Modelling growth for two-storied stands: Incorporating an effect of shading on the tree growth into growth and yield models

The objective of this paper is to review research into growth and yield modelling for two-storied plantation forests. Difficulty with modelling growth for this kind of forest, compared with pure even-aged forests, is mentioned. Then research in this field is divided into three parts and described. First, growth models and equations which were driven theoretically from biological assumptions and may be available to develop growth models for two-storied stands are reviewed. Secondly, research into methods of controling light levels and the relationship between the growth of trees and light level beneath a canopy are reviewed. Thirdly, growth models developed for two-storied stands, especially two-storied plantation forests, are reviewed. In conclusion, some problems which are still left in this field are pointed out.     

Effects of using certain tree species in forest regeneration on volume growth,timber yield,and carbon stock of boreal forests in Finland under different CMIP5 projections

We studied how the use of certain tree species in forest regeneration affected the volume growth, timber yield, and carbon stock of boreal forests in Finland under the current climate (1981–2010) and recent-generation global climate model (GCM) predictions (i.e., multi-model means and individual GCMs of CMIP5), using the representative concentration pathways RCP4.5 and RCP8.5 over the period 2010–2099. Forest ecosystem model simulations were conducted on upland national forest inventory plots throughout Finland. In a baseline management regime, forest regeneration was performed by planting the same tree species that was dominant before the final cut. In alternative management regimes, either Scots pine, Norway spruce, or silver birch were planted on medium-fertility sites. Other management actions over rotation were done as in a baseline management. Compared to baseline management, an increased planting of birch resulted in relative sense highest increase in the volume growth, timber yield, and carbon stock in forests in the south, especially under severe climate projections (e.g., multi-model mean RCP8.5, and GCMs such as HadGEM2-ES RCP8.5 and GFDL-CM3 RCP8.5). This situation was opposite for Norway spruce. In the north, the volume growth, timber yield, and carbon stock of forests increased the most under severe climate projections (e.g., multi-model mean RCP8.5 and CNRM-CM5 RCP8.5), regardless of tree species preference. The magnitude of the climate change impacts depended largely on the geographical region and the severity of the climate projection. Increasing the cultivation of birch and Scots pine, as opposed to Norway spruce, could be recommended for the south. In the north, all three species could be cultivated, regardless of the severity of climate change.     

Early seedling establishment of two tropical montane cloud forest tree species: The role of native and exotic grasses

Tropical montane cloud forest has been undergoing a drastic reduction because of its widespread conversion to pastures. Once these forests have been cleared exotic grasses are deliberately introduced for forage production. Exotic grass species commonly form monodominant stands and produce more biomass than native grass species, resulting in the inhibition of secondary succession and tree regeneration. The purpose of this study was to assess the effect of native vs. exotic grass species on the early establishment of two native tree seedlings (Mexican alder, Alnus acuminata and Jalapa oak, Quercus xalapensis) on an abandoned farm in central Veracruz, Mexico. Seedling survival and growth were monitored (over 46 weeks) in relation to grass cover and height, and available photosynthetic active radiation (PAR). More seedlings survived in the presence of the native grass Panicum glutinosum than those growing with the exotic grass Cynodon plectostachyus (92% vs. 48%). The causes of seedling mortality varied between species; Q. xalapensis was affected by herbivory by voles but mainly in the exotic grass-dominated stands, whereas A. acuminata seedlings died due to competition with the exotic grass. A. acuminata seedlings increased more in height in the exotic grass-dominated stands (102 ± 7.8 cm) compared to native grass-dominated stands (51 ± 4.7 cm). Grass layer height, cover and available PAR were correlated (Pearson; p < 0.05). In the exotic grass dominated plots, grass layer height was correlated with the relative height growth rates of Q. xalapensis (Pearson; p < 0.05). These results indicate that the exotic grass may be affecting tree regeneration directly (grass competition) and indirectly (higher herbivory). Passive restoration may occur once P. glutinosum dominated pastures are abandoned. However, when C. plectostachyus dominates, introduction of early and mid successional tree seedlings protected against vole damage is needed.     

The effects of species mixture on the growth and yield of mid-rotation mixed stands of Scots pine and silver birch

The effect of tree species mixture on stand volume yield and on tree-species-specific diameter and height growth rates were analysed in managed mixed stands of Scots pine (Pinus sylvestris L.) and silver birch (Betula pendula Ehrn.).Data were obtained from 14 repeatedly measured stands located in Southern Finland on mineral soil sites with varying admixture of Scots pine and silver birch. Statistical analysis was carried out for studying the effect of species mixture on the development of stand characteristics. For the analysis, the plots were categorised into three groups (plot types) according to the species dominance. In order to analyse species-specific growth rates, individual-tree mixed linear growth models for tree diameter and height growth were developed for both tree species.The results clearly show that the yield of the managed mid-rotation, mixed stands was greater for stands dominated by Scots pine than for stands dominated by birch, and the stand volume increment decreased with an increasing proportion of silver birch. Analysis of diameter and height growth by tree species revealed that the main reason for this pattern is the negative impact of birch competition on the growth of pine trees. The increase in diameter of pine was clearly hampered if the proportion of birch was high. An abundance of birch also slightly decreased the growth in height of Scots pine, although the effect was less than on diameter growth. Species mixture did not affect the diameter growth of birch but did have a significant effect on height development. Height growth of birch was considerably greater in pine-dominated stands than in birch-dominated stands. In pine-dominated mixed stands, the height growth of birch was quite close to that of dominant pine trees, and birches can endure in competition with pines for light.The results apply for even-aged and single-storey managed stands, where stocking density and structure are controlled with pre-commercial and commercial thinnings. The results are not applicable to unmanaged mixed stands undergoing self-thinning. This study provides new information on mixed stands from a silvicultural perspective, which can be applied in decisions involving the management of mixed stands.