A lightweight, low cost autonomously operating terrestrial laser scanner for quantifying and monitoring ecosystem structural dynamics

摘要

The three-dimensional (3-D) structure of ecosystems is inherently dynamic. However, this is often ignored in ecological studies because it is difficult to characterize using traditional field methods. Terrestrial laser scanning (TLS) is a rapidly maturing technique to complement and enhance traditional field methods for quantifying 3-D geometric properties of ecosystems. Two major limitations of TLS include the low temporal resolution that often exists between each data acquisition, and the relatively high cost of such systems (entry level systems cost >$40,000 USD) that puts this method out of reach for many potential users. Consequently, TLS is currently limited as a mainstream method for capturing 3-D geometric ecosystem dynamics. The objectives of this study were to (i) describe the design of a lightweight (3.85 kg), low-cost ($<12,000 USD), autonomously operating terrestrial laser scanner (ATLS) and to (ii) test its ability to provide data to quantify and monitor ecological characteristics that exhibit structural change. We tested the utility of the ATLS data to quantify plant growth by measuring plants with different heights and diameter at breast height (DBH). Specifically, we derived the canopy heights of a conifer tree (Engelmann spruce, Picea engelmannii), broadleaf tree (Quaking aspen, Populus tremuloides), graminoid (Calamagrostis x acutiflora), and forb (Hemerocallis lilioasphodelus), and the DBH of Ponderosa Pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) trees. The ATLS was also tested under varying weather conditions (including rain, snowfall and temperature ranging from -9.1 to 21.1 degrees C), to quantify canopy structural changes in quaking aspen during leaf drop relative to a Ponderosa Pine that retained its leaves over the same time period. We also compared canopy structural changes quantified by ATLS canopy laser returns with those quantified using a commercial TLS. Our results showed strong agreements between observed and ATLS derived conifer tree canopy height (RMSE = 0.96 cm, r(2) = 1.00, slope = 0.96, intercept = 1.43), broadleaf tree canopy height (RMSE = 0.08 m, r(2) = 0.99, slope = 1.01, intercept = -0.38), graminoid and forb canopy height (RMSE = 1.56 cm, r(2) = 0.98, slope = 1.04, intercept = -2.22), and DBH (RMSE = 2.24 cm, r(2) = 0.99, slope = 0.99, intercept = 0.45). A strong relationship (r(2) = 0.86) also existed between the number of TLS and ATLS canopy laser returns. Our results indicate that the ATLS is suitable for monitoring and quantifying dynamics of plant growth and potentially many other 3-D properties of ecosystems. While further research is needed to better understand the effect of scan resolution, beam divergence, and atmospheric conditions on the accuracy of ATLS derived metrics, this instrument has great promise for providing new insights into dynamic ecosystem processes that are currently difficult to monitor at high temporal and spatial resolution