Investigation of Temperature and Aridity at Different Elevations of Mt

Int J Biometeorol DOI 10.1007/s00484-012-0570-6

SHORT NOTE

Investigation of temperature and aridity at different elevations of Mt. Ailao, SW China

Guangyong You & Yiping Zhang & Yuhong Liu & Douglas Schaefer & Hede Gong & Jinbo Gao & Zhiyun Lu & Qinghai Song & Junbin Zhao & Chuansheng Wu & Lei Yu & Youneng Xie

Received: 28 July 2011 /Revised: 8 June 2012 /Accepted: 8 June 2012 # ISB 2012

Abstract Our current understanding is that plant species 2,680 m than at 2,480 m, especially for the surface soil distribution in the subtropical mountain forests of Southwest layer. China is controlled mainly by inadequate warmth. Due to We conclude that the decrease in temperature does not abundant annual precipitation, aridity has been less considered effectively explain the sharp transition between these forest in this context, yet rainfall here is highly seasonal, and types. During the dry season, plants growing at 2,680 m are the magnitude of drought severity at different elevations likely to experience more drought stress. In seeking to has not been examined due to limited access to higher understand the mountain forest distribution, further studies elevations in this area. should consider the effects of drought stress alongside those In this study, short-term micrometeorological variables of altitude. were measured at 2,480 m and 2,680 m, where different forest types occur. Drought stress was evaluated by combin- Keywords Drought . Water evaporation demand . Soil ing measurements of water evaporation demand (Ep) and volumetric water content . Bulk density . Subtropical soil volumetric water content (VWC). The results showed mountain forest that: (1) mean temperature decreased 1 °C from 2,480 m to 2,680 m and the minimum temperature at 2,680 m was above freezing. (2) Elevation had a significant influence Introduction on Ep; however, the difference in daily Ep between 2,480 m and 2,680 m was not significant, which was possi- In subtropical mountain forests with high annual precipita- bly due to the small difference in elevation between these tion, temperature patterns are considered to be controlling two sites. (3) VWC had larger range of annual variation at factors of species distribution (Jobbágy and Jackson 2000).

G. You : Y. Zhang : Y. Liu : D. Schaefer : Z. Lu : Q. Song : H. Gong J. Zhao : C. Wu : L. Yu Faculty of Ecotourism, Southwest Forestry University, Key Laboratory of Tropical Forest Ecology, Xishuangbanna Kunming, Yunnan 650224, China Tropical Botanical Garden, Chinese Academy of Sciences, Menglun, J. Gao Mengla, Yunnan 666303, China Institute of the Urban Environment, Chinese Academy of Sciences, Y. Zhang : Y. Liu : Z. Lu : C. Wu Xiamen, Fujian 361021, China Ailaoshan Station for Subtropical Forest Ecosystem Studies, Chinese Ecosystem Research Network, Y. Xie Jingdong, Yunnan 676209, China Jingdong Bureau of National Nature Reserve, Jingdong, Yunnan 676209, China Y. Zhang : Y. Liu : Z. Lu : C. Wu National Forest Ecosystem Research Station at Ailaoshan, Y. Zhang (*) Jingdong, Yunnan 676209, China Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden (Kunming Section), Chinese Academy of Sciences, G. You : J. Gao : Q. Song : J. Zhao : L. Yu 88 Xuefu Road, Graduate School of the Chinese Academy of Sciences, Kunming, Yunnan 650223, China Beijing 100049, China e-mail: [email protected] Int J Biometeorol

