Abstract
This paper evaluates the relationship between fire occurrence (number and burned area) and climate variability (precipitation and maximum temperatures) across central and south‐central Chile ...(32°–43° S) during recent decades (1976–2013). This region sustains the largest proportion of the Chilean population, contains ecologically important remnants of endemic ecosystems, the largest extension of forest exotic plantations, and concentrates most of the fire activity in the country. Fire activity in central Chile was mainly associated with above‐average precipitation during winter of the previous year and with dry conditions during spring to summer. The later association was particularly strong in the southern, wetter part of the study region. Maximum temperature had a positive significant relationship with burned area across the study region, with stronger correlations toward the south. Fires in central Chile were significantly related to El Niño–Southern Oscillation, through rainfall anomalies during the year previous to the fire season. The Antarctic Oscillation during winter through summer was positively related to fires across the study area due to drier/warmer conditions associated with the positive polarity of this oscillation. Climate change projections for the region reveal an all‐season decrease in precipitation and increases in temperature, that may likely result in an increment of the occurrence and the area affected by fires, as it has been observed during a multi‐year drought afflicting central Chile since 2010.
We estimated the amount of carbon (C) stored in terrestrial ecosystems of the Chilean Patagonia and the proportion within protected areas. We used existing public databases that provide information ...on C stocks in biomass and soils. Data were analysed by ecosystem and forest type in the case of native forests. Our results show that some ecosystems have been more extensively studied both for their stocks in biomass and soils (e.g. forests) compared with others (e.g. shrublands). Forests and peatlands store the largest amount of C because of their large stocks per hectare and the large area they cover. The total amount of C stored per unit area varies from 261.7 to 432.8 Mg C ha−1, depending on the published value used for soil organic C stocks in peatlands, highlighting the need to have more precise estimates of the C stored in this and other ecosystems. The mean stock in national parks (508 Mg C ha−1) is almost twice the amount stored in undisturbed forests in the Amazon. State and private protected areas contain 58.9% and 2.1% of the C stock, respectively, playing a key role in protecting ecosystems in this once pristine area.
The forests of south‐central Chile are facing a drying climate and a megadrought that started in 2010. This study addressed the physiological responses of five Nothofagus obliqua stands across the ...Mediterranean‐Temperate gradient (35.9°−40.3°S) using carbon isotope discrimination (Δ13 C) and intrinsic water use efficiency (iWUE) in tree rings during 1967–2017. Moreover, tree ring δ18O was evaluated in the northernmost site to better understand the effects of the megadrought in this drier location. These forests have become more efficient in their use of water. However, trees from the densest stand are discriminating more against 13C, probably due to reduced photosynthetic rates associated with increasing light competition. The strongest associations between climate and Δ13C were found in the northernmost stand, suggesting that warmer and drier conditions could have reduced 13C discrimination. Tree growth in this site has not decreased, and δ18O was negatively related to annual rainfall. However, a shift in this relationship was found since 2007, when both precipitation and δ18O decreased, while correlations between δ18O and growth increased. This implies that tree growth and δ18O are coupled in recent years, but precipitation is not the cause, suggesting that trees probably changed their water source to deeper and more depleted pools. Our research demonstrates that forests are not reducing their growth in central Chile, mainly due to a shift toward the use of deeper water sources. Despite a common climate trend across the gradient, there is a non‐uniform response of N. obliqua forests to climate drying, being their response site‐specific.
Plain Language Summary
The forests of south‐central Chile are facing a drying climate and a megadrought that started in 2010. This study addressed the physiological responses of five Nothofagus obliqua stands across the Mediterranean‐Temperate gradient (35.9°−40.3°S) to climate drying using stable isotopes in tree rings during the period 1967–2017. These forests have become more efficient in their use of water. However, trees from the densest stand are showing a poor adaptability to climate change, probably due to a decrease in photosynthetic rates associated with the increase in competition. Our research demonstrates for the first time that forests are not reducing their growth in central Chile, mainly due to a shift toward the use of deeper water sources. However, it is not certain until which point this acclimation will persist as these pools get depleted in the future with continued drying. Despite a common climate trend across the latitudinal gradient, this research points to a non‐uniform response of N. obliqua second‐growth forests to a drying climate, being their response stand and site‐specific.
