Atmospheric nitrogen and phosphorus deposition reduces tropical dry forest tree seed germination
National Centre For Biological Sciences
Varun Varma, Mahesh Sankaran
Human activities since industrialisation have resulted in increasing amounts of atmospheric nitrogen (N) and phosphorus (P) deposition. This has resulted in considerable changes in productivity and diversity of vegetation communities across ecosystems globally. However, vegetation responses to this fertilisation in tropical ecosystems, such as tropical dry forests (TDFs), India’s largest forest type, remain under-represented in assessments.
We used a germination experiment to quantify the effects of N and P addition on germination of TDF tree seeds. Seeds from eight species (four legumes and four non-legumes) of commonly occurring TDF trees were subjected to five nutrient treatments. The design also allowed us to determine (i) differences in responses between legumes and non-legumes, (ii) if germination responses were contingent on the relative availability of N and P and (iii) the effects of seed size.
Nutrient addition significantly reduced germination of TDF tree seeds, with legumes experiencing stronger declines than non-legumes. Absolute nutrient quantities affect the germination of seeds as opposed to relative nutrient availability. Seed size is not correlated with germination responses to nutrient addition.
Results demonstrate a reduction in germination, the first stage of recruitment of trees into the community. If negative effects of nutrient addition carry forward to seedling growth and early survival, this may result in reduced tree cover over time. Additionally, stronger negative effects on legumes relative to non-legumes may signal a re-ordering of community dominance patterns. In all, this may result in changes to this ecosystem’s diversity and function.