[1] Feedbacks among vegetation dynamics, pedogenesis, and topographic development affect the "critical zone"-the living filter for Earth's hydrologic, biogeochemical, and rock/ sediment cycles. Assessing the importance of such feedbacks, which may be particularly pronounced in water-limited systems, remains a fundamental interdisciplinary challenge. The sky islands of southern Arizona offer an unusually well-defined natural experiment involving such feedbacks because mean annual precipitation varies by a factor of five over distances of approximately 10 km in areas of similar rock type (granite) and tectonic history. Here we compile high-resolution, spatially distributed data for Effective Energy and Mass Transfer (EEMT: the energy available to drive bedrock weathering), above-ground biomass, soil thickness, hillslope-scale topographic relief, and drainage density in two such mountain ranges (Santa Catalina: SCM; Pinaleño: PM). Strong correlations exist among vegetation-soil-topography variables, which vary nonlinearly with elevation, such that warm, dry, low-elevation portions of these ranges are characterized by relatively low above-ground biomass, thin soils, minimal soil organic matter, steep slopes, and high drainage densities; conversely, cooler, wetter, higher elevations have systematically higher biomass, thicker organic-rich soils, gentler slopes, and lower drainage densities. To test if eco-pedo-geomorphic feedbacks drive this pattern, we developed a landscape evolution model that couples pedogenesis and topographic development over geologic time scales, with rates explicitly dependent on vegetation density. The model self-organizes into states similar to those observed in SCM and PM. Our results highlight the potential importance of eco-pedo-geomorphic feedbacks, mediated by soil thickness, in water-limited systems.

Processes within the critical zone-spanning groundwater to the top of the vegetation canopy-have important societal relevance and operate over broad spatial and temporal scales that often are not included in existing frameworks for ecosystem services evaluation. Here we expand the scope of ecosystem services by specifying how critical zone processes extend context both spatially and temporally, determine constraints that limit provision of services, and offer a potentially powerful currency for evaluation. Context: A critical zone perspective extends the context of ecosystem services by expressly addressing how the physical structure of the terrestrial Earth surface (e.g., parent material, topography, and orography) provides a broader spatial and temporal template determining the coevolution of physical and biological systems that result in societal benefits. Constraints: The rates at which many ecosystem services are provided are fundamentally constrained by rate-limited critical zone processes, a phenomenon that we describe as a conceptual "supply chain" that accounts for rate-limiting soil formation, hydrologic partitioning, and streamflow generation. Currency: One of the major challenges in assessing ecosystem services is the evaluation of their importance by linking ecological processes to societal benefits through market and nonmarket valuation. We propose that critical zone processes be integrated into an evaluation currency, useful for valuation, by quantifying the energy flux available to do thermodynamic work on the critical zone. In short, characterization of critical zone processes expands the scope of ecosystem services by providing context, constraints, and currency that enable more effective management needed to respond to impacts of changing climate and disturbances.Abbreviations: EEMT, Effective Energy and Mass Transfer.

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Abbreviations: MAP, mean annual precipitation; PDI, profile development index; PET, potential evapotranspiration; SCM, Santa Catalina Mountains; SOC, soil organic carbon. I n the hot, dry ecosystems of the southwestern United States, water availability and topography are central to understanding soil and landscape evolution. Water availability is the dominant driver of soil formation in water-limited climates where precipitation regulates soil production and erosion, vegetation distribution and primary production, soil organic C (SOC) storage, and the degree of primary mineral alteration to secondary products. Topography and landscape position are particularly important in water-limited systems through their control on the redistribution of limited water resources that can lead to significant variation in hillslope-scale soil development and physiochemical properties (Muhs, 1982). It is necessary to understand the coupling of soil, climate, and landscape position in the Desert Southwest where predicted changes in temperature extremes and precipitation variability may drive significant shifts in water availability and biological production (Seager et al., 2007; Intergovernmental Panel on Climate Change,

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