In contrast, drought stress with elevation has not been ex- Materials and methods amined except in some studies of arid areas (Vargas- Rodriguez et al. 2005; Köhler et al. 2006; Urbieta et al. Study site 2008). However, in some climates rainfall is highly seasonal and limited mostly to a short wet season. In the following Sampling was conducted in the northern part of Ailaoshan dry season, there are high water losses via evapotranspira- National Natural Reserve, which is the largest remaining tion and soil-water depletion. area of natural subtropical broad-leaved forest in China. The Potential evapotranspiration and soil water content Ailaoshan Station for Subtropical Forest Ecosystem Studies are two important indicators used to evaluate drought (ASSFE; 24°32′N, 101°01′E; 2,480 m a.s.l.) of the Chinese stress. For potential evapotranspiration estimations with Academy of Sciences is located in Jingdong County, temperature-dependent methods, water evaporation de- Yunnan Province (Fig. 1). A standard meteorological sta- mand decreases with elevation (Staudinger and Rott tion, operated by ASSFE, has been in operation at the site 1981; Lambert and Chitrakar 1989; Shevenell 1999;Liu since 1981. Based on long-term meteorological observa- et al. 2007). When more climatic variables are consid- tions from that station, the annual mean temperature is ered, however, water evaporation demand can increase 11.0 °C, with a monthly mean temperature of 5.2 °C in the with elevation (Leuschner 2000; Köhler et al. 2006). coldest month (January) and 15.2 °C in the warmest month Soil water content is influenced by precipitation, water (July). Average annual rainfall is 1,902 mm, with 1,630 mm evaporation demand, and soil properties, which results in falling during the wet season (May–October) and 272 mm in site-specific relationships between soil water content and the dry season (November–April). The Penman potential elevation (Denslow et al. 2006; Köhler et al. 2006; evaporation is 441.5 mm in dry season and 383.7 mm in Cierjacks et al. 2008). Therefore, observed water evapo- wet season (unpublished data). Consequently, there is a net ration demand and soil water content will assist in eval- accumulation of water in the wet season, and a net loss in uating aridity along elevation gradients. the dry season. The Hengduan Mountains of Southwestern China, and particularly the well-preserved area of Ailaoshan Microclimatic observations and water evaporation demands National Natural Reserve (ANNR), contain an abundant subtropical mountain humid evergreen broad-leaved for- During four sampling periods (in July 2008, October 2008, est (MHEF) with the forest canopy typically being 25– January 2009, and April 2009), simultaneous measurements 30 m in height. At altitudes from 2,600 m to the sum- were made of micrometeorological variables in the open mits (approximately 3,100 m), the forest canopy height lands of MHEF (2,480 ma.s.l.) and MDF (2,680 ma.s.l.). decreases to 5–7 m, accompanied by changes in forest In this study, measurement in MHEF was made by the auto- species composition, community structure, and forest recording meteorological station (affiliated to ASSFE), physiognomy (Wu et al. 1983; Qiu and Xie 1998). where wind speed was recorded at 10 m height. Based on Therefore, it has been termed top-mountain dwarf mossy previous simultaneous observations (14 September 2005–17 forest (MDF). Due to limited access to higher elevations, September 2005) of wind speed at the heights of 2 m and few studies have explored transitions in forest types in relation to altitude (Shi and Zhu 2009). Traditional understanding of species distributions at different altitudes has focused on the temperature regimes (Fang et al. 1996; Liu et al. 2007;Yangetal.2009), and seasonal drought stress has been largely disregarded in this context due to high annual precipitation in this region. This study investigated temperature and aridity at two elevations with different forest types. The objectives of this study were to answer the following questions: (1) related to temperature-controlled transition of forest types, what is temperature difference between these two elevations? (2) Does aridity, the combined effect of water evaporation demand and soil water content, increase with altitude? This study aims to obtain further knowledge on the transition of the forest types found at different elevations in subtropical mountain forests of Southwest China. Fig. 1 Location of the study site Int J Biometeorol

10 m, we found a linear transfer coefficient of 0.377 (n075; replicate measurements of VWC at MDF, soil samples at R200.900) for 10 m to 2 m. The micrometeorological var- each depth were collected with a corer. The sample was iables in MDF were recorded by micrometeorological ob- dried to constant weight at 80 °C and the VWC was servation system with data collected by a data logger calculated. (CR1000, Campbell Scientific, Logan, UT). We need one For comparison of soil bulk densities, we randomly se- clear day to install the micrometeorological observation lected three sites in both MHEF and MDF and collected the system, and another clear day to dismantle it after the soil samples at different depths with a corer. The samples observations. Therefore the lengths of the observation peri- were dried to constant weight at 80 °C and the bulk densities ods were not equal. Each sampling period lasted more than were calculated. 10 days (11 days in July 2008; 12 days in October 2008; 12 days in January 2009; 17 days in April 2009), and data Data analysis were collected at 30-min intervals. Air temperature (T) and vapor saturation deficit (VPD) were measured by a temper- For micrometeorological measurements at the two eleva- ature and relative humidity probe (HMP45C; Campbell tions, altitude and month were the categorical predictor Scientific). Wind speed at 2 m height (U2) was measured variables. Effects of altitude and month were analyzed joint- by an anemometer (05103 R.M. Young, Campbell ly using Two-Way-ANOVA, followed by Tukey HSD post-

