Joaquim S. Silva

Regional variations in wildfire susceptibility of land-cover types in Portugal: implications for landscape management to minimize fire hazard

Patterns of wildfire occurrence at the landscape level were characterised during the period 1990–94 in Portugal. Based on land-cover information within 5591 burned patches (larger than 5 ha) and in the surrounding landscape, selection ratio functions were used to measure fire preference or avoidance for different land-cover types in 12 regions of the country. Shrublands were the most fire-prone land cover, whereas annual crops, permanent crops and agro-forestry systems were the most avoided by fire. In terms of forest types, conifer plantations were more susceptible to fire than eucalyptus, and broadleaved forests were the least fire-prone. There were regional variations in land-cover susceptibility to fire, which may be explained by differences in climate, management, ignition patterns, firefighting strategies, and regional availability. A cluster analysis of regional variations in selection ratios for all land covers allowed the identification of three main geographical areas with similar fire selection patterns. These results can be used for planning landscape-scale fuel management in order to create landscapes with a lower fire hazard.

Towards developmental modelling of tree root systems

Abstract Knowledge of belowground structures and processes is essential for understanding and predicting ecosystem functioning, and consequently in the development of adaptive strategies to safeguard production from trees and woody plants into the future. In the past, research has mainly been concentrated on growth models for the prediction of agronomic or forest production. Newly emerging scientific challenges, e.g. climate change and sustainable development, call for new integrated predictive methods where root systems development will become a key element for understanding global biological systems. The types of input data available from the various branches of woody root research, including biomass allocation, architecture, biomechanics, water and nutrient supply, are discussed with a view to the possibility of incorporating them into a more generic developmental model. We discuss here the main focus of root system modelling to date, including a description of simple allometric biomass models, and biomechanical stress models, and then build in complexity through static growth models towards architecture models. The next progressive and logical step in developing an inclusive developmental model that integrates these modelling approaches is discussed.

Effects of prescribed fire on vegetation structure and breeding birds in young Pinus pinaster stands of northern Portugal

The effects of prescribed fire on understory vegetation structure and bird populations of Pinus pinaster stands in northern Portugal were assessed by using a synchronic sampling approach. During the spring of 1998 and 1999, 49 plots (of which 40 had been burned from 0.5 to 5 years ago) were characterised in terms of vegetation structure, surrounding landscape composition and breeding bird populations. Linear regression analysis and ANOVA were used to evaluate the effects of prescribed fire on vegetation and birds. Herbaceous vegetation showed a modal growth with a maximum development ca. 3 years after fire. Shrub development increased linearly with time since fire. Understory vegetation structure 5 years after fire was similar to control plots. Bird abundance and richness were influenced by plot variables (increased with stand age) and landscape variables (decreased with the amount of shrublands and eucalyptus stands surrounding the plot). After controlling for the effect of these variables, time since fire affected bird abundance, particularly for shrub/ground nesters. Minimum abundance occurred in the second or third breeding season after fire, possibly due to site tenacity or increased food availability. Prescribed burns did not affect bird richness. The size of the burned plots did not affect bird abundance or richness, probably because most burns were very small. Measures to increase bird diversity in these forests include maintaining other tree species and snags, avoiding the use of eucalyptus, and keeping a 5-year interval between consecutive prescribed burns in a given stand. The average size of burns could probably be increased without detrimental effects on bird populations.

Root distribution of a Mediterranean shrubland in Portugal

The distribution of roots of an Erica (Erica scoparia and Erica lusitanica) dominated Mediterranean maquis was studied using three different approaches: root counts on trench walls (down to 120 cm), estimation of the maximum rooting depth using an allometric relationship and estimation of fine root biomass and fine root length using soil cores (down to 100 cm). Roots were classified according to diameter (fine, ≤1.0 mm; small, 1.1–5.0 mm; medium, 5.1–10.0 mm; coarse, >10.0 mm) and species (Erica sp., Pteridium aquilinum, Rubus ulmifolius and Ulex jussiaei). The depth corresponding to 50% of all roots (D50) was determined by fitting a new model to the cumulative root distribution. Fine roots represented 96% of root counts. Root counts of Erica represented 59%, Ulex 34%, Rubus 6% and Pteridium 1%. Overall root counts showed a D50 of 26 cm. D50 was higher for Ulex (40 cm) and Erica (22 cm), than for Pteridium (9 cm) and Rubus (3 cm). D50 for fine roots was 27 cm, for small roots 11 cm, for medium roots 6 cm and for coarse roots 4 cm. The estimated average maximum rooting depth of the 28 deepest Erica roots was 222 cm. The deepest Erica root was estimated to reach 329 cm. A total of 82% of roots growing deeper than 125 cm were not reaching more than 175 cm. The overall fine root length density ranged from 4.6 cm/cm3 at 10 cm to 0.8 cm/cm3 at 80 cm. The overall fine root biomass ranged from 7.7 mg/cm3 at 10 cm to 0.6 mg/cm3 at 40 cm. D50 for root biomass was 12 cm and D50 for root length was 14 cm. Fine root biomass was estimated as 1.6 kg/m2 and the respective root length as 18.7 km/m2.

