Fertilization Effects on the Ecohydrology of a Southern California Annual Grassland
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UC Irvine Faculty Publications Title Fertilization effects on the ecohydrology of a southern California annual grassland Permalink https://escholarship.org/uc/item/1x3885dw Journal Geophysical Research Letters, 39(8) Authors Parolari, A. J. Goulden, M. L. Bras, R. L. Publication Date 2012-04-26 License https://creativecommons.org/licenses/by/3.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L08405, doi:10.1029/2012GL051411, 2012 Fertilization effects on the ecohydrology of a southern California annual grassland A. J. Parolari,1 M. L. Goulden,2 andR.L.Bras3 Received 18 February 2012; revised 26 March 2012; accepted 28 March 2012; published 26 April 2012. [1] Nitrogen limits leaf gas exchange, canopy development, conductances, such that ET and leaf water potential were and evapotranspiration in many ecosystems. In dryland eco- relatively unchanged [Ewers et al., 2000]. These responses systems, it is unclear whether increased anthropogenic nitro- were not observed in irrigated stands, indicating drought gen inputs alter the widely recognized dominance of water exposure as an important determinant of hydraulic capacity. and energy constraints on ecohydrology. We use observa- In another study, fertilized savanna trees did not fully adjust tions from a factorial irrigation and fertilization experiment hydraulic capacity, resulting in increased ET and decreased in a nitrogen-limited southern California annual grassland leaf water potential [Bucci et al., 2006]. These results dem- to explore this hypothesis. Our analysis shows growing onstrate the ET response to fertilization is mediated by plant season soil moisture and canopy-scale water vapor conduc- hydraulic characteristics that respond to both soil moisture tance are equivalent in control and fertilized plots. This and nitrogen availability, although it is unclear which is the consistency arises as fertilization-induced increases in dominant factor. leaf area index (LAI) are offset by reduced leaf area-based [4] Dryland ecosystems present a compelling forum in stomatal conductance, gs. We interpret this as evidence which to study the relative effects of soil moisture and of a hydraulic feedback between LAI, plant water status, nitrogen on ecohydrology. These systems are characterized and gs, not commonly implemented in evapotranspira- by strong interactions between vegetation and hydrology, tion models. These results support the notion that canopy experience frequent periods of water stress [Rodriguez- physiology and structure are coordinated in water-limited Iturbe et al., 2001], and are often co-limited by water and ecosystems to maintain a transpiration flux tightly controlled nitrogen [Hooper and Johnson, 1999]. We use an ecohy- by hydraulic constraints in the soil-vegetation-atmosphere drological framework to explore fertilization effects on pathway. Citation: Parolari, A. J., M. L. Goulden, and R. L. Bras interactions between canopy characteristics, ET, and the (2012), Fertilization effects on the ecohydrology of a southern growing season water balance in a southern California California annual grassland, Geophys. Res. Lett., 39,L08405, annual grassland. Inferences are made from data collected doi:10.1029/2012GL051411. in a factorial irrigation and nitrogen fertilization experi- ment, where biomass production responds to the addition 1. Introduction of either resource. [2] Plant physiological activity links the carbon and 2. Background nitrogen cycles with the surface water and energy balances [Schimel et al., 1997]. Ecophysiological traits that control 2.1. Canopy Structure, Physiology, and Transpiration evapotranspiration (ET) and photosynthesis vary with the [5] Ecohydrological dynamics in drylands are the result relative supplies of soil water and nitrogen. Therefore, of strong interaction between transpiration, controlled by increased exogenous nitrogen supply from either fertiliza- vegetation canopies, and soil moisture. This interaction tion or deposition may alter interaction between plants and can be understood through a mechanistically simple tran- the water balance. spiration model [Laio et al., 2001], similar to that com- [3] Fertilization induces changes in leaf area index (LAI), monly implemented in land surface models (LSMs). The stomatal conductance, and the conductivity of leaf, stem, transpiration rate T [mm sÀ1] is defined as the product of and root tissues. In fertilized Pinus taeda stands, increased a climate- and vegetation-specific potential transpiration À1 LAI was compensated by decreased tissue and stomatal rate Tp [mm s ] and a linear scaling factor b that accounts for soil water deficit, 8 q ≤ q <> 0 w 1 q À q Department of Civil and Environmental Engineering, Massachusetts ðÞ¼q Á b ¼ Á w q < q ≤ q∗ ð Þ Institute of Technology, Cambridge, Massachusetts, USA. T Tp Tp > q∗ À q w 1 2 : w Department of Earth System Science, University of California, Irvine, 1 q > q∗ California, USA. 3 Schools of Civil and Environmental