Abstract
Drought stress causes a reduction in tree growth and forest productivity, which could be aggravated by climate warming and defoliation due to moth outbreaks. We investigate how European gypsy moth (Lymantria dispar dispar L., Lepidoptera: Erebidae) outbreak and related climate conditions affected growth and wood features in host and non-host tree species in north-western Spain. There, radiata pine (Pinus radiata D. Don) plantations and chestnut (Castanea sativa Mill.) stands were defoliated by the moth larvae, whereas Maritime pine (Pinus pinaster Ait.) was not defoliated. The gypsy moth outbreak peaked in 2012 and 2013, and it was preceded by very warm spring conditions in 2011 and a dry-warm 2011–2012 winter. Using dendrochronology we compared growth responses to climate and defoliation of host species (radiata pine, chestnut) with the non-host species (Maritime pine). We also analyzed wood density derived from X-ray densitometry in defoliated and non-defoliated trees of radiata pine. We aimed to: (i) disentangle the relative effects of defoliation and climate stress on radial growth, and (ii) characterize defoliated trees of radiata pine according to their wood features (ring-width, maximum and minimum density). Radial growth during the outbreak (2012–2013) decreased on average 74% in defoliated (>50% of leaf area removed) trees of radiata pine, 43% in defoliated trees of chestnut, and 4% in non-defoliated trees of Maritime pine. After applying a BACI (Before-After-Control-Impact) type analysis, we concluded that the difference in the pattern of radial growth before and during the defoliation event was more likely due to the differences in climate between these two periods. Radiata pines produced abundant latewood intra-annual density fluctuations in 2006 and 2009 in response to wet summer conditions, suggesting a high climatic responsiveness. Minimum wood density was lower in defoliated than in non-defoliated trees of radiata pine prior to the outbreak, but increased during the outbreak. The pre-outbreak difference in minimum wood density suggests that the trees most affected by the outbreak produced tracheids with wider lumen and were more susceptible to drought stress. Results of this study illustrate (i) that the pattern of radial growth alone may be not a good indicator for reconstructing past defoliation events and (ii) that wood variables are reliable indicators for assessing the susceptibility of radiata pine to defoliation by the gypsy moth.
Highlights
In the Anthropocene, forests face unprecedented pest problems that are outside managers’ range of experience (Liebhold, 2012)
Taking into account that differences in tree radial growth can arise because of different microsite conditions, and the experimental design did no factor out microsite differences, we examined whether spatially autocorrelated factors, such as microsite differences, were present
Abnormal warm and dry conditions in the winter prior to a gypsy moth outbreak reduced radial growth in both host and non-host (Maritime pine) species and might have resulted in the defoliation event occurred in 2012–2013
Summary
In the Anthropocene, forests face unprecedented pest problems that are outside managers’ range of experience (Liebhold, 2012). Rapid land-use changes during the twentieth century have resulted in millions of hectares of pine plantations, in areas in the southern Hemisphere where pines are introduced species (FAO, 2010). These commercial plantations involve biota alterations because they are monocultures of non-native tree species which may face new herbivores (Wingfield et al, 2015). In some cases, these plantations can reduce herbivory relative to their native environments via geographical escape from their natural herbivores. Sometimes the native herbivores can become important pests of non-native tree species (Nuñez et al, 2008)
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