Monday, August 21, 2017

Natural Born Allelopaths

The Sussex Downs after rainfall is as lovely as it gets. This is a lyric from ‘Forget’, an introspective track from Ben Watt’s 2014 alt-folk album, ‘Hendra’. This song has been buzzing intermittently around my head for almost three years, and I was recently given an opportunity to put the writer’s assertion to the test.

The South Downs Way (Figure 110.1) is a 100-mile (160-km) route across the southern English coastal counties of Hampshire, West Sussex and East Sussex. It is largely a combination of footpath and bridleway, although there are sections of road, weaving through the chalk hills towards the south coast. Cycling along, at my usual 10 miles per hour, midday sunlight filtered through the trees, traffic noise was negligible, and gently-sloping embankments were bursting with masses of delicate, hat-shaped flowers on tall red stems (Figure 110.2). Watt’s word picture seemed perfectly apt – until I looked more closely.

Figure 110.1: From Winchester to Eastbourne, the South Downs Way is contained entirely within a National Park.

Copyright © 2017 Britainexpress

Figure 110.2: A single plant species dominating a West Sussex roadside

Copyright © 2017 Paul Spradbery

I lay my bike on a grassy verge and inspected the one-and-only species of plant. Most of the flowers were pink (Figure 110.3), the remainder pure white (Figure 110.4). Its lanceolate leaves had serrated edges and gave off a sweet balsam scent after I had crushed a few between my hands and cupped them to my face. Everything about this distinctive plant was beautiful. I clasped the nearest three or four stems and ripped them from the earth.

Figures 110.3 & 110.4: In accordance with the Monohybrid Phenotypic Ratio, the pink variety outnumbers the white exactly 3 to 1.

Copyright © 2017 Paul Spradbery

You see, the plant species was Impatiens glandulifera, a.k.a. Himalayan Balsam, and a botanical thug. It is currently one of the most invasive plants in England and fast becoming a major headache for gardeners and environmental agencies alike. It was originally imported in the 1830s on the pretext that its rapid growth and spread would make gardening a less expensive pastime for the masses. If a single plant could yield hundreds of seeds capable of surviving up to two years, then what was not to like?

The Law of Unintended Consequences was waiting in the weeds. Within a decade, the impatient Impatiens had successfully negotiated the nation’s garden fences. The dispersal capability of its seeds was even more astounding than Victorian horticulturists had anticipated. Once its flowering period has ended, usually by early autumn, the seed pods literally explode with even the gentlest disturbance, producing up to 800 seeds which can be propelled as far as seven metres from the parent plant. Prolific does not begin to describe its propagation capacity. Riverbanks are particularly vulnerable to colonization, as seeds can be transported downstream as far as the water flows. The Downs are especially susceptible to unintentional seed transfer, by wildlife, ramblers and vehicles – including, I am disheartened to admit, my own bike tyres and boot soles.

This thuggish invader – the plant, not I – is, in a Darwinian sense, stupendously fit. It is the tallest annual presently growing in England, enabling it to impede smaller native species by depriving them of sunlight and attracting a greater proportion of bees for pollination. As if that does not provide sufficient evolutionary advantage, it has another, more subtle, weapon in its competitive biological armoury. There is evidence to suggest that the Himalayan Balsam exhibits allelopathy. This means that it can excrete, as a by-product of its metabolism, powerful toxins capable of inhibiting the survival and growth of neighbouring species.

Throughout England, volunteers of ‘balsam-bashers’ have begun to tackle the growing threat to public spaces (Figure 110.5). One method of control is to uproot whole plants before seeds develop; another involves chemical controls. Perhaps the most promising method, however, currently being researched, involves the parasitic use of rust fungi, which act by infecting fast-growing invasive plants.

Figure 110.5: Busy balsam-bashing

Copyright © 2017 Julio Lopez

It would seem impossible to know how problematic this plant will be, a decade from now. Will it be semi-controlled? Will it be everywhere? I cannot predict its fate, yet I am confident that the Sussex Downs, especially after rainfall, will remain as lovely as Ben Watt described.

Copyright © 2017 Paul Spradbery

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