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Keywords:

  • Climate change;
  • Competition;
  • Drought;
  • Old growth;
  • Ponderosa pine;
  • Restoration;
  • Southwest USA;
  • Thin and burn treatments;
  • Treatment effects

Abstract

Questions

Do landscape-scale thin and burn restoration treatments have a long-term, landscape-scale impact on old Pinus ponderosa growth? Is there a relationship between old P. ponderosa growth and climatic factors, in particular, drought, before and after restoration treatments?

Location

Northwestern Arizona, USA.

Methods

We looked at old P. ponderosa growth across the landscape in both an area ‘treated’ by thin and burn restoration treatments, and a neighbouring untreated area. We re-visited each old P. ponderosa located on permanent 0.1-ha plots installed across the landscape prior to treatment implementation and recorded tree status, diameter, aspect, slope and competition from neighbouring trees. Growth was analysed from shallow increment cores taken from each tree at breast height (1.37 m). Comparisons of growth between the treated and untreated areas were carried out using regional proxy and instrumental Palmer drought severity index values and instrument precipitation data.

Results

We found significant differences in precipitation and temperature between the treated and untreated areas, indicating a drier, less advantageous climate in the untreated area. Old trees in the treated area responded less negatively in diameter growth to treatments; both treatment and abiotic site factors were important in predicting post-treatment growth. All old trees grew slowly during drought years; however, old trees in the treated area grew better after three recent drought years than old trees in the untreated area.

Conclusions

Old P. ponderosa diameter growth increased following restoration, though not immediately. Old trees in the treated area also grew better in the years after drought than old trees in the untreated area. Restoration, or similar treatments removing small, neighbouring trees may be critical in maintaining old P. ponderosa in the landscape, particularly under future climate change and increasing drought frequency in the western USA.