Enrique Jiménez

Effectiveness of three post-fire treatments at reducing soil erosion in Galicia (NW Spain)

This study assessed the effectiveness of different methods of reducing soil erosion after a severe wildfire in Galicia (NW Spain). The treatments compared were: straw mulch (2.5 Mg ha–1), wood-chip mulch (4 Mg ha–1), cut-shrub barriers and control. Straw mulch provided an initial ground cover of 80% and the wood chips only 45%. Sediment yields were measured by means of sediment fences in 500-m2 bordered plots. During the first year after wildfire, mean precipitation was 1520 mm. The mean sediment yield in the control plots was 35 Mg ha–1. During this period, only straw mulch application significantly reduced soil erosion relative to controls (66%). The mean sediment yields in the wood-chip mulch and erosion barrier treatments, 33 and 30 Mg ha–1 respectively, were similar to rates in the untreated plots (35 Mg ha–1). Soil erosion decreased sharply during the second year after wildfire when mean precipitation was 1194 mm. Vegetation regrowth was very fast and treatments had no significant effect on the rate of recovery of vegetation cover, which was ~80% at the end of the study. The results obtained showed that ground cover was a key factor in determining post-fire soil loss. Stabilisation treatments such as wood-chip mulch and erosion barriers were not effective in reducing soil loss relative to the untreated control.

Effects of soil burn severity on germination and initial establishment of maritime pine seedlings, under greenhouse conditions, in two contrasting experimentally burned soils

The effects of soil burn severity on initial establishment of maritime pine in burned areas are not well known. Many factors may interact in the field, thus making it difficult to determine the exact role played by soil burn severity in the post-fire regeneration process. Monoliths of two contrasting soils – an acid, coarse-textured soil, with high organic matter content, and a neutral heavy-textured soil with low organic matter content – were experimentally burned to provide two markedly different levels of soil burn severity. The burned monoliths were sown with Pinus pinaster seeds and then placed in a greenhouse under a preselected water regime to determine the effect of burn severity on emergence and initial establishment of pine seedlings. High soil burn severity in the coarse-textured soils delayed germination, increased mortality and temporarily decreased the height of pine seedlings in the first year after sowing. This response was affected by: soil heating level, soil C consumption, post-fire soil C, depth of burn and post-fire duff-depth. Ash had no influence on the above processes. These factors did not explain the variability in the response of regeneration variables in the heavy-textured soils. The applicability of the results to field conditions is discussed.

Response of Pinus pinaster Ait. trees to controlled localised application of heat to stem and crown

Key messageExperimental damage was induced by controlled application of heat to Pinus pinaster trees. The results suggest that low intensity fires and prescribed burning will scarcely affect the trees.AbstractPinus pinaster is a prominent component of the western Mediterranean forest and is frequently affected by wildfires. However, little is known about the susceptibility of this species to heat-induced damage. Experimental damage was induced by controlled application of heat to P. pinaster trees, and short-term effects on growth and selected physiological parameters were evaluated. The treatments were partial crown damage (71% of crown volume scorched), partial stem damage (69% of the stem circumference girdled), combined stem and crown damage, and no damage (control trees). Only trees affected by combined damage showed a significant decrease in radial growth after the treatment, despite a significant increase in sap flow density. Neither the morphological characteristics of needles formed during the growing season after heat treatment nor the water potential reflected any changes as a consequence of the damage. Nonetheless, the results suggest that P. pinaster trees may be adversely affected by fires of sufficient intensity to cause the level of damage induced here. By contrast, low intensity fires without surface root damage will scarcely affect the trees.

Interspecific variation of constitutive chemical compounds in Pinus spp. xylem and susceptibility to pinewood nematode ( Bursaphelenchus xylophilus )

Pinewood nematode (PWN), Bursaphelenchus xylophilus, the causal agent of pine wilt disease (PWD), was detected in Spain in 2008. This gives rise to serious concern, as the disease has caused severe environmental and economic losses in Portugal and in Asian countries. We studied interspecific variation in susceptibility to pine wilt disease and differences in constitutive chemical compounds in the xylem tissue of the seven pine species -P. canariensis, P. halepensis, P. pinaster, P. pinea, P. sylvestris, P. radiata and P. taeda. Two-year-old trees were inoculated with B. xylophilus. Water potential and nematode densities were measured for each species on specific dates; whereas, wilting symptoms were recorded weekly until the end of the assay. Chemical compounds in the xylem were determined prior to inoculation. Three different resistance groups can be established in terms of the pine species susceptibility to PWN: non- to slightly-susceptible (P. canariensis, P. halepensis, P. taeda and P. pinea), susceptible (P. pinaster and P. radiata), and highly-susceptible (P. sylvestris). Nematodes migrated downward to the roots in all seven species. Constitutive xylem nitrogen, total polyphenols, and marginally phosphorus were negatively correlated with mortality caused by PWN. The most susceptible species, Pinus sylvestris, presented high levels of constitutive lipid-soluble substances and low levels of manganese, pointing to a possible relation between these components and PWN susceptibility. The results suggest P. sylvestris, P. pinaster and P. radiata forests could be severely damaged by PWN in Spain and highlight how constitutive chemical compounds such as nitrogen might play a role in resistance mechanisms against PWN.

Modelling canopy fuel dynamics of maritime pine stands in north-west Spain

Effective silvicultural strategies for reducing the likelihood and severity of crown fires include increasing canopy base height (CBH) and reducing canopy bulk density (CBD). These variables depend to a certain degree on stand structure and are therefore responsive to stand density management through thinning. In this study, data from permanent sample plots and thinning trials were used to model the dynamics of canopy fuel variables in maritime pine stands in north-western Spain. On the basis of the state–space modelling approach, the canopy fuel conditions at any point in time were assumed to be adequately defined by three state variables: number of stems per hectare (N), canopy fuel load (CFL) and CBH. These variables were projected by simultaneous fitting of three transition functions, which explained more than 77, 96 and 97% of the observed variability in N, CFL and CBH. The effect of thinning was modelled by including a thinning response function. Once the state variables were determined for a given point in time, CBD was derived from CFL, CBH and average stand height, thus ensuring compatibility between estimates. The system of equations developed, together with fire management decision support systems, will enable assessment of the crown fire potential associated with different silvicultural alternatives.