Scientific). Net radiation (Rn) was measured by a net radi- hoc test for the difference among months. In each observa- ometer (CNR1, Kipp and Zonen, Delft, the Netherlands). tion period, normal data distribution was tested by Shapiro- For measurements in open lands, spatial replication was not Wilk. Statistical significance of difference between eleva- necessary as a result of uniformity in site conditions. tions was conducted by the Paired-samples-t test for periods Water evaporation demand was evaluated by PenPan with normally distributed data. Non-parameteric Two- model (Roderick et al. 2007), which is based on Penman’s independent-samples test was conducted for the periods potential evapotranspiration method (Eq. 1). with non-normally distributed data. All the statistical analyses were conducted by SPSS (Norušis 1998). Δ Rn ag Ep ¼ Ep;R þ Ep;A ¼ þ fqðÞU VPD Δ þ ag l Δ þ ag 2 ð1Þ Results Where, E (mm) is estimated pan evaporation, Δ (kPa °C−1) p Microclimate and water evaporation demand is slope of saturation vapor pressure with temperature (T,°C), R (MJ m−2) is net radiation, α (02.4 here) is the ratio of n Table 1 lists differences in micrometeorological variables effective surface area for heat and vapor transfer, λ − − between 2,480 m and 2,680 m. From 2,480 m to 2,680 m, T (02.45 MJ kg 1) is the latent of vaporization, γ (kPa °C 1)is decreased 1.0 °C overall (P<0.01). During our observation the psychrometric constant adjusted for elevation, f (U )isa q 2 in January (coldest month), daily mean T decreased 1.6 °C function based on wind speed at 2 m height (U ), VPD (kPa) is 2 and the minimum T decreased from 2.6 °C to 1.2 °C on relative vapor saturation deficit. average.

The mean Ep in each observation period increased from Soil volumetric water content 2,480 m to 2,680 m. As weather conditions during our short-

term study periods were not uniform, daily Ep in January A soil profile with measurements of soil volumetric water and April followed the normal distribution and daily Ep in content (VWC) at different depths would have done consid- July and October did not. Two-way-ANOVA showed that erable damage to the roots of nearby plants. As this study is elevation had significant influence on the Ep (P<0.05), and located in the Ailaoshan National Nature Reserve, such Ep in April was significantly higher than other months (P< experiments were not generally allowed. VWC was mea- 0.01). The difference in Ep between the two elevations was sured by a neutron probe (CNC503DR, China) with mea- not significant. surement accuracy of 3 %. To represent the forests, we chose sites for VWC measurements with moderate terrain Differences in VWC and soil properties conditions. Five tubes were installed randomly in the forest floor of MHEF. As soil at higher elevations is rocky, only Figure 2 shows variations of VWC at MHEF and MDF. In two tubes were placed successfully in the forest floor of both forest types, VWC was high in wet season and low in MDF. Using these tubes, VWC at 0–10, 20–30, 40–50 cm dry season. The annual range of VWC variation at MDF depths were continuously measured with the neutron probe was larger than those of MHEF, especially for the surface atevery10days(startingon5August2009).Totake soil layer. Bulk density was higher in MDF than in MHEF, Int J Biometeorol

Table 1 Short-term micrometeorological observations and Sites Elevation January April July October water evaporation demands −2 (mean ± standard error) at Rn (W m ) 2,680 m 69.9±10.6 121.1±10.3 97.4±12.1 64.3±13.6 2,480 m and 2,680 m (12 days in 2,480 m 47.3±10.1 115.6±12.7 54.7±9.5 40.2±8.6 January 2009, 17 days in April T (°C) 2,680 m 2.7±0.3 12.1±0.5 14.4±0.3 9.9±0.4 2009, 11 days in July 2008 and 2,480 m 4.3±0.5 13.3±0.3 15.2±0.2 10.3±0.6 12 days in October 2008) −1 U2 (m s ) 2,680 m 1.5±0.2 1.8±0.1 0.9±0.2 1.2±0.1 2,480 m 1.3±0.1 1.3±0.1 0.8±0.1 1.0±0.1 VPD (hPa) 2,680 m 0.4±0.1 3.6±0.5 1.0±0.2 0.5±0.1 2,480 m 0.5±0.2 3.9±0.5 1.3±0.2 0.7±0.1 −1 Ep (mm day ) 2,680 m 0.9±0.2 2.7±0.3 1.8±0.3 1.2±0.2 2,480 m 0.7±0.2 2.5±0.3 1.1±0.2 0.8±0.2 and bulk density increased with depth and was nearly dou- between the two elevations was small and not a compelling ble that of the surface soil layer in MDF (Fig. 3). explanation for the sharp change in forest type. Moreover, minimum temperature during our observation period did not freeze in MDF, implying that coldness was not fatal to MDF Discussion and implications plants. Another fact that could not be explained by inadequate warmth is the drought-adopted leaf morphology in MDF. For Microclimatic differences in forest types example, Shi and Zhu (2009) reported that leaf sizes were small, and few tree and shrub species have drip tip leaf apex Previous studies have suggested that forest ecotones are con- and papery leaves in MDF. A preliminary study showed that trolled by temperature patterns (Ohsawa 1995;Fangetal. MDF had a lower specific leaf area than MHEF, possibly 1996;Yangetal.2009). From 2,480 m to 2,680 m, mean related to increased insolation with elevation (Ackerly et al. temperature decreased 1.0 °C and approximately 350 °C day−1 2002). Therefore, a strictly temperature-controlled forest eco- for accumulated temperature. Although a decrease in temper- tone between 2,480 m to 2,680 m is questionable. To under- ature was observed at 2,680 m, this difference in temperature stand the transition of forest types, further study should pay

