Biology subjects

Phillips, O. L.

Publications and source records attributed to Phillips, O. L..

4 recordsLinked to original sources

bioRxiv2025

Thresholds of drought and terrain complexity shape biomasspatterns in South America's Caatinga

Seasonally Tropical Dry Forests (STDFs) are widespread throughout the world and store significant amounts of carbon; however, they are often overlooked in spatial assessments compared to humid tropical forests. The Caatinga, which is the largest seasonally dry tropical forest in South America, covers approximately 862,000 km{superscript 2} in northeastern Brazil and supports millions of people. Unfortunately, its carbon dynamics has not been thoroughly quantified, especially after centuries of land-use transformation and wood extraction that have significantly diminished its biomass stocks. In this study, we model the potential aboveground biomass (AGB) that Caatinga could sustain under current climatic, atmospheric, and topographic conditions. We integrated data from 301 geo-referenced plots along with high-resolution environmental predictors. The principal Component Analysis revealed two main gradients: a hydro-thermal axis dominated by precipitation, temperature, and severity of drought, and a topographic axis reflecting slope, ruggedness, and terrain position. Random Forest models, validated through both random and spatial cross-validation, explained a significant amount of variation in AGB (R{superscript 2} = 0.81, RMSE = 21 Mg ha). Our findings indicated that AGB is highly sensitive to water availability. In particular, biomass increased sharply when the maximum cumulative water deficit (MCWD) was less than -500 mm and annual rainfall exceeded approximately 1,100 mm. In contrast, elevated vapour pressure deficit (VPD) and potential evapotranspiration (PET) were associated with reduced carbon storage. Topographic heterogeneity further influenced AGB, with rugged and concave terrains supporting potential biomass levels more than twice as high as those found in flat, convex areas. Our predictive map reveals a mosaic of low-biomass cores and localized high-biomass refugia, highlighting the dual influence of hydroclimatic and topographic factors. These findings reposition Caatinga as a heterogeneous and dynamic potential carbon reservoir - historically degraded by changes in land-use but still capable of storing substantial carbon. This research offers critical insights for the restoration, conservation and mitigation efforts for climate change in tropical drylands worldwide.

ecology↗

bioRxiv2023

Forest demography and biomass accumulation rates are associated with transient mean tree size vs density scaling relations

Linking individual and stand-level dynamics during forest development reveals a scaling relationship between mean tree size and tree density in forest stands, which integrates forest structure and function. However, the nature of this so-called scaling law and its variation across broad spatial scales remains unquantified and its linkage with forest demographic processes and carbon dynamics remains elusive. Here we develop a theoretical framework and compile a broad-scale dataset of long-term sample forest stands (n = 1433) from largely undisturbed forests to examine the association of temporal mean tree size vs density scaling trajectories (slopes) with biomass accumulation rates and the sensitivity of scaling slopes to environmental and demographic drivers. The results empirically demonstrate a large variation of scaling slopes, ranging from -4 to -0.2, across forest stands in tropical, temperate and boreal forest biomes. Steeper scaling slopes are associated with higher rates of biomass accumulation, resulting from a lower offset of forest growth by biomass loss from mortality. In North America, scaling slopes are positively correlated with forest stand age and rainfall seasonality, thus suggesting a higher rate of biomass accumulation in younger forests with lower rainfall seasonality. These results demonstrate the strong association of the transient mean tree size vs density scaling trajectories with forest demography and biomass accumulation rates, thus highlighting the promise of leveraging forest structure properties to predict forest demography, carbon fluxes and dynamics at broad spatial scales. Significance StatementMean tree size vs density scaling relationships are thought to predict forest function at broad spatial scales. Here we develop a theoretical framework based upon demographic processes and empirical evidence from forest inventory data to demonstrate a strong association of the transient mean tree size and density scaling trajectories (slopes) with forest demography and biomass accumulation rates. This strong association is pervasive across forest biomes and suggests a negative relationship between scaling slope and biomass accumulation rate (resource availability). Our results highlight the promise of leveraging forest structure (i.e., inferred from high resolution remote sensing data or fused into size-structured demographic models) to evaluate forest demography, carbon fluxes and dynamics at broad spatial scales.

ecology↗

bioRxiv2021

Tropical tree growth sensitivity to climate is driven by species intrinsic growth rate and leaf traits

A better understanding of how climate affects growth in tree species is essential for improved predictions of forest dynamics under climate change. Long-term climate averages (mean climate) and short-term deviations from these averages (anomalies) both influence tree growth, but the rarity of long-term data integrating climatic gradients with tree censuses has so far limited our understanding of their respective role, especially in tropical systems. Here, we combined 49 years of growth data for 509 tree species across 23 tropical rainforest plots along a climatic gradient to examine how tree growth responds to both climate means and anomalies, and how species functional traits mediate these tree growth responses to climate. We showed that short-term, anomalous increases in atmospheric evaporative demand and solar radiation consistently reduced tree growth. Drier forests and fast-growing species were more sensitive to water stress anomalies. In addition, species traits related to water use and photosynthesis partly explained differences in growth sensitivity to both long-term and short-term climate variations. Our study demonstrates that both climate means and anomalies shape tree growth in tropical forests, and that species traits can be leveraged to understand these demographic responses to climate change, offering a promising way forward to forecast tropical forest dynamics under different climate trajectories.

ecology↗

bioRxiv2021

The Maximum Entropy Formalism of statistical mechanics in a biological application: a quantitative analysis of tropical forest ecology

In a time of rapid global change, the question of what determines patterns in species abundance distribution remains a priority for understanding the complex dynamics of ecosystems. The constrained maximization of information entropy provides a framework for the understanding of such complex systems dynamics by a quantitative analysis of important constraints via predictions using least biased probability distributions. We apply it to over two thousand hectares of Amazonian tree inventories across seven forest types and thirteen functional traits, representing major global axes of plant strategies. Results show that constraints formed by regional relative abundances of genera explain eight times more of local relative abundances than constraints based on directional selection for specific functional traits, although the latter does show clear signals of environmental dependency. These results provide a quantitative insight by inference from large-scale data using cross-disciplinary methods, furthering our understanding of ecological dynamics.

ecology↗