Quantifying water use by temperate deciduous forests in South Korea : roles of species diversity, canopy structure, and complex terrain

About seventy percent of South Korea is covered with forests, most of which are found in the mountain regions since mountains receive more rainfall and are difficult terrains not suitable for agriculture. Because mountains are important water sources for cities and human population downstream, performing water balance for forest catchments has become a research priority. The ongoing shift from coniferous to species-rich deciduous forests due to a changing government policy and the anticipated changes in future climate, associated with increasing amount of rainfall and temperature will also impact forest water use, calling for an urgent need to understand how forests, in their current status, use water. The knowledge is vital for predicting water requirements for the future forest. The warm-deciduous temperate forests found in South Korea, however, have a high diversity of tree species, have multi-layered canopies and are mostly located on rugged mountainous terrains, which make it difficult to quantify forest water use, a basic requirement for catchment water budgeting. The main objectives of this study were to: (1) identify the roles of species diversity in tree and forest water use, (2) examine the impact of canopy structure on forest transpiration, and (3) evaluate the influence of terrain on forest water use. Site-specific studies were carried out in three different natural deciduous forests, namely, Gyebang (GB), Gwangneung (GN) and Haean (HA) forest sites, representing the general structure of S. Korean forests. GB site is known for its high species diversity, GN site is an old forest growth at climax, with clearly defined understory and overstory canopy layers while the HA site was located with in a catchment, with strong elevation changes within short horizontal distances, rising from 400 to 1,000 m a.s.l., and in different aspects. Four locations with varying elevations and aspects were chosen in the HA site. Tree water use (TWU) and canopy transpiration (EC) were estimated from sap flux density measured with thermal dissipation probes. Understory transpiration (EU) was measured using stem heat balance while ecosystem evapotranspiration (Eeco) was determined using eddy covariance technique. Air temperatures (Ta), precipitation, solar radiation, vapor pressure deficit (VPD), wind speed were measured from weather stations and soil water content was measured from frequency domain reflectometry (FDR) sensors at the respective study sites. Vegetation surveys, including diameter at breast height (DBH), tree density, species composition, sapwood area (AS), and leaf area index were performed in all the sites. Canopy conductance (GC) and stomatal sensitivity to VPD were assessed based on transpiration and microclimate measured at each site. A functional allometric relationship was established between AS and DBH, and also between TWU and DBH for all the study sites; first for single species and then combining all the species either in a single site or in all the sites. Irrespective of tree species, AS and maximum TWU were significantly correlated with DBH in a power function for AS (R2 = 0.77, P <0.0001) and both in power (R2 = 0.63, P <0.0001) and sigmoid functions (R2 = 0.66, P <0.0001) for TWU, for the co-occurring species as well as across the sites, suggesting that DBH can be a good predictor of stand AS and maximum TWU, based on the established allometric functions. Early bud break and development of the understory compared to the overstory canopy resulted in an earlier onset of forest transpiration, with EU contributing 22% and 14% between April and May to the total forest transpiration. This high contribution was favored by high radiation and VPD in the understory, since the overstory was still undeveloped and open. Despite diminishing VPD and light conditions in the understory between June and August, the understory continued to transpire a substantial amount of water, contributing 10% of the total transpiration. The seasonal patterns of both EO and EU were synchronized to canopy development, while VPD and radiation determined daily trends. EO and EU accounted for 80% of Eeco in spring but only 60% during the monsoon period due to lowered radiation input, VPD, and plant area index (PAI). Thus, Eeco is largely influenced by transpiration rate and its seasonal variation and also canopy structure. Early saturation of EC at relatively low VPD and also a rapid decrease in GC with increasing VPD were observed in the forest stand located at the highest elevation studied (950 m) in the HA site, compared to the GN and the other forest stands in HA. These differences in transpiration rates and stomatal response can be explained by greater stomatal sensitivity to VPD of 0.83 found at the 950 m site compared to 0.63–0.66 in the other study sites. However, the main controlling factor of the change in stomatal sensitivity at the 950 m stand is uncertain. Although maximum daily EC were correlated with AS of the forest stands at different sites (R2 = 0.78, P <0.01), annual EC declined with increasing elevation, i.e., 176 >175 >110 >90 mm year−1 at 340 >450 >650 >950 m, respectively. Decline in total EC was due to the decline in annual Ta, daytime VPD, and length of growing season at higher elevations. The GB site, which was located at 960 m elevation, however, did not display a same response pattern as those observed at the 950 m site. It is likely because these sites were under different environmental conditions, i.e., GB site is exposed to higher Ta and higher humidity, and is sheltered (lower wind speeds). These observations emphasize the complexity associated with estimation of transpiration in rugged terrains, since general principles do not always apply and the spatial patterns of forest transpiration are complex. Complexity arising from multiple tree species composition when estimating forest water use can be reduced by applying functional allometric relationship linking tree size and water use. Forest canopy structure and physical location should be taken into account since they influence the way forests use water resources by altering microclimate and plant physiology. Based on our findings, estimation of forest water use on rugged terrains require repeated measurements at relatively small spatial scales since the driving factors change rapidly over very narrow vertical distances.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2013

Author dc:creator

• Jung, Eun-Young

Contributors dc:contributor

• Otieno, Dennis

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/114/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:114