A conceptual model of sprouting responses in relation to fire damage: an example with cork oak (Quercus suber L.) trees in Southern Portugal

The sprouting response types of 1,151 cork oak (Quercus suber) trees one and half years after a wildfire in southern Portugal were characterised. It was hypothesised that different response types should occur according to the following conceptual model: an increased level of damage (fire severity) on a sprouting tree that suffered a crown fire was expected to be reflected in a sequence of four alternative events, namely (a) resprouting exclusively from crown, (b) simultaneous resprouting from crown and base, (c) resprouting exclusively from base and (d) plant death. To assess whether the level of expected damage was influenced by the level of protection from disturbance, we explored the relationships between response types and tree size, bark thickness and cork stripping, using an information-theoretic approach. The more common response type was crown resprouting (68.8% of the trees), followed by plant death (15.8%), simultaneous resprouting from crown and base (10.1%) and basal resprouting (5.3%). In agreement with the conceptual model, trees which probably suffered a higher level of damage by fire (larger trees with thinner bark; exploited for cork) died or resprouted exclusively from base. On the other hand, trees that were well protected (smaller trees with thicker bark not exploited for cork) were able to rebuild their canopy through crown resprouting. Simultaneous resprouting from the crown and base was determined mainly by tree size, and it was more common in smaller trees.

Assessing the relative fire proneness of different forest types in Portugal

Abstract We have assessed the fire proneness of the main forest types in Portugal classified according to the main species, using three different approaches: the use of resource selection ratios applied to burned patches, the proportion of randomly located plots that were burned and the proportion of burned National Forest Inventory plots. The results allowed ranking fire proneness according to the following decreasing order: maritime pine forests, eucalyptus forests, unspecified broadleaf forests, unspecified conifer forests, cork oak forests, chestnut forests, holm oak forests and stone pine forests. In order to understand the obtained results we have assessed the structure of the different forest types using the percent cover of seven vegetation layers (C1–C7), a Tree Dominance Index, a Height Index and a Cover Index (IC). Structural variables and stand composition were used to predict fire probability according to binary logistic modelling. Only four structural variables and stand composition provided significant results, the latter being the most important variable for explaining fire probability. These models were used to predict fire probability for different stand types as a function of IC.

Forest fires in cork oak (Quercus suber L.) stands in Portugal

Fire and cork can be considered both the most serious problem and the most valuable resource of Portuguese forests. The existence of important areas of burned cork oak stands resulting from severe fire seasons in recent years is an environmental problem which deserves special attention. This paper makes a multidisciplinary survey of the problem of burned cork oak stands focusing on: the dimensions of the problem in Portugal, the remarkable resistance of cork oak to fire, the post‐fire management of cork oak stands, and the economic and ecological effects of fire in cork oak stands. Particular emphasis is given to knowledge gaps in this context.

Soil water dynamics after fire in a Portuguese shrubland

This paper presents a study where soil water content (SW) was measured before and after an experimental fire in a shrubland dominated by Erica scoparia L. in Portugal. Two plots were established: one was kept as a control plot and the other was burned by an experimental fire in June 2001. Measurements were taken before fire (2000), and after fire (2001, 2002, and 2003) at six depths down to 170 cm, from June to December. Measurements before fire allowed comparison of the two plots in terms of the SW differential, using 2000 as a reference. Results for 2001 showed that SW decreased less during the drying season (June-September) and increased more during the wetting season (October-December) in the burned plot than in the control plot. The magnitude of these effects decreased consistently in 2002 and 2003, especially at surface layers. The maximum gain of SW for the total profile in the burned plot was estimated as 105.5 mm in 2001, 70.2 mm in 2002, and 35.6 mm in 2003. The present paper discusses the mechanisms responsible for the increase in SW taking into account the characteristics of the plant community, including the root distribution, and the results of other studies.

Root distribution of Mediterranean woody plants. Introducing a new empirical model

ABSTRACT The root distributions of 42 plants from 18 Mediterranean woody species were characterised by using an adjustable new empirical model. Plants were classified in terms of developmental stage and regenerative strategy. Vertical distributions of root length and root biomass were determined, standardised, and computed as cumulative data. The root distribution data were used to test the new proposed model against three other models taken from the literature. Tests have shown that the new model presented the best performance among the four. Significant differences were obtained between developmental stages and between regenerative strategies, using model-derived parameters. Despite the heterogeneous characteristics within the group of studied plants, different rooting patterns could be detected using the new modelling approach. The paper discusses these patterns in terms of the ecological characteristics of the different species.

Post-Fire Management of Exotic Forests

Forests dominated by exotic tree species represent around 5% of all European forests, including plantations and self-sown exotic forests. The management of burned exotic forests is characterized taking into account specific aspects related with the protection of soils and the restoration of natural vegetation. Given its increasing importance, the chapter includes a synthetic review on the ecology of invasive tree species and their relationship with fire. Different post-fire management practices and alternatives are described for some of the most common sub-types of exotic forests in southern Europe. Two case studies are presented, including relevant information for the post-fire management of Acacia longifolia woodlands and Eucalyptus globulus plantations.