Estimating the Water Requirements for Plants of Floodplain Wetlands

Note 26Canopy condition indexA visual assessment of tree canopycondition initially developed insouth-eastern Queensland forassessing health of eucalypt forests,has been used on floodplain trees, inparticular on black box Eucalyptuslargiflorens.The original index (Grimes 1987)was in four parts: crown size, crowndensity, dead branches andepicormic growth. Modifications bydifferent users to make this moresuitable for ecological purposesmake it difficult to comparefloodplains. Nonetheless, the indexhas proven useful so continues to beused.In modified form, this index hasbeen used for E. largiflorens on theMirrool Creek floodplain (Robertsand Wylks 1992) and on the Chowillafloodplain (Jolly et al. 1996).Estimates of tree health have beenused to assess and monitor noneucalyptspecies around wetlands inWestern Australia (Froend et al.1987).the other is oxygen deprivation in the root zone, resulting in reducedphotosynthesis, possibly phytotoxin trauma in the roots and evenculms, stunted roots, and reduced nutrient uptake. Symptoms forestablished plants include leaf-burning, leaf-shedding, prematurecanopy senescence, stunted growth, reduced reserves and dieback ofbranches. All these can be detected at the species level.Sustained stress, such as altered water regime, affects plant vigour. Thiscan alter competitive interactions between species, or even alterdominance and so create an opportunity for other species. At thecommunity level, this can be detected as a change in area, by shifts incommunity boundaries, and by increased incidence of opportunisticspecies or of species more closely associated with the ‘new’ conditions,whether they be drier or wetter. Thus, measures of communitycondition can include presence and abundance or area of invasivespecies. An indicator species may point to wetter conditions, or drierconditions, or seasonal shifts. Useful examples of indicator species fromeastern Australia are cumbungi Typha spp. for wet conditions, andlippia Phyla canescens for drier conditions (Figure 13). Indicatorspecies need to be clearly associated with the change, and not withanother environmental condition, such as stock grazing: someunpalatable Juncus species increase under grazing pressure.For species and community, lack of regeneration is a serious long-termloss of vigour. In choosing lack of regeneration as a measure (eg. lack ofseedlings, no juveniles or saplings, only ageing and senescent treespresent), it will be necessary to establish that this is an outcome ofwater regime alone or in combination with something else, and not theresult of other factors such as rabbits.Condition and growth-formLeaves are the basic unit of growth for most plants so are also a goodstarting point to assess plant vigour and condition. Some aspects of leafvigour are common across growth-forms. For example, it is a useful butsimplistic assumption that a plant with larger leaves, or greaterphotosynthetic area (more leaves, higher LAI) will be in better healththan a plant with smaller photosynthetic area or which has some leafarea that is not fully photosynthetic (yellowing, brown spots, deadleaves) or that is damaged or incomplete (insect damage, leaves beingshed, burnt). Condition of leaves can be assessed remotely, using NDVI(Note 25) or assessed from the ground using indices of canopy vigour(Note 26), or its physiological condition measured in the field.Many of the methods for measuring vigour have been developed foragricultural or forestry species. Consequently, the techniques andequipment for estimating health and vigour are better developed forfloodplain trees than for other growth-forms, especially aquatic species.There are few means for measuring the health and vigour of leaflessspecies (see below).TreesCanopy condition index. A visual assessment of canopy condition iscompared with reference material, then scored. When using suchindices, it is important to use printed reference material in the fieldthroughout the investigation in order to standardise observations. It isalso essential to identify examples of the full condition range, from deador very poor to vigorous (Figure 15). Canopy indices are ordinal data,58 Estimating the Water Requirements for Plants of Floodplain Wetlands