Epsilon Archive for Student Projects

Abstract

Drought as a stressor has a strong influence on the tree growth, leading to reduced growth and die-backs. Possible ways are researched to mitigate this impact on trees; mixed species forests are often discussed in this context. Existing research points out a possible positive effect on the growth under drought, based on reduced competition and facilitation. However, differences between the effect, depending on the combined species seem to exist. Pine and oak species are often found coexisting, especially as well in dry climates, which suggests the question if complementarity might play a role. Research of the two genera showed often positive but varying effects of mixture, dbh1.3, I therefore investigate, if this applies as well for two common species of the US American Southwest: Pinus ponderosa and Quercus gambelii. Due to the regular droughts in the region, I expect a strong positive impact on the tree growth by mixture. Based on analysed increment cores I found a stronger, but also more varying growth of Pinus ponderosa compared to Quercus gambelii. The high correlation of growth patterns with the drought index SPEI, shows the dominating growth limitation by the dry climate in the region. The use of a generalized linear model gave more insight in the influence of species mixture as well as the diameter and competition on the growth. Though statistically not significant the results suggest a clear negative influence of increasing tree diameters but higher growth under drought for Pinus ponderosa with a higher share of Quercus gambelii in their neighbourhood, compared to trees growing in a monospecific mixture. The clear differences between the increment as well as its variability between the two species, is likely caused by the different adaptions to drought. I found possible explanations for the positive mixing effect for Pinus ponderosa in the decreased competition above and below ground, facilitation by Quercus gambelii through hydraulic redistribution of deep soil water as well as decreased evaporation due to a protecting microclimate under the canopy of Quercus gambelii. The results point out the possibilities of species mixtures in mitigating the increasingly difficult growth conditions due to dry climate. Especially for the two researched species a positive effect can be expected, which suggests the use of its potential in the forest management of the Southwestern US.