Abstract
The strong effects of climate change are expected to negatively impact the long-term resilience and function of forest ecosystems, which could lead to changes in forest carbon balance and productivity. However, these forest responses may vary with local conditions and forest types. Accordingly, this study was carried out to determine gross primary productivity (GPP) sensitivity to changes in environmental parameters. Central European beech (at Štítná) and spruce species (at Bílý Kr̆íz̆ and Rájec), growing under contrasting climatic conditions, were studied. The comparative analyses of GPP were based on a five-year-long dataset of eddy covariance fluxes during the main growing season (2012–2016). Results of forest GPP responses with changes in environmental factors from a traditional Stepwise multiple linear regression model (SMLR) were used and compared with Random forest (RF) analyses. To demonstrate how actual GPP trends compare to potential GPP (GPPpot) courses expected under near-optimal environmental conditions, we computed normalized GPP (GPPnorm) with values between 0 and 1 as the ratio of the estimated daily sum of GPP to GPPpot. The study confirmed the well-known effect of total intensity of the photosynthetically active radiation and its diffuse fraction on GPPnorm across all the forest types. However, the study also showed the secondary effects of other environmental variables on forest productivity depending on the species and local climatic conditions. The reduction in forest productivity at the beech forest in Štítná was presumed to be mainly induced by edaphic drought (anisohydric behaviour). In contrast, reduced forest productivity at the spruce forest sites was presumably induced by both meteorological and hydrological drought events, especially at the moderately dry climate in Rájec. Overall, our analyses call for more studies on forest productivity across different forest types and contrasting climatic conditions, as this productivity is strongly dependent on species type and site-specific environmental conditions.
Highlights
Gross primary productivity (GPP) constitutes an essential part of net ecosystem CO2 exchange (NEE) between the atmosphere and the forest ecosystems [1,2,3]
There were statistically significant differences (p < 0.01) in the mean monthly vapour pressure deficit (VPD) and soil volumetric water content (SVWC) values between the months of June–September in Bílý Kríz as compared to that in both Rájec and Štítná. These results indicate a drier climate at the dry spruce forest site in Rájec as compared to a humid climate in Bílý Kríz
This study used two sets of regression models (SMLR and Random forest (RF)) to identify the main environmental variables influencing the ratio between actual and optimal gross primary productivity across central European beech and Norway spruce species growing under contrasting climatic conditions
Summary
Gross primary productivity (GPP) constitutes an essential part of net ecosystem CO2 exchange (NEE) between the atmosphere and the forest ecosystems [1,2,3]. Seasonal trends of GPP demonstrate the forest response to variations in environmental factors These forest responses vary from site to site depending on the species, age, local climate, and additional factors. Higher temperatures (beyond the optimum level) may induce varying physiological and biochemical responses to heat stress at different forest ecosystems based on the acclimation processes of specific plant species to changes in their environment [13,14]. Such contrasting GPP responses (due to the shift in plant functional traits) of specific species to different environmental stress conditions at different sites are poorly understood [15]. As multiple climatic factors affect forest carbon uptake, there is the need to identify near-optimal environmental conditions for different species, under which GPP can reach maximal theoretically attainable values termed potential GPP (GPPpot )
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