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Ecosystem Carbon and Water Cycling from a Sky Island Montane Forest T ⁎ John F Agricultural and Forest Meteorology 281 (2020) 107835 Contents lists available at ScienceDirect Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet Ecosystem carbon and water cycling from a sky island montane forest T ⁎ John F. Knowlesa,b, , Russell L. Scotta, Rebecca L. Minorb, Greg A. Barron-Gaffordb a Southwest Watershed Research Center, USDA-ARS, Tucson, AZ, United States b University of Arizona, Tucson, AZ, United States ARTICLE INFO ABSTRACT Keywords: “Sky islands” are characteristic of sequential mountain-valley terrain where mountains form an island archi- Monsoon pelago rising from surrounding valleys of desert “sea”. At high elevations in the Madrean sky islands of the Flux southwestern United States (USA) and Mexico, mixed evergreen conifer forests occur near the latitudinal extent Productivity of their distribution. This setting provides a unique opportunity to explore the ecosystem response to warmer CO 2 and drier conditions that are forecasted to become more common throughout the species range. Accordingly, this Evapotranspiration work used the eddy covariance method to quantify carbon and water cycling dynamics from a Madrean sky Arizona island forest ecosystem for nine years between 2009 and 2018. The forest functioned as net sink of carbon dioxide throughout the year, which resulted in more carbon sequestration than other monitored montane forests in the continental western USA. Sustained forest activity was made possible by the combination of mild winter temperatures and a bimodal precipitation regime that delivered moisture during both summer and winter. Seasonally, gross primary production (GPP) was temperature limited in winter and could become moisture limited during the dry early summer period depending on antecedent snowmelt moisture. Ecosystem respiration was more sensitive than GPP to moisture availability throughout the rest of the non-winter period. Forecasted warming could thus stimulate forest carbon gains during the winter and either increase or decrease respiratory carbon loss during summer as a function of moisture. Overall, a metric of snow aridity that included snow depth and potential evapotranspiration was the best predictor of the warm season carbon balance (R2 = 0.86). The seasonally dissimilar impacts of warming and drying identified by this work inform current understanding of how climate change and/or variability may affect forest water and carbon cycling dynamics throughout the montane forest biome. 1. Introduction pattern that results in convective thunderstorm activity that peaks in July and August and concludes the highly arid pre-summer period (pre- Mountain forests are responsible for the majority of sustained monsoon) across the southwestern USA and northwestern Mexico carbon sequestration in the semiarid western USA (Schimel et al., 2002; (Adams and Comrie, 1997; Higgins et al., 1999). Since the relative Desai et al., 2011). However, these ecosystems may be increasingly importance of monsoon precipitation is dependent on latitude, this vulnerable to disturbance as a result of climate change (Dale et al., produces a north-south gradient in the ratio of winter to summer pre- 2001; Williams et al., 2010). In particular, increasing drought is fore- cipitation throughout the region (Douglas et al., 1993). Previous work casted for this area, and forest carbon uptake is already seasonally has shown that monsoon precipitation can be critically important for moisture limited in many places (Seager et al., 2007; Knowles et al., rescuing individual trees from winter drought (Peltier and Ogle, 2019), 2018). The potential consequences of drought in semiarid regions are but the relative impact of summer precipitation on the annual montane especially severe, and have been linked to widespread vegetation forest carbon cycle is a currently open ecohydrological question mortality and reductions in the global carbon balance (Ahlstrom et al., (Berkelhammer et al., 2017; Hu et al., 2010; Szejner et al., 2016). 