Article Growth Relationships in Silver Fir Stands at Their Lower-Altitude Limit in Romania Gheorghe-Marian Tudoran 1, *, Avram Cics , a 1,2 , Albert Ciceu 1,2 and Alexandru-Claudiu Dobre 1,2   Citation: Tudoran, G.-M.; Cics , a, A.; Ciceu, A.; Dobre, A.-C. Growth Relationships in Silver Fir Stands at Their Lower-Altitude Limit in Romania. Forests 2021, 12, 439. https://doi.org/10.3390/f12040439 Academic Editors: Alessandra De Marco and Timothy A. Martin Received: 20 January 2021 Accepted: 3 April 2021 Published: 5 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Forest Engineering, Forest Management Planning and Terrestrial Measurements, Faculty of Silviculture and Forest Engineering, “Transilvania” University, 1, Ludwig van Beethoven Str., 500123 Bra¸ sov, Romania; cicsa_avram@yahoo.co.uk (A.C.); albert.ciceu@yahoo.ro (A.C.); dobre.alexandruclaudiu@gmail.com (A.-C.D.) 2 Department of Forest Monitoring, “Marin Drăcea” Romanian National Institute for Research and Development in Forestry, 128 Eroilor Blvd., 077190 Voluntari, Romania * Correspondence: tudoran.george@unitbv.ro; Tel.: +40-268-418-600 Abstract: This study presents the biometric relationships among various increments that is useful in both scientific and practical terms for the silvicultural of silver fir. The increments recorded in the biometric characteristics of trees are a faithful indicator of the effect of silvicultural work measures and of environmental conditions. Knowing these increments, and the relationships among them, can contribute to adaptations in silvicultural work on these stands with the purpose of reducing risks generated by environmental factors. We carried an inventory based on tree increment cores. The sample size was determined based on both radial increment and height increment variability of the trees. The sample trees were selected in proportion to their basal area on diameter categories. Current annual height increment (CAI h ) was measured on felled trees from mean tree category. For CAI h we generated models based on the mean tree height. Percentages of the basal area increment and of form-height increment were used to compute the current annual volume increment percentage (PCAI v ). For the mean tree, the CAI h estimated through the used models had a root-mean-square error (RMSE) of 0.8749 and for the current annual volume increment (CAI v ) the RMSE value was 0.1295. In even-aged stands, the mean current volume increment tree is a hypothetical tree that may have the mean basal area of all the trees and the form-height of the stand. Conclusions: The diameter, height, and volume increments of trees are influenced by structural conditions and natural factors. The structures comprising several generations of fir mixed with beech and other deciduous trees, which have been obtained by the natural regeneration of local provenances, are stable and must become management targets. Stable structures are a condition for the sustainable management of stands. Keywords: silver fir; current annual increment; percentage volume increment; basal area; tree diameter; tree form-height 1. Introduction In Romania, the fir (Abies alba Mill.) is frequently found mixed with beech (Fagus sylvatica L.) at altitudes between 700 and 1200 m. Mixed of fir and beech forests are for- mations representative of the lower mountain zone in Romania. Of the coniferous trees, Norway spruce (Picea abies (L.) Karst.) descends to these altitudes sporadically, but heat and reduced precipitation become limiting factors to its lower range; the proportion of spruce increases with altitude. Fir and beech influence climatic factors differently—especially hu- midity, light, and heat—so that, under the shelter they provide, beech seedlings frequently become established under fir, and vice versa. Consequently, the structures of stands are greatly varied in terms of the relative proportions of the two species. The stands structures present a wide range of diameter categories and offer the most favorable conditions for promoting structure of the uneven-aged stand type, which is characteristic of the natural Forests 2021, 12, 439. https://doi.org/10.3390/f12040439 https://www.mdpi.com/journal/forests