Key Points
Despite a common climate trend, the response of N. obliqua to climate change is not uniform, but rather specific to the stand and site
Tree growth and δ18O have been coupled during the megadrought in the northernmost site, but rainfall does not influence this pattern
N. obliqua northern forests are able to maintain radial growth rates and acclimate to persistent droughts by tapping deeper water sources
Measurements of ecosystem carbon (C) fluxes in temperate forests are concentrated in the Northern Hemisphere, leaving the functionally diverse temperate forests in the Southern Hemisphere ...underrepresented. Here, we report 3 years (February 2018 to January 2021) of C fluxes, studied with eddy‐covariance and closed chamber techniques, in an endangered temperate evergreen rainforest of the long‐lived paleoendemic South American conifer Fitzroya cupressoides. Using classification and regression trees, we analyzed the most relevant drivers and thresholds of daily net ecosystem exchange (NEE) and soil respiration. The annual NEE showed that the forest was a moderate C sink during the period analyzed (−287 ± 38 g C m−2 year −1). We found that the capacity to capture C of the Fitzroya rainforests in the Coastal Range of southern Chile is optimal under cool and rainy conditions in the early austral spring (October–November) and decreases rapidly toward the summer dry season (January–February) and autumn. Although the studied forest type has a narrow geographical coverage, the gross primary productivity measured at the tower was highly representative of Fitzroya and other rainforests in the region. Our results suggest that C fluxes in paleoendemic cool F. cupressoides forests may be negatively affected by the warming and drying predicted by climate change models, reinforcing the importance of maintaining this and other long‐term ecological research sites in the Southern Hemisphere.
Plain Language Summary
Forest ecosystems play a central role in climate policy because their growth removes nearly a third of the anthropogenic carbon emissions to the atmosphere. Yet, warming and drying with climate change threaten the capacity of forests to absorb carbon in some regions. Measuring the carbon sink of forests and its climatic control is thus essential for ecosystem management and climate policy, but current measurements are overwhelmingly concentrated on the Northern Hemisphere leaving a gap in the Southern Hemisphere. Here, we present the first measurements of carbon fluxes of an adult 300‐year‐old temperate rainforest of the ultra‐long‐lived conifer Fitzroya cupressoides in southern Chile. This conifer is a paleoendemic tree confined to a few cool and very wet climate refugia. Our results show that the forest can photosynthesize year‐round because it is particularly adapted to cold and rainy conditions. Photosynthesis consistently exceeds ecosystem carbon losses by respiration between July and January, resulting in a moderate carbon sink compared with temperate forests in the Northern Hemisphere. Summer drying and warming enhance carbon losses by respiration and thus weaken the carbon sink capacity, implying that the ongoing and future climate drying and warming in the region threaten the carbon sink of this unique rainforest.
