Claire E. Lukens

Bedrock composition regulates mountain ecosystems and landscape evolution

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. 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.

Climate and topography control the size and flux of sediment produced on steep mountain slopes

Significance Rivers carve through landscapes using sediment produced on hillslopes by biological, chemical, and physical weathering of underlying bedrock. Both the size and supply rate of sediment influence the pace of river incision and landscape evolution, but the connections remain poorly understood, because the size distributions of sediment supplied from slopes have been difficult to quantify. This study combined existing sediment-tracing techniques in a previously unidentified approach to quantify sediment production across an alpine catchment in the High Sierra, California. Results show that colder, steeper, and less vegetated slopes produce coarser sediment that erodes faster into the channel network. These results demonstrate that the sediment-tracing approach can be used to quantify feedbacks between climate, topography, and erosion. Weathering on mountain slopes converts rock to sediment that erodes into channels and thus provides streams with tools for incision into bedrock. Both the size and flux of sediment from slopes can influence channel incision, making sediment production and erosion central to the interplay of climate and tectonics in landscape evolution. Although erosion rates are commonly measured using cosmogenic nuclides, there has been no complementary way to quantify how sediment size varies across slopes where the sediment is produced. Here we show how this limitation can be overcome using a combination of apatite helium ages and cosmogenic nuclides measured in multiple sizes of stream sediment. We applied the approach to a catchment underlain by granodiorite bedrock on the eastern flanks of the High Sierra, in California. Our results show that higher-elevation slopes, which are steeper, colder, and less vegetated, are producing coarser sediment that erodes faster into the channel network. This suggests that both the size and flux of sediment from slopes to channels are governed by altitudinal variations in climate, vegetation, and topography across the catchment. By quantifying spatial variations in the sizes of sediment produced by weathering, this analysis enables new understanding of sediment supply in feedbacks between climate, tectonics, and mountain landscape evolution.

Grain size bias in cosmogenic nuclide studies of stream sediment in steep terrain

Cosmogenic nuclides in stream sediment are widely used to quantify catchment-average erosion rates. A key assumption is that sampled sediment is representative of erosion from the entire catchment. Here we show that the common practice of collecting a narrow range of sizes—typically sand—may not yield a representative samplewhen the grain size distribution of sediment produced on slopes is spatially variable. A grain size bias arises when some parts of the catchment produce sandmore readily than others. To identify catchments that are prone to this bias, we used a forward model of sediment mixing and erosion to explore the effects of catchment relief and area across a range of altitudinal gradients in sediment size and erosion rate. We found that the bias increases with increasing relief, because higher-relief catchments have a larger fraction of high elevations that are underrepresented in the sampled sand when grain size increases with altitude. The bias also increases with catchment area, because sediment size reduction during transport causes an underrepresentation of more distal, higher elevations within the catchment. Our analysis indicates that grain size bias may be significant at many sites where cosmogenic nuclides have been used to quantify catchment-average erosion rates. We discuss how to quantify and account for the bias using cosmogenic nuclides and detrital thermochronometry in multiple