The influence of soil treatments on post fire regeneration of native vegetation in a context of reforestation in an arid landscape in southern Portugal

Abstract

Wildfires have increased in intensity and extent in Portugal over the past decade. Such events significantly alter ecosystems and negatively affect soil properties and vegetation cover, leading to erosion, nutrient loss, and vegetation depletion. Under such poor conditions, human interventions to improve soil richness are often needed to accelerate forest recovery and native vegetation regeneration. The objective of this study is to evaluate the effects of soil treatments on native vegetation and their impact on the survival of seedlings planted for reforestation purposes. As part of a broader research project, several soil treatments were applied on recently burned soil: control zone; prior vegetation removal; creation of contour bunds; creation of contour bunds and woodchips disposal; creation of contour bunds and green waste compost disposal. This research will focus on addressing (1) the influence of each treatment on soil properties of which soil organic matter (SOM) and soil moisture (SM); (2) the influence of soil treatments on native vegetation biomass, cover, and moisture and their interactions with soil properties; (3) the impact of the presence and development of native vegetation on the survival rate of tree and shrub seedlings planted on site. Particular attention will be pay to the effects of the treatments on one native species, Cistus ladanifer by examining its abundance and some of its phenological parameters. Finally, (2bis) this work investigated the development of a simple tool to assess biomass in the field using green vegetation cover as a proxy. The results showed that soil organic matter is higher where there have been no treatments or only vegetation removal. This is likely due to (i) field preparation, which may have removed burned organic materials and turned over the soil layer, and (ii) higher vegetation cover in these plots. Biomass and cover of native vegetation are also higher in these plots, likely due to (i) vegetation removal by machinery and (ii) the layer of supplemental organic material (woodchips or compost) preventing plant growth. Native vegetation is more humid where contour bunds were created, likely due in part to (i) increased infiltration rates and rooting depth, and (ii) less abundant vegetation and lignified species. Cistus ladanifer grows more humid, taller, and with larger leaf areas in the same plots, indicating that this species may be a good indicator of treatment effects. There is a negative correlation between the presence of native vegetation (including Cistus ladanifer) and the survival rate of trees and shrubs, indicating that native vegetation may compete with them. However, it is more likely that the planted trees and shrubs survived better thanks to the positive effect of the organic amendments, which simultaneously reduced vegetation cover. Finally, a linear regression between biomass and GVC showed that GVC is a good proxy for biomass estimation.