Abstract
One of the biggest uncertainties of climate change is determining the response of vegetation to many co-occurring stressors. In particular, many forests are experiencing increased nitrogen deposition and are expected to suffer in the future from increased drought frequency and intensity. Interactions between drought and nitrogen deposition are antagonistic and non-additive, which makes predictions of vegetation response dependent on multiple factors. The tools we use (Earth system models) to evaluate the impact of climate change on the carbon cycle are ill equipped to capture the physiological feedbacks and dynamic responses of ecosystems to these types of stressors. In this manuscript, we review the observed effects of nitrogen deposition and drought on vegetation as they relate to productivity, particularly focusing on carbon uptake and partitioning. We conclude there are several areas of model development that can improve the predicted carbon uptake under increasing nitrogen deposition and drought. This includes a more flexible framework for carbon and nitrogen partitioning, dynamic carbon allocation, better representation of root form and function, age and succession dynamics, competition, and plant modeling using trait-based approaches. These areas of model development have the potential to improve the forecasting ability and reduce the uncertainty of climate models.
Highlights
Earth system models (ESMs) have been used to predict the climate’s response to increased CO2 emissions, but uncertainty in land carbon (C) feedbacks results in a wide spread of uncertainty in model results [1]
The interactions between N and drought are difficult to determine because (1) the effects can depend on the timing of N deposition relative to drought; (2) most experiments are done with young trees or herbaceous plants and not with mature vegetation; (3) many studies impose only weak drought conditions that might not result in drought–N feedbacks [74]; and (4) the impacts vary with ecosystem and plant traits
A method to capture succession was implemented by Fisher et al [135] in the Community Land Model by separating vegetation into cohorts of age, plant functional type (PFT), and height; this method was tested against deciduous–evergreen forest boundaries [126]
Summary
Earth system models (ESMs) have been used to predict the climate’s response to increased CO2 emissions (or concentrations), but uncertainty in land carbon (C) feedbacks results in a wide spread of uncertainty in model results [1] Part of this uncertainty lies in a general lack of knowledge of the physical processes responsible for the land feedbacks on the C cycle, which makes estimating the land C sink difficult. Nitrogen deposits from industrial and agriculture activities have led to significant N loading in soils, in regions of Europe and the eastern United States [6,7] These N additions are within the critical load of N for sensitive ecosystems [8,9]. In order to simulate N N deposition-drought interactions, wewe hypothesize arenecessary necessary to deposition‐drought interactions, hypothesizethat thatadditional additional model model developments developments are mimic ecosystem stress responses, if ESMs are to represent the.
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