related to <i>g</i>_m. The contribution of <i>g</i>_m to the leaf CO₂ gradient was similar to that of <i>g</i>_s. <i>g</i>_s<span class="thinspace"></span>∕<span class="thinspace"></span><i>A</i>, <i>g</i>_m<span class="thinspace"></span>∕<span class="thinspace"></span><i>A</i> and <i>g</i>_t<span class="thinspace"></span>∕<span class="thinspace"></span><i>A</i> was negatively related to <i>V</i>_cmax<span class="thinspace"></span>∕<span class="thinspace"></span><i>A</i>. The relative limitations of <i>g</i>_s (<i>l</i>_s), <i>g</i>_m (<i>l</i>_m), and <i>V</i>_cmax (<i>l</i>_b) to <i>A</i> for the whole group (combined six life forms) were significantly different from each other (<i>P</i><span class="thinspace"></span>&amp;lt;<span class="thinspace"></span>0.05). <i>l</i>_m was the largest (0.38<span class="thinspace"></span>±<span class="thinspace"></span>0.12), followed by <i>l</i>_b (0.34<span class="thinspace"></span>±<span class="thinspace"></span>0.14), and <i>l</i>_s (0.28<span class="thinspace"></span>±<span class="thinspace"></span>0.07). No significant difference was found between <i>l</i>_s, <i>l</i>_m, and <i>l</i>_b for trees and tree/shrubs, while <i>l</i>_m was the largest, followed by <i>l</i>_b and <i>l</i>_s for shrubs, grasses, vines and ferns (<i>P</i><span class="thinspace"></span>&amp;lt;<span class="thinspace"></span>0.05). iWUE varied about 3-fold (from 29.52 to 88.92<span class="thinspace"></span>µmol<span class="thinspace"></span>CO₂<span class="thinspace"></span>mol⁻¹<span class="thinspace"></span>H₂O) across all species, and was significantly correlated with <i>g</i>_s, <i>V</i>_cmax, <i>g</i>_m<span class="thinspace"></span>∕<span class="thinspace"></span><i>g</i>_s, and <i>V</i>_cmax<span class="thinspace"></span>∕<span class="thinspace"></span><i>g</i>_s. These results indicated that karst plants maintained relatively high <i>A</i> and low iWUE through the covariation of <i>g</i>_s, <i>g</i>_m, and <i>V</i>_cmax as an adaptation to a karst environment.</p>