Niko Kulha discusses his recent article: Competition-induced tree mortality across Europe is driven by shade tolerance, proportion of conspecifics and drought.
Climate change is altering the conditions and context for tree growth in myriad ways, including increasing forest stand density and tree biomass. These changes in tree neighborhoods suggest increased tree-tree competition, which may increase mortality, especially among trees with weak competitive positions, such as saplings in closed canopy stands. This could mean that the baseline of background mortality is changing across biomes. However, anticipating the consequences of increased competition is difficult because competition-induced mortality may depend on the occurrence of extreme climatic events such as drought, but also on stand composition, as tree species differ in their ability to compete and tolerate competition.
🌲 What we did
Our study examines the influence of competition type, competitive ability – quantified as species-level differences in shade tolerance – and drought events on the probability of competition-induced tree mortality using a large dataset derived from the Finnish, French and German National Forest Inventories (NFI). We reasoned that data with a latitudinal gradient from the Mediterranean to the Arctic would provide results that could be generalised to different stands and environmental conditions, thus allowing us to assess the general patterns of competition-induced tree mortality in different environmental contexts. As expected, the harmonisation of multinational NFI data, collected with widely differing protocols in different countries (Fig. 1), the formulation of the analytical approach, and various tests of the robustness of the results took a considerable amount of time. In the end, this exercise has not only shed light on how competition-induced mortality occurs, but also on the usability of different NFI data in a joint continental-scale analysis.
🔎 What we found
We found that competition was the main driver of background mortality, with its relative importance exceeding that of tree size and climate. Furthermore, individuals of the same species had a greater competitive effect than heterospecifics, with the probability of competition-induced mortality increasing more rapidly with increasing competition in monospecific than in mixed stands. While increasing shade tolerance increased the survival of a tree, increasing the shade tolerance of its neighbours had the opposite effect, most likely due to increased shading effect as shade tolerance increased. Drought anomalies increased the competition effect, resulting in a higher mortality probability for the most suppressed trees.
Middle: A view from the Belledonne Massif in the French Alps, included in our study area. Photo by Julien Barrere.
Right: Ongoing competition in a disturbance-induced canopy opening in the hiking area of Evo, southern Finland. Photo by Katharina Albrich.
⭐️ Lessons learnt
Our results indicate that increases in stand density and tree biomass do not linearly increase the probability of competition-induced tree mortality. Rather, the rate of change in competition-induced tree mortality probability depends on the composition of a stand, who is competing with whom, on the ability of competitors to shade their neighbours and tolerate shading, and on the occurrence of droughts. Our results allow us to rank the drivers of background mortality – including competition – in the order of importance and to show the relative contribution of the level and type of competition to background mortality probability. These results are valuable tools for better predicting tree mortality in the face of climate change.
Our work also demonstrates the feasibility (and challenges) of using NFI data, often collected to monitor changes in the growing stock, to analyse tree mortality. Considering that many administrations operatively collect and openly share NFI data that may already go back a century, multinational NFI data can be a real treasure trove for addressing various questions of forest dynamics at a variety of spatial and temporal scales. However, when planning such an analysis, one needs to prepare for rigorous harmonising and testing for robustness. In my opinion, collaborating with researchers and professionals who are familiar with the NFI data in their region is a crucial step in avoiding the most common pitfalls.
Niko Kulha, Natural Resources Institute Finland.
Read the full article online: Competition-induced tree mortality across Europe is driven by shade tolerance, proportion of conspecifics and drought.
