A remote-sensing approach to studying drought resistance in Swedish old-growth and production forests

إنجليزي. Boreal forest ecosystems are predicted to experience hotter and drier summers due to climate change, leading to more frequently reduced soil water availability and an increased risk of droughts, as was already the case in Northern Europe during the severe summer drought of 2018. Prolonged water stress in forest ecosystems can lead to impacts ranging from reduced photosynthesis to forest dieback. However, it is currently unknown whether characteristics associated with undisturbed old-growth forests, such as a natural age structure, and the presence of understory and dead wood, are linked to increased drought resistance. Therefore, this study investigated 2018 drought impacts of over 300 Swedish forest pairs through a comparative analysis of spatially proximate old-growth and production forest stands, and their associated Landsat EVI2 Z-scores. Prior to this, several candidate satellite vegetation indices (Landsat EVI2, MODIS EVI2, MODIS NDWI, MODIS CCI) were evaluated on their ability to capture anomalies in gross primary production (GPP) measured at different Eddy-Covariance ecosystem monitoring stations. The candidate indices were also compared at the landscape level across a highly heterogenous ‘case study’ area in northern Sweden. While neither of the indices consistently captured GPP anomalies at the tested stations, the increasingly degraded spatial resolution associated with the MODIS vegetation indices proved to cause severe smoothing effects when investigating impacts across a topographically varied landscape; in contrast, Landsat EVI2 captured negative anomalies along drier slopes, while moister valleys were associated with positive Z-scores. It was therefore chosen as the most suitable index, and for further analysis, forested areas were separated into five distinct topographical soil moisture classes. Despite their older age, old-growth forests were on average associated with significantly higher Z-scores than their surrounding production forests. Overall Z-scores increased with increasing soil moisture, whereas relative differences (old-growth – production Z-score per forest pair) decreased. When normalized by the frequency distribution of the soil moisture classes, the mean difference amounted to +0.13. Furthermore, variability in drought impacts was higher for old-growth than production forests, and consequently, both the stands associated with the highest positive and negative anomalies were old-growth. The largest negative Z-score differences can be reconciled by the fact that those old-growth forests were on significantly steeper slopes and had a higher fraction of purely deciduous stands. Apart from that, however, relative differences in drought impacts were not associated with a clear spatial pattern, and no general relationship was found with drought severity, nor relative differences in age, elevation, and slope. Differences were therefore likely caused by other factors not considered.

إنجليزي. During the hot and dry summer of 2018, Sweden experienced one of its worst droughts in recent history, but consequences for forest ecosystems are still unexplored. When a forest experiences long-term water stress, impacts range from reduced photosynthesis to permanent tree damage or even death. While most of Sweden’s surface is covered by forests, most of them are managed for wood production. In contrast, very few undisturbed ‘old-growth’ forests are left like ‘islands’ within those production forests. It is currently unknown whether differences between these two types of forests (e.g. forest age and structure and the presence of dead wood) could lead to differences in drought resistance, that is, a forest’s ability to withstand a drought during its occurrence. Therefore, this study investigated the 2018 drought impacts by analyzing and comparing over 300 old-growth forest stands with their surrounding production forests. Droughts not only affect chlorophyll absorption, but also canopy water content and carotenoid pigment pools, and all three can be studied by calculating different vegetation indices (VIs) based on satellite data. Before choosing the most suitable metric to quantify drought resistance, four different VIs based on NASA’s Landsat and MODIS satellite sensors were evaluated on their ability to capture anomalies in net photosynthesis (expressed as ‘GPP’) as measured at different monitoring stations. Due to their different spatial resolutions, the candidate VIs were also compared across a topographically varied test area. This was done by computing per-pixel Z-scores, which express 2018 summer anomalies from the long-term mean. While neither of the VIs consistently captured GPP anomalies, the coarser spatial resolution associated with the MODIS vegetation indices (250m – 1km) proved to cause severe smoothing effects across the test area. In contrast, Landsat EVI2 Z-scores, an index expressing anomalies in chlorophyll absorption at a resolution of 30m, captured negative anomalies along dry slops, while moisture valleys were associated with positive anomalies, highlighting the importance of working with high-resolution data. Landsat EVI2 was therefore chosen as the most suitable index. Despite their older age, old-growth forests were on average associated with significantly higher EVI2 Z-scores than their surrounding production forests, suggesting a higher drought resistance. Furthermore, the difference between old-growth and production forests became smaller within the wetter soil moisture classes, and old-growth forests also showed a higher variability in impacts, with both the most negatively and most positively impacted forests being old-growth. The largest negative differences in drought resistance between old-growth and production forests can be linked to their differences in slope and fraction of tree types (more deciduous trees). Apart from that, however, no general relationship was found to other explanatory variables, such as relative differences in age and elevation. Therefore, the higher average drought resistance of old-growth forests is likely caused by other factors not studied here.