Fig. 2 Soil volumetric water content (VWC) at depths of 0– 10, 20–30, 40–50 cm in mountain humid evergreen broad-leaved forest (MHEF; solid symbols) and top- mountain dwarf mossy forest (MDF; open symbols). “Days” on 5 August 2009 Int J Biometeorol

Soil properties in different forest types

A decrease in soil water content with elevation was reported in Mt. Teide (Köhler et al. 2006) and in Hawaii (Denslow et al. 2006). The soil volumetric water content of 10 % at high elevation was thought to have a relationship with the for- mation of the treeline in Mt. Teide (Köhler et al. 2006). In this study, bulk density is much higher in MDF than in MHEF, which implies that the soil in MDF was rocky and lower in water holding capacity. Therefore, compared with MHEF, water loss through drainage and depletion could be higher in MDF. In addition, soil water content is the balance between recharge from up-slope and drainage toward the down-slope. As MDF is located at the summit of the mountain, moisture recharge from the upslope drainage is limited compared with MHEF. These factors could contribute to VWC having larger temporal variation in MDF than in MHEF (Fig. 2). Moreover, soil pH decreased from 4.4– 4.9 in MHEF (Gong et al. 2011) to 3.6 in MDF (Chen et al. 2010), due possibly to higher percolation and lower Fig. 3 Bulk density (mean ± stand deviation) at different depths of MHEF (solid symbols) and MDF (open symbols) rate soil organic decomposition in MDF (Shi 2007). Consequently, during the dry season, MDF plants could experience more drought stress than those of MHEF. more attention to the drought and edaphic conditions of these two sites (Doležal and Šrůtek 2002; Denslow et al. 2006; Seasonal aridity and implication Köhler et al. 2006). There is a difference of only 200 m in elevations of the At the present study site, pan evaporation was higher and micrometeorological observation sites, and the difference in rainfall was lower in the dry season. Consequently, seasonal daily Ep between these two sites was not significant. drought was revealed by the co-occurrence of high water However, we suggest that Ep differences between these evaporation demand and low soil water content during the two sites could be detected if their altitudinal difference dry season (Schaap and Bouten 1997; Schaap et al. 1997). was larger. In addition, the PenPan model might perform This pattern has been observed in previous studies, which better on the weekly or monthly estimations due to its have shown that the highest seedling mortality and litterfall exclusion of a heat-storage component (Thom et al. 1981). occur at the end of the dry season (Liu et al. 2002;Gongetal.

Therefore, mean level of Ep in more than 10 days observa- 2011). As a consequence of reduced soil water availability tions could better represent the water evaporation demand in with elevation during the dry period, adaptation to drought each observation period. Consequently, this study revealed stress could be important for plant growth, especially for MDF the increased Ep from 2,480 m to 2,680 m. This result seedling establishment (Leuschner 2000; Denslow et al. supports the reports that water evaporation demand 2006). In seeking to understand forest distribution patterns increases with elevation (Leuschner 2000;Köhleretal. with elevation, further study should focus on the effect of 2006) and contradicts Liu et al. (2007), who reported de- strengthened drought stress in MDF and its possible influence creased water evaporation demand with elevation through a on seedling establishment and species distribution. temperature-based estimation.

