Showing papers by "Andy Hector published in 2020"

General destabilizing effects of eutrophication on grassland productivity at multiple spatial scales

Abstract: Eutrophication is a widespread environmental change that usually reduces the stabilizing effect of plant diversity on productivity in local communities. Whether this effect is scale dependent remains to be elucidated. Here, we determine the relationship between plant diversity and temporal stability of productivity for 243 plant communities from 42 grasslands across the globe and quantify the effect of chronic fertilization on these relationships. Unfertilized local communities with more plant species exhibit greater asynchronous dynamics among species in response to natural environmental fluctuations, resulting in greater local stability (alpha stability). Moreover, neighborhood communities that have greater spatial variation in plant species composition within sites (higher beta diversity) have greater spatial asynchrony of productivity among communities, resulting in greater stability at the larger scale (gamma stability). Importantly, fertilization consistently weakens the contribution of plant diversity to both of these stabilizing mechanisms, thus diminishing the positive effect of biodiversity on stability at differing spatial scales. Our findings suggest that preserving grassland functional stability requires conservation of plant diversity within and among ecological communities.

The global abundance of tree palms

Abstract: Aim: Palms are an iconic, diverse and often abundant component of tropical ecosystems that provide many ecosystem services. Being monocots, tree palms are evolutionarily, morphologically and physiologically distinct from other trees, and these differences have important consequences for ecosystem services (e.g., carbon sequestration and storage) and in terms of responses to climate change. We quantified global patterns of tree palm relative abundance to help improve understanding of tropical forests and reduce uncertainty about these ecosystems under climate change. Location: Tropical and subtropical moist forests. Time period: Current. Major taxa studied: Palms (Arecaceae). Methods: We assembled a pantropical dataset of 2,548 forest plots (covering 1,191 ha) and quantified tree palm (i.e., ≥10 cm diameter at breast height) abundance relative to co-occurring non-palm trees. We compared the relative abundance of tree palms across biogeographical realms and tested for associations with palaeoclimate stability, current climate, edaphic conditions and metrics of forest structure. Results: On average, the relative abundance of tree palms was more than five times larger between Neotropical locations and other biogeographical realms. Tree palms were absent in most locations outside the Neotropics but present in >80% of Neotropical locations. The relative abundance of tree palms was more strongly associated with local conditions (e.g., higher mean annual precipitation, lower soil fertility, shallower water table and lower plot mean wood density) than metrics of long-term climate stability. Life-form diversity also influenced the patterns; palm assemblages outside the Neotropics comprise many non-tree (e.g., climbing) palms. Finally, we show that tree palms can influence estimates of above-ground biomass, but the magnitude and direction of the effect require additional work. Conclusions: Tree palms are not only quintessentially tropical, but they are also overwhelmingly Neotropical. Future work to understand the contributions of tree palms to biomass estimates and carbon cycling will be particularly crucial in Neotropical forests.

Monitoring tropical forest degradation and restoration with satellite remote sensing: A test using Sabah Biodiversity Experiment

Abstract: Selective logging has been so extensive that harvested forest now exceeds unlogged areas in most tropical forest regions outside of the Amazon. In response, in Southeast Asia, enrichment planting with dipterocarp tree species is carried out in an attempt to accelerate restoration of forest structure and functioning. However, assessing the impacts of degradation (from selective logging and other causes) and subsequent restoration with field measurements is expensive and time-consuming. There is therefore a need to develop methods for the assessment of forest quality using remote sensing. Here, we use high spatial resolution satellite imagery and advanced remote sensing products to monitor the pattern and dynamics of estimated vegetation cover, Leaf Area Index (LAI), and the biomass of plots within a field-scale (500 ha) replicated and randomized manipulation that compares different forest restoration treatments with naturally regenerating controls within the Sabah Biodiversity Experiment (SBE). We also compare the biodiversity experiment plots with the surrounding area of the Malua Forest Reserve that was selectively logged for the second time in 2007. In general, satellite remote sensing detected differences in degradation between the once- and twice-logged areas as well as between the different experimental restoration treatments. We found that approximately 70% of the Malua Forest Reserve experienced a decrease of vegetation cover after the selective relogging in 2007, while the Sabah Biodiversity Experiment area that was not relogged showed increasing vegetation cover. Within the experiment, we found that plots restored using Enrichment line planting, had higher remote sensed vegetation cover (Mean ± SE: 66.90 ± 0.06 vs. 61.96 ± 0. 16) and LAI (Mean ± SE: 5.09 ± 0.03 vs. 4.61 ± 0.11) than that of unenriched plots. Among the enrichment planted plots, those planted with mixtures of (4 or 16) species exhibited higher vegetation cover (Mean ± SE: 67.72 ± 0.06 vs. 65.35 ± 0. 09) and LAI (Mean ± SE: 5.29 ± 0.04 vs. 4.82 ± 0.06) than that of monoculture plots. Overall, when our test case of the Sabah Biodiversity Experiment was viewed through the lens of remote sensing indicators, satellite imagery was able to detect changes in forest quality due to selective logging and restoration enrichment planting. Furthermore, our results suggest that diverse mixtures of planted tree seedlings enhance restoration of forest canopies compared to planting with single species. Confirmation with ground data will be needed to validate these results and to better understand the biological processes determining tropical forest degradation and restoration.

