Recent studies show that past and ongoing environmental changes have been substantial and have likely already affected conservation efforts in Hawaii. Much of the state has experienced substantial drying, including decreases in mean annual precipitation since the 1920s, longer rainless periods, and decreasing stream flow. Temperatures have been increasing in the state for the last 40 years, especially at higher elevations where most native habitats and species currently persist. Unfortunately there are few long term monitoring efforts that allow us to understand plant species responses to these past, ongoing and future shifts in environmental conditions. Consequently, we know little about how environmental shifts may be limiting the success of current conservation and management efforts. We have adapted high resolution digital dendrometer methods to the harsh field conditions of Hawaiian forests. To help define optimal management for these species under a shifting climate, we have begun deploying recently developed autonomous dendrometer sensor arrays to monitor growth of individual native and non-native trees and local environmental conditions at sub-daily intervals. Similar methods have been used elsewhere in precision tree plantations and agricultural settings to link optimal/detrimental conditions to individual plant performance (Drew & Downes 2009; Pajares 2011). A recent test in Hawaiian wet forests indicates we can use these techniques to monitor growth of native plants as an integrated measure of plant fitness and relate it to variable environmental conditions such as precipitation, temperature, light, and soil moisture. The digital dendrometer sensor array is unobtrusive, autonomous and provides real-time data relevant to a large number of conservation related potential applications.
We are determining best modeling approaches to define the relationship between daily, weekly, monthly and seasonal tree diameter change to multiple interacting environmental variables such as PAR, temperature, precipitation, and soil moisture. This will require a complex time series analysis as diameter size variations are a consequence of two related processes: daily fluctuations of stem water and longer-term diameter increment from woody growth. Additionally, most relevant environmental variables are strongly correlated making spurious correlations possible. There are a few delineated set of approaches for analyzing these datasets in the literature to be explored (e.g. Kocher et al 2012, McMahon and Parker 2015). The project explores methodological and ecological questions related to the application of dendrometer methods in Hawaii and will result in a set of r scripts that can be used to analyze similar data for similar studies elsewhere in Hawaii. Because of the general lack of long-term plant demography data in Hawaii, high resolution plant dendrometer studies can offer major insights into how species of conservation concern are reacting to current shifts in climate means and variability. As such, this study may provide a tool to monitor ecological change necessary for effective conservation planning. High resolution digital dendrometers show great potential in advancing the understanding of climatic controls to plant growth and overall fitness. However, there are multiple analytical approaches identified in the literature that need to be assessed for their relevance to tropical settings in Hawaii and beyond.
