See the forest not the trees! Ecosystem-based assessment of response, resilience, and scope for growth of global forests

Abstract

Anthropogenic activities are degrading forest health globally. Detection of changes in forest growth rates is possible with field measures of total net primary productivity (tNpp), however, assessing forest adaptive capacity was historically challenging due to a scarcity of tNpp data, knowledge of carbon allocation to above- and belowground biomass, and inability to calculate forest productive potential. This study used a global data set of 307 published research studies to identify tNpp thresholds and site-level factors constraining growth for forests in boreal, temperate, and tropical biomes. This data makes it possible to: (i) calculate the ratio of measured tNpp to a theoretical maximum tNpp from variables external to the site, its “ecosystem fit,” (ii) identify environmental thresholds by scale and biogeography, and (iii) determine stand level conditions that limit growth. At the global scale, climatic variables explain most of the variance in tNpp whereas, at the biome scale different combinations of climatic/edaphic variables interact with phenological traits to explain productivity. For example, deciduous boreal forests were less resilient if precipitation increased, but deciduous tropical forests were less resilient if minimum annual temperature increased. At the biome scale, boreal forest mean tNpp (7.3 Mg ha−1 yr−1) was significantly lower than tropical forests (19.0 Mg ha−1 yr−1) but the mean tNpp of temperate forests tNpp (14.2 Mg ha−1 yr−1) did not significantly differ from boreal or tropical forests, representing a first-order latitudinal forcing of productivity. Comparison of ranked clusters of tNpp (low, medium, high) indicated that the tropics had a larger proportion of highly productive forests (20%) than temperate (12%) or boreal forests (14%). Tropical forests are highly adapted to small-scale environmental heterogeneity but their unique evolutionary trajectories make them more sensitive to land-use change, habitat fragmentation, and climate disruption. There was minimal overlap among the high tNpp groups between boreal forests (12.0–18.2 Mg ha−1 yr−1) and temperate forests (17.6–37.7 Mg ha−1 yr−1), and moderate overlap between temperate and tropical forests (26.8–45.9 Mg ha−1 yr−1), indicating increased adaptive capacity to environmental variability by climatic zone. All tropical forests had similar ecosystem fit, but Paleotropic forests growing on Inceptisol and Entisol soils were significantly more productive than Neotropical forests revealing similar adaptive capacity although there were less favorable growing conditions in the Neotropics. This study supports including site-level variations in edaphic and climatic factors to understand changes in primary productivity due to disturbance and proposes using Ecosystem fit to identify forest adaptive capacity in response to climate destabilization.