Phenology and water relations of treeline species of Western Himalaya, India

Many regions of Himalayas are warming more than global average rate, and treeline is assumed to be sensitive to changes in the climate. The Indian Himalayan treeline species are very less studied on water stress level and its relation to phenology. In the present study, we have tried to relate water relations of selected treeline species with the timing of phenological phases in treeline areas of western Himalayan region of Uttarakhand. The soil moisture content (S_m) varied between 33.21 and 59.16%. Most phenological phases occurred between pre-summer and summer season in all studied treeline species. The water potential (Ψ) of all the studied tree species increased just before the commencement of leafing and flowering phases when the temperature rose. The species never encountered severe water stress and the pre-dawn water potential (Ψ_PD) remained above − 1.04 MPa across all the phenological phases. Ψ_PD was most negative − 1.04 MPa during flowering period and least negative − 0.14 MPa during fruit maturation period. Morning leaf conductance (gw_AM) remained above 304.59 m mol m² s⁻¹ across all the seasons for all the studied species. Rhododendron’s species had the maximum phenological phases during summer season. The highest gw_AM was measured in Betula utilis during rainy season and minimum in R. campanulatum during winter season. Leafing and flowering both showed a positive correlation with temperature while fruiting showed a positive correlation with rainfall. The study reveals that the water potential does not reach lethal level to curtail phenological and physiological activities in treeline species. The timing of phenophases in these species is highly sensitive to seasonal rainfall and soil water availability, with temperature also directly influencing the controlling/shifting of the phenophases.