92 PROCEEDINGS OF THE NEW ZEALAND ECOLOGICAL SOCIETY, VOL. 23, 1976 NOTES ON THE INFLUENCE OF DROUGHT ON THE BUSH REMNANTS OF THE MANAWATU LOWLANDS JOHN OGDEN' Department of Botany and ZOO/DRY, Massey University, Palmerston North SUMMAR Y: The data of Atkinson and Greenwood (1972) on drought damage to trees in 1969.70 in two bush patches in the Manawatu district are reassessed, and an index of drought susceptibility based on their data used to classify species into three categories of susceptibility. The results of this categorisation are compared with independent data on the effects of the 1972-73 drought on the nearby bush remnant at Ashhurst Domain. The behaviour of species in all three bush patches is shown to be similar, Comparison is also made with scanty data from other bush remnants, and with the upper altitudinal limits of the species in each category. The results suggest that the different tree species involved have intrinsically different suscept- ibilities to water stress, which partially explains why in nature they are sorted into hydrologi- cally different situations. The potential application of knowledge on the drought susceptibility of species to reserve management on the one hand, and to past climatic reconstruction using dendrochronology and population structure on the other, is pointed out. INTRODUCTION The present composition and structure of both Podocarpu,~' and Nothofagus dominated forest stands With the upsurge of interest in the recent history has been attributed in part to the climatic conditions of our climate, and in particular the numerous (the "little Ice Age") occurring between 1600-1800 demonstrations that quite significant climatic changes AD (see, e.g., Wardle, 1963; Park, 1971; Ogden, 1971; have occurred during the lives of many forest trees Salinger, 1975), If, in general, present forest com- living today, the plant ecologist will perhaps begin position is in part a function of past climate, then to pay more attention to the historical climatic or future composition will be influenced by the marked climate-related factors which have influenced the secular increase in temperature which has been present composition of forest stands. Although this occurring throughout New Zealand since c, 1935 approach has a venerable history in New Zealand (Salinger and Gunn, 1975), Rainfall during this (e,g. Holloway, 1954), its full aJ:plication has been period has shown no significant variation (Salinger hindered by lack of adequate dating for events and Gunn, 1975), Any tendency towards jncreased hundreds of years in the past, by insufficient know- dryness will have its main effect on small isolated ledge of the population dynamics of the species bush remnants, because "drying of the forest floor involved, and, related to this, by uncertainty about is a vital factor militating against the survival of the ways in which forest communities as a whole many species" in such situations (Esler, 1962). If the respond to extreme climatic events or secular changes. processes of vegetation change occurring in small The first of these inadequacies will probably be over- remnant bush patches can be adequately documented, come by advances in dendrochronology, and inform- and if the climatic factors can be separated from ation on the population dynamics of individual the anthropogenic ones, then information of value species is gradually accumulating (e.g, Wardle, 1970, to reserve management in a wider context is almost June and Ogden, 1975.) In the field of vegetation sure to be obtained, Documentation of the effects dynamics there are very few data, yet it is in this of drought, or other extreme events, is particularly field that information is urgently needed if sound va]uable because it is during the extremes that other- long-term management strategies for forest reserves, wise imperceptible secular changes become concen- national parks etc., are to be implemented, trated and quantifiable. Vegetation change associated with drought occurs * Present address: Department of Biogeography and because certain species are more susceptible to Geomorphology, Research School of Pacific Studies, Australian National University, P.O. Box 4, Canberra, drought than others, just as changes in composition A.C.T, 2600, Australia, resulting from browsing by introduced mammals OGDEN: INFLUENCE OF DROUGHT ON BUSH REMNANTS IN THE MANAWATU 93 occurs through differential palatability*. Depletion RESULTS or extinction of certain susceptible species may be followed in subsequent non-drought years by expan I. A reassessment of the data in Tables 2 and 3 of sion or invasion of more tolerant species. Unfortun Atkinson and Greenwood (1972). ately the degree of drought susceptibility of a species Table 2 of Atkinson and Greenwood (1972) com- cannot be determined simply by scoring the apparent prises a list of species from different sites within damage