Effects of microclimatic fluctuations on a subalpine Larix decidua Mill. forest: implications for water use and gas exchanges

Climate extremes, such as drought and heat waves, were predicted to become more frequent in the future, due to water cycle modifications and increasing temperatures. Drought and heat waves have the potential to alter forest function, that plays a fundamental role in the regulation of the atmospheric CO2 concentration and, consequently, in the stabilization of the global climate. Indeed, climate extremes may limit CO2 fixation by trees, inducing plant stomatal closure to prevent water loss by transpiration, and consequently causing the interruption of carbon assimilation. Therefore, the occurrence of severe climate extremes limiting plant carbon assimilation has the potential to transform forests from carbon sinks to carbon sources, with important consequences for climate, biodiversity and ecosystem services. This study was carried out in a subalpine Larix decidua Mill. forest, located on the western Italian Alps at 2000 m asl. There, we monitored microclimatic conditions (air temperature, precipitation, soil temperature, soil water content, solar radiation and vapor pressure deficit) with a meteorological station, and H2O and CO2 exchanges (evapotranspiration (ET), gross primary production (GPP) and net ecosystem exchange (NEE)) using the eddy covariance technique, from 2013 to 2017. Moreover, we measured the sap flow (SF), using the thermal dissipation method, and the stem growth, using dendrometers, of three larch individuals of the forest from 2015 to 2017. Finally, stem radial increment (SRI) values were used to calculate the tree water deficit (TWD). Based on microclimatic data, we identified 2015 and 2017 as ‘anomalous’ years, due to particularly high air temperatures and scarce precipitation. Furthermore, these two years showed different microclimatic conditions, being 2015 very hot, but not as dry as 2017. We compared anomalous years with all the other years (i.e. 2013, 2014 and 2016), that were grouped as ‘normal’ years and averaged, in order to investigate the larch forest responses to the climatic fluctuations occurred during the study. The results about ET, GPP and NEE showed that ET and, consequently, carbon assimilation were not negatively affected by climate extremes occurred in 2015 and 2017. On the contrary, we observed an increase of the total annual GPP and NEE, due to a shift forward of the growing season onset, as the consequence of higher spring temperature than normal years. Larch SF data did not show any decrease during the anomalous years, confirming that L. decidua was not negatively affected by climate extremes at the study site. Nevertheless, the results about TWD and SRI showed that during late summer 2017, the driest year, soil water content became limiting for larches, which maintained their SF at the expense of their own water content. Finally, we observed a higher and a lower stem growth respectively in 2015 and 2017, compared to normal years. Since we did not found pronounced differences in the total annual carbon assimilation between the two anomalous years, we suggest that a change in carbon allocation occurred in 2017. This study highlighted the importance of larch forests for their ability to counteract the effects of climatic fluctuations, thanks to the response of L. decidua, which was able to maintain its optimal SF regardless of high temperatures and drought. On the other hand, we found signs of increasing TWD, which requires further investigations to improve the predictions of future scenarios.