Abstract
This paper examines the possible ecological controls on the diameter increment of oriental beech (Fagus orientalis Lipsky) in a high altitude forest in northern Iran. The main objectives of the study are computer-generated abiotic surfaces and associated plot estimates of (i) growing-season-cumulated potential solar radiation, (ii) seasonal air temperature, (iii) topographic wetness index in representing soil water distribution, and (iv) wind velocity generated from the simulation of fluid-flow dynamics in complex terrain. Plot estimates of the tree growth are based on averaged plot measurements of diameter at breast height increment during a growing period of nine years (2003–2012). Biotic variables related to the tree diameter increment involve averaged 2003 tree diameter and basal area measured in individual forest plots. In the modelling data (144 plots), the assemblage of modelled and observed site variables explained 75% of the variance in plot-level diameter increment. In the validation data (32 plots), the degree of explained variance was 77%. Mean tree diameter at breast height showed the strongest correlation with diameter increment, explaining 32% of the variation between-plot, followed by the configuration of topography and re-distribution of surface water (19.5%) and plot basal area (16.9%). On average, localised estimates of solar radiation and wind velocity potentially contribute to about 20% of the control on plot-level mean increment in oriental beech of the area. The results of the genetic programming showed that controlling the stand basal area and tree size by thinning and/or selective harvesting can have a favourable impact on the future distribution of mean diameter in oriental beech.
Highlights
The determination of tree growth in the forest is recognized as one of the most important information for long-term forest management [1] that plays an important role in future forest management decisions [2]
Our results demonstrated the capability of gaussian process (GP) to study the biophysical controls on diameter-growth of Fagus orientalis in northern Iran and to identify the predictors (WIND, Topographic wetness index (TWI), BAL, diameter, and temperature) that contribute the most to growth rate
The current manuscript provides a semi-empirical approach based on GP for analyzing biophysical controls on diameter increment of oriental beech in a subsection of the Hyrcanian forest in northern Iran
Summary
The determination of tree growth in the forest is recognized as one of the most important information for long-term forest management [1] that plays an important role in future forest management decisions [2]. An accurate prediction of tree growth is of great importance in the quality of forest management. Tree growth models are divided into two broad categories: Experimental and process-based models. The first group predicts dimensions such as diameter at breast height (DBH) or volume through direct measurement or modelling, and the second group is based on tree physiology [3,4]. When the trees grow beyond a threshold size, i.e., ~80–90 cm in DBH for oriental beech (Fagus orientalis Lipsky), their annual growths start to decrease as a result of increased hydraulic resistance and lowered transpiration and photosynthetic capacity in larger trees [5]
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