Factors shaping the distribution of old-growthness attributes in the forests of Spain

Conservation and enhancement of old-growth forests are key in forest planning and policies. In order to do so, more knowledge is needed on how the attributes traditionally associated with old-growth forests are distributed in space, what differences exist across distinct forest types and what natural or anthropic conditions are affecting the distribution of these old-growthness attributes. Using data from the Third Spanish National Forest Inventory (1997–2007), we calculated six indicators commonly associated with forest old-growthness for the plots in the territory of Peninsular Spain and Balearic Islands, and then combined them into an aggregated index. We then assessed their spatial distribution and the differences across five forest functional types, as well as the effects of ten climate, topographic, landscape, and anthropic variables in their distribution. Relevant geographical patterns were apparent, with climate factors, namely temperature and precipitation, playing a crucial role in the distribution of these attributes. The distribution of the indicators also varied across different forest types, while the effects of recent anthropic impacts were weaker but still relevant. Aridity seemed to be one of the main impediments for the development of old-growthness attributes, coupled with a negative impact of recent human pressure. However, these effects seemed to be mediated by other factors, specially the legacies imposed by the complex history of forest management practices, land use changes and natural disturbances that have shaped the forests of Spain. The results of this exploratory analysis highlight on one hand the importance of climate in the dynamic of forests towards old-growthness, which is relevant in a context of Climate Change, and on the other hand, the need for more insights on the history of our forests in order to understand their present and future.