Since the late 1990s, the world has grappled with an escalating issue of increasing vapor pressure deficit (VPD), an index measuring the shortfall of air moisture from saturation. This challenge is anticipated to persist into the upcoming century. Multiple studies have revealed that the mounting VPD significantly hampers vegetation growth, primarily due to constrained water availability. A recent study adds a novel dimension by highlighting that this growth suppression phenomenon diminishes in northern peatlands, shedding light on the underlying reasons.
Commonly accepted cognitions for VPD effects
Ample evidence indicates that VPD exerts a suppressive effect on vegetation worldwide. In response to rising VPD, which signifies atmospheric dryness, plants commonly exhibit stomatal closure to minimize water loss and prevent excessive tension within the xylem, albeit at the cost of reduced or halted photosynthesis. Consequently, the well-established understanding is that the impact of increasing VPD, driven by warming and declining relative humidity, consistently yields a negative response in vegetation, particularly in regions where water is limited.
Are VPD effects always negative?
Research has reported that in the Amazon rainforest, photosynthesis increases in response to atmospheric dryness, attributed to a compensatory effect of new leaves mitigating stomatal limitation through the rise in VPD. This perspective challenges conventional understanding. However, it is essential to recognize that the unique surface characteristics supporting this viewpoint may not apply to northern peatlands, another notably humid region. Despite its significance akin to the Amazon rainforest, the response of vegetation to increasing VPD in these peatlands has been overlooked. Various pieces of evidence suggest that the elevation in VPD in northern peatlands may be solely driven by warming. This could be due to a wet and cold environment and extensive moss cover, generating sufficient atmospheric moisture to fulfill the increased demand for rising VPD in these peatlands. Nevertheless, direct observational evidence is lacking, leaving uncertainties about whether increasing VPD is solely driven by warming and how vegetation growth responds to the warming-induced elevation of VPD in northern peatlands.
Do the commonly accepted cognitions represent the VPD effects in the northern peatlands?
Analysis of multisource datasets revealed that increasing VPD solely due to warming did not hinder vegetation growth in northern peatlands. A site-level manipulation experiment and a synthesis of multiple sites indicated a neutral impact of rising VPD on vegetation growth. The regional analysis further demonstrated a pronounced declining gradient of VPD suppression effects, with sparsely distributed peatlands experiencing a stronger decline than densely distributed ones. To discern the unique VPD impacts on northern peatlands, we compared them with global non-peatlands, challenging the prevailing notion of VPD suppression on vegetation. Consistent observations at field and grid scales suggested that the commonly held viewpoint, derived from global non-peatland regions, may exaggerate the VPD suppression impact in northern peatlands (Fig. 1).
Mechanisms for the divergent VPD effects
The contrasting effects of VPD stem from disparities in water availability and soil hydraulic properties governing plant water-use strategies and stomatal activity in response to increasing VPD (Fig. 1). Specifically, in global non-peatland regions characterized by relatively dry soils and air, plants adopted a "reduce expenditure" water-use strategy in reaction to escalating VPD, consequently restricting vegetation growth (Fig. 1). In the moss-dominated, wet system of northern peatlands, the typical response to increasing VPD did not hinder vegetation growth. This is because plants in these environments have evolved an "open" water-use strategy, adapted to the moist soil and air conditions (Fig. 1). The observed neutral impacts of VPD in northern peatlands stand in contrast to the reported vegetation suppression in global non-peatland areas experiencing elevated VPD due to concurrent warming and decreasing relative humidity. This contrast underscores the imperative need to refine models to accurately represent VPD impacts in northern peatlands.