Rhododendrons in Yunnan, China - pH of Associated Soils
David J. J. Kinsman
Windermere, Cumbria, UK
Reprinted from The New Plantsman, March 1998

Summary

Soil pH measurements were made at several sites in NW Yunnan where rhododendrons grow in shallow soils overlying limestone. At almost all sites the soils had pH values of less than 6. Only Rhododendron primuliflorum was determined definitely to be growing under alkaline soil conditions (pH 7.4-7.9).

The high summer rainfall in Yunnan leads to the flux of acid precipitation and acid soil waters being downward through the soil profile throughout the active growing season and quite possibly at all seasons of the year; this climatic control adequately explains the occurrence of rhododendrons growing in even the thinnest of soils overlying limestone. In the United Kingdom, in contrast, except in the wettest areas of the country, net water deficit develops in soils during late spring, summer and autumn; this leads to an upward flux of soil pore waters and thus in areas of alkaline rocks and soils the root zones of rhododendrons become affected by these alkaline waters unless artificial irrigation is applied.

Introduction

Nearly all rhododendrons are intolerant of alkaline soils and this also holds true for most other members of the Ericaceae . A small number of rhododendrons has some tolerance of alkaline soils, but for most species and cultivars exposure to alkaline soil conditions leads fairly rapidly to death of the plant. Yet it is widely recorded throughout the Himalayas and western China in particular that many species grow in limestone areas and observation shows these plants to be thriving. Indeed, George Forrest reported many times the widespread occurrence of rhododendrons overlying limestone in western China, but botanists in Britain took little note of his observations. Cowan (1952) in his book entitled The Journeys and Plant Introductions of George Forrest concluded a discussion about this phenomenon with this rather extraordinary statement: "...and the theory is now perhaps of academic interest rather than practical application. The subject need not therefore detain us. "Yet these were factual observations, not mere speculation on the part of Forrest. Perhaps because no botanist had a rational explanation for the rhododendron/limestone association and maybe because Yunnan was a long way away it was considered that to dismiss and ignore the problem was an acceptable way forward.

Over the past 50 years many botanists have studied the ecological requirements of ericaceous plants, although studies on rhododendrons are somewhat few. The published literature is now replete with suggestions relating to this intriguing dilemma. A fairly common suggestion relates to calcium metabolism although authors frequently become confused by the terms alkalinity, hardness, calcium, magnesium and lime.

As a contribution to the debate I present below some field measurements of soil pH and also a brief analysis of the Yunnan climate as it relates to this problem. These measurements were made during a botanical expedition to Yunnan in September/October 1996, led by Alan Clark, in association with the Kunming Institute of Botany. John Nichols helped with field sampling which he did selflessly and on one occasion at considerable personal cost.

Methods

a)  SAMPLE COLLECTION. Soil samples were collected from measured depths beneath actively growing rhododendrons. All rhododendrons were identified by Alan Clark. Note was also made of the underlying rock formation, i.e., whether limestone or other rock type.

b)  LABORATORY MEASUREMENTS. Within 48 hours of collection soil samples of 20-30ml were well mixed to a slurry with a similar volume of bottled water and left to equilibrate for about half an hour before pH measurement.

Distilled water was in short supply, but it was found by repeated measurement that the commercially available bottled water had a pH close to 7, i.e., was pH neutral and would therefore contribute to the text sample equivalent amounts of hydrogen and alkalinity ions. In addition it should be noted that the contribution of hydrogen and alkalinity species from the added water would have been trivial compared with that contributed by the soil.

Samples were measured at least twice with a delay of at least 2 hours between measurements. No significant change in pH was recorded between sequential measurements even when 12 hours had elapsed.

A battery operated portable pH meter Qenway, Model 3070) was used, together with an Amphel pH probe (silver/silver chloride for both sensor and primary reference systems) and a temperature probe. Standard calibration procedures were followed, as recommended by the equipment manufacturer, using pH 4 and 7 buffer solutions.

The system proved remarkably stable and pH measurements are reproducible to ± 0.1 pH units. (See Table 1 for results.)

Interpretation

Samples 5 and 12 show pH values typical of surface waters which have equilibrated with limestone. The fact that after 12 hours Sample 6 did not differ significantly from Sample 5 also confirms this.

Three of the four areas where samples were collected were dominated by hard, fairly pure limestone, overlain by thin mineral and/or organic soils. Even where soils were only a few cm in thickness, measured soil pH values were mostly below 6. The most acid soils were generally those richest in humus and these occurred usually as the uppermost part of the soil profile and were commonly overlain by sphagnum moss.

Rhododendrons have shallow, fibrous root systems which in nearly all sample locations were seen to be restricted to the upper, acid horizons of the soil. This relationship was found for both large and small leafed species. The only exception was Rhododendron primuliflorum which commonly occurred anchored into small cracks and joints within the limestone, often hanging precariously off sheer limestone cliffs. The plants had generally accumulated a small quantity of organic debris at their base, but even this material had a measured pH of 7.4-7.9  (Samples 10 & 11). There seems to be no obvious way for one to argue for acid soil conditions in this instance; this truly does seem to be a species of rhododendron which is completely tolerant of alkaline conditions at its roots. It is instructive to consider the sources of acidity in these soils, most of which are so closely associated with limestone. One obvious source is decaying organic detritus, particularly sphagnum mosses. The other major source of hydrogen ions is precipitation, although no measurements of rainfall pH were made. Rain fall essentially everywhere is acidic, with typical pH values of 4.5-5.5. With precipitation somewhat greater than 1000 mm/yr, the natural addition of acidity from precipitation will be on the order of 10 gm of H+/m ² /yr.