The increase in Ep with elevation was due mostly to the increase in Rn (Table 1). The increase in sunshine Conclusion hours with elevation in the dry season has been reported (Wang 1993), implying that solar radiation could increase Due to poor access to higher elevations, our previous with elevation. In addition, MDF is characterized by understanding of local meteorological conditions was in- higher soil organic content (Wu et al. 1983;Shi2007), complete. This study reports direct observation of micro- which decreases the surface albedo in MDF. Moreover, meteorology at higher elevation, which is helpful in net downward long-wave (net infrared) radiation could be understanding the transition of forest types with elevation. higher in MDF because MDF is more enclosed in the As temperature decreases only slightly from 2,480 m to atmosphere (Fu 1983). 2,680 m, it is concluded that transition of forest types at Int J Biometeorol these two elevations is likely not driven by the decease in Jobbágy EG, Jackson RB (2000) Global controls of forest line eleva- temperature, nor by winter freezing. Therefore, predictions tion in the northern and southern hemispheres. Global Ecol Bio- geogr 9(3):253–268. doi:10.1046/j.1365-2699.2000.00162.x of forest distribution following climate change should not Köhler L, Gieger T, Leuschner C (2006) Altitudinal change in soil and depend exclusively on temperature. From 2,480 m to foliar nutrient concentrations and in microclimate across the tree 2,680 m, water evaporation demand increased and VWC line on the subtropical island mountain Mt. Teide (Canary – decreased, especially in the dry season. Consequently, plants Islands). Flora 201(3):202 214. doi:10.1016/j.flora.2005.07.003 Lambert L, Chitrakar B (1989) Variation of potential evapotranspira- growing at the higher elevation experience higher water tion with elevation in Nepal. Mt Res Dev 9(2):145–152 evaporation demand, lower VWC and poorer edaphic con- Leuschner C (2000) Are high elevations in tropical mountains arid dition especially in the dry season. One possible conse- environments for plants? Ecology 81(5):1425–1436 quence of the enhanced drought stress with elevation is Liu WY, Fox JED, Xu ZF (2002) Litterfall and nutrient dynamics in a montane moist evergreen broad-leaved forest in Ailao Mountains, that seedling establishment is stunted. Therefore, further SW China. Plant Ecol 164(2):157–170 study should include seedling establishment when seeking Liu Y, Zhang YP, He DM, Cao M, Zhu H (2007) Climatic control of an understanding of forest distribution patterns in this area. plant species richness along elevation gradients in the longitudinal range-gorge region. Chinese Sci Bull 52:50–58. doi:10.1007/ s11434-007-7006-4 Norušis M (1998) SPSS Base 8.0 for windows user's guide. SPSS, Acknowledgments This study was supported by Key of the Yunnan Chicago, IL Natural Science Foundation of Yunnan Province, China (No. Ohsawa M (1995) Latitudinal comparison of altitudinal changes in 2011FA025), the Development Program in Basic Science of China forest structure, leaf-type, and species richness in humid monsoon (No. 2010CB833501), the Strategic Priority Research Programs of Asia. Vegetatio 121(1–2):3–10 the Chinese Academy of Sciences (No. XDA05050601, No. Qiu XZ, Xie SC (1998) Studies on the forest ecosystem in Ailao Moun- XDA05050502) and the Knowledge Innovation Program of the Chi- tains Yunnan. Yunnan Science and Technology Press, Kunming nese Academy of Sciences (No. KZCX2-YW-Q1-05-04). We thank the Roderick ML, Rotstayn LD, Farquhar GD, Hobbins MT (2007) On the Ailaoshan Station for Subtropical Forest Ecosystem Studies (ASSFE) attribution of changing pan evaporation. Geophys Res Lett 34: for providing long-term data and accommodation. Three anonymous L17403. doi:17410.11029/12007GL031166 reviews are appreciated for their kind suggestions for improving this Schaap MG, Bouten W (1997) Forest floor evaporation in a dense manuscript. We also thank Prof. Yong Tang, Prof. Zheng Zhen, Dr. douglas fir stand. J Hydrol 193(1–4):97–113 Yongjiang Zhang, Dr. Zhenghong Tan, and Dr. Lu Qiao for engaging in Schaap MG, Bouten W, Verstraten JM (1997) Forest floor water content insightful and useful discussions. Yugang Yao, Mingda Zhang, dynamics in a douglas fir stand. J Hydrol 201(1–4):367–383 Pengchao Zhang, Linhui Li, Zenghe Bai, and Fengxiang Qu pro- Shevenell L (1999) Regional potential evapotranspiration in arid cli- vided assistance with field measurements and data analyses. Jiafu mates based on temperature, topography and calculated solar Wu provided his help in figure drawing with Geographic Informa- radiation. Hydrol Process 13(4):577–596. doi:10.1002/(SICI) tion Systems. 1099-1085(199903)13:4<577::AID-HYP757>3.0.CO;2-P Shi JP (2007) Community ecology and biogeography of the mossy dwarf forest in Yunnan. 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