Species Mixing Effects on Height–Diameter and Basal Area Increment Models for Scots Pine and Maritime Pine

José Riofrío,Felipe Bravo,Douglas Maguire,Miren Del Río

doi:10.3390/f10030249

Abstract

Models that incorporate known species-mixing effects on tree growth are essential tools to properly design silvicultural guidelines for mixed-species stands. Here, we developed generalized height–diameter (h-d) and basal area growth models for mixed stands of two main forest species in Spain: Scots pine (Pinus sylvestris L.) and Maritime pine (Pinus pinaster Ait.). Mixed-effects models were fitted from plot measurement and tree rings data from 726 Scots pine and 693 Maritime pine trees from mixed and pure stands in the Northern Iberian Range in Spain, with the primary objective of representing interactions between the species where they are interspersed in mixtures of varying proportions. An independent dataset was used to test the performance of the h-d models against models previously fitted for monospecific stands of both species. Basal area increment models were evaluated using a 10-fold block cross-validation procedure. We found that species mixing had contrasting effects on the species in both models. In h-d models, the species-mixing proportion determined the effect of species interactions. Basal area growth models showed that interspecific competition was influential only for Maritime pine; however, these effects differed depending on the mode of competition. For Scots pine, tree growth was not restricted by interspecies competition. The combination of mixed-effect models and the inclusion of parameters expressing species-mixing enhanced estimates of tree height and basal area growth compared with the available models previously developed for pure stands. Although the species-mixing effects were successfully represented in the fitted models, additional model components for accurately simulating the stand dynamics of mixtures with Scots pine and Maritime pine and other species mixtures require similar model refinements. Upon the completion of analyses required for these model refinements, the degree of improvement in simulating growth in species mixtures, including the effects of different management options, can be evaluated.

Highlights

Despite evidence that many mixed-species forests offer greater potential to supply ecological and socio-economical goods and services than monospecific forests [1], quantitative silviculturalForests 2019, 10, 249; doi:10.3390/f10030249 www.mdpi.com/journal/forestsForests 2019, 10, 249 guidelines that facilitate the efficient management of mixed-species stands are still largely based on research information from pure even-aged stands [2]
Results support the general conclusion that the stand and species productivity [7], size distribution and other stand structural attributes [8], tree allometry [9,10], mortality [11], and carrying capacity [12] of mixed-species stands are different from the weighted means of the respective pure stand
This study aimed to develop generalized height–diameter (h-d) and basal area growth models for individual trees in mixed-species stands of Maritime pine and Scots pine in a manner that tested the effect of potential species interactions or species composition on model form, model parameters, and associated growth estimates

Summary

Introduction

Despite evidence that many mixed-species forests offer greater potential to supply ecological and socio-economical goods and services than monospecific forests [1], quantitative silviculturalForests 2019, 10, 249; doi:10.3390/f10030249 www.mdpi.com/journal/forestsForests 2019, 10, 249 guidelines that facilitate the efficient management of mixed-species stands are still largely based on research information from pure even-aged stands [2]. Results support the general conclusion that the stand and species productivity [7], size distribution and other stand structural attributes [8], tree allometry [9,10], mortality [11], and carrying capacity [12] of mixed-species stands are different from the weighted means of the respective pure stand These effects are not generalizable, because they do not necessarily apply to all combinations of tree species [13], or even to specific species mixes with differences in stand structure [14] or under different site conditions [15]. The development of models that consider known species-mixing effects on tree and stand productivity, tree allometry, mortality, and maximum density is essential to properly design the initial composition and subsequent spatial arrangement and management of mixed-species stands [6]

Objectives

Methods

Results

Discussion

Conclusion