Key Points
We present the first analysis of carbon fluxes of a paleoendemic temperate rainforest in a global drying hotspot
The paleoendemic ecosystem thrives under cool and wet conditions during the austral spring
Projected warming and drying in the region threaten the carbon sink capacity of this globally unique, ultra‐long‐lived rainforest
There is an ongoing debate on whether a drought induced carbohydrate limitation (source limitation) or a direct effect of water shortage (sink limitation) limit growth under drought. In this study, ...we investigated the effects of the two driest summers recorded in southern Chile in the last seven decades, on the growth and non-structural carbohydrates (NSC) concentrations of the slow-growing conifer Fitzroya cupressoides. Specifically, we studied the seasonal variation of NSC in saplings and adults one and two years after the occurrence of a 2 year-summer drought at two sites of contrasting precipitation and productivity (mesic-productive vs. rainy-less productive). We also evaluated radial growth before, during and after the drought, and predicted that drought could have reduced growth. If drought caused C source limitation, we expected that NSCs will be lower during the first than the second year after drought. Conversely, similar NSC concentrations between years or higher NSC concentrations in the first year would be supportive of sink limitation. Also, due to the lower biomass of saplings compared with adults, we expected that saplings should experience stronger seasonal NSC remobilization than adults. We confirmed this last expectation. Moreover, we found no significant growth reduction during drought in the rainy site and a slightly significant growth reduction at the mesic site for both saplings and adults. Across organs and in both sites and age classes, NSC, starch, and sugar concentrations were generally higher in the first than in the second year following drought, while NSC seasonal remobilization was generally lower. Higher NSC concentrations along with lower seasonal NSC remobilization during the first post-drought year are supportive of sink limitation. However, as these results were found at both sites while growth decreased slightly and just at the mesic site, limited growth only is unlikely to have caused NSC accumulation. Rather, these results suggest that the post-drought dynamics of carbohydrate storage are partly decoupled from the growth dynamics, and that the rebuild of C reserves after drought may be a priority in this species.
Forest disturbances influence
Fitzroya
cupressoides forest structure and carbon stocks at multiple spatial and temporal scales. Natural disturbances such as landslides and volcanism affect and give ...rise to the mostly pristine
Fitzroya
stands present in the Andean cordillera. On the other hand, mostly human-caused fires and logging have been the main processes shaping the structure of
Fitzroya
stands in the Coastal range and of
Fitzroya
small remnants in the Central depression. The main goal of this study was to assess the carbon stocks and accumulation rates of
Fitzroya
forest stands according to their development stage under different disturbance regimes and environmental conditions given by the three physiographic units where the species grows (Coastal range, Central depression, and Andean range). The site selection included an age sequence of stands, known as a chronosequence approach. We identified
Fitzroya
post-disturbance stands in three different stages of development: young forest stage (mean stand age of the main cohort ≤ 200 years old), mature forest stage (200–800 years old), and old growth forest stage (800–1,500 years old). The following biomass components were considered: living standing trees, dead standing trees (snags), and logs from dead trees laying on the ground (coarse woody debris). Old-growth
Fitzroya
forests reached a mean total carbon stock (standing live trees, snags, and coarse woody debris) of 507, 279, and 331 Mg C ha
−1
in the Andean and Coastal ranges, and Central depression, respectively.
Fitzroya cupressoides
contributes, in average, more than 80% to the total carbon stock in the Andean and Coastal ranges, and 63% in the Central depression. The remainder corresponds mainly to
Nothofagus
spp. The high carbon stocks in old-growth stands in the Andean range are explained by
Fitzroya
longevity, larger size, wood decay resistance, and the low recurrence of volcanic events. Carbon accumulation rates differ between the forests in the three physiographic units (Central depression>Andean range>Coastal range), mainly due to the different growth rates and environmental conditions present in each unit. In the context of climate change, conserving old-growth stands with large biomass and carbon stocks and restoring
Fitzroya
forests should be recognized as a key contribution toward national and global goals to mitigate global warming.
South American (SA) societies are highly vulnerable to droughts and pluvials, but lack of long-term climate observations severely limits our understanding of the global processes driving climatic ...variability in the region. The number and quality of SA climatesensitive tree ring chronologies have significantly increased in recent decades, now providing a robust network of 286 records for characterizing hydroclimate variability since 1400 CE. We combine this network with a self-calibrated Palmer Drought Severity Index (scPDSI) dataset to derive the South American Drought Atlas (SADA) over the continent south of 12°S. The gridded annual reconstruction of austral summer scPDSI is the most spatially complete estimate of SA hydroclimate to date, and well matches past historical dry/wet events. Relating the SADA to the Australia–New Zealand Drought Atlas, sea surface temperatures and atmospheric pressure fields, we determine that the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are strongly associated with spatially extended droughts and pluvials over the SADA domain during the past several centuries. SADA also exhibits more extended severe droughts and extreme pluvials since the mid-20th century. Extensive droughts are consistent with the observed 20th-century trend toward positive SAM anomalies concomitant with the weakening of midlatitude Westerlies, while low-level moisture transport intensified by global warming has favored extreme rainfall across the subtropics. The SADA thus provides a long-term context for observed hydroclimatic changes and for 21st-century Intergovernmental Panel on Climate Change (IPCC) projections that suggest SA will experience more frequent/ severe droughts and rainfall events as a consequence of increasing greenhouse gas emissions.
