Abstract

Significance: 

This investigation focused on the factors that influence forest cover in the Sierra Nevada, California, where Giant Sequoia, the largest trees on Earth, grow in groves immediately next to expanses of rock devoid of soil and vegetation. The differences in forest cover correspond to twofold differences in erosion rates, suggesting that vegetation is an important regulator of landscape evolution across the region. Analyses presented here show that differences in forest cover can be explained by variations in geochemical composition of underlying bedrock. These results are important because they demonstrate that bedrock geochemistry is on par with climate as a regulator of vegetation in the Sierra Nevada and likely in other granitic mountain ranges around the world.

Abstract: 

Earth’s land surface teems with life. Although the distribution of ecosystems is largely explained by temperature and precipitation, vegetation can vary markedly with little variation in climate. Here we explore the role of bedrock in governing the distribution of forest cover across the Sierra Nevada Batholith, California. Our sites span a narrow range of elevations and thus a narrow range in climate. However, land cover varies from Giant Sequoia (Sequoiadendron giganteum), the largest trees on Earth, to vegetation-free swaths that are visible from space. Meanwhile, underlying bedrock spans nearly the entire compositional range of granitic bedrock in the western North American cordillera. We explored connections between lithology and vegetation using measurements of bedrock geochemistry and forest productivity. Tree-canopy cover, a proxy for forest productivity, varies by more than an order of magnitude across our sites, changing abruptly at mapped contacts between plutons and correlating with bedrock concentrations of major and minor elements, including the plant-essential nutrient phosphorus. Nutrient-poor areas that lack vegetation and soil are eroding more than two times slower on average than surrounding, more nutrient-rich, soil-mantled bedrock. This suggests that bedrock geochemistry can influence landscape evolution through an intrinsic limitation on primary productivity. Our results are consistent with widespread bottom-up lithologic control on the distribution and diversity of vegetation in mountainous terrain.

Citation

Hahm, W.J., Riebe, C.S., Lukens, C.E., and Araki, S. (2014): Bedrock composition regulates mountain ecosystems and landscape evolution. Proceedings of the National Academy of Sciences 111(9):3338-3343. DOI: 10.1073/pnas.1315667111

This Paper/Book acknowledges NSF CZO grant support.