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 nutrientrich, 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.erosion rates | bedrock weathering | critical zone | forest distribution V egetation captures solar energy and sends it cascading through ecosystems, creating habitats for other organisms and fixing nutrients and carbon from the atmosphere. Vegetation also plays an important although still incompletely understood role in the breakdown and erosion of rock (1-3) and thus the evolution of Earth's topography (4). Understanding the factors that determine where vegetation thrives-and where it does not-is therefore fundamental to many disciplines, including ecology, geomorphology, geochemistry, and pedology. As a substrate for life, lithology can influence overlying vegetation, spurring endemism due to the presence of toxins (5, 6) and limiting productivity where rock-derived nutrients are scarce (7-9). However, lithologic effects on vegetation are generally considered secondary to climatic factors such as the length of the growing season and the amount of moisture available for plant growth (10). Here we show that bedrock composition can drive differences in vegetation on par with the systematic altitudinal differences found in mountains between their hot, dry foothills and cold, wet alpine summits.

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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.weathering | erosion | critical zone | detrital thermochronometry T he interplay of climate and life drives weathering on mountain slopes (1-4), converting intact bedrock into mobile sediment particles ranging in size from clay to boulders (5, 6). Water, wind, and biota sweep these particles across slopes under the force of gravity and erode them into channels, where they serve as tools that cut into underlying bedrock during transport downstream (7). Both the size and flux of particles eroded from slopes into channels can influence incision into bedrock (8, 9), which in turn governs the pace of erosion from slopes where the sediment is produced (10, 11). The relationships between sediment production, hillslope erosion, and channel incision imply that they are central to feedbacks that drive mountain landscape evolution (12). When channel incision and hillslope erosion are relatively fast, sediment particles spend less time exposed to weathering on slopes (13) and thus may be coarser when they enter the channel (14), promoting faster incision into bedrock (7). Integrated over time, channel incision and hillslope erosion generate topography (15), imposing altitudinal gradients in precipitation, temperature, and hillslope form (16), and thus ultimately influencing erosion (17), weathering (1), and the sizes of sediment produced on slopes (2). Thus, the size and erosional flux of sediment may both depend on and regulate rates of channel incision into bedrock via feedbacks spanning a range of scales and processes.Feedbacks between climate, erosion, and tectonics have been widely studied (8,16,(18)(19)(20)(21)(22)(23). However, understanding the role of sedi...

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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 sample when the grain size distribution of sediment produced on slopes is spatially variable. A grain size bias arises when some parts of the catchment produce sand more 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 sediment sizes.