Optimal Irrigation Regime for Woody Species Potentially Suitable for Effective and Sustainable Afforestation in the Desert Region of Mongolia

Long-term studies on plant response mechanisms to different irrigation regimes will provide a better understanding of the survivability and establishment of plant communities in a desert environment. Thus, across 10 years, we regularly investigated the effects of the rainfall (control), rainfall + 4 L h⁻¹, rainfall + 8 L h⁻¹, and rainfall + 12 L h⁻¹ irrigation regimes on the growth and leaf morpho-physiology of <i>Tamarix ramosissima</i> Ledeb., <i>Ulmus pumila</i> L., <i>Elaeagnus moorcroftii</i> Wall. ex Schltdl., and <i>Hippophae rhamnoides</i> L. to suggest an optimal irrigation regime for each woody species for effective and sustainable afforestation in Mongolia. We measured the root collar diameter (RCD), annual height growth, survivability, leaf area (LA), specific leaf area (SLA), leaf biomass (LB), total chlorophyll concentration, and predawn (ψ_p) and midday (ψ_m) leaf water potentials across the treatments and species. Results showed that trees grown at 12 L h⁻¹ grew taller per year and generally resulted in a higher SLA, but generally resulted in a lower survival rate compared with those in the other treatments in all species. Total chlorophyll content was higher in trees grown under 4 and/or 8 L h⁻¹, particularly for <i>T. ramosissima</i> and <i>E. moorcroftii.</i> Lastly, leaf water potentials were found more negative for trees subjected to 4 L h⁻¹, especially in <i>T. ramosissima</i> and <i>U. pumila</i>, but still resulted in a higher survival rate and LB compared with 12 L h⁻¹. <i>H. rhamnoides</i> showed higher survivability at 8 and/or 12 L h⁻¹ than at 4 L h⁻¹. Therefore, we suggest 4 L h⁻¹ to be the optimal irrigation regime for irrigating <i>T. ramosissima</i>, <i>U. pumila</i> and <i>E. moorcroftii</i>, and 8 and/or 12 L h⁻¹ for <i>H. rhamnoides.</i> Our findings are relevant to ensuring the sustainability of afforestation programs in arid and semiarid landscapes in Mongolia.