Replacement of oak forest with pine in the Himalaya affects the nitrogen cycle

Abstract

Pine and oak are the two predominant evergreen forests in the central and western Himalaya between altitudes 1,200 and 2,200 m. The pine is a light-demanding, fire-adapted but fire-promoting species. The surface fires averaging once every 2 or 3 years cause substantial nitrogen losses in pine forest. Pine forest naturally occurs on the driest and rockiest slopes, and has spread greatly under the influence of cutting and burning, replacing oak forest in vast areas. Oak forest is non-inflammable but has suffered a good deal from fire spreading from the pine forest1. Deforestation has also accounted for loss of oak forest1–3, and although the replacement of oak by pine is common1,2, the reverse has not been observed. Our studies indicate that the greater nutrient-conserving ability of pine and the creation of a nitrogen shortage makes it difficult for oak to re-invade areas occupied by pine4–8.