Emergent climate and CO₂ sensitivities of net primary productivity in ecosystem models do not agree with empirical data in temperate forests of eastern North America

Abstract

Ecosystem models show divergent responses of the terrestrial carbon cycle to global change over the next century. Individual model evaluation and multimodel comparisons with data have largely focused on individual processes at subannual to decadal scales. Thus far, data-based evaluations of emergent ecosystem responses to climate and CO at multidecadal and centennial timescales have been rare. We compared the sensitivity of net primary productivity (NPP) to temperature, precipitation, and CO in ten ecosystem models with the sensitivities found in tree-ring reconstructions of NPP and raw ring-width series at six temperate forest sites. These model-data comparisons were evaluated at three temporal extents to determine whether the rapid, directional changes in temperature and CO in the recent past skew our observed responses to multiple drivers of change. All models tested here were more sensitive to low growing season precipitation than tree-ring NPP and ring widths in the past 30 years, although some model precipitation responses were more consistent with tree rings when evaluated over a full century. Similarly, all models had negative or no response to warm-growing season temperatures, while tree-ring data showed consistently positive effects of temperature. Although precipitation responses were least consistent among models, differences among models to CO drive divergence and ensemble uncertainty in relative change in NPP over the past century. Changes in forest composition within models had no effect on climate or CO sensitivity. Fire in model simulations reduced model sensitivity to climate and CO , but only over the course of multiple centuries. Formal evaluation of emergent model behavior at multidecadal and multicentennial timescales is essential to reconciling model projections with observed ecosystem responses to past climate change. Future evaluation should focus on improved representation of disturbance and biomass change as well as the feedbacks with moisture balance and CO in individual models.

A Combined Tree Ring and Vegetation Model Assessment of European Forest Growth Sensitivity to Interannual Climate Variability

“…The enhanced climate sensitivity of NPP from DGVMs has been previously reported (Babst et al, ; Ciais et al, ; Huang et al, ; Piao et al, ; Rollinson et al, ; Zhang et al, ) and may result from inadequate model parameterizations related to soil moisture availability and plant water transport. Circularity issues introduced by correlating NPP with the same climate data that initially drove the model (Babst et al, ) may further contribute to increased climate sensitivity.…”

Linking drought legacy effects across scales: From leaves to tree rings to ecosystems

“…Previous attempts to uncover legacy effects in larger scale C cycle processes have been limited by their large spatial and temporal breadth (Schwalm et al, 2017;Wu et al, 2017), while other studies linking tree rings to GPP have not examined this question in the context of environmental stress (Mund et al, 2010;Ouimette et al, 2018;Rocha et al, 2006;Rollinson et al, 2017). Thus, we lack a comprehensive understanding of how legacy effects affect the C cycle.…”

A Framework for Determining Population-Level Vulnerability to Climate: Evidence for Growth Hysteresis in Chamaecyparis thyoides Along Its Contiguous Latitudinal Distribution

“…Researching growth-climate relationships at the individual tree level helps to understand how forests might respond to climatic change, and their potential productivity moving into the future (e.g., Carrer, 2011;Clark et al, 2012). However, some of the next steps in understanding the potential level of persistence and resilience of trees includes understanding how climate-growth relations might change over time (Cook and Zedaker, 1992;Visser et al, 2010;Carrer, 2011), the nonlinear response of tree growth to climate (Rollinson et al, 2017), and then linking these responses to individual genetics within a population (Housset et al, 2018). The ultimate growth vulnerability and potential mortality of tree populations likely depends on relative changes in climate, the genetic, phenotypic, and morphological adaptability of trees to change, and ecosystem development processes that influence ecological factors of growth.…”

“…However, we also note that results for a species of restricted distribution, such as Chamaecyparis thyoides, limited to the specific sites might be hard to extrapolate to other systems. A good pursuit would be to continue efforts like ours and continued data-model assimilations (Dietze et al, 2013;Rollinson et al, 2017;Fisher et al, 2018).…”