Drought Influences the Accuracy of Simulated Ecosystem Fluxes: A Model-Data Meta-analysis for Mediterranean Oak Woodlands

Abstract

Water availability is the dominant control of global terrestrial primary productivity with concurrent effects on evapotranspiration and ecosystem respiration, especially in water-limited ecosystems. Process-oriented ecosystem models are critical tools for understanding land-atmosphere exchanges and for up-scaling this information to regional and global scales. Thus, it is important to understand how ecosystem models simulate ecosystem fluxes under changing weather conditions. Here, we applied both time-series analysis and meta-analysis techniques to study how five ecosystem processoriented models-simulated gross primary production (GPP), ecosystem respiration (Reco), and evapotranspiration (ET). Ecosystem fluxes were simulated for 3 years at a daily time step from four evergreen and three deciduous Mediterranean oak woodlands (21 site-year measurements; 105 siteyear-simulations). Mediterranean ecosystems are important test-beds for studying the interannual dynamics of soil moisture on ecosystem mass and energy exchange as they experience cool, wet winters with hot, dry summers and are typically subject to drought. Results show data-model disagreements at multiple temporal scales for GPP, Reco, and ET at both plant functional types. Overall there was a systematic underestimation of the temporal variation of Reco at both plant functional types at temporal scales between weeks and Modeled Reco was systematically overestimated during drought for all sites, but daily GPP was systematically underestimated only for deciduous sites during drought. In contrast, daily estimates of ET showed good data-model agreement even during drought conditions. This meta-analysis brings attention to the importance of drought conditions for modeling purposes in representing forest dynamics in water-limited ecosystems.

Evaluating the agreement between measurements and models of net ecosystem exchange at different times and timescales using wavelet coherence: an example using data from the North American Carbon Program Site-Level Interim Synthesis

“…Models that calculate NEE as GPP minus ER tend to fit better on monthly to seasonal timescales at the coniferous forests CA-Obs and US-Ho1. In general, simulating NEE and HR results in poorer NEE model fit at seasonal and annual timescales in coniferous stands, and simulating GPP and ER presents more of a challenge in grasslands, wetlands, and deciduous forests (Vargas et al, 2013). Many of the subplots in Fig.…”

“…5. spectral analysis of model residuals (Dietze et al, 2011;Vargas et al, 2010Vargas et al, , 2013, and/or quantification of causal relationships among measurements and models across time and spectra using the Granger definition (Detto et al, 2012). Here, it is pertinent to point out that different approaches to creating null spectra or surrogate time series for significance testing are favored for different time-series methodologies, and to reiterate that the estimation of significance resides partially with the choice of null or surrogate spectra.…”

“…Such model diagnostics are able to identify whether a different model, different model parameterization, or different subroutine represents an improvement (Akaike, 1974), but are not intended to identify the symptoms of model-data disagreement across time and scales in time in order to identify the conditions that result in discrepancies. Residual analyses and detailed investigations of model performance during different time periods give important insight into the mechanisms underlying model failure, but are rarely interpreted with respect to patterns of model/measurement mismatch in the frequency (or timescale) domain (see however Dietze et al, 2011;Mahecha et al, 2010;Vargas et al, 2010, andVargas et al, 2013). In this paper, we quantify periods in time and scales in time when ecosystem models are not significantly related to eddy covariance measurements of net ecosystem exchange (NEE) to identify periods in which models can and should be improved .…”

The Responses of Plant Leaf CO2/H2O Exchange and Water Use Efficiency to Drought: A Meta-Analysis

“…Water scarcity has become one of the most adverse environmental factors for plant growth and productivity, limiting terrestrial primary productivity worldwide, especially in the semi-arid regions of the northern hemisphere [2,3]. As a primary resource for plant growth and productivity, water is directly or indirectly involved in all physiological processes.…”

Drought severity–duration–frequency curves: a foundation for risk assessment and planning tool for ecosystem establishment in post-mining landscapes

“…Ecosystem attributes such as the distribution of native tropical species (Engelbrecht et al, 2007;Kuster et al, 2013), the structure and functioning of forests (Zhang and Jia, 2013;Vargas et al, 2013), biodiversity and ecosystem resilience (Brouwers et al, 2013;Lloret, 2012;Jongen et al, 2013), and primary productivity and respiration of vegetation (Shi et al, 2014) are sensitive to the occurrence of drought events. In the context of mined land rehabilitation, droughts also play a critical role for the early establishment of plants (Nefzaoui and Ben Salem, 2002;Gardner and Bell, 2007) and long-term resilience of novel (Doley et al, 2012;Doley and Audet, 2013) and/or native ecosystems on post-mining land (Bell, 2001).…”