However, acidity derived from precipitation and soil humus sources is readily neutralised by reaction with the underlying limestone as water percolates downwards. This is indicated by Samples 5 & 12 which both contain exceedingly low hydrogen concentrations (<10 ^-8 moles/l).

As limestone is situated so closely beneath the root zone of so many of these rhododendrons one might ask why it is that alkaline soil waters do not inhibit rhododendron growth. To provide a possible answer it is instructive to consider the climate of Yunnan.

Climate of Yunnan

The climate of the mountainous western areas of Yunnan is temperate and is typified by that of the Cangshan Mountains to the west of Dali (Duan Chengzhong, 1995). The aim of our expedition was to collect seed of plants which would be hardy in the UK. This might lead one to assume a close similarity between the climate of the two areas but this is only in part true. The hardiness requirement is largely related to the temperature regimes of the two areas.

The much lower latitude of Dali (26°N, compared with 54°N for the English Lake District) leads to greatly enhanced light levels in Yunnan. In addition, the annual total of sunshine hours is about 2200 at Dali, in contrast to little more than 1000 hours in much of the Lake District.

Seasonal temperatures at a 2940m station on the Cangshan (where we were collecting) average 8.2°C and average monthly temperatures are shown in Figure 1. For comparison are shown typical western UK temperature data, where we have a similar annual average of around 9°C. The Cangshan summer temperatures are presumably depressed by the cloudiness and monsoon precipitation, as one would expect such a continental site to show a rather higher amplitude seasonal temperature variation. In contrast, the western UK has a typical low amplitude maritime temperature curve.

Figure 1. Average monthly air temperatures in UK and Yunnan.

The 2940m station on the Cangshan averages close to 1800mm of precipitation per year, which is similar to that of Windermere, UK. Note, however, the very different season distribution (Fig. 2). In Yunnan, the six drier months of the year (November-April) provide barely 15% of the total, whereas the months of May-October are very much wetter. In contrast, in the UK there is little difference in the monthly precipitation throughout the year, i.e., we have no strongly developed dry or wet season.

Figure 2. Monthly precipitation in western UK (Windermere), the Thames region and Yunnan (Cangshan).

In both the Cangshan and the western UK annual evapotranspiration losses are about 30-40% of the annual precipitation input, but in the drier parts of Britain evapotranspiration losses are higher, exceeding 60% of annual precipitation over large areas. These losses are strongly temperature dependent and are at a maximum in the warmer months of the year when plant growth, transpiration rates and simple evaporation from soils are all at a maximum (Fig. 3). Seasonal evapotranspiration curves for the water authority areas of England and Wales are remarkably similar and all exceed 60mm/month from late April to mid/late August. However, monthly mean precipitation values differ markedly from area to area. In Fig. 3a are shown evapotranspiration and precipitation curves for Windermere, a wetter western UK site; in contrast, Fig. 3b gives curves for the Thames water authority area. The latter clearly indicates that evapotranspiration exceeds precipitation for about four months of the year and that this deficit will take until late October or November before excess precipitation makes up the deficit.

Figure 3. Long-term monthly mean (1961-1990) evapotranspiration and precipitation for Windermere (Fig. 3a, upper) and Thames region (Fig. 3b, lower).

Over much of the UK a situation similar to that portrayed in Fig. 3b leads to a net water deficit progressively developing in soil profiles especially all areas southeast of a line from the Humber to Lyme Bay. Maximum deficits are often reached in late September and early October. Interstitial waters within the soil profile consequently are drawn upwards to satisfy plant water requirements (Fig. 4). In Yunnan, the period of maximum evapotranspiration loss will coincide with the summer period of high precipitation; as a result, soil profiles in Yunnan will not develop a net water deficit, i.e., in Yunnan the water flux is downward through the soil profile at all seasons of the year (Fig. 4).

Figure 4. Direction of water flux in soils in Yunnan and UK throughout the year.

Thus in Yunnan rhododendrons are bathed in acid water which percolates downward through the shallow soils all year long. As long as a thin soil is present overlying the limestone, the plants effectively have no knowledge of the chemistry of the underlying rock formation. In contrast, in areas such as the UK with a seasonal net water deficit, rhododendron root zones will be affected by the chemistry of the deeper soil waters when these are drawn upward. To grow rhododendrons in the UK where deeper soils are alkaline one needs to irrigate and prevent net water deficit conditions establishing themselves (Only in the highest rainfall areas of the UK does summer rainfall exceed summer evapotranspiration rates).

In summary, I would propose that the frequent observation through the Himalayan and western China areas by Forrest, Kingdon Ward and other plant collectors, of rhododendrons growing on limestone rock formations can be adequately explained by the constant downward flux of acid soil waters through the root zone; this situation is related to the very high summer rainfall of these areas, i.e., the summer monsoon.

Acknowledgments

Professor Jim Wallace, Institute of Hydrology, helpfully made available certain climatic data. I thank my wife for much help with graphics.

References
Cowan, J. M. 1952. The Journeys and Plant Introductions of George Forrest . Oxford Univ. Press. London. 252pp.
Duan Chengzhong (Ed). 1995. Scientific Investigation of the Plants on Cangshan Mountain. (In Chinese) Yunnan Science & Technology Press. 238pp.
Thomson, N., Barrie, L. A. and Ayles, M. 1981. The Meteorological Office's Rainfall and Evaporation Calculation system; MORECS. Met. Office Hydrological Memorandum No. 45.

Dr. Kinsman holds a doctoral degree in geology and for many years was a professor of geochemistry at Princeton University. He is a life-long gardener in both American and England.