Thermal optima of gross primary productivity are closely aligned with mean air temperatures across Australian wooded ecosystems

Abstract

Gross primary productivity (GPP) of wooded ecosystems (forests and savannas) is central to the global carbon cycle, comprising 67%–75% of total global terrestrial GPP. Climate change may alter this flux by increasing the frequency of temperatures beyond the thermal optimum of GPP (Topt). We examined the relationship between GPP and air temperature (Ta) in 17 wooded ecosystems dominated by a single plant functional type (broadleaf evergreen trees) occurring over a broad climatic gradient encompassing five ecoregions across Australia ranging from tropical in the north to Mediterranean and temperate in the south. We applied a novel boundary‐line analysis to eddy covariance flux observations to (a) derive ecosystem GPP–Ta relationships and Topt (including seasonal analyses for five tropical savannas); (b) quantitatively and qualitatively assess GPP–Ta relationships within and among ecoregions; (c) examine the relationship between Topt and mean daytime air temperature (MDTa) across all ecosystems; and (d) examine how down‐welling short‐wave radiation (Fsd) and vapour pressure deficit (VPD) influence the GPP–Ta relationship. GPP–Ta relationships were convex parabolas with narrow curves in tropical forests, tropical savannas (wet season), and temperate forests, and wider curves in temperate woodlands, Mediterranean woodlands, and tropical savannas (dry season). Ecosystem Topt ranged from 15℃ (temperate forest) to 32℃ (tropical savanna—wet and dry seasons). The shape of GPP–Ta curves was largely determined by daytime Ta range, MDTa, and maximum GPP with the upslope influenced by Fsd and the downslope influenced by VPD. Across all ecosystems, there was a strong positive linear relationship between Topt and MDTa (Adjusted R2: 0.81; Slope: 1.08) with Topt exceeding MDTa by >1℃ at all but two sites. We conclude that ecosystem GPP has adjusted to local MDTa within Australian broadleaf evergreen forests and that GPP is buffered against small Ta increases in the majority of these ecosystems.

Species and Competition Interact to Influence Seasonal Stem Growth in Temperate Eucalypts

“…We acknowledge that relationships of stem growth with temperature are not necessarily causative, particularly because temperature effects were likely confounded with soil moisture content (i.e., indicated by a strong negative correlation). Nonetheless, bell-shaped relationships between monthly stem growth and temperature were consistent with similar shaped relationships between gross primary productivity and mean air temperature that controlled for other climate variables across a broad range of Australian wooded ecosystems [91]. The importance of temperature was not evident in seasonal LMEMs, when competition and rainfall were of greater importance.…”

Elevated [CO2] raises the temperature optimum of photosynthesis and thus promotes net photosynthesis of winter wheat and rice

“…Moreover, rice crop has a higher benefit from e[CO 2 ] than wheat. The optimal temperature for gross primary productivity (GPP) was found to be associated with growth temperature; the higher the growth temperature, the higher the optimal temperature (Bennett et al, 2021;Huang et al, 2019) Previous studies showed an exponential response of Tr and VPD to leaf temperature (Greer, 2019) or air temperature (Grossiord et al, 2020). A positive response was also determined under optimal leaf temperatures in this study (Figure 2C,D), suggesting the response of Tr and VPD to leaf or air temperature and to optimal leaf temperature in a similar manner.…”

“…The shift in T opt reflects a considerable capacity to adjust their photosynthetic characteristics to their growth environment (Yamori et al, 2013). A higher T opt is associated with ecosystems from warmer climates (Bennett et al, 2021; Huang et al, 2019).…”

Bridge to the future: Important lessons from 20 years of ecosystem observations made by the OzFlux network

“…High temperatures and associated deficits in atmospheric vapour pressure provide challenges for the ability of plants to regulate water loss and to maintain photosynthesis. A synthesis across 17 OzFlux wooded ecosystems demonstrates strong alignment between the thermal optima of GPP and mean daytime air temperatures, indicating ecosystem scale photosynthesis has adjusted to past thermal regimes (Bennett et al, 2021). Although it currently seems that GPP in Australian broadleaf evergreen forests is buffered against small increases in air temperature, the shape of this relationship and the response of ER to rising temperatures will determine the sustainability of Australian carbon sinks into the future (Bennett et al, 2021; Duffy et al, 2021; Griebel, Bennett, et al, 2020; van Gorsel et al, 2016).…”