Fast and furious: Early differences in growth rate drive short-term plant dominance and exclusion under eutrophication.

Abstract: The reduction of plant diversity following eutrophication threatens many ecosystems worldwide. Yet, the mechanisms by which species are lost following nutrient enrichment are still not completely understood, nor are the details of when such mechanisms act during the growing season, which hampers understanding and the development of mitigation strategies.Using a common garden competition experiment, we found that early-season differences in growth rates among five perennial grass species measured in monoculture predicted short-term competitive dominance in pairwise combinations and that the proportion of variance explained was particularly greater under a fertilization treatment.We also examined the role of early-season growth rate in determining the outcome of competition along an experimental nutrient gradient in an alpine meadow. Early differences in growth rate between species predicted short-term competitive dominance under both ambient and fertilized conditions and competitive exclusion under fertilized conditions.The results of these two studies suggest that plant species growing faster during the early stage of the growing season gain a competitive advantage over species that initially grow more slowly, and that this advantage is magnified under fertilization. This finding is consistent with the theory of asymmetric competition for light in which fast-growing species can intercept incident light and hence outcompete and exclude slower-growing (and hence shorter) species. We predict that the current chronic nutrient inputs into many terrestrial ecosystems worldwide will reduce plant diversity and maintain a low biodiversity state by continuously favoring fast-growing species. Biodiversity management strategies should focus on controlling nutrient inputs and reducing the growth of fast-growing species early in the season.

Biotic and abiotic drivers of soil microbial functions across tree diversity experiments

Abstract: Aim: Soil microorganisms are essential for the functioning of terrestrial ecosystems. Although soil microbial communities and functions may be linked to the tree species composition and diversity of forests, there has been no comprehensive study of how general potential relationships are and if these are context-dependent. A global network of tree diversity experiments (TreeDivNet) allows for a first examination of tree diversity-soil microbial function relationships across environmental gradients. Location: Global. Major Taxa Studied: Soil microorganisms. Methods: Soil samples collected from eleven tree diversity experiments in four biomes across four continents were used to measure soil basal respiration, microbial biomass, and carbon use efficiency using the substrate-induced respiration method. All samples were measured using the same analytical device in the same laboratory to prevent measurement bias. We used linear mixed-effects models to examine the effects of tree species diversity, environmental conditions, and their interactions on soil microbial functions. Results: Across biodiversity experiments, abiotic drivers, mainly soil water content, significantly increased soil microbial functions. Potential evapotranspiration (PET) increased, whereas soil C-to-N ratio (CN) decreased soil microbial functions under dry soil conditions, but high soil water content reduced the importance of other abiotic drivers. Tree species richness and phylogenetic diversity had overall similar, but weak and context-dependent (climate, soil abiotic variables) effects on soil microbial respiration. Positive tree diversity effects on soil microbial respiration were most pronounced at low PET, low soil CN, and high tree density. Soil microbial functions increased with the age of the experiment. Main conclusions: Our results point at the importance of soil water content for maintaining high levels of soil microbial functions and modulating effects of other environmental drivers. Moreover, overall tree diversity effects on soil microbial functions seem to be negligible in the short term (experiments were 1-18 years old). However, context-dependent tree diversity effects (climate, soil abiotic variables) have greater importance at high tree density, and significant effects of experimental age call for longer-term studies. Such systematic insights are key to better integrate soil carbon dynamics into the management of afforestation projects across environmental contexts, as today9s reforestation efforts remain focused largely on aboveground carbon storage and are still dominated by less diverse forests stands of commercial species.