Photorespiration, Drought and the Assimilation–Growth Gap: Bounding the Forgone Wood in High-Elevation Cedrus libani and Juniperus excelsa Forests of the Eastern Mediterranean

Türkiye holds the world’s largest natural stands of Lebanon cedar (Cedrus libani A. Rich.) and extensive Crimean juniper forest (Juniperus excelsa M. Bieb.), together comprising 884,361 ha of closed, drought-exposed forest. Both assimilate at needle water potentials near −3.0 MPa, yet lose part of their potential assimilation to photorespiration and to stomatal closure under water deficit. A literature-parameterized scenario model relaxes both constraints against a radiation-limited ceiling. The zonal photorespiratory and stomatal input fractions are structured expert estimates, not measurements, which are: 8–16% and 14–28% of potential gross primary production (GPP), or 13–23% and 11–23% after coupling. Together they reduce weighted-mean net primary productivity from 3.29 to 2.17 Mg C ha−1 yr−1, a 34.1% reduction in potential GPP (Monte Carlo interval 31.0–36.4%, conditional on the elicitation). Of that loss, 39–46% is obligate photorespiration beyond management reach, depending on how the jointly caused interaction is assigned; the obligate share falls along the aridity gradient. Radial growth in these species occupies a narrow climatic window that does not coincide with peak assimilation, so forgone wood is reported as a function of the source-control coefficient φ, with φ = 1 as an upper bound. At φ = 1, the estate forgoes up to 1.5 million m3 yr−1 of stemwood, and the 3.6 million Mg yr−1 of unfixed carbon (as CO2-equivalent, not net sequestration) is worth, together with that wood, about US$ 0.20 billion; at φ = 0.5 the wood and the value halve, and at φ = 0 no wood is forgone.