Bedrock composition regulates mountain ecosystems and landscape evolution

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 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.

Conflicting selection from fire and seed predation drives fine-scaled phenotypic variation in a widespread North American conifer

“…If changes in the frequency of serotiny over time lead to changes to stand structure that ultimately affect squirrel population densities, then there is the potential for feedbacks in the system that could either reinforce or dampen spatial patterns in selection on serotiny. Alternatively, stand structure may be more influenced by factors such as bedrock (38), which could lead to similar stand structure from one tree generation to the next, resulting in consistent spatial variation in both squirrel densities and selection.…”

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…By contrast, for rocks that produce low plasma : skeleton ratios such as granite and quartz-rich sandstone, we expect that uplift (erosion) will impose an absolute constraint on P availability that is far less buffered by proximal controls such as plasma sorption. Those lithologies that form soils with low plasma : skeleton ratios are more likely to have Plimited ecosystems (Hahm et al, 2014) and therefore be influenced by differences in dust inputs (Aciego et al, 2017). Such low plasma : skeleton lithologies are also more likely to develop strong local P gradients due to hydrological redistribution along hillslopes Bern et al, 2015).…”

“…As biotic engines, trees thus strongly impact the energy, water, and element cycles in forested and savannah ecosystems, shaping and sculpting landscapes and soils over long timescales (Reneau and Dietrich, 1991;van Breemen et al, 2000;Balogh-Brunstad et al, 2008a;Pawlik et al, 2016). Soils and landscapes in turn affect plant species composition and size as well as above-and below-ground productivity and rooting depth (Bennie, 1991;Clark et al, 2003;Hahm et al, 2014;Marshall and Roering, 2014). Only by studying the entire CZ using concepts from hydrology, soil science, geomorphology, geochemistry, and ecology will a synthetic view of tree-soil-landscape co-evolution emerge.…”

Abrupt Change in Forest Height along a Tropical Elevation Gradient Detected Using Airborne Lidar

“…Vegetation cover has also been shown to be strongly influenced by soil properties. Although such properties are also partly determined by climate, they can vary with bedrock geological composition and with erosion rate [4,5]. Bedrock geology and erosion rates can vary along elevation gradients in a way that can reinforce or obscure the patterns produced by the climatic gradient.…”