Regeneration and mortality of woody species in the Mediterranean landscape
Regeneration and mortality of woody species in the Mediterranean landscape - Effect of interface treatments, competition and environmental conditions
Danielle Bashan and Avi Bar-Massada
Mediterranean regions are characterized by a variety of plant formations, due to dry climatic conditions and a history of human based disturbances. These plants have developed drought resisting mechanisms and improved regenerative capacity. Human based disturbances such as grazing and shrub clearing have created a complex landscape mosaic characterized by high levels of spatial heterogeneity at different spatial scales. This heterogeneity contributes to high species diversity (mainly herbaceous plants) and reduced risk of wildfire spread (due to fine scale fragmentation of vegetative fuels). Therefore, disturbance-based management has become a common management strategy. On the side of the spectrum of the effects of human activities, anthropogenic climate change may increase the frequency of droughts, which can lead to woody species mortality. In Ramat HaNadiv Nature Park, a nature reserve in northern Israel, regenerative processes occur side by side with woody species mortality, mainly of common oaks (Quercus calliprinos). Tree mortality is primarily driven by two factors: first, a-biotic disturbances such as fire, floods and droughts. Secondly, mortality can be caused by biotic effects such as disease, old age, human utilization and competition effects. My research focused on two processes that drive the dynamics of Mediterranean woody vegetation: the response of woody vegetation to human based disturbances, and quantifying various factors that lead to oak mortality. The main objectives of this research were: (1) to quantify the effects of different disturbance based management treatments on the spatiotemporal dynamics of woody vegetation in the LTER (long term ecological research) area in Ramat HaNadiv; and (2) to characterize and quantify the relative weights of environmental conditions compared to different types of competition in causing oak mortality in Ramat HaNadiv. I reconstructed the historical spatial patterns of woody vegetation under human based disturbances using high resolution aerial photographs. I then conducted a spatial analysis to quantify the roles of different disturbance treatments in forming these dynamics. In the second part of the research, I conducted an analysis based on field measurements and remote sensing data to disentangle the roles of biotic and a-biotic factors in driving oak mortality. I then conducted an empirical statistical analysis to quantify the relative weights of environmental conditions vs. intra-and-interspecific competition in models that explain tree mortality. The results of the research indicate that woody vegetation dynamics vary among treatments: goat grazing led to a continuous decrease in woody vegetation cover. A combination of shrub clearing followed by goat grazing led to steady spatial patterns of woody vegetation, and hence presented the most effective management strategy for generating and maintaining fuel breaks to prevent wildfire spread. After analyzing the different factors that explain oak mortality, I found that height, woody cover (both of which interact with a measure of intraspecific competition) and oak density (number of trees per unit area), are the main predictors of oak mortality. Hence, tree mortality is driven by a complex network of interactions between a-biotic and biotic factors. Therefore, management treatments aimed at increasing the resilience of oak stands in the face of climate change should account for multiple ecological processes, which I elaborate on in this study.
The results of this research present us with a better understanding of various processes that control woody vegetation dynamics in Mediterranean landscapes. This understanding may allow us to predict environmental changes that would alter woody vegetation across landscapes. This information, ultimately, is invaluable for creating effective management strategies that create and maintain fuel breaks to protect habitats from wildfires on one hand, and may promote oak stands that are more resilient to environmental change.