2015; Breshears et al., 2005). For these reasons, it is essential to un- Further, changes in the seasonal timing and the magnitude of monsoon derstand the carbon cycling dynamics of mountain forest ecosystems precipitation may occur with climate change (Cook and Seager, 2013; across the full range of hydrometeorological and species composition Pascale et al., 2019), with implications for the seasonality of drought gradients on the landscape. and forest production. As a result, montane forests in the southwestern The North American Monsoon (monsoon) describes an atmospheric USA provide a unique opportunity to constrain the impact of ⁎ Corresponding author at: USDA-ARS Southwest Watershed Research Center, 2000 E. Allen Road, Tucson, AZ 85719, United States. E-mail address: [email protected] (J.F. Knowles). https://doi.org/10.1016/j.agrformet.2019.107835 Received 11 July 2019; Received in revised form 17 October 2019; Accepted 8 November 2019 Available online 20 November 2019 0168-1923/ Published by Elsevier B.V. J.F. Knowles, et al. Agricultural and Forest Meteorology 281 (2020) 107835 precipitation seasonality on growing season moisture availability and the north and east (Fig. 1b). In 2012, portions of forest near the in- subsequent ecosystem carbon and water fluxes. strument tower were selectively thinned to promote horizontal and Forest production is commonly assumed to be limited by growing vertical diversity in the form of more dense forest stands spaced farther season length in evergreen conifer forests (Churkina et al., 2005; apart. This treatment was generally restricted to south-facing slopes to Piao et al., 2007). However, recent work has shown that this may not be the southeast of the tower; hence, away from the zone of peak mea- the case in some Mediterranean-climate montane forest ecosystems surement sensitivity. Qualitative analysis indicates that ~30% of the where winters are mild and production can occur year-round forest was thinned in these areas. The soil texture is sandy loam with (Barnard et al., 2018; Kelly and Goulden, 2016). Monsoon-affected “sky 32% sand, 41% silt, and 26% clay and a pH of 5.4 (Sánchez- island” forests occur in areas of continental mountain-valley terrain, Cañete et al., 2018). Depth to bedrock is 1 m or less. where vegetation communities stack vertically to encompass a range of The 1965–1980 (longest available record) mean annual precipita- ecosystems usually distributed across broad latitudinal gradients, and tion (PPT) and air temperature (Tair) were 790 mm (including snow) represent a potentially similar but understudied ecosystem due to in- and 9.3 °C, respectively, at the Palisades Ranger Station, approximately herently remote and complex terrain. The Madrean archipelago spans a 340 m below and 1.2 km southeast of the study site. The region ex- globally unique temperate-to-tropical climatic gradient and contains periences a semiarid climate and is subject to both the effects of the approximately 40 sky islands between southern Arizona and New North American Monsoon and the El Niño-Southern Oscillation (ENSO; Mexico, USA and northwestern Mexico (Warshall, 1995). Madrean sky Adams and Comrie, 1997; Brown-Mitic et al., 2007). This results in island forests are warmer than higher latitude continental forests, but distinct summer and winter precipitation maxima separated by a pro- also more arid due to persistent subtropical high atmospheric pressure. nounced dry season. For the current study, we defined five seasons There is broad consensus that a lengthened dry season and increased according to the following classification: winter (November to Feb- plant water demand will accompany increased aridification that is ruary), spring (March and April), pre-monsoon (May and June), mon- forecasted for the southwestern USA (Archer and Predick, 2008; soon (July and August), and fall (September and October). During the Seager et al., 2007); hence, the degree to which current temperature winter, snow is common, but a seasonally-continuous snowpack does versus moisture or other factors limit production from sky island forests not always develop. In its absence, ephemeral snow cover can accu- could inform the trajectory of more northerly continental forests under mulate and ablate many times in one season. Recognizing the effect of climate change. The results of this scenario will have significant im- winter precipitation on ensuing warm season ecosystem dynamics, the plications for the current and future western USA carbon sink water year (WY) was defined as the period from 1 November to31 (Schwalm et al., 2012). October. The overarching goal of this study was to evaluate the seasonal ecohydrological dynamics of a sky island montane forest that may serve 2.2. Instrumentation and measurements as a proxy for future conditions along the Rocky Mountain cordillera. To address this, we leveraged nine years of continuous eddy covariance An eddy covariance system was used to quantify the surface-atmo- data from the Santa Catalina Mountains, Arizona, USA that included sphere exchange of heat, water vapor, and carbon dioxide (CO2) be- periods of distinct hydrometeorological variability. The specific objec- tween 1 November 2009 and 31 October 2018 (AmeriFlux site code US- tives of this work were to: (1) characterize baseline patterns of ever- MtB). Turbulent fluxes were calculated as the 30-minute covariances green conifer forest carbon and water cycling variability near the lati- between 10 Hz deviations in vertical wind
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