Drought, fire and grazing precursors to large‐scale pine forest decline

Simon E Connor,Maria Da Conceição Freitas,João Araújo,Boris Vannière,Ana Gomes,César Morales‐Molino,Sandra D Gomes,Manel Leira,Tomasz Boski,Rufus B Akindola,Fátima Franco‐Múgica,Cesar Andrade

doi:10.1111/ddi.13261

Abstract

AbstractAimTemperate forests are currently facing multiple stresses due to climate change, biological invasions, habitat fragmentation and fire regime change. How these stressors interact with each other influences how, when and whether ecosystems recover, or whether they adapt or transition to a different ecological state. Because forest recovery or collapse may take longer than a human lifetime, predicting the outcomes of different stressor combinations remains difficult. A clearer vision of future forest trajectories in a changing world may be gained by examining collapses of forests in the past. Here, we use long‐term ecological data to conduct a post‐mortem examination of the decline of maritime pine forests (Pinus pinaster Ait.) on the SW Iberian Peninsula 7000–6500 years ago.LocationPortugal and Spain.MethodsWe compared four palaeoecological records—two with pine declines and two without—using a multiproxy approach. Bioclimatic differences between the four sites were explored. Proxies for past vegetation and disturbance (fire and grazing) were compared with independent palaeoclimatic records. We performed functional traits analysis and used phase plots to examine the causes of pine decline.ResultsThe pine decline represents a critical transition in SW Iberia, which lies close to maritime pine's bioclimatic limits. Prolonged drought likely killed trees and suppressed the fires that normally stimulate pine germination and pinewood recovery. Increased grazing pressure facilitated the rapid spread of resprouter shrubs. These competed with pine trees and ultimately replaced them. Our data highlight complex interactions between climate, fire, grazing and forest resilience.Main ConclusionsThe pine decline occurred at least a century after post‐fire resprouters overtook obligate seeders in the vegetation, constituting an early‐warning signal of forest loss. Fire suppression, resprouter encroachment and grazing may threaten the persistence of Mediterranean forests as droughts become more frequent and extreme.

Highlights

In the current era of rapid environmental change, ecosystems are under stress on various fronts
This contrasts with the more central position of El Carrizal (ELCA) and Espinosa de Cerrato (ESCE) sites within or near the Spanish Tierra de Pinares, where pollen and macrofossil data indicate pine forest continued uninterrupted through the mid-Holocene (Franco Múgica et al, 2001; Franco-Múgica et al, 2005; Morales-Molino et al, 2012, 2017)
Ecological models that simulate the effects of drought–fire interactions in Mediterranean vegetation provide little indication of this potential outcome (Batllori et al, 2017; Mouillot et al, 2002). This may be because the duration of mid-Holocene drought in SW Iberia was longer than those simulated in models (e.g. 15 years in Batllori et al, 2017)

Summary

| INTRODUCTION

In the current era of rapid environmental change, ecosystems are under stress on various fronts. Soil type, grazing, pathogens and seed predation further complicate recovery trajectories (Baeza et al, 2007; Foster et al, 2016; Pausas, 2004; Vega et al, 2011) The complexity of these interactions requires multiple-stressor models to forecast ecosystem responses to environmental change. Validation of these models is a major challenge, as observational records tend to encompass shorter time frames than the fire- and drought-frequency parameters currently being modelled (Barros et al, 2018; Batllori et al, 2017; Mouillot et al, 2002). We compare a new multiproxy record with three previous sequences (a)

| METHODS

| DISCUSSION

Findings

| CONCLUSIONS