Engineering and Earth and q Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, where [-] is the volumetric water content averaged over the USA. root zone, qw [-] is the permanent wilting point, below which transpiration ceases, and q* [-] is the incipient stress point, Corresponding Author: A. J. Parolari, Department of Civil and Environ- mental Engineering, Massachusetts Institute of Technology, 77 Massachusetts above which transpiration occurs at the potential rate. Ave., Bldg. 48-216, Cambridge, MA 02139, USA. ([email protected]) [6] This formulation captures the two most general aspects of plant-water interactions. First, when soil moisture is plen- Copyright 2012 by the American Geophysical Union. tiful, vegetation transpires at a potential rate that is determined 0094-8276/12/2012GL051411 by soil hydraulic properties, the hydraulic conductance of the L08405 1of5 L08405 PAROLARI ET AL.: FERTILIZATION EFFECTS ON ECOHYDROLOGY L08405 root-stem-leaf system, canopy structure, and atmospheric This implies more rapid soil moisture depletion and earlier variables governing the demand for water (e.g., wind speed, arrival of water stress, stomatal closure, and drought-induced air temperature, humidity). Second, when subject to hydraulic tissue loss. We refer to this scenario as the “soil moisture” limitations associated with soil water deficit (i.e., q < q*), plants plant-water feedback mechanism below and anticipate reduce stomatal opening (and subsequently T)tomaintainleaf nitrogen fertilization to decrease q with a corresponding water potential and avoid damage due to water stress. decrease in gs (i.e., through b). [7] T(q) in equation (1) can be expressed in terms of [12] Equation (4) implies that the plant-water feedback observable canopy characteristics. Net carbon assimilation, mechanism and T are mediated by the hydraulic capacity transpiration, and stomatal conductance vary with depth in of the plant tissues, gsrp. If the soil-root-plant pathway is the canopy as light is extinguished by upper canopy layers. limiting, an increase in either gs or LAI cannot increase T, We assume the energy available for photosynthesis decrea- generating a so-called “hydraulic” feedback. In this case, ses exponentially with cumulative leaf area index (LAI, we predict T is constrained such that the q and T dynamics L [m2 leaf mÀ2 ground]) [Monsi and Saeki, 2005] and sto- are relatively insensitive to changes in LAI. As a conse- matal conductance is proportional to the local irradiance quence, the predicted decrease in gs is associated with through the carbon assimilation rate [Oleson et al., 2010]. increased LAI and not decreased q. À2 The total canopy water vapor conductance Gc [mol H2Om À ground s 1] is then, 3. Loma Ridge Observations Z L [13] The Loma Ridge experimental site is located in the ¼ maxb ðÞÀ ¼ maxb À1½ðÀ ðÞÀ Þ Gc gs exp kl dl gs k 1 exp kL 2 Santa Ana foothills in central Orange County, California 0 (33.742N, 117.704W). The climate is Mediterranean with max À2 À1 where gs [mol H2Om leaf s ] is the light-saturated cool, wet winters and dry, hot summers. Annual rainfall leaf-specific stomatal conductance measured at the top-of- follows a lognormal distribution with median 281 mm yrÀ1, À canopy and k [m2 ground m 2 leaf] is the canopy light of which approximately 90% falls between November and extinction coefficient. Under the diffusion analogy, April. The grassland is composed primarily of invasive annual grasses and forbs, whose composition varies with ¼ ¼ À1½À ðÞÀ ð Þ T GcD gsk 1 exp kL D 3 precipitation. In a typical year, this mixture includes Bromus diandrus, Lolium multiflorium, and Avena spp. at approxi- where D [-] is the vapor pressure deficit and gs represents the mately 15–20% relative cover each. Forbs and remnants of actual stomatal conductance under water-stressed conditions the historically dominant perennial grass Nassella pulchra maxb (i.e., the product gs ). (<5%) are also present. The local soil is homogeneous and [8] An alternative model also constrains T by the capacity characterized as sandy loam. of the soil, roots, and xylem to deliver water to the leaf [14] Factorial plot experiments at Loma Ridge were estab- surface for evaporation, lished in 2007 to study ecosystem responses to changing À climate and increased nitrogen deposition. Twenty-four plots T ¼ g ½¼y ðÞÀq y g k 1½1 À expðÞÀkL D ð4Þ srp s l s (approx. 6.1  8.5 m) are divided into eight replicates of three À1 À1 water input treatments: ambient, ambient minus 40% (dry), where gsrp [mol MPa s ] is the soil-root-plant conductance and y (q)andy [MPa] are the soil and leaf water potentials, and ambient plus 40% (wet). All plots were burned in the s l October 2007 Santiago wildfire. Rainfall is excluded from respectively. gsrp is reduced at low ys and yl [Daly et al., 2004]. Equations (3) and (4) are now used to generate dry plots with retractable polyethylene roofs that were closed hypotheses of the hydrologic response to fertilization. during approximately half of the rain events (closed <5% of the days during a year). Water draining from the roofs was 2.2.