Simulations disentangling temperature vs. VPD effects on tropical forest GPP

Tropical forest photosynthesis can decline at high temperatures due to (1)
biochemical responses to increasing temperature and (2) stomatal responses
to increasing vapor pressure deficit (VPD), which is associated with
increasing temperature. It is challenging to disentangle the influence of
these two mechanisms on photosynthesis in observations, because
temperature and VPD are tightly correlated in tropical forests.
Nonetheless, quantifying the relative strength of these two mechanisms is
essential for understanding how tropical gross primary productivity (GPP)
will respond to climate change, because increasing atmospheric CO2
concentration may partially offset VPD-driven stomatal responses, but is
not expected to mitigate the effects of temperature-driven biochemical
responses. We used two terrestrial biosphere models to quantify how
physiological process assumptions (temperature acclimation of
photosynthesis and dynamic hydraulic stress) and functional traits (e.g.
maximum xylem conductivity) influence the relative strength of modeled
temperature vs. VPD effects on light-saturated GPP at an Amazonian forest
site, a seasonally dry tropical forest site, and an experimental tropical
forest mesocosm. By simulating idealized climate change scenarios, we
quantified the divergence in GPP predictions under model configurations
with stronger VPD effects compared to stronger direct temperature effects.
Assumptions consistent with stronger direct temperature effects resulted
in larger GPP declines under warming, while assumptions consistent with
stronger VPD effects resulted in more resilient GPP under warming. Our
findings underscore the importance of quantifying the role of 
direct temperature and indirect VPD effects for projecting the resilience
of tropical forests in the future, and demonstrate that the relative
strength of temperature vs. VPD effects in models is highly sensitive to
plant functional parameters and structural assumptions about
photosynthetic temperature acclimation and plant hydraulics.

热带森林的光合作用在高温条件下会出现衰退，其诱因主要包含两点：一是温度升高引发的生化响应，二是水汽压差（Vapor Pressure Deficit, VPD）升高引发的气孔响应，而水汽压差的升高与温度上升密切相关。在野外观测研究中，厘清这两种机制对光合作用的具体影响颇具挑战，这是因为热带森林生态系统内温度与VPD往往呈现高度相关的关系。尽管如此，量化这两种机制的相对作用强度，对于理解热带陆地总初级生产力（Gross Primary Productivity, GPP）如何响应气候变化至关重要。究其原因，大气CO₂浓度升高可部分抵消VPD驱动的气孔响应，但无法缓解温度驱动的生化响应所带来的负面影响。本研究采用两个陆地生物圈模型（Terrestrial Biosphere Model），量化了生理过程假设（光合作用的温度驯化与动态水力胁迫）以及功能性状（如最大木质部导水率）如何影响模型中温度与VPD对光饱和GPP的相对影响强度。研究覆盖了亚马逊森林样地、季节性干旱热带森林样地以及热带森林人工中型生态系统实验平台三个研究站点。通过模拟理想化气候变化情景，我们量化了两种模型配置下GPP预测结果的差异：一种配置偏向更强的VPD影响，另一种则偏向更强的直接温度影响。当模型假设更倾向于直接温度影响占主导时，升温会导致GPP出现更大幅度的衰退；而当假设更偏向VPD影响占主导时，升温情境下的GPP则表现出更强的恢复力。本研究结果凸显了量化直接温度影响与间接VPD影响的重要性——这对于预测未来热带森林的生态恢复力至关重要，同时也证实：模型中温度与VPD影响的相对强度，对植物功能参数以及光合温度驯化、植物水力结构相关的模型结构假设具有高度敏感性。