Remotely sensed canopy nitrogen correlates with nitrous oxide emissions in a lowland tropical rainforest.

Tropical forests exhibit significant heterogeneity in plant functional and chemical traits that may contribute to spatial patterns of key soil biogeochemical processes, such as carbon storage and greenhouse gas emissions. Although tropical forests are the largest ecosystem source of nitrous oxide (N2 O), drivers of spatial patterns within forests are poorly resolved. Here, we show that local variation in canopy foliar N, mapped by remote-sensing image spectroscopy, correlates with patterns of soil N2 O emission from a lowland tropical rainforest. We identified ten 0.25 ha plots (assemblages of 40-70 individual trees) in which average remotely-sensed canopy N fell above or below the regional mean. The plots were located on a single minimally-dissected terrace (<1 km2 ) where soil type, vegetation structure and climatic conditions were relatively constant. We measured N2 O fluxes monthly for 1 yr and found that high canopy N species assemblages had on average three-fold higher total mean N2 O fluxes than nearby lower canopy N areas. These differences are consistent with strong differences in litter stoichiometry, nitrification rates and soil nitrate concentrations. Canopy N status was also associated with microbial community characteristics: lower canopy N plots had two-fold greater soil fungal to bacterial ratios and a significantly lower abundance of ammonia-oxidizing archaea, although genes associated with denitrification (nirS, nirK, nosZ) showed no relationship with N2 O flux. Overall, landscape emissions from this ecosystem are at the lowest end of the spectrum reported for tropical forests, consist with multiple metrics indicating that these highly productive forests retain N tightly and have low plant-available losses. These data point to connections between canopy and soil processes that have largely been overlooked as a driver of denitrification. Defining relationships between remotely-sensed plant traits and soil processes offers the chance to map these processes at large scales, potentially increasing our ability to predict N2 O emissions in heterogeneous landscapes.

Episodic Canopy Structural Transformations and Biological Invasion in a Hawaiian Forest.

The remaining native forests on the Hawaiian Islands have been recognized as threatened by changing climate, increasing insect outbreak, new deadly pathogens, and growing populations of canopy structure-altering invasive species. The objective of this study was to assess long-term, net changes to upper canopy structure in sub-montane forests on the eastern slope of Mauna Kea volcano, Hawai'i, in the context of continuing climate events, insect outbreaks, and biological invasion. We used high-resolution multi-temporal Light Detection and Ranging (LiDAR) data to quantify near-decadal net changes in forest canopy height and gap distributions at a critical transition between alien invaded lowland and native sub-montane forest at the end of a recent drought and host-specific insect (Scotorythra paludicola) outbreak. We found that sub-montane forests have experienced a net loss in average canopy height, and therefore structure and aboveground carbon stock. Additionally, where invasive alien tree species co-dominate with native trees, the upper canopy structure became more homogeneous. Tracking the loss of forest canopy height and spatial variation with airborne LiDAR is a cost-effective way to monitor forest canopy health, and to track and quantify ecological impacts of invasive species through space and time.

Nutrient acquisition, soil phosphorus partitioning and competition among trees in a lowland tropical rain forest.

We hypothesized that dinitrogen (N2 )- and non-N2 -fixing tropical trees would have distinct phosphorus (P) acquisition strategies allowing them to exploit different P sources, reducing competition. We measured root phosphatase activity and arbuscular mycorrhizal (AM) colonization among two N2 - and two non-N2 -fixing seedlings, and grew them alone and in competition with different inorganic and organic P forms to assess potential P partitioning. We found an inverse relationship between root phosphatase activity and AM colonization in field-collected seedlings, indicative of a trade-off in P acquisition strategies. This correlated with the predominantly exploited P sources in the seedling experiment: the N2 fixer with high N2 fixation and root phosphatase activity grew best on organic P, whereas the poor N2 fixer and the two non-N2 fixers with high AM colonization grew best on inorganic P. When grown in competition, however, AM colonization, root phosphatase activity and N2 fixation increased in the N2 fixers, allowing them to outcompete the non-N2 fixers regardless of P source. Our results indicate that some tropical trees have the capacity to partition soil P, but this does not eliminate interspecific competition. Rather, enhanced P and N acquisition strategies may increase the competitive ability of N2 fixers relative to non-N2 fixers.

Environmental controls on canopy foliar nitrogen distributions in a Neotropical lowland forest.

Distributions of foliar nutrients across forest canopies can give insight into their plant functional diversity and improve our understanding of biogeochemical cycling. We used airborne remote sensing and partial least squares regression to quantify canopy foliar nitrogen (foliar N) across ~164 km2 of wet lowland tropical forest in the Osa Peninsula, Costa Rica. We determined the relative influence of climate and topography on the observed patterns of foliar N using a gradient boosting model technique. At a local scale, where climate and substrate were constant, we explored the influence of slope position on foliar N by quantifying foliar N on remnant terraces, their adjacent slopes, and knife-edged ridges. In addition, we climbed and sampled 540 trees and analyzed foliar N in order to quantify the role of species identity (phylogeny) and environmental factors in predicting foliar N. Observed foliar N heterogeneity reflected environmental factors working at multiple spatial scales. Across the larger landscape, elevation and precipitation had the highest relative influence on predicting foliar N (30% and 24%), followed by soils (15%), site exposure (9%), compound topographic index (8%), substrate (6%), and landscape dissection (6%). Phylogeny explained ~75% of the variation in the field collected foliar N data, suggesting that phylogeny largely underpins the response to the environmental factors. Taken together, these data suggest that a large fraction of the variance in foliar N across the landscape is proximately driven by species composition, though ultimately this is likely a response to abiotic factors such as climate and topography. Future work should focus on the mechanisms and feedbacks involved, and how shifts in climate may translate to changes in forest function.