Randall J. Donohue

Long‐term CO2 fertilization increases vegetation productivity and has little effect on hydrological partitioning in tropical rainforests

Understanding how tropical rainforests respond to elevated atmospheric CO2 concentration (eCO2) is essential for predicting Earths carbon, water, and energy budgets under future climate change. Here we use long-term (1982–2010) precipitation (P) and runoff (Q) measurements to infer runoff coefficient (Q/P) and evapotranspiration (E) trends across 18 unimpaired tropical rainforest catchments. We complement that analysis by using satellite observations coupled with ecosystem process modeling (using both “top-down” and “bottom-up” perspectives) to examine trends in carbon uptake and relate that to the observed changes in Q/P and E. Our results show there have been only minor changes in the satellite-observed canopy leaf area over 1982–2010, suggesting that eCO2 has not increased vegetation leaf area in tropical rainforests and therefore any plant response to eCO2 occurs at the leaf level. Meanwhile, observed Q/P and E also remained relatively constant in the 18 catchments, implying an unchanged hydrological partitioning and thus approximately conserved transpiration under eCO2. For the same period, using a top-down model based on gas exchange theory, we predict increases in plant assimilation (A) and light use efficiency (e) at the leaf level under eCO2, the magnitude of which is essentially that of eCO2 (i.e., ~12% over 1982–2010). Simulations from 10 state-of-the-art bottom-up ecosystem models over the same catchments also show that the direct effect of eCO2 is to mostly increase A and e with little impact on E. Our findings add to the current limited pool of knowledge regarding the long-term eCO2 impacts in tropical rainforests.

Global estimation of effective plant rooting depth: Implications for hydrological modeling

Plant rooting depth (Zr) is a key parameter in hydrological and biogeochemical models, yet the global spatial distribution of Zr is largely unknown due to the difficulties in its direct measurement. Additionally, Zr observations are usually only representative of a single plant or several plants, which can differ greatly from the effective Zr over a modelling unit (e.g., catchment or grid-box). Here, we provide a global parameterization of an analytical Zr model that balances the marginal carbon cost and benefit of deeper roots, and produce a climatological (i.e., 1982-2010 average) global Zr map. To test the Zr estimates, we apply the estimated Zr in a highly transparent hydrological model (i.e., the Budyko-Choudhury-Porporato (BCP) model) to estimate mean annual actual evapotranspiration (E) across the globe. We then compare the estimated E with both water balance-based E observations at 32 major catchments and satellite grid-box retrievals across the globe. Our results show that the BCP model, when implemented with Zr estimated herein, optimally reproduced the spatial pattern of E at both scales (i.e., R2=0.94, RMSD=74 mm yr−1 for catchments, and R2=0.90, RMSD=125 mm yr−1 for grid-boxes) and provides improved model outputs when compared to BCP model results from two already existing global Zr datasets. These results suggest that our Zr estimates can be effectively used in state-of-the-art hydrological models, and potentially biogeochemical models, where the determination of Zr currently largely relies on biome type-based look-up tables. This article is protected by copyright. All rights reserved.

An analytical model for relating global terrestrial carbon assimilation with climate and surface conditions using a rate limitation framework

We develop an analytical model for estimating mean annual terrestrial gross primary productivity (GPP) based on a rate limitation framework. Actual GPP (climatological mean from 1982 to 2010) is calculated as a function of the balance between two GPP potentials defined by the climate (i.e., precipitation and solar radiation) and a third parameter that encodes other environmental variables and modifies the GPP-climate relationship. The model was tested using observed GPP from 94 flux sites and modeled GPP (using the model tree ensemble approach) at 48,654 (0.5°) grid cells globally. Results show that the model could account for the spatial GPP patterns, with a root-mean-square error of 0.70 and 0.65 g C m−2 d−1 and R2 of 0.79 and 0.92 for the flux site and grid cell scales, respectively. This analytical GPP model shares a similar form with the Budyko hydroclimatological model, which opens the possibility of a general analytical framework to analyze the linked carbon-water-energy cycles.

Lags in hydrologic recovery following an extreme drought: Assessing the roles of climate and catchment characteristics

Drought, generally characterized by below-average water supply, propagates through the hydrologic system with consequent ecological and societal impacts. Compared with other drought aspects, the recovery of drought especially in the hydrological components, which directly relates to the recovery of water resources for agricultural, ecological and human needs, is less-understood. Here, taking the Millennium drought in southeast Australia (∼1997-2009) as an illustrating case, we comprehensively examined multiple aspects of the meteorological (i.e., precipitation) and hydrological (i.e., streamflow and baseflow) droughts across 130 unimpaired catchments using long-term hydro-meteorological observations. Results show that the duration and intensity of the meteorological drought are both lengthened and amplified in the hydrological drought, suggesting a nonstationarity in the rainfall-runoff relationship during a prolonged drought. Additionally, we find a time lag commonly exists between the end of the meteorological droughts and the end of the hydrological drought, with the recovery of baseflow showing a longer lag than the recovery of streamflow. The recovery rate of precipitation after drought was found to be the dominant factor that controls the recovery of hydrological droughts while catchment landscape (i.e., valley bottom flatness) plays an important but secondary role in controlling the lags in the hydrological recovery. Other hydro-climatic factors and catchment properties appear to have only minor influences governing hydrological drought recovery. Our findings highlight a delayed response in the terrestrial components of the hydrological cycle to precipitation after prolonged drought, and provide valuable scientific guidance to water resources management and water security assessment in regions facing future droughts.

A simple hypothesis of how leaf and canopy‐level transpiration and assimilation respond to elevated CO2 reveals distinct response patterns between disturbed and undisturbed vegetation

Elevated CO2 increases leaf-level water-use efficiency (ω) almost universally. How canopy-level transpiration and assimilation fluxes respond to increased ω is currently unclear. We present a simple, resource-availability-based hypothesis of how equilibrium (or mature) leaf and canopy transpiration and assimilation rates, along with leaf area index (L), respond to elevated CO2. We quantify this hypothesis in the form of a model and test it against observations from eight Free Air CO2 Enrichment sites that span a wide range of resource availabilities. Sites were grouped according to vegetation disturbance status. We find the model adequately accounts for the responses of undisturbed vegetation (R2 = 0.73, 11% error) but cannot account for the responses of disturbed vegetation (R2 = 0.47, 17% error). At undisturbed sites, the responses of L and of leaf and canopy transpiration vary predictably (7% error) with resource availability, whereas the leaf assimilation response is less predictable. In contrast, the L and transpiration flux responses at the disturbed (mostly forested) sites are highly variable and are not strongly related to resource availability. Initial analyses suggest that they are more strongly related to regrowth age than to resource availability. We conclude that (i) our CO2 response hypothesis is valid for capturing the responses of undisturbed vegetation only, (ii) that the responses of disturbed vegetation are distinctly different from undisturbed vegetation, and (iii) that these differences need to be accounted for when predicting the effects of elevated CO2 on land surface processes generally, and on leaf area and water fluxes in particular.