The research results presented in preceding chapters show that shrubland is more persistent where wildfire has occurred, particularly on south‑ ‑facing slopes. Cistus shrubs are active pyrophytes, thus their persistence can be supported by a positive feedback mechanism triggered by frequent wildfires, especially on southern aspects, where conditions are drier and more hostile for seedling recruitment and cork oak survival (Chapter 2). In addition, the results also show that post ‑fire survival of cork oak trees was lower on the slopes facing south to east in the study area (Chapter 4), which are more xerophitic and also the most exposed to the predominant hot summer winds in the region (Ribeiro et al., 1987). On these sites the trees are therefore subjected to greater physiological stress. So, it seems that the combination of wildfire occurrence and aspect accounts for the lower post ‑fire survival rates of cork oak trees, since physiological stress will in‑ crease a tree’s vulnerability to fire. As demonstrated (in Chapter 2), wild‑ fires caused a decrease in forest persistence and triggered transitions from forests to shrublands after 1995. It will be recalled that it was argued that the observed transitions were probably reinforced and accelerated by the rise in mean annual temperatures since the 1970s (the six hottest recorded years occurred in the last 12 years of the period 1931 ‑2000: Cabrinha and Santo, 2000, Miranda et al., 2002) and increased frequency of wildfires in recent decades (Pausas, 2004; Anon., 2006) in southern Portugal.
From the findings presented in earlier chapters, patch type transi‑ tions at the landscape scale are determined not only by ecological proc‑ esses, but also by human intervention, such as intensity of land use and management practices. Cork oak agro ‑silvopastoral systems are inher‑ ently unstable and have to be maintained by human management of the understorey. If the understorey under cork oak trees is not managed, it is expected to develop shrubs and cork oak trees, which will establish
and lead to forest recovery. However, in Chapter 5 it was shown that for the 45 ‑year observation period, the interactive effect of past understorey management, cork oak planting, wildfire occurrence and less steep slopes (although slope was less important than the other variables) correlated significantly with patches that underwent shrub encroachment, patches where tree cover diminished, and patches with no tree cover. It was con‑ cluded that the interaction of such management and ecological factors halted forest recovery and led to shrub encroachment.
Past understorey management in oak agroforestry systems of southern Portugal has been pointed out as a major reason for the current soil ero‑ sion and lack of fertility, particularly due to long ‑term intensive wheat cul‑ tivation from the 1930s to the 1960s (Baptista, 1993). In addition, it is also generally known that livestock grazing led to a gradual decrease in cork oak natural regeneration and seedling establishment. As livestock grazing pres‑ sure and understorey use intensified, the understorey vegetation cover grad‑ ually diminished, due to continuous ploughing, cereal cultivation and no protection of acorns or oak natural regeneration from grazing livestock. On already degraded soils, inadequate cork oak planting interventions will fur‑ ther increase soil erosion (Zavala et al., 2004). Indeed, it has been reported that the use of inadequate techniques for cork oak planting combined with summer drought has led to high seedling mortality in southern Portugal (Almeida et al., 2009). In addition, vegetation diversity and soil cover will decrease due to the vegetation clearings that are necessary for any reforesta‑ tion project (Zavala et al., 2004). As a result, if planted seedling mortality is high in the first months and remaining vegetation cover is sparse, and there is no project supervision, the soil is quickly invaded by Cistus shrubs. In in‑ fertile and degraded soils, Cistus shrubs will outcompete the remaining oak seedlings for soil water and nutrients, increasing oak mortality. If a wildfire occurs, Cistus shrubs will spread their seeds and expand, some cork oak seedlings will be destroyed, and the ones that are able to survive or recover will face several limitations to establish in shrubland patches (Chapter 3).
Figure 1 is a schematic depiction of the conclusions reached from the findings reported here, i.e. that the persistence of shrubland patches, absence of cork oak recruitment and regression of forests result from a combination of several management and ecological factors. As reported in Chapter 2, most of the changes are gradual and transitions between forests and shrublands are not immediate, instead, the tree canopy disap‑ pears gradually: forest patches first change into savannas and then these savannas change into shrublands. However, severe wildfires coupled with past intensive land use practices and drought may lead to abrupt transi‑ tions from forests to shrublands (Figure 1). Furthermore, as shown in Chapter 4, post ‑fire survival of adult cork oaks is also essentially deter‑ mined by management decisions, namely when to start debarking and the timing of successive cork extractions. The reason Figure 1 does not include grasslands is because, as noted in Chapter 2, today they occupy very small and scattered patches in the landscape and are very unstable.
Figure 1 – Ecological and management factors that trigger transitions between cork oak forests, cork oak savannas and shrublands
Dashed lines indicate critical transitions, circular arrows indicate persistency of vegetation states and the symbol (+) indicates a combination of factors