Little is known about how old‐growth and massive forests are responding to environmental change. We investigated tree‐ring growth and carbon isotopes of the long‐lived and high biomass Fitzroya ...cupressoides in two stands growing in contrasting environmental conditions in the Coastal Range (~300 years old) and Andean Cordilleras (>1500 years old) of southern Chile. The interannual variability in δ13C was assessed for the period 1800–2010, and changes in discrimination and intrinsic water use efficiency (iWUE) were evaluated in relation to changes in climate and tree‐ring growth during the last century. 13C discrimination has significantly decreased, and iWUE has increased since the 1900s in both sites. However, these trends in isotopic composition have been accompanied by different growth patterns: decreasing growth rates in the Coastal Range since the 1970s and increasing growth rates in the Andes since the 1900s. Trees growing in the Coastal Range have become more efficient in their use of water, probably due to reduced stomatal conductance caused by increases in CO2 and warming. Trees growing in the Andes have also become more water use efficient, but this has been likely due to increased photosynthetic rates. Fitzroya forests, including particularly old‐growth stands, are responding to recent environmental changes, and their response has been site dependent. The growth of forests under a more Mediterranean climate influence and restrictive soil conditions in the Coastal Range has been more negatively affected by current warming and drying; while the growth of old stands in the wet Andes has been positively affected by changes in climate (decreasing cloudiness) and increasing CO2. Permanent monitoring of these endangered forests under ongoing environmental changes is needed in order to reassure the long‐term preservation of this millennial‐aged species.
Key Points
Tree‐ring width and carbon isotope chronologies for two contrasting Fitzroya sites are presented
Tree growth has decreased in the drier site and increased in the rainier site in recent decades
Fitzroya trees from both sites have become more efficient in their water use in recent decades
The soil carbon dynamics of southern hemisphere temperate rainforests have rarely been studied. Here, we report for the first time soil CO2 effluxes and their partitioning for medium‐age and ...old‐growth Fitzroya cupressoides forests growing under contrasting environmental conditions in the Coastal Range and Andean Cordillera of southern Chile. Fitzroya is a high biomass and one of the longest lived species in the world. We characterized soil respiration patterns over almost 2 yr. Annual soil respiration was slightly higher in younger forests from the Coastal Range compared with Andean forests during the first studied year (6.37–6.66 vs. 5.06–6.14 Mg C·ha−1·yr−1), and significantly higher during the second year mainly due to a warmer and drier summer (8.08–8.64 vs. 4.98–5.35 Mg C·ha−1·yr−1). Therefore, warmer and drier conditions, likely to become more common in this region under future climate change, were associated with significantly higher respiration in the shallow soils of the coast, but not in the Andes. A higher proportion of autotrophic respiration was found in the Coastal Range forest probably due to a much higher fine root biomass in this site. However, fine root productivity, an important contributor of belowground carbon fluxes, was a little lower (not significantly) in the coastal site (0.81 ± 0.60 vs. 1.50 ± 0.42 Mg C·ha−1·yr−1), indicating a longer root residence time in forests from this area. Soil CO2 effluxes from these forests and their root productivity are at the lower end of values recorded for other mature and old‐growth temperate wet forests worldwide. The intrinsic longevity and the particularly poor soils and rainy conditions where these forests grow may influence these facts. Interannual climate variability appears to be especially important for soil respiration in the Coastal Range due to the more Mediterranean climate influence and shallow, poor water retention soils in this area.
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