Anthropogenic‐driven rapid shifts in tree distribution lead to increased dominance of broadleaf species

Abstract

Over the past century, major shifts in the geographic distribution of tree species have occurred in response to changes in land use and climate. We analyse species distribution and abundance from about 33 000 forest inventory plots in Spain sampled twice over a period of 10-12 years. We show a dominance of range contraction (extinction), and demographic decline over range expansion (colonization), with seven of 11 species exhibiting extinction downhill of their distribution. Contrary to expectations, these dynamics are not always consistent with climate warming over the study period, but result from legacies in forest structure due to past land use change and fire occurrence. We find that these changes have led to the expansion of broadleaf species (i.e. family Fagaceae) over areas formerly dominated by conifer species (i.e. family Pinaceae), due to the greater capacity of the former to respond to most disturbances and their higher competitive ability. This recent and rapid transition from conifers to broadleaves has important implications in forest dynamics and ecosystem services they provide. The finding raises the question as to whether the increasing dominance of relatively drought-sensitive broadleaf species will diminish resilience of Mediterranean forests to very likely drier conditions in the future.

Increased aridity drives post‐fire recovery of Mediterranean forests towards open shrublands

“…In many short-term simulations, communities included pines, shrubs and grasses, with oaks comprising only a minor proportion (e.g. However, direct observations also show that fires can transform pine forests into oak forests if the latter are present in the understory (Retana et al, 2002;Torres et al, 2016;Vayreda et al, 2016;Mart ın-Alc on & Coll, 2016). 2a; Fig.…”

“…Specifically, shrubs typically are 'seeders' that spread rapidly after fire through their large, and often persistent, seed banks. Conversely, the late successional trees, mainly oaks, are slow-growing resprouters (Clarke et al, 2013;Zeppel et al, 2015), and can outcompete pines, shrubs and grasses (Ac acio et al, 2007;Vayreda et al, 2016). The combination of fast post-fire responses and high flammability enables shrubs and grasses to maintain the system in a highly fire-prone condition.…”

Contrasting Hydraulic Architectures of Scots Pine and Sessile Oak at Their Southernmost Distribution Limits

“…Our results suggest that sessile oak holds the capacity to plastically adjust its hydraulic architectures to dryness, whereas Scots pine does not have such adaptable hydraulic structure, having both strategies different advantages but also disadvantages. Although recent publications have highlighted the ongoing replacement of pine by oak at the lower edge of the elevation range in some regions of the Mediterranean basin and inner alpine dry valleys (Galiano et al, 2010; Rigling et al, 2013; Vayreda et al, 2016), the intensity, duration and recurrence of forthcoming dry spells might be a crucial factor shaping the southernmost distribution limits of both tree species.…”

Impacts of Global Change on Mediterranean Forests and Their Services

“…Pine species (more drought resistant) were widely replaced by broadleaf species, mainly of the Fagaceae family, in Spanish forests between 1990 and 2002 [105,147], mainly due to past land uses, fire history, abandonment of forest management and abandonment of remote areas by humans and much less due to climate change. This drive to a new land cover with fewer drought-resistant species decreases the resistance and resilience of global forests against drought.…”

“…The defoliation was associated mainly with drought due to climate change and paralleled significant increases in tree mortality, a reduction in tree density and changes in insect and fungal dynamics [129]. The health and relative (per unit of surface area) capacity of forests to fix C have thus decreased, despite the increase in the percentage of total area occupied by forests, mainly in the second half of the 20th century [147]. Negative net primary production becomes even more negative when drought coincides with high defoliation in Mediterranean woodlands [227,228].…”