Atmospheric changes that may affect physiological and biogeochemical processes in old-growth tropical forests include: (i) rising atmospheric CO2 concentration; (ii) an increase in land surface temperature; (iii) changes in precipitation and ecosystem moisture status; and (iv) altered disturbance regimes. Elevated CO2 is likely to directly influence numerous leaf-level physiological processes, but whether these changes are ultimately reflected in altered ecosystem carbon storage is unclear. The net primary productivity (NPP) response of old-growth tropical forests to elevated CO2 is unknown, but unlikely to exceed the maximum experimentally measured 25% increase in NPP with a doubling of atmospheric CO2 from pre-industrial levels. In addition, evolutionary constraints exhibited by tropical plants adapted to low CO2 levels during most of the Late Pleistocene, may result in little response to increased carbon availability. To set a maximum potential response for a Central Amazon forest, using an individual-tree-based carbon cycling model, a modelling experiment was performed constituting a 25% increase in tree growth rate, linked to the known and expected increase in atmospheric CO2. Results demonstrated a maximum carbon sequestration rate of ca. 0.2 Mg C per hectare per year (ha(-1) yr(-1), where 1 ha = 10(4) m2), and a sequestration rate of only 0.05 Mg C ha(-1) yr(-1) for an interval centred on calendar years 1980-2020. This low rate results from slow growing trees and the long residence time of carbon in woody tissues. By contrast, changes in disturbance frequency, precipitation patterns and other environmental factors can cause marked and relatively rapid shifts in ecosystem carbon storage. It is our view that observed changes in tropical forest inventory plots over the past few decades is more probably being driven by changes in disturbance or other environmental factors, than by a response to elevated CO2. Whether these observed changes in tropical forests are the beginning of long-term permanent shifts or a transient response is uncertain and remains an important research priority.
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机译：可能影响古老的热带森林的生理和生物地球化学过程的大气变化包括：（i）大气CO2浓度上升； （ii）地表温度上升； （iii）降水和生态系统水分状况的变化； （iv）更改干扰制度。升高的CO 2可能直接影响许多叶片水平的生理过程，但是尚不清楚这些变化是否最终反映在改变的生态系统碳储存中。未知热带老林对二氧化碳升高的净初级生产力（NPP）响应，但大气中的二氧化碳含量比工业化前水平增加一倍时，它不可能超过实验测得的最大NPP增加25％。此外，在大多数晚更新世中适应低二氧化碳水平的热带植物表现出的进化限制可能导致对碳可利用性增加的反应很小。为了设置亚马逊中部森林的最大潜在响应，使用基于单个树的碳循环模型，进行了模拟实验，该实验使树木的生长速度提高了25％，这与大气中二氧化碳的已知增加和预期增加有关。结果表明，最大固碳速率约为。每年每公顷0.2 Mg C（ha（-1）yr（-1），其中1 ha = 10（4）m2），并且隔离的速率仅为0.05 Mg C ha（-1）yr（-1）以1980-2020日历年为中心的时间间隔。这种低比率是由于树木生长缓慢以及碳在木质组织中的停留时间长所致。相比之下，扰动频率，降水模式和其他环境因素的变化会导致生态系统碳储量出现明显且相对较快的变化。我们认为，在过去的几十年中观察到的热带森林资源积蓄的变化很可能是由干扰或其他环境因素的变化驱动的，而不是对二氧化碳浓度升高的响应。这些观察到的热带森林变化是长期永久性变化的开始还是短暂的响应尚不确定，仍然是重要的研究重点。
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