Productivity of old-growth beech forests in the Mediterranean Basin was measured by average stem basal area increment (BAI) of dominant trees at two mountain sites in the Italian Apennines. Both forests could be ascribed to the old-growth stage, but they differed markedly with regard to elevation (1000 vs. 1725m a.s.l.), soil parent material (volcanic vs. calcareous), mean tree age (less than 200 years vs. 300 years), and stand structure (secondary old-growth vs. primary old-growth forest). Drought at the two sites was quantified by the self-calibrated Palmer Moisture Anomaly Index (Z-index), and by the self-calibrating Palmer Drought Severity Index (PDSI) for summer (June through August) and the growing season (May through September). Dendroclimatological analyses revealed a moisture limitation of beech BAI at interannual (water availability measured by Z-index) and decadal scales (water availability measured by PDSI). Both BAI and water availability increased from 1950 to 1970, and decreased afterwards. Trees were grouped according to their BAI trends in auxological groups (growth-type chronologies), which confirmed that growth of most trees at both sites declined in recent decades, in agreement with increased drought. Because BAI is not expected to decrease without an external forcing, the patterns we uncovered suggest that long-term drought stress has reduced the productivity of beech forests in the central Apennines, in agreement with similar trends identified in other Mediterranean mountains, but opposite to growth trends reported for many forests in central Europe.

/pdf/drought-driven-growth-reduction-in-old-beech-fagus-sylvatica-3z65z6yqde.pdf

Drought‐driven growth reduction in old beech (Fagus sylvatica L.) forests of the central Apennines, Italy

Summary
1.Mixed species forests can often be more productive and deliver higher levels of ecosystem services and functions than monocultures. However, complementarity effects for any given tree species are difficult to generalize because they can vary greatly along gradients of climatic conditions and resource availability. Identifying the conditions where species diversity can positively influence productivity is crucial. To date, few studies have examined how growth complementarity across species and mixture types is modulated by stand and environmental factors, and fewer have considered more than one or two factors.

2.We investigated how complementarity effects for several major central European tree species change with climatic and edaphic conditions, and with stand structural characteristics, including species composition. We used data from the Swiss National Forest Inventory, which is based on 3231 plots of pure and mixed stands (19 mixture types) across a broad environmental gradient, to test (i) how mixing effects change depending on the identity of the admixed species and (ii) if complementarity consistently increases when environmental conditions become harsher.

3.The magnitude, whether positive or negative, of complementarity increased with increasing stand density and stand developmental stage, but no general pattern could be identified across mixture types. Complementarity for many species increased as drought intensity and temperature increased, but not for all species and mixture types. While soil conditions, nitrogen and site topography influenced complementarity for many species, there was no general pattern (increases and decreases were observed).

4.Synthesis. Our study indicates that complementarity varies strongly with stand density and stand development as well as with topographic, climatic and soil conditions. This emphasizes the need to account for site-dependent conditions when exploring mixture effects in relation to forest productivity. We found that under certain conditions (i.e., increasing drought, higher temperature), mixed forests can promote individual-tree growth in central European temperate forests. However, careful assessments depending on the species composing the stands are required under changing resource availability as well as under different levels of stand density and development.

This article is protected by copyright. All rights reserved.

https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1111/1365-2745.12846

Multiple factors modulate tree growth complementarity in Central European mixed forests

The impact of windthrow on carbon sequestration in Switzerland: a model-based assessment

Forest ecosystem management heads towards the use of partial cuttings. However, the wide variation in growth response of residual trees remains unexplained, preventing a suitable prediction of forest productivity. The aim of the study was to assess individual growth and identify the driving factors involved in the responses of residual trees. Six study blocks in even-aged black spruce [Picea mariana (Mill.) B.S.P.] stands of the eastern Canadian boreal forest were submitted to experimental shelterwood and seed-tree treatments. Individual-tree models were applied to 1039 trees to analyze their patterns of radial growth during the 10 years after partial cutting by using the nonlinear Schnute function on tree-ring series. The trees exhibited different growth patterns. A sigmoid growth was detected in 32% of trees, mainly in control plots of older stands. Forty-seven percent of trees located in the interior of residual strips showed an S-shape, which was influenced by stand mortality, harvested intensity and dominant height. Individuals showing an exponential pattern produced the greatest radial growth after cutting and were edge trees of younger stands with higher dominant height. A steady growth decline was observed in 4% of trees, represented by the individuals suppressed and insensitive to the treatment. The analyses demonstrated that individual nonlinear models are able to assess the variability in growth within the stand and the factors involved in the occurrence of the different growth patterns, thus improving understanding of the tree responses to partial cutting. This new approach can sustain forest management strategies by defining the best conditions to optimize the growth yield of residual trees.

/pdf/understanding-tree-growth-responses-after-partial-cuttings-a-9n6kf7kavk.pdf

Understanding tree growth responses after partial cuttings: A new approach.

We used data from representatively sampled trees to identify key drivers of tree growth for central European tree species. Nonlinear mixed models were fitted to individual-tree basal area increments (BAI) from the Swiss national forest inventory. Data from 1983 to 2006 were used for model fitting and data from 2009 to 2013 for model evaluation. We considered 23 potential explanatory variables specifying individual-tree characteristics, site and stand conditions, management, climate, and nitrogen deposition. Model selection was processed separately for Picea abies, Abies alba, Pinus sp., Larix sp., other conifers, Fagus sylvatica, Quercus sp., Fraxinus sp./Acer sp., and other broadleaves. The selected models explained 56–70% of the BAI variance in the model fitting dataset and 21–64% in the evaluation dataset. While some variables were relevant for all species, the combination of further variables differed among the species, reflecting their physiological properties. In general, BAI was positively related to DBH and temperature and negatively related to basal area of larger trees, stand density, mean DBH of the 100 thickest trees per ha, slope, and soil pH. For most species, harvesting had a positive effect on BAI. In general, nitrogen deposition was positively related to BAI, except for spruce and fir, for which the inverse effect was found. Increasing drought reduced BAI for most species, except for pine and oak. These BAI models incorporate many influencing factors while representing large spatial extents, making them useful for both nationwide scenario analyses and deepening the understanding of the main drivers modulating tree growth throughout central Europe.

Predicting individual-tree growth of central European tree species as a function of site, stand, management, nutrient, and climate effects

Evaluation of the growth function of an empirical forest scenario model

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Evaluation of the growth function of an empirical forest scenario model