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Tree generation replacement in a European natural forest – unravelling the role of canopy gaps using a spatially explicit approach

Periodic Reporting for period 1 - ForestRege (Tree generation replacement in a European natural forest– unravelling the role of canopy gaps using a spatially explicit approach)

Reporting period: 2016-05-01 to 2018-04-30

Replacement of successive tree generations is a fundamental process which shapes the structure, dynamics and functioning of forest ecosystems. To accurately predict the course of generation replacement in natural forests, the project produced an explicit and comprehensive evaluation of the temporal and spatial patterns of tree species establishment. The general objective of this project was to achieve a holistic grasp of the long-term dynamics of tree generations in a well-preserved natural forest of Europe.
Natural forest regeneration was investigated in an exceptionally well-preserved area of subalpine spruce forest on Mt. Babia Góra (Western Carpathians, Poland), which has retained its nearly primeval character. Recent extensions of spatial point pattern analysis (SPPA) were used to determine the spatial patterns of tree species regeneration in a natural subalpine forest. SPPA deals with statistical analysis of mapped point patterns (e.g. trees), which comprise the coordinates, as well as additional ecological features (e.g. age, size, species). Point pattern analysis can help identify different proximate mechanisms of tree regeneration and quantify the overall spatial pattern and potential associations of saplings and canopy gaps at a range of spatial scales.
The project’s results are of great significance for European forest preservation and for reconstruction of forest cover across the continent. Most studies of forest regeneration are short-term and focus on the present status of the young tree generation. More comprehensive knowledge requires long-term data, as forest ecosystem dynamics proceed on the scale of decades and even centuries. Moreover, forest studies in Europe are most often conducted in forests highly altered by human activity. In this project, the long-term dataset comes from a European forest of nearly primeval character.
The problem addressed in the project is increasingly important in the context of contemporary global changes. For example, climate change can increase the frequency of disturbances such as windthrows and insect outbreaks, and in consequence can initiate replacement of tree stand generations. To understand the implications of such changes, deep knowledge of natural forest dynamics is needed. To predict the natural stages following a disturbance event, the conditions that permit a new generation of trees to establish must be known.
During the project, the data were collected in Babia Góra National Park. In the field, the current locations of all rowan and spruce saplings and living trees were mapped in a permanent plot located in a subalpine spruce forest (14.4 ha), and their size was measured (approx. 7200 trees in total). The spatial data of saplings and trees were analyzed using the techniques of marked point pattern analysis with Programita software.

Main results:
(1) Two models of the relationship between stand dynamics and stand structure were proposed for Western Carpathian subalpine spruce stands. In the first one, severe stand-replacing disturbances hit forests over a large area, resulting in low variability of the age and size of trees recruited synchronously over extensive areas. In the second model, gap-phase dynamics produce spruce stands composed of distinct age cohorts of trees that originated over extensive areas.
(2) Studying the changes in the spatial distribution of adult trees that died attacked by bark beetles (Ips typographus) in this Carpathian subalpine spruce forest, we found that the density of snags originating during 10 years was positively correlated spatially with the density of snags from the previous decade, indicating that trees attacked by I. typographus appeared mostly in the same places during two consecutive decades. This suggests that existing gaps expand more often than new ones are created. (3) Considering the microsite-specific survival of spruce saplings, we found two factors relevant to the survival of young spruces: the share of canopy gap area in their vicinity, and substrate type. The highest 20-year rate of sapling survival was recorded on windthrow mounds, and the lowest on decaying logs. Although coarse woody debris is more favorable for the establishment of spruces, they survive better on the forest floor, on windthrow mounds in particular.
(4) Herbivore pressure on palatable young rowan trees can be limited by coarse woody debris and hiking trails. In subalpine spruce forest, which is under strong herbivore pressure, rowans can leave the seedling bank and succeed to higher forest strata in places that have an abundance of logs lying on the forest floor and in places close to hiking trails. The presence of logs seems to be a sufficient barrier to deer activity, protecting the young trees; also, deer are likely to be startled near trails reducing their pressure on rowan sapling growth.
(5) We found negative density dependence of mortality among spruce saplings: that is, conspecific facilitation among niche competitors. Tree debarking caused by antler rubbing by red deer (Cervus elaphus) was the main identified cause of sapling mortality and showed a strong density- and distance-dependent pattern. The deer killed young trees, choosing isolated trees for antler rubbing more frequently than they chose trees growing in aggregations.
(6) Rare events of massive plant reproductive investment lead to long-term density-dependent reproductive success. Strong negative density-dependent effects on rowan reproduction were found in only a few of the 16 studied years. Such processes operated mainly during large-crop years and at relatively small spatial scales. These years of large reproductive investment had a decisive influence on the overall spatial pattern of the fruit crop.
The results of the project were published in top-ranking international journals covered by the ISI Science Citation Index (Journal of Ecology, Forest Ecology and Management). Besides appearing in academic journals, the research results were disseminated through publication in a popular-science monograph, and were presented at international and national conferences addressed to scientists (Symposium of Polish Botanical Society, 2016, Lublin, Poland; IAVS Annual Symposium, Palermo, Italy). Results were presented at a conference addressed to forest service professionals (8° Congreso Florestal Nacional – Floresta em Português: Raízes do Futuro”, 2017, Viana do Castelo, Portugal), and at monthly public seminars organized by CEABN in collaboration with the Forestry Department of the ISA. Finally, the project findings are accessible online.
The results of the project enlarge the body of knowledge about natural ecological processes governing forest ecosystems. The findings are useful mainly for nature conservation but also for forest services. Nature conservation typically concentrates on static systems, and any abrupt events that change a known static configuration are worrying to the staff of nature protection services. Disturbances, however, are a natural and inherent part of the natural forest ecosystem cycle. Spontaneous regeneration occurs as part of this cycle and is inseparably associated with the disturbance regime. An understanding of the relationship between tree species regeneration and canopy gaps will allow conservation guidelines to be better applied to forest ecosystems and will also permit the costs of nature protection to be managed more effectively.