Understanding the structure and composition of native forests is a prerequisite in developing an adaptive forest management plan for Himalayan forest ecosystems where climate change is rapid. However, basic information on forest structure and composition are still limited in many places of the Eastern Himalayas. In this study, we aimed to understand the diversity, structure, and composition of forests and their variations along an altitudinal gradient in Himalayan forests. The study was conducted in the Indian federal state of Sikkim, Eastern Himalayas. We carried out a comprehensive and comparative evaluation of species diversity, stand basal area, and stem density along the altitudinal gradient from 900 m a.s.l. to 3200 m a.s.l. We used stratified random sampling to survey eighty-three plots each 0.1 ha in forest communities that occurred along the altitudinal gradient: (a) lower (900-1700 m) altitude forest (N = 24), (b) mid (1700-2500 m) altitude forests (N = 37), and (c) higher (2500-3200 m)altitude forests (N = 22). We measured and identified all living trees with a >3 cm diameter at breast height in each plot. We counted 10,344 individual plants, representing 114 woody species belonging to 42 families and 75 genera. The family Fagaceae and its species Lithocarpus pachyphyllus (Kurz) Rehder. were reported as the most dominant forest trees with the highest Importance Value Index. The Shannon diversity index was recorded as being the highest for the low-altitude forests, whereas measures of structural diversity varied among forests along with altitude: the mid-altitude forests recorded the highest stem density and the high-altitude forests showed the highest mean stem DBH (diameter at 1.3 m height). One significant finding of our study was the disparity of the size class distribution among forests along the altitudinal gradient. Overall, we found a reverse J-shape distribution of tree diameter signifying the uneven-agedness. However, we showed, for the first time, a complete lack of large trees (>93 cm DBH) in the lower altitude forests. Our study highlights conservation concerns for the low-altitude forests that record high species diversity, although lacked large-diameter trees. We anticipate that our study will provide a comprehensive understanding of forest diversity, composition, and structure along the altitudinal gradient to design conservation and sustainable management strategies

Vegetation studies establishing direct mechanistic linkages between stand transpiration and streamflow are rare from sub-tropical and tropical montane forests (TMFs) like Himalaya. We quantified the impact of diurnal and seasonal transpiration on lean season streamflow in a broad-leaved evergreen secondary TMF in Eastern Himalaya. Whole-tree and stand transpiration were measured using Granier’s thermal dissipiation sap flow probes at one of the wettest (4500 mm yr-1) and highest elevation (2100 m) sites in the world to date. The observed daily and annual transpiration rates were double of the reported values from TMFs in relatively drier Central Himalaya, but at the lower bound of TMFs globally. Solar radiation was the key driver of transpiration in energy-limited winter under hydrated conditions. Vapour pressure deficit (D) controlled transpiration in energy-abundant summer. We also found that moderate precipitation events (10-30 mm) followed by clear skies can induce significant increase (93±110 %) in stand transpiration. In turn, transpiration was the main driver of lean season streamflow in dry winter and to a lesser extent in wet summer. Thus, in winter, the transpiration-driven abstraction induced corresponding diurnal cycles in soil moisture and streamflow with an average lag of 1.3±1.8 hours and 2.9±2.5 hours, respectively, and strong negative correlations (-0.8±0.1). Thus, changes in vegetation cover and precipitation patterns are likely to impact local and regional moisture recycling by vegetation and lean season flow, thereby affecting regional water security in the 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.

The interactive role of life-history traits and environmental forcing on plant-water relations is crucial for understanding species response to climate change but remains poorly understood in secondary tropical montane forests (TMFs). Comparing contrasting life-history traits (pioneer vs late-successional species) in a biodiverse Eastern Himalayan secondary TMF, we investigated sap flow responses in co-occurring pioneer species, Symplocos racemosa (n=5) and Eurya acuminata (n=5), and late-successional species, Castanopsis hystrix (n=3), using modified Granier’s Thermal Dissipation probes. The fast-growing pioneers S. racemosa and E. acuminata) had 2.1- and 1.6-times higher sap flux density than the late-successional C. hystrix, respectively, and exhibited characteristics of long-lived pioneer species. Significant radial and azimuthal variability in sap flow (V) between species was observed and attributed to life history traits and the canopy’s access to sunlight. Nocturnal V (1800-0500 hr) was 13.8 % of daily V and is attributed to stem recharge for evening V (1800-2300 hr) and to endogenous stomatal controls for pre-dawn V (0000-0500 hr). Both the shallow-rooted pioneer species exhibited midday depression in V attributed to photosensitivity and diel moisture stress response. In contrast, deep-rooted C. hystrix transpired unaffected across the dry season likely accessing groundwater. Thus, the secondary broadleaved TMFs, with the dominance of shallow-rooted pioneers, are more prone to the negative impacts of drier and warmer winters than primary forests, which are dominated by deep-rooted species. The study provides an empirical understanding of life-history traits and microclimate modulating plant-water use in widely distributed secondary TMFs in Eastern Himalaya and highlights their vulnerability against warmer winters and reduced snowfall due to climate change.