the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Feb 2026
Tropical forest responses to climate extremes: an analysis using an individual-based demographic vegetation model
Abstract. Tropical forests play a crucial role in the global carbon and water cycles, yet their response to the climate extremes remains uncertain. Here, an individual-based demographic vegetation model is used to investigate the effects of warming and drought on ecosystem dynamics across three neotropical sites that span a precipitation gradient. By explicitly resolving plant hydraulic constraints and demographic processes, the study provides a mechanistic understanding of forest responses to climate stressors. The results reveal that warming had the strongest impact on carbon assimilation in the wettest sites (Paracou and Barro Colorado Island). This reduction was primarily driven by a rising vapor pressure deficit, which induced hydraulic failure even in the absence of soil moisture depletion. In contrast, the driest site (Tapajos National Forest) exhibited the highest sensitivity to drought, driven by severe soil moisture depletion. The analysis also shows that the timescale of imposed stress matters: short daily hot-dry events led to weaker impacts due to partial recovery between pulses, whereas yearly-scale warming and drought produced much stronger, persistent reductions in productivity. These findings highlight the site-specific vulnerabilities of tropical forests to climate extremes, where VPD-induced hydraulic stress limits carbon assimilation under warming in moist sites, while soil moisture constraints dominate in drier ecosystems.
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Status: open (until 15 May 2026)
- RC1: 'Comment on egusphere-2026-145', Hisashi Sato, 02 Apr 2026 reply
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- 1
Dear authors,
This study investigates how tropical forests respond to drought, warming, and their compound effects across multiple timescales using a demographic vegetation model (BiomeE). By applying controlled perturbations to precipitation and temperature, the authors aim to disentangle the relative roles of soil water supply and atmospheric demand in shaping ecosystem stability. The results highlight the importance of vapor pressure deficit (VPD), baseline hydroclimate, and stress persistence in determining forest resilience and collapse thresholds across contrasting tropical sites.
I find that the topic is relevant and timely, and the modeling framework provides useful mechanistic insights into the interactions between drought and warming in tropical ecosystems. The manuscript is well-structured and addresses an important question in the context of future climate extremes. However, I have several concerns about the consistency between the figures and their interpretations, as well as the clarity of some key definitions and the conceptual framing. These issues should be addressed to improve the robustness and readability of the manuscript.
_____ Major Issue _____
(1) Figure 2
I find this figure somewhat difficult to interpret due to inconsistent axes and response variables across panels (a-c). While each panel individually provides useful information, their combined presentation obscures the underlying logic and makes cross-panel comparison challenging.
Specifically, panel (a) uses rainfall as the x-axis and GPP as the response, panel (b) uses soil water content and WUE, and panel (c) uses transpiration and NPP. These shifts in both explanatory and response variables make it difficult for the reader to follow the causal chain from water availability to ecosystem response.
I would suggest the authors consider the following improvements:
A. Clarify the conceptual role of each panel in the main text, explicitly stating that panels (a-c) represent different stages of the water-carbon pathway (e.g., external forcing --> internal state --> flux response).
B. Unify response variables where possible (e.g., use GPP consistently across panels, or explicitly justify the use of NPP in panel (c)).
C. Consider reorganizing the figure, for example:
Using a consistent x-axis (e.g., soil moisture or rainfall) across panels, or
Splitting the figure into multiple figures with clearer thematic grouping (e.g., thresholds vs. water-use strategies vs. flux relationships).
D. In panel (c), the linear relationship appears to mask important nonlinear behavior (as noted for TNF). It would be helpful to explicitly highlight this in the figure (e.g., by showing the raw points more clearly, adding a breakpoint, or avoiding a simple linear fit).
Overall, improving the structural consistency of Figure 2 would significantly enhance readability and clarify the key model behavior.
(2) Figure 3
The definition of the x-axis variable "T/Ta" in Figure 3 is unclear. The figure caption does not explicitly define this quantity, and it appears that different panels represent different temperature metrics (e.g., mean temperature vs. temperature range at various timescales).
This lack of clarity makes it difficult to interpret the magnitude of warming and to compare across panels. I suggest that the authors:
A. Explicitly define "T/Ta" in the figure caption, including whether it refers to mean temperature, temperature range, or another metric.
B. Use distinct labels for different temperature metrics if they are not directly comparable (e.g., mean temperature vs. diurnal or interannual range).
(3) Figure 5 & 6
The patterns shown in Figures 5 and 6 appear to be inconsistent with the interpretation provided in the text, particularly for GYF.
In Figure 5 (yearly-scale perturbations), GPP declines relatively gradually with increasing VPD, whereas in Figure 6 (daily-scale perturbations), the decline is more abrupt. This seems counter to the general expectation stated in the manuscript that longer-term stress should lead to stronger or more abrupt ecosystem responses.
Would the authors clarify this apparent discrepancy? Specifically:
A. Is the difference due to the way VPD or GPP is normalized or aggregated across timescales? If so, the normalization procedure should be clearly defined and justified, as it may affect the apparent sensitivity across figures.
B. Are the VPD ranges directly comparable between Figures 5 and 6?
(4) Conceptual inconsistency between soil moisture limitation and VPD dominance
The manuscript presents soil moisture limitation (Section 4.1) and atmospheric demand (VPD; Sections 4.2-4.3) as primary drivers of ecosystem instability, but the relationship between these two mechanisms is not clearly articulated.
In particular, Section 4.1 emphasizes cumulative soil water supply as the determinant of ecosystem decline, whereas Section 4.3 suggests that VPD dominates ecosystem response under compound stress. It remains unclear whether these are complementary processes or competing explanations.
I recommend that the authors clarify the conceptual framework linking water supply (soil moisture) and atmospheric demand (VPD), and explicitly state the conditions under which each mechanism becomes dominant.
_____ Minor Technical Issues _____
(1) Definition of normalized variables
The manuscript frequently refers to normalized variables (e.g., normalized GPP and normalized VPD), but the normalization procedures are not clearly defined. It would be helpful to explicitly state how these variables are normalized (e.g., relative to mean ambient conditions, baseline simulations, or maximum values), as this affects the interpretation of sensitivity across scenarios.
(2) Definition of "threshold."
The term “threshold” is used throughout the manuscript, but it is not formally defined. Please clarify how thresholds are identified (e.g., visually, statistically, or based on a predefined criterion), as this would improve reproducibility and interpretation.
(3) Use of GPP versus NPP
The manuscript switches between GPP and NPP across figures (e.g., Figure 2), but the rationale for this distinction is not clearly explained. A brief justification of when and why each metric is used would improve clarity.
(4) Figure captions
Some figures (e.g., Figure 3) lack sufficient information in the captions to be interpreted independently of the main text. Improving figure captions to be more self-contained (e.g., defining variables and explaining axes clearly) would enhance readability.