Oak Competition Dominates Interspecific Interactions in Growth and Water-Use Efficiency in a Mixed Pine–Oak Mediterranean Forest

Juan Pedro Ferrio,Tatiana A Shestakova,Jordi Voltas,Jorge Del Castillo

doi:10.3390/f12081093

Abstract

In the Mediterranean, mixed forests of Aleppo pine and holm oak are widespread. Generally considered a transition stage in the succession towards climax oak communities, niche segregation may also contribute to the prevalence of these communities. So far, there is increasing evidence of hydrological niche segregation, with the two species showing complementary water use and seasonal growth patterns. However, it remains unknown whether interspecific interactions affect the response to climate and the mid-term (decadal) growth and water-use efficiency of pines and oaks in mixed stands. Here, we combined tree-ring chronologies, built on different competition classes within a mixed stand, with a spatially explicit assessment of individual growth and wood carbon isotope discrimination (∆13C), as a proxy of intrinsic water-use efficiency, and compared these results with previously reported water uptake patterns. We found that competition with pines modulated the climate response of oaks, whereas pine climate response was insensitive to competition. On the other hand, pine density affected only pine growth, whereas oak competition affected both species. We conclude that the presence of pines had negligible or even positive effects on the oaks, but competition with neighbor oaks limited their ability to recover after drought. Conversely, pines experienced greater drought stress under competition, with both oaks and pines.

Highlights

The Mediterranean Basin is considered a biodiversity hotspot [1,2], where the current increase in seasonality and intensity of summer drought caused by climate change, together with fires and insect outbreaks, is challenging the resilience of forests [3]
To characterize the main climate drivers of tree growth and how their influence is modulated by competition for each species, we evaluated the relationships between highfrequency fluctuations in ring-width chronologies (TRW) and climate either at whole-stand level or for each local density class independently (Figure 2)
Our results show that competition with pines modulates the climate responses of oaks, whereas climate responses of pines are virtually insensitive to competition

Summary

Introduction

The Mediterranean Basin is considered a biodiversity hotspot [1,2], where the current increase in seasonality and intensity of summer drought caused by climate change, together with fires and insect outbreaks, is challenging the resilience of forests [3]. In coastal areas of the western Mediterranean, mixed stands of Mediterranean pines and evergreen oaks are common forest types [8]. These mixed forests have been traditionally considered transient states in the progression towards a climax vegetation dominated by oaks [9]. Water scarcity may lead to an increase in competition for water in mixed-species stands [15], and climate change is likely to disturb the current equilibrium between co-existing species [16]. The thresholds determining the prevalence of competition or facilitation effects in response to resource limitations are still a matter of debate [17,18,19]

Methods

Results

Conclusion