The potential of alternative conifers to replace larch species in Ireland, in response to the threat of Phytophthora ramorum.

Forest ecosystems are facing many challenges in the wake of recent pest and disease outbreaks, coupled with uncertain future climate conditions. A particular challenge emerges from the recent outbreak of Phytophthora ramorum identified in Japanese larch (Larix kaempferi) in 2010. Its subsequent spread has caused widespread damage to Japanese larch stands and has resulted in the Japanese, European (Larix decidua) and hybrid (Larix × eurolepis) larches no longer being grant-aided in the Irish afforestation programme in Ireland. Over 20% of forest stands contain some quantity of larch, with a total area of 32,057 ha. Japanese larch is the predominant species with 27,859 ha, 86% occurring as mixed stands and 79% in mixture with Sitka spruce (Picea sitchensis). The objective of the study was to examine the range of alternative conifer species that may be suitable to replace larch which potentially have similar or higher levels of productivity, acceptable timber properties, while affording reduced levels of risk from pest/disease outbreak. To assess productivity, yield class of a range of species in mixture with larch across a gradient of soil types was assessed. Analysis of this data indicated that Sitka spruce, western red cedar (Thuja plicata), grand fir (Abies grandis), western hemlock (Tsuga heterophylla), Norway spruce (Picea abies), European silver fir (Abies alba), noble fir (Abies procera) and Douglas fir (Pseudotsuga menziesii) provide suitable alternatives, showing higher levels of productivity across a range of soil types. A strong positive correlation was found between the yield classes of (a) Japanese larch and western hemlock (r = 0.70), (b) hybrid larch and Douglas fir (r = 0.73) and (c) European larch and Sitka spruce (r = 0.61) growing on the same sites. Regression equations were developed between the site yields of Japanese, European and hybrid larches and those of alternative species, as a useful tool to predict growth performance of potential alternative species across a range of soil types where larch is currently growing. The predictive power varied for different species pairings (r2 of 0.24 to 0.87) with the strongest relationships between the yields of Japanese larch and Norway spruce on basin peat (r2 = 0.71) and Japanese larch and Douglas fir on podzol soils (r2 = 0.76; y = 1.2632x + 2.6316). Given the significance of Sitka spruce/Japanese larch mixtures in Irish forestry, future research should focus on the potential for mixtures combining Sitka spruce and alternative Pacific conifers (e.g. Douglas fir, grand fir, western hemlock and western red cedar) that may enhance the resilience of and maintain productivity.