it suffers in a particular drought, because Bledisloe Park, Palrnerston North, classified accord- susceptibility must be assessed on a time scale similar ing to size classes and according to the degree of to the generation time of the population. For drought damage recorded in 1969-70, The drought instance. if the droughfs primary effect is to increase damage classes recognised were: seedling mortality, then species with annual seed U plant apparently unaffected production and abundant seedlings will have a higher D many leaves drooping or affected in some probability of long-term survival than species with other way by drought few seeds or seedlings which require conditions for W all leaves wilted, but green foliage stiJl present; germination or establishment that are not fulfilled with trees and shrubs, death of branches was annually, despite the much greater apparent damage sufficient to alter shape of crown to the former species, Also, a ranking of species in X no green foliage, plant apparently dead, terms of drought susceptibility derived from one Table 3 of Atkinson and Greenwood (1972) records area will almost certainly not apply to all situations the same data for species from Keeble's Bush. in which some of these species occur. Just as the Manawatu District. relative growth rates of different tree species change If for any plant species, jrrespective of size-class according to site conditions (see, e,g. Cameron, 1959) and site, the number of individuals showing severe so that a species which is a fast growing dominant drought stress (Wand X in Table 2 of Atkinson and in one forest type may be a slow growing subordinate Greenwood) is expressed as a proportion of the total in another, so also will other physiological characters, number of individuals of the species observed, an such as drought resistance, vary in their phenotypic "index of drought susceptibility" is obtained, This expression according to habitat conditions. Also, it index varies from 0, where no individuals were placed seems likely that in widespread species, particularly in the Wand X columns of Table 2. to 1.00 where those which span a broad altitudinal range, there all the individuals were in these two columns, Thus, will be clines of drought resistance, so that different for the drought resistant A /ectryol1 exce/sus the index populations of the same species may differ in their is ° in both patches of bush studied, while for the ability to withstand extreme drying. Finally, one susceptible Beilschmiedia tawa the index is 0.67 for has to remember that the present distribution of a BIedisloe Park and 0.92 for Keeble's Bush. The species may be in part determined by the history of :Jbsolute value of this index will of course vary with earlier droughts. the severity of the drought in which the observations These considerations led Atkinson and Greenwood are made, but nevertheless it allows the species to (1972) to conclude that it was not possible to use be ranked in order of observed damage. For Bledis- their data-on observed damage to plants in the loe Park the mean index for the 33 trees and shrubs 1969-71 drought in two bush remnants in the Mana- listed is 0.19, for 9 woody climbers it is 0,35 and watu lowlands-to rank the species in order of for J5 ferns it is 0.53. For Keeble's Bush the mean drought tolerance. This paper, in calling for more index for 16 trees is 0,24 (Table 3 of Atkinson and attention to be paid to this question, reassesses that Greenwood, excluding Cyathea dealbata). While conclusion. For comparison, data on the effects of these results suggest that, in general, ferns show more drought in 1973 at Ashhurst Domain (Manawatu signs of drought stress than woody climbers which district) are presented, and reference made to historic- in turn show more stress than trees, they do not ally recorded vegetation changes in a similar bush necessarily teIJ us anything about the order in which patch and to the altitudinal limits of the species in the groups might become extinct in any bush patch. the nearby mountains. Taking indices of drought susceptibility derived from the data in Tables 2 and 3 of Atkinson and Greenwood it is possible to classify the trees and shrubs (and Cyathea dealbata) into three broad categories, * In practice, of course, these two causes of change are Category 1 Plants placed only in classes U and often confounded. D by Atkinson and Greenwood, 94 PROCEEDINGS OF THE NEW ZEALAND EcOLOGICAL SOCIETY, VOL. 23, 1976 therefore with a drought susceptibil- ate1y) and 'P' <0.01. A second analysis in which the ity index of O. ("Resistant") first five species in the list (tied ranks) were amalgam- Category 2 Plants with more individuals in ated and the degrees of freedom consequently classes U and D than in classes W reduced from to to 6 gave 'R' = .708, 't' = 2.456 and X, and therefore with a drought (approximately) and 'P' <0.05.