Potocic, N. | Search bioRxiv

bioRxiv2022

Drought effects of annual and long-term temperature and precipitation on mortality risk for 9 common European tree species

Risk factors for natural tree mortality in managed forests, excluding wind and snow induced breakage, fires and thinning, can be difficult to identify due to correlated confounders of long- and short-term weather patterns with tree age. This study quantified the association of annual and long-term 30-year average temperature and precipitation effects on individual tree death across Europe from 2011 to 2020 for European beech, sessile and pedunculate oak, silver birch, black pine, Austrian oak, Scots pine, European hornbeam, and Norway spruce. For each species, logistic regression approaches for predicting annual mortality evaluated the influence of age, exposition and weather effects on individual tree death, while accounting for multi-collinearity of risk factors. For all species except sessile oak, higher 30-year-temperature averages were associated with higher odds of tree mortality. Effect size of other risk factors varied among species, with similar weather associations between Austrian and sessile oak on the one hand, and Scots pine, Norway spruce and pedunculate oak on the other hand. In particular, warmer winters reduced mortality for silver birch, sessile and Austrian oaks, while having the opposite association for the other species. Sessile oak was most robust against drought effects and could serve as an important tree species under climate change scenarios.

plant biology↗

bioRxiv2021

European-wide forest monitoring substantiate the neccessity for a joint conservation strategy to rescue European ash species (Fraxinus spp.)

European ash (Fraxinus excelsior) and narrow-leafed ash (F. angustifolia) are keystone forest tree species in Europe with a broad ecological amplitude and significant economic importance. Besides global warming both species are currently under significant thread by an invasive fungal pathogen that has been progressively spreading throughout the continent for almost three decades. Ash dieback caused by the invasive ascomycete Hymenoscyphus fraxineus is capable of damaging ash trees of all age classes and often leads to the ultimate death of a tree after years of progressively developing crown defoliation. While studies at national and regional level already suggested rapid decline of ash populations as a result of ash dieback, a comprehensive survey at European level with harmonized crown assessment data across countries could shed more light into the population decline from a pan-European perspective and could also pave the way for a new conservation strategy beyond national boarders. Here, we present data from the ICP Forests Level I crown condition monitoring including 27 countries, covering the timespan from 1987-2020. In total, 407 survey plots randomly distributed across these countries were analyzed resulting in >36,000 individual observations. We found a substantial increase in defoliation and mortality over time indicating that crown defoliation has almost doubled during the last three decades. Hotspots of mortality are currently situated in southern Scandinavia and north-eastern Europe, well corresponding to the fact that the disease spread fast from north-east to north-west. Overall survival probability after nearly 30 years of infection has already reached a critical value of 0.51, but with large differences among regions (0.00-0.907). Both a Cox proportional hazard model as well as an Aalen additive regression model strongly suggest that survival of ash is significantly lower in locations with excessive water regime and which experienced more extreme precipitation events during the last two decades. Our results underpin the neccessity for fast governmental acting and joint rescue efforts beyond national boarders since overall mean defoliation will likely reach 50% as early as 2030 as suggested by time series forecasting. We strongly recommend to develop a pan-European conservation strategy before the decline will reach its tipping point resulting into non-reversible loss of diversity in the European forest landscape.

ecology↗