Laia Andreu-Hayles

The impact of rising temperatures on tropical treeline ecosystems remains understudied. Here we report on the growth history of Polylepis pepei trees growing in Madidi National Park, a tropical forest setting at elevational treeline in the Andes-Amazon ecotone of Bolivia. Using dendrochronological methods, we developed an annually-resolved tree-ring width chronology spanning from 1850 to 2018 CE. To our knowledge this is the longest tree-ring record for this species (168 years). The standardized ring-width chronology revealed a significant radial growth decline after the 1962–1963 growth ring (i.e. 1963–2018). From 1960–2015, smaller ring-widths were associated with drier and warmer conditions during the previous wet season (∼ November–March), while wetter and cooler conditions led to enhanced growth in the following year. P. pepei radial growth decline coincided with a significant increase in minimum temperature and decrease in precipitation and diurnal temperature range that was observed both locally and regionally across tropical South America between 1960–2015. These climate trends at treeline may indicate a reduction in moisture convergence and transport to such higher elevation settings in the Andes. If temperatures continue to rise at current rates, P. pepei growth may continue to decline, threatening the survival of tropical treeline ecosystems in Bolivia.

The magnitude of the terrestrial carbon sink remains a key uncertainty in future climate projections, in part due to poorly understood links between carbon uptake and its allocation to woody biomass in vegetation. Here, in this study, we show that photosynthesis and aboveground growth occur asynchronously across diel to seasonal scales in eight North American oak species. Across 137 tree ring sites, current-year annual growth was insensitive to climate variability after midsummer despite 26 to 36% of annual gross primary productivity (GPP) occurring during this period. Hourly GPP flux and growth measurements at four sites spanning seven site years further demonstrate that wood formation ceases earlier than photosynthesis and is restricted to periods of low atmospheric aridity and temperature. This photosynthesis-growth decoupling intensifies with interannual variability in vapor pressure deficit ( r = 0.86, P < 0.05), suggesting that by assuming tight coupling between photosynthesis and woody biomass, current earth system models may overestimate long-term carbon sequestration in forests.

Central America faces increasing risks from climate variability and extreme weather events. Limited observational records and model biases have constrained our ability to understand the ocean–atmosphere dynamics that influence precipitation variability in the region over longer timescales. Paleoclimate proxies, including the stable oxygen isotope ratio of the cellulose of tropical trees, can extend the climate record, allowing recent trends and variability to be evaluated in a long‐term context and improving our understanding of forced and unforced variability of the climate system. Here, we present a new multidecadal record of tree‐ring from Abies guatemalensis (Guatemalan Fir) from Guatemala and Honduras. We demonstrate that this proxy records boreal summer rainfall and is tightly coupled to neotropical ocean–atmosphere dynamics. This precisely dated, high‐resolution proxy can be used for multicentury hydroclimate reconstructions of the Intertropical Convergence Zone dynamics and its interactions with the eastern Pacific Ocean and Caribbean Sea.

Summary: Tree rings are considered the gold standard for observing variation in past atmospheric radiocarbon ( ¹⁴ C), yet little attention has been paid to whether different trees record tropospheric ¹⁴ C evenly. The discovery of Miyake events, rapid increases in ¹⁴ C production occurring in past millennia, has led to repeated measurements of ¹⁴ C in tree rings across species and locations. These records demonstrate remarkable synchrony, yet significant variability between samples remains, limiting reliable use of tree rings as a precise indicator of the timing and scale of past ¹⁴ C production. Understanding how trees and species record tropospheric ¹⁴ C would improve reconstructions of past ¹⁴ C production events, refine geochronological control, and because of the pulse‐like nature of past ¹⁴ C production events, shed light on a fundamental question in tree physiology and dendrochronology: what is the lag time between photosynthesis, storage, and allocation to wood and how might this lag blur isotopic signals in tree rings? Here, we evaluate the implicit assumption that all trees record tropospheric ¹⁴ C evenly by focusing on the path carbon takes within trees including: phenology of carbon uptake and wood formation, storage and use of nonstructural carbohydrates, and how these factors interact to affect the tropospheric ¹⁴ C signal in wood.

