Species Mixing Proportion and Aridity Influence in the Height–Diameter Relationship for Different Species Mixtures in Mediterranean Forests

Jorge Aldea,Diego Rodríguez de Prado,Felipe Bravo,James McDermott,Jose Riofrío,Celia Herrero de Aza

doi:10.3390/f13010119

Abstract

Estimating tree height is essential for modelling and managing both pure and mixed forest stands. Although height–diameter (H–D) relationships have been traditionally fitted for pure stands, attention must be paid when analyzing this relationship behavior in stands composed of more than one species. The present context of global change makes also necessary to analyze how this relationship is influenced by climate conditions. This study tends to cope these gaps, by fitting new H–D models for 13 different Mediterranean species in mixed forest stands under different mixing proportions along an aridity gradient in Spain. Using Spanish National Forest Inventory data, a total of 14 height–diameter equations were initially fitted in order to select the best base models for each pair species-mixture. Then, the best models were expanded including species proportion by area (mi) and the De Martonne Aridity Index (M). A general trend was found for coniferous species, with taller trees for the same diameter size in pure than in mixed stands, being this trend inverse for broadleaved species. Regarding aridity influence on H–D relationships, humid conditions seem to beneficiate tree height for almost all the analyzed species and species mixtures. These results may have a relevant importance for Mediterranean coppice stands, suggesting that introducing conifers in broadleaves forests could enhance height for coppice species. However, this practice only should be carried out in places with a low probability of drought. Models presented in our study can be used to predict height both in different pure and mixed forests at different spatio-temporal scales to take better sustainable management decisions under future climate change scenarios.

Highlights

Mixed stands have been evidenced as a smart strategy to adapt forest to climate change since they have been shown to provide positive productive and ecosystem service outcomes compared with monocultures [1,2,3,4,5]
For all pairs of species mixtures, expanded H–D models significantly improved the goodness of fit, in terms of Akaike Information criterion (AIC) and root mean squared error (RMSE), compared with the base H–D models
The highest RMSE values were found for Pinus sylvestris, Pinus uncinata, Quercus petraea, Quercus pyrenaica and Quercus robur with values close to 1.5 m

Summary

Introduction

Mixed stands have been evidenced as a smart strategy to adapt forest to climate change since they have been shown to provide positive productive and ecosystem service outcomes compared with monocultures [1,2,3,4,5]. Development of models that consider known species-mixing effects on tree and stand productivity and the potential to improve the resiliency of forestry under expected climate change is essential to properly design the initial composition and subsequent spatial arrangement and management for mixed-species stands [11]. In this context, estimating diameter at breast height and total tree height is fundamental to both developing and applying many growth and yield models [12,13]. Increasing the accuracy of tree height estimates in mixed stands has important implications, because differences in tree morphology directly affect crown competition, stem volume, biomass production, mechanical stability and wood quality predictions [37,42]

Methods

Results

Conclusion