EFFECT OF SOIL COMPACTION ON TREE GROWTH: A METAANALYSIS.

In recent decades, many papers about the effects of forest operation on soil compaction and thus tree growth and regeneration have been published. Most of them examined one or few compaction degrees, tree species, taxonomic groups or soil types, providing interesting result but without interconnections among studies. In order to fill this gap, our study aims to draw general conclusions concerning the impact of mechanized harvesting and soil compaction on tree growth and regeneration by means of a meta-analysis. We specifically addressed (i) the effects of soil compaction on shoot morphological and physical traits as well as root parameters (ii) the effects of compaction degree, soil type and tree species (iii) the experimental factors which affect the variation in growth responses to compaction. The data of several studies were collected taking into consideration peer-reviewed literature indexed in several databases and published between 1948 and 2017. A cross-disciplinary approach was applied including researches under natural environmental conditions and manipulative experiments. The studies were initially filtered by title and subsequently the abstracts were examined with regard to the possible relevance to the research questions. Additional criteria used for inclusion into the final stage of the meta-analysis were: the presence of the mean value of the response variable (root, shoot or total dry biomass: length, width or root diameter; photosynthetic and leaf traits) before and after soil compaction; a number of replicates available together with their standard deviations or standard errors; individual observations, values obtained from different cultivars/clones within a plant species, different soil compaction treatments and measurements made on different dates in the same experiment, to be statistically independent. Only the papers reporting a metric to quantify soil compaction were included. At the end of data collection, 58 articles with all the relevant details were identified and included in the meta-analysis. Our results suggest that compaction affects seedling growth by an overall decrease of shoot and root growth with increasing levels of soil compaction; the effect resulted higher in loamy soil than in sandy soil, and significant on main root length and total and roots dry biomass. On the contrary, significant differences did not occurred comparing morphological characters of conifers and hardwoods species. Compaction significantly reduced net photosynthesis, did not affect leaf nitrogen content (as a proxy of nutrient uptake from roots) but decreased leaf water potential (as a proxy of water uptake from roots). Therefore, the reduction of water availability was a primary cause for the decline of photosynthesis. Interestingly, those negative impacts of soil compaction on leaf physiology occurred in loamy soils rather than in sandy soils