Bark thickness and related parameters of tree species along an elevation transect leading to treeline in Central Himalaya.

Since treelines are generally fire-free, the trees growing there are expected to have thin bark, unless adaptation to other factors than fire results in the selection of a thick bark. Related to this is also higher proportional investment in inner bark in such an environment of infrequent fire. This study has considered stem bark thickness both in absolute and relative terms and also in the frame of the composition of outer and inner bark components of 20 tree species along an elevation transect (2100-3300 m) in high ranges of the Central Himalaya leading to treelines. The study species varied from 2.1 to 16.2 mm for total bark thickness and from 1.2 to 18.85% for relative bark thickness. The average absolute total bark thickness across the tree species decreased with elevation from forest to treeline, both when trees of all diameters (10.2 ± 0.84 mm for forest and 6.9 ± 1.79 mm for treeline) and those of the same stem diameter range (18-20 m) were compared (9.10 ± 1.30 mm for forest species and 6.38 ± 1.31 mm for treeline species). Nevertheless, the treeline bark thickness was similar to those of several forest communities considered to have comparatively thick bark. Like many other biological structures, bark carries out multiple functions; therefore, its thickness could be affected by more than one environmental factor. We suggest that the requirement of mechanical resistance to the snowfall, rainstorms, wind and adaptation to a high sunlight and UV radiations or storage of water, and non-structural carbohydrates could affect total, outer and inner bark thickness. Studies on these aspects in similar ecosystems may help understand the multi-functional attributes of the bark. For trees of comparable sizes (trees with 18-20 cm diameter at breast height) treeline species also had lower relative bark thickness (< 6%) than trees of forest below it (> 7%). The median proportion of inner bark of the total bark (70.5%) for our 20 species was more than that for savannas (~ 50%), exposed to frequent fire regime and similar to those of in cool sclerophyllous forests and temperate rain forests where fire return time is > 100 years. However, it was lower than the inner bark proportion reported for tropical rain forests. To conclude, in spite of a fire-free environment, the Himalayan treeline and adjoining forest species show mixed bark characters.

Chir pine and banj oak responses to pre-monsoon drought across slope aspects and positions in Central Himalaya.

The difference in maintaining a safety margin with regard to hydraulic conductance between pine and oak species influences their distribution in a region. Chir pine (Pinus roxburghii) and banj oak (Quercus leucotrichophora) are the principal species of Central Himalayan forests between 1000 and 2000 m elevations. Nearly 80% of annual precipitation of ~ 1400 mm in the region occurs during monsoon, from mid-June to September, whereafter droughts of varying length and intensity are common. The main objective of the study is to find out the responses of these two evergreen tree species to pre-monsoon (March to mid-June) water stress and topographical heterogeneity that occur in Central Himalaya. We measured soil and tree water potential and osmotic adjustment across five seasons on three slope positions, namely, hill base, mid-slope, and hill top, on north and south slope aspects. Chir pine showed an early response to pre-monsoon drought by restraining daily change in Ψ to 0.89 MPa, while predawn Ψ (ΨPD) was still moderate (isohydric tendency). In contrast, the daily reduction in Ψ of banj oak kept on increasing up to 1.49 MPa, despite severely low ΨPD (anisohydric tendency). In both tree species, Ψ was invariably lower on south aspect than north aspect and declined from hill base to hill top. Such responses to slope aspect and position, however, were relatively less apparent in chir pine, which tended to maintain a wide safety margin when under stress. As for soil Ψ, banj oak site retained monsoon rainwater more effectively than chir pine.

Chir pine forest and pre-monsoon drought determine spatial, and temporal patterns of forest fires in Uttarakhand Himalaya.

Associated with farming practices (between 300 and 2000 m elevations), human-ignited small, and patchy surface forest fires occur almost every year in Uttarakhand (between 28°43`- 31°27` N and 77°34`- 81°02`E; area 51,125 km2), a Himalayan state of India. Using fire incidence data of 19 years (2002-2020) generated by MODIS, we analysed the factors which drive temporal and spatial patterns of fire in the region. The fire incidence data were organized by 24 forest divisions, the unit of state forest management and administration. The standardized regression model showed that pre-monsoon temperature (March to May or mid-June), proportional area of the forest division under chir pine (Pinus roxburghii) forest (positive effect), and pre-monsoon and winter precipitation (negative effect) accounted for 56% of the variance in fire incidence density (FID). The pre-monsoon temperature (warmer) and precipitation (lower) were significantly different in 2009, 2012, 2016 and 2019, the years with high FID (average 54.9 fire/100 km2) than the rest of years with low FID (average 20.9 fire/100 km2). During the two decades of warming, high FID (> 30 incidence per year /100 km2) occurred after every three to four years, and fire peaks tended to increase with time. The study suggests that effective fire management can be attained by improving pre-monsoon precipitation forecasting and targeting forest compartments with a higher occurrence of chir pine and fire-vulnerable oaks. Furthermore, since fires are human-ignited, periodical analysis of changes in population distribution and communities' dependence on forests would need to be conducted. Supplementary Information: The online version contains supplementary material available at 10.1007/s42965-023-00306-9.