Global Vegetation Photosynthetic Phenology Products Based on MODIS Vegetation Greenness and Temperature: Modeling and Evaluation

Land surface phenology (LSP) products that are derived from different data sources have different definitions and biophysical meanings. Discrepancies among these products and their linkages with carbon fluxes across plant functional types and climatic regions remain somewhat unclear. In this study, to differentiate LSP related to gross primary production (GPP) from LSP related to remote sensing data, we defined the former as vegetation photosynthetic phenology (VPP), including the starting and ending days of GPP (SOG and EOG, respectively). Specifically, we estimated VPP based on a combination of observed VPP from 145 flux-measured GPP sites together with the vegetation index and temperature data from MODIS products using multiple linear regression models. We then compared VPP estimates with MODIS LSP on a global scale. Our results show that the VPP provided better estimates of SOG and EOG than MODIS LSP, with a root mean square error (RMSE) for SOG of 12.7 days and a RMSE for EOG of 10.5 days. The RMSE was approximately three weeks for both SOG and EOG estimates of the non-forest type. Discrepancies between VPP and LSP estimates varied across plant functional types (PFTs) and climatic regions. A high correlation was observed between VPP and LSP estimates for deciduous forest. For most PFTs, using VPP estimates rather than LSP improved the estimation of GPP. This study presents a useful method for modeling global VPP, investigates in detail the discrepancies between VPP and LSP, and provides a more effective global vegetation phenology product for carbon cycle modeling than the existing ones.

Differences in C, N, δ13C and δ15N among plant functional types after a wildfire in a black spruce forest, interior Alaska

We measured differences in %C, %N,  13C and  15N of plant functional types 17 (PFTs) between burned and unburned ground surfaces soon after a wildfire on a north-18 facing slope in interior Alaska. The C and N were measured for 16 species and 19 Sphagnum litter.  13C differed among the PFTs and was low for trees and shrubs, 20 suggesting that woody stems slowed C dynamics or showed low water use efficiency. 21  15N concentrations suggested that the herbaceous plants depended less on the 22 mycorrhizal associations that became weak on the burned surfaces. The shrub leaves 23 showed the lowest  15N of PFTs and showed higher  15N on the burned surface, showing 24 that N transfer from the soils to the leaves in the shrubs was slowed by the wildfire. 25 Mosses showed the highest C/N ratio. Sphagnum litter decomposed faster on the burned 26 surface, and %N and  15N in the litter increased from the second to third year on both 27 burned and unburned surfaces, while %C changed little. In conclusion, the responses to 28 the wildfire differed among the PFTs as characterized by their C and N dynamics. 29 30 Key words: Burned and unburned ground surface, carbon (C) and nitrogen (N), Alaskan 31 taiga, plant functional type, stable isotope

Simplicity or complexity? Important aspects of high nature value grassland management in nature conservation

Local, adaptive traditional grassland management systems have played a fundamental role in the creation, maintenance and conservation of high nature value (HNV) grasslands. The state of diverse HNV grasslands has deteriorated across Europe in conjunction with changes in various management factors, such as management type and management intensity. To conserve the species-rich vegetation of HNV grasslands and to avoid undesirable shifts in plant functional type dominance, it is important to explore the effects of management factors crucial for nature conservation and to adapt them to local circumstances. In our study, we focus on three of the main factors in the management of valuable meadow steppes in the Great Hungarian Plain region (Central Hungary). We studied management types (mowing, grazing and combined), different levels of herbage removal intensity (low, medium, high) and spatio-temporal complexity (low, medium and high) of grassland management. Altogether 172 plots (1 m × 1 m) were designated in 17 sites. Plant diversity indexes and plant functional types were calculated according to the presence and percentage cover of plant species in the plots. Regarding plant diversity and the dominance of plant functional types, herbage removal intensity and spatio-temporal complexity of management had, for the most part, stronger effects than the type of management. Higher spatio-temporal complexity of management resulted in higher plant diversity, while higher intensity of management led to significantly lower diversity. Proper application of type, intensity and spatio-temporal complexity of management practices (separately and in combination) proved to be determining factors in the long-term maintenance and conservation of diversity and species composition of HNV grasslands.

Responses of Sap Flux Densities of Different Plant Functional Types to Environmental Variables Are Similar in Both Dry and Wet Seasons in a Subtropical Mixed Forest

Subtropical mixed forest ecosystems are experiencing dramatic changes in precipitation and different plant functional types growing here are expected to respond differently. This study aims to unravel the water use patterns of different plant functional types and their responses to environmental changes in a typical subtropical mixed forest in southern China. Diurnal and seasonal sap flux densities of evergreen broad-leaved trees (EBL), deciduous broad-leaved trees (DBL), and conifers (CON), as well as environmental variables, were recorded simultaneously from May 2016 to March 2019. The results showed that the sap flux density of EBL was significantly higher than those of CON and DBL in all seasons, irrespective of dry or wet seasons. Path analysis revealed that seasonal differences in sap flux density were mainly due to variations in photosynthetic photon flux density (PPFD). At saturating PPFD, changes in sap flux density during the day were in response to vapor pressure deficit (VPD). Regression analyses showed that sap flux density increased logarithmically with PPFD, irrespective of functional type. The hysteresis loops of sap flux density and VPD were different among different plant functional types in wet and dry seasons. Our results demonstrated converging response patterns to environmental variables among the three plant functional types considered in this study. Our findings contribute to a better understanding of the water use strategies of different plant functional types in subtropical mixed forests.