#L11 Vegetation demographics in ACME

Abstract

Vegetation demographics in ACME: Capturing forest response, plant co-existence, and functional shifts to droughts

ACME PI Meeting Abstract:

The inclusion of dynamic vegetation demography in Earth System Models (ESMs) has been repeatedly identified as a critical step in moving ESMs towards more realistic representations of plant ecology, and the processes that govern fluxes of carbon, energy, and water mediated by vegetation. However, incorporating dynamic vegetation demography poses huge challenges owing to their increased model complexity. These processes have not been represented in the newly developed ACME Land Model (ALM) until the recent inclusion of the Functionally-Assembled Terrestrial Ecosystem Simulator (FATES) into ALM, which is also the base model for further development in the NGEE-Tropics Project. We summarize current progress so far, assess new model features necessitated by these developments, and consider the emergence of alternative model formulations and opportunities for progress. We present multiple process approaches to represent plant competition for light, such as the Perfect Plasticity Approximation (PPA), discretized PPA used in ALM-ED, flat top crowns, and individual based approach. We will also present model evaluation on accurately representing vegetation mortality using multiple mechanistic approaches and recovery from disturbances. Lastly, we present latest model development towards a hydraulic-traits enabled FATES, which include incorporating plant hydraulic traits which will control response to drought.

A second goal of this study was to compare FATES’s ecosystem demography model predictions to field measurements across gradients of drought conditions. Over the past couple years California has been experiencing a monumental drought, and drought conditions are likely to increase worldwide. McIntyre et al. (2015) reported that compared to historical records the contemporary Californian forests have declined in large trees, biomass has decreased, and forests are becoming denser. Due to the demographic capabilities in FATES, we will compare model simulations of California forests from 2000-2015 to Forest Inventory Analysis (FIA) datasets over the same period, and to results reported from McIntyre et al. (2015). We present preliminary work on multiple temperate plant functional type (PFT) coexistence. This is development that will be used to investigate future outcomes of differential tree mortality, growth, recruitment, and shifts in plant functional type (PFT) as a result of various drought scenarios and climate change in California, over the 21^st century. We hypothesize that under drought conditions in California the mortality of all trees will increase, but there will be higher mortality for large trees. Secondly, we hypothesize that changing climates in the 21^st century will cause the climate of southern California to migrate to northern California. This will lead to a reduction in needle-leaf, evergreen trees (i.e., PFTs representing redwoods, sequoia, pines, red fir) and an increase in broadleaf deciduous trees (i.e., oaks). The goals of this work are too improve and validate functioning of FATES in ACME, in addition to broadening applicability of FATES across non-tropical biomes.

Mcintyre PJ, Thorne JH, Dolanc CR, Flint AL, Flint LE, Kelly M, Ackerly DD (2015) Twentieth-century shifts in forest structure in California: Denser forests, smaller trees, and increased dominance of oaks. Proceedings of the National Academy of Sciences, 112, 1458-1463.