Differential water-use strategies in co-occurring pioneers and late-successional tree species in secondary tropical montane forests of Eastern Himalaya

Plant-water relations in secondary tropical montane forests (TMFs) are driven by complex interactions between environmental conditions, species composition, and forest structure. We investigated the differential water-use strategies of cooccurring pioneers and late-successional tree species in a secondary TMF of Eastern Himalaya, India. It is the wettest (mean annual precipitation = 4500 mm yr -1 ) high-elevation (> 2000 m) site in the world. The observed maximum daily stand transpiration (5.3 mm) is highest among other tropical montane or lowland forests. Although energy-limited, increased moisture availability allowed the observed sap flux densities from the studied species, Symplocos racemosa, Eurya acuminata, and Castanopsis racemosa , to be 3-9 times higher than their conspecifics from relatively drier TMFs. Interestingly, differential access to solar radiation, a characteristic of the forest canopy in secondary forests, induced significant radial and azimuthal variability in sap flow. Solar radiation was the key driver of transpiration in energy-limited winters and Vapour pressure deficit in energy-abundant summers. Nocturnal (1800-0500h) transpiration was significant (13.8%) part of daily T and was dominated by pre-dawn flux. Shallow-rooted pioneers, S. racemosa and E. acuminata , exhibited strong midday depression in sap flow in response to environmental extremes and soil moisture fluctuations, whereas the deep-rooted late-successional C. hystrix transpired unaffected. The complex interactions between different successional groups for accessing changing energy and moisture conditions are highlighted for prioritized conservation and management of these secondary forests in Eastern Himalaya.