Central America faces increasing risks from climate variability and extreme weather events. Limited observational records and model biases have constrained our ability to understand the ocean–atmosphere dynamics that influence precipitation variability in the region over longer timescales. Paleoclimate proxies, including the stable oxygen isotope ratio of the cellulose of tropical trees, can extend the climate record, allowing recent trends and variability to be evaluated in a long-term context and improving our understanding of forced and unforced variability of the climate system. Here, we present a new multidecadal record of δ18O from Abies guatemalensis from Guatemala and Honduras. We show that this proxy records boreal summer rainfall and is tightly coupled to neotropical ocean–atmosphere dynamics. This precisely dated, high-resolution proxy can be used for multicentury hydroclimate reconstructions of the Intertropical Convergence Zone dynamics and its interactions with the eastern Pacific Ocean and Caribbean Sea.

Paleoclimatology makes it possible to place recent climate changes in a longer‐term context than is available from the instrumental record. Tree‐ring reconstructions, often used to quantify temperature variations over the Common Era, contain multiple uncertainties that affect estimates of the magnitude of recent trends and past variability. The use of maximum latewood density (MXD) proxy mitigates many biases, but intra‐annual measurement resolution remains a limitation. We develop a quantitative wood anatomy (QWA)‐based temperature reconstruction from the Firth River in northeastern Alaska from 1150 CE to 2021 CE to improve regional past temperature estimates and to test the sensitivity of MXD‐based climate reconstructions to measurement constraints. We find that high‐resolution wood‐anatomy measurements reduce biological noise and enhance the representation of low‐frequency variability, resulting in stronger temperature signals and a larger magnitude of preindustrial to modern change. QWA data provide novel high‐resolution information that improves tree‐ring temperature reconstructions.

Relative to research efforts in higher latitiudes, the impact of climate shifts in the tropical treeline remains understudied. Little is known about the tree growth dynamics and climate response at this treeline over the past few centuries, and at present under a rapidly changing environment. Here we provide information on recent changes in tree-ring patterns of Polylepis pepei BB.Simpson, a tropical tree species that grows in a monospecific forest at the elevational treeline in the Andes-Amazon ecotone of Bolivia and identify factors that limit its radial growth. We first developed a ring width (RW) chronology spanning 1867–2018 C.E. using dendrochronological methods and independently verified annual periodicity with radiocarbon dating. The RW chronology indicates a significant (p < 0.01) radial growth decline in P. pepei since 1997, a trend that mirrors a decrease reported in other Polylepis species from the drier central Andes of South America. P. pepei tree-ring width (RW) was mostly limited by mean, minimum, and maximum temperature and precipitation during austral summer (November–January). Over the instrumental period (1981–2019) prior-year temperatures negatively affected current-year tree growth (p < 0.05), while prior-year wet conditions were associated with higher growth (p < 0.05). Gridded temperature records (1901–2019) showed a significant increase in minimum temperatures and a decline in the diurnal temperature range since 1967, which may reduce orographic convection and water availability at higher elevations where our forest is located. In situ daily measurements from dataloggers in the forest recorded higher temperatures and lower relative humidity values when data was available. Our results suggest less moisture availability associated with warming conditions was related to the observed tree-growth decline. If temperature continues to rise at current rates, one of the highest-elevation tree species on the globe, P. pepei, could face severe consequences. This work provides insights into the past and historical trends of a tropical Andean treeline, which shows a recent decline also observed in other high-elevation forests (4657–4800 m.a.s.l.) of tropical South America (>17° S).

