Holocene Dynamics of Temperate Rainforests in West-Central Patagonia

Abstract

Analyses of long-term ecosystem dynamics offer insights into the conditions that have led to stability vs. rapid change in the past and the importance of disturbance in regulating community composition. In this study, we (1) used lithology, pollen, and charcoal data from Mallín Casanova (47 • S) to reconstruct the wetland, vegetation, and fire history of west-central Patagonia; and (2) compared the records with independent paleoenvironmental and archeological information to assess the effects of past climate and human activity on ecosystem dynamics. Pollen data indicate that Nothofagus-Pilgerodendron forests were established by 9,000 cal yr BP. Although the biodiversity of the understory increased between 8,480 and 5,630 cal yr BP, forests remained relatively unchanged from 9,000 to 2,000 cal yr BP. The charcoal record registers high fire-episode frequency in the early Holocene followed by low biomass burning between 6,500 and 2,000 cal yr BP. Covarying trends in charcoal, bog development, and Neoglacial advances suggest that climate was the primary driver of these changes. After 2,000 cal yr BP, the proxy data indicate (a) increased fire-episode frequency; (b) centennial-scale shifts in bog and forest composition; (c) the emergence of vegetation-fire linkages not recorded in previous times; and (d) paludification in the last 500 years possibly associated with forest loss. Our results therefore suggest that Nothofagus-Pilgerodendron dominance was maintained through much of the Holocene despite long-term changes in climate and fire. Unparalleled fluctuations in local ecosystems during the last two millennia were governed by disturbance-vegetation-hydrology feedbacks likely triggered by greater climate variability and deforestation.

Disentangling the Medieval Climatic Anomaly in Patagonia and its impact on human societies

“…1100-1400. Though less robustly according to our critical analysis, records from Mallín Casanova (Iglesias et al, 2016(Iglesias et al, , 2018 this work; ID 24, ca. 450-950 CE, evergreen forest), La Tercera (Bamonte and Mancini, 2011;this work;ID 27, ca.…”

Combining Contemporary and Paleoecological Perspectives for Estimating Forest Resilience

“…Study C combines lake-sediment charcoal record with fire scars on living spruce trees (Picea) to assess response of this forest system to increasing human pressure since the gold rush (c. 1902) (Gaglioti et al, 2016). Study D presents a study of Nothofagus that revealed a long and stable persistence of this forest system over time (Iglesias et al, 2018). Study E shows a 90,000 year pollen record from an Afromontane site in Cameroon, indicating highly unstable species composition with past climate changes (Lézine et al, 2019).…”

“…Paleoecological methods for detecting stability or variability can rely on the rate of change in a pollen time-series (Iglesias et al, 2018) as well as turnover in pollen composition (e.g., Birks and Birks, 2008). First, in Patagonia, the Nothofagus (southern beeches) forest was found to have prevailed relatively unchanged for the past 9,800 years until the twentieth century by calculating the rate of change in pollen records, which was equal to zero (Iglesias et al, 2018) (Figure 3, Study D). This stability could be attributed to Nothofagus' resistance to environmental change, its ability to rapidly recover or trade-offs in the plasticity within Nothofagus' species traits.…”

If the trees burn, is the forest lost? Past dynamics in temperate forests help inform management strategies

“…Mallín Casanova (47.4°S) is a broad wetland surrounded by rainforests of Nothofagus pumilio and Pilgerodendron uviferum. Radiocarbon-dated reconstructions show that the modern bog developed at 5800 cal yr BP, probably in response to the onset of wetter conditions that impacted all of Patagonia[30]. Approximately 2000 years ago, charcoal data indicate that fires became more frequent.…”

“…(a) Wetland dynamics at Mallín Casanova (47.4°S; Chile) inferred from squared-chord distances (SCD) between every pollen sample and the first sample of the record (9500 cal yr BP). SCDs were based on a dissimilarity matrix produced for all aquatic and wetland taxa and interpreted as temporal changes in wetland composition[30]. (b) Changes in SCD between times t + 1 and t as a function of SCD at time t. The nearly horizontal, positive regression line (slope = 0.22; equation (2.1)) indicates that the wetland was in unstable equilibrium.…”