Carbon and nitrogen allocation in montane vegetation: Understanding the impact of environmental change on ecosystem processes

Abstract. Mountain forests, comprising nearly 20 % of the world’s forest cover, are among the most ecologically fragile ecosystems due to their strong topographic and climate sensitivity. Understanding how climate influences these ecosystems requires examining the allocation of key nutrients like carbon (C) and nitrogen (N). We investigated foliar C and N allocation in the Himalayan vegetation by sampling 141 leaf samples from 14 species across three climatically different transects spanning 1,900–5,200 m. The mean total nitrogen (TN) was 6.6 ± 4.5 %, with Juniperus and Abies exhibiting the highest TN (8.3 %) and grasses the lowest (1.6 %). Foliar δ¹⁵N ranged from +2.1 ‰ in Juniperus to +8.9 ‰ in grasses, varying inversely with TN. Total organic carbon (TOC) averaged 37.5 ± 6.2 %, peaking in Juniperus (40.6 %) and lowest in Abies (34.2 %). Foliar δ¹³C in C₃ species clustered near –27 ‰, while C₄ grasses reached –13.4 ‰. In the Central Himalaya, Juniperus maintained high TN and TOC at altitude, whereas in the West, Rhododendron had elevated TN and TOC relative to other genera. Statistical analyses showed that warmer growing seasons strongly reduced TN in Abies and Rhododendron, and wetter conditions increased δ¹³C in Abies and TN in Pinus. Clustering of δ¹³C and TN effectively distinguished mycorrhizal types, with arbuscular‐associated grasses displaying higher δ¹³C and lower TN than ectomycorrhizal and ericoid‐associated taxa. Geostatistical modelling produced a dual‐isotope map revealing “arid hotspots” (high δ¹⁵N and δ¹³C, low TN) in dry western valleys and “humid cold spots” (low δ¹⁵N and δ¹³C, high TN and TOC) in moist eastern slopes. These findings demonstrate that steep precipitation and temperature gradients drive N much more than C in Himalayan plants, underscore the tight coupling of C and N, and identify areas most sensitive to climate‐induced shifts in nutrient allocation.