Relative to research efforts in higher latitiudes, the impact of climate shifts in the tropical treeline remains understudied. Little is known about the tree growth dynamics and climate response at this treeline over the past few centuries, and at present under a rapidly changing environment. Here we provide information on recent changes in tree-ring patterns of Polylepis pepei BB.Simpson, a tropical tree species that grows in a monospecific forest at the elevational treeline in the Andes-Amazon ecotone of Bolivia and identify factors that limit its radial growth. We first developed a ring width (RW) chronology spanning 1867–2018 C.E. using dendrochronological methods and independently verified annual periodicity with radiocarbon dating. The RW chronology indicates a significant (p < 0.01) radial growth decline in P. pepei since 1997, a trend that mirrors a decrease reported in other Polylepis species from the drier central Andes of South America. P. pepei tree-ring width (RW) was mostly limited by mean, minimum, and maximum temperature and precipitation during austral summer (November–January). Over the instrumental period (1981–2019) prior-year temperatures negatively affected current-year tree growth (p < 0.05), while prior-year wet conditions were associated with higher growth (p < 0.05). Gridded temperature records (1901–2019) showed a significant increase in minimum temperatures and a decline in the diurnal temperature range since 1967, which may reduce orographic convection and water availability at higher elevations where our forest is located. In situ daily measurements from dataloggers in the forest recorded higher temperatures and lower relative humidity values when data was available. Our results suggest less moisture availability associated with warming conditions was related to the observed tree-growth decline. If temperature continues to rise at current rates, one of the highest-elevation tree species on the globe, P. pepei, could face severe consequences. This work provides insights into the past and historical trends of a tropical Andean treeline, which shows a recent decline also observed in other high-elevation forests (4657–4800 m.a.s.l.) of tropical South America (>17° S).

Sea surface temperature (SST) variability in the equatorial Pacific plays a crucial role in shaping global climate patterns. As El Niño–Southern Oscillation (ENSO)‐related extremes intensify, understanding SST fluctuations and their connection to ENSO becomes increasingly important. However, limited instrumental records hinder the ability to simulate SST variability and distinguish natural variability from anthropogenic trends. Tree rings provide annual data essential for reconstructing past SST and ENSO dynamics. While previous research has successfully reconstructed SST variability in the central equatorial Pacific, capturing signals from the eastern Pacific remains more challenging. This study analyzes P. rodolfovasquezii trees from the central Peruvian Andes (11.72°S, 75.14°W) using ring width and stable oxygen and carbon isotopes to assess their sensitivity to Pacific SST variability. We focus on the two leading modes of SST variability derived from Empirical Orthogonal Functions: EOF1 representing SST variability in the Niño 3.4 region, and EOF2 depicting a west‐east dipole between the Niño 4 and 1 + 2 regions, explaining 66.22% and 10.79% of the variance in tropical Pacific SSTs, respectively. We developed a 129‐year multiproxy tree‐ring record. Trees' sensitivity to local and regional hydroclimate is reflected in the temporal variability of their isotopic content. Tree‐ring oxygen isotopes showed significant correlations with both SST modes, although their ability to capture EOF2 variability was limited—likely due to the trees' distance from the influence region of the Niño 1 + 2. Overall, oxygen isotopes outperformed ring width and carbon isotopes, underscoring their potential for future SST reconstructions in the tropical Andes.

Relative to research efforts in higher latitiudes, the impact of climate shifts in the tropical treeline remains understudied. Little is known about the tree growth dynamics and climate response at this treeline over the past few centuries, and at present under a rapidly changing environment. Here we provide information on recent changes in tree-ring patterns of Polylepis pepei BB.Simpson, a tropical tree species that grows in a monospecific forest at the elevational treeline in the Andes-Amazon ecotone of Bolivia and identify factors that limit its radial growth. We first developed a ring width (RW) chronology spanning 1867–2018 C.E. using dendrochronological methods and independently verified annual periodicity with radiocarbon dating. The RW chronology indicates a significant (p < 0.01) radial growth decline in P. pepei since 1997, a trend that mirrors a decrease reported in other Polylepis species from the drier central Andes of South America. P. pepei tree-ring width (RW) was mostly limited by mean, minimum, and maximum temperature and precipitation during austral summer (November–January). Over the instrumental period (1981–2019) prior-year temperatures negatively affected current-year tree growth (p < 0.05), while prior-year wet conditions were associated with higher growth (p < 0.05). Gridded temperature records (1901–2019) showed a significant increase in minimum temperatures and a decline in the diurnal temperature range since 1967, which may reduce orographic convection and water availability at higher elevations where our forest is located. In situ daily measurements from dataloggers in the forest recorded higher temperatures and lower relative humidity values when data was available. Our results suggest less moisture availability associated with warming conditions was related to the observed tree-growth decline. If temperature continues to rise at current rates, one of the highest-elevation tree species on the globe, P. pepei, could face severe consequences. This work provides insights into the past and historical trends of a tropical Andean treeline, which shows a recent decline also observed in other high-elevation forests (4657–4800 m.a.s.l